Somatic stability of the expanded CAG trinucleotide repeat in X-linked spinal and bulbar muscular atrophy

Expansion of trinucleotide repeats has now been associated with eight inherited diseases: X-linked spinal and bulbar muscular atrophy, two fragile X syndromes, myotonic dystrophy, Huntington's disease, spinocerebellar ataxia type I, dentatorubral pallidoluysian atrophy and Machado-Joseph disease. It has been shown that these expanded DNA repeats are unstable in number when transmitted from parents to offspring (“meiotic instability”), while somatic variation in repeat number has also been found in the fragile X syndrome and myotonic dystrophy. Moderate meiotic instability has been demonstrated in X-linked spinal and bulbar muscular atrophy (SBMA, Kennedy's disease). In order to determine if the expanded CAG repeat in SBMA also shows somatic instability, we compared different tissues from two patients with SBMA. We then examined the in vitro stability of the CAG repeat expansion by analyzing fibroblast cell cultures. Length comparison of expanded CAG repeats from all these materials clearly demonstrates that the CAG trinucleotide repeat in SBMA does not exhibit somatic variation. © 1996 Wiley-Liss, Inc.     

Genetic variation and evolutionary stability of the FMR1 CGG repeat in six closed human populations

In an attempt to understand the allelic diversity and mutability of the human FMR1 CGG repeat, we have analyzed the AGG substructure of this locus within six genetically-closed populations (Mbuti pygmy, Baka pygmy, R. surui, Karitiana, Mayan, and Hutterite). Most alleles (61/92 or 66%) possessed two AGG interspersions occurring with a periodicity of one AGG every nine or ten CGG repeats, indicating that this pattern is highly conserved in all human populations. Significant differences in allele distribution were observed among the populations for rare variants possessing fewer or more AGG interruptions than the canonical FMR1 CGG repeat sequence. Comparisons of expected heterozygosity of the FMR1 CGG repeat locus with 30 other microsatellite loci, demonstrated remarkably similar levels of polymorphism within each population, suggesting that most FMR1 CGG repeat alleles mutate at rates indistinguishable from other microsatellite loci. A single allele (1 out of 92) was identified with a large uninterrupted tract of pure repeats (42 pure CGG triplets). Retrospective pedigree analysis indicated that this allele had been transmitted unstably. Although such alleles mutate rapidly and likely represent evolving premutations, our analysis suggests that in spite of the estimated frequency of their occurrence, these unstable alleles do not significantly alter the expected heterozygosity of the FMR1 CGG repeat in the human population. © 1996 Wiley-Liss, Inc.     

Decrease in the size of the myotonic dystrophy CTG repeat during transmission from parent to child: Implications for genetic counselling and genetic anticipation

Recently an unstable trinucleotide CTG repeat, located within the 3′ untranslated region of a gene on 19q13.3 was discovered in kindreds with myotonic dystrophy (DM). The age-of-onset/severity of DM shows a good correlation with CTG repeat size, and pedigrees and data reported to date have shown a striking trend toward amplification of the size of the CTG repeat during transmission from parent to child. The amplification has been accepted as the biological explanation for anticipation in the clinical severity observed in many families with DM. In this paper we report on 3 families where CTG amplification decreased during transmission from parent to child. In one case there was a gene conversion event, while in the remaining 2 there was a simpler reduction in the size of the repeat length. The changes appear to have been accompanied by a reduction in clinical severity in the child when compared to the parent. These observations are discussed in terms of their clinical implications and the biases that may exist in much of the reported data. © 1993 Wiley-Liss, Inc.     

