The Second Annual Symposium of the Global Cancer Genomics Consortium (GCGC) was held at the Tata Memorial Center in Mumbai, India, from November 19 to 20, 2012. Founded in late 2010, the GCGC aims to provide a platform for highly productive, collaborative efforts on next-generation cancer research through bridging the latest scientific and technology developments with clinical oncology challenges. This year’s presenters brought together highly innovative interdisciplinary views and strategies to meet major challenges in cancer research. The symposium featured 3 major themes: OMICS approaches toward the identification of cancer molecular drivers, single-cell analysis in cancer, and clinical and translational genomics. Each theme was represented in presentations of new findings, with an obvious implication in cross-disciplinary components of OMICs and an overwhelming participation by students. In summary, the GCGC symposium provided a discussion and congregation of the latest advances in basic and translational cancer research and offered the participants with a highly cooperative network environment for future collaboration.
genomics medicine; anticancer target; cancer therapy
HER2 is overexpressed in a subset of breast cancers and controls an oncogenic signaling network that inhibits tumor cell death through the specific biochemical regulation of apoptotic pathways. In particular, the mitochondrial pathway for apoptosis is important for death induced by inhibitors of HER2. This review focuses on the connections between this oncogenic signaling network and individual components of the mitochondrial pathway. A comprehensive view of this signaling network is crucial for developing novel drugs in this area and to gain an understanding of how these regulatory interactions are altered in drug-refractory cancers.
lapatinib; trastuzumab; BAD; BCL-2; AKT; mTOR
Several recent reports have identified TET1 as the main enzyme modulating DNA methylation and gene transcription via hydroxylation of 5-methylcytosine. However, little is known about the protein network that controls TET1 activity. By using a new proximity ligation in situ assay, we identified MeCP2, HDAC1/6/7, EZH2, mSin3A, PCNA, and LSD1 as TET1-interacting proteins. We also discerned that TET1/PCNA acts as a demethylator of the cyclical methylation/demethylation process, the perturbation of which promotes the aberrant methylation hallmarks frequently observed in cancer cells.
DNA methylation; demethylation; TET1; glioma
The gene encoding EWS (EWSR1) is involved in various chromosomal translocations that cause the production of oncoproteins responsible for multiple cancers including Ewing sarcoma, myxoid liposarcoma, soft tissue clear cell sarcoma, and desmoplastic small round cell sarcoma. It is well known that EWS fuses to FLI to create EWS/FLI, which is the abnormal transcription factor that drives tumor development in Ewing sarcoma. However, the role of wild-type EWS in Ewing sarcoma pathogenesis remains unclear. In the current study, we identified EWS-regulated genes and cellular processes through RNA interference combined with RNA sequencing and functional annotation analyses. Interestingly, we found that EWS and EWS/FLI co-regulate a significant cluster of genes, indicating an interplay between the 2 proteins in regulating cellular functions. We found that among the EWS–down-regulated genes are a subset of neuronal genes that contain binding sites for the RE1-silencing transcription factor (REST or neuron-restrictive silencer factor [NRSF]), neuron-restrictive silencer element (NRSE), suggesting a cooperative interaction between REST and EWS in gene regulation. Co-immunoprecipitation analysis demonstrated that EWS interacts directly with REST. Genome-wide binding analysis showed that EWS binds chromatin at or near NRSE. Furthermore, functional studies revealed that both EWS and REST inhibit neuronal phenotype development and oncogenic transformation in Ewing sarcoma cells. Our data implicate an important role of EWS in the development of Ewing sarcoma phenotype and highlight a potential value in modulating EWS function in the treatment of Ewing sarcoma and other EWS translocation–based cancers.
