Practice question

Bloom syndrome is rare autosomal recessive disorder characterized by growth retardation, photosensitive facial rash, immunodeficiency, and markedly elevated sister chromatid exchanges leading to predisposition to many cancers. Molecular defect lies in BLM gene encoding RecQ helicase homologous to yeast Sgs1, possessing 3'→5' helicase activity. BLM together with Top3α and RMI1 forms dissolvasome that dissolves double Holliday junctions into non-crossovers. Deficiency causes accumulation of unresolved recombination intermediates resolved aberrantly by nucleases, producing excessive crossovers and aneuploidy, contrasting with Werner syndrome caused by WRN helicase.