A Role for BLM in Double-Strand Break Repair Pathway Choice: Prevention of CtIP/Mre11-Mediated Alternative Nonhomologous End-Joining

Abstract

The choice of the appropriate double-strand break (DSB) repair pathway is essential for the maintenance of genomic stability. Here, we show that the Bloom syndrome gene product, BLM, counteracts CtIP/MRE11-dependent long-range deletions (>200 bp) generated by alternative end-joining (A-EJ). BLM represses A-EJ in an epistatic manner with 53BP1 and RIF1 and is required for ionizing-radiation-induced 53BP1 focus assembly. Conversely, in the absence of 53BP1 or RIF1, BLM promotes formation of A-EJ long deletions, consistent with a role for BLM in DSB end resection. These data highlight a dual role for BLM that influences the DSB repair pathway choice: (1) protection against CtIP/MRE11 long-range deletions associated with A-EJ and (2) promotion of DNA resection. These antagonist roles can be regulated, according to cell-cycle stage, by interacting partners such as 53BP1 and TopIII, to avoid unscheduled resection that might jeopardize genome integrity.

53BP1, BRCA1, and the Choice between Recombination and End Joining at DNA Double-Strand Breaks

“…Cells from Bloom syndrome patients fail to recruit 53BP1 to DNA damage sites (69). BLM-deficient cells also show evidence of increased resection during microhomology-mediated end joining, which is surprising given that BLM helps catalyze one branch of long-range resection (69).…”

Aberrant recombination and repair during immunoglobulin class switching in BRCA1-deficient human B cells

“…Thus, it might seem contradictory that BRCA1 could also inhibit resection and promote NHEJ during the G1 cell cycle. However, several DNA repair proteins have opposing functions during HR and NHEJ, including ATM, H2AX, BLM, and RIF1 (48,49). Furthermore, by studying RNF168-deficient cells, we showed that combined deficiency of 53BP1 and BRCA1 did not rescue the NHEJ defect, as it did for HR.…”

Repair of Accidental DNA Double-Strand Breaks in the Human Genome and Its Relevance to Vector DNA Integration

“…In addition to the classical pathway of NHEJ, recent evidence indicates the existence of a more error-prone mechanism of NHEJ called alternative endjoining that plays a role in DSB repair [3,13]. Alternative end-joining is Ku/Lig4 independent and the precise mechanism remains largely unclear, although PARP1, Ligase III, and several factors involved in end resection (to initiate HR) have been implicated in DSB repair via alternative end-joining [14][15][16].Which DSB repair pathway is beneficial for cells to preserve genome integrity? NHEJ (the classical NHEJ pathway) repairs broken DNA ends with little or no homology and is often associated with nucleotide loss, whereas HR allows for accurate repair of DSBs with the use of homologous DNA sequence, usually located on a sister chromatid [3,4,12].…”

“…Recent evidence suggests that Ku-bound DSBs, where end resection does not occur, are directed to NHEJ, while end-resected DSBs, to which Ku cannot bind, are channeled to HR (or alternative end-joining) [20][21][22][23][24]. Thus, in addition to the end binding protein Ku, various factors that regulate end resection are involved in DSB repair pathway choice [16,[25][26][27][28][29][30]. Apparently, the type of DSB is also a determinant of pathway choice [31,32]; for example, replication-associated one-ended DSBs are preferentially repaired by HR, while topoisomerase II-mediated DSBs are almost exclusively repaired by NHEJ [33,34].…”

“…In addition to the classical pathway of NHEJ, recent evidence indicates the existence of a more error-prone mechanism of NHEJ called alternative endjoining that plays a role in DSB repair [3,13]. Alternative end-joining is Ku/Lig4 independent and the precise mechanism remains largely unclear, although PARP1, Ligase III, and several factors involved in end resection (to initiate HR) have been implicated in DSB repair via alternative end-joining [14][15][16].…”