Population screening at the FRAXA and FRAXE loci: molecular analyses of boys with learning difficulties and their mothers.

Preliminary results on a large population-based molecular survey of FRAXA and FRAXE are reported. All boys with unexplained learning difficulties are eligible for inclusion in the study and data are presented on the first 1013 tested. Individuals were tested for the number of trinucleotide repeats at FRAXA and FRAXE and typed for four flanking microsatellite markers. Mothers of 760 boys were tested to determine the stability of the FRAXA and FRAXE repeats during transmission and to provide a population of control chromosomes. The frequency of FRAXA full mutations was 0.5%, which gives a population frequency of 1 in 4994, considerably less than previous reports suggest. No FRAXE full mutations were detected, confirming the rarity of this mutation. In the boys' X chromosomes, we detected one FRAXA premutation with 152 repeats and one putative FRAXE premutation of 87 repeats. No full or premutations were seen in the control chromosomes. A significant excess of intermediate alleles at both FRAXA and FRAXE was detected in the boys' X chromosomes by comparison with the maternal control chromosomes. This suggests that relatively large unmethylated repeats of sizes 41-60 for FRAXA and 31-60 for FRAXE may play some role in mental impairment. No instability was found in transmissions of minimal or common alleles in either FRAXA or FRAXE, but we saw two possible instabilities in transmission of FRAXA and two definite instabilities in transmission of FRAXE among 43 meioses involving intermediate or premutation sized alleles. We found no linkage disequilibrium between FRAXA and FRAXE but did find significant linkage disequilibrium between large alleles at FRAXE and allele 3 at the polymorphic locus DXS1691 situated 5 kb distal to FRAXE.