EWS; REST; oncogenic transformation; neuronal phenotype; Ewing sarcoma
S100A4, a calcium-binding protein, is known for its role in the metastatic spread of tumor cells, a late event of cancer disease. This is the first report showing that S100A4 is not merely a metastatic protein but also an oncoprotein that plays a critical role in the development of tumors. We earlier showed that S100A4 expression progressively increases in prostatic tissues with the advancement of prostate cancer (CaP) in TRAMP, an autochthonous mouse model. To study the functional significance of S100A4 in CaP, we generated a heterozygously deleted S100A4 (TRAMP/S100A4+/−) genotype by crossing TRAMP with S100A4−/− mice. TRAMP/S100A4+/− did not show a lethal phenotype, and transgenes were functional. As compared to age-matched TRAMP littermates, TRAMP/S100A4+/− mice exhibited 1) an increased tumor latency period (P < 0.001), 2) a 0% incidence of metastasis, and 3) reduced prostatic weights (P < 0.001). We generated S100A4-positive clones from S100A4-negative CaP cells and tested their potential. S100A4-positive tumors grew at a faster rate than S100A4-negative tumors in vitro and in a xenograft mouse model. The S100A4 protein exhibited growth factor–like properties in multimode (intracellular and extracellular) forms. We observed that 1) the growth-promoting effect of S100A4 is due to its activation of NFκB, 2) S100A4-deficient tumors exhibit reduced NFκB activity, 3) S100A4 regulates NFκB through the RAGE receptor, and 4) S100A4 and RAGE co-localize in prostatic tissues of mice. Keeping in view its growth-promoting role, we suggest that S100A4 qualifies as an excellent candidate to be exploited for therapeutic agents to treat CaP in humans.
S100A4; TRAMP; prostate cancer; NFB; RAGE
Osteopontin (OPN) Spp1 is involved in differentiation of the mammary gland. We engineered mice to overexpress OPN in mammary epithelium and describe an altered mammary phenotype. Three transgenic (Tg) founder lines FVB/N Tg(MMTV-Opn)(1-3BOR) were propagated after FVB/NJ pronuclear injections. Mammary glands from Tg-OPN mice compared to littermate controls showed, at 4 weeks of age, exaggerated terminal end buds; at 8 and 12 weeks, more numerous and complex ducts with increased luminal protein; and at 16 weeks, increased lobulogenesis. Lactational Tg-OPN mammary glands showed more rapid lobulogenesis and lactational changes with slower gland involution and regression following weaning. Ex vivo lobulogenesis was noticeably increased from organoids of Tg-OPN mice. Immunohistochemistry revealed cytoplasmic OPN accumulation and increased Ki-67 positive mammary epithelial cells in Tg-OPN mammary glands. OPN appears to convey a proliferative stimulus for mammary epithelial cells and alters development and differentiation. These OPN mammary overexpressing mice provide a means to study the role of OPN in cancer progression.
mammary gland; osteopontin; lobulogenesis; involution; organoid culture; flow cytometry
NAD+-dependent deacetylase SIRT1 is a master regulator of nucleosome positioning and chromatin structure, thereby reprogramming gene expression. In acute inflammation, chromatin departs from, and returns to, homeostasis in an orderly sequence. This sequence depends on shifts in NAD+ availability for SIRT1 activation and deacetylation of signaling proteins, which support orderly gene reprogramming during acute inflammation by switching between euchromatin and heterochromatin. In contrast, in chronic inflammation and cancer, limited availability of NAD+ and reduced expression of SIRT1 may sustain aberrant chromatin structure and functions. SIRT1 also influences inflammation and cancer by directly deacetylating targets like NFκB p65 and p53. Here, we review SIRT1 in the context of inflammation and cancer.
SIRT1; chromatin regulation; NFκB; inflammation; cancer
Sirtuins play an essential role in the cellular response to environmental stress, promoting DNA repair, telomere stability, cell cycle arrest, cellular senescence, and apoptosis. Much attention has been given to the role of sirtuins in aging and cancer development; however, less is known about their role in stem cell regulation. This review focuses in this topic and discusses the possible implications in adult stem cell aging.
sirtuins; stem cells; development; aging
SIRT3 is a NAD+-dependent deacetylase that regulates the function of numerous mitochondrial proteins with roles in metabolism, oxidative stress, and cell survival. It is emerging as an instrumental regulator of the mitochondrial adaptive responses to stress, including metabolic reprogramming and enhancing antioxidant defense mechanisms. Here, we discuss the role that SIRT3 plays at both a cellular and physiological level and consider its involvement in disease. Mitochondrial dysfunction is a key contributing factor in many diseases; however, the mechanisms involved are often not well understood, and few targeted therapies exist. If manipulation of SIRT3 proves to be beneficial in disease states, then it could be a promising target for novel therapies.
sirtuins; mitochondria; stress; cell protection
Aging is a degenerative process resulting in compromised tissue maintenance and increased susceptibility to diseases, such as cancer. Recent advancements support the notion that aging is a highly regulated process governed by evolutionarily conserved pathways. In mammals, tissue-specific adult stem cells (ASCs) persist throughout the lifetime to maintain and repair tissues. While reduced ASC self-renewal is thought to contribute to compromised tissue maintenance, increased self-renewal of cancer stem cells (CSCs) may lead to tumorigenesis. It is speculated that genetic regulators of aging, such as sirtuins, are likely to impinge upon the ASC compartments to regulate tissue maintenance and tumorigenesis. In this review, we discuss the emerging evidence linking sirtuins to normal and malignant ASC self-renewal, tissue maintenance, and tumorigenesis.
