Fragile X full mutation alleles composed of few alleles: implications for CGG repeat expansion.

Southern analysis of the FMR1 repeat region has suggested that individuals with the full mutation usually carry a heterogeneous array of FMR1 alleles in somatic tissue that can range from 200 to more than 1,000 repeats. Our studies indicate that this heterogeneity is an artifact generated by ethidium bromide commonly used in Southern analysis. When analyzed in the absence of ethidium bromide, nearly all full mutation individuals carried only one to four major alleles and did not exhibit the heterogeneity often referred to as a "smear" in the literature. Full mutations in chorionic villi, however, exhibited much greater heterogeneity. Nine transmissions from mothers with full mutation alleles to offspring indicated that the full mutations continued to expand in transmission to the next generation. In contrast, analysis of leukocyte DNA from three full mutation males revealed no change in somatic full mutation alleles over many years. Our studies support the hypothesis that the FMR1 CGG repeat instability is limited to very early embryogenesis in the soma. These studies also have clinical importance because the omission of ethidium bromide will facilitate the diagnosis of females with full mutation alleles.

Expansion of the fragile X CGG repeat in females with premutation or intermediate alleles.

The CGG repeat in the 5' untranslated region of the fragile X mental retardation 1 gene (FMR1) exhibits remarkable instability upon transmission from mothers with premutation alleles. A collaboration of 13 laboratories in eight countries was established to examine four issues concerning FMR1 CGG-repeat instability among females with premutation (approximately 55-200 repeats) and intermediate (approximately 46-60 repeats) alleles. Our central findings were as follows: (1) The smallest premutation alleles that expanded to a full mutation (>200 repeats) in one generation contained 59 repeats; sequence analysis of the 59-repeat alleles from these two females revealed no AGG interruptions within the FMR1 CGG repeat. (2) When we corrected for ascertainment and recalculated the risks of expansion to a full mutation, we found that the risks for premutation alleles with <100 repeats were lower than those previously published. (3) When we examined the possible influence of sex of offspring on transmission of a full mutation-by analysis of 567 prenatal fragile X studies of 448 mothers with premutation and full-mutation alleles-we found no significant differences in the proportion of full-mutation alleles in male or female fetuses. (4) When we examined 136 transmissions of intermediate alleles from 92 mothers with no family history of fragile X, we found that, in contrast to the instability observed in families with fragile X, most (99/136 [72.8%]) transmissions of intermediate alleles were stable. The unstable transmissions (37/136 [27.2%]) in these families included both expansions and contractions in repeat size. The instability increased with the larger intermediate alleles (19% for 49-54 repeats, 30.9% for 55-59, and 80% for 60-65 repeats). These studies should allow improved risk assessments for genetic counseling of women with premutation or intermediate-size alleles.