RELIABILITY OF DIAGNOSTIC-ASSESSMENT OF NORMAL AND PREMUTATION STATUSIN THE FRAGILE-X-SYNDROME USING DNA TESTING

Until recently, fragile X [fra(X)] syndrome was diagnosed by cytogenet ic techniques and/or linkage analysis. Investigation of the mutation a t the molecular level has shown that amplification of a polymorphic tr inucleotide repeat (CGG) is diagnostic of this syndrome. Fu et al. [19 91] observed that between 6-54 copies of the repeat were associated wi th alleles found in the general population, whereas 50-200 copies were associated with the premutation. In general, differences in copy numb er between the normal and premutated states are sufficiently large so that the probability of misclassification is, for all practical purpos es, zero. However, there is a grey area in which members from both pop ulations overlap. The purpose of our study was to determine the probab ility of misclassifying an individual from either the general or premu tation population. DNA obtained from the general population and transm itting fra(X) females were analyzed from 3 centers in North America: H ouston, Texas; Rochester, Minnesota; and Kingston, Ontario. The distri bution of normal alleles from Houston was not significantly different from those obtained from Rochester. Therefore, these 2 samples were co mbined and the pooled distribution of normal alleles was compared with the pooled distribution of premutations. Results indicated that if 50 repeats were used as the cutoff criterion, sensitivity is 100%, speci ficity is 99.6%, and the probability that an individual has the fra(X) premutation given that the number of repeats is greater than 50 is 95 %. Other cutoff criteria (45, 55, 60, 65) employed produced like findi ngs, although 55 repeats appears to be a marginally superior criterion to 50. An independent sample from Kingston was used to verify the ori ginal assessments. Results indicated no significant differences when t he pooled Houston/Rochester distribution was compared with the distrib ution of repeats found in the general population from Kingston. Our fi ndings indicate that family studies should be done to establish stabil ity/instability of alleles when more than 55 repeats are found in the proband. (C) 1994 Wiley-Liss, Inc.