Chromosome Breakage and Recombination at Fragile Sites

Chromosomal fragile sites are points on chromosomes that usually appear as nonstaining chromosome or chromatid gaps. It has frequently been suggested that fragile sites may be involved in chromosome breakage and recombination events. We and others have previously shown that fragile sites predispose to intrachromosomal recombination as measured by sister-chromatid exchanges. These findings suggested that fragile site expression often, if not always, is accompanied by DNA strand breakage. In the present report, fragile sites are shown to predispose to deletions and interchromosomal recombination. By use of somatic cell hybrids containing either human chromosome 3 or the fragile X chromosome, deletions and translocations were induced by FUdR or aphidicolin with breakpoints at the fragile sites Xq27 or 3p14.2 (FRA3B) or at points so close to the fragile sites as to be cytogenetically indistinguishable. Southern blot analysis of DNA from a panel of chromosome 3 deletion and translocation hybrids was then utilized to detect loss or retention of markers flanking FRA3B and to corroborate the cytogenetic evidence that the breakpoints were at this fragile site. One cell line with a reciprocal translocation between human chromosome 3 (with breakpoint at 3p14.2) and a hamster chromosome showed cytogenetically that the fragile site was expressed on both derivative chromosomes, supporting the hypothesis that the fragile site represents a repeated sequence. The approach described provides a means of generating specific rearrangements in somatic cell hybrids with a breakpoint at a fragile site.(ABSTRACT TRUNCATED AT 250 WORDS)