A series of critical pathways are responsible for the detection, signaling and restart of replication forks that encounter blocks during S-phase progression. Small base lesions may obstruct replication fork progression and processing, but the link between repair of small lesions and replication forks is unclear. In this study, we investigated an hypothesized role for DNA-PK, an important enzyme in DNA repair, in cellular responses to DNA replication stress. The enzyme catalytic subunit DNA-PKcs was phosphorylated on S2056 at sites of stalled replication forks in response to short hydroxyurea treatment. Using DNA fiber experiments, we found that catalytically active DNA-PK was required for efficient replication restart of stalled forks. Further, enzymatically active DNA-PK was also required for PARP-dependent recruitment of XRCC1 to stalled replication forks. This activity was enhanced by preventing Mre11-dependent DNA end resection, suggesting that XRCC1 must be recruited early to an unresected stalled fork. We also found that XRCC1 was required for effective restart of a subset of stalled replication forks. Overall, our work suggested that DNA-PK and PARP-dependent recruitment of XRCC1 is necessary to effectively protect, repair, and restart stalled replication forks, providing new insight into how genomic stability is preserved.

Follicular lymphoma (FL), the most common indolent subtype of non-Hodgkin’s lymphoma, is associated with a relatively long overall survival rate ranging from 6 to 10 years from time of diagnosis. However, in 20-60% of FL patients, transformation to aggressive diffuse large B-cell lymphoma (DLBCL) reduces median survival to only 1.2 years. The specific functional and genetic determinants of FL transformation remain elusive, and genomic alterations underlying disease advancement have only been identified for a subset of cases. Therefore, to identify candidate drivers of FL transformation, we performed systematic analysis of a B-cell-specific regulatory model exhibiting FL transformation signatures using the Master Regulator Inference algorithm (MARINa). This analysis revealed FOXM1, TFDP1, ATF5, HMGA1, and NFYB to be candidate master regulators (MR) contributing to disease progression. Accordingly, validation was achieved through synthetic lethality assays in which RNAi-mediated silencing of MRs individually or in combination reduced the viability of (14;18)-positive DLBCL (t-DLBCL) cells. Furthermore, specific combinations of small molecule compounds targeting synergistic MR pairs induced loss of viability in t-DLBCL cells. Collectively, our findings indicate that MR analysis is a valuable method for identifying bona fide contributors to FL transformation and may therefore guide the selection of compounds to be used in combinatorial treatment strategies.

Production of mitochondrial reactive oxygen species and integrity of mitochondrial DNA (mtDNA) are crucial in breast cancer progression and metastasis. Therefore, we evaluated the role of mtDNA damage in breast cancer by genetically modulating the DNA repair enzyme 8-oxoguanine DNA glycosylase (OGG1) in the PyMT transgenic mouse model of mammary tumorigenesis. We generated mice lacking OGG1 (KO), mice overexpressing human OGG1 subunit 1alpha in mitochondria (Tg), and mice simultaneously lacking OGG1 and overexpressing human OGG1 subunit 1alpha in mitochondria (KO/Tg). We found that Tg and KO/Tg mice developed significantly smaller tumors than KO and wildtype mice after 16 weeks. Histological analysis revealed a roughly two-fold decrease in the incidence of lung metastases in Tg mice (33.3%) compared to wildtype mice (62.5%). Furthermore, lungs from Tg mice exhibited nearly a 15-fold decrease in the average number of metastatic foci compared to WT mice (p<0.05). Primary tumors isolated from Tg mice also demonstrated reduced total and mitochondrial oxidative stress, diminished mtDNA damage, and increased mitochondrial function. Targeting hOGG1 to the mitochondria protected cells from mtDNA damage resulting in downregulation of HIF-1alpha and attenuated phosphorylation of Akt. Collectively, we demonstrate proof of concept that mtDNA damage results in breast cancer progression and metastasis in vivo. Moreover, our findings offer new therapeutic strategies for modulating the levels of mtDNA repair enzymes to delay or stall metastatic progression.

Therapy resistance and poor outcome in prostate cancer is associated with increased expression of Cyclin D1. Androgens promote DNA double strand break repair to reduce DNA damage, and cyclin D1 was also shown to enhance DNA damage repair (DDR). In this study, we investigated the significance of cyclin D1 in androgen-induced DDR using established prostate cancer cells and prostate tissues from cyclinD1 knockout mice. We demonstrate that endogenous cyclin D1 further diminished the dihydrotestosterone (DHT)-dependent reduction of gammaH2AX foci in vitro. We also show that cyclin D1 was required for the androgen-dependent DNA damage response both in vitro and in vivo. Furthermore, cyclin D1 was required for androgen-enhanced DDR and radioresistance of prostate cancer cells. Moreover, microarray analysis of primary prostate epithelial cells from cyclin D1-deficient and wild-type mice demonstrated that most of the DHT-dependent gene expression changes are also cyclin D1-dependent. Collectively, our findings suggest that the hormone-mediated recruitment of cyclin D1 to sites of DDR may facilitate the resistance of prostate cancer cells to DNA damage therapies, and highlight the need to explore other therapeutic approaches in prostate cancer to prevent or overcome drug resistance.

Tumor-associated macrophages (TAMs) play complex and pivotal roles during cancer progression. A subset of metastasis-associated macrophages accumulate within metastatic sites to promote the invasion and growth of tumor cells. Src Kinase Associated Phosphoprotein 2 (SKAP2), a substrate of Src family kinases, is highly expressed in macrophages from various tumors, but its contribution to the tumor-promoting behavior of TAMS is unknown. Here, we report that SKAP2 regulates podosome formation in macrophages to promote tumor invasion and metastasis. SKAP2 physically interacted with Wiskott-Aldrich syndrome protein (WASP) and localized to podosomes, which were rarely observed in SKAP2-null macrophages. The invasion of peritoneal macrophages derived from SKAP2-null mice was significantly reduced compared to wild-type macrophages, but could be rescued by the restoration of functional SKAP2 containing an intact tyrosine phosphorylation site and the ability to interact with WASP. Furthermore, SKAP2-null mice inoculated with lung cancer cells exhibited markedly decreased lung metastases characterized by reduced macrophage infiltration compared to wild-type mice. Moreover, intravenously-injected SKAP2-null macrophages failed to efficiently infiltrate established tumors and promote their growth. Taken together, these findings reveal a novel mechanism by which macrophages assemble the appropriate motile machinery to infiltrate tumors and promote disease progression, and implicate SKAP2 as an attractive candidate for therapeutically targeting TAMs.