sirtuin; hematopoietic stem cell (HSC); leukemia; leukemic stem cell (LSC); chronic myelogenous leukemia (CML)
The sirtuin family has emerged as important regulators of diverse physiological and pathological events, including life-span extension, neurodegeneration, age-related disorders, obesity, heart disease, inflammation, and cancer. In mammals, there are 7 members (SIRT1-SIRT7) in the sirtuin family, with the function of SIRT1 being extensively studied in the past decade. SIRT1 can deacetylate histones and a number of nonhistone substrates, which are involved in multiple signaling pathways. Numerous studies have suggested that SIRT1 could act as either a tumor suppressor or tumor promoter depending on its targets in specific signaling pathways or in specific cancers. This review highlights the major pathways regulated by SIRT1 involved in tumorigenesis.
SIRT1; deacetylase; cancer
SIRT1 is a NAD+-dependent protein deacetylase that has a very large number of established protein substrates and an equally impressive list of biological functions thought to be regulated by its activity. Perhaps as notable is the remarkable number of points of conflict concerning the role of SIRT1 in biological processes. For example, evidence exists suggesting that SIRT1 is a tumor suppressor, is an oncogene, or has no effect on oncogenesis. Similarly, SIRT1 is variably reported to induce, inhibit, or have no effect on autophagy. We believe that the resolution of many conflicting results is possible by considering recent reports indicating that SIRT1 is an important hub interacting with a complex network of proteins that collectively regulate a wide variety of biological processes including cancer and autophagy. A number of the interacting proteins are themselves hubs that, like SIRT1, utilize intrinsically disordered regions for their promiscuous interactions. Many studies investigating SIRT1 function have been carried out on cell lines carrying undetermined numbers of alterations to the proteins comprising the SIRT1 network or on inbred mouse strains carrying fixed mutations affecting some of these proteins. Thus, the effects of modulating SIRT1 amount and/or activity are importantly determined by the genetic background of the cell (or the inbred strain of mice), and the effects attributed to SIRT1 are synthetic with the background of mutations and epigenetic differences between cells and organisms. Work on mice carrying alterations to the Sirt1 gene suggests that the network in which SIRT1 functions plays an important role in mediating physiological adaptation to various sources of chronic stress such as calorie restriction and calorie overload. Whether the catalytic activity of SIRT1 and the nuclear concentration of the co-factor, NAD+, are responsible for modulating this activity remains to be determined. However, the effect of modulating SIRT1 activity must be interpreted in the context of the cell or tissue under investigation. Indeed, for SIRT1, we argue that context is everything.
scale-free network; protein deacetylation; nutritional stress; oncogenesis
Sirtuins (SIRT1-SIRT7), the mammalian homologs of the silent information regulator 2 (Sir2) in Saccharomyces cerevisiae, have been a major focus of study in the scientific community this past decade because of their emerging role in cancer biology and other age-related diseases. Emerging functions for this unique family of enzymes include roles in genomic stability, angiogenesis, metabolism, and anoikis. Here, we review recent developments on the role of sirtuins in cancer with a particular focus on SIRT3 and its role in the hallmarks of cancer and as a potential drug target for cancer treatment.
sirtuin-3; SIRT3; sirtuins; cancer; tumor promoter; tumor suppressor; anoikis
Sirtuins are a class of histone deacetylases that have a wide range of regulatory roles in the cell. Three sirtuins, SIRT3 to SIRT5, localize to and function within the mitochondria. Mitochondrial dysfunction is thought to be the underlying mechanism of several age-related diseases, such as metabolic syndrome, cancer, and neurodegeneration. This review examines current evidence that mitochondrial sirtuins are involved in regulating mitochondrial function and pathogenesis.
sirtuins; mitochondria; metabolic syndrome; cancer; neurodegeneration; therapeutic targets; aging
The cellular NAD+/NADH level controls Sir2 (silent information regulator 2) deacetylase activity in regulating aging in lower species. Much work has been put forth to identify ways to activate SIRT1, the mammalian ortholog of Sir2. The identification of p53 as a bona fide substrate of SIRT1 deacetylation has linked SIRT1 to a role in tumorigenesis. Here, we review the various SIRT1 endogenous and small molecular activators and inhibitors that regulate p53 acetylation and subsequent activation of p53 tumor suppression activity.
