Summary MEK inhibitors are clinically active in BRAFV600E melanomas, but only marginally so in KRAS-mutant tumors. Here we found that MEK inhibitors suppress ERK signaling more potently in BRAFV600E- than in KRAS-mutant tumors. To understand this, we performed an RNAi screen in a KRAS-mutant model and found that CRAF knockdown enhanced MEK inhibition. MEK activated by CRAF was less susceptible to MEK inhibitors than when activated by BRAFV600E. MEK inhibitors induced RAF-MEK complexes in KRAS mutant models, and disrupting such complexes enhanced inhibition of CRAF-dependent ERK signaling. Newer MEK inhibitors not only target MEK catalytic activity, but also impair its reactivation by CRAF: either by disrupting RAF-MEK complexes or by interacting with Ser 222 to prevent RAF-mediated phosphorylation in the complex.

SUMMARY Transcription by RNA polymerase II (RNAPII) is pervasive in the human genome. However, the mechanisms controlling transcription at promoters and enhancers remain enigmatic. Here, we demonstrate that Integrator subunit 11 (INTS11), the catalytic subunit of the Integrator complex, regulates transcription at these loci through its endonuclease activity. Promoters of genes require INTS11 to cleave nascent transcripts associated with paused RNAPII and induce their premature termination in the proximity of the +1 nucleosome. The turnover of RNAPII permits the subsequent recruitment of an elongation-competent RNAPII complex, leading to productive elongation. In contrast, enhancers require INTS11 catalysis not to evict paused RNAPII but rather to terminate enhancer RNA transcription beyond the +1 nucleosome. These findings are supported by the differential occupancy of negative elongation factor (NELF), SPT5, and tyrosine-1-phosphorylated RNAPII. This study elucidates the role of Integrator in mediating transcriptional elongation at human promoters through the endonucleolytic cleavage of nascent transcripts and the dynamic turnover of RNAPII.

The human actin-binding protein filamin-A (also known as ABP-280) cross-links actin into a dynamic three-dimensional structure. It interacts with >45 proteins of diverse functions, serving as the scaffold in various signaling networks. BRCA2 is a protein that regulates RAD51-dependent recombinational repair of DNA double strand breaks (DSB). Proximate to the COOH terminus of the BRCA2 protein, a conserved and DNA binding domain (BRCA2-DBD) interacts with filamin-A and BCCIP. In this study, we sought to test the hypothesis that filamin-A influences homologous recombinational repair of DSB and the maintenance of genomic stability. We used three pairs of cell lines with normal and reduced filamin-A expression, including breast cancer and melanoma cells. We found that lack or reduction of filamin-A sensitizes cells to ionizing radiation, slows the removal of DNA damage-induced ;H2AX nuclear foci, reduces RAD51 nuclear focus formation and recruitment to chromatin in response to irradiation, and results in a 2-fold reduction of homologous recombinational repair of DSB. Furthermore, filamin-A-deficient cells have increased frequencies of micronucleus formation after irradiation. Our data illustrate the importance of the cytoskeleton structure in supporting the homologous recombinational DNA repair machinery and genome integrity, and further implicate a potential of filamin-A as a marker for prognosis in DNA damage-based cancer therapy. [Cancer Res 2009;69(20):7978-85]

Filamin-A (FLNA), also called actin-binding protein 280 (ABP-280), was originally identified as a non-muscle actin binding protein, which organizes filamentous actin into orthogonal networks and stress fibers. Filamin-A also anchors various transmembrane proteins to the actin cytoskeleton and provides a scaffold for a wide range of cytoplasmic and nuclear signaling proteins. Intriguingly, several studies have revealed that filamin-A associates with multiple non-cytoskeletal proteins of diverse function and is involved in several unrelated pathways. Mutations and aberrant expression of filamin-A have been reported in human genetic diseases and several types of cancer. In this review, we discuss the implications of filamin-A in cancer progression, including metastasis and DNA damage response.

