Evidence for BLM and Topoisomerase III alpha interaction in genomic stability

The genomic instability of persons with Bloom's syndrome (BS) features part icularly an increased number of sister-chromatid exchanges (SCEs), The prim ary cause of the genomic instability is mutation at SLM, which encodes a DN A helicase of the RecQ family. BLM interacts with Topoisomerase IIIa (Topo III alpha), and both BLM and Topo III alpha localize to the nuclear organel les referred to as the promyelocytic leukemia protein (PML) nuclear bodies. In this study we show, by analysis of cells that express various deletion constructs of green fluorescent protein (GFP)-tagged BLM, that the first 13 3 amino acids of BLM are necessary and sufficient for interaction between T opo III alpha and BLM, The Topo III alpha -interaction domain of BLM is not required for BLM's localization to the PML nuclear bodies; in contrast, To po III alpha is recruited to the PML nuclear bodies via its interaction wit h BLM, Expression of a full-length BLM (amino acids 1-1417) in BS cells can correct their high SCEs to normal levels, whereas expression of a BLM frag ment that lacks the Topo III alpha interaction domain (amino acids 133-1417 ) results in intermediate SCE levels. The deficiency of amino acids 133-141 7 in the reduction of SCEs was not explained by a defect in DNA helicase ac tivity, because immunoprecipitated 133-1417 protein had 4-fold higher activ ity than GFP-BLM, The data implicate the BLM-Topo III alpha complex in the regulation of recombination in somatic cells.