Download Warren, ST and Nelson, DL: Trinucleotide repeat expansions in neurological disease. Current Opinion in Neurobiology 3:752-759 (1993).

Stephen T. Warren
Emory University
in neurological
repeat expansions
Trinucleotide
School of Medicine,
During
have
the
been
as their
triplet
coding.
past
cause: the
repeats
Atlanta, USA
year,
discovered
and David
new
have
remarkable
are normally
an
or by thousands
gene expression,
Current
of
expansion
neurological
of trinucleotide
markedly
of repeats
or protein
1993,
These
not always
and may expand
influencing
structure.
3:752-759
able within families where the gene becomes penetrant
only when maternally transmitted and the chance of penetrance increases in successive generations. This peculiar
inheritance pattern has been referred to as the Sherman
paradox [4]. In 1991 a series of reports delineated the
mutation and gene involved in FraX syndrome
[5-81.
gene was
The FMRl (Fragile X Mental Retardation-l)
discovered to contain a CGG-repeat within the first exon
that was normally polymorphic, exhibiting between 6-52
copies with a mean of 29. Among affected individuals, the
repeat length was dramatically increased well beyond 230
repeats, usually 600 or more. Concomitant with the large
expansion, referred to as the full mutation, was abnormal
methylation of restriction sites within a region rich in
CG dinucleotides
(a CpG-island) immediately upstream
of the gene [9,10]. Pieretti et al. [ 111 demonstrated
the
from full mutation alleabsence of FMRl transcription
les. Non-penetrant
carrier males and many such females
had repeat lengths of intermediate
size without abnormal methylation, called premutations. These sizes ranged
from approximately 50 to 200 and, upon transmission to
offspring, displayed remarkable instability, with offspring
usually exhibiting allele sizes different from the transmitting parent and distinct from other siblings. The change
tends to increase the repeat length and, in the maternal
premutation size range, the repeat length is proportional
with the risk of full expansion and therefore penetrance
For decades, mutational mechanisms that lead to human
genetic disease have followed rules and examples set
forth in model systems such as Drosophila and yeast. In
recent years, however, new mechanisms
responsible for
genetic disease have emerged where little or no precedent had been established in other genetically studied
organisms. One such mechanism is trinucleotide
repeat
expansions [ l*,2,3*]. Since early 1991, when the mutation
responsible for the fragile X syndrome was uncovered as
an astounding expansion of an exonic CGG-repeat, four
other human genetic diseases have been similarly demonstrated to be due to such mutations. In most cases, these
mutations occurred in genetic diseases that display some
degree of unusual inheritance patterns, such as incomplete penetrance or genetic anticipation, where severity
of symptoms increases and/or age-of-onset decreases in
subsequent generations of a single kindred. These previously poorly understood genetic phenomena can now be
satisfactorily explained by the behavior of unstable and
expanding trinucleotide repeats. However, much remains
to be understood, in particular the mechanisms responsible for this unprecedented
mutational change. Below
are short descriptions
of the genetic diseases currently
recognized to exhibit these mutations and what is known
about each trinucleotide repeat, its behavior and the gene
it influences.
[121.
Sutcliffe et al. [13**] studied chorionic villi and fetal
tissue of a male fetus with a full mutation where the
chorionic villi were unmethylated
and the fetal tissue
methylated. Expression of FMRl was demonstrated
from
the unmethylated sample, suggesting gene expression is
repressed by the abnormal methylation. The abnormal
methylation was shown to include not only the CpGisland but the entire repeat itself, in a pattern similar to
that observed on the inactive X-chromosome
[14*,15*].
Fragile X syndrome
Fragile X (FraX) syndrome is an X-linked dominant disorder with reduced penetrance.
The syndrome is the
leading cause of inherited mental retardation
in hum
mans and is associated with a chromosomal
fragile site
at Xq27.3 (see Fig, 1). The reduced penetrance
is vati-
Abbreviations
AR-androgen
receptor;
FraX-Fragile
SBMA-spinal
752
DM-mytonic
dystrophy;
X; HD-Huntington’s
and bulbar
muscular
@ Current
USA
disease
repeats.
though
unstable
Houston,
of mutation
in a single generation,
in Neurobiology
Introduction
type
and exonic,
message stability
Opinion
human
unprecedented
polymorphic
In disease states they become
moderately
1. Nelson
and Baylor College of Medicine,
examples
that
disease
FMRl-Fragile
X Mental Retardation-l
disease; Mt-PK-myotonic-protein
atrophy;
Biology
SCAl-spinocerebellar
Ltd ISSN 0959-4388
gene;
kinase;
ataxia
type
1.
Trinucleotide
The notion that the absence of FMKl gene product is
solely responsible
for this phenotype
is supported
by
patients with FraX phenotype (without the cytogenetic
fragile site) that have deletions including FMRl [ 16*,17*]
and one patient with a missense mutation in FMRl [ 18**].
The nature of the repeat expansion
remains unclear.
Richards et al. [ 19”] and Oudet et al [20**] demonstrated linkage disequilibrium
between nearby polymorphic markers and the FraX chromosome
suggesting a
limited pool of founder chromosomes.
Smits et al. [21*]
presented evidence that the premutation
may be quite
long-lived in the population supporting a step-wise mutation rate from a normal allele ( < 35 CGGs) to normal
alleles with larger repeats (3550 CGGs) to premutation
and then full mutation [ 22*,23*]. The parent-of-origin
differences in expansion to full mutations is perhaps being
resolved. Willems et al. [24] showed that a mosaic male
(with both full mutations and premutations)
transmitted
a premutation to his daughter who subsequently
had an
affected son. Reyniers et al [25**] showed only premutation sperm in affected males with full mutations. Tissue
expression studies of FMRl showed normal expression
in testes [ 26.1. Bachner et al. [ 27**] presented evidence
to support the idea of FMRl expression being required
for normal spermatogenesis,
such that full mutation or
mosaic males would produce premutation
sperm due
to selection for FMRl expression.
