SEQUENCE-ANALYSIS OF THE FRAGILE-X TRINUCLEOTIDE REPEAT - IMPLICATIONS FOR THE ORIGIN OF THE FRAGILE-X MUTATION

This study addresses mechanism of instability of the FMR-1 (CGG)(n)-re peat, and investigates features which may distinguish between normal s table and fragile X unstable repeats. To achieve this, we have sequenc ed 178 alleles to analyze patterns of AGG interruptions within the CGG repeat, and have typed the (CA)(n)-repeat at DXS548 for 204 chromosom es. Overall, our data is consistent with the idea that the length of u ninterrupted CGG repeats determines instability. We predict that certa in sequence configurations [no AGG, and (CGG)(9-11)AGG(CGG)(greater th an or equal to 20)] present in the general population, are predisposed towards replication slippage. Association between these proposed pred isposing repeats and DXS548 alleles may explain the previously reporte d frequencies of fragile X mutations and large-size normal repeats on specific haplotype backgrounds. We propose that predisposing alleles a rise in the general population by as yet undefined mechanism(s) which introduce a relatively long stretch of pure CGG repeat at the 3'-end ( relative to the direction of transcription) of the FMR-1 repeat region . The 3' pure repeat may then be susceptible to further expansion by r eplication slippage. Slippage on these predisposing chromosomes could accumulate over many generations until a threshold size is reached, at which point the repeat is susceptible to greater instability (i.e. pr emutation stage). Thus, results suggest that evolution of fragile X fu ll mutations could involve 4 definable stages: 1) ancestral events lea ding to the formation of predisposing alleles which have large total r epeat length (e.g. between 35 to 50) but no AGG or 1 AGG; 2) gradual s lippage of these predisposing alleles to small premutations (S alleles ); 3) conversion from S alleles to larger premutations (Z); 4) massive expansion from a Z allele to a full mutation (L).