Case control analysis of repeat expansion size in ataxia

Spinocerebellar ataxias (SCAs) are a group of clinically and genetically heterogeneous neurological diseases. The expansion of unstable microsatellite repeats has been identified as the underlying pathogenic cause of 10 subtypes of autosomal dominant SCAs. The aetiology of sporadic SCA is unknown. The aim of this study was to investigate the effect of large normal repeats in patients presenting with sporadic or familial ataxia compared to a control population. The size of the expansion was determined using a fluorescent PCR approach in 10 common SCA genes: SCA-1 (ATXN1), SCA-2 (ATXN2), SCA-3 (ATXN3), SCA-6 (CACNA1A), SCA-7 (ATXN7), SCA-8 (ATXN8OS), SCA-10 (ATXN10), SCA-12 (PPP2R2B), SCA-17 (TBP) and DRPLA (ATN1), in 165 ataxia patients and 307 controls of Welsh origin. There was no difference between cases and controls in the distribution of the large normal alleles, or in the distribution of the combined CAG repeats. The normal allele distribution in the Welsh population was largely similar to that of other Caucasian populations. Our study failed to demonstrate an effect of large normal repeats on the susceptibility to develop ataxia.     

Population genetics of the FRAXE and FRAXF GCC repeats,and a novel CGG repeat,in Xq28

Most of the rare folate sensitive fragile sites cloned to date arise from expansion of a CGG: CCG trinucleotide repeat array. Analysis of the CAG repeat at the Huntington Disease (HD) locus showed a positively skewed repeat distribution leading to the proposal that microsatellites are subject to a mutational bias toward expansion. Such a mutational bias predicts an increase in mean repeat size at all microsatellite loci. We present an analysis of repeats at two fragile site loci, FRAXE and FRAXF, and a novel CGG repeat in Xq28, in five different human populations, which suggests that these loci may also be subject to the same mutation process. The novel repeat array may represent the first evidence for the existence of a fourth fragile site in Xq27.3-28. Am. J. Med. Genet. 73:463–469, 1997. © 1997 Wiley-Liss, Inc.     

Tremor in X-linked recessive spinal and bulbar muscular atrophy (Kennedy's disease)

OBJECTIVE:

To study tremor in patients with X-linked recessive spinobulbar muscular atrophy or Kennedy''s disease.

METHODS:

Ten patients (from 7 families) with a genetic diagnosis of Kennedy''s disease were screened for the presence of tremor using a standardized clinical protocol and followed up at a neurology outpatient clinic. All index patients were genotyped and showed an expanded allele in the androgen receptor gene.

RESULTS:

Mean patient age was 37.6 years and mean number of CAG repeats 47 (44-53). Tremor was present in 8 (80%) patients and was predominantly postural hand tremor. Alcohol responsiveness was detected in 7 (88%) patients with tremor, who all responded well to treatment with a β-blocker (propranolol).

CONCLUSION:

Tremor is a common feature in patients with Kennedy''s disease and has characteristics similar to those of essential tremor.     

MICA gene and ankylosing spondylitis: linkage analysis via a transmembrane-encoded triplet repeat polymorphism

In order to address the possibility that the MICA gene located 47 kb upstream from HLA-B is involved in the pathogenesis of ankylosing spondylitis (AS), we have investigated microsatellite polymorphism in the transmembrane region of MICA in Caucasian patients with AS. The microsatellite allele consisting of 4 repetitions of GCT/AGC was present at significantly higher frequency in the patient group (Pc<0.0000001) than in the ethnically matched control group. However, the frequency of the (GCT/AGC)₄ allele was significantly low in the B27-positive patients than in the B27-positive healthy controls (Pc=0.0145). These observations suggest that B27 itself remains the primary genetic marker for AS, although the significantly dissimilar pheno-type frequency of the (GCT/AGC)₄ allele in B27-positive patients and healthy individuals may reflect the existence of other genetic factor(s) in the HLA-B27 haplotype involved in the development of AS.     

Decrease in androgen binding and effect of androgen treatment in a case of X-linked bulbospinal neuronopathy

X-linked recessive bulbospinal neuronopathy is a motoneuron disorder to be distinguished from amyotrophic lateral sclerosis. Effective treatment is not known. Patients with X-linked recessive bulbospinal neuronopathy may show gynecomastia and testicular atrophy, and a mutation in the androgen receptor gene has been found associated with the disease. Intermediate steps leading from the androgen receptor abnormality to the clinical syndrome have not yet been elucidated. Therefore, binding of androgen ([³H]dihydrotestosterone) to its specific receptor by genital skin fibroblasts cultured from a patient with X-linked recessive bulbospinal neuronopathy and confirmed androgen receptor mutation was studied. Markedly decreased binding capacity was found. We treated the patient for 6 months with nandrolone-decanoate. No effect on his neuromuscular status was observed during 2 years of follow-up.Abbreviations AR androgen receptor - BSN X-linked recessive bulbospinal neuronopathy     

