Download Developmental and genetic disorders in

Human Reproduction Update 1999, Vol. 5, No. 2 pp. 120–140
European Society of Human Reproduction and Embryology
Developmental and genetic disorders in
spermatogenesis
T.Diemer1,2,3 and C.Desjardins1
1Department
of Physiology and Biophysics and Department of Urology, University of Illinois at Chicago (UIC),
College of Medicine, 1819 West Polk Street, Chicago, IL 60611, USA and 2Department of Urology, University
Hospital, Justus-Liebig-University Giessen, Klinikstrasse 29, 35392 Giessen, Germany
The most common cause of male infertility is idiopathic. Fresh insights based on genetic and molecular analysis of the
human genome permit classification of formerly unexplained disorders in spermatogenesis. In this article, we review
new procedures that expand diagnostic and therapeutic approaches to male infertility. Recombinant DNA technology
makes it possible to detect specific chromosomal and/or genetic defects among infertile patients. The identification of
genes linked to disorders in spermatogenesis and male sexual differentiation has increased exponentially in the past
decade. Genetic defects leading to male factor infertility can now be explained at the molecular level, even though the
germ cell profile of infertile patients is too variable to permit classification of the clinical phenotype. Increasing knowledge of genes that direct spermatogenesis provides important new information about the molecular and cellular events
involved in human spermatogenesis. Molecular analysis of chromosomes and/or genes of infertile patients offers unique
opportunities to uncover the aetiology of genetic disorders in spermatogenesis. Increasing numbers of cases, previously
classified as idiopathic, can now be diagnosed to facilitate the treatment of infertile men. Advanced knowledge also poses
ethical dilemmas, since children conceived with assisted reproductive technologies such as intracytoplasmic sperm
injection (ICSI) are at risk for congenital abnormalities, unbalanced complements of chromosomes and male infertility.
Key words: chromosomal aberrations/infertility/male sexual differentiation/microdeletions/testis
TABLE OF CONTENTS
Introduction
120
Therapeutic approaches to disorders in
spermatogenesis
121
Chromosomal aberrations
121
Numerical aberrations in sex chromosomes
129
Endocrine disorders of genetic origin and their impact on
spermatogenesis
131
Defects in peptide hormone response elements
136
Conclusions
136
Acknowledgements
137
References
137
Introduction
Analysis of infertility cases worldwide indicates that the
largest percentage of these patients experience idiopathic infertility (Figure 1), typified by severe oligozoospermia or
azoospermia (Fisch and Lipshultz, 1992; Martin-DuPan et al.,
1997; Nieschlag, 1997; Sigmann et al., 1997; Zargar et al.,
1997). Investigation of chromosomes from patients with
azoospermia or oligozoospermia offers fresh insights that
begin to explain some of the causes of idiopathic infertility at
the level of specific genes, particularly those associated with
the X and Y chromosomes (Jaffe and Oates, 1997). Chromosomal defects formerly identified by cytological techniques
can now be explained on the basis of substitutions in individual base pairs within a gene (Tuerlings et al., 1997; Simoni
et al., 1998).
Genes on sex chromosomes have been implicated in a
growing number of spermatogenetic disorders because recessive mutations and deletions on the X and Y chromosomes
exert a direct impact on phenotypic sex. The absence of effective crossing-over in meiosis caused by negligible homology
of large parts of the sex chromosomes makes them susceptible
to recessive defects (Handel and Hunt, 1992). Recombinant
DNA techniques provide the potential for explaining as much
as 10% of the idiopathic forms of infertility that have heretofore gone undiagnosed (Simoni et al., 1998; Tuerlings et al.,
1998b). Adoption of new molecular procedures for detecting
3To whom correspondence should be addressed at: Department of Physiology and Biophysics, University of Illinois at Chicago (UIC), College of Medicine,
Room 150 CMW, 1918 West Polk Street, Chicago, IL 60612, USA. Telephone: +312-413-8466; fax: +312-413-1325; e-mail: [email protected].
Disorders in spermatogenesis
121
Therapeutic approaches to disorders in
spermatogenesis
Figure 1. Distribution of primary diagnoses among a cohort of 7802
infertile men attending an infertility clinic (Nieschlag, 1997). Only
the most probable cause of infertility is represented. The percentage
of hypogonadal patients may be over-represented due to consecutive
referrals to an established infertility clinic. Comparison of these data
with those reported from other clinics (Fisch and Lipshultz, 1992;
Martin-Du Pan et al., 1997; Sigmann et al., 1997; Zargar et al., 1997)
confirms that idiopathic infertility is the most frequent diagnosis
among infertile men.
aberrations in chromosomes or defects in genes has had profound investigative, clinical and ethical implications.
From an investigative standpoint, the use of recombinant
DNA techniques provides new insights into the mechanisms
directing germ cell maturation at all stages of spermatogenesis. At the clinical level, many new diagnostic approaches are
available, and these enlarge the therapeutic options for managing clinical disorders in spermatogenesis that were formerly untreatable. New diagnostic protocols also expand the
opportunities to make patients aware of their disorder and
facilitate prediction of the risks of transmitting a genetic disorder to offspring. From an ethical perspective, prudence and
caution should be exercised in recommending assisted reproductive protocols to infertile couples. Children born following
the use of intracytoplasmic sperm injection (ICSI) are more
likely to experience developmental abnormalities and male
infertility than those conceived naturally or with in-vitro fertilization (IVF) (Andrews et al., 1998; Bonduelle et al.,
1998a,b; Bowen et al., 1998).
In this article, we review the chromosomal, genetic and
clinical aspects of disorders in spermatogenesis, with a focus
on identifying genes responsible for disorders in germ cell
differentiation. Attention then shifts to genetic disorders interfering with testicular development and male sexual differentiation. New diagnostic and therapeutic approaches are
emphasized that will assist physicians with diagnosis and
treatment of male infertility.
Primary endocrine failure involving either gonadotrophin-releasing hormone (GnRH) or gonadotrophin deficiency is
associated with severe disorders in spermatogenesis (Behre
et al., 1997). Such disorders can be treated successfully by
replacement therapy with either GnRH or a combination of
follicle stimulating hormone (FSH) and luteinizing hormone
(LH) (Behre et al., 1997).
Other disorders in spermatogenesis fail to respond to endocrine therapy and require interventional approaches such as
ICSI (Palermo et al., 1992) when only a few spermatozoa are
available for recovery from an ejaculate (Harari et al., 1995).
Patients with azoospermia can be assisted if spermatozoa are
recoverable from the epididymal duct following epididymal
aspiration or can be isolated from the seminiferous epithelium
after testicular biopsy (Silber et al., 1995). Round spermatids
have been used successfully with ICSI when spermatogenesis
is arrested at the late spermatid stage, but optimal clinical
success is achieved with motile spermatozoa (Fishel et al.,
1995; Tesarik et al., 1995; Vanderzwalmen et al., 1995). Fertilization rates of 35% and pregnancy rates of ~25% represent
current standards for ICSI when spermatozoa are recovered
from azoospermic patients (Kahraman et al., 1996). Genetic
risks and developmental delays following use of the ICSI
procedure should be considered, since, according to some
studies, the probability that offspring may be affected adversely is significantly greater than after IVF or natural fertilization (Andrews et al., 1998; Bonduelle et al., 1998a,c;
Bowen et al., 1998).
No known therapeutic approaches are available to men
when spermatogenesis is halted prior to the completion of
meiosis. The fraction of patients who lack post-meiotic germ
cells is relatively small (Silber et al., 1995), but the most
profound disorders in spermatogenesis are prevalent in these
patients. Patients lacking post-meiotic germ cells may be assisted in the future, but as yet no known treatment can prompt
germ cells to complete meiosis and proceed with spermiogenesis.
Chromosomal aberrations
Chromosomal aberrations occur as two types: numerical and
structural. Numerical aberrations arise from a missing or accessory chromosome due to meiotic non-disjunction in either maternal or paternal germ cells. Accessory chromosomes, such as
trisomies and other polyploidies, may involve either an autosome or sex chromosome or both. Numerical abnormalities in
sex chromosomes typically occur in newborns without apparent defects in the external genitalia. Somatic structures are
usually normal, but profound deviations can occur. The severity of defects appears to depend on the grade of sex chromosome polyploidy, and such defects are more likely in patients
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T.Diemer and C.Desjardins
with cell lines containing multiple X chromosomes or an X0
cell line. Trisomies, involving autosomes, usually are associated with severe somatic defects, including mental retardation, abnormal stature or heart failure.
Structural aberrations involve the loss or duplication of
genetic information, including the translocation of genetic
information from one chromosome to another without loss in
a numerically normal complement of chromosomes. Gross
structural aberrations in a chromosome are detectable by
microscopic inspection of the karyotype, while the microdeletion of a gene or a single base pair mutation can only be
detected by analysis of DNA from a patient of interest. Point
mutations represent the smallest structural aberration in a
single base pair within a gene. Aberrations involving structural rearrangements in a chromosome are known to cause
errors in promoter or repressor sequences of multiple genes,
leading to profound disturbances in spermatogenesis
(Nieschlag et al., 1997; Tuerlings et al., 1998b). Structural
aberrations explain a wide range of phenotypic abnormalities,
including a host of defects in spermatogenesis that are impossible to classify because hundreds of genes are required for the
production of spermatozoa (Engel et al., 1996).
Classification of structural aberrations in
chromosomes
Deletion refers to the loss of genetic information. Deletions
occur interstitially or terminally when one arm of a chromosome is affected. Deletions range from the microdeletion of a
single gene to macroscopic losses in parts of a chromosome
(Mak and Jarvi, 1996).
Peri- and paracentric inversion involves the rearrangement
of genetic information without the loss of chromosomal material. Defects attributed to gene reorganization depend upon
the localization of the breakpoints and the number and function of affected genes (Meschede et al., 1994).
Translocation involves the transfer of a part of a chromosome to another arm of the same or to another chromosome.
Translocations can be balanced or unbalanced. Unbalanced
translocations imply that the portion of the transferred
chromosome is lost or duplicated, with the retention of all
information in the balanced carrier (Xing and Lawrence,
1993; Chandley et al., 1975; Mak and Jarvi, 1996).
Robertsonian translocation refers to a special condition
where fusion occurs between two acrocentric chromosomes
with the loss of genetic information from the short arms of
participating chromosomes (Mak and Jarvi, 1996).
Base pair mutations can occur in all genes. Substitution or
deletion of a single base sequence in a strand of DNA can have
a profound impact on gene function (Quinton et al., 1996;
Cameron and Sinclair, 1997). The mutation may produce a
failure in transcription or a frameshift resulting in the complete loss of an encoded protein. Recessive mutations have no
impact on the affected individual but are inherited and transferred to future generations.
Sex chromosomes
X-chromosome aberrations
Infertility is caused by minor deletions or translocations of
genes on the X chromosome (Meschede et al., 1998). The
most profound defects in testicular development are caused
by deletion of a major portion of the X chromosome. Such
deletions are incompatible with the development of a male
fetus, since the loss of one or more genes on the X chromosome in males is not compensated by the presence of genes
from a second X chromosome, as is the case for females.
Xp22 contiguous gene syndrome. The Xp22 contiguous
gene syndrome is a rare disorder and treatment of infertility is
not a clinical objective due to multiple defects such as anosmia, mental retardation and short stature (Kletter et al., 1991).
De-novo deletion of Xp22-pter from the X chromosome
(Figure 2) affects several genes, including those responsible
for glycerol kinase deficiency (GKD), adrenal hypoplasia
congenita (DAX-1), Duchenne type muscular dystrophy
(DMD) (Kletter et al., 1991) and Kallmann’s syndrome
(KAL-1) (Figure 2). KAL-1 is deleted in ~50% of the patients
with the Xp22 contiguous gene syndrome, causing a primary
deficiency in GnRH production. Patients lacking GnRH are
hypogonadal and fail to produce spermatozoa.
Translocations of X-chromosome genes to autosomes and
the Y chromosome. Translocation of genes on the X chromosome to either autosomes or the Y chromosome is not a common cause of male infertility (Chandley et al., 1975;
Gabriel-Robez et al., 1990; Yoshida et al., 1997). X chromosome translocations are hereditary and the impact on offspring
is unpredictable because multiple genes are affected.
Balanced translocations of genes on the X chromosome to
either autosomes or the Y chromosome interfere with male
sexual differentiation or spermatogenesis or both (Meschede
et al., 1998). Such translocations typically occur at breakpoints in the pseudoautosomal boundary region of the X
chromosome (Xp22-pter) and in the Yq11 region of the Y
chromosome. The Yq11 region contains several genes located
in the AZF (azoospermia factor) region (Figure 2) that regulate transcription and are required for spermatogenesis (Vogt,
1998). No genes from the pseudoautosomal boundary region
of the X chromosome have been linked to spermatogenetic
arrest (Gabriel-Robez et al., 1990).
Normal spermatogenesis depends on X chromosome inactivation, a process directed by an X-linked gene during the
spermatocyte stage of germ cell development. The pairing and
recombination of the X and Y chromosomes, followed by X
inactivation, is limited to the pseudoautosomal regions and
occurs when both chromosomes are transcriptionally inactive, as indicated by chromatin formation (Handel and Hunt,
1992). X-chromosome inactivation functions to prevent re-
Disorders in spermatogenesis
123
Figure 2. Diagrammatic representation of sex chromosomes showing genes on the X and Y chromosomes that have been identified to be
associated with disorders in either spermatogenesis or the differentiation of the male phenotype. Numbers in the centre of the Y chromosome
designate intervals, and localizations are designated in the centre of the figure based on cytogenetic banding patterns. Genes affecting spermatogenesis are designated in bold type, other loci purported to control spermatogenesis are listed in brackets, and DNA markers for specific regions
are shown in italicized type. Specific regions enclosed in brackets may be similar to those genes shown in bold type where they appear adjacent to
each other. New information on genes mapping to the X and Y chromosomes can be obtained at http://www.nhgri.nih.gov./Data, a website maintained by the National Human Genome Research Institute.
combination between the X and the Y chromosomes during
meiosis (Handel and Hunt, 1992; Jamieson et al., 1996).
Translocations affecting the X chromosome appear to interfere with the process of X inactivation, resulting in meiotic
arrest.
Translocations involving a portion of the X chromosome
have a profound impact on spermatogenesis, as indicated by
the failure of most spermatocytes to enter into meiosis
(Jamieson et al., 1996). In certain cases, spermatogenesis can
proceed to the formation of elongated spermatids, but the
process is remarkably inefficient, as indicated by the presence
of a few spermatozoa.
Table I. Disorders in spermatogenesis attributable to mutations and defects of the Y chromosome
Locus
Gene
Disorder
Phenotype
Gonadal
manifestation
Extra-gonadal
manifestation
Spermatogenesis
Yp11.3
SRY
XX-male
male
gonadal
sex
germ cells
XY-female
female
dysgenesis
reversal
absent
Yq11.21
unknown
azoospermia
male
defective
none
germ cells absent
AZF ’a’
factor ’a’
Yq11.22–23
SMCYa
azoospermia
AZF ’b’
RBM1a
factor ’b’
male
defective
Sertoli only
none
spermatogenesis
spermatogenic arrest
at level of
TSPYa
spermatocytes or
EIF1AYa
spermatids
Yq11.22–23
DAZa
azoospermia
AZF ’c’
DAZ2a
factor ’c’
TTY1+2a
aTentative
spermatogenesis
male
defective
spermatogenesis
none
reduced spermatogenesis or arrest in
spermatid development
identification of a gene or multiple genes appear to be involved, but definitive identification and significance in the differentiation of germ cells remain to be established.
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T.Diemer and C.Desjardins
Infertile patients with X-linked translocations are candidates for assisted reproductive technologies, provided the
translocations are not associated with other somatic defects.
Patient selection should be based on the degree of spermatogenic failure; no treatment can be offered unless meiosis is
completed and spermatogenesis proceeds to the formation of
spermatids.
Sexual differentiation is affected by translocations in
Xp21.2-pter (GDXY) (Figure 2). The Xp21.2-pter region appears to be crucial for the differentiation of the male phenotype, since duplication and translocation can give rise to a 46,
XY dose-sensitive sex reversal with a feminine phenotype
and gonadal dysgenesis (Pringle and Page, 1997).
DAX-1, a gene located at Xp21 (Figure 2), is a decisive X
gene for sexual differentiation (Swain et al., 1998). Expression of DAX-1 in mice indicates it plays a critical role in early
differentiation of adrenal and Sertoli cells in the genital ridge
(Swain et al., 1998), but prolonged expression of DAX-1 interferes with testicular development. DAX-1 expression is
down-regulated during the formation of the testis, while transcription persists during the development of the ovary in mice
(Swain et al., 1998). Overexpression of DAX-1 in transgenic
mice antagonizes the expression of SRY, a gene on the Y
chromosome (Figure 2) required for the differentiation of
male secondary sexual characteristics (Swain et al., 1998).
Antagonism between DAX-1 and SRY explains X-linked
forms of gonadal dysgenesis in humans (Swain et al., 1998).
Y-chromosome aberrations
Translocations involving the Y chromosome. Cytogenetic survey of infertile patients indicates that chromosomal abnormalities, including translocations, are 4–10 times more prevalent
among infertile than fertile men (Chandley et al., 1975; Mak
and Jarvi, 1996; Meschede et al., 1998). Translocation of a
portion of the Y chromosome to autosomes typically occurs in
chromosomes 1, 3 and 11 and impairs spermatogenesis (Sasagawa et al., 1993). Breakpoints in the Y chromosome frequently arise in the Yq11 or Yq12 region (Shapiro, 1991).
Genes affected by these translocations have not been identified, but probably include those in the AZF region (Vogt,
1998) of the Y chromosome (Delobel et al., 1998). This region
has been divided into three subregions (AZFa–c; Table I),
where genetic defects have been associated with different
grades of impairment and arrest of spermatogenesis (Vogt et
al., 1996; Vogt, 1998). Translocations between the Y chromosome and autosomes have an adverse effect on spermatogenesis due to interference with sex chromosome pairing and
incomplete inactivation of the X chromosome (Handel and
Hunt, 1992).
Translocations involving the Y chromosome cause severe
disorders in spermatogenesis, but the cellular lesion in germ
cell differentiation cannot be established from the site of a
translocation (Delobel et al., 1998). Analysis of testicular
biopsies suggests that translocations of the Y chromosome
explain multiple disorders in spermatogenesis, ranging from
failure of spermatogonial differentiation to quantitative losses
in the formation of postmeiotic germ cells (Chandley et al.,
1986; Chandley and Speed, 1995).
Other Y chromosome abnormalities disrupting spermatogenesis include the dicentric short arm (Chandley et al.,
1986; Taniuchi et al., 1991) and ring formation involving the
loss of large parts of the Yq arm, a region that encompasses
most of the AZF region (Figure 2). Spermatogonia typically
fail to differentiate in patients with a dicentric short arm Y
chromosome or Y ring formation, and certain patients may
have short stature or tooth anomalies, suggesting that genes
other than those directing the differentiation of germ cells are
affected (Chandley et al., 1986; Chandley, 1994; Graves,
1995; Grootegoed et al., 1995).
No known therapeutic treatments are available for patients
with Y chromosome translocations or malformations. Germ
cell differentiation is arrested prior to the formation of spermatids in most cases, but elongated spermatids have been
recovered from testicular biopsies in some patients and used
for ICSI. Such practice must be viewed with caution given
new evidence from some studies pointing to developmental
delays in children following use of ICSI (Andrews et al.,
1998; Bowen et al., 1998), although such delays were not
apparent in other data sets using similar methods (Bonduelle
et al., 1998c).
Deletions and mutations of the SRY gene. SRY is required
for the regression of Müllerian ducts and the subsequent
formation of testes, two sequential steps in male sexual differentiation. SRY is located on the short arm of the Y chromosome at Yp11.3 (Figure 2), adjacent to the pseudoautosomal
boundary on interval 1 (Page et al., 1987; Sinclair et al., 1990).
Over 26 inactivating point mutations in SRY have been discovered so far, and all are associated with gonadal dysgenesis
or sex reversal syndromes or both (Pringle and Page, 1997).
Point mutations in SRY produce structural and functional
effects similar to those seen following deletion or translocation.
The SRY gene spans over 35 kilobase pairs, with an HMGbox region crucial for gene function. Mutations in the HMGbox region of SRY have the most deleterious impact on the
male phenotype compared with mutations in other areas of the
gene. SRY is absent in some patients due to deletion or translocation to the X chromosome during paternal meiosis. Paternal translocation of SRY to the X chromosome during meiosis
results in either XY-female or XX-male genotypes in the offspring (Table I).
SRY encodes for a DNA-binding protein and is transcribed
as a single exon (Cameron and Sinclair, 1997). This protein
functions as a transcription factor regulating the formation of
the testis in fetal development. Either the absence of SRY or a
gene mutation explains sex reversal syndromes like XXmales and XY-females. Testes may fail to differentiate in pa-
Disorders in spermatogenesis
tients with a 46, XY complement of chromosomes and normal
SRY gene expression, indicating that genes from other
chromosomes are required for spermatogenesis and the differentiation of the male phenotype (Page et al., 1987).
Deletion or translocation of SRY during paternal meiosis
typically results in a feminine phenotype with gonadal dysgenesis in spite of a normal male 46,XY complement of chromosomes. Dysplasia of ovaries and an undifferentiated female
genital tract are present, with underdeveloped but feminine
secondary sexual features such as breasts, body proportions
and external genitalia.
A similar consequence emerges in people who are phenotypically male but have a normal female complement of
chromosomes, 46,XX, referred to as XX-male syndrome.
Translocation of SRY to an X chromosome during paternal
meiosis occurs in ~2% of patients with severe disorders in
spermatogenesis (Nieschlag et al., 1997). Patients with an
XX-male genotype have masculine secondary sexual features
and gynaecomastia is common. Body height and weight is
frequently below normal and typically feminine. The testes of
XX-males are diminutive and the seminiferous epithelium is
devoid of elongated spermatids. The prevalence of this condition among infertile patients underscores the importance of
undertaking a complete genetic evaluation before considering
the use of ICSI.
Deletions in the Yq11 (AZF) region. Major deletions in the
euchromatic part of the Y chromosome interfere with spermatogenesis, as demonstrated by severe oligozoospermia and
azoospermia (Tiepolo and Zuffardi, 1976; Hargreave et al.,
1996). The euchromatic part of the Y chromosome contains a
family of genes important for normal spermatogenesis, commonly referred to as AZF (azoospermia factor; Figure 2).
Deletions occur in three different regions of Yq11 referred to
as AZFa to AZFc and appear to be linked to specific lesions in
spermatogenesis (Vogt, 1997, 1998). The incidence of deletions in the Yq11.21–23 region (Figure 2) among patients with
severe disorders in spermatogenesis is estimated to range between 7 and 20% (Nakahori et al., 1996; Bonhoff et al., 1997;
Roberts, 1998; Simoni et al., 1998). The relatively high occurrence of this disorder points to the advantage of adopting
recombinant DNA techniques to assess genetic errors among
infertile patients with apparent idiopathic infertility (Najmabadi et al., 1996; Qureshi et al., 1996).
Several genes have been proposed as AZF candidates based
on microdeletion mapping in men with severe disorders in
spermatogenesis (Vogt et al., 1992; Vogt, 1997). The number
and location of AZF genes are a topic of intense investigation,
as is their potential interaction within and among genes located on intervals 5 and 6 of the Y chromosome (Roberts,
1998; Simoni et al., 1998; Figure 2). Some 15 genes have been
proposed as AZF candidates so far. Microdeletions appear to
map to three different regions of Yq11.21–23 within intervals
5 and 6 (AZFa, AZFb, AZFc; Figure 2) (Vogt et al., 1995;
Pryor et al., 1997; Roberts, 1998; Vogt, 1998). It remains to be
125
elucidated if disorders in spermatogenesis can result from a
single defective candidate gene or if the extinction of genetic
information from a whole region is required (Vogt, 1998).
Microdeletions in the Yq11.21–23 region occur as de-novo
mutations since they are usually absent from the paternal genome (Vogt, 1998).
Severe disorders in spermatogenesis involve microdeletions at the start of Yq11.21 (AZFa) distal of DYS3 (Table I
and Figure 2), but candidate genes have not been reported in
this region (Vogt et al., 1995; Vogt, 1997; Roberts, 1998).
De-novo deletion of genes in the Yq11.21 region (AZFa; Figure 2) causes the complete loss of germ cells (Table I), typical
of the Sertoli cell-only syndrome (Vogt et al., 1995; Roberts,
1998; Vogt, 1998).
Within the Yq11.22–23 region (AZFb; Figure 2), four
candidate genes (RBM1, SMCY, TSPY and EIF1AY) with suspected implications for spermatogenesis have been identified
(Delbridge et al., 1997; Vogt, 1998). RBM1 (RNA binding
motif 1) and multiple repeats of the RBM gene family are
distributed on the Y chromosome (Ma et al., 1993; Delbridge
et al., 1997). Deletion of regions containing either RBM1,
SMCY or the others leads to the early arrest of spermatogenesis at the level of pre- and post-meiotic spermatocytes, based
on histological assessments of the testis (Delbridge et al.,
1997). Ejaculates from patients with deletions in the AZFb
region are most commonly azoospermic, but severe oligozoospermia may be encountered (Table I). No known mechanism has been proposed to explain why spermatogenesis can
proceed in a few cases despite the absence of candidate genes
(Nakahori et al., 1996; Delbridge et al., 1997).
The Yq11.22–23 region also contains AZFc (Figure 2),
with important genes that appear to regulate spermatogenesis:
DAZ (deleted in azoospermia) on interval 6 between DYS7C
and DYS233 (Reijo et al., 1995, 1996), and SPGY (spermatogenesis gene on the Y, also referred to as DAZ2) located
distal of DAZ. Recently, two new candidate genes have been
discovered, TTY1 and TTY2, but their translation in germ cell
development has not been confirmed. DAZ and SPGY (DAZ2;
Figure 2) share remarkable structural homologies, as indicated by identical 72 basepair exons, but appear to function as
independent genes of the same family (Habermann et al.,
1998; Vogt, 1998). Patients with deletions in DAZ and SPGY
(AZFc) present a range of spermatogenic disorders (Reijo
et al., 1995, 1996; Nakahori et al., 1996; Pryor et al., 1997;
Roberts, 1998). Analysis of the seminiferous epithelium, following testicular biopsy, reveals quantitative reductions in
spermatogenesis or the arrest of germ cell development at the
level of spermatids (Table I). Azoospermic or severe oligozoospermic ejaculates confirm lesions predicted by the morphological assessment of biopsied tissue (Shinka and
Nakahori, 1996).
Proteins encoded by DAZ and SPGY (DAZ2) were identified exclusively in late spermatids, suggesting that these proteins have a role in the storage and transport of mRNA within
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T.Diemer and C.Desjardins
late spermatids (Habermann et al., 1998). Deletions of DAZ
and DAZ2 interfere with the maturation of spermatids, but not
with the development of spermatogonia or the formation of
spermatocytes (Habermann et al., 1998). DAZL1 resides on
chromosome 17 in mice and functions to restrict transcription
in pre-meiotic spermatocytes, implying a regulatory role in
early germ cell differentiation in the mouse (Niederberger et
al., 1997). The human autosomal homologue of DAZL1 resides on chromosome 3p (DAZLA), but the function of this
autosomal copy in spermatogenesis is unknown.
New microdeletions have been described on subinterval 6e
of Yq (Figure 2), though no specific genes have been identified (Nicolai et al., 1998). Tentative indications, based on a
cohort of patients with microdeletions in this region, suggest
that spermatogenesis is impaired, as shown by oligozoospermia among all patients studied (Nicolai et al., 1998).
Genes in the Yq11.21–23 region (Figure 2) appear to share
similar functions in directing spermatogenesis since they
regulate the transcription of other genes involved in the development of germ cells (Jaffe and Oates, 1994). Some of these
genes encode for RNA-binding proteins and are structurally
similar to other human RNA-binding proteins (Delbridge
et al., 1997), while others function as translation initiators and
proteases. Certain genes, such as RBM1, are only transcribed
in the testis at specific stages of germ cell development
(Delbridge et al., 1997). The molecular site of action of such
genes is unknown, since the cohort of genes regulated by
those in the Yq11.21–23 region has not been established.
Assessment of the role of genes that direct germ cell differentiation will be difficult, because the number of genes required for human spermatogenesis is large and the products of
multiple genes are likely to interact to direct one or more steps
in spermatogenesis (Engel et al., 1996).
Interactions among genes within Yq11.21–23 are not yet
known. Understanding the functional significance of these
interactions is important, but deletion of one gene critical to
spermatogenesis could result in a range of disturbances unrelated to the lesions triggered by other candidate genes. Neither
point mutations in any of the candidate genes within the
Yq11.21–23 region are known at this time nor has one candidate gene been confirmed as AZF (Vogt, 1998). Detection of
mutations in candidate genes could offer new insights into the
impact of genes within the Yq11.21–23 region.
Deletion of genes in the Yq11.21–23 region of the Y
chromosome are not associated with other known somatic
disorders or defects in sexual differentiation despite their profound influence on spermatogenesis (Jaffe and Oates, 1994,
1997). Circulatory concentrations of FSH in patients with
gene deletions in Yq11.21–23 are usually elevated or remain
in the normal range (Kremer et al., 1997). Elevated FSH
secretion coincides with the reduced inhibin production by
Sertoli cells and aplastic seminiferous tubules (Roberts,
1998).
No therapeutic options are available to restore spermatogenesis among patients with microdeletions in Yq11.21–23.
ICSI can be employed in the fraction of patients in whom the
testis contains elongated spermatids, since a few spermatozoa
may be recovered following testicular biopsy (Bonhoff et al.,
1997). Male offspring are infertile due to the absence of genes
on Yq11.21–23 which must be expressed to allow spermatogenesis to proceed. Physicians should make their patients
aware of the risk of transmitting infertility to prospective male
offspring or inducing other developmental disorders in both
genders (Andrews et al., 1998; Bonduelle et al., 1998b) before assisted reproductive techniques are adopted.
Autosomes
Disorders in spermatogenesis cannot be predicted from the
cytological analysis of chromosomal structure for two reasons. First, chromosomal aberrations differ from individual to
individual since different sets of genes can be involved due to
slightly different breakpoints and impact on different genes
that cannot be assessed by cytological techniques. Second,
neither deletions nor mutations in a gene can be established at
the level of the light microscope. Cytogenetic analyses of
chromosomes from fertile and infertile men indicate that autosomal translocations occur ~4–10 times more frequently in
infertile than in fertile men (Chandley et al., 1975; Elliott and
Cooke, 1997). The most typical structural aberration involves
pericentric inversions on the following chromosomes: 1, 3, 5,
6, 9, 10 or 21 (Meschede et al., 1994). Even if all of the genes
required for spermatogenesis could be detected by molecular
techniques, the defects attributable to a single gene on an
autosome or sex chromosome would be difficult to resolve
since hundreds of genes have a role in spermatogenesis (Engel
et al., 1996). However, a coincidence of autosomal translocations and infertility must be considered since genes affected
by these translocations are not known in most cases (Meschede et al., 1994)
The impact of autosomal translocations on spermatogenesis typically involves the formation of synaptonemal complexes in the early pachytene stage of spermatogenesis (de
Perdigo et al., 1991; Guichaoua et al., 1992). Normal homosynapsis in meiotic cells supports effective crossing over between homologous pairs, directing proper disjunction in the
anaphase of meiosis (Guichaoua et al., 1992). Following
autosomal translocations, asynaptical and heterosynaptical
complexes are formed between non-homologous chromosomes and, most importantly, with the sex chromosome bivalent. Interactions between heterosynaptical quadrivalents and
the XY pair purportedly interfere with the X chromosome
inactivation required for proper meiosis (Johannisson et al.,
1993). Transcription of X genes during the pachytene stage of
meiosis can interrupt the meiotic cycle, as shown by sperma-
Disorders in spermatogenesis
togenic arrest in patients with different types of translocated
autosomes (de Perdigo et al., 1991; Johannisson et al., 1993).
Robertsonian translocations
Translocations between acrocentric chromosomes occur following centromere fusion and result in the loss of a centric
fragment (Mak and Jarvi, 1996; Nieschlag et al., 1997).
Robertsonian translocations are among the most common
structural rearrangements in humans, where they involve
chromosomes 13 and 14 in ~15 per 1000 births (Uccellatore
et al., 1983). The cellular impact of Robertsonian translocations on spermatogenesis is unpredictable, but it appears to
depend on the extent of trivalent/XY-pair formation during
the pachytene stage (Johannisson et al., 1993). Pairing seems
to interfere with the X chromosome inactivation required for
normal spermatogenesis (Handel and Hunt, 1992). Alterations of the X chromosome may be a precondition for the
formation of pairing sites between the Robertsonian trivalent
and the X chromosome, thus explaining phenotypic differences in spermatogenesis and other traits among individuals
with similar Robertsonian translocations.
Deleted genes attributable to the loss of centric fragments
remain to be identified (Johannisson et al., 1993). Spermatogenic arrest, at the spermatocyte stage, is most common, but
defects in spermatogenesis range from severe losses of spermatogonia to little or no change in the seminiferous epithelium (Johannisson et al., 1993; Meschede et al., 1998).
Remarkably, the same Robertsonian translocations have been
reported in fathers and sons with no apparent effect on spermatogenesis or fertility in the fathers while spermatogenesis
was impaired in the sons (Johannisson et al., 1993). The high
incidence of Robertsonian translocations in humans suggests
that patients with such translocations are found frequently
among infertile men (Meschede et al., 1998). FSH may be
elevated in conjunction with this spermatogenic disorder, as is
typical of patients with disorders in spermatogenesis (Uccellatore et al., 1983). Treatment strategies rely on ICSI provided
that elongated spermatids or spermatozoa are present.
Peri- and paracentric inversions
Peri- and paracentric inversions are the least frequently occurring chromosomal rearrangements in humans, with an incidence of <0.01% in adult men. The occurrence among
infertile men appears to be higher, but patients with inversions
are rarely observed in infertility clinics.
Inversions of both types have been described in infertile
patients on chromosomes 1 (Meschede et al., 1994), 7
(Navarro et al., 1993), 3, 6,13, 20 and 21 respectively
(Gabriel-Robez et al., 1988). Pericentric inversions involve
the centromere, while paracentric inversions take place on the
p or q arm of the affected chromosome. The current consensus
127
is that pericentric inversions produce a greater incidence of
unbalanced complements of chromosomes based on sperm
karyotypes. This finding suggests that offspring of such patients have a higher probability of carrying an unbalanced
complement of chromosomes, with consequent genetic defects occasioned by the loss or duplication of multiple genes
(Navarro et al., 1993).
Inversions interfere with spermatogenesis, but the specific
genes have not been identified (Mak and Jarvi, 1996). The
nexus between inversions and spermatogenic disorders is
based on a few cases, making it difficult to develop any generalization (Meschede et al., 1994). The impact of pericentric
and paracentric inversions on spermatogenesis varies from
patient to patient. Arrest at the spermatocyte stage has been
described for a particular pericentric inversion on chromosome 1 (p34q23) (Meschede et al., 1994), whereas pericentric
inversions involving other chromosomes have been associated with severe oligozoospermia or azoospermia (Meschede et al., 1998). Neither sexual differentiation nor other
somatic tissues appear to be affected by inversions of either
type.
Therapy depends on the extent of the pathophysiological
lesion in spermatogenesis. The risk for inheriting unbalanced
complements of chromosomes is higher in the offspring of
patients with inversions, and the probability of spontaneous
abortion should be anticipated when fertilization is achieved
by ICSI or IVF (Navarro et al., 1993; Andrews et al., 1998).
Campomelic dysplasia
The gene for campomelic dysplasia has been identified as
SOX-9 (SRY-type HMG-box gene 9) on chromosome 17.
SOX-9 was the first sex-determining gene to be discovered on
autosomes. Deletion, translocation and inactivating mutations
involving SOX-9 cause campomelic dysplasia, including sex
reversal in individuals with a normal male karyotype, 46, XY
(Pringle and Page, 1997). The typical features of campomelic
dysplasia and sex reversal have been observed in patients with
translocations at breakpoints located in the q24–25 region of
chromosome 17. Similar consequences result when breakpoints are localized outside the coding sequence of SOX-9.
Translocations, mutations and deletions all appear to involve a
defect in the promoter element for SOX-9, but a molecular
explanation for the error in promoter activity is still uncertain.
Disorders in SOX-9, originating at any level, cause a semilethal, recessive disorder with a range of skeletal defects. Patients typically have feminine external genitalia with multiple
somatic defects (Nieschlag et al., 1997). Testes are present as
anlagen with few if any Sertoli or germ cells.
Patients with campomelic dysplasia are not seen in infertility clinics but residual testes, in the inguinal or abdominal
128
T.Diemer and C.Desjardins
position, should be removed surgically to limit the risk of
malignancies.
Autosomal translocations
Some 20 different balanced and unbalanced chromosomal
translocations associated with disorders in spermatogenesis
involve autosomes and more are expected to be discovered
(Table II). Nearly all autosomes implicated in balanced or
reciprocal translocations can be associated with infertility
(Meschede et al., 1998).
Table II. Summary of balanced translocations reported to impair
spermatogenesis and fertility. t (...) (...) = t (affected
chromosomes) (cytogenetic location)
Translocations between autosomes
t (1;9) (q32.1; q34.3)
t (1;16) (q22; p 12)
t (1;16) (q23; p13)
t (1;18) (p31.2; q23)
t (1;19) (p13; q13), 1qh+
t (2;4) (q23; q27)
Distinct balanced translocations associated with infertility
are limited to a few patients. The basis for impaired spermatogenesis in men with autosomal translocations has begun to
emerge (Debiec-Rychter et al., 1992). Translocations appear
to interfere with the inactivation of the X chromosome, as
indicated by the pairing of autosomal quadrivalents with
either the X or the Y chromosome during meiosis (Guichaoua
et al., 1992). A significant correlation was observed between
the extent of this interference and the grade of spermatogenic
disorder (Micic and Micic, 1984; Gabriel-Robez et al., 1986,
Luciani et al., 1987; Guichaoua et al., 1992). Other groups
could not identify interference of the autosomal quadrivalents
(de Perdigo et al., 1991) and hexavalents (Saadallah and
Hulton, 1985) with the XY configuration. However, neither
genes nor gene regulators that contribute to the disorder in
spermatogenesis have been identified in infertile patients with
balanced translocations, so the mechanism by which balanced
translocations interfere with germ cell differentiation is not
understood.
Therapeutic protocols rely on ICSI. Balanced and unbalanced chromosomal translocations, however, may be passed
to offspring, posing an ethical dilemma for physicians treating
these patients.
t (2;8) (q23; p21)
t (2;22) (q33; q22)
t (3;4) (p25; q21.3)
Noonan’s syndrome
t (3;5) (q29; q14) (p12; q34)
t (3;14) (q27; q11)
t (3;20) (q11; p13)
t (4;6) (q26; q27)
t (4; 13) (q11; q12.3)
t (5; 8) (q22; q24.1)
t (9;12) (q22; q22)
t (9;13) (q22; q32)
t (9;15) (q22; q15)
t (9;17) (q11.3; q11.3)
t (9;17) (q11; q11)
t (9;20) (q34; q11)
t (10; 14)
t (11;15) (q25-qter; pter)
t (13;14)
t (14;21) (q13; p13)
t (14;22) (p11; q11.1)
t (17;21) (p13; q11)
t (19;22) (p13.1; q11.1)
Translocations between autosomes and sex chromosomes
t (13q;15q)/46, XY
t (Y;1) (q12; p34.3)
t (Y;3) (q12; p21)
t (Y;11) (q11.2; q24)
Translocations between sex chromosomes
t (X;Y)
t (X;Y) (p22.3; q11)
Noonan’s syndrome is a congenital disorder of which half of
all cases are due to autosomal dominant inheritance while the
remainder occur spontaneously. The incidence is estimated at
one per 1000 to one per 5000 live births. The hallmarks of this
disorder include short stature, congestive heart failure, and a
facial phenotype with posteriorly rotated ears (Allanson,
1987).
The gene defect has been localized to chromosome
12q22-qter between D12S84 and D12S366 for the autosomal
form of the disorder (Jamieson et al., 1994). Other candidate
loci have been proposed, but linkage analysis failed to prove
the involvement of these loci (Jamieson et al., 1994).
The remarkably high incidence of Noonan’s syndrome
should be considered in the evaluation of infertility patients.
Interindividual variation in germ cell differentiation among
Noonan’s syndrome patients is too broad to provide any systematic classification of the lesion in spermatogenesis. Primary hypogonadism occurs as a facultative feature.
Hormonal therapy is indicated for cases including hypogonadism and late onset puberty. Patients should be informed
about the genetic basis of their disease and the possibility that
the trait can be transferred to offspring if spermatogenesis is
stimulated or ICSI is used. Patients with Noonan’s syndrome
should be examined for cryptorchidism, since ~60% of
affected males are bilaterally cryptorchid (Allanson, 1987).
Surgical relocation of the testes to the scrotum should be
performed to reduce the risk of testicular malignancy.
Disorders in spermatogenesis
129
Table III. Numerical aberrations in sex chromosomes and their impact on spermatogenesis
Karyotype
Disorder
Phenotype
Gonadal
manifestation
Extra-gonadal
manifestation
Spermatogenesis
47,XXY-
Klinefelter’s
male
hypogonadism,
gynaecomastia,
germ cells absent or
48,XXXY
syndrome
defect in
tall stature
early spermatogenic arrest,
more X and
spermatogenesis
few if any spermatozoa
mosaics
47,XYY
XYY-male
male
impaired
none
spermatogenesis
present but cell proliferation
possible
46, XX+SRY
all stages of spermatogenesis
is variable
XX-male
male
hypospadias,
cryptorchidism
feminine stature
Sertoli only
XY/X0
mixed
male only in
intersexual
multiple extra-
germ cells absent,
mosaic
gonadal
exceptional
genitalia, sex-
gonadal lesions
Sertoli only
dysgenesis
cases
reversal with
reported
(SRY transl.)
gynaecomastia,
germ cells absent,
abdominal testes
Kartagener’s syndrome and axoneme defects
The genetic basis of Kartagener’s syndrome has not been
elucidated. It appears to be inherited as an autosomal recessive
mutation in most cases, but a dominant inheritance pattern has
been established (Nieschlag et al., 1997). Patients with the
syndrome lack dynein, a protein connecting triplets of
microtubules within the axoneme. Dynein is required for the
movement of cilia in all tissues where they are found, including spermatozoa. Spermatozoa produced by patients with this
syndrome are immotile and the patients are infertile. Definitive diagnosis relies on demonstrating by electron microscopy
that dynein connections between the microtubules are absent,
since the spermatozoa appear normal in every respect
(Nieschlag et al., 1997).
Infertility, bronchiectasis and situs inversus are the hallmarks of Kartagener’s syndrome. Cilia of the respiratory tract
are affected and patients experience recurrent infections and
formation of bronchiectasis.
A second axoneme defect affects the central doublet of
microtubules. This defect, known as the 9+0 syndrome, is of
unknown genetic origin. Affected patients produce spermatozoa that are immotile and incapable of fertilization (Nieschlag
et al., 1997).
Assisted reproductive techniques rely on ICSI. Neither intrauterine insemination (IUI) nor IVF is effective, since spermatozoa from patients with Kartagener’s syndrome and other
axoneme defects fail to fertilize in vitro. The risk of transmitting
this defect via ICSI is 50% for the autosomal dominant forms.
Globozoospermia
Globozoospermia is a special but rare hereditary condition
where acrosomal caps are absent from all spermatozoa present in an ejaculate. Neither the chromosomal origin nor the
gene defect responsible for this condition are known (Schill,
1991).
Globozoospermia is attributed to a defect in the formation
of the Golgi apparatus in spermatids and the eventual development of an acrosome. Spermatozoa from fertile men appear
to have elongated heads when viewed by light and electron
microscopy, while those from patients with globozoospermia
are round and fail to bind to the zona pellucida (Schill, 1991).
ICSI is the therapeutic option of choice for patients with globozoospermia, since spermatozoa are easily prepared from
the ejaculate.
Numerical aberrations in sex chromosomes
Accessory chromosomes arise because of errors in parental
meiosis. Parental non-disjunction of sex chromosomes can
result in numerical aberrations of chromosomes, 47+, with
multiple X or Y chromosomes (Table III). The functional
consequences of accessory sex chromosomes are mild compared with the effects occasioned by numerical aberrations in
an autosome. Loss of a sex chromosome such as the X
chromosome, as indicated by a 45– genotype, produces multiple somatic defects and profound disturbances in sexual differentiation of females (Turner syndrome, 45, X0).
Klinefelter’s syndrome: 47,XXY
Klinefelter’s syndrome is the most common chromosomal
defect causing hypogonadism and infertility in men
(Nieschlag et al., 1997). The hallmarks of Klinefelter’s
syndrome include hypogonadism with mild to severe losses in
spermatogenesis, tall stature, gynaecomastia and obesity with
a feminine pattern of fat distribution (Klinefelter et al., 1942;
Klinefelter, 1984). Klinefelter’s patients have one or more
accessory X chromosomes (Table III). The additional X
chromosome arises from the non-disjunction of parental germ
cells during meiosis (Klinefelter et al., 1942; Klinefelter,
1984). About two-thirds of meiotic non-disjunctions are of
130
T.Diemer and C.Desjardins
maternal origin and about one-third are paternal (Nieschlag
et al., 1997). The typical karyotype is 47, XXY, but chromosomal mosaics with 46, XY/47 XXY and complements with
multiple X chromosomes like 48, XXXY are known (Zang,
1984). The incidence of Klinefelter’s syndrome is one per
500–700 male births.
The molecular basis for hypogonadism in Klinefelter’s patients is unclear. Klinefelter’s patients with a mosaic karyotype of 46, XY/47 XXY have been known to induce
spontaneous pregnancies, and even patients with the typical
form of Klinefelter’s syndrome have fathered children in exceptional cases (Schill et al., 1984). The seminiferous epithelium of patients with a mosaic karyotype including a 46,XY
cell line has mild defects in spermatogenesis (Gordon et al.,
1972; Schill et al., 1984). Testes of patients with both a 46,XY
or 47, XXY chromosomal complement undergo meiosis to
form spermatozoa with either a normal, 23, X or Y, or abnormal, 24,XY, karyotype (Cozzi et al., 1994; Foresta et al.,
1998). Sperm karyotypes of Klinefelter’s patients indicate
that 2–20% of spermatozoa have sex chromosome aneuploidy
(Foresta et al., 1998).
In patients with the typical form of Klinefelter’s syndrome,
the testicular volume is 80–90% below normal and ejaculates
contain few or no spermatozoa (Schill et al., 1984). The seminiferous tubules are fibrotic and hyalinized, with infrequent
spermatogonia (Bandmann and Perwein, 1984). The spermatogonia fail to differentiate beyond the primary spermatocyte
stage of spermatogenesis, but spermatogenesis can proceed to
completion, at least in a few areas of the germinal epithelium.
The diagnosis of Klinefelter’s syndrome is often delayed
until adult patients consult an infertility specialist. The therapeutic strategies available to patients with Klinefelter’s syndrome include those adopted for hypogonadal men (Nieschlag,
1984). Spermatozoa have been recovered from testicular
biopsies or isolated from ejaculates from these patients, and
their subsequent use for fertilization via ICSI has resulted in
pregnancies and birth of healthy children (Bourne et al., 1997;
Hinney et al., 1997; Kruse et al., 1998; Palermo et al., 1998).
However, sperm karyotypes are abnormal in Klinefelter’s patients, as indicated by recombinant DNA technology such as
fluorescence in-situ hybridization (Cozzi et al., 1994).
The genetic and ethical dilemmas of using spermatozoa
from Klinefelter’s patients in assisted reproductive technologies require careful consideration (Andrews et al., 1998).
Spermatozoa from patients with Klinefelter’s syndrome have
a greater frequency of containing accessory X chromosomes
and passing the disorder on to offspring (Cozzi et al., 1994).
The probability of transmitting the chromosomal defect causing Klinefelter’s syndrome can be estimated by determining
the percentage of aneuploid spermatozoa following in-situ
hybridization of sex chromosomes in interphase sperm nuclei
(Kruse et al., 1998).
XYY syndrome: 47,XYY
Paternal non-disjunction during meiosis can result in a double
Y chromosome with a 47, XYY complement of chromosomes
(Table III). Secondary sexual characteristics are unaffected
and spermatogenesis is normal among most XYY-men since
the extra Y chromosome is eliminated during meiosis. Incomplete inactivation of the X chromosome during the pachytene
stage of meiosis is the genetic explanation for the arrest in
spermatogenesis that is occasionally observed in affected patients (Handel and Hunt, 1992).
Patients with an XYY genotype, however, produce spermatozoa with disomic and hyperhaploid karyotypes, such as
24,XY or 24,YY at a much greater rate than is observed in
individuals with a normal complement of 46 chromosomes
(Blanco et al., 1997; Chevret et al., 1997). The risk of sex
chromosome trisomies in the offspring of such patients is
enhanced, but can be estimated on the basis of the number of
hyperhaploid karyotypes in ejaculated spermatozoa (Blanco
et al., 1997; Chevret et al., 1997). Infertile patients with this
spermatogenic disorder can be assisted with ICSI, but consideration should be given to the genetic risks and ethical
issues that stem from assisted reproductive protocols.
XX-male syndrome: 46,XX +SRY
Approximately 2% of the men seen at infertility clinics have a
46,XX genotype (Nieschlag et al., 1997). Despite a normal
female karyotype, these individuals have fully developed
masculine features due to the presence of SRY in their complement of chromosomes (Tables I and III). SRY, a gene required
for testicular development, and other genes in the pseudoautosomal boundary region of the Y chromosome (Figure 2) are
partially translocated to an X chromosome during paternal meiosis, thus explaining this form of sex reversal (Pringle and
Page, 1997).
SRY encodes for a nucleotide-binding protein directing the
transcription of other genes required for the formation of a
testis. Testicular volume is reduced, the seminiferous epithelium usually is devoid of germ cells and ejaculates are azoospermic, since the subset of genes on the Y chromosome that are
required for spermatogenesis are absent (Cooper and Sandlow,
1996). The secondary sexual features of XX-males are fully
differentiated, but gynaecomastia is common, with a feminine
stature. Infertility in these patients is not treatable since spermatogenesis is arrested prior to the formation of spermatids.
Mixed gonadal dysgenesis: mosaic 45, X0/46, XY
Mixed gonadal dysgenesis occurs among patients with a mosaic arrangement of chromosomes such as 45, X0/46, XY
(Table III). Gonadal and phenotypic sexual features are typically female, but testicles are retained in the abdomen and may
occur unilaterally. In exceptional cases, males with abdominal
Disorders in spermatogenesis
testes have been described, but both spermatogenesis and
Leydig-cell function are reminiscent of that seen in the prepubertal state. Testosterone replacement therapy is required
for the completion of sexual differentiation and the expression
of a male phenotype. Remnants of testicular tissue should be
removed surgically in patients with feminine secondary sexual features.
Endocrine disorders of genetic origin and
their impact on spermatogenesis
The differentiation of the male phenotype, including the seminiferous epithelium, requires the secretion of FSH and LH
131
from the pituitary gland and the production of testosterone by
Leydig cells. Disorders of male sexual differentiation, including defects such as micropenis and hypospadias, occur when
androgen is insufficient or absent during organogenesis
(Table IV). Patients with a 46,XY karyotype may be genetic
but not phenotypic males as judged by the failure of secondary
sexual tissues to differentiate according to a male pattern.
Spermatogenesis is not initiated when GnRH is lacking or
when either FSH or LH secretion fails, resulting in testosterone insufficiency (Behre et al., 1997). The range of developmental disorders in spermatogenesis arising from a defect in
one or more genes controlling endocrine function is summarized in Table IV and presented in more detail below.
Table IV. Summary of gene mutations causing disturbances in sexual differentiation, spermatogenesis, or both
Locus
Gene
Disorder
Gonadal
manifestation
Extra-gonadal
manifestation
Spermatogenesis
Xp22.3
KAL-1
Kallmann’s
hypotrophic testes,
anosmia,
spermatogenic arrest
syndrome
cryptorchidism,
renal dysplasia
or quantitative reductions
hypogonadism
15q11–13
Xp21.2–21.3
IC
DAX-1
in germ cell proliferation
Prader– Labhardt–Willi
hypogenitalism,
stature and
spermatogenic arrest at
syndrome
micropenis,
skeletal
spermatid stage,
hypogonadism
anomalies
normal fertility possible
spermatogenic arrest
congenital hypoplasia
hypogonadism,
adrenal
of adrenals and testes
cryptorchidism
insufficiency
11q13
BBS1
Laurence-Moon
hypogonadism,
mental
wide range of defects
16q21
BBS2
and Bardet-Biedel
hypogenitalism,
retardation,
in spermatogenesis,
15q22.3
BBS3
syndrome
micropenis,
obesity,
normal spermatogenesis
3p12
BBS4
hypospadias
retinal dystrophy
possible
3p
PIT-1
hypogonadism
hypopituitarism
quantitative reductions in
stature anomalies
germ cell proliferation
none
quantitative reductions in
none
quantitative reductions in
congenital
hypopituitarism
Unknown
unknown
isolated deficiency of
defective
FSH
spermatogenesis
2p21-p16
FSHR
FSH receptor mutation
defective
Unknown
unknown
isolated deficiency
production of spermatozoa
spermatogenesis
hypogonadism
production of spermatozoa
none
of LH
quantitative reductions in
production of spermatozoa
fertility possible
10q24.3
CYP 17
9q22
HSD17B3
17,20-desmolase/
gonadal dysgenesis,
extra-gonadal effects
range of defects in
17α-hydroxylase defect
cryptorchidism
of hypogonadism
spermatogenesis
17β-hydroxysteroid
hypogonadism,
extra-gonadal effects
range of defects in
dehydrogenase defect
cryptorchidism,
of hypogonadism
spermatogenesis
gonadal dysgenesis
2p23
SRD5A2
5α-reductase 2
gonadal dysgenesis
feminine stature
deficiency
Xq11–12
germ cells absent,
Sertoli only
AR
testicular
feminine genitalia,
feminine
germ cells,
(androgen
feminization
abdominal testes
phenotype
Sertoli only
Reifenstein syndrome
cryptorchidism
receptor)
Xq11–12
AR
hypospadias
Xq11–12
AR
hypogonadism,
spermatogenic arrest
undervirilization
at spermatid level
Undervirilized fertile
spermatogenic
undervirilization,
quantitative reductions in
male syndrome
lesion
male phenotype
spermatogenesis possible
132
T.Diemer and C.Desjardins
Kallmann’s syndrome
Kallmann’s syndrome is caused by a deletion or mutation in
KAL-1 located on the X chromosome at Xp22.3 (Figure 2 and
Table IV). Deletions or mutations in KAL-1 result in hypogonadotrophic hypogonadism and anosmia and occur in one per
7000–8000 births (Behre et al., 1997). KAL-1 is susceptible to
deletion along with other genes in Xp22-pter, as noted above
in the section about the Xp22 contiguous gene syndrome.
Deletions or mutations in KAL-1 have been described in patients with sporadic Kallmann’s syndrome, but families with
X-linked recessive inheritance have been confirmed (Behre et
al., 1997).
KAL-1 encodes for a cell adhesion protein and shares structural homology with other cell adhesion molecules made up of
repeated sequences of fibronectin type III (Pringle and Page,
1997). The gene product interferes with the migration of
GnRH neurons to the hypothalamus during brain development. Migration failure coincidences with the lack of GnRH,
the neural hormone responsible for the synthesis and release
of LH and FSH from the anterior pituitary gland (Pringle and
Page, 1997).
Typical patients with Kallmann’s syndrome have an almost
total lack of FSH and LH, depending on the extent of GnRH
insufficiency (Behre et al., 1997). The syndrome frequently
coincides with anosmia because the same adhesion molecules
guide the development of both hypothalamic GnRH and olfactory neurons (Bick et al., 1989). Other defects, such as
unilateral renal aplasia or renal dysplasia, are facultative features of Kallmann’s syndrome (Table IV; Bick et al., 1989).
The male phenotype is normal in patients with Kallmann’s
syndrome since testosterone concentrations are adequate to
support sexual differentiation during organogenesis. All secondary sexual features are infantile due to the lack of the
pituitary gonadotrophic hormones required for steroidogenesis. Affected patients retain the classical hallmarks of hypogonadism evident at puberty or in adult life. Diagnostic
approaches rely on physical examination and are confirmed
by near undetectable concentrations of FSH, LH and testosterone in peripheral blood (Behre et al., 1997). Definitive
diagnosis is based on provocative infusion of GnRH and the
detection of a brisk and robust increase in the secretion of both
gonadotrophins and testosterone.
Spermatogenesis is absent or highly reduced depending on
the severity of the hypothalamic deficiency in GnRH secretion. Phenotypic variations among adult patients with Kallmann’s syndrome range from normogonadotrophic fertile
patients to those reminiscent of prepubertal boys. A clear
relationship between genotype and phenotype has not been
established because mutations in KAL-1 involve over 10 different basepair deletions and mutations. Multiple genetic defects result in remarkably diverse phenotypes that defy
systematic classification of the disorder at the level of sperma-
togenesis or other endocrinological endpoints (Quinton et al.,
1996).
Kallmann’s syndrome accounts for ~5% of infertile men
with hypogonadotrophic hypogonadism (Behre et al., 1997).
Patients respond to induction of puberty with GnRH and testosterone therapy supporting the development of secondary
male sexual features including erectile function and ejaculation. Spermatogenesis can be stimulated to near normal levels
once patients wish to father children, as documented in multiple clinical experiences (Behre et al., 1997).
Prader–Labhardt–Willi syndrome
Prader–Labhardt–Willi syndrome is caused by the absence of
a small part of a paternal chromosome. The syndrome occurs
is one per 15 000 births and is associated with hypogonadotrophic hypogonadism including spermatogenic arrest and
testicular atrophy (Table IV). The genetic defect may be
caused by a deletion, mutation, uniparental disomy or translocation of parts of chromosome 15q11–13 (Robinson et al.,
1998). Deletions and mutations involving chromosome
15q11–13 can impair the transcription of genes in the 1 megabase vicinity of the imprinting centre (Saitoh et al., 1996).
Microdeletions causing gene defects in the imprinting centre
on chromosome 15q11–13 appear to be the most frequent
basis for this syndrome (Robinson et al., 1991)
Failure of the imprinting centre (Table IV) interferes with
the expression of genes in the vicinity of paternal alleles (Lyko
et al., 1998). Deletion mapping suggests that the deleted segment contains ~200 kilobases located between D15S13 and
the first exon of SNRPN. Multiple genes in the 1 megabase
vicinity are subsequently repressed including ZNF127,
SNRPN, IPW, PAR-1 and PAR-5 (Saitoh et al., 1996). The
primary lesion caused by gene failure is located in the hypothalamus, where it appears to involve a defect in GnRH-releasing neurons. Pituitary gland involvement, as viewed by
magnetic resonance imaging, has been established (Miller et
al., 1996). Estimates of circulatory concentrations of FSH,
LH and testosterone confirm deficits in hypothalamic GnRH
release.
Short stature, mental retardation, obesity and cryptorchidism are the dominant clinical features of Prader–Labhardt–
Willi syndrome (Robinson et al., 1991, 1998). Testicular
biopsies reveal that Leydig cells are undifferentiated and spermatogenesis is arrested at the level of late spermatids
(Hamilton et al., 1972). The seminiferous epithelium is aplastic and disorganized due to the combined influence of hypogonadism and cryptorchidism (Wu et al., 1981).
Hypogenitalism is common, but not a consistent feature, including micropenis, scrotal dysplasia and Leydig cell failure
(Butler, 1990).
Testes should be translocated to the scrotum within the first
2 years of life if they remain in an inguinal or abdominal
position. Treatment with pulsatile GnRH was shown to trigger
Disorders in spermatogenesis
FSH and LH release, with the subsequent induction of spermatogenesis, but fertility was not assessed in the treated subjects (Muller, 1997; Swaab, 1997). Rescue of the
seminiferous epithelium is not a routine practice among patients with Prader–Labhardt–Willi syndrome since the disorder involves multiple somatic defects, including mental
retardation.
Congenital hypoplasia of adrenal and testes
Congenital hypoplasia of the adrenal gland and testis is a rare
condition affecting the development of steroidogenic tissues
during organogenesis (Habiby et al., 1996). Multiple forms of
primary adrenal insufficiency are known, with either X-linked
or autosomal recessive inheritance and both can occur in association with hypogonadotrophic hypogonadism (Partsch
and Sippell, 1989; Nieschlag et al., 1997).
The X-linked form has been localized to the Xp21.2–21.3
region of the X chromosome, where two-thirds of affected
patients express a mutation in DAX-1 (Table IV). Most defects
in DAX-1 involve the cytomegalic form of the disorder typified by congenital adrenal hypoplasia and hypogonadotrophic
hypogonadism. Three types of defects have been identified in
DAX-1: major deletions, single base substitutions and basepair deletions. The syndrome can also be inherited via maternal
alleles, as indicated by affected families (Zanaria et al., 1994;
Yanase et al., 1996).
DAX-1 consists of two exons encoding for a member of the
nuclear hormone receptor superfamily with a novel DNAbinding domain (Zanaria et al., 1994; Yanase et al., 1996).
The gene product is a dominant negative regulator protein that
functions in transcription and is mediated by the retinoic acid
receptor (Zanaria et al., 1994). Inactivating mutations in
DAX-1 or the loss of DAX-1 typically result in hypoplasia of
the adrenals and testis (Lalli et al., 1997). Defects attributed to
DAX-1 are traceable to a failure in steroidogenic cell differentiation at the time that the anlagen for the adrenal glands and
gonads develop from the genital ridge.
DAX-1 expression also has implications for the differentiation of GNRH neurons in the hypothalamus and gonadotrophins of the anterior pituitary gland, but the mechanisms of
DAX-1 action in the hypothalamus and anterior pituitary are
not yet known (Partsch and Sippell, 1989). Experimental investigations in the mouse indicate that DAX-1 expression is
required for the function of hypothalamic neurons that regulate the secretion of adrenocorticotrophic hormone (ACTH)
and gonadotrophic hormones in the pituitary gland (Habiby et
al., 1996). It has not been elucidated whether DAX-1 mutations primarily affect the hypothalamus or the pituitary gland
or both (Habiby et al., 1996).
Congenital defects in the adrenal gland and testis, due to
DAX-1, result in simultaneous deficiencies in glucocorticoids,
mineralocorticoids and testosterone (Kletter et al., 1991). The
adrenals of patients with a failure in DAX-1 are severely hypo-
133
plastic and disorganized and usually go undetected in computed tomographic examinations. Most newborns die within
the first 3 weeks unless supplemented with adrenal cortical
hormones (Behre et al., 1997). Lifelong substitution of glucocorticoids and mineralocorticoids is required to replace hormones made by the adrenal cortex. Surviving, untreated males
show classical features of Addison’s disease and hypogonadotrophic hypogonadism requiring substitution of testosterone (Table IV). Secondary sexual features of males develop
normally in most cases but cryptorchidism occurs frequently,
owing to the relative lack of testosterone during fetal development (Kletter et al., 1991).
Spermatogenesis among patients lacking DAX-1 is reduced
or arrested completely (Table IV). The cellular architecture of
the testis is disorganized due in part to the absence of testosterone (Kletter et al., 1991). Fertility is not a therapeutic objective among patients with congenital adrenal hypoplasia.
Autosomal recessive inheritance explains a variant of congenital adrenal hypoplasia not linked to DAX-1 mutations
(Muscatelli et al., 1994). Patients with the alternative form
have mild to modest deficiencies in both adrenal and testicular
steroidogenesis, but hypothalamic control of anterior pituitary
hormone secretion is usually normal. Modest reductions in
spermatogenesis are evident but patients usually are fertile.
FTZ1, a gene encoding for the nuclear orphan receptor and
transcription factor SF1 residing on chromosome 9q33, is a
candidate gene for the alternative form of congenital adrenal
hypoplasia since the mouse homologue (Ftz-F1) also interferes with the differentiation of steroidogenic tissues (Lala
et al., 1992). Expression of Ftz-F1 in mice indicates that it
precedes the transcription of DAX-1 and participates in sexual
differentiation. The gene product SF1 itself seems to interfere
with DAX-1 transcription via direct binding to repressor regions of DAX-1 (Nachtigal et al., 1998).
Laurence–Moon and Bardet–Biedel syndromes
Both syndromes are rare autosomal recessive disorders that
involve a wide range of defects, including mental retardation,
polydactylia, obesity, retinal pigmentary dystrophy and hypogenitalism (Beales et al., 1997; Bruford et al., 1997). Hypogonadism is a facultative feature in both syndromes and
spermatogenesis can be impaired in affected males (Whitaker
et al., 1987). The clinical distinction between the less observed Laurence–Moon syndrome and the more prevalent
Bardet–Biedl syndrome is based on polydactylia, a feature
limited to the Laurence–Moon syndrome (Leppert et al.,
1994).
The chromosomal origin and genetic basis for Laurence–
Moon syndrome remain to be established. In sharp contrast,
genetic defects in the Bardet–Biedel syndrome have been
localized to four loci, BBS1 to BBS4, in families with hereditary Bardet–Biedl syndrome (Table IV). Most families show
linkage to the BBS1 locus on chromosome 11q13 (Leppert
134
T.Diemer and C.Desjardins
et al., 1994), followed by defects in BBS2 on chromosome
16q21, BBS3 on chromosome 15q22.3–23 and BBS4 on
chromosome 3p12 (Beales et al., 1997; Bruford et al., 1997).
Approximately 30% of families do not present linkage to any
BBS locus, implicating one or more additional loci in the
disorder. Candidate genes within the BBS loci have not been
isolated (Beales et al., 1997). BBS1 on chromosome 11q13
contains several genes of physiological importance such as
PYGM (muscle-type glycogen phosphorylase), GCK (human
B-lymphocyte serine/threonine protein kinase), and ZFM1
(zinc finger protein) (Kedra et al., 1997).
Apart from hypogenitalism, which is characteristic of both
Laurence–Moon and Bardet–Biedel syndromes, other clinical
features include mental retardation, obesity, neurological defects, renal abnormalities and retinal dystrophy (Whitaker
et al., 1987). Hypogenitalism in patients with Laurence–
Moon or Bardet–Biedel syndrome is typified by micropenis,
hypospadias and cryptorchidism in males, but genital abnormalities, such as vaginal atresia, are also known in females
(Stoler et al., 1995). The failure in pituitary gonadotrophin
secretion impairs both steroidogenesis and spermatogenesis
(Table IV). Infertility is common but not always present, since
patients with normal testicular function and spermatogenesis
are known to carry Laurence–Moon or Bardet–Biedel syndrome. Testicular biopsies contain elongated spermatids, but
the number of spermatids that complete spermatogenesis is
well below normal (Chanmugam et al., 1977; Whitaker et al.,
1987).
The clinical management of Laurence-Moon and BardetBiedel syndrome is focused primarily on treating the multiple
somatic defects and not on restoring spermatogenesis. In
those instances when the somatic defects are mild, treatment
for hypogonadism can be accomplished by hormone replacement. Spermatogenesis responds to gonadotrophin stimulation in some cases (Chanmugam et al., 1977; Whitaker et al.,
1987).
Congenital hypopituitarism
PIT-1, a transcription factor, is located on chromosome 3p
(Raskin et al., 1996). Mutations in the PIT-1 gene contribute
to the pathogenesis of congenital hypopituitarism (Table IV),
but extra-pituitary effects of mutations in PIT-1 are known,
indicating that PIT-1 regulates multiple genes (PellegriniBouiller et al., 1996). Mutations affecting the DNA-binding
domain of PIT-1 appear to have a profound impact on fetal
neurons during pituitary differentiation (Pfaffle et al., 1996).
PIT-1 is encoded by six exons on chromosome 3p (Raskin
et al., 1996). PIT-1 regulates the expression of prolactin,
growth hormone and the β unit of thyrotrophin by binding to
the promoter region of genes for each of these peptide hormones (Pellegrini-Bouiller et al., 1996). Mutations in PIT-1
typically involve exons 5 and 6, causing structural and functional abnormalities in the DNA-binding domain.
Mutations in PIT-1 cause congenital hypopituitarism, with
profound structural and functional alterations in the pituitary
gland. The clinical phenotype of disorders in PIT-1 differs
among patients, suggesting a multifactorial origin of the
mutations. Aplasia is possible in certain patients, while in
others fusion defects occur between the anterior and posterior
lobes (Pfaffle et al., 1996). The clinical picture includes the
almost total lack of growth hormone (GH), thyrotrophin
(TSH) and prolactin, and thus the arrest of body growth early
in life. Gonadotrophins are not affected in most instances, but
patients who lack gonadotrophins are known (Nogueira et al.,
1997). Insufficient stimulation by FSH and LH is believed to
impair spermatogenesis (Nogueira et al., 1997).
Congenital hypopituitarism is a rare disorder. No consensus
has emerged for treating infertility in these patients since clinical experience is limited. Substitution of growth hormone and
other critical hormonal peptides is required (Behre et al.,
1997).
Isolated deficiency of FSH and LH
Isolated deficiency of FSH and LH occurs infrequently
among patients seen at infertility clinics. Neither the chromosomal origin nor the genetic basis for isolated deficiencies in
LH or FSH secretion is known (Table IV). GnRH-secreting
neurons in the hypothalamus are implicated because replacement therapy with repeated but periodic GnRH evokes the
production of both FSH and LH from the anterior pituitary
gland (Al Ansari et al., 1984).
Isolated FSH deficiency is a rare condition in which LH and
testosterone concentrations remain in the normal range in infertile patients presenting with severe disorders of spermatogenesis. FSH release can be induced by repeated pulsatile
treatment with GnRH. Differentiation of the male sexual phenotype is complete. Defects in spermatogenesis range from
modest reductions in the number of ejaculated spermatozoa
(Haegg et al., 1978) to a failure in spermatogenesis at the level
of spermatocytes or spermatids (Maroulis et al., 1977). In
certain patients, only Sertoli cells are present within the testis
(Al Ansari et al., 1984). The wide range of differences in
spermatogenesis among patients with isolated FSH deficiency points to the interaction of multiple gene products in
this genetic disorder. Treatment can be accomplished by periodic but repeated administration of GnRH to evoke the secretion of FSH (Al Ansari et al., 1984) and restore
spermatogenesis (Maroulis et al., 1977). Clinical success is
limited to the fraction of patients in whom spermatogenesis
has halted at the spermatid stage or where all classes of germ
cells are present but markedly reduced.
Isolated deficiency in LH secretion was found first among
patients who also had Kallmann’s syndrome (Wortsman and
Hughes, 1996). The hallmarks of isolated LH deficiency, also
known as the Pasqualini or fertile eunuch syndrome, include
hypogonadism with hypoplastic Leydig cells, subnormal con-
Disorders in spermatogenesis
centrations of LH and testosterone, and normal concentrations
of FSH (Table IV). All classes of germ cells are present, including spermatozoa in the ejaculate (Faiman et al., 1968;
Williams et al., 1975). Spermatogenic failure is attributable to
testosterone insufficiency. Despite the apparent hypogonadism, patients have been reported to father children (Faiman et
al., 1968; Williams et al., 1975). Testosterone replacement in
patients with isolated LH deficiency alleviates the symptoms
of hypogonadism and may stimulate spermatogenesis (Behre
et al., 1997). Therapeutic strategies to restore spermatogenesis have been attempted in exceptional cases of both disorders,
but routine experience is missing (Wortsmann and Hughes,
1996).
135
Genetic disorders of testosterone synthesis
droxysteroid dehydrogenase (Nieschlag et al., 1997). The extent of the enzyme defect and the level of testosterone
insufficiency is variable, providing wide deviations in secondary sexual differentiation (Nieschlag et al., 1997). Testes
remain in the abdomen or in the inguinal canal in patients with
partial or complete enzyme defects. Germ cell development
fails to proceed beyond the spermatocyte stage due to the
absence of testosterone or elevated temperatures resulting
from cryptorchidism. Testicles should be removed in patients
with feminine sexual features or, in male patients, relocated
surgically to the scrotum to reduce the risk of testicular cancer.
Most patients with male pseudohermaphroditism have feminine genitalia (Table IV).
Androstendione concentrations are elevated in post-pubertal patients with male pseudohermaphroditism due to a
17β-hydroxysteroid dehydrogenase deficiency (Anderson,
1995). Patients with the most severe forms of pseudohermaphroditism, occasioned by the complete to partial lack of
testosterone during embryonic development, are usually not
seen in infertility clinics.
Patients with abnormal male sexual differentiation and
minor lesions such as mild hypospadias are rare (Yanase,
1995). Testosterone substitution is recommended from the
time that puberty is anticipated in patients who have near
normal secondary sexual features (Nieschlag et al., 1997).
Spermatogenesis may be induced by testosterone replacement in the subset of patients who manifest modest losses in
steroidogenic enzyme activity resulting in a normal male phenotype, but there is a lack of systematic therapeutic approaches.
17,20-Desmolase/17α-hydroxylase and
17β-hydroxysteroid dehydrogenase defect
5α-Reductase 2 deficiency
Testosterone biosynthesis depends on the cytochrome
P-450c17 enzyme, involving 17,20-desmolase and 17α-hydroxylase activity, and the P-450 enzyme 17β-hydroxysteroid
dehydrogenase (Payne and Youngblood, 1995; Suzuki et al.,
1998). Testosterone insufficiency, inducing disturbances in
male sexual differentiation and infertility, has been attributed
to a range of enzymatic defects involving the complete to
partial lack of testosterone due to autosomal recessive mutations of steroidogenic enzymes (Table IV). More than 21
mutations are known in the gene for P-45017c on chromosome 10q24.3, resulting in partial or no activity of 17,20-desmolase and 17β-hydroxylase (Monno et al., 1997; Suzuki
et al., 1998). Over 14 mutations have been identified in the
gene encoding for 17β-hydroxysteroid dehydrogenase III, the
testis-specific isoenzyme, on chromosome 17 in patients with
17β-hydroxysteroid dehydrogenase deficiency (Andersson,
1995).
The differentiation of the male phenotype requires testosterone. Variable forms of male pseudohermaphroditism are
evident among patients who fail to produce testosterone due
to defects in 17,20-desmolase/17α-hydroxylase or 17β-hy-
The conversion of testosterone to dihydrotestosterone is catalysed by the enzyme 5α-reductase 2 (Table IV). Enzyme production is a prerequisite for the differentiation of the
urogenital tubercle and sinus, the anlagen of male external
genitalia. The gene encoding for 5α-reductase 2 is located on
chromosome 2 and consists of five transcribed exons (Wilson
et al., 1993). Either deletion or mutations of the gene SRD5A2
results in different grades of male pseudohermaphroditism
(Meschede et al., 1997). Mutations in SRDA2 affect all exons
of the gene, resulting in a deficiency of 5α-reductase 2. The
catalytic activity of the enzyme is reduced substantially following mutations (Wilson et al., 1993). A total of 28 inactivating mutations have been demonstrated in 52 families with this
disorder (Wilson et al., 1993). All transcribed exons are affected and most of them involve point mutations with recessive inheritance (Wilson et al., 1993).
The external genitalia of newborns with 5α-reductase deficiency are feminized and most children are raised as females.
The structural features of the disorder include perineoscrotal
hypospadias with a pseudovagina (Meschede et al., 1997).
The prostate gland is dysplastic, rudimentary, and located in a
Mutations of the FSH receptor
Mutations of the FSH receptor typically involve profound
reductions in spermatogenesis (Tapanainen et al., 1997). The
gene encoding for the FSH receptor, FSHR, resides on
chromosome 2p21-p16 and consists of 10 transcribed exons
(Table IV). An inactivating point mutation in FSHR causes a
recessively inherited form of hypogonadotrophic failure in
homozygous women, resulting in ovarian failure (Tapanainen
et al., 1997). The same point mutation in exon 7 (566c to T) in
homozygous men results in severely oligozoospermic ejaculates and elevated FSH concentrations. Screening tests for
mutations of the FSH receptor in all exons in a group of
infertility patients indicated that receptor failure is an uncommon cause of male infertility (Tuerlings et al., 1998b).
136
T.Diemer and C.Desjardins
post-urethral position (Meschede et al., 1997). Testes remain
in the inguinal position, but seminal ducts and epididymides
are evident since neither of these structures requires dihydrotestosterone for differentiation. Spermatogenesis is halted at
early steps since germ cell differentiation fails at body temperature (Wilson et al., 1993).
Substitution therapy with either testosterone or dihydrotestosterone has been attempted, but current clinical experience
is too limited to recommend a standard treatment. External
genitalia may be enlarged and corrected surgically. No therapeutic approaches for infertility have been reported since the
deficiency in 5α-reductase 2 is a rare form of male pseudohermaphroditism limited to about 50 families (Wilson et al.,
1993). Testes should be removed from patients with a female
phenotype, and oestrogen substitution should be initiated
when puberty is anticipated (Meschede et al., 1997). Patients
with a predominantly feminine phenotype often consult their
physicians because of amenorrhoea or hirsutism. Male patients are usually not seen in infertility clinics.
Disorders of androgen action at target organs:
androgen resistance
Androgens fail to exert their effects when the intracellular
androgen receptor is functionally absent or impaired. Mutations in the androgen receptor have been localized to the X
chromosome (Figure 2 and Table IV), Xq11–12, and are estimated to occur at a frequency of one per 50 000 births
(McPhaul et al., 1991, 1993).
Over 200 different mutations in the gene encoding for the
androgen receptor have been identified (Gottlieb et al., 1998).
The relatively large number of mutations accounts for multiple syndromes of androgen resistance that are typified by
complete to partial androgen insensitivity (McPhaul et al.,
1993; Gottlieb et al., 1998). Most mutations affect the hormone-binding site and the DNA-binding domain of the receptor protein, explaining the absence of ligand binding and
qualitative or quantitative reductions in ligand binding
(Marcelli et al., 1992; Quigley et al., 1995). The grades of
androgen resistance find their expression in multiple syndromes characterized by different phenotypes (Table IV).
The undervirilized fertile male syndrome represents the
mildest form of androgen insensitivity. Patients are not well
characterized since the phenotype is normal and the incidence
among infertile patients is rare (Meschede et al., 1997).
Spermatogenesis may be reduced among patients with this
syndrome, but fertility is unaffected (Meschede et al., 1997).
The typical cellular defect in spermatogenesis is unknown
since systematic studies are lacking.
The Reifenstein syndrome represents a moderate type of
androgen resistance. Defects in male genitalia such as hypospadias and cryptorchidism are frequently evident. Patients
with Reifenstein syndrome have impaired spermatogenesis
and may consult their physicians because of infertility. For
mild forms of androgen resistance, infertility therapy relies on
assisted reproductive technology for those cases where spermatozoa are recoverable from the ejaculate or testicular
biopsy. Genetic counselling is recommended in these cases.
Testicular feminization represents the most severe form of
androgen resistance. Patients have a normal male karyotype
of 46, XY but have phenotypic sexual features of females,
including female body proportions and pseudovagina; however, ovaries are absent, but inguinal testicles are present. The
seminiferous epithelium is limited to Sertoli cells (Table IV).
Testes should be removed surgically to reduce the risk of
testicular cancer.
Defects in peptide hormone response
elements
Spermatogenesis is supported by FSH which binds to Sertolicell receptors to trigger cAMP-dependent pathways, including nuclear transcription factors such as cAMP-responsive
element-binding protein (CREB) and the cAMP-responsive
element modulator (CREM). CREM and CREB ultimately
influence the expression of genes involved in spermatogenesis such as protamine 1 and 2, transition protein 1 and 2, and
others (Peri et al., 1998). Different CREM-dependent genes
appear to regulate the formation of CREM repressors and
activators acting at different points in the meiotic cycle (Weinbauer et al., 1998). Switching between the expression of genes
directing CREM repressors and CREM activators, during the
early steps of spermatogenesis, appears to be a decisive regulatory event in completing spermatid development since
CREM activator expression is missing in certain severely oligozoospermic men (Peri et al., 1998; Weinbauer et al., 1998).
The genes responsible for the failure in the CREM activator
switch remains to be identified and these preliminary findings
with oligozoospermic patients must be extended to larger
groups of infertile men.
Conclusions
Widespread use of ICSI (Palermo et al., 1993) protocols raises
medical concerns about the transfer of genetic defects to future generations since men with severe disorders of spermatogenesis also manifest genetic errors as part of the underlying
aetiology of their defect (de Kretser, 1995; Engel et al., 1996;
Tuerlings et al., 1998a). Indeed, recent investigations confirm
a higher incidence of chromosomal abnormalities, such as
unbalanced complements of chromosomes and delays in firstyear developmental scores, in children conceived via the use
of ICSI than in those conceived naturally or with other assisted reproductive procedures (Andrews et al., 1998;
Bonduelle et al., 1998a,b; Bowen et al., 1998). However,
most men seen in infertility clinics are free of genetic
alterations that are known to cause severe somatic defects, so
the transfer of defective genes to future generations via repro-
Disorders in spermatogenesis
ductive technologies is unlikely to differ from that of normal
fertile men. Severe chromosomal defects, such as polyploidies or unbalanced translocations, usually result in embryonic death of the offspring in the first trimester of pregnancy
so inheritance of major somatic defects is not expected following use of assisted reproductive techniques (Engel et al.,
1996).
The percentage of infertile males with severe disorders in
spermatogenesis associated with microdeletions on the Y
chromosome can be expected to increase if the 10% of infertile men with such deletions rely on ICSI. Microdeletions on
the Y chromosome are passed to 100% of the sons if spermatozoa from patients with microdeletions are used in assisted reproduction (de Kretser, 1995; Engel et al., 1996).
Infertility in this cohort of men can be treated with assisted
reproductive technologies; however, such protocols pose a
dilemma for future male offspring who seek treatments similar to those experienced by their fathers (de Kretser, 1995).
Patients should be informed of the risk of infertility in their
male offspring. Genetic counselling is recommended in all
cases where chromosomal abnormalities are linked to disorders of spermatogenesis (Meschede et al., 1998).
New opportunities exist to access the genetic makeup of all
cells, including individual spermatozoa by molecular analyses. Adoption of these new methods expands the opportunity
to diagnose former idiopathic causes of infertility and, most
importantly, to advise patients about the potential for passing
on inheritable defects via the paternal genome. Current reliance on sperm karyotypes and fluorescence in-situ hybridization of sex chromosomes offers a superficial estimate of the
risk involved for patients with numerical chromosomal aberrations, especially when spermatozoa are recovered from
individuals with Klinefelter’s syndrome. Providing patients
with access to contemporary diagnostic protocols will expand
the range of treatments that can be offered to assist the infertile
male as well as providing advice on the risks of transmitting
abnormal genes to future children.
Acknowledgements
Thorsten Diemer is currently supported by a grant from the
‘Deutsche Forschungsgemeinschaft’ (Germany), project Di
723/1–1.
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Received on September 2, 1998; accepted on December 2, 1998