Homologous recombination and non-homologous end-joining pathways of DNA double-strand break repair have overlapping roles in the maintenance of chromosomal integrity in vertebrate cells

Abstract

Eukaryotic cells repair DNA double-strand breaks (DSBs) by at least two pathways, homologous recombination (HR) and non-homologous end-joining (NHEJ).Rad54 participates in the first recombinational repair pathway while Ku proteins are involved in NHEJ. To investigate the distinctive as well as redundant roles of these two repair pathways, we analyzed the mutants RAD54 -/-, KU70 -/-and RAD54 -/-/KU70 -/-, generated from the chicken B-cell line DT40. We found that the NHEJ pathway plays a dominant role in repairing γ-radiation-induced DSBs during G 1 -early S phase while recombinational repair is preferentially used in late S-G 2 phase. RAD54 -/-/KU70 -/-cells were profoundly more sensitive to γ-rays than either single mutant, indicating that the two repair pathways are complementary. Spontaneous chromosomal aberrations and cell death were observed in both RAD54 -/-and RAD54 -/-/KU70 -/-cells, with RAD54 -/-/KU70 -/-cells exhibiting significantly higher levels of chromosomal aberrations than RAD54 -/-cells. These observations provide the first genetic evidence that both repair pathways play a role in maintaining chromosomal DNA during the cell cycle.

The plant‐specific CDKB 1‐ CYCB 1 complex mediates homologous recombination repair in Arabidopsis

“…Consistently, ku70 and ku80 mutants are hypersensitive to the DSB‐inducing agents bleomycin (BLM) and methyl methane sulfonate (MMS) (Riha et al , 2002). However, NHEJ can be imprecise, leading to the loss of nucleotides when overlaps are not compatible (Takata et al , 1998). …”

XRCC4 in G1 suppresses homologous recombination in S/G2, in G1 checkpoint-defective cells

“…The cell cycle dependency of NHEJ is even more contested, and in many reviews it is proposed to act only in G1 phase. This is due to the fact that a large majority of NHEJ-defective cells are hypersensitive to IR when irradiated in G1 but not in G2 (Stamato et al, 1988;Jeggo, 1990;Lee et al, 1997;Takata et al, 1998;Wang et al, 2001). Recently, this interpretation found molecular support by data showing that KU80 preferentially binds to DSBs in the G1 phase (Rodrigue et al, 2006).…”

“…HR appears to be active from mid-S to G2 as attested by the fact that: (i) HR-defective vertebrate cells are more sensitive when irradiated in the late S/G2 phase (Cheong et al, 1994;Takata et al, 1998;Rothkamm et al, 2003;Hinz et al, 2005); (ii) Rad51 foci do not occur in G1 (Yuan et al, 2003;Aten et al, 2004); (iii) phosphorylation by cyclin-dependent kinase of BRCA2, an essential factor for Rad51 foci assembly, consistently disrupts BRCA2-Rad51 interaction only in G1 phase (Esashi et al, 2005); (iv) conservative HR has been proposed to occur preferentially in S phase (SalehGohari and Helleday, 2004). However, one study challenged such interpretations as it found Rad51 foci in G1 (Kim et al, 2005).…”

Minding the gap: The underground functions of BRCA1 and BRCA2 at stalled replication forks

“…The DSG has received less attention than its illustrious cousin, the DSB, but recent work in model organisms has reawakened interest in the DSG as a potential intermediate in genomic instability and cancer [7]. DSBs and DSGs can be repaired in an error-free manner by sister chromatid recombination (SCR), a mechanism whereby the damaged chromatid uses the intact sister as a template for repair by homologous recombination (HR) [5,8,9]. SCR competes with alternative error-prone pathways for repair of DSBs and DSGs and is therefore an important bulwark against the threats of genomic instability and cancer.…”