Under this hypothesis, affected males cannot transmit full mutations, fitting
the observations of Sherman et al. [4].
repeat
expansions
muscular
atrophy
Spinal and bulbar muscular atrophy (SBMA), also known
as Kennedy’s disease, is a rare X-linked recessive with occasional manifesting females. Clinically, SBMA is characterized by a slowly progressive spinal and bulbdr muscular atrophy affecting the anterior horn cells, resulting
in weakness and cramping of the proximal limb muscles
(spinal) and of th e faci‘al an d c h ewing muscles, including
the tongue (bulbar) with fasciculations. Gynecomastia is
also frequently seen, as is testicular atrophy and azospermia, at the later stages of the disease. Age of onset is usually in the third to fifth decade, although by history, many
patients describe mild symptoms much earlier.
Fischbeck et al. [ 281 mapped SBMA to Xql l-12 (see Fig.
1), which overlapped the position of the androgen receptor (AR). As patients exhibit some features of androgen
insensitivity and the occasional finding of diminished androgen binding to SBMA fibroblasts, Ia Spada et al. [29]
studied the AR gene in SBMA and correlated the length
of an exonic CAG-repeat with the disease.
The CAG-repeat of the AR is normally polymorphic
[30]
and encodes a long tract of glutamine residues near the
amino-terminus
of the protein. Among normal individuals, there is an average of 21 repeats with a range of 12
to 30 [30,31*]. Among patients with SBMA, the repeats
range from 40 to 62 [ 29,32,33**], The SBMA repeat allele
is relatively unstable when transmitted, varying by 2-3
disease
Warren
and Nelson
repeats [31*,33**]. This instability was observed by ~a
Spada et al. [YY] to be more common upon paternal transmission
rather than maternal (57% compared
to 13%). Igarashi et al. [34] and La Spada et al. [33**]
demonstrated
a strong correlation between the length of
the CAG-repeat and the age of onset of muscle weakness:
those with the smallest abnormal length (43 repeats)
had onset in the sixth decade, whereas those with the
largest expansion
(51 repeats) had onset between 22
and 31 years of age. Apart from the onset of muscle
weakness, no correlation with other clinical parameters
was observed.
SBMA is clearly a gain-of-function,
as can be exquisitely
demonstrated
by considering
testicular
feminization,
which is due to the absence of the AR. Testicular feminization individuals, some with deletions of the entire AR
gene, exhibit abnormal fetal sexual differentiation leading
to an external female phenotype,
despite an XY karyotype, with no evidence of muscle weakness. SBMA patients have normal fetal differentiation with comparatively
mild signs of androgen insensitivity, therefore suggesting
that the AR with the expanded glutamine tract is at least
partially functional, As ARs are normally concentrated
in
spinal and bulbar motor neurons [35], the expanded glutamine repeat may allow a more promiscuous interaction
with gene promoters, resulting in degeneration
of these
cells in SBMA
Myotonic
Spinal-bulbar
in neurological
dystrophy
Myotonic dystrophy (DM) is an autosomal
dominant
multi-systemic disorder characterized by dystrophic muscular weakness with myotonia and muscle wasting. Patients often exhibit additional signs of cataracts, cardiac
conduction defects and, in males, premature balding and
testicular atrophy. This clinical picture is highly variable
and the mildest form (cataracts with little or no muscle involvement) tends to become progressively worse in subsequent generations, sometimes leading to a congenital
form of the disease, which is associated with muscular
hypoplasia, mental retardation and neonatal mortality. In
addition, the age-of-onset decrease in subsequent generations, resulting in a classic example of genetic anticipation, was until recently without molecular explanation.
In early 1992 several groups published the identification
of the mutant gene involved in DM [36,37,38**40**].
The DM mutation was localized to chromosome
19q13.3
(see Fig. 1) and identified as a CTG-repeat. The CTGrepeat was found to reside within the 3’ untranslated
region of a gene, designated myotonin kinase, because
the predicted amino acid sequence shares homologies
with other serine-threonine
protein kinases. The repeat
is normally polymorphic with a mean size of 5 triplets.
Affected individuals display repeat lengths over a considerable range from approximately
50 triplets to over
2000. Within this range, the trinucleotide
repeat is unstable in both meiosis and mitosis. The DM mutation
is in strong linkage disequilibrium
with a nearby polymorphism
[37,38**,41], leading Imbert et al. [42**] to
753
754
Disease,
16
transplantation
and regeneration
HD
Y
15.3
25
15.2
15.1
24
23
SCAl
22.3
22.2
22.1
22.3
22.2
22.1
13.1
13.2
13.3
R
21.3
21.2
21.1
21.1
21.2
21.3
11.4
11.3 6
11.23
22
11:;:
11.1
23
24
25
26
15
ii.1
12.2
16.1
13
SBMA
1%
27
13.3 n
13.2
21.3
22.1
22.2
22.3
23
131
1:
1:
33.3
24
25
32
26
33
13.2
34
351
I
27
DM
13.3
u
X
19
Repeat
Disease
FRAXA
26
134
sequences in human disorders due to trinucleotide
Gene
Repeat
Location
Normal
lengtfi
repeat expansion
Disease length’“’
HD
Huntingtin
CAG
poly-gin?
Chromosome 4
1St axon
translated?
11-36
42-l 00
SCAl
???
CAG
Chromosome 6
??!
25-36
43-81
DM
DM kinase
CTC
(CAG)
Chromosome 19
last exon
untranslated
5-30
so->2000
SBMA
AR
CAG
poly.gin
Chromosome X
1St exon
translated
12-30
40-62
FRAXA
FMRl
CGG
Chromosome X
1 st exon
untranslated
h-52
230.>2000
Fig. 1. Summary
of human disorders
due
to trinucleotide
repeat expansions.
The
chromosomal
locations
indicated on the ideograms
the disease notation.
‘“‘Number of trinucleotide
repeats
The length of the abnormal DM repeat has been correlated with disease severity and age-of-onset
[43,44].
Harley et al.[45*] has shown that patients with repeats
toward the lower abnormal
range (- 5Ck-100 triplets)
The
adjacent to
repeat is ori-
ented relative to the sense strand, when
known.
propose that DM mutations are derived from normal alleles of 19-30 repeats, which themselves are derived from
the 5 repeat alleles in a multi-step mechanisms similar to
that proposed for FraX syndrome.
of the loci are
FRAXA-FraX
locus.
have the mildest disease, presenting with cataracts and
minimal or no muscle weakness, whereas classic DM
and congenital
DM patients have progressively
larger
expansions.
