Functions of the Bloom Syndrome Helicase N-terminal Intrinsically Disordered Region.

Bloom Syndrome helicase (Blm) is a RecQ family helicase involved in DNA repair, cell-cycle progression, and development. Pathogenic variants in human BLM cause the autosomal recessive disorder Bloom Syndrome, characterized by predisposition to numerous types of cancer. Prior studies of Drosophila Blm mutants lacking helicase activity or protein have shown sensitivity to DNA damaging agents, defects in repairing DNA double-strand breaks (DSBs), female sterility, and improper segregation of chromosomes in meiosis. Blm orthologs have a well conserved and highly structured RecQ helicase domain, but more than half of the protein, particularly in the N-terminus, is predicted to be intrinsically disordered. Because this region is poorly conserved across metazoa, we compared closely related species to identify regions of conservation that might be associated with important functions. We deleted two Drosophila-conserved regions in D. melanogaster using CRISPR/Cas9 gene editing and assessed the effects on several Blm functions. Each deletion had distinct effects. Deletion of either conserved region 1 (CR1) or conserved region 2 (CR2) compromised DSB repair through synthesis-dependent strand annealing and resulted in increased mitotic crossovers. In contrast, CR2 is critical for embryonic development but CR1 is less important. Loss of CR1 leads to defects in meiotic crossover designation and patterning but does not impact meiotic chromosome segregation, whereas deletion of CR2 does not result in significant meiotic defects. Thus, while the two regions have overlapping functions, there are distinct roles facilitated by each. These results provide novel insights into functions of the N-terminal region of Blm helicase.