Molecular analysis of mutations in the gene FMR-1 segregating in fragile X families

Increase of FMRP expression, raised levels of FMR1 mRNA, and clonal selection in proliferating cells with unmethylated fragile X repeat expansions: a clue to the sex bias in the transmission of full mutations?

Fragile X syndrome is a triplet repeat disorder caused by expansions of a CGG repeat in the fragile X mental retardation gene (FMR1) to more than 220 triplets (full mutation) that usually coincide with hypermethylation and transcriptional silencing. The disease phenotype results from deficiency or loss of FMR1 protein (FMRP) and occurs in both sexes. The underlying full mutations arise exclusively on transmission from a mother who carries a premutation allele (60-200 CGGs). While the absolute requirement of female transmission could result from different mechanisms, current evidence favours selection or contraction processes acting at gametogenesis of pre- and full mutation males. To address these questions experimentally, we used a model system of cultured fibroblasts from a male who presented heterogeneous unmethylated expansions in the pre- and full mutation size range. On continual cell proliferation to 30 doublings we re-examined the behaviour of the expanded repeats on Southern blots and also determined the expression of the FMR1 gene by FMRP immunocytochemistry, western analysis, and RT-PCR. With increasing population doublings, expansion patterns changed and showed accumulation of shorter alleles. The FMRP levels were below normal but increased continuously while the cells that were immunoreactive for FMRP accumulated. The level of FMR1 mRNA was raised with even higher levels of mRNA measured at higher passages. Current results support the theory of a selection advantage of FMRP positive over FMRP deficient cells. During extensive proliferation of spermatogonia in fragile X males, this selection mechanism would eventually replace all full mutations by shorter alleles allowing more efficient FMRP translation. At the proliferation of oogonia of carrier females, the same mechanism would, in theory, favour transmission of any expanded FMR1 allele on inactive X chromosomes.

A five-year experience with fragile X screening of high-risk gravid women

OBJECTIVE

We sought to compare our 5-year program of fragile X screening of high-risk gravid women with our program of fragile X testing of affected individuals (probands).

STUDY DESIGN

All women referred to the prenatal genetics clinic from 1994 to 1998 who had a family history of unspecified mental retardation or learning or behavioral disorders (known fragile X families excluded) were offered fragile X screening. Results were compared with those of probands with the same diagnoses who underwent fragile X testing during the same time period.

RESULTS

We counseled 12,349 prenatal patients from 1994-1998, of whom 263 (2.1%) had a positive family history and underwent fragile X screening. No mutations or premutations were identified. In contrast, 31 (1.9%) of 1637 affected probands who underwent fragile X testing during the same time period had positive results, which was a significant difference (0/263 vs 31/1637; P <.05).

CONCLUSIONS

Testing the affected proband is superior to screening the pregnant relative of the proband for identification of families at risk for fragile X syndrome.

Unusual mutations in high functioning fragile X males: apparent instability of expanded unmethylated CGG repeats

We report on further cases of high functioning fragile X males showing decreased expression of FMR1 protein, absence of detectable methylation at the EagI site in the FMR1 gene promoter, and highly unusual patterns of fragile X mutations defined as smear of expansions extending from premutation to full mutation range. Very diffuse and therefore not easily detectable patterns of full mutations were also observed on prenatal testing using DNA from chorionic villi sampled at a time of development when full mutations were still unmethylated in this particular tissue. In the search for possible determinants of such unusual patterns, repeat expansions in the premutation and in the lower full mutation range were identified on genomic PstI blots previously prepared for fragile X DNA testing. Cases with 130 or more triplets, and a number of shorter repeats, were reinvestigated on EcoRI plus EagI digests. Among the 119 expansions, there were 22 in our sample showing either blurred bands or smears on PstI blots. This particular characteristic was strongly associated with the coincidence of a repeat size of more than 130 triplets and absence of EagI site methylation. Our data set also includes cases of mosaic patterns consisting of smears of unmethylated expansions to more than 130 CGGs and of clear bands of methylated expansions. We therefore suggest that in fragile X syndrome unusual smeared patterns of mutations result from somatic instability of larger repeats under circumstantial absence of repeat methylation.

Characterization of FMR1 promoter elements by in vivo-footprinting analysis

Fragile X syndrome is associated with silencing of the FMR1 gene. We studied the transcriptional regulation, by analysis of the FMR1 promoter region for the presence of in vivo protein/DNA interactions and for cytosine methylation at the single-nucleotide level. Four protein-binding sites were present in the unmethylated promoter of the active FMR1 gene. In the methylated promoter of inactive genes no footprints were detected, and no evidence of active repression was found in the region investigated. We propose that the silencing of FMR1 gene transcription results from a lack of transcription-factor binding.