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Exon-level transcriptome profiling in murine breast cancer reveals splicing changes specific to tumors with different metastatic abilities

ABSTRACT: BACKGROUND: Breast cancer is the second most frequent type of cancer affecting women. We are increasingly aware that changes in mRNA splicing are associated with various characteristics of cancer. The most deadly aspect of cancer is metastasis, the process by which cancer spreads from the primary tumor to distant organs. However, little is known specifically about the involvement of alternative splicing in the formation of macroscopic metastases. Our study investigates transcript isoform changes that characterize tumors of different abilities to form growing metastases. METHODS AND FINDINGS: To identify alternative splicing events (ASEs) that are associated with the fully metastatic phenotype in breast cancer, we used Affymetrix Exon Microarrays to profile mRNA isoform variations genome-wide in weakly metastatic (168FARN and 4T07) and highly metastatic (4T1) mammary carcinomas. Statistical analysis identified significant expression changes in 7606 out of 155,994 (4%) exons and in 1725 out of 189,460 (1%) intronic regions, which affect 2623 out of 16,654 (16%) genes. These changes correspond to putative alternative isoforms-several of which are novel-that are differentially expressed between tumors of varying metastatic phenotypes. Gene pathway analysis showed that 1224 of genes expressing alternative isoforms were involved in cell growth, cell interactions, cell proliferation, cell migration and cell death and have been previously linked to cancers and genetic disorders. We chose ten predicted splice variants for RT-PCR validation, eight of which were successfully confirmed (MED24, MFI2, SRRT, CD44, CLK1 and HNRNPH1). These include three novel intron retentions in CD44, a gene in which isoform variations have been previously associated with the metastasis of several cancers. CONCLUSION: Our findings reveal that various genes are differently spliced and/or expressed in association with the metastatic phenotype of tumor cells. Identification of metastasis-specific isoforms may contribute to the development of improved breast cancer stage identification and targeted therapies.

ORGANISM(S): Mus musculus

PROVIDER: GSE21994 | GEO | 2010/05/29

SECONDARY ACCESSION(S): PRJNA127213

REPOSITORIES: GEO

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Project description:Background:Breast cancer is the second most frequent type of cancer affecting women. We are increasingly aware that changes in mRNA splicing are associated with various characteristics of cancer. The most deadly aspect of cancer is metastasis, the process by which cancer spreads from the primary tumor to distant organs. However, little is known specifically about the involvement of alternative splicing in the formation of macroscopic metastases. Our study investigates transcript isoform changes that characterize tumors of different abilities to form growing metastases. Results:To identify alternative splicing events (ASEs) that are associated with the fully metastatic phenotype in breast cancer, we used Affymetrix Exon Microarrays to profile mRNA isoform variations genome-wide in weakly metastatic (168FARN and 4T07) and highly metastatic (4T1) mammary carcinomas. Statistical analysis identified significant expression changes in 7606 out of 155,994 (4%) exons and in 1725 out of 189,460 (1%) intronic regions, which affect 2623 out of 16,654 (16%) genes. These changes correspond to putative alternative isoforms - several of which are novel - that are differentially expressed between tumors of varying metastatic phenotypes. Gene pathway analysis showed that 1224 of genes expressing alternative isoforms were involved in cell growth, cell interactions, cell proliferation, cell migration and cell death and have been previously linked to cancers and genetic disorders. We chose ten predicted splice variants for RT-PCR validation, eight of which were successfully confirmed (MED24, MFI2, SRRT, CD44, CLK1 and HNRNPH1). These include three novel intron retentions in CD44, a gene in which isoform variations have been previously associated with the metastasis of several cancers. Conclusions:Our findings reveal that various genes are differently spliced and/or expressed in association with the metastatic phenotype of tumor cells. Identification of metastasis-specific isoforms may contribute to the development of improved breast cancer stage identification and targeted therapies. Keywords: Seek pre-mRNA changes associated with the fully metastatic phenotype in breast cancer We used RNA tumor tissues derived from three murine mammary carcinoma cell lines (168FARN, 4T07 and 4T1); four biological replicates of 168FARN, four biological replicates of 4T07, and four biological replicates of 4T1 were hybridized independently at McGill university site.