Abeliovich et al. [46],Mulley et al. [47*]
and Lavedan et al. [48] have demonstrated
that when an
expanded DM paternal allele is transmitted, the repeat
length often contracts, whereas similar sized maternal
alleles continue
to demonstrate
continued
expansion.
Therefore, the earlier observation
that congenital DM
Trinucleotide
are born to affected mothers rather than to affected
fathers may be explained by this paternal size reduction, as congenital DM is always associated with very
large CTG-repeat expansions
[49*]. O’Hoy et al. [ 50**]
showed that some apparent reductions in repeat length
are due to gene conversion events where the expanded
DM repeat is converted to the size of the repeat on the
normal allele.
The mechanism(s)
by which the DM repeat expansion
leads to disease is largely unknown.
Fu et al. [51.*]
and Sabourin et al. [52**] have shown, however, that
mRNA levels of the myotonin kinase gene are abnormal in DM tissue. Although it remains unclear what the
precise change is in the normal allele, as well as the DM
allele, changes in message half-life could alter protein levels [5I**] resulting in abnormal kinase activity.
Huntington’s
disease
Huntington’s disease (HD) is an autosomal dominant disorder and has a frequency of approximately
1 per 10 000
individuals. It is a neurodegenerative
disease characterized by choreiform movement and progressive intellectual deterioration
associated with atrophy of the caudate and putamen. Age-of-onset is usually in middle age
(35-50 years), though juvenile cases, which are typically
more severe, are occasionally seen and are usually paternally inherited [ 531.
In one of the first successful attempts at linkage analysis
between a restriction fragment length polymorphism
and
an unmapped human disease, Gusella et al. [ 541 assigned
HD to chromosome 4. Intense efforts over the following
decade refined the position to 4~16.3 (see Fig. 1) and
generated a high-resolution
physical map of the area
likely to include the HD gene. MacDonald et al. [55]
identified the presence of strong linkage disequilibrium
between the HD locus and several polymorphic
markers suggesting a region of 500kilobases
containing the
HD mutation, which is found on a common haplotype
in approximately one-third of HD chromosomes,
The Huntington’s Disease Collaborative Research Group
[56**] recently reported the identification
of the HD
gene by exon-trapping
from a cosmid contig of this
region. The HD transcript, referred to as IT15, identifies a 10.4kilobase message predicting a 348kDa protein. Near the 5’ end of the message, a repeat of 21
CAG trinucleotides
was observed predicting a polyglutamine stretch in the protein. This trinucleotide
repeat
was found to be normally polymorphic with allele length
ranging from 11 to 36 CAG-repeats with 98% of normals
having alleles at or below 24 triplets. Among patients with
HD, this repeat expands to sizes between 42 and approximately 100 repeats. When the HD alleles are transmitted,
they exhibit instability usually resulting in larger alleles,
though occasional reductions are observed. The length
of the repeat appears to be correlated with the severity
of the disease such that juvenile cases fall within the high
end of the abnormal allele lengths, Two new mutations
repeat
expansions
in neurological
disease
Warren
and
Nelson
were reported where the abnormal alleles were inherited
from a parent with an allele at the high end of the normal range (e.g. a 36 to 44 repeat transition from parent
to affected offspring).
The change in the CAG trinucleotide
repeat would predict a variable number of glutamines within the HD
protein (termed huntingtin). Since normal message levels
were observed in cell lines derived from patients, including a homozygous affected, the increase of glutamines beyond 44 residues likely result in a gain-of-function.
Spinocerebellar
ataxia type 1
Spinocerebellar
ataxia type 1 (SCAl) is a progressive neurodegenerative
disease with variable presentation
that is
inherited as an autosomal dominant. The disorder is characterized by ataxia, ophthalmoparesis
and motor weakness believed to be due to the selective neuronal loss in
the cerebellum, inferior olive, and spinocerebellar
tracts.
The age of onset is typically in the third or fourth decade
of life, although juvenile cases have been reported with
onset as early as age 4. Indeed, these juvenile onset individuals appear in later generations, thus exhibiting anticipation [ 57,581.
Linkage analysis has placed SCAl in the region 6p22-~23
spanning
[59,6O] (see Fig. 1). Using a YACcontig
1.2 megabases
inclusive of the candidate region, Orr and
colleagues [ 61**] identified cosmid subclones containing a common CAG-repeat. The repeat was normally
polymorphic with alleles ranging from 25 to 36 triplets
(99% of normal alleles less than 34 repeats). In patients
with SCAl, an abnormal allele in excess of 43 repeats
was observed in addition to a normal allele. The largest
abnormal allele length observed consisted of 81 repeats
and those patients with juvenile onset SCAl had repeat
lengths in this range (59 to 81 repeats). Instability was
observed in transmission
of the abnormal allele with
a preponderance
of male transmissions
of the juvenile
cases. A remarkable correlation was demonstrated
between the length of the abnormal CAG-repeat and the
age-of-onset, providing compelling evidence for this repeat expansion to be the mutation site in SCAl [61**].
Although the data thus far has analyzed genomic DNA,
the sequence surrounding
the CAG-repeat is an open
reading frame and is detectable
by reverse transcriptase polymerase chain reaction, both suggestive of an
exon. The CAG-repeat in one direction would predict
a polyglutamine
tract which would abnormally expand
in length, similar to SBMA and HD.