应用D21S11、D21S1440和Penta D短串联重复序列诊断唐氏综合征

目的 探讨应用短串联重复序列(short tandem repeat,STR)诊断唐氏综合征(Down's syndrome,DS)的可行性,为建立一种快速、准确诊断DS的技术提供依据.方法 用定量荧光聚合酶链反应扩增719份样本(包括外周血389例、羊水282例和绒毛48例)21号染色体上D21S11、D21S1440和Penta D STR基因座,通过对扩增产物条带的分析达到诊断DS的目的.结果 核型正常的652份样本中,635份表现为荧光强度为1∶1的2条带或1条带,17例表现3条带,为假阳性.67份DS样本均得到了诊断,53份样本出现1∶1∶1的3条带(峰),14份样本出现2:1的2条带(峰).D21S11、D21S1440和PentaD STR基因座诊断DS的灵敏度和特异度分别为76.12％和98.62％、71.64％和98.93％、89.55％和99.85％.联合应用3个基因座诊断DS的灵敏度为100％(67/67),特异度为97.39％(635/652).结论 用定量荧光聚合酶链反应扩增STR基因座具有灵敏度高、特异度强、简便、快速等优点,在临床上有广阔的应用前景,是大规模产前筛查DS的理想工具.     

Mutation screening and CAG repeat length analysis of the androgen receptor gene in Klinefelter's syndrome patients with and without spermatogenesis

BACKGROUND: Mutations of the androgen receptor (AR) gene give rise to a wide array of phenotypic abnormalities. A systematic analysis of the AR gene in patients with 47,XXY has not previously been performed. METHODS: Mutations of the AR gene and expansion of the CAG repeats in exon 1 of the AR gene were studied in 13 patients with Klinefelter's syndrome either with (n = 1) or without (n = 12) spermatogenesis. RESULTS: No abnormalities in the AR gene were detected by single strand conformational polymorphism analysis. The CAG lengths ranged from 17 to 27 (mean +/- SD 22.8 +/- 3.3, median 23) for Klinefelter patients or from 17 to 28 (mean +/- SD 23.2 +/- 2.6, median 23) for control subjects. X-inactivation analysis for the methylation status of the AR gene was performed in seven patients who were heterozygous for CAG repeats of different length, showing that the longer CAG repeat alleles underwent random but more frequent inactivation in five patients and skewed inactivation in two. CONCLUSIONS: An AR gene abnormality does not constitute an important factor for impaired spermatogenesis in patients with Klinefelter's syndrome.     

A novel duplication in the FMR1 gene: implications for molecular analysis in fragile X syndrome and repeat instability

We have observed a 49 bp tandem duplication adjacent to the triplet repeat of the FMR1 gene and have shown it to occur as a variant in Finland. It affects the primers commonly used in molecular analysis of fragile X syndrome by polymerase chain reaction (PCR) methods. One concern is that females with the full mutation and variant alleles might be missed because of the two PCR products generated by the variant. We suggest that the duplication has arisen by a misalignment of the proximal end of the repeat tract and the non-adjacent GGCGGCGGCGG-sequence located 37 bp upstream and may indicate a mutation hot spot. The discovery of this duplication and the previous observations on deletions associated with full mutations in FMR1 indicate that realignment between the repeat tract and dispersed non-adjacent homologous repetitive sequences may also play a role in repeat instability in fragile X.     

