Unusual mutations in high functioning fragile X males: apparent instability of expanded unmethylated CGG repeats.

We report on further cases of high functioning fragile X males showing decreased expression of FMR1 protein, absence of detectable methylation at the EagI site in the FMR1 gene promoter, and highly unusual patterns of fragile X mutations defined as smear of expansions extending from premutation to full mutation range. Very diffuse and therefore not easily detectable patterns of full mutations were also observed on prenatal testing using DNA from chorionic villi sampled at a time of development when full mutations were still unmethylated in this particular tissue. In the search for possible determinants of such unusual patterns, repeat expansions in the premutation and in the lower full mutation range were identified on genomic PstI blots previously prepared for fragile X DNA testing. Cases with 130 or more triplets, and a number of shorter repeats, were reinvestigated on EcoRI plus EagI digests. Among the 119 expansions, there were 22 in our sample showing either blurred bands or smears on PstI blots. This particular characteristic was strongly associated with the coincidence of a repeat size of more than 130 triplets and absence of EagI site methylation. Our data set also includes cases of mosaic patterns consisting of smears of unmethylated expansions to more than 130 CGGs and of clear bands of methylated expansions. We therefore suggest that in fragile X syndrome unusual smeared patterns of mutations result from somatic instability of larger repeats under circumstantial absence of repeat methylation.