Conclusions
Within the past two years, five human diseases have been
shown to be the result of the novel mutation mechanisms
of trinucleotide
repeat expansion. All five disorders are
neurological
and share other similarities. Although the
755
756
Disease, transplantation
and regeneration
mechanism(s)
of the repeat expansion
remain poorly
understood,
it is clear from these five examples that
even if the mechanism of repeat expansion
is similar,
the consequence
of that expansion
may be unique. In
FraX syndrome expansion
leads to transcriptional
silencing, whereas in DM similar expansions
appear to result
in changes in message stability. In SBMA, HD and SCAl,
the expansion
changes the length of glutamine tracts
perhaps separating critical domains, leading to a gainof-function mutation. Neufeld et al. [62] has shown that
in Drosophila proteins with similar amino acid repeats
(opa repeats) the length of the repeats are evolutionarily conserved supporting the notion of glutamine repeats
serving as spacers between domains. Unlike the repeats
in these human disorders, the opa repeats of Drosophila
are cryptic, that is, not pure repeats of identical trinucleotides. Gostout et al [63*] has found evidence suggesting that pure repeats are evolutionarily derived from
cryptic repeats. Therefore, the homologous genes for the
live human disorders may not contain pure triplet repeats, and thus may account for the current limitation
of trinucleotide repeat expansion mutations to humans.
It is likely that additional examples of trinucleotide repeat
mutations will be found in humans. Riggins et al. [64-l
demonstrated the occurrence of triplet repeats in numerous human genes: seemingly more frequent in genes
expressed in the brain. Schalling et al. [65*] recently
described an approach to search for expanded triplet
repeats, documenting
a locus on chromosome
18, in an
apparently normal family, which contains an expanded
and unstable CTG-repeat. As the live disorders thus far
known to be due to expansion mutations result in vatiable expressivity, different ages-of-onset, and/or incomplete penetrance, disorders that are known not to follow
strict patterns of Mendelian inheritance, such as psychiatric disorders, may be particularly good candidates for
further study.
References
and recommended
reading
Papers
review,
.
..
of particular interest, published
have been highlighted as:
of special interest
of outstanding
interest
1.
.
Triplet Repeat Mutations in Human Disease. Science 1992,
CA.WEY CT, Puzrln
within
A, FU YH,
the annual
FENWICK
RG,
period
NELSON
of
DL:
256~784789.
This paper reviews FraX syndrome, DM and SBMA from a number of
perspectives, including the clinical/diagnostic
areas as well as the molecular findings. Hypotheses to account for the high mutation frequencies
of the triplet repeat alleles as well as the pathophysiology
are proposed.
2.
RICHARDSRI, SUTHERLAND GR: Dynamic
Class of Mutations
Causing
Human
70:70%712.
Mutations,
a New
Disease.
Cell 1992,
MANDEL JL: Questions
of Expansion.
Nature
Genet 1993,
48-9.
kis is a recent review of the molecular findings in FraX syndrome,
SBMA, DM and HD. It provides a valuable comparison of the four triplet
repeat diseases.
3.
4.
SHERMAN
SL,
JACOBS
HOWARD-PEEBLFZ
PN,
PA,
MORTON
NIEISEN
NE,
KB,
FROSTER-ISKENIUS U,
PARTINGTON
MW,
SIJTHERLAND GR, TURNER G, WATSON M: Further
Segregation
Analysis of the Fragile X Syndrome with Special Reference
to Transmitting
Males. Hum Genet 1985, 69:283-299.
5.
VERKERK AJMH,
S~JTCUFFE JS, FU YH, KUHL DPA,
RICHARDSS, VICTOFU MF, ZHANG F,
ET AL.: Identification
of a Gene (FMRZ) Containing
a
CGG Repeat Coincident
with a Breakpoint
Cluster Region Exhibiting
Length Variation in Fragile X Syndrome.
Cell 1991, 65:305-914.
PUUTI
6.
A,
OBERLE I,
BouB
PIERETTI M,
REINER
0,
Pair DNA Segment
Syndrome. Science
7.
Yu S,
HEITZ
ROUSSEAUF,
J, BERTHEA~ MF,
D,
KRETz C, DEWS
MANDEL
JL: Instability
and Abnormal Methylation
1991, 252:1097-l 102.
D, HANAUER A,
of a 55-Base
in Fragile X
PI~TCHARD M, KREMER E, LYNCH M, NANCARROW J, BAKER
E, HOU~AN
K, MIJLLEY JC, WARREN ST,
SCHLESSINGER D,
Fragile X Genotype
Characterized
by an Unstable
of DNA. Science 1991, 252:117~1181.
8.
ET AL.:
Region
KREMER EJ, PRITCHARD M, LYNCH M, Yu
S, HOW
K, BAKER E,
ST: Mapping of DNA Instability at the Fragile X to
a Trinucleotide
Repeat Sequence
p(CCG)n.
Science 1991,
252:1711-1714.
WARREN
9.
VINCENT A, HEI’I~
D, PETIT C, KRETZ C, OBERL~ I, MANDEL JL:
Abnormal Pattern Detected in FragiIe X Patients by PulsedField Gel Electrophoresis.
Nature 1991, 349:624626.
10.
BELL MV,
HIRST MC,
NAKAHORI Y,
FLINT TJ, JACOBS PA,
TOMMERUP
MACKINNON
N,
RN,
ROCHE A,
TRANEBJAERG L, FROSTER-
ISKENIUS U,
ET AL.: Physical
Mapping Across the Fragile X:
Hypermethylation
and Clinical Expression
of the Fragile X
Syndrome.
Cell 1991, 64861-866.
11.
PIER!XII
M, ZHANG F, FU YH, WARREIN ST, OOSTRA BA,
CASKEYCT, NELSONDL: Absence of Expression of the FMRZ
Gene in Fragile X Syndrome.
Cell 1991, 66:1-20.
12.
Fti
YH,
KUHL
DPA,
P~ZZUTI A,
PIEREWI
M,
SUTCUFFE
JS,
RICHARDS S, VERKEFX
AJMH, HOIDEN JJA, FENWICK RG, WARREN
of the CGG Repeat at the Fragile X Site
ST, ETA(_: Variation
Results in Genetic Instability: Resolution
of the Sherman
Paradox. Cell 1991, 67:1047-1058.
13.
..
SUTCUFFE JS, NEISON DL, ZHANG F, PIERET~I M, CASKEY CT, SAI.E
DNA Methylation
Represses FMRl Transcription in Fragile X Syndrome. Hum Mol Genet 1992, 1:397PioO.
This paper describes identification of unmethylated
chorionic tillus
cells in a male fetus with a full FraX mutation. The finding of FMRl
mRNA in the cells of the chorion in spite of the presence of a full mutation argues quite strongly for methylation as the mediator of FMRl
down-regulation,
Fetal tissue was found to be nearly completely methyl
lated and fo express little FMRl mRNA.