Fragile X mutation and FG syndrome-like phenotype

We present data on 4 mentally retarded brothers, 2 of whom were dizygotic twins with congenital hypotonia, constipation, head size disproportionately large for length or height, and a combination of minor anomalies suggestive of FG syndrome. These brothers have a mentally retarded full sister with similar minor anomalies and an older half-brother with the Martin-Bell syndrome. The mother is mentally retarded; 4 of 7 individuals are positive for fragile X, but all have a CGG expansion ranging from 0.2–2 to 4 kb. Although the phenotype is not completely typical of the FG syndrome and the coincidence of the FMR1 mutation and segregation of the MCA/MR phenotype are highly unlikely, the FMR1 mutation may affect morphogenesis more extensively and differently than the Martin-Bell syndrome does to effect an FG syndromelike phenotype in certain families. This phenotype does not appear to be a contiguous gene syndrome, but an effect of the FMR1 mutation on an adjacent gene must be considered. © 1996 Wiley-Liss, Inc.     

Aarskog syndrome in a Danish family: an illustration of the need for dysmorphology in paediatrics

The first Danish case of Aarskog syndrome is reported. The child had attended several specialized out-patient clinics before the diagnosis was suggested. This underlines the need for dysmorphology in paediatrics.     

CGG repeat in the FMR1 gene: size matters

The FMR1 gene contains a CGG repeat present in the 5'-untranslated region which can be unstable upon transmission to the next generation. The repeat is up to 55 CGGs long in the normal population. In patients with fragile X syndrome (FXS), a repeat length exceeding 200 CGGs (full mutation: FM) generally leads to methylation of the repeat and the promoter region, which is accompanied by silencing of the FMR1 gene. The absence of FMR1 protein, FMRP, seen in FM is the cause of the mental retardation in patients with FXS. The premutation (PM) is defined as 55-200 CGGs. Female PM carriers are at risk of developing primary ovarian insufficiency. Elderly PM carriers might develop a progressive neurodegenerative disorder called fragile X-associated tremor/ataxia syndrome (FXTAS). Although arising from the mutations in the same gene, distinct mechanisms lead to FXS (absence of FMRP), FXTAS (toxic RNA gain-of-function) and FXPOI. The pathogenic mechanisms thought to underlie these disorders are discussed. This review gives insight on the implications of all possible repeat length categories seen in fragile X families.     

年龄和组织类型对X连锁Alport综合征女性患者X染色体失活方式的影响

目的探讨不同年龄和组织类型对X连锁Alport综合征（XLAS）女性患者X染色体失活方式的影响。方法纳入1997年1月至2006年12月北京大学第一医院（我院）儿科肾脏病遗传门诊就诊符合XLAS诊断标准的家系。根据年龄,将XLAS家系中女性患者分为两组,年龄≤30岁为A组,年龄〉30岁为B组。采集患者及其家属静脉血3mL,盐析法提取淋巴细胞基因组DNA。对家系中的女性患者在前臂下1/3处进行皮肤活检,并进行成纤维细胞培养。通过PCR扩增雄性激素受体（AR）基因第一外显子CAG重复序列的多肽性分析X染色体失活。每一标本X染色体失活分析均检测2次,取其平均值用于两组X染色体失活的差异分析,以X染色体失活率≥80%作为X染色体失活倾斜的标准。结果研究期间我院儿科肾脏病遗传门诊共诊断XLAS家系186个,符合纳入和排除标准的XLAS家系共32个,包括36例女性患者,32例男性患者。A组和B组分别为13和23例,平均年龄分别为（17.9±11.7）和（38.6±6.2）岁。①36例女性患者中,外周血中AR基因位点杂合者32例,异质性为88.9%;AR基因位点纯合者4例。外周血中X染色体失活倾斜比例为12.5%（4/32例）,其中A组0例,B组4/20例（20.0%）,两组差异无统计学意义（χ2=2.743,P=0.098）。②12例女性患者同时分析外周血和皮肤成纤维细胞X染色体失活,结果显示两种组织中X染色体失活方式明显不同,3例患者仅皮肤成纤维细胞显示为X染色体失活倾斜而外周血无失活倾斜;1例外周血显示X染色体失活倾斜而皮肤成纤维细胞无失活倾斜。7/12例（58.3%）患者两种组织中以同一条X染色体失活为主;5/12例（41.7%）患者则相反,两种组织间X染色体失活明显不同。结论不同组织类型而不是年龄可影响XLAS女性患者的X染色体失活方式,这可能是有关XLAS女性患者和X染色体失活研究结果不一致的一个主要原因。     

Early clinical signs in Coffin-Lowry syndrome

Two unrelated patients with Coffin-Lowry syndrome are described. The main characteristics of a typical face, thick hands with tapering fingers and a transverse hypothenar crease, general hypotonia with extensible joints made diagnosis possible before the age of 6 months. A persistent large anterior fontanel beyond the age of two years may be another associated finding. Retarded bone age, coarsity of the face and skeletal malformations considered characteristic in adult patients were not present.
Early recognition of Coffin-Lowry syndrome is important for genetic counseling and prevention of severe skeletal malformations.     

