The role of BRCA1 in DNA damage response

FOXM1 (Forkhead box M1) in tumorigenesis: overexpression in human cancer, implication in tumorigenesis, oncogenic functions, tumor-suppressive properties, and target of anticancer therapy

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor and is also intimately involved in tumorigenesis. FOXM1 stimulates cell proliferation and cell cycle progression by promoting the entry into S-phase and M-phase. Additionally, FOXM1 is required for proper execution of mitosis. In accordance with its role in stimulation of cell proliferation, FOXM1 exhibits a proliferation-specific expression pattern and its expression is regulated by proliferation and anti-proliferation signals as well as by proto-oncoproteins and tumor suppressors. Since these factors are often mutated, overexpressed, or lost in human cancer, the normal control of the foxm1 expression by them provides the basis for deregulated FOXM1 expression in tumors. Accordingly, FOXM1 is overexpressed in many types of human cancer. FOXM1 is intimately involved in tumorigenesis, because it contributes to oncogenic transformation and participates in tumor initiation, growth, and progression, including positive effects on angiogenesis, migration, invasion, epithelial-mesenchymal transition, metastasis, recruitment of tumor-associated macrophages, tumor-associated lung inflammation, self-renewal capacity of cancer cells, prevention of premature cellular senescence, and chemotherapeutic drug resistance. However, in the context of urethane-induced lung tumorigenesis, FOXM1 has an unexpected tumor suppressor role in endothelial cells because it limits pulmonary inflammation and canonical Wnt signaling in epithelial lung cells, thereby restricting carcinogenesis. Accordingly, FOXM1 plays a role in homologous recombination repair of DNA double-strand breaks and maintenance of genomic stability, that is, prevention of polyploidy and aneuploidy. The implication of FOXM1 in tumorigenesis makes it an attractive target for anticancer therapy, and several antitumor drugs have been reported to decrease FOXM1 expression.

Associations between gene expression variations and ovarian cancer risk alleles identified from genome wide association studies

Functional genetic variations play important roles in shaping phenotypic differences among individuals through affecting gene expression, and thus, very likely to influence disease susceptibility, such as cancer susceptibility. One critical question in this era of post-genome wide association studies (GWAS) is how to assess the functional significance of the genetic variations identified from GWAS. In the current study, with lymphoblastoid cell lines (LCLs) from 74 non-related women with familial ovarian cancer and 47 unrelated controls matched on gender and race, we explored the associations between seven ovarian cancer risk variants identified from GWAS (rs3814113 on 9p22.2, rs2072590 on 2q31, rs2665390 on 3q25, rs10088218, rs1516982, rs10098821 on 8q24.21, and rs2363956 on 19p13) and whole genome mRNA expression profiles. We observed 95 significant trans-associations at a permutation level of 0.001. Compared to the other risk variants, rs10088218, rs1516982, and rs10098821 on 8q24.21 had the greatest number of significant associations (25, 16, and 38, respectively). Two possible cis-associations were observed between rs10098821 and c-Myc, and rs2072590 and HS.565379 (Permutated P = 0.0198 and 0.0399, respectively). Pathway enrichment analysis showed that several key biological pathways, such as cell cycle (P = 2.59×10⁻⁰⁶), etc, were significantly overrepresented. Further characterization of significant associations between mRNAs and risk alleles might facilitate understanding the functions of GWAS discovered risk alleles in the genetic etiology of ovarian cancer.

[BRCA1: a new predictive genomic marker for chemotherapy and radiotherapy of non-small cell lung cancer]

Non-small cell lung cancer (NSCLC) is one of the most common malignant tumors worldwide. Chemotherapy and radiotherapy are the main methods in therapy of NSCLC. The identification and characterization of predictive markers of radio- and chemo-sensitivity will be beneficial with customized treatment of NSCLC patients. This article will review the evidence regarding breast cancer susceptibility gene 1 (BRCA1) as a new predictive marker for chemotherapy and radiotherapy of NSCLC.

Regulatory mechanisms and clinical perspectives of miRNA in tumor radiosensitivity

MicroRNA (miRNA) influences carcinogenesis at multiple stages and it can effectively control tumor radiosensitivity by affecting DNA damage repair, cell cycle checkpoint, apoptosis, radio-related signal transduction pathways and tumor microenvironment. MiRNA also efficiently modulates tumor radiosensitivity at multiple levels by blocking the two essential non-homologous end-joining repair and homologous recombination repair pathways in the DNA damage response. It interferes with four radio-related pathways in ionizing radiation, including the PI3-K/Akt, NF-κB, MAPK and TGFβ signaling pathways. Moreover, the regulatory effect of miRNA in radiosensitivity can be enhanced when interacting with various key molecules, including H2AX, BRCA1, ATM, DNA-PK, RAD51, Chk1, Cdc25A, p53, PLK1, HIF-1 and VEGF, which are involved in these processes. Therefore, thoroughly understanding the mechanism of miRNA in tumor radiosensitivity could assist in finding novel targets to improve the radiotherapeutic effects and provide new clinical perspectives and insights for developing effective cancer treatments.

