CARTOGRAPHIC STUDY - BREAKPOINTS IN 1574 FAMILIES CARRYING HUMAN RECIPROCAL TRANSLOCATIONS

Reciprocal translocations (rep) are among the most common constitution al chromosomal aberrations in man, Using a European database of 1574 f amilies carrying autosomal rep, a cartographic study was done on the b reakpoints involved. The breakpoints are non-randomly distributed alon g the different chromosomes, indicating ''hot spots''. Breakpoints of rep that result in descendants that are unbalanced chromosomally at bi rth 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 G C islands and poor in Alu repetitive sequences are more frequently the site of breakpoints, as well as bands that include a fragile site. Th is result suggests that the variation in degree of methylation in GC i slands could be involved in chromosomal breakage and hence in chromoso mal rearrangements.