Uncommon deletions of the Smith-Magenis syndrome region can be recurrent when alternate low-copy repeats act as homologous recombination substrates.

Several homologous recombination "hotspots," or sites of positional preference for strand exchanges, associated with recurrent deletions and duplications have been reported within large low-copy repeats (LCRs). Recently, such a hotspot was identified in patients with the Smith-Magenis syndrome (SMS) common deletion of approximately 4 Mb or a reciprocal duplication within the KER gene cluster of the SMS-REP LCRs, in which 50% of analyzed strand exchanges resulting in deletion and 23% of those resulting in duplication occurred. Here, we report an additional recombination hotspot within LCR17pA and LCR17pD, which serve as alternative substrates for nonallelic homologous recombination that results in large (approximately 5 Mb) deletions of 17p11.2, which include the SMS region. Using polymerase-chain-reaction mapping of somatic cell hybrid lines, we refined the breakpoints of six deletions within these LCRs. Sequence analysis of the recombinant junctions revealed that all six strand exchanges occurred within a 524-bp interval, and four of them occurred within an AluSq/x element. This interval represents only 0.5% of the 124-kb stretch of 98.6% sequence identity between LCR17pA and LCR17pD. A search for potentially stimulating sequence motifs revealed short AT-rich segments flanking the recombination hotspot. Our findings indicate that alternative LCRs can mediate rearrangements, resulting in haploinsufficiency of the SMS critical region, and reimplicate homologous recombination as a major mechanism for genomic disorders.