D, WARREN ST:
RS, GARTER
SM, Sco-rr
CR, CHEN SH, IAIR~ CD:
Methylation
Analysis of CGG Sites in the CpG Island of
the Human FMRl Gene. Hum Mol Genet 1992, 1:571-578.
A description of methylation of the region including the CGG-repeats at
the FraX site in normal and FraX individuals using methylation-sensitive
restriction enzymes. This is the first demonstration
of methylation of the
CGG-repeats, and a more complete analysis of CpG methylation in the
region. The authors suggest abnormal X-inactivation plays a role in FraX
syndrome, due to similarities in the methylation patterns of inactive and
FraX chromosomes.
14.
.
HANSEN
15.
.
HORNSTRA IK, NELSON DL, WARREN
16.
WOHRLE
D,
KOTZOT
.
SCHMIDT
A,
BARBI
ST, YANG TP: High Resolution Methylation
Analysis of the FMRI Gene Trinucleotide
Repeat Region in Fragile X Syndrome. Hum Mel Genet 1993,
in press.
The definitive description of methylation of the CGG repeats and other
CpGs at the FraX site using genomic sequence analysis to assess each
nucleotide in the region.
STEINBACH P:
G,
D,
HIR~T
Roar
HD,
MC,
MANCA
POUSTKA
A,
A,
KORN
DAVIES
B,
KE,
A Microdeletion
of Less than 250kb, Including the Proximal Part of the FMRI Gene and the FragiIe
X Site, in a Male with the Clinical Phenotype
of Fragile X
Syndrome. Am J Hum Genet 1992, 51:29+306.
Trinucleotide
repeat
These authors provide evidence for FMRl’s role in FraX syndrome by
identification of a patient with FraX features exhibiting a deletion encompassing the gene. Other closely linked genes cannot be ruled out
as playing a role in the disorder from these data.
17.
GEDEON AK, BAKER E, ROBINSON H, PARTINGTON IMW, GROSS
.
B, MANCA 4
18.
DE BOIJ~LEK, VERKERKAJMH, REYNIERSE, VITS L, HENDRICKX
J, VAN ROY B, VAN DEN Bos F, DE GRAFF E, OOSTRA BA,
in the FMRl Gene AssociWILLEMS PJ: A Point Mutation
ated with Fragile X Mental Retardation.
Nature Genet 1993,
3:31-35.
The finding of a retarded male with features of FraX syndrome exhibiting a new missense mutation in the FMRl gene offers definitive evidence
of the role of FMRl in the etiology of FraX syndrome. The finding of a
more severe phenotype in this patient suggests that this aberrant protein is more deleterious than the absence of the FMRl gene product.
Whether this mutation is dominant cannot be determined as it is absent
in the patient’s mother.
RICHARDSRI, HOLMAN K, FRJEND K, KREMER E, HUN
D,
STAPLES A, BROWN WI’, G~~NEWARDENA P, TARLETON J,
SCHWARTZC, SUTHERLANDGR: Evidence of Founder
Chromosome in Fragile X Syndrome. Nature Genet 1992, 1:257-260.
This paper reports identification of common haplotypes on FraX chromosomes. This was a surprising finding, since X~linked genetically
‘lethal’ disorders typically involve new mutations, and the frequency
of FraX syndrome in the population
has been taken to indicate a
very high new mutation frequency ( - 50% of carrier mothers would
have new mutations). Whether this indicates a common chromosome
susceptible to the mutation or long-term inheritance of ‘predisposed
chromosomes
is unclear.
19.
..
20.
..
OUDET C, MORNET E, SEFXEJI THOMAS F, LENTES-ZENGERLING
S, KRETZC, DELUCHATC, TEJADAI, BouB J, BouB A, MANDELJL:
Linkage Disequilibrium between the Fragile X Mutation
and Two Closely Linked CA Repeats Suggests that Fragile X Chromosomes are Derived from a Small Number of
Founder Chromosomes. Am J Hum Genet 1993, 52:297-304.
Extending the observation of common haplotypes in FraX chromosomes, this paper suggests as few as six founder chromosomes
making
up the majority of present-day FraX mutations.
21.
.
SMITXAPT, DREESENJCFM, POST JG, SMEETS DFCM, DE DIESMUIDERSC, SPAANS-VANDER BIJI.T, GOVAERTS
LCP, WARRENST,
ROSTRA BA, VAN Oos’r BA: The Fragile X Syndrome:
No
Evidence for any Recent Mutations.
J Med &net 1993,
30:9496.
Identification of a series of affected FraX males with common ancestry
dating to the mid-1700s sugRest that initial mutations in FraX can persist
for many generations prior to phenotypic effect. This, combined with
the observation of no new mutations in a large number of families,
suggest that predisposition
to FraX is not deleterious and begIns to
explain the obsetvation of common haplotypes in FraX chromosomes.
MORTON NE, MACPHERSONJN: Population Genetics of the
Fragile X Syndrome. Multiallelic Model for the FMRl Locus.
Proc Nat1 Acud Sci USA 1992, 89~42154217.
A mathematical treatment of the origin of FraX chromosomes,
suggesting that several transitions in size and/or stability are required (each
with different rates) for eventual FraX development. This model would
explain founder chromosomes
as well as the high frequency of the sym
drome without high mutation frequency.
22.
.
CHAKRAvARnA: Fragile X Founder Effect? Nature Genet 1992,
23.
.
1:237-238.
This news and views regarding [ 19**] suggests that tluctuation anal@s
is the appropriate view for the common haplotypes seen in modemday FraX chromosomes.
In this view, the most common haplotypes
obsetved are those on which the initial mutation arose at the appropriate number of generations ( -30) in the past for them to be causing
in neurological
disease
Warren
FraX in great numbers. This view is entirely compatible
of Morton and MacPherson [22-j.
and Nelson
with the mode1
24.
WILLEMSPJ, VAN ROY B, DE BOUILI! K, Vr13 L, REYNIERXE,
BECK 0, DUMON JE, VERKERKAJMH, OOSTRA BA: Segregation
of the Fragile X Mutation from an Affected Male to His
Normal Daughter.
