Population cytogenetics of folate-sensitive fragile sites. II. Autosomal rare fragile sites.

The frequencies of folate-sensitive autosomal rare fragile sites (ARFS) were compared in populations of mentally retarded, mentally subnormal, and mentally normal children and of patients referred for diagnostic chromosome study. The frequencies did not differ significantly. Altogether, an autosomal rare fragile site was found in 16 of 1,445 individuals (1 in 90). Of six different folate-sensitive ARFS detected, the most common one was FRA9A, with a frequency of 1 in 241 individuals. In addition, FRA17A, classified as a distamycin A-inducible fragile site, was found with a frequency of 1 in 206. It was regarded as a spontaneously expressive fragile site. In 19 families in which transmission of an autosomal rare fragile site was studied, the mother was the carrier in 16 families and the father, in one family. The mean percentage (+/- SD) of cells expressing ARFS in 55 individuals was 19% (+/- 11.4). The age did not affect the rate of expression. When the rate of expression was calculated separately in a group of mentally retarded (mean = 23.4%) and in a group of mentally normal individuals (mean = 16.0%), the difference was statistically significant.