Alternative splicing in the fragile X gene FMR1

Human Molecular Genetics 2 pp. 399–404 (1993) The authors wish to note a mistake which was incorporated infigure 3 where both Asp and Asn were given the letter code N.A correct version of the figure and its legend is printed below.     

Mosaicism for an FMR1 gene deletion in a fragile X female

Most cases of fragile X syndrome result from expansion of CGG repeats in the FMR1 gene; deletions and point mutations of FMR1 are much less common. Mosaicism for an FMR1 full mutation with a deletion or with a normal allele has been reported in fragile X males. Here we report on a fragile X female who is mosaic for an FMR1 full mutation and an intragenic deletion. The patient is a 4-year-old girl with developmental delay, autistic-like behaviors, and significant speech and language abnormalities. Southern blotting demonstrated the presence of a methylated full mutation, a normal allele in methylated and unmethylated forms, and an additional fragment smaller than the normal methylated allele. This result indicates that the patient is mosaic for a full mutation and a deletion, in the presence of a normal allele. By DNA sequence analysis, we mapped the 5' breakpoint 63/65 bp upstream from the CGG repeat region and the 3' breakpoint 86/88 bp downstream of the CGG repeats within the FMR1 gene. The deletion removed 210 bp, including the entire CGG repeat region. The full mutation was inherited from a premutation in the patient's mother. The deletion, which remained methylated at the Eag I and Nru I sites, was probably derived from the full mutation allele. Mosaicism of this type is rare in females with a fragile X mutation but should be kept in mind in the interpretation of Southern blots.     

脆性X综合征FMR1基因CpG岛的甲基化程度分析

目的 建立用甲基化敏感性限制性内切酶定量聚合酶链反应(methylation-sensitiverestriction enzymes-based quantitative PCR,MSRE-qPCR)分析FMR1基因CpG岛甲基化程度的方法,并探讨其对脆性X综合征的诊断价值.方法 以常规PCR初筛存在FMR1基因5′(CGG)n异常扩增的30例智力低下男童和20名母亲作为研究对象,用Eag Ⅰ酶消化DNA样品,针对FMR1基因CpG岛设计引物,定量PCR扩增Eag Ⅰ酶切前、后DNA,用2-△△Ct法计算CpG岛甲基化程度;以Southern印迹杂交确诊的3例患儿和正常体检男、女各30例DNA样品为质控样本,从而建立优化的MSRE-qPCR方法.结果 确立了正常甲基化、部分异常甲基化、全甲基化的区间值,并明确30例常规PCR初筛异常患儿中3例存在部分甲基化,27例为全甲基化,其中3例经Southern印迹杂交验证;13例母亲处于正常甲基化,7例存在异常甲基化.结论 MSRE-qPCR可以对FMR1基因CpG岛的甲基化程度进行快速可靠分析,为脆性X综合征的分子诊断提供新的策略.     

Haplotype analysis of the fragile X syndrome gene FMR1 in the Czech Republic.

We report on the haplotype analysis with polymorphic repeat markers DXS548 and FRAXAC1 next to the FMR1 gene in 37 unrelated fragile X and 36 control chromosomes from Bohemia and Moravia. Our results suggest a significant linkage disequilibrium between fragile X mutations and certain DXS548-FRAXAC1 haplotypes. Allele frequencies obtained differ slightly from those of other European populations with allele 194 being less frequent in our control sample. Rare DXS548 alleles 6.5 (195) and 0 (208) were also present.     

