Sequence analysis of the fragile X trinucleotide repeat: Correlations with stability and haplotype and implications for the origin of fragile X alleles
- Mayo Clinic, Rochester, MN (United States)
Fragile X (FX) syndrome is associated with amplification of a CGG trinucleotide repeat in the 5{prime} untranslated region of the gene FMR-1. To address mechanism of instability and concern related to overlap between sizes of normal stable alleles and FX unstable alleles, we have sequenced 165 alleles to analyze patterns of AGG interruptions within the CGG repeat, and have typed the (CA)n at DXS548 for 204 chromosomes. Overall, our data is consistent with the idea that the length of uninterrupted CGG repeats determines instability. For 17 stably transmitted alleles with total repeat lengths between 33 and 51, the longest stretch of uninterrupted CGGs was 41. In contrast, for 13 premutation alleles, the shortest stretch of uninterrupted CGGs was 48, suggesting a threshold for expansion between 41 and 48 pure CGGs. For expansion from a premutation to a full mutation, the threshold appears to be {ge}70 uninterrupted repeats. Interestingly, an AGG was detected in some carriers of a full mutation. Comparison of the number of {open_quote}shadow bands{close_quote} in PCR products from similar size alleles with different AGG interruption patterns supports replication slippage as a potential mechanism, i.e. replication slippage occurs more readily as the length of pure repeat increases. Alleles with high total repeat lengths but up to 3 AGGs may be relatively protected against expansion, whereas smaller alleles with pure CGG sequence could be at higher risk for instability. Comparison of sequence data and DXS548 (CA)n data revealed specific sequence trends for each of the DXS548 alleles, explaining the previously reported haplotype association with FX. Incorporating these observations into models for the origin of FX alleles, we consider replication slippage, unequal crossover within the CGG repeat region, recombination between FMR-1 and DXS548, and loss of AGGs by A to C transversion.
- OSTI ID:
- 134329
- Report Number(s):
- CONF-941009-; ISSN 0002-9297; TRN: 95:005313-1062
- Journal Information:
- American Journal of Human Genetics, Vol. 55, Issue Suppl.3; Conference: 44. annual meeting of the American Society of Human Genetics, Montreal (Canada), 18-22 Oct 1994; Other Information: PBD: Sep 1994
- Country of Publication:
- United States
- Language:
- English
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