SIRT1; p53; acetylation; deacetylation; tumorigenesis
Cells must continuously respond to stressful insults via the upregulation of cytoprotective pathways. The longevity factor and deacetylase SIRT1 plays a critical role in coordinating this cellular response to stress. SIRT1 activity and levels are regulated by cellular stressors, including metabolic, genotoxic, oxidative, and proteotoxic stress. As a stress sensor, SIRT1 impacts cell survival by deacetylating substrate proteins to drive the cell towards a cytoprotective pathway. Extreme stress conditions, however, can cause SIRT1 to lead cells down an apoptotic pathway instead. SIRT1 is frequently dysregulated in cancer cells and has been characterized to have a dual role as both an oncogene and a tumor suppressor, likely due to its pivotal function in regulating cytoprotection. Recently, the ability of SIRT1 to regulate HSF1-dependent induction of the heat shock response has highlighted another pathway through which SIRT1 can modulate cytoprotection. Activation of HSF1 results in the production of cytoprotective chaperones that can facilitate the transformed phenotype of cancer cells. In this review, we discuss the stress-dependent regulation of SIRT1. We highlight the role of SIRT1 in stress management and cytoprotection and emphasize SIRT1-dependent activation of HSF1 as a potential mechanism for cancer promotion.
SIRT1; metabolic stress; stress responses; heat shock response; HSF1; chaperones; cytoprotection; cancer; apoptosis; genotoxic stress
Innate resistance to various therapeutic interventions is a hallmark of cancer. In recent years, acquired resistance has emerged as a daunting challenge to targeted cancer therapy, which abolishes the efficacy of otherwise successful targeting drugs. Cancer cells gain the resistance property through a variety of mechanisms in primary and metastatic cancers, involving cellular intrinsic and extrinsic factors. Increasing evidence suggests that the mammalian stress response gene sirtuin 1 (SIRT1) plays a critical role in multiple aspects of cancer drug resistance. SIRT1 decreases drug penetration, confers proliferation and antiapoptotic survival advantages to cancer cells, facilitates acquired resistance through genetic mutations, promotes the survival of cancer stem cells, and changes the tumor microenvironment for resistance in cell-autonomous and -nonautonomous manners. This article provides an overview of research advances in the roles of SIRT1 in cancer drug resistance and highlights the prospect of targeting SIRT1 as a new strategy to overcome cancer drug resistance and improve therapeutic outcomes.
SIRT1; drug resistance; acquired resistance; cancer stem cells; genetic mutation; tumor microenvironment
The members of the Sir2 family, or sirtuins, are major regulators of the response to different types of stress. The members of the family have adapted to increasing complexities throughout evolution and have become diversified by increasing their number, specificity, and localization and acquiring novel functions. Sirtuins have been consistently implicated in the cross-talk between the genomic information and environment from the prokaryotes onward. Evidence suggests that in the transition to eukaryotes, histones became one of the basic and most conserved targets of the family, to the extent that in yeast and mammals, sirtuins were originally described as NAD+-dependent histone deacetylases and classified as class III histone deacetylases. A growing number of studies have determined that sirtuins also target a wide range of nonhistone proteins. Many of these targets are also directly or indirectly related to chromatin regulation. The number of targets has grown considerably in the last decade but has provoked an ill-founded discussion that neglects the importance of histones as sirtuin targets. In this review, we summarize our knowledge regarding the range of sirtuin targets described to date and discuss the different functional implications of histone and nonhistone targets throughout evolution.
