MCL-1 Depletion Impairs DNA Double-Strand Break Repair and Reinitiation of Stalled DNA Replication Forks

Abstract

Myeloid cell leukemia 1 (MCL-1) is a prosurvival BCL-2 protein family member highly expressed in hematopoietic stem cells (HSCs) and regulated by growth factor signals that manifest antiapoptotic activity. Here we report that depletion of MCL-1 but not its isoform MCL-1S increases genomic instability and cell sensitivity to ionizing radiation (IR)-induced death. MCL-1 association with genomic DNA increased postirradiation, and the protein colocalized with 53BP1 foci. Postirradiation, MCL-1-depleted cells exhibited decreased ␥-H2AX foci, decreased phosphorylation of ATR, and higher levels of residual 53BP1 and RIF1 foci, suggesting that DNA double-strand break (DSB) repair by homologous recombination (HR) was compromised. Consistent with this model, MCL-1-depleted cells had a reduced frequency of IRinduced BRCA1, RPA, and Rad51 focus formation, decreased DNA end resection, and decreased HR repair in the DR-GFP DSB repair model. Similarly, after HU induction of stalled replication forks in MCL-1-depleted cells, there was a decreased ability to subsequently restart DNA synthesis, which is normally dependent upon HRmediated resolution of collapsed forks. Therefore, the present data support a model whereby MCL-1 depletion increases 53BP1 and RIF1 colocalization at DSBs, which inhibits BRCA1 recruitment, and sensitizes cells to DSBs from IR or stalled replication forks that require HR for repair.KEYWORDS MCL-1, BCL-2, HR, ICL, apoptosis, 53BP1, DSB repair M CL-1 is a member of the prosurvival BCL-2 family and plays an important role in the regulation of the intrinsic or mitochondrial apoptotic pathway by inhibiting both BH3-only proteins and the proapoptotic proteins (1-3). MCL-1 is mainly located at the outer mitochondrial membrane and inhibits the progression of apoptosis by sequestering executioner proapoptotic proteins BAK and BAX, which are capable of inducing pore formation in the mitochondrial membrane. The subsequent release of cytochrome c into the cytoplasm activates caspases which are responsible for the majority of the macromolecular degradation observed during apoptosis (3). Suppression of BAK and BAX polymerization by MCL-1 is prevented either by MCL-1 degradation or by saturating and inhibiting the MCL-1 binding sites on BAK/BAX with BH3 proteins or mimetics.Under normal growth conditions, MCL-1 is important for mouse embryonic survival (4) and critical for the survival of neutrophils, lymphocytes, hematopoietic stem cells, and neurons (5). MCL-1 overexpression is the hallmark of several cancers, including hematological malignancies as well as solid tumors. Elevated cellular MCL-1 expression correlates with resistance to drug toxicity and ionizing radiation (IR), whereas its inhibition sensitizes cells to both. The BCL-2 family of proteins is characterized by the presence of BCL-2 homology (BH) domains (1, 2). The MCL-1 protein itself is unique among BCL-2 members in also containing multiple N-terminal PEST motifs in addition to BH1, BH2, BH3, and C-terminal transmembrane (TM) domains. PEST is ...

Targeting Mcl-1 enhances DNA replication stress sensitivity to cancer therapy

“…Knockout of Mcl-1 significantly delayed the disappearance of γ-H2AX foci following DNA replication stress ( Figure 2, A and C). Because the repair of DNA replication stress-induced DSBs occurs through the HR DNA repair pathway (6), our results indicate a potential role for Mcl-1 in HR-dependent DSB repair, which is consistent with and/or extends recent findings from others (24). These data indicate that slower repair of Hu-induced DSBs in Mcl-1-deficient cells may result from inhibition of the HR repair pathway.…”

“…A cell cycle activation step is required to initiate the process of DNA end resection, but the mechanism remains unclear (59). Mcl-1 has been reported to play an important role in DNA repair/DNA damage response (21,23,24). However, the exact mechanism is not fully understood.…”

“…Targeting Mcl-1 enhances DNA replication stress sensitivity to cancer therapy repair and reinitiation of stalled DNA replication forks, leading to sensitization of cells to radiation (24).…”

Idelalisib and bendamustine combination is synergistic and increases DNA damage response in chronic lymphocytic leukemia cells

“…During DNA damage response, it is translocated to nucleus [27, 33–35]. Nuclear Mcl-1 interacts with many DNA repair/damage proteins such as H2AX, Nbs1, Ku70, and co-localizes with 53BP1 and is involved in homologous recombination pathway [35]. …”

Role of HP1β during spermatogenesis and DNA replication

“…To determine whether increased number of stalled forks in HP1b ed cells could be due to replication fork dynamics, we measured fork progression. Cells were first treated with chlorodeoxyuridine (CldU) to label ongoing DNA replication, washed out and then iododeoxy-uridine (IdU) incorporated to identify replication restarts and new origins (Mattoo et al, 2017). Replication fork dynamics in cells with and without loss of HP1β was measured after treatment with hydroxyurea (HU), which stalls replication by depleting the nucleotide pool.…”