Hum Mol Genet 1991, 1:511-515.
25.
REYNIER.5
E, VITS L, DE BOULLE K, VAN ROY B, VAN VEIZEN D,
DE GRAFF E, VERKERKAJMH, JORENS HZJ, DARSY JK, OOSTRA B,
WILI~MS PJ: The Full Mutation
in the FMRI Gene of Male
KORN B, POLJSTKAA, YU S, SUTHERIAND GR,
MULLEYJC: Fragile X Syndrome
Without
CCG AmplIfication has an FMRl Deletion. Nature Genet 1992, 1:341&344.
This paper also describes a deletion of FM/?1in a patient with FraX
features. Here, the deletion is extensive, with loss of the entire FMRl
gene and over 2.5 million flanking base pairs of DNA, suggesting that
no other essential genes are in this region.
..
expansions
..
Fragile X Patients is Absent in their Sperm. Nature Genet
1993, 4:14>146.
This surprising finding begins to unravel the peculiar maternal-only
expansion of premutation-sized
repeats to the full mutations in FraX
families. The finding that males with full mutations in somatic cells
(e.g. blood) demonstrate
solely premutation-bearing
sperm suggests
two possibilities, either expansion is specific to somatic cells and the
germ-line is exempt, or FMRl protein is required in sperm development and selection for premutation-carrying
sperm producing cells is
operating in the germ line. The observation of transiently high levels
of FMRl mRNA in developing spermatagonia
(see [27**] ) suggests the
latter.
26.
.
HINDS HL, ASHLEY CT, NELSON DL, WARREN ST, HOUSMAN DE,
!%XALLtNCM: Tissue Specify Expression of FMRl Provides
Evidence for a Functional Role in Fragile X Syndrome.
Nature Genet 1993, 3:3643.
A survey of FMRl mRNA expression in the developing and adult mouse.
The finding of higher levels of expression in brain and testes suggests
that FMRl is involved in the phenotype of FraX syndrome.
27.
..
BACHNER D, STEINBACH P, WOHIUE D, JUST W, VOGEL M,
HAMEISTERH: Enhanced Fmr-1 Expression in Testis. Nature
Genet 1993, 4:ll>ll6.
This correspondence
regarding FMRl mRNA expression levels in
murine testes suggests that developing spermatagonia
produce high
levels of FMRl transiently prior to production
of sperm. This observation may help to explain the lack of transmission of full mutations
from males.
28.
FISCHBECKKH, IONA~ESCUV, RITIER AW, IONASESCUR, DAVIESK,
BALL S, BOSCH P, BLJRNS T, HAUSMANOWA-PETRUSEWICZ
I,
BORKOWSKAJ, ET AL: Localization of the Gene for X-Lied
Spinal Muscular Atrophy. Neurology 1986, 36:1595-1598.
29.
L4 SPADA AR, WILsON
FISCHBECKKH: Androgen
Linked Spinal
352177-79.
30.
EM,
I.U&~HN DB,
Receptor
and Bulbar Muscular
HARDING &
Gene Mutations
In XAtrophy. Nature 1991,
EDWARDSA, HAMMON HA, JIN L, CASKF~ CT, CHAKRAB
OR’I?’R:
Genetic Variation at Five Trimeric and Tetrameric Tandem
Repeat Loci in Four Human Population Groups. Genomics
1992, 12:241-255.
BIANCAIANAV, SERVILLEF, POMMIER J, JUUEN J, HANAUER A,
MANDELJL: Moderate Instabiity of the Trinucleotide Repeat
in Spin0 Bulbar Muscular Atrophy. Hum Mol Genet 1992,
1:255-258.
This paper describes small changes in the number of repeats upon
transmission of mutant alleles in SBMA in a four generation pedigree.
Of 17 transmissions, 7 increased in size, 9 remained the same and 1
decreased. The mutant alleles varied in length from 46 to 53 repeats.
31.
.
32.
YAhL4MOTOY, KAWAI H, NAKAHARAK, 0s~
M, NAKAT~UJIY,
KISHIMOTO T, SAKODAS: A Novel Primer Extension Method
to Detect the Number of CAG Repeats In the Androgen
Receptor Gene in Families with X-Linked Spinal and Bulbar Muscular Atrophy. Biccbem Biophys Res Commun 19992,
182:507-513.
33.
..
IA SPADAAR, ROLING DB, HARDING AE, WARNER CL, SPIEGELR,
HAUSMANOWA-PETRUSEWZI, YEE WC, FISCHBECKKH: Meiotic
Stability and Genotype-Phenotype Correlation of the Trinucleotide Repeat in X-Linked Spinal and BuIbar Muscular
Atrophy. Nature Genet 1992, 2:301-304.
This extensive analysis of instability in SBMA observed 45 transmissions
of mutant alleles, with 12 changes in length, of which 2 were clear de-
757
758
Disease, transplantation
creases. Correlation between
of symptoms was noted.
34.
35.
36.
37.
38.
..
and regeneration
repeat length and age of onset of a variety
M, SAT-0 S,
IGAFMHI S, TANNO Y, ONODERA 0, Y -1
ISHIKAWA A, MNATANI N, NAGASHIMAM, ISHIKAWAY, SAHAsHIK,
ETA: Strong Correlation
Between
the Number of CAG Repeats in Androgen Receptor
Genes and the Clinical Onset
of Features of Spinal and Bulbar Muscular Atrophy. Neuro
logy 1992, 42:230&2302.
SAR M, STUMPFWE: Androgen Concentration in Motor Neurons of Cranial Nerves and Spinal Cord. Science 1977,
197:77-80.
A.SL&NIDIS
C, JANSENG, AMEMNA C, SHUTLERG, MAHADEVANM,
TSILFIDIS C, CHEN C, ALLEMANJ, WORMSKAMPNGM, VOOIJS M,
ET AL.:Cloning of the Essential
Myotonic
Dystrophy
Region and Mapping of the Putative
Defect.
Nature 1992,
355:54%551.
