Disappearing trisomy 8 mosaicism

A case is presented of a patient with disappearing trisomy 8 mosaicism initially thought to have stigmata of the fragile X syndrome. This case is interesting for two reasons. First, it demonstrates the occurrence of "disappearing mosaicism," a phenomenon first described by LaMarche et al, in 1967. Our patient, initially studied in 1991 by two laboratories and found to be mosaic for chromosome 8 trisomy, was apparently normal by both GTG-banding and fluorescent in situ hybridization (FISH) when studied in 1996. Second, this case further underscores the fact that except under special circumstances, it is important that GTG-banding analysis be performed so that the entire human genome be examined in addition to scoring for the fragile X mutation on Xq27.3. In a recent review of the existing database at Rhode Island Hospital on chromosomal abnormalities found in patients referred because of a question of the fragile X syndrome during the period from January 1, 1990 to June 30, 1995, it was found that the frequency of other chromosomal abnormalities in patients referred because of a question of fragile X syndrome equaled or exceeded that of patients found to be positive for fragile X. Our figures, consistent with those reported in the literature, underscore the value of routine karyotyping in this population of patients.