The Replication Stress Response (RSR) is a signaling network that recognizes challenges to DNA replication and coordinates diverse DNA repair and cell-cycle checkpoint pathways. Gemcitabine is a nucleoside analogue that causes cytotoxicity by inducing DNA replication blocks. Using a synthetic lethal screen of a RNAi library of nuclear enzymes to identify genes that when silenced cause gemcitabine sensitization or resistance in human triple-negative breast cancer cells, we identified NIMA (never in mitosis gene A)-related kinase 9 (NEK9) as a key component of the RSR. NEK9 depletion in cells leads to replication stress hypersensitivity, spontaneous accumulation of DNA damage and RPA70 foci, and an impairment in recovery from replication arrest. NEK9 protein levels also increase in response to replication stress. NEK9 complexes with CHK1, and moreover, NEK9 depletion impairs CHK1 autophosphorylation and kinase activity in response to replication stress. Thus, NEK9 is a critical component of the RSR that promotes CHK1 activity, maintaining genome integrity following challenges to DNA replication.

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BACKGROUND:To assess the potential mechanisms that may underlie increased local failure in triple negative (TN) breast cancers, an analysis was performed of the risk of residual carcinoma after lumpectomy with correlation to pathologic factors, including molecular phenotype. METHODS: A review of pathologic specimens was performed for women with invasive breast cancer treated with lumpectomy followed by reexcision. Data were collected on age; tumor size, grade, and nodal stage; estrogen receptor, progesterone receptor, and human endothelial growth factor receptor 2 (Her2); extensive intraductal component; lymphovascular invasion; margins; and reexcision findings. Univariate and multivariate logistic regression analyses were performed to evaluate for associations between pathologic features of the lumpectomy specimen and reexcision findings. Molecular phenotypes were defined by conventionally used immunohistochemical pattern. RESULTS: Data were collected on 369 patients with breast cancer. The median age was 57 years, median tumor size was 1.5 cm, 36% had positive margins, 32% had positive lymph nodes, 73.5% had the luminal A subtype, 9.5% had the luminal B subtype, 4.5% were Her2-enriched, and 12.5% were TN. Overall, 32% of patients had invasive cancer in their reexcision specimens, and 51% of those with the TN subtype had residual invasive disease on reexcision compared with 30% to 31% for other subtypes. On univariate analysis, age, tumor size, margin status, lymphovascular invasion, nodal status, and TN subtype were associated with elevated risk of residual invasive cancer. On multivariate analysis using a forward stepwise model, TN subtype maintained significance, with an odds ratio of 3.28 (P ¼ .002). CONCLUSION: TN subtype has a statistically significant association with an increased risk of residual tumor. This suggests the putative increase in the risk of local failure in TN patients may be related to increased residual