The Breast Cancer Susceptibility Gene BRCA2 Is Required for the Maintenance of Telomere Homeostasis

Abstract

Inactivating mutations in the breast cancer susceptibility gene BRCA2 cause gross chromosomal rearrangements. Chromosome structural instability in the absence of BRCA2 is thought to result from defective homology-directed DNA repair. Here, we show that BRCA2 links the fidelity of telomere maintenance with genetic integrity. Absence of BRCA2 resulted in signs of dysfunctional telomeres, such as telomere shortening, erosions, and end fusions in proliferating mouse fibroblasts. BRCA2 localized to the telomeres in S phase in an ATR-dependent manner, and its absence resulted in the accumulation of common fragile sites, particularly at the G-rich lagging strand, and increased the telomere sister chromatid exchange in unchallenged cells. The incidence of common fragile sites and telomere sister chromatid exchange increased markedly after treatment with replication inhibitors. Congruently, telomereinduced foci were frequently observed in the absence of Brca2, denoting activation of the DNA damage response and abnormal chromosome end joining. These telomere end fusions constituted a significant portion of chromosome aberrations in Brca2-deficient cells. Our results suggest that BRCA2 is required for telomere homeostasis and may be particularly important for the replication of G-rich telomeric lagging strands.The inheritance of one mutant allele of BRCA2 predisposes carriers to early onset breast cancer through loss of heterozygosity; thus, BRCA2 is a tumor suppressor (1, 2). Recently, it has been shown that BRCA2 heterozygosity also promotes Kras G12D -driven carcinogenesis (3), indicating that mutation of BRCA2 is critical for both the initiation and progression of cancer.A truncated Brca2 allele (Brca2 Tr ) in mice causes embryonic lethality and growth retardation due to accumulation of DNA double-stranded breaks (DSBs) 3 and consequent checkpoint activation (4). Metaphase chromosome spreads of the mouse embryonic fibroblasts (MEFs) from Brca2 Tr/Tr mice display chromatid, chromosome breaks, and radial structured chromosomes, strongly indicating that DSB repair is impaired in Brca2Tr/Tr mice (4). Congruently, molecular and biochemical studies of BRCA2 have revealed that BRCA2 regulates homologous recombination (HR), also called homology-directed repair (HDR) (5), by interacting with the recombinase Rad51, through the BRC repeats in exon 11 (6 -8) and the C terminus (6, 9). These studies confirmed the well defined role of BRCA2 as a tumor suppressor and a critical regulator of error-free DNA repair.HDR begins when a damaged DNA strand invades the undamaged duplex of its sister DNA strand. The damaged strand is then repaired by DNA synthesis using the sister strand as a template. Thus, HDR is an error-free DSB repair pathway that takes place during the S or G 2 phases of the cell cycle (10). Notably, HDR is implicated in the repair and rescue of stalled DNA replication forks (11). The inefficient resolution of stalled replication forks that occurs in the absence of BRCA2 greatly contributes to the accumulation of gross c...

Rint1 inactivation triggers genomic instability, ER stress and autophagy inhibition in the brain

“…Anaphase bridges are found among cells exhibiting impaired telomere homeostasis [39][40][41][42] and defective DNA DSB repair. [36][37][38][39][40][41][42][43][44][45][46][47][48][49] RINT1 is known to be involved in both pathways through interaction with p130 15 or Rad50. 14 The structure of chromosome fusion in the Rint1-deficient cells supports the hypothesis of defective DSB repair (Figures 4c and d).…”

ATM Inhibition Potentiates Death of Androgen Receptor-inactivated Prostate Cancer Cells with Telomere Dysfunction

“…As reported previously with LNCaP cells (4), 22Rv1 cells also exhibited several telomere aberration phenotypes including broken (fragile) telomeres and telomere loss (Fig. 2C), which occur as a result of homologous recombination (HR) of damaged telomeres (37,38), and sister-chromatid telomere fusion and telomere end-to-end fusion (Fig. 2D), which result from non-homologous end joining (NHEJ) of damaged telomeres (14,38).…”

BRCA2 functions: from DNA repair to replication fork stabilization

“…The detection of telomere fusions in BRCA2 cKO cells suggests that it facilitates telomere capping potentially by modulating the formation of T-loops. Furthermore, the presence of telomere abnormalities in these cells suggests that BRCA2 is required to limit replication stress at telomeres (Badie et al 2010, Min et al 2012. Consistent with a role of BRCA2 in assisting the replication of G-rich sequences at telomeres, a stabilizer of G-quadruplex structures (pyrodostatin (PDS)) was shown to induce lethality in BRCA2-deficient cells .…”

“…The idea that BRCA2 is involved in telomere maintenance originates from the observation that the conditional knock-out (cKO) of Brca2 leads to telomere shortening in mice (Badie et al 2010, Min et al 2012. Although BRCA2 is required to load RAD51 on telomeres (Badie et al 2010), the exact mechanism by which it safeguards telomere integrity is still not clearly defined.…”

“…Consistent with a role of BRCA2 in assisting the replication of G-rich sequences at telomeres, a stabilizer of G-quadruplex structures (pyrodostatin (PDS)) was shown to induce lethality in BRCA2-deficient cells . Thus, the current model proposes that BRCA2 safeguards telomeric integrity by facilitating their replication (Badie et al 2010, Min et al 2012. Interestingly, BRCA1 participates in the protection of stalled forks in G-quadruplex structures but is dispensable for the maintenance of telomere length (Badie et al 2010).…”