REFINED MOLECULAR CHARACTERIZATION OF THE BREAKPOINTS IN SMALL INV DUP(15) CHROMOSOMES

Inv dup(15) is the most common supernumerary marker chromosome in huma ns. To investigate the mechanism responsible for this frequent chromos ome rearrangement, we characterized the breakpoints in 18 individuals with small inv dup(15) chromosomes [i.e., negative for the Prader-Will i (PWS)/Angelman syndrome (AS) critical region]. Since two proximal br eakpoint regions (''hotspots'') for PWS/AS deletions have been previou sly identified with the most proximal 15q markers D15S541/S542 and S54 3, we hypothesized that formation of the small inv dup(15) chromosomes may involve one or both of these breakpoint hotspots. By analysis wit h S542, both breakpoint regions were found to be involved in approxima tely equal frequencies. In ten cases, the inv dup(15) was negative for S542 (Class I), indicating the breakpoint is between the centromere a nd the most proximal marker on chromosome 15. For the other eight case s, S542 was positive by fluorescence in situ hybridization (5/5) and/o r microsatellite analysis (7/7), but S543 was negative (Class II). The se two breakpoint regions appear to be the same as the two proximal br eakpoints reported in the common PWS/AS deletions. To initiate cloning and sequencing of the Class II breakpoint, the gap in the yeast artif icial chromosome (YAC) contig between S541/S542 and S543 was filled by screening the CEPH YAC and me,oa-YAC libraries. YACs 705C2 and 368H3 were found to bridge this gap, and therefore contain the more distal b reakpoint region. The finding of consistent breakpoints in small inv d up(15), like that: found in PWS/AS deletions, provides strong evidence for hotspots for chromosome breakage in this region. In addition, our results show that two extra copies (tetrasomy) of the region from 15c en to the euchromatic region containing S542 are present in individual s with Class II breakpoints. Since most individuals carrying a small i nv dup(15) are phenotypically normal, the euchromatin region included in the small inv dup(15) chromosomes does not appear to contain genes with clinically significant dosage effects.