Olga Anczuków

The splicing factor SRSF1 regulates apoptosis and proliferation to promote mammary epithelial cell transformation

The splicing-factor oncoprotein SRSF1 (also known as SF2/ASF or ASF/SF2) is upregulated in breast cancers. We investigated the ability of SRSF1 to transform human and mouse mammary epithelial cells in vivo and in vitro. SRSF1-overexpressing COMMA-1D cells formed tumors, following orthotopic transplantation to reconstitute the mammary gland. In three-dimensional (3D) culture, SRSF1-overexpressing MCF-10A cells formed larger acini than control cells, reflecting increased proliferation and delayed apoptosis during acinar morphogenesis. These effects required the first RNA-recognition motif and nuclear functions of SRSF1. SRSF1 overexpression promoted alternative splicing of BIM (also known as BCL2L11) and BIN1 to produce isoforms that lack pro-apoptotic functions and contribute to the phenotype. Finally, SRSF1 cooperated specifically with MYC to transform mammary epithelial cells, in part by potentiating eIF4E activation, and these cooperating oncogenes are significantly coexpressed in human breast tumors. Thus, SRSF1 can promote breast cancer, and SRSF1 itself or its downstream effectors may be valuable targets for the development of therapeutics.

OLego: fast and sensitive mapping of spliced mRNA-Seq reads using small seeds

A crucial step in analyzing mRNA-Seq data is to accurately and efficiently map hundreds of millions of reads to the reference genome and exon junctions. Here we present OLego, an algorithm specifically designed for de novo mapping of spliced mRNA-Seq reads. OLego adopts a multiple-seed-and-extend scheme, and does not rely on a separate external aligner. It achieves high sensitivity of junction detection by strategic searches with small seeds (∼14 nt for mammalian genomes). To improve accuracy and resolve ambiguous mapping at junctions, OLego uses a built-in statistical model to score exon junctions by splice-site strength and intron size. Burrows–Wheeler transform is used in multiple steps of the algorithm to efficiently map seeds, locate junctions and identify small exons. OLego is implemented in C++ with fully multithreaded execution, and allows fast processing of large-scale data. We systematically evaluated the performance of OLego in comparison with published tools using both simulated and real data. OLego demonstrated better sensitivity, higher or comparable accuracy and substantially improved speed. OLego also identified hundreds of novel micro-exons (<30 nt) in the mouse transcriptome, many of which are phylogenetically conserved and can be validated experimentally in vivo. OLego is freely available at http://zhanglab.c2b2.columbia.edu/index.php/OLego.

Isolated pseudo–RNA-recognition motifs of SR proteins can regulate splicing using a noncanonical mode of RNA recognition

Significance Serine/arginine (SR) proteins are key regulators of eukaryotic gene expression and have been associated with multiple human diseases including cancers. Several members of this protein family contain a noncanonical RNA recognition motif (RRM), the pseudo-RRM, for which the mode of RNA recognition is unknown. Here, we solved the structure of SRSF1 pseudo-RRM bound to RNA. It reveals the RNA motif recognized and a very unusual mode of interaction, which is conserved for all the SR proteins containing pseudo-RRMs. Finally, we show that the pseudo-RRM in isolation often is sufficient to regulate splicing, and we reveal its mechanism of action. Serine/arginine (SR) proteins, one of the major families of alternative-splicing regulators in Eukarya, have two types of RNA-recognition motifs (RRMs): a canonical RRM and a pseudo-RRM. Although pseudo-RRMs are crucial for activity of SR proteins, their mode of action was unknown. By solving the structure of the human SRSF1 pseudo-RRM bound to RNA, we discovered a very unusual and sequence-specific RNA-binding mode that is centered on one α-helix and does not involve the β-sheet surface, which typically mediates RNA binding by RRMs. Remarkably, this mode of binding is conserved in all pseudo-RRMs tested. Furthermore, the isolated pseudo-RRM is sufficient to regulate splicing of about half of the SRSF1 target genes tested, and the bound α-helix is a pivotal element for this function. Our results strongly suggest that SR proteins with a pseudo-RRM frequently regulate splicing by competing with, rather than recruiting, spliceosome components, using solely this unusual RRM.

Splicing-factor alterations in cancers

Tumor-associated alterations in RNA splicing result either from mutations in splicing-regulatory elements or changes in components of the splicing machinery. This review summarizes our current understanding of the role of splicing-factor alterations in human cancers. We describe splicing-factor alterations detected in human tumors and the resulting changes in splicing, highlighting cell-type-specific similarities and differences. We review the mechanisms of splicing-factor regulation in normal and cancer cells. Finally, we summarize recent efforts to develop novel cancer therapies, based on targeting either the oncogenic splicing events or their upstream splicing regulators.

