Screening and instability of FMR1 alleles in a prospective sample of 24,449 mother-newborn pairs from the general population.

To study the instability of FMR1 triplet repeats in the general population, we screened a prospective sample of 24,449 anonymized mother-offspring pairs and analyzed transmissions of intermediate-size (45-54 triplets) and premutation-size (55-200 triplets) alleles. We screened all mothers for alleles > or = 45 triplets by Southern blot and studied transmission of 545 maternal alleles to their offspring using polymerase chain reaction. Out of 21,411 maternal samples with conclusive results, we identified 250 carriers of at least one intermediate-size allele and 39 carrying a premutation-size allele. Out of a subsample of 430 transmissions of normal-size alleles (< 45 triplets), we observed four (< 1%) unstable transmissions. There were 6/90 intermediate-size unstable alleles (7%) and 11/25 unstable premutation-size alleles (44%). Two mothers transmitted a typical full mutation. The incidence of fragile X syndrome was thus 1/12,225 newborns (upper limit of 95% confidence interval: 1/4638 newborns), but larger in males (1/6209) than females (none detected in over 12,000 newborn females). Intermediate-size alleles were more unstable than normal-size alleles (p = 0.0027), but more stable (about sixfold) than premutation-size alleles (p < 0.0001). Unstable premutation-size alleles harbored the major fragile X haplotype (T50-T42-T62), and this haplotype appeared to be a good predictor of instability in premutations (p = 0.02). Incidence and instability are important to determine the feasibility and cost effectiveness of putative FMR1 screening programs. Carriers of FMR1 alleles of 55+ triplets with no family history of the disease may have a significant risk of expansion to a full mutation in a single generation.

Premutation and intermediate-size FMR1 alleles in 10572 males from the general population: loss of an AGG interruption is a late event in the generation of fragile X syndrome alleles.

We previously reported a 1:259 prevalence of female carriers of FMR1 premutation-size alleles (greater than 54 triplet repeats) in the general population. We now have screened 10 572 independent males from the same population for similar alleles using high-throughput Southern blotting. We identified 13 male carriers of an allele with more than 54 repeats. This corresponds to a prevalence of 1:813 males (95% confidence interval 1:527 to 1:1781). Haplotype analysis of four markers flanking the triplet array revealed that the prevalence of the major fragile X mutation-associated haplotype was increased among FMR1 alleles of 40-54 repeats. Although sequencing of highly unstable premutation alleles from fragile X families revealed only pure CGG tracts, this was not the case for alleles of similar size that were identified in males from the general population. Forty-eight out of forty-nine alleles of 40 or more triplets had one or two AGG interruptions. This observation, combined with the observation of the enrichment of major fragile X syndrome haplotypes in all alleles of this size, is evidence that the loss of an AGG interruption in the triplet repeat array is not necessary for expansion of normal alleles of 29-30 triplets to intermediate size. The loss of AGG interruptions thus appears to be a late event that leads to greatly increased instability and may be related to the haplotype background of specific FMR1 alleles.

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 leukocytes 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.

Prevalence of carriers of premutation-size alleles of the FMRI gene--and implications for the population genetics of the fragile X syndrome.

The fragile X syndrome is the second leading cause of mental retardation after Down syndrome. Fragile X premutations are not associated with any clinical phenotype but are at high risk of expanding to full mutations causing the disease when they are transmitted by a carrier woman. There is no reliable estimate of the prevalence of women who are carriers of fragile X premutations. We have screened 10,624 unselected women by Southern blot for the presence of FMR1 premutation alleles and have confirmed their size by PCR analysis. We found 41 carriers of alleles with 55-101 CGG repeats, a prevalence of 1/259 women (95% confidence interval 1/373-1/198). Thirty percent of these alleles carry an inferred haplotype that corresponds to the most frequent haplotype found in fragile X males and may indeed constitute premutations associated with a significant risk of expansion on transmission by carrier women. We identified another inferred haplotype that is rare in both normal and fragile X chromosomes but that is present on 13 (57%) of 23 chromosomes carrying FMR1 alleles with 53-64 CGG repeats. This suggests either (1) that this haplotype may be stable or (2) that the associated premutation-size alleles have not yet reached equilibrium in this population and that the incidence of fragile X syndrome may increase in the future.