HARLEY HG, BROOK JD, RUNDLE SA, CROW S, REARDON W,
BUCKLER AJ, HARPER PS, HOUSMAN DE, SHAW DJ: Expansion
of an Unstable
DNA Region and Phenotypic
Variation
in
Myotonic Dystrophy.
Nature 1992, 355:545-546.
WEVAN
M, TSILFIDIS C, SABOURIN L, SHUTIER G, AMEMNA
C, JANSEN G, NEVILLEC, NARANGM, BARCELO J, O’Hou K, ~7
AL: Myotonic Dystrophy Mutation: An Unstable CTG Repeat
in the 3’ Untranslated
Region of the Gene. Science 1992,
255:1253-1255.
See [4000].
39.
..
FU YH, Pzzu’n A, FENWOCK RG, KING J, RAJNARAYAN
S, DUNNE
PW, DUBEL J, NASSER GA, A.SHUAWAT, DE JONG P, ~7’AL: An
Unstable
Triplet
Repeat
in a Gene Related
to Myotonic
Muscular Dystrophy.
Science 1992, 255:125&1258.
see [40**1.
BRCOK JD,
MCCURRACH ME, HARLEY HG, BUCKLER AJ,
CHURCH D, AEXIRATANIH, HUNTER K, STANTONVP, THIRIONJP,
HUDSON T, ET a: Molecular
Basis of Myotonic
Dystrophy:
Expansion
of a Trinucleotide
(CTG) Repeat at the 3’ End
of a Transcript
Encoding a Protein Kinase Family Member.
Cell 1992, 68:79%808.
The papers by Mahadevan et al. [3Boo], Fu et a(. [39**], and Brook ef
al. [40**] all describe the simultaneous discovery of the mutation responsible for DM as an expanding CTG-repeat in a gene with homology
40.
..
with serine-threonine protein kinase.
41.
42.
..
YAMAGATA H, MIKI T, OGIHARA T, NAKAGAWAM, HIGUCHI I,
0s~
M, SHELBOURNE P, DAVIES J, JOHNSON K: Expansion
of Unstable DNA Region in Japanese
Myotonic
Dystrophy
Patients. Lancet 1992, 339~692.
IMBERT G, KRETz C, JOHNSON K, MANDEL JL: Origin of the
Expansion
Mutation in Myotonic
Dystrophy.
Nature Genet
1993, 4~72-76.
An insightful study of the linkage disequilibrium
between the DM mutation and a nearby polymorphic
marker. The authors interpret their
data as an initial predisposing event of transition from a 5-repeat allele
to alleles with 1930 repeats, which would constitute a resenroir for
recurrent expansion mutations. This interpretation
would suggest that
alleles with 1 l-13 repeats rarely, if ever, undergo expansion mutations.
43.
HARLF( HG, RLJNDLE SA, REARDON W, MYR~NG J, CROW S,
BROOK JD, HARPER PS, SHAW DJ: Unstable
DNA Sequence
in Myotonic
Dystrophy.
Lancet 1992, 339:1125-l
128.
44.
KURDON
W, HARLEY HG, BROOK JD, RUNDLE SA, CROW S,
HARPER PS, SHAW DJ: Minimal Expression
of Myotonic
Dystrophy: a Clinical and Molecular Analysis. J Med Genet 1992,
that congenital DM caSes have the longest
mothers have larger mean repeat sizes.
46.
AE%EL~OV~CH
D, LEER I, PASHUT-IAVON I, SCHMUEU E, RAASROTHSCHILO A, FRYDMAN M: Negative
Expansion
of the
Myotonic
Dystrophy
Unstable Sequence.
Am J Hum Genet
47.
MULLEYJC, STAPLES A, DONNELLYA, GEDEON AK, HECKT BK,
NICHOLSON GA, HAAN EA, SUTHERLANDGR: Explanation
for
Exclusive
Maternal
Inheritance
for Congenital
Form of
Lancet 1993, 34 1~236237.
Myotonic
Dystrophy.
1993, 52:1175-1181.
.
A brief letter describing
data showing that paternal DM alleles with long
repeats are less likely to expand, and even contract, when transmitted as
compared to similar sized maternal alleles. The authors suggest that the
exclusive maternal origin of congenital DM is due to this phenomenon.
48.
IAXDAN C, HOFFMAWRALWAWIH, RABEs JP, ROUMEJ, JUNIEN C:
Different
Sex-Dependent
Constraints
in CTG Length Variation as Explanation
for Congenital
Myotonic
Dystrophy.
Lancet 1993, 341:237.
TSILFIDIS C,
MACKENZIE AE,
METTLER G,
BARCEL.~ J,
KORNEL~JKRAG: Correlation
Between
CG Repeat Length and
Frequency
of Severe Congenital
Dystrophy.
Nature Genet
1992, 1:192-195.
A determination
of the DM repeat length in 272 patients that demonstrates a direct correlation between repeat length and disease severity.
49.
.
O’Hou KL, TSILFIDIS C, MAHADEVAN
MS, NE~ILLECE, BARCEL~)J,
HUNTER AGW, KORNELUK RG: Reduction
in Size of the
Myotonic
Dystrophy
Trinucleotide
Repeat
Mutation
During Transmission.
Science 1993, 259:80+812.
Describes three cases of reduction of the repeat upon transmission.
50.
..
One case wa.s most likely due to a gene conversion event that changed
the expanded repeat of the DM allele to the repeat length of the normal allele in normal offspring who inherited the DM haplotype from an
affected father.
Fu YH, FRIEIXVIANDL, FUcbmx
S, PEARIMAN JA, GIBBS
R&
PI~~IJTI A, A.sHI~_AWAT, PERRYW
MB,
SCARLA1’0
G, FENWOCK RG, C~SKEY CT: Decreased
Expression
of
Myotonin-Protein
Kinase
Messenger
RNA and Protein
in Adult Form
of Myotonic
Dystrophy.
Science 1993,
260:235-238.
The paper shows evidence for reduced levels of myotonic~protein
kinase (Mt.PK) mRNA as well as protein in muscle from adult DM patients. Also described are antibodies against Mt~PK protein.
51.
..
SABOIWN LA, WEVAN
MS, NARANGM, LEE DSC, SURH LC,
KORNELUKRG: Effect of the Myotonic
Dystrophy
(DM) Mutation on mRNA Levels of the DM Gene. Nature Genel
1993, in press.