Abstract 1: Role of the splicing factor SF2/ASF in mammary epithelial cell transformation

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC Cancer cells often display aberrant profiles of alternative splicing, leading to the production of isoforms that can stimulate cell proliferation and migration or improve resistance to apoptosis. Some of these alterations are caused by mutations at splice sites or in splicing-regulatory elements, but changes can also be caused by alterations in the expression of proteins involved in splice-site selection. Changes in the expression of splicing factors have been reported in different types of cancers, such as colon, ovarian, and breast. Our lab recently demonstrated that the splicing factor SF2/ASF can be oncogenic and is often upregulated in human cancers. Several studies have suggested a role for splicing factors in the development of breast tumors in both human and mice. Thus, our goal is to examine the role of splicing factors in transformation and tumor maintenance of mammary epithelial cells. We have analyzed the ability of the splicing factor SF2/ASF to promote transformation of MCF-10A human mammary epithelial cells, which form organized acinar structures in three-dimensional (3D) culture. Cells slightly overexpressing SF2/ASF form significantly larger acini than the control, but retain a proper acinar organization with a hollow lumen. They show increased proliferation and delayed apoptosis in 3D culture, and can also form colonies in soft agar, a measure of anchorage-independent growth. Interestingly, these features require the first RNA-recognition motif (RRM1) of SF2/ASF, as a deletion mutant lacking this motif has the same phenotype as the control. Furthermore, we find that SF2/ASF can cooperate with the myc oncogene but not with ErbB2 to transform mammary epithelial cells grown in 3-D, and also to promote anchorage-independent growth in soft agar. In contrast, when SF2/ASF is overexpressed together with the HPV16 E7 oncogene, MCF-10A acini are smaller than the controls, suggesting competitive pathways. We are currently investigating if epithelial cells overexpressing SF2/ASF alone or with the above oncogenes can form tumors in a xenograft model, after transplantation into epithelium-free fat pads of three-week-old mice. We are also investigating the signaling pathways involved in SF2/ASF-mediated transformation, focusing on mTOR signaling, as well as on the isoforms of the downstream kinase S6K1, which has been linked to SF2/ASFs oncogenic properties in murine fibroblasts. In addition, we are in the process of identifying the splicing targets of SF2/ASF in MCF-10A in 3D culture. This study will allow us to assess the role of SF2/ASF in breast cancer. By identifying oncogenic splicing factors and their specific targets, we hope in the future to be able to contribute to the development of alternative cancer therapies based on modulating the expression or activity of these factors or their targets. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1.

The spliceosome, a potential Achilles heel of MYC-driven tumors

Alterations in RNA splicing are frequent in human tumors. Two recent studies of lymphoma and breast cancer have identified components of the spliceosome — the core splicing machinery — that are essential for malignant transformation driven by the transcription factor MYC. These findings provide a direct link between MYC and RNA splicing deregulation, and raise the exciting possibility of targeting spliceosome components in MYC-driven tumors.

Abstract A50: Nonredundant functions of splicing factors in breast-cancer initiation and metastasis

Alternative splicing is a key control point in gene expression, whose misregulation contributes to cancer malignancy. Although certain splicing factors (SFs) and their targets are altered in human tumors, the functional significance of these alterations remains unclear. We previously demonstrated that the splicing factor SRSF1 is upregulated in human breast tumors and promotes transformation in vivo and in vitro. SRSF1 is a prototypical member of the SR protein family, composed of 12 structurally related proteins. However, little is known about differences and redundancies in their splicing targets and biological functions. Here, we investigated whether additional SFs also promoted breast cancer, using transformation models that mimic the relevant biological context. In parallel, we used RNA sequencing (RNA-seq) to systematically identify their oncogenic splicing targets. By mining a large collection of human tumors from the TCGA project, we defined the molecular portraits of SFs alterations in breast tumors. We identified five SFs amplified and/or overexpressed in at least 10% of breast tumors. We then used SF-overexpressing human mammary epithelial MCF-10A cells grown in organotypic 3-D culture; these cells form polarized growth-arrested acinar structures, similar to the terminal units of mammary ducts. Various breast-cancer oncogenes are known to disrupt acinar growth and/or architecture. Interestingly, only certain SFs were oncogenic in this context, differentially affecting cell proliferation, apoptosis, or acinar organization, suggesting non-redundant functions. We then characterized the splicing targets relevant for SF-mediated transformation. We developed a bioinformatics pipeline to identify and quantify splicing variation in RNA-seq data. We defined the global repertoire of SF-regulated splicing events in 3-D culture and compared the target specificities of various SR proteins. In addition, we identified splicing targets regulated both in 3-D culture as well as in human breast tumors. Strikingly, SFs that promoted similar phenotypic changes shared a significant number of splicing targets, suggesting that they regulate common genes to promote tumor initiation. Furthermore, specific SFs affected targets previously associated with epithelial to mesenchymal transition, and increased cell migration or invasion. Finally, we uncovered that the splicing regulator TRA2β is required for the maintenance of metastatic properties of human breast-cancer cells in 3-D culture and in mouse orthotopic models. Furthermore, TRA2β levels correlate with increased metastatic incidