Download The Jumping SHOX Gene—Crossover in the Pseudoautosomal

J C E M
O N L I N E
B r i e f
R e p o r t — E n d o c r i n e
R e s e a r c h
The Jumping SHOX Gene—Crossover in the
Pseudoautosomal Region Resulting in Unusual
Inheritance of Leri-Weill Dyschondrosteosis
Sarina G. Kant, Hetty J. van der Kamp, Marjolein Kriek, Egbert Bakker,
Boudewijn Bakker, Mariette J. V. Hoffer, Patrick van Bunderen,
Monique Losekoot, Saskia M. Maas, Jan M. Wit, Gudrun Rappold,
and Martijn H. Breuning
Center for Human and Clinical Genetics (CHCG)-Department of Clinical Genetics (S.G.K., M.K., M.H.B.),
Department of Paediatrics (H.J.v.d.K., B.B., J.M.W.), and CHCG-Laboratory for Diagnostic Genome
Analysis (E.B., M.J.v.H., P.v.B., M.L.), Leiden University Medical Center, 2300 RC Leiden, The
Netherlands; Departments of Clinical Genetics (S.M.M.) and Paediatrics (S.M.M.), Academic Medical
Centre, 1105 AZ Amsterdam, The Netherlands; and Department of Human Molecular Genetics (G.R.),
University of Heidelberg, 69120 Heidelberg, Germany
Context: During meiosis I, the recombination frequency in the pseudoautosomal region on Xp and
Yp (PAR1) in males is very high. As a result, mutated genes located within the PAR1 region can be
transferred from the Y-chromosome to the X-chromosome and vice versa.
Patients: Here we describe three families with SHOX abnormalities resulting in Leri-Weill dyschondrosteosis or Langer mesomelic dysplasia.
Results: In about half of the segregations investigated, a transfer of the SHOX abnormality to the
alternate sex chromosome was demonstrated.
Conclusions: Patients with an abnormality of the SHOX gene should receive genetic counseling
as to the likelihood that they may transmit the mutation or deletion to a son as well as to a
daughter. (J Clin Endocrinol Metab 96: E356 –E359, 2011)
he classic clinical triad in Leri-Weill dyschondrosteosis
(LWD) is short stature, mesomelia, and Madelung deformity. In mesomelia, the middle portion of a limb is shortened in relation to the proximal portion, whereas Madelung
deformity includes abnormal alignment of the radius, ulna,
and carpal bones at the wrist (1, 2). Penetrance of the phenotype appears to be incomplete within families. Besides,
both bilateral Madelung deformity and short stature tend to
be more common and severe in females than males (3).
LWD is caused in a large proportion of patients by mutations or deletions in the short stature homeobox (SHOX)
gene or by deletions downstream of the SHOX gene (4). The
SHOX gene is located in the pseudoautosomal 1 region
(PAR1) of the X- and Y-chromosomes. The SHOX gene
escapes X-chromosome inactivation. Two functional
T
ISSN Print 0021-972X ISSN Online 1945-7197
Printed in U.S.A.
Copyright © 2011 by The Endocrine Society
doi: 10.1210/jc.2010-1505 Received July 1, 2010. Accepted October 7, 2010.
First Published Online November 10, 2010
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SHOX genes are needed for normal growth. When both
SHOX genes are mutated, this results in Langer mesomelic
dysplasia, a more severe form of skeletal dysplasia.
Due to crossover during meiosis I in males, transfer of
the deleted SHOX gene to the alternate sex chromosome
may be observed in some pedigrees. Here we present three
families in which this transfer event occurs in almost 50%
of the segregations investigated.
Patients and Methods
Family 1
The index case (III:2) was born at term with a normal birth
weight and length. At the age of 8.9 yr, her height (H) was 121.1
cm [⫺2.6 SD score (sds)] with a sitting height (SH)/H ratio of 1.4
Abbreviations: FISH, Fluorescence in situ hybridization; H, height; LWD, Leri-Weill dyschondrosteosis; MLPA, multiplex ligation probe amplification; PAR1, pseudoautosomal 1
region; sds, SD score; SH, sitting height; SHOX, short stature homeobox gene.
J Clin Endocrinol Metab, February 2011, 96(2):E356 –E359
J Clin Endocrinol Metab, February 2011, 96(2):E356 –E359
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The fathers came from a family of four. In one other brother
(II:4), their sister (II:3), and the (grand-) parents (I:1 and I:2),
karyotyping and fluorescence in situ hybridization (FISH) analysis were performed, but clinical measurements were not.
Family 3
The index case (II:2) was seen at the age of 40 yr. She was born
after an uneventful pregnancy of 40 wk with a birth weight of 4
kg. Her arms and legs were relatively short at birth. Until the age
of 35 yr, she was healthy, but since 5 yr ago she had joint pain,
especially of her fingers, for which she presented at the hospital.
At physical examination, she was 130 cm tall, and she had rhizomelic shortening of the arms and legs. X-rays showed Madelung deformity and shortening of the ulnae.
According to the index case, her brother (II:1) has exactly the
same H and stature as the proband. He is mentally retarded, and
it was not possible to examine him.
The mother (I:2) has a H of 160 cm with relatively short arms
and legs. Three of the eight sibs of the mother are disproportionately short. The father (I:1) had a normal stature, and his H
was 165 cm. He died from leukemia.
Cytogenetic and molecular analysis
FIG. 1. Clinical photograph and x-rays of the left hand and wrist of
patient III:2 of family 1. Note the Madelung deformity.
sds (5, 6). Her arm span was ⫺2.2 sds (7). She had a Madelung
deformity of both wrists (Fig. 1). Bone age according to Greulich
and Pyle (8) was 6.9 yr.
Her cousin (III:5) was also known with short stature. At the
age of 9.7 yr, her H was 126.8 cm (⫺2.4 sds), and her SH/H ratio
was 2.2 sds. Her arm span was ⫺2.0 sds. Her bone age according
to Greulich and Pyle (8) was 7.9 yr. She had a mild Madelung
deformity.
The father of the index case (II:2) had a H of 169.6 cm (⫺2.0 sds)
and a SH/H ratio of 1.5 sds. His arm span was ⫺2.0 sds. He had no
Madelung deformity. The father of the cousin (II:3) had a H of
179.5 cm (⫺0.6 sds) and a SH/H ratio of 1.5 sds. His arm span was
⫺2.7 sds. He had mild Madelung deformity. A third brother (II:1)
did not have short stature or disproportion. The paternal grandfather (I:1) had a H of 165.5 cm (⫺2.6 sds) and a SH/H ratio of 1.8
sds. X-rays of his wrists did not show any abnormality.
Treatment with GH and decapeptyl in both cousins was
started and resulted in a H gain with an increase of the H sds,
whereas their bone age increased only slightly.
Family 2
The index case (III:5) in the second family was seen at the age
of 11.7 yr. Her H was 134.7 cm (⫺2.7 sds), with a SH/H ratio
of 2.5 sds and an arm span of 1.0 sds. An x-ray of the wrist
showed no abnormalities. Bone age according to Greulich and
Pyle (8) was 11.8 yr. Her father (II:2) had a H of 176 cm (⫺1.1
sds), but had experienced disproportionately short legs. He had
a SH/H ratio well above 2.5 sds. His arm span was ⫺1.0 sds.
A cousin of the index case (III:2) was also known with short
stature, and at the age of 12.8 yr she had a H of 144 cm (⫺2.3
sds), a SH/H ratio of 1.4 sds, and an arm span of ⫺1.0 sds. No
abnormalities of the wrist were noted. Bone age was normal. Her
father (II:1) had disproportionate short stature with a H of 165.7
cm (⫺2.6 sds) and a SH/H ratio of 2.7 sds.
Karyotyping was performed in families 1 and 2 using standard
procedures. FISH analysis of the PAR1 region was also performed
using standard procedures. In family 1, the FISH probe 29B11 was
used, which is located 3⬘ of the SHOX gene. In family 2, the FISH
probes 34F5 (SHOX specific) and RP11– 839D20 (subtelomere
specific) were used. For family 1 and family 3, sequencing and multiplex ligation probe amplification (MLPA) of the SHOX gene and
MLPA of the enhancer region were used to detect mutations or copy
number variants using standard procedures.
Results
Family 1
Karyotyping showed a normal result in the index patient
(III:2) and her cousin (III:5). Sequence analysis and MLPA of
the SHOX gene revealed no abnormalities in both affected
girls. However, FISH analysis showed a deletion in PAR1
downstream of the SHOX gene in the girls, in their fathers
(II:2 and II:3), and in the paternal grandfather (I:1). Results
of this analysis were described earlier (9). The deletion in this
family was confirmed using MLPA techniques.
The deletion in both fathers (II:2 and II:3) and the
grandfather (I:1) was situated on the Y-chromosome (Fig.
2). The deletion was not observed in the two sisters of the
index case (III:3 and III:4), the two brothers of her cousin
(III:6 and III:7), and her uncle (II:1).
Family 2
Karyotyping in the index patient (III:5) showed a normal female karyotype. FISH analysis demonstrated a terminal deletion of the tip of Xp, including the SHOX gene
region, with its breakpoint in Xp22.3 [ish del(X)(p22.3)
(34F5-, 839D20-)]. This deletion was located on the X-
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Kant et al.
The Jumping SHOX Gene
FIG. 2. Pedigrees of families 1, 2 and 3. Crossing over of the PAR1
abnormality has occurred in family 1 in patients II:1, III:2, III:5, III:6, and
III:7; in family 2 in patients II:3, II:4, III:1, III:2, and III:5; and in family 3
in either II:1 or II:2. Light gray symbols, Persons with SHOX
haploinsufficiency; dark gray symbols, persons with compound
heterozygous mutation of SHOX. Y, SHOX-mutation/deletion on Y
chromosome; X, SHOX-mutation/deletion on X chromosome; (?),
SHOX-deletion on either X or Y chromosome, derived from the results
of both children; n, no deletion. The index persons are indicated by an
arrow.
chromosome (Fig. 2) in the index case, her aunt (II:3), one
of the sons of this aunt (III:9), and the daughter of the
father’s brother (III:2). It was demonstrated on the
Y-chromosome in the father of the index case (II:2), one of
his brothers (II:1), and the grandfather (I:1). The deletion
was not observed in the three sisters of the index case (III:3,
4, 6), three of her cousins (III:1, 7, 8), and one of the
brothers of her father (II:4).
Family 3
In the index case (II:2), MLPA analysis showed a deletion
of the promoter region until exon 8 of the SHOX gene, and
mutation analysis showed a mutation in exon 4 of the SHOX
gene (c.517C⬎T p.Arg173Cys), in accordance with the clinical diagnosis of Langer mesomelic dysplasia. Her brother
(II:1) was shown to have the same deletion and mutation.
The mother (I:2) appeared to have the deletion of the SHOX
gene. DNA of the father (I:1) could not be analyzed.
Discussion
LWD caused by mutations or deletions of the SHOX gene is
known to be clinically highly variable. Family 1 shows that
this is also true for LWD caused by a deletion downstream
the SHOX gene in PAR1. The index patient and her cousin
J Clin Endocrinol Metab, February 2011, 96(2):E356 –E359
have clear disproportionate short stature, whereas their fathers have normal H, with only slight disproportion.
PAR1 is located at the terminal end of Xp and Yp. All
genes within PAR1 escape X inactivation in women. However, the only gene in this region associated with human
disease is the SHOX gene (10). Inheritance of LWD is
pseudoautosomal. The SHOX gene in the normal situation is present in two functional copies. In fact, one might
consider PAR1 as a very small extra pair of autosomes
(11), except that segregation in autosomes is independent
of sex. Here we describe three families in which an abnormality in PAR1 segregates from one sex chromosome
to the other in the next generation.
PAR1 is highly homologous and is therefore necessary
for X-Y chromosome-pairing during male meiosis (11). As
with autosomes, it undergoes one obligatory crossover
event during this process, which always occurs within the
PAR1 region. In males, the recombination frequency in
PAR1 is extraordinarily high, about 20 times higher than
the genome average (12, 13). It decreases rapidly at the
proximal boundary of the 2.6 Mb pseudoautosomal segment in conjunction with a decrease in homology (10, 11).
One explanation for the high recombination rate is found
in the fact that telomeric regions usually are more recombinogenic than the rest of a chromosome (14, 15), but in
the sex chromosomes a special situation exists because
recombination in male meiosis is limited to the pseudoautosomal regions. This leads to a recombination frequency across the pseudoautosomal regions being about
10 times higher in male than in female meiosis (12, 14).
The phenomenon of transfer of a mutated or deleted
SHOX gene from one sex chromosome to the other occasionally led to minimal comment in some reports (16,
17) but has never been described in a high percentage
within the same family as in the families that we report
here. A recombination event resulting in transfer of a
PAR1 abnormality between X- and Y-chromosomes occurred in 11 of the 24 segregations investigated. This high
percentage could be due to the fact that males with LWD
are overrepresented in the families.
The possibility of a crossing over seems not to be dependent on the kind of abnormality in PAR1. Flanagan et
al. (16) described one family with at least a deletion of the
whole SHOX gene and one family with a splice-site mutation in the SHOX gene, whereas Sabherwal et al. (17)
reported on a family with a missense mutation. The families we describe have a deletion downstream the SHOX
gene, a deletion of the terminal part of PAR1 including
SHOX, a deletion of the whole SHOX gene, and a mutation in the SHOX gene.
The fact that SHOX can “jump” from one sex chromosome to the other has consequences for genetic coun-
J Clin Endocrinol Metab, February 2011, 96(2):E356 –E359
seling. When giving genetic advice to male patients with
LWD, transfer of the mutation or deletion in PAR1 to the
other sex chromosome should be discussed. The possibility of such a transfer is also present in female patients, but
chances are smaller than in males, and it does not influence
their risk of having an affected son or daughter.
Acknowledgments
Address all correspondence and requests for reprints to: Sarina
G. Kant, Center for Human and Clinical Genetics-Department of Clinical Genetics, Leiden University Medical Center,
P.O. Box 9600, 2300 RC Leiden, The Netherlands. E-mail:
[email protected].
Disclosure Summary: The authors have nothing to disclose.
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