Novel roles for the Drosophila melanogaster ortholog of SMARCAL1 in DNA damage repair

Schimke immuno-osseous dysplasia (SIOD) is a monogenic, autosomal recessive disorder with highly variable penetrance and expressivity caused by biallelic mutations in the gene SMARCAL1. SMARCAL1 and its orthologs have been implicated in multiple repair pathways including replication-associated DNA damage repair and stability, gene expression in response to environmental stress, and non-homologous end joining. Early studies of SMARCAL1 suggest a role in double strand break (DSB) repair but have not been thoroughly tested. DSBs pose a serious threat to genomic integrity. If unrepaired, they can lead to chromosome fragmentation and cell death. If repaired incorrectly, they can cause mutations and chromosome rearrangements. DSBs are repaired using end-joining or homology-directed repair strategies, with the predominant form of homology-directed repair being synthesis-dependent strand annealing (SDSA). SDSA is the first defense against genomic rearrangements and information loss during DSB repair, making it a vital component of cell health and an attractive target for chemotherapeutic development. SDSA has also been proposed to be the primary mechanism for integration of large insertions during genome editing with CRISPR/Cas9. Despite the central role for SDSA in genome stability, little is known about the defining step: annealing. I hypothesized that annealing during SDSA is performed by SMARCAL1, which can anneal RPA-coated single DNA strands during replication-associated DNA damage repair. I utilized unique genetic tools in Drosophila melanogaster to test whether the fly ortholog of SMARCAL1, Marcal1, mediates annealing during SDSA. Repair that requires annealing is significantly reduced in Marcal1 null mutants in both a synthesis-dependent and synthesis-independent (single-strand annealing) assays. Elimination of the ATP binding activity of Marcal1 also reduced annealing-dependent repair, suggesting that the annealing activity requires translocation along DNA. Unlike the null mutant, however, the ATP binding-defect mutant showed reduced end-joining, shedding light on the interaction between SDSA and end-joining pathways. Lastly, I found that Marcal1 genetically interacts with Blm in SDSA and replication-associated repair. Blm prevents replication fork damage that is often repaired via Marcal1-mediated pathways. These data contribute to our understanding of DNA damage repair mechanisms and regulation.Doctor of Philosoph