sirtuins; deacetylation; SIRT1-SIRT7; epigenetics; stress response; genome stability; histones
MicroRNAs (miRNA) are small, noncoding RNAs with important regulatory roles in development, differentiation, cell proliferation, and death as well as the complex process of acquired drug resistance. The goal of this study was to identify specific miRNAs and their potential protein targets that confer acquired resistance to gemcitabine in urothelial carcinoma of the bladder (UCB) cell lines. Gemcitabine-resistant cells were established from 6 cell lines following exposure to escalating concentrations of the drug and by passaging cells in the presence of the drug over a 2- to 3-month period. Differential miRNA expression was identified in a microarray format comparing untreated controls with resistant cell lines, representing the maximum tolerated concentration, and results were validated via qRT-PCR. The involvement of specific miRNAs in chemoresistance was confirmed with transfection experiments, followed by clonogenic assays and Western blot analysis. Gemcitabine resistance was generated in 6 UCB cell lines. Microarray analysis comparing miRNA expression between gemcitabine-resistant and parental cells identified the differential expression of 66 miRNAs. Confirmation of differential expression was recorded via qRT-PCR in a subset of these miRNAs. Within this group, let-7b and let-7i exhibited decreased expression, while miR-1290 and miR-138 displayed increased expression levels in gemcitabine-resistant cells. Transfection of pre–miR-138 and pre–miR-1290 into parental cells attenuated cell death after exposure to gemcitabine, while transfection of pre–miR-let-7b and pre–miR-let-7i into the resistant cells augmented cell death. Mucin-4 was up-regulated in gemcitabine-resistant cells. Ectopic expression of let-7i and let-7b in the resistant cells resulted in the down-regulation of mucin-4. These results suggest a role for miRNAs 1290, 138, let-7i, and let-7b in imparting resistance to gemcitabine in UCB cell lines in part through the modulation of mucin-4. Alterations in these miRNAs and/or mucin-4 may constitute a potential therapeutic strategy for improving the efficacy of gemcitabine in UCB.
miRNA; gemcitabine; resistance; bladder cancer
Individual genetic variations may have a significant influence on the survival of metastatic prostate cancer (PCa) patients. We aimed to identify target genes and their variations involved in the survival of PCa patients using a single nucleotide polymorphism (SNP) panel. A total of 185 PCa patients with bone metastasis at the initial diagnosis were analyzed. Germline DNA in each patient was genotyped using a cancer SNP panel that contained 1,421 SNPs in 408 cancer-related genes. SNPs associated with survival were screened by a log-rank test. Fourteen SNPs in 6 genes, XRCC4, PMS1, GATA3, IL13, CASP8, and IGF1, were identified to have a statistically significant association with cancer-specific survival. The cancer-specific survival times of patients grouped according to the number of risk genotypes of 6 SNPs selected from the 14 SNPs differed significantly (0-1 v. 2-3 v. 4-6 risk genotypes; P = 7.20 × 10−8). The high-risk group was independently associated with survival in a multivariate analysis that included conventional clinicopathological variables (P = 0.0060). We identified 14 candidate SNPs in 6 cancer-related genes, which were associated with poor survival in patients with metastatic PCa. A panel of SNPs may help predict the survival of those patients.
prostate cancer; bone metastasis; survival; single nucleotide polymorphism
The Tousled-like kinases (TLKs) are involved in chromatin assembly, DNA repair, and transcription. Two TLK genes exist in humans, and their expression is often dysregulated in cancer. TLKs phosphorylate Asf1 and Rad9, regulating double-strand break (DSB) repair and the DNA damage response (DDR). TLKs maintain genomic stability and are important therapeutic intervention targets. We identified specific inhibitors of TLKs from several compound libraries, some of which belong to the family of phenothiazine antipsychotics. The inhibitors prevented the TLK-mediated phosphorylation of Rad9(S328) and impaired checkpoint recovery and DSB repair. The inhibitor thioridazine (THD) potentiated tumor killing with chemotherapy and also had activity alone. Staining for γ-H2AX revealed few positive cells in untreated tumors, but large numbers in mice treated with low doxorubicin or THD alone, possibly the result of the accumulation of DSBs that are not promptly repaired as they may occur in the harsh tumor growth environment.
inhibitors of Tousled kinases; radiomimetic sensitizers; mechanism of DSB repair; DNA damage response
The human GT198 gene (gene symbol PSMC3IP) is located at chromosome 17q21, 470 kb proximal to BRCA1, a locus previously linked to breast and ovarian cancer predisposition. Its protein product (also known as TBPIP and Hop2) has been shown to regulate steroid hormone receptor–mediated gene activation and to stimulate homologous recombination in DNA repair. Here, we screened germline mutations in GT198 in familial and early-onset breast and ovarian cancer patients. We have identified 8 germline variants in a total of 212 index patients including reoccurring nonsense mutation c.310C>T (p.Q104X) and 5′ UTR mutation c.-37A>T, each found in 2 unrelated families. Most identified index patients from cancer families had early onsets with a median age of 35 years. c.310C>T was absent in a total of 564 control individuals analyzed. GT198 gene amplification with an imbalanced mutant copy gain was identified in the blood DNA of one of the patients carrying c.310C>T. When tested, this truncating mutation abolished DNA damage–induced Rad51 foci formation. In addition, we have identified 15 somatic mutations in 2 tumors from 1 patient carrying germline mutation c.-37A>T. The presence of a somatic mutation on the wild-type allele showed that GT198 was biallelically mutated in the tumor. The somatic mutations identified near a splicing junction site caused defective alternative splicing and truncated the open reading frame. Therefore, distinct mutations may cause a similar consequence by truncating the full-length protein and inducing a loss of the wild type. Our study provides the first evidence of the presence of inactivating mutations in GT198 in familial and early-onset breast and ovarian cancer patients. Mutations in GT198, a gene regulating DNA repair, potentially contribute to an increased risk in familial breast and ovarian cancers.