Project description:Vascular endothelial growth factor is a multifunctional cytokine playing important roles in angiogenesis, tumor progression and metastasis. Alternative splicing results in the production of several different isoforms of VEGF. We have previously generated human breast cancer cells overexpressing VEGF165 or VEGF189 isoforms (referred to as the V165 and V189 clones, respectively) and showed that VEGF189-transfected cells were less tumorigenic. In this study, we used bioluminescence imaging to analyze the metastasis capacity of breast cancer cell lines (MDA-MB-321) overexpressing VEGF isoforms in nude mice. V165, V189 and control cV clones were transfected with a luciferase plasmid to generate bioluminescent clones (the V165-B, V189-B and cV clones, respectively). These clones were then injected into the left heart ventricle of nude mice. Analysis of the location of metastases shows that V189-B cells induced fewer metastases in lung and bone than V165-B and cV-B cells. Moreover, there was a delay on metastasis appearance in mice receiving VEGF V189-B clone, consistent with increase survival in these mice. We investigated the possible mechanisms underlying these effects and found that VEGF189 increased adhesion and decreased cell invasion and survival in vitro. In the other side, transcriptomic analyses shown expression of genes, implicated in breast cancer and metastatic dissemination, in VEGF165 overexpressing cells. After in silico analyses of genes differentially expressed between V189 and V165 cells,we used Q-PCR assays to quantify mRNA levels of some gene of interest in 120 breast tumors from patients with or without metastasis and/or specially lung metastasis. We found that genes have prognostical significance and /or showed an influence on relapse-free survival. These data provide the first evidence that different VEGF isoforms have different effects on breast cancer cell line invasion and colonization. Human MDA-MB-231 breast cancer cells were used to generate stable transfected clones overexpressing VEGF165 isoform, VEGF189 isoform, or control vector (referred to as 42ctl8, 13ctl3 and PCIctl3, respectively). Analysis of the metastasis capacity of these clones in nude mice.

Project description:Tumor-initiating cells with reprogramming plasticity and/or de-differentiation attributes have been thought to initiate primary tumor development as well as to regenerate secondary tumors in metastatic organs; however, the molecular mechanisms are not fully understood. We previously found that breast tumor-initiating cell marker, CD44, directs multicellular aggregation and cluster formation of circulating tumor cells (CTCs), which further enhance stemness and survival of such cells, enabling metastatic colonization to the lungs. To further elucidate the molecular network underlying CTC cluster formation, we performed global proteomic profiling and discovered that the tetraspanin protein CD81, which is normally enriched in exosomes (small extracellular vesicles), is a new driver of cancer initiation and metastasis as a facilitator and target of CD44. Loss of CD81 compromises tumorigenicity and mammosphere formation of triple negative breast cancer (TNBC) cells. Assisted by machine learning-based algorithms and mutagenesis approach, we found that CD81 interacts with CD44 on the cellular membrane through their extracellular regions. Notably, genetic knockout of CD44 or CD81 results in loss of both CD81 and CD44 in secreted exosomes, a state which abolishes exosome-induced self-renewal of recipient cells, such as mammosphere formation. In addition, RNA sequencing analysis showed that CD81 knockdown up-regulates expression of a cell differentiation marker, SEMA7a, whose down-regulation partially rescues mammosphere formation inhibition by CD81 depletion. Clinically, CD81 expression was observed in >80% of CTCs and specifically enriched and co-expressed along with CD44 in clustered CTCs of breast cancer patients. Mimicing the phenotypes of CD44 deficiency, loss of CD81 also inhibited tumor cell aggregation and lung metastasis of TNBC in both human and mouse tumor models, supporting the clinical significance of CD81 in association with patient outcomes. Our study highlights a new driving role of CD81 in cancer exosome-induced stemness, clustered CTCs, and metastasis initiation of TNBC, reported for the first time to our knowledge.

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Exon-level transcriptome profiling in murine breast cancer reveals splicing changes specific to tumors with different metastatic abilities

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