RELATIONSHIP OF EXPANSION OF CGG REPEATS AND X-INACTIVATION WITH EXPRESSION OF FRA(X)(Q27.3) IN HETEROZYGOTES

Expression of the fragile site Xq27.3 was investigated in 82 heterozyg otes (58 normal and 24 mentally impaired) diagnosed by DNA analysis. E coRI and EagI DNA digests were probed with StB12.3. This allowed the d etection of the expansion of the CGG repeat of the FMR1 gene and the m ethylation pattern of the adjacent CpG island. Heterozygotes with Delt a less than or equal to 400 bp (52/82) were all mentally normal and ma nifested fra(X) in less than 3% of the cells or did not express it. Un methylated mutant alleles were always observed. About two thirds of fe males with Delta >500 bp (21/30) showed fra(X) frequencies above 3% (3 normal and 18 mentally impaired). Lower frequencies of fra(X) or nega tive results were observed in the remaining 9 females (3 normal and 6 affected). The large mutant alleles were always methylated. Therefore, while Delta less than or equal to 400 bp is always associated with ne gative or low expression of fra(X), larger expansions are not present exclusively in heterozygotes with high frequencies of fra(X). In 25 of 30 heterozygotes with a >500 bp, active and inactive normal alleles w ere observed. Three fra(X)-negative or low manifesting heterozygotes s howed completely skewed X-inactivation, with the normal allele either active or inactive. Two females with high frequencies of fra(X) always had the normal allele inactivated. Densitometry studies showed no dif ference in the inactivation of the normal allele between heterozygotes who manifested fra(X) or not. Thus fra(X) expression does not seem to be influenced by X-inactivation. (C) 1994 Wiley-Liss, Inc.