BLM's balancing act and the involvement of FANCJ in DNA repair

Abstract

Timely recruitment of DNA damage response proteins to sites of genomic structural lesions is very important for signaling mechanisms to activate appropriate cell cycle checkpoints but also repair the altered DNA sequence to suppress mutagenesis. The eukaryotic cell is characterized by a complex cadre of players and pathways to ensure genomic stability in the face of replication stress or outright genomic insult by endogenous metabolites or environmental agents. Among the key performers are molecular motor DNA unwinding enzymes known as helicases that sense genomic perturbations and separate structured DNA strands so that replacement of a damaged base or sugar-phosphate backbone lesion can occur efficiently. Mutations in the BLM gene encoding the DNA helicase BLM leads to a rare chromosomal instability disorder known as Bloom's syndrome. In a recent paper by the Sengupta lab, BLM's role in the correction of double-strand breaks (DSB), a particularly dangerous form of DNA damage, was investigated. Adding to the complexity, BLM appears to be a key ringmaster of DSB repair as it acts both positively and negatively to regulate correction pathways of high or low fidelity. The FANCJ DNA helicase, mutated in another chromosomal instability disorder known as Fanconi Anemia, is an important player that likely coordinates with BLM in the balancing act. Further studies to dissect the roles of DNA helicases like FANCJ and BLM in DSB repair are warranted.

Keywords: BLM; Bloom’s syndrome; DNA repair; FANCJ; Fanconi Anemia; double-strand break; genomic instability; helicase.

Figure 1.

Models depicting the recruitment of BLM, FANCJ and other DNA repair proteins to double-strand breaks. A, Homologous recombination repair. Early recruitment: Early responders including the MRN complex recruit to the DSB and activate ATM during S or G2 phases of the cell cycle. 1) ATM is activated by autophosphorylation which in-turn can amplify MRN-mediated signaling. MRN with its protein partner CTIP begin resection at the DSB, thereby initiating DSB repair by HR. 2) BLM is recruited to the DSB along with BRCA1 and FANCJ. It is unclear whether FANCJ and BRCA1 form a complex in solution before recruitment to the DSB or interact at the DSB. The interaction between FANCJ-BRCA1 and BLM may potentiate DSB repair by the HR pathway. 3) The short DNA overhangs created by the MRE11 nuclease of the MRN complex are stabilized by RPA. Late recruitment: Long 5ʹ strand resection occurs through the BLM-DNA2 or EXO1 pathway. FANCJ may functionally interact with BLM and MRE11 leading to the formation of RPA nucleated stable 3ʹ ssDNA. 1) The long RPA filament is replaced by RAD51 and this replacement is facilitated by BRCA2. Both BLM and FANCJ can negatively regulate HR by inhibiting RAD51 interaction with ssDNA filaments. Further, BLM’s retention at the DNA is aided by ubiquitylation, which is important for interaction of NBS1 with the MRN complex. B, Non-homologous end-joining. Early recruitment: Early responders including ATM and the MRN complex recruit to the DSB and set up a cascade for NHEJ during any phase of the cell cycle. 1) Upon detection of DSBs, ATM becomes phosphorylated and in turn phosphorylates other proteins functioning later in NHEJ. 2) The recruitment of DNA-PKcs followed by KU 70/80 complex at DSBs displaces MRN complex and initiates NHEJ. 3) BLM and potentially FANCJ may be recruited at this stage. Late recruitment: Whether BLM and FANCJ interact to modulate NHEJ is an important area of investigation. 1) BLM regulates the recruitment of the repair protein XRCC4 which enhances the joining activity of LIG4.