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Haplotype Study of Intermediate-Length Alleles at the Fragile X (FMR1) Gene: ATL1, FMRb, and Microsatellite Haplotypes Differ from Those Found in Common-Size FMR1 Alleles

YVETTE CURLIS, CUILING ZHANG, JEANETTE J. A. HOLDEN, KEN KIRKBY, DANUTA LOESCH and R. JOHN MITCHELL
Human Biology
Vol. 77, No. 1 (February 2005), pp. 137-151
Stable URL: http://www.jstor.org/stable/41466310
Page Count: 15
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Haplotype Study of Intermediate-Length Alleles at the Fragile X (FMR1) Gene: ATL1, FMRb, and Microsatellite Haplotypes Differ from Those Found in Common-Size FMR1 Alleles
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Abstract

The CGG repeat within the X-chromosome-linked FMR1 gene, which in hyperexpansion (> 200 copies) results in fragile X syndrome, is highly polymorphic. The mechanism of expansion is not well understood, but CGG repeats called intermediate-length or gray zone alíeles (= 35-60 repeats) are thought to make up the FMR1 alleles showing initial steps in this expansion process. It has been hypothesized that the background haplotype of these alleles plays a role in their susceptibility to expansion. In this study we investigate whether or not the frequencies of alleles and haplotypes at four marker loci in the FMR1 gene region (microsatellites DXS548 and FRAXAC1 and SNPs ATL1 and FMRb) in 84 intermediate-length male chromosomes differ from those in 94 common-size male alíeles. The ATL1*G and FMRB*A alleles were more frequent among intermediate-length alíeles than among common alleles. In addition, the DXS548-FRAXAC1 T50-T42 and T40-T42 haplotypes were strongly associated with intermediate-length alleles between 41 and 60 CGG repeats (p < 0.001). Two extended haplotypes, DXS548-FRAXAC1-ATL1-FMRb T50-T42-G-A and T40-T42-G-A, are strongly associated (p< 0.001) with intermediate-length alíeles between 41 and 60 CGG repeats, and these haplotypes have also been reported as fragile X associated haplotypes in European populations. These data suggest that these haplotypes are among the most susceptible to further expansion among the intermediate-length alíeles. T50-T42-G-A was also much more prevalent in males with 35-40 CGG repeats than in males with common-size alleles. ATL1 did not increase discrimination among intermediate-length alleles beyond that detected by DXS548-FRAXAC1 haplotypes, but the FMRb locus did, particularly for the DXS548-FRAXAC1-ATL1 T50-T42-G and T40-T42-G haplotypes. Comparison with fragile X associated haplotypes, from the literature, suggests that repeat hyperexpansion occurs most frequently on chromosomes carrying FMRB*A. Within the intermediate-length alíele category, however, there were some significant differences in haplotype frequencies between smaller and larger alíeles, and this finding has implications for future studies.

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