Atypical skeletal changes in otopalatodigital syndrome type II: Phenotypic overlap among otopalatodigital syndrome type II,boomerang dysplasia,atelosteogenesis type I and type III,and lethal male phenotype of Melnick-Needles syndrome

We report on 2 cases of otopalatodigital syndrome type II (OPD II) with atypical skeletal changes, overlapping those of boomerang dysplasia, atelosteogenesis type I (AO I) and type III (AO III), and the lethal male phenotype of Melnick-Needles syndrome. One patient exhibited strikingly broad, bowed femora, which resembled those of boomerang dysplasia. The other patient possessed conspicuous undertubulation of the long bones, defective ossification of the spine, and severe undermineralization of the calvaria, which may have caused diagnostic confusion with AO I, AO III, and the lethal male phenotype of Melnick-Needles syndrome. OPD II is transmitted as an X-linked recessive trait, whereas AO I, AO III, and boomerang dysplasia are considered to result from a new dominant mutation, and Melnick-Needles syndrome is inherited as an X-linked dominant trait. Accordingly, differential diagnosis is mandatory to provide the affected families with adequate genetic counseling. Awareness of these skeletal changes in OPD II will prevent the misdiagnosis of this entity as other disorders. Furthermore, the phenotypic overlap among these disorders may expand the entities that constitute the OPD-Larsen dysplasia family proposed by Spranger [1985]. Am. J. Med. Genet. 73:132–138, 1997. © 1997 Wiley-Liss, Inc.     

Prader-Willi-like phenotype in fragile X syndrome

Henk Meyer¹
A 3-year-old boy was referred to the pediatric department because of unexplained extreme obesity. Height and occipitofrontal circumference were just above the 90th centile. Endocrine studies failed to show any significant abnormality. Motor and speech development were generally delayed. On clinical-cytogenetic-molecular grounds, Prader-Willi syndrome was excluded. Fragile X syndrome was diagnosed by the presence of the classical FMR-1 mutation and confirmed by cytogenetic studies, revealing 20% fragile X positive cells. We compare the clinical features in the present patient with the nine reported patients with fra(X) syndrome and extreme obesity. In pathogenesis, hypothalamic dysregulation is hypothesized. In differential diagnosis of Prader-Willi syndrome, fragile X has to be considered, especially when laboratory workup for Prader-Willi syndrome is negative. Clinical behavior can be of help.     

Mental retardation in Nance-Horan syndrome: Clinical and neuropsychological assessment in four families

Nance-Horan syndrome (NHS) is a rare X-linked condition comprising congenital cataract with microcornea, distinctive dental, and evocative facial anomalies. Intellectual handicap was mentioned in seven published NHS patients. We performed a clinical study focused on psychomotor development, intellectual abilities, and behavior in 13 affected males in four NHS families, and present the results of a neuropsychological evaluation in 7 of them. Our study confirms that mental retardation (MR) can be a major component of the NHS. Combining our data with those from the literature leads to a frequency of MR in NHS of around 30%. In most cases, MR is mild or moderate (80%) and not associated with motor delay. Conversely, a profound mental handicap associated with autistic traits may be observed. MR has intra- and inter-familial variability but does not appear to be expressed in carriers. Awareness of MR in NHS may be of importance in the management of the patients, especially in terms of education. Cloning and characterization of the gene and analysis of mutations will be an important step towards understanding the molecular basis of mental deficiency in NHS, and in delineation from the other XLMR conditions at Xp22. Am. J. Med. Genet. 71:305–314, 1997. © 1997 Wiley-Liss, Inc.