FMR1 gene expression in olfactory neuroblasts from two males with fragile X syndrome

The fragile X mental retardation 1 gene (FMR1) mutation is strongly correlated with specific and marked neurobehavioral and neuroanatomical abnormalities. The protein product, FMRP, is highly expressed in neurons of the normal mammalian brain, and absent or in low levels in leukoctyes from individuals with fragile X (FraX)–associated mental impairment. Inferences which arise from these findings are that FMRP has a critical role in the development and functioning of the brain, and that leukocyte-derived molecular assessments provide a good indicator of FMR1 expression in that organ. This latter conclusion appears true in most cases even though the typical FMR1 mutation is an unstable triplet repeat expansion which demonstrates somatic heterogeneity within and across tissues. Blood to brain correspondence in FraX patients has only rarely been confirmed by the direct study of human brain specimens and, to our knowledge, it has never been studied in living individuals with the FMR1 mutation. In this report, we describe the FMR1 patterns in olfactory neuroblasts (ON) from two living brothers with expansion mutations in their leukocytes who are mentally retarded and autistic. ON were chosen for study because they are accessible neurons closely linked to the brain. In both subjects, the ON genotype was highly, but not perfectly, consistent with that observed in leukocytes. Protein phenotypes across tissues were completely consistent showing the absence of FMRP-immunoreactivity (-ir). These results augment the limited amount of direct evidence which indicates that FMR1 mutation patterns in leukocytes are a good, albeit potentially fallible, reflection of such patterns in the brain. This report further demonstrates the feasibility of using ON samples to evaluate the FMR1 mutation in humans in vivo. Am. J. Med. Genet. 82:25–30, 1999. © 1999 Wiley-Liss, Inc.     

脆性X综合征FMR1基因CGG重复序列与甲基化的检测

目的建立一种快速、可靠的脆性X综合征的群体筛查方法。方法应用热启动PCR和甲基化特异性PCR（MS-PCR）方法对62例智力低下儿童、12例父母外周血液以及5例高危胎儿的脐带血中FMR1基因CGG重复序列与甲基化状态进行检测。结果采用热启动PCR方法检测79例标本,77例标本的CGG重复数在21～40之间,与正常对照组无明显差异;2例标本未扩增出明显条带。采用MS-PCR方法检测出2例FMR1基因甲基化但CGG重复数在正常范围的患者。结论应用热启动PCR结合MS-PCR方法检测FMR1基因CGG重复数和甲基化,能提高诊断效率,可作为筛查脆性X综合征的首选方法。     

A novel polymorphism in the FMR1 gene: implications for clinical testing of fragile X syndrome

Fragile X syndrome is the most common cause of inherited mental retardation among males. In most cases, the molecular basis of fragile X syndrome is the expansion and subsequent methylation of a CGG trinucleotide repeat in the 5' untranslated region of the fragile X mental retardation 1 (FMR1) gene. Laboratory diagnosis usually relies on a combination of Southern blot and polymerase chain reaction analyses. In this case report we describe an unusual Southern blot result in a patient who presented with developmental delay and had a normal CGG repeat number by polymerase chain reaction analysis. Further investigation revealed a novel G3310C transversion in the FMR1 gene resulting in a new recognition site for the BssHII restriction enzyme. This novel restriction site could potentially mimic a partial deletion of the FMR1 gene on Southern blot analysis and thus represents a possible pitfall in the diagnosis of fragile X syndrome.     

Mosaicism in a fragile X male including a de novo deletion in the FMR1 gene.

In most cases the fragile X syndrome is caused by an amplification of the CGG trinucleotide repeat in the 5' untranslated region of the FMR1 gene, in combination with the hypermethylation of the proximal CpG island. Recently, also a few cases with deletions or a mosaic of a deletion and a full mutation in the FMR1 gene, leading to the same phenotype, have been described. Here we report the molecular analysis of a patient with typical fragile X phenotype and mosaicism of the FMR1 genomic region consisting of a premutation, a full mutation of the CGG repeats, and a 215 bp deletion, diagnosed by Southern blot hybridisation and polymerase chain reaction (PCR). Sequence analysis of the deletion demonstrated that the 5' breakpoint of the deletion is located within a putative hotspot region 75-53 bp proximal to the CGG repeat.     

