FANCJ Helicase Facilitates DNA End Resection to Suppress Long Tract Gene Conversions

Mutations in FANCJ helicase is linked to hereditary breast and ovarian cancers as well as bone marrow failure disorder Fanconi anemia (FA). Although FANCJ has been implicated in the repair of DNA double-strand breaks (DSBs) by homologous recombination (HR), the molecular mechanism underlying the tumor suppressor functions of FANCJ remains obscure. Previously, we had uncovered a novel role of FANCJ in the regulation of the balance between short and long-tract gene conversions during sister chromatid recombination (SCR). We had demonstrated that FANCJ plays an important role in the suppression of long-tract gene conversions in a manner that is dependent on its interaction with BRCA1 tumor suppressor and its helicase activity. However, the mechanism by which FANCJ participates in DSB repair and suppress extended repair synthesis is unclear.

In the current study, we find that FANCJ deficient cells exhibit defective processing of DSB ends as measured by native BrdU tracts and RPA staining, implying that FANCJ helicase plays a role in processing DNA ends. Using AsiSI-ER system that allows induction of sequence-specific DSBs and quantitative measurement of resection tracts, we find that the ssDNA generation is compromised in FANCJ deficient cells. Chromatin immunoprecipitation (ChIP) analyses show that FANCJ localizes to the DSB sites and interacts with the effectors of end resection such as MRE11, CtIP, BRCA1 and BLM. Notably, we find that the underlying end resection defect in FANCJ deficient cells is due to defective recruitment of CtIP to the DSB sites. These results suggest a novel and an extended role of FANCJ helicase in the regulation of DNA end resection and imply that a defect in this process might lead to defective SCR and SCR associated gene amplification/duplications.