Homologous recombination (HR) is critical for maintaining genome stability through precise repair of DNA double-strand breaks (DSBs) and restarting stalled or collapsed DNA replication forks. HR is regulated by many proteins through distinct mechanisms. Some proteins have direct enzymatic roles in HR reactions, while others act as accessory factors that regulate HR enzymatic activity or coordinate HR with other cellular processes such as the cell cycle. The breast cancer susceptibility gene BRCA2 encodes a critical accessory protein that interacts with the RAD51 recombinase and this interaction fluctuates during the cell cycle. We previously showed that a BRCA2- and p21-interacting protein, BCCIP, regulates BRCA2 and RAD51 nuclear focus formation, DSB-induced HR and cell cycle progression. However, it has not been clear whether BCCIP acts exclusively through BRCA2 to regulate HR and whether BCCIP also regulates the alternative DSB repair pathway, non-homologous end joining. In this study, we found that BCCIP fragments that interact with BRCA2 or with p21 each inhibit DSB repair by HR. We further show that transient down-regulation of BCCIP in human cells does not affect non-specific integration of transfected DNA, but significantly inhibits homology-directed gene targeting. Furthermore, human HT1080 cells with constitutive down-regulation of BCCIP display increased levels of spontaneous single-stranded DNA (ssDNA) and DSBs. These data indicate that multiple BCCIP domains are important for HR regulation, that BCCIP is unlikely to regulate non-homologous end joining, and that BCCIP plays a critical role in resolving spontaneous DNA damage.

Filamin-A, also called actin binding protein 280 (ABP-280), cross-links the actin filaments into dynamic orthogonal network to serve as scaffolds in multiple signaling pathways. It has been reported that filamin-A interacts with DNA damage response proteins BRCA1 and BRCA2. Defects of filamin-A impair the repair of DNA double strand breaks (DSBs), resulting in sensitization of cells to ionizing radiation. In this study, we sought to test the hypothesis that filamin-A can be used as a target for cancer chemotherapy and as a biomarker to predict cancer response to therapeutic DNA damage. We found that reduction of filamin-A sensitizes cancer cells to chemotherapy reagents bleomycin and cisplatin, delays the repair of not only DSBs but also single strand breaks (SSBs) and interstrand crosslinks (ICLs), and increases chromosome breaks after the drug treatment. By treating a panel of human melanoma cell lines with variable filamin-A expression, we observed a correlation between expression level of filamin-A protein and drug IC50. We further inhibited the expression of filamin-A in melanoma cells, and found that this confers an increased sensitivity to bleomycin and cisplatin treatment in a mouse xenograft tumor model. These results suggest that filamin-A plays a role in repair of a variety of DNA damage, that lack of filamin-A is a prognostic marker for a better outcome after DNA damage based treatment, and filamin-A can be inhibited to sensitize filamin-A positive cancer cells to therapeutic DNA damage. Thus filamin-A can be used as a biomarker and a target for DNA damage based cancer therapy.

The tumor suppression function of p53 is mostly conferred by its transactivation activity, which is inactivated by p53 mutations in ϳ50% of human cancers. In cancers harboring wild type p53, the p53 transactivation activity may be compromised by other mechanisms. Identifying the mechanisms by which wild type p53 transactivation activity can be abrogated may provide insights into the molecular etiology of cancers harboring wild type p53. In this report, we show that BCCIP, a BRCA2 and CDKN1A-interacting protein, is required for the transactivation activity of wild type p53. In p53 wild type cells, BCCIP knock down by RNA interference diminishes the transactivation activity of p53 without reducing the p53 protein level, inhibits the binding of p53 to the promoters of p53 target genes p21 and HDM2, and reduces the tetrameric formation of p53. These data demonstrate a critical role of BCCIP in maintaining the transactivation activity of wild type p53 and further suggest down-regulation of BCCIP as a novel mechanism to impair the p53 function in cells harboring wild type p53.

Dubowitz Syndrome is an autosomal recessive disorder with a unique set of clinical features including microcephaly and susceptibility to tumor formation. Although more than 140 cases of Dubowitz syndrome have been reported since 1965, the genetic defects of this disease has not been identified. In this study, we systematically analyzed the DNA damage response and repair capability of fibroblasts established from a Dubowitz Syndrome patient. Dubowitz syndrome fibroblasts are hypersensitive to ionizing radiation, bleomycin, and doxorubicin. However, they have relatively normal sensitivities to mitomycin-C, cisplatin, and camptothecin. Dubowitz syndrome fibroblasts also have normal DNA damage signaling and cell cycle checkpoint activations after DNA damage. These data implicate a defect in repair of DNA double strand break (DSB) likely due to defective non-homologous end joining (NHEJ). We further sequenced several genes involved in NHEJ, and identified a pair of novel compound mutations in the DNA Ligase IV gene. Furthermore, expression of wild type DNA ligase IV completely complement the DNA repair defects in Dubowitz syndrome fibroblasts, suggesting that the DNA ligase IV mutation is solely responsible for the DNA repair defects. These data suggests that at least subset of Dubowitz syndrome can be attributed to DNA ligase IV mutations.