Biochemical properties of naturally occurring human bloom helicase variants

Abstract

Bloom syndrome helicase (BLM) is a RecQ-family helicase implicated in a variety of cellular processes, including DNA replication, DNA repair, and telomere maintenance. Mutations in humanBLMcause Bloom syndrome (BS), an autosomal recessive disorder that leads to myriad negative health impacts including a predisposition to cancer. BS-causing mutations inBLMoften negatively impact BLM ATPase and helicase activity. WhileBLMmutations that cause BS have been well characterized bothin vitroandin vivo, there are other less studiedBLMmutations that exist in the human population that do not lead to BS. Two of these non-BS mutations, encoding BLM P868L and BLM G1120R, when homozygous, increase sister chromatid exchanges in human cells. To characterize these naturally occurring BLM mutant proteinsin vitro, we purified the BLM catalytic core (BLMcore, residues 636–1298) with either the P868L or G1120R substitution. We also purified a BLMcoreK869A K870A mutant protein, which alters a lysine-rich loop proximal to the P868 residue. We found that BLMcoreP868L and G1120R proteins were both able to hydrolyze ATP, bind diverse DNA substrates, and unwind G-quadruplex and duplex DNA structures. Molecular dynamics simulations suggest that the P868L substitution weakens the DNA interaction with the winged-helix domain of BLM and alters the orientation of one lobe of the ATPase domain. Because BLMcoreP868L and G1120R retain helicase functionin vitro, it is likely that the increased genome instability is caused by specific impacts of the mutant proteinsin vivo. Interestingly, we found that BLMcoreK869A K870A has diminished ATPase activity, weakened binding to duplex DNA structures, and less robust helicase activity compared to wild-type BLMcore. Thus, the lysine-rich loop may have an important role in ATPase activity and specific binding and DNA unwinding functions in BLM.