GT198; mutation; gene amplification; breast and ovarian cancer
Adenosine monophosphate–activated protein kinase (AMPK) is a metabolic regulator that promotes energy conservation and restoration when cells are exposed to nutrient stress. Given the high metabolic requirement of cancer cells, AMPK activation has been suggested as a potential preventative and therapeutic target. However, previous findings have shown that AMPK activity is diminished in some cancers. Expression of the 2 catalytic isoforms, AMPKα1 and AMPKα2, was evaluated in primary breast cancer and matched nontumor-adjacent tissue samples using immunohistochemistry. AMPK-dependent growth signaling events were examined in primary human mammary epithelial cells (HMECs) using RNAi to understand the importance of AMPKα2 in normal growth regulation. To test whether AMPKα2 would reinstate growth control and apoptotic mechanisms in breast cancer cells, metabolic stress assays and tumor xenografts were performed in MCF-7 cells, expressing low levels of AMPKα2, with stable transfection of either green fluorescent protein (GFP) or AMPKα2 expression constructs. AMPKα2 was found to be significantly suppressed in breast cancer tissue samples, whereas AMPKα1 was not. In normal HMECs, low glucose stress resulted in AMPK-driven growth inhibition. Interestingly, this response was ablated when AMPKα2 was silenced. Metabolic stress assays in MCF-7 cells indicated that AMPKα2 expression reduced both mTOR signaling and cyclin D1 expression, contributing to G1-phase cell cycle arrest. Cells expressing AMPKα2 underwent apoptosis more readily than GFP control cells. Xenograft studies demonstrated that MCF-7 tumors expressing AMPKα2 display reduced proliferation and increased apoptotic events. Furthermore, AMPKα2 xenografts exhibited diminished cyclin D1 levels along with an increased amount of nuclear p53, thereby implicating the AMPKα2-p53 signaling axis as a mediator of cell apoptosis. Together, these results highlight the significance of reduced AMPK activity contributing to human carcinogenesis and, specifically, the role of AMPKα2 with respect to its control of normal mammary epithelial cell growth and its reduced expression in breast cancer.
breast cancer; AMPK; apoptosis; cell cycle arrest; p53
Alternative pre-mRNA splicing yields functionally distinct splice variants in regulating normal cell differentiation as well as cancer development. The putative tumor suppressor gene GT198 (PSMC3IP), encoding a protein also known as TBPIP and Hop2, has been shown to regulate steroid hormone receptor–mediated transcription and to stimulate homologous recombination in DNA repair. Here, we have identified 6 distinct GT198 splice variant transcripts generated by alternative promoter usage or alternative splicing. Various splice variant transcripts preserve a common open reading frame, which encodes the DNA binding domain of GT198. The splice variants act as dominant negatives to counteract wild-type GT198 activity in transcription and to abolish Rad51 foci formation during radiation-induced DNA damage. In fallopian tube cancer, we have identified 44 point mutations in GT198 clustered in 2 mutation hotspot sequences. The mutation hotspots coincide with the regulatory sequences responsible for alternative splicing, strongly supporting that imbalanced alternative splicing is a selected consequence in cancer. In addition, splice variant–associated cytoplasmic expression is found in tumors carrying germline or somatic GT198 mutations. An altered alternative splicing pattern with increased variants is also present in lymphoblastoid cells derived from familial breast cancer patients carrying GT198 germline mutations. Furthermore, GT198 and its variant are reciprocally expressed during mouse stem cell differentiation. The constitutive expression of the GT198 variant but not the wild type induces tumor growth in nude mice. Our results collectively suggest that mutations in the GT198 gene deregulate alternative splicing. Defective alternative splicing promotes antagonizing variants and in turn induces a loss of the wild type in tumorigenesis. The study highlights the role of alternative splicing in tumor suppressor gene inactivation.
alternative splicing; GT198; tumor suppressor gene; somatic mutation; DNA repair