Cartographic study: Breakpoints in 1574 families carrying human reciprocal translocations

Reciprocal translocations (rep) are among the most common constitutional chromosomal aberrations in man, Using a European database of 1574 families carrying autosomal rep, a cartographic study was done on the breakpoints involved. The breakpoints are non-randomly distributed along the different chromosomes, indicating ''hot spots''. Breakpoints of rep that result in descendants that are unbalanced chromosomally at birth are more frequent in a distal position on chromosomal arms, and 65% of them are localised in R-bands. Among the R-bands, bands rich in GC islands and poor in Alu repetitive sequences are more frequently the site of breakpoints, as well as bands that include a fragile site. This result suggests that the variation in degree of methylation in GC islands could be involved in chromosomal breakage and hence in chromosomal rearrangements.