Subcellular localization regulates BRCA1 function, and BRCA1 is exported to the cytoplasm following DNA damage in a p53-dependent manner. Because more than 50% of solid tumors harbor p53 mutations, it is possible that genetically wild-type (wt) BRCA1 is functionally abnormal through compromised nuclear-cytoplasmic shuttling in sporadic breast cancer patients with dysfunctional p53. In this study, we have investigated the mechanisms of p53-dependent BRCA1 subcellular distribution and DNA damage-induced nuclear export, as well as the impact on the resulting cytotoxic response to therapy in human breast cancer. We first show that p53 mediates BRCA1 nuclear export via protein-protein binding, rather than by modulation of its transcription. Furthermore, it is the C-terminal (BRCT) region of BRCA1 that is critical for its interaction with p53, and p53 may promote BRCA1 nuclear export by interrupting the association of BRCA1 with BARD1. In sporadic breast cancer specimens, dysfunctional p53 strongly correlates with nuclear retention of sequence-verified wt BRCA1. This p53-dependent BRCA1 shuttling determines cellular susceptibility to DNA damage as augmentation of cytosolic BRCA1 significantly enhances cancer cell susceptibility to ionizing radiation. Taken together, our data suggest that p53 dysfunction compromises nuclear export of wt BRCA1 as a mechanism to increase cellular resistance to DNA damage in sporadic breast cancer. We propose that targeting nuclear BRCA1 to the cytoplasm may offer a unique strategy to sensitize p53-deficient sporadic breast cancers to DNA damage-based therapy. Cancer Res; 71(16); 5546-57. Ó2011 AACR.

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The tumor suppressor BRCA1 is a nuclear shuttling protein. However, the role of BRCA1 localization in the control of its functions remains to be elucidated. Given the central role of BRCA1 in DNA damage repair, we hypothesized that depletion of nuclear BRCA1 would compromise its nuclear function in DNA repair and thereby result in enhanced cytotoxic response to DNA damage. In this study, we showed that repair of DNA double-strand breaks required BRCA1 in the nucleus. In addition, sequestering BRCA1 in the cytosol enhanced the cytotoxic response to ionizing radiation or cisplatin in human breast and colon cancer cells. However, further genetic dissection of the mechanism of this enhanced cytotoxicity using BRCA1 mutants deficient in double-strand break repair unexpectedly revealed a dissociation of BRCA1's function in DNA repair from its effects on cellular sensitivity to DNA damage. Interestingly, we observed a dependence of the DNA damage-induced cell killing on the translocation and accumulation of BRCA1 in the cytosol. Together, these data suggest a novel role of cytoplasmic translocation of BRCA1, not only in controlling its DNA repair functions, but also in the regulation of cell death processes following DNA damage. Further dissection of the mechanism of cytotoxicity induced by BRCA1 cytoplasmic translocation revealed the involvement of the apoptotic pathway. We propose that the status of BRCA1 nuclear/cytoplasmic shuttling might provide a molecular marker to predict tumor response and a potential novel target to sensitize cancer cells to DNA damage-based therapy. Cancer Res; 70(15); 6258-67. ©2010 AACR.

An increased risk of non-small cell lung cancer (NSCLC) in cystic fibrosis (CF) patients and carriers of CF transmembrane conductance regulator (CFTR) mutations has been proposed. However, the role of CFTR in lung cancer remains controversial. In the present study, CFTR expression was assessed in 165 NSCLC tumors and 22 normal lung samples with validation in an independent series of 131 samples. The effect of gain and loss of CFTR on the malignant behavior of NSCLC was examined. The effect of CFTR manipulation on tumor metastasis was examined in a mouse model. Expression of CFTR was downregulated in NSCLC (p=0.041). Low CFTR expression was correlated with advanced stage (p<0.001) and lymph node metastasis (p=0.009). Low CFTR expression was significantly associated with poor prognosis (overall survival: 45 vs. 36 months, p<0.0001; progression-free survival: 41 vs. 30 months, p=0.007). Knockdown of CFTR in NSCLC cells enhanced malignant behavior (epithelial-mesenchymal transition, invasion and migration); in contrast, overexpression of CFTR suppressed cancer progression in vitro and in vivo. The tumor-suppressing effect of CFTR was associated with inhibition of multiple uPA/uPAR-mediated malignant traits in culture. These results show that CFTR plays a role in inhibition of NSCLC metastasis and suggest that CFTR may serve as a novel indicator for predicting adverse prognosis and metastasis in NSCLC patients.

One of the DNA repair machineries is activated by Poly (ADP-ribose) Polymerase (PARP) enzyme. Particularly, this enzyme is involved in repair of damages to single-strand DNA, thus decreasing the chances of generating double-strand breaks in the genome. Therefore, the concept to block PARP enzymes by PARP inhibitor (PARPi) was appreciated in cancer treatment. PARPi has been designed and tested for many years and became a potential supplement for the conventional chemotherapy. However, increasing evidence indicates the appearance of the resistance to this treatment. Specifically, cancer cells may acquire new mutations or events that overcome the positive effect of these drugs. This paper describes several molecular mechanisms of PARPi resistance which were reported most recently, and summarizes some strategies to reverse this type of drug resistance.

EB-OCT, a minimally invasive imaging modality with high-resolution, is useful and clinically practical for assessing proximal and distal airways of human compared with CT and histology.

High frequency loss of 3p21.3 region is a common event in various kinds of tumors including nasopharyngeal carcinoma (NPC). RASSF1A has been identified as a putative tumor suppressor gene residing in this region. Chromosome alterations and epigenetic changes are commonly observed as mechanisms for inactivation of RASSF1A function. In this study, we applied the PCR-cloning-sequencing strategy to examine somatic mutations in RASSF1A in NPC tissues as compared with the sequences detected in the matched peripheral blood lymphocytes. Our results revealed a high incidence of RASSF1A mutation in primary tumor tissues of NPC. There are totally 35 mutations identified in 74% (17/23) of these NPC cases, including 30 transitions, three transversions and two deletions. Most of these mutations result in amino acid changes: three nonsense (stop codon) mutations, two-1 bp deletion (frameshift), 26 missense and the remaining four are synonymous (silent). No obvious 'hot-spot' mutations were observed in this study. A similarly high rate (74%) of promoter methylation of RASSF1A was also detected in the same group of NPC tissues, but no significant correlation between mutation and methylation was detected. Our results suggest various mechanisms involved in inactivation of RASSF1A function and indicate a critical role of RASSF1A in NPC development.

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