Genetic Proof of Unequal Meiotic Crossovers in Reciprocal Deletion and Duplication of 17p11.2

A number of common contiguous gene syndromes have been shown to result from nonallelic homologous recombination (NAHR) within region-specific low-copy repeats (LCRs). The reciprocal duplications are predicted to occur at the same frequency; however, probably because of ascertainment bias and milder phenotypes, reciprocal events have been identified in only a few cases to date. We previously described seven patients with dup(17)(p11.2p11.2), the reciprocal of the Smith-Magenis syndrome (SMS) deletion, del(17)(p11.2p11.2). In >90% of patients with SMS, identical .3.7-Mb deletions in 17p11.2 have been identified. These deletions are flanked by large (.200 kb), highly homologous, directly oriented LCRs (i.e., proximal and distal SMS repeats [SMS-REPs]). The third (middle) SMS-REP is inverted with respect to them and maps inside the commonly deleted genomic region. To investigate the parental origin and to determine whether the common deletion and duplication arise by unequal crossovers mediated through NAHR between the proximal and distal SMS-REPs, we analyzed the haplotypes of 14 families with SMS and six families with dup(17)(p11.2p11.2), using microsatellite markers directly flanking the SMS common deletion breakpoints. Our data indicate that reciprocal deletion and duplication of 17p11.2 result from unequal meiotic crossovers. These rearrangements occur via both interchromosomal and intrachromosomal exchange events between the proximal and distal SMS-REPs, and there appears to be no parental-origin bias associated with common SMS deletions and the reciprocal duplications.