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Human Molecular Genetics, 2000, Vol. 9, No. 12 1759-1769
© 2000 Oxford University Press

Fragile X CGG repeat structures among African-Americans: identification of a novel factor responsible for repeat instability

Dana C. Crawford1, Fuping Zhang2,4, Beth Wilson1, Stephen T. Warren1,2,3,4 and Stephanie L. Sherman1,+

Departments of 1Genetics, 2Biochemistry and 3Pediatrics, Emory University School of Medicine, 1462 Clifton Road NE, Atlanta, GA 30322, USA and 4Howard Hughes Medical Institute, Atlanta, GA 30322, USA

The cryptic CGG repeat responsible for the fragile X syndrome, located in the 5'-UTR of FMR1, is unique compared with the many other triplet repeat-causing diseases, making it ideal for identifying factors involved in repeat expansion that may be common to other triplet repeat diseases. To date, a number of factors have been identified which may influence repeat instability, including the number and position of interspersed AGGs, length of the 3' pure CGG repeat and haplotype background. However, nearly all such data were derived from studies of Caucasians. Using a large African-American population, we present the only comprehensive examination of factors associated with CGG repeat instability in a non-Caucasian population. Among Caucasians, susceptible alleles were thought to come from those in the intermediate repeat range (41–60 repeats); however, we find that susceptible alleles may come from a larger repeat pool (35–60 repeats) and are better defined by their pure CGG repeat and/or ­presence of only one AGG interruption. These results demonstrate the existence of different susceptible alleles among world populations and may account for the similar prevalence of the fragile X syndrome in African-Americans compared with Caucasians despite the lower frequency of inter­mediate sized alleles in the African-American population. Finally, we show that repeat structures among unaffected African-Americans with the most frequent fragile X haplotype background are either pure or contain a single distal interruption. We propose that the lack of a proximal most interruption is a novel factor involved in CGG repeat instability.

+ To whom correspondence should be addressed. Tel: +1 404 727 5862; Fax: +1 404 727 3949; Email: ssherman@genetics.emory.edu


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D. C. Crawford, B. Wilson, and S. L. Sherman
Factors involved in the initial mutation of the fragile X CGG repeat as determined by sperm small pool PCR
Hum. Mol. Genet., November 1, 2000; 9(19): 2909 - 2918.
[Abstract] [Full Text] [PDF]


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