Further evidence for genetic heterogeneity in the fragile X syndrome

The X-linked fragile X [fra(X)] syndrome, associated with a fragile site at Xq27.3, is the most common Mendelian inherited form of mental deficiency. Approximately 1 in 1060 males and 1 in 677 females carry the fra(X) chromosome. However, diagnosis of carrier status can be difficult since about 20% of males and 44% of females are nonpenetrant for mental impairment and/or expression of fra(X). We analyzed DNA from 327 individuals in 23 families segregating fra(X) for linkage to three flanking polymorphic probes: 52A, F9, and ST14. This allowed probable nonpenetrant, transmitting males and carrier females to be identified. A combined linkage analysis was conducted using these families and published probe information on F9 in 27 other families, 52A in six families, and ST14 in five families. The two-point recombination fraction for 52A-F9 was 0.13 (90% confidence interval, 0.10-0.16), for F9-fra(X) was 0.21 (0.17-0.24), and for fra(X)-ST14 was 0.12 (0.07-0.17). Tight linkage between F9 and fra(X) was observed in some families; in others loose linkage was seen suggesting genetic linkage heterogeneity. Risk analysis of carrier status using flanking DNA probes showed that probable nonpenetrant transmitting males were included in families showing both tight and loose linkage. Thus, in contrast to our previous conclusions, it appears that the presence or absence of nonpenetrant, transmitting males in a family is not an indicator of heterogeneity. To determine if heterogeneity was present, we employed the admixture test. Evidence for linkage heterogeneity between F9 and fra(X) was found, significant at P less than 0.0005. Nonsignificant heterogeneity was seen for 52A-F9 linkage. No heterogeneity was found for fra(X)-ST14. The frequency of fra(X) expression was significantly lower in families with tight F9-fra(X) linkage than in families with loose linkage. Cognition appeared to relate to linkage type: affected males in tight linkage families had higher IQs than those in loose linkage families. These findings of genetic heterogeneity can account in part for the high prevalence and apparent high new mutation rate of fra(X). They will affect genetic counseling using RFLPs. An understanding of the basis for genetic heterogeneity in fra(X) will help to clarify the nature of the unusual pattern of inheritance seen in this syndrome.