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-repeat, and investigates features which may distinguish between normal stable and fragile X unstable repeats. To achieve this, we have sequenced 178 alleles to analyze patterns of AGG interruptions within the CGG repeat, and have typed the (CA)n-repeat at DXS548 for 204 chromosomes. Overall, our data is consistent with the idea that the length of uninterrupted CGG repeats determines instability. We predict that certain sequence configurations [no AGG, and (CGG)9-11AGG(CGG) > or = 20] present in the general population, are predisposed towards replication slippage. Association between these proposed predisposing repeats and DXS548 alleles may explain the previously reported frequencies of fragile X mutations and large-size normal repeats on specific haplotype backgrounds. We propose that predisposing alleles arise 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 replication 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. premutation stage). Thus, results suggest that evolution of fragile X full mutations could involve 4 definable stages: 1) ancestral events leading to the formation of predisposing alleles which have large total repeat length (e.g. between 35 to 50) but no AGG or 1 AGG; 2) gradual slippage 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).
Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN 55905.
K
K
Snow
D J
DJ
Tester
K E
KE
Kruckeberg
D J
DJ
Schaid
S N
SN
Thibodeau
D000483:Alleles; D001483:Base Sequence; D005075:Biological Evolution; D002873:Chromosome Fragility; D004247:DNA / Q000235:genetics; D005260:Female; D005600:Fragile X Syndrome / Q000235:genetics*; D005819:Genetic Markers; D006801:Humans; D008297:Male; D008957:Models, Genetic; D008969:Molecular Sequence Data; D009154:Mutation; D009838:Oligodeoxyribonucleotides / Q000235:genetics; D010375:Pedigree; D010802:Phylogeny; D012091:Repetitive Sequences, Nucleic Acid; D014960:X Chromosome
D005819:Genetic Markers; D009838:Oligodeoxyribonucleotides; D004247:DNA