This study shows evidence for elevated levels of Mt-PK mFWA from the
brain of an infant with congenital DM and suggests the expansion sta-
52.
..
bilizes the message resulting in increased levels of Mt-PK protein. Also
described as an exonic polymorphism
useful for distinguishing products of each allele.
53.
HARPER PS, MORRIS MJ, QUARREU. 0, SHAW DJ, ‘PnER A,
YOUNGMANS: Huntington’s
Disease. Philadelphia: W.B. Saun-
54.
GUSEIIA JF, WEXLER NS, CONNEALLY PM, NAYLOR SL,
ANDERSONMA, TANZI RE, WATKINS PC, O’ITINA K, WALIACE MR,
SAKAGUCHI AY, ET AL.: A polymorphic
DNA Marker
Genetically
Linked
to Huntington’s
Disease.
Nature
1983,
ders; 1991.
306:234-238.
55.
MACDONALD ME, NOVELLETTO A, LIN C, TAGLE D, BARNES G,
BATES G, TAYLOR S, ALurro B, ALTHERR M, MVERS R, ET AL.:
The Huntington’s
Disease Candidate Region Exhibits Many
Different Haplotypes. Nature &net 1992, 1:9%103.
56.
..
MACDONALEME, AMBROSE CM, DUYAO MP, MVERS RH, LIN C,
SIUNIDHIL, BARNES G, TAYWR SA, JAMES M, GROOT N, ET AL.
[THE HUNTING’I’ON’SDISEASE COUABORATNE RESEARCHGROUP]:
A Novel Gene Containing
a Trinucleotide
Repeat that is
Expanded
and Unstable on Huntington’s
Disease Chromosomes. Ceil 1993, 72:971-983.
29:77&773.
HARLEY HG, RUNDLE SA, MAcMll~rl JC, MYRING J, BROOK JD,
CROW S, REARDON W, FENTON I, SHAW DJ, HARPER PS: Size
of the Unstable CTG Repeat Sequence
in Relation to Phenotype and Parental Transmission
in Myotonic
Dystrophy.
Am J Hum Genet 1992, 52:1164-1174.
A very complete analysis of 439 individuals of 101 kindreds, which
demonstrates anticipation quite nicely as there is a correlation between
repeat length and disease severity and age-of-onset.
The data shows
repeat length and that their
45.
.
Trinucleotide
repeat
The long~standing search for this elusive gene is resolved in this paper,
which identifies the HD gene and describes the mutation as a trinucleotide repeat expansion.
57.
ZOCHBI HY: Spinocerebellar
Ataxia: Variable Age of Onset
and Linkage to Human Leukocyte Antigen in a Large Kindred. Ann Neural 1988, 23:58&584.
58.
HAINES
JL, SCHUT LJ, WErr’&+MP
LR Spinocerebellar Ataxis in
Large Kindred: Age at Onset, Reproduction and Genetic
Linkage Studies. Neurology 1984, 34:1542-1548.
59.
RANIJM LPW: Localization of the Autosomal Dominant, HLALinked Spinocerebellar At&a (SCAl) Locus in Two Kindreds Within an 8cM Subregion of Chromosome 6p. Am J
Hum Genet 1991, 49:31-41.
60.
K~IATKOWSKI
TJ: The Gene for Autosomal Dominant
Spinocerebellar Ataxia (SCAl) Maps Centromeric to D6S89
and Shows no Recombination, in Nine Large Kindreds, with
a Dinucleotide Repeat at the AM10 Locus. Am .J Hum Genet
1993,
61.
..
in press.
ORR HT,
CH~NC
BEAUDET
AL,
M,
BANFI
MCCALL AE,
S,
KWAITKOWSKI
DIJVCK
IA,
TJ, SERVA~IO A,
RANIJM LPW,
ZOCHIU
Expansion of an Unstable Trinucleotide (CAG) Repeat in Spinocerebellar Ataxia Type 1. Nature Genet 1993,
4:221-226.
The latest example of a human neurological disease due to a trinucleotide repeat expansion.
expansions
in neurological
disease Warren
and Nelson
LKI Q, SOMERS SS: ‘Cryptic’ Repeating
Triplets
of Purines and Pyrimidines (cRRY (i)) are Frequent and
Polymorphic: Analysis of Coding cRRY(i) in the Proopiomelanocortin
(POMC) and TATA-Binding
Protein (TBP)
Genes. Am J Hum Genet 1993, 52:1182-1190.
An interesting study of trinucleotide repeats which suggests that conselved perfect triplet repeats within genes may be derived from cryptic
rep&,&.
63.
GOSTOLIT H,
.
RIGGINSGJ, LOKEY LK, CFIASTAINJL, LEINER HA, SHEKMAN SL,
KD,
WAJUEN
ST: Human
Genes
Containing
Polymorphic
Trinucleotide
Repeats.
Nature
Genet 1992,
2:186191.
Through
cDNA library screening
with triplet repeat probes and
database searches, numerous examples of human genes containing normally polymorphic exonic repeats were identified, indicating that other
loci exist, which potentially could undergo similar mutational changes.
64.
.
WILKINSON
SCHALUNG M, HUDSON TJ, BUETOW KH, HOUSMAN DE: Direct
Detection of Novel Expanded Trinucleotide Repeats in the
Human Genome. Nature Genet 1993, 4:13%139.
A method of detecting expanded trinucleotide repeats in genomic DNA
that should prove useful in screening families with suspected expansion
mutations. Unfortunately, the present method does not allow identification of flanking sequences to specifically clone the locus.
65.
.
HY:
62.
NEUFELD SJ, SCHMID AT, YEDVOBNICK B: Homopolymer Length
Variation in the Drosophila
Gene Mastermind. J Mel Et101
1993, in press.
ST Warren, Howard Hughes Medical Institute, Emoty University School
of Medicine, Rollins Research Centre, Atlanta, Georgia 30322, USA
DL Nelson, Institute for Molecular Genetics and The Human Genome
Center, Baylor College of Medicine, One Baylor Plaza, Houston, Texas
77030, USA
759