Identification of a break-prone structure in the 9q1 heterochromatic region.

The unusual behaviour of the 9q1 human chromosome region is studied in various conditions. In controls with normal chromosomes 9, del(9q1) is the most frequent spontaneously occurring deletion. This deletion is highly inducible by melphalan, an S phase-dependent alkylating agent. This may correspond to the uncovering of pre-existing DNA breaks in this region. In a 46,XX,9qh+ control, melphalan does not induce deletions any more efficiently than in donors with normal chromosomes 9. In a 46,XY,inv(9) (p11q1205) donor, all deletions of inv(9) affect the short, but not the long, arm. This indicates that the sensitive segment is not the whole heterochromatic region, but rather a limited structure. The high rate of rearrangements affecting this structure may be responsible for somatic crossing over, leading to loss of heterozygosity for 9q, and to the frequent occurrence of inv(9) in human populations.