Loss of BRCA1-A complex function in RAP80 null tumor cells

Receptor Associated Protein 80 (RAP80) is a subunit of the BRCA1-A complex and targets BRCA1 to DNA damage sites in response to DNA double strand breaks. Since mutations of BRCA1 are associated with familial ovarian cancers, we screened 26 ovarian cancer-derived cell lines for RAP80 mutations and found that TOV-21G cells harbor a RAP80 mutation (c.1107G >A). This mutation generates a stop codon at Trp369, which deletes the partial AIR region and the C-terminal zinc fingers of RAP80. Interestingly, both the mutant and wild type alleles of RAP80 lose their expression due to promoter hypermethylation, suggesting that TOV-21G is a RAP80-null cell line. In these cells, not only is the BRCA1-A complex disrupted, but the relocation of the remaining subunits in the BRCA1-A complex including BRCA1, CCDC98, NBA1, BRCC36 and BRE is significantly suppressed. Moreover, TOV-21G cells are hypersensitive to ionizing radiation, which is due to the compromised DNA damage repair capacity in these cells. Reconstitution of TOV-21G cells with wild type RAP80 rescues these cellular defects in response to DNA damage. Thus, our results demonstrate that RAP80 is a scaffold protein in the BRCA1-A complex. Identification of TOV-21G as a RAP80 null tumor cell line will be very useful for the study of the molecular mechanism in DNA damage response.

The C-terminal proteolytic fragment of the breast cancer susceptibility type 1 protein (BRCA1) is degraded by the N-end rule pathway

The breast cancer susceptibility type 1 gene product (BRCA1) is cleaved by caspases upon the activation of apoptotic pathways. After proteolysis the C-terminal fragment has been reported to translocate to the cytoplasm and promote cell death. Here we report that the C-terminal fragment is unstable in cells as it is targeted for degradation by the N-end rule pathway. The data reveals that mutating the wild type N-terminal aspartate, of the C-terminal fragment, to valine stabilizes the fragment. If the N terminus is mutated to another N-terminal destabilizing residue, like arginine, the C-terminal fragment remains unstable in cells. Last, the C-terminal fragment of BRCA1 is stable in cells lacking ATE1, a component of the N-end rule pathway.

Evaluation of microRNA expression profiles and their associations with risk alleles in lymphoblastoid cell lines of familial ovarian cancer

Interindividual variations of microRNA expression are likely to influence the expression of microRNA target genes and, therefore, contribute to phenotypic differences in humans, including cancer susceptibility. Whether microRNA expression variation has any role in ovarian cancer development is still unknown. Here, we evaluated microRNA expression profiles in lymphoblastoid cell lines from 74 women with familial ovarian cancer and 47 unrelated controls matched on gender and race. We found that the cases and unrelated controls can be clustered using 95 differentially expressed microRNAs with 91% accuracy. To assess the potential implications of microRNAs in ovarian cancer, we investigated the associations between microRNA expression and seven ovarian cancer risk variants discovered from genome-wide association studies (GWAS), namely, rs3814113 on 9p22.2, rs2072590 on 2q31, rs2665390 on 3q25, rs10088218, rs1516982, rs10098821 on 8q24.21 and rs2363956 on 19p13. We observed 130 significant associations at a permutation level of 0.01. Compared with other risk variants, rs3814113 and rs2072590 had the greatest number of significant associations (68 and 37, respectively). Interestingly, 14 microRNAs that were associated with ovarian cancer risk alleles belong to five microRNA clusters. The most notable cluster is the tumorigenic miR-17-92 cluster with five microRNAs, all of which are significantly associated with rs3814113. Using pathway analysis, several key biological pathways were significantly overrepresented, such as cellular response to stress (P = 2.87 × 10(-06)), etc. Further characterization of significant associations between microRNAs and risk alleles could facilitate the understanding of the functions of these GWAS discovered risk alleles in the genetic etiology of ovarian cancer.

Molecular mechanisms of ultraviolet radiation-induced DNA damage and repair

DNA is one of the prime molecules, and its stability is of utmost importance for proper functioning and existence of all living systems. Genotoxic chemicals and radiations exert adverse effects on genome stability. Ultraviolet radiation (UVR) (mainly UV-B: 280-315 nm) is one of the powerful agents that can alter the normal state of life by inducing a variety of mutagenic and cytotoxic DNA lesions such as cyclobutane-pyrimidine dimers (CPDs), 6-4 photoproducts (6-4PPs), and their Dewar valence isomers as well as DNA strand breaks by interfering the genome integrity. To counteract these lesions, organisms have developed a number of highly conserved repair mechanisms such as photoreactivation, base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR). Additionally, double-strand break repair (by homologous recombination and nonhomologous end joining), SOS response, cell-cycle checkpoints, and programmed cell death (apoptosis) are also operative in various organisms with the expense of specific gene products. This review deals with UV-induced alterations in DNA and its maintenance by various repair mechanisms.