AGG interruptions within the maternal FMR1 gene reduce the risk of offspring with fragile X syndrome

PurposeThe ability to accurately predict the likelihood of expansion of the CGG repeats in the FMR1 gene to a full mutation is of critical importance for genetic counseling of women who are carriers of premutation alleles (55–200 CGG repeats) and who are weighing the risk of having a child with fragile X syndrome. The presence of AGG interruptions within the CGG repeat tract is thought to decrease the likelihood of expansion to a full mutation during transmission, thereby reducing risk, although their contribution has not been quantified.MethodsWe retrospectively analyzed 267 premutation alleles for number and position of AGG interruptions, length of pure CGG repeats, and CGG repeat lengths present in the offspring of the maternal transmissions. In addition, we determined the haplotypes of four markers flanking the 5′-UTR locus in the premutation mothers.ResultsWe found that the presence of AGG interruptions significantly increased genetic stability, whereas specific haplotypes had a marginal association with transmission instability.ConclusionThe presence of AGG interruptions reduced the risk of transmission of a full mutation for all maternal (premutation) repeat lengths below ~100 CGG repeats, with a differential risk (0 vs. 2 AGG) exceeding 60% for alleles in the 70- to 80-CGG repeat range.Genet Med 2012:14(8):729–736     

Penetrance of FMR1 premutation associated pathologies in fragile X syndrome families

Within the past few years, there has been a significant change in identifying and characterizing the FMR1 premutation associated phenotypes. The premutation has been associated with elevated FMR1 mRNA levels and slight to moderate reductions in FMRP levels. Furthermore, it has been established that ∼20% of female premutation carriers present primary ovarian insufficiency (POI) and that fragile X-associated tremor/ataxia syndrome (FXTAS) occurs in one-third of all male premutation carriers older than 50 years. Besides POI and FXTAS, new disorders have recently been described among individuals (especially females) with the FMR1 premutation. Those pathologies include thyroid disease, hypertension, seizures, peripheral neuropathy, and fibromyalgia. However there are few reports related to FXTAS penetrance among female premutation carriers or regarding these disorders recently associated to the FMR1 premutation. Therefore, we have evaluated 398 fragile X syndrome (FXS) families in an attempt to provide an estimation of the premutation associated phenotypes penetrance. Our results show that signs of FXTAS are detected in 16.5% of female premutation carriers and in 45.5% of premutated males older than 50 years. Furthermore, among females with the FMR1 premutation, penetrance of POI, thyroid disease and chronic muscle pain is 18.6, 15.9 and 24.4%, respectively. The knowledge of this data might be useful for accurate genetic counselling as well as for a better characterization of the clinical phenotypes of FMR1 premutation carriers.     

General overgrowth in the fragile X syndrome: variability in the phenotypic expression of the FMR1 gene mutation

The fragile X syndrome, which often presents in childhood with overgrowth, may in some cases show some diagnostic overlap with classical Sotos syndrome. We describe four fragile X patients with general overgrowth, all of whom are from families with other affected relatives who show the classic Martin-Bell phenotype. Molecular studies of the FMR1 gene in all cases showed the typical full mutation as seen in males affected by the fragile X syndrome. Endocrine studies were unremarkable, except in one case where there were raised levels of insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-3 (IGFBP-3)     

Molecular-intelligence correlations in young fragile X males with a mild CGG repeat expansion in the FMR1 gene

Several mechanisms can explain the occurrence of full-mutation fragile X males with an IQ level above −2 SD below mean, also called “high-functioning fragile X males.” Incomplete methylation of the CpG island at the 5′ end of the FMR1 gene is one of these mechanisms. The present study describes the physical and behavior phenotypes in 7 fragile X boys with CGG repeat insertions in the FMR1 gene between 600–2,400 base pairs. The degree of methylation at the FMR1-associated CpG island ranges in peripheral blood lymphocytes from 0–95%. Subjects with a low degree of methylation at this site have mild or absent physical characteristics of the fragile X syndrome, while subjects with a high degree of methylation at this site have more severe physical characteristics. In this range of CGG repeat insertion (600–2,400 base pairs), the degree of methylation at the FMR1-associated CpG island is a good predictor of intelligence, while CGG repeat insertion length is not. © 1996 Wiley-Liss, Inc.