THE EFFECT OF FMR1 CGG REPEAT INTERRUPTIONS ON MUTATION FREQUENCY AS MEASURED BY SPERM TYPING

Fragile X syndrome results from the unstable expansion of a CGG repeat within the FMR1 gene. Three classes of FMR1 alleles have been identif ied, normal alleles with 6-60 repeats, premutations with 60-200 repeat s, and full mutations with >230 repeats. Premutations are exquisitely unstable upon transmission. Normal alleles, while generally stable upo n transmission, are thought to have different intrinsic mutation frequ encies, such that some normal alleles may be predisposed towards expan sion while others may be more resistant to such change. One variable t hat may account for this difference is the occurrence of one or more A GG triplets punctuating the normal CGG repeat, The AGG interruptions l ead to alleles that have equivalent overall length but different lengt hs of perfect repeats. To test the influence of the length of perfect repeats on stability, we examined the CGG repeat of single sorted sper m from two males, each with 39 total repeats, but distinct AGG interru ption patterns. Sorted sperm of each donor showed -15% variation in re peat length, consistent with previous studies of sorted sperm at other triplet repeat loci. However, when discounting the majority variation of +/-1 repeat, the male with 29 perfect repeats showed 3% expansion changes while the donor with only 19 perfect repeats had none (<0.9%). Moreover, >90% of all variant sperm, including all those observed wit h expansions, showed expansion or contraction of the 3' end of the rep eat array. These data are consistent with the hypothesis that perfect repeat tracts influence the repeat stability and that changes of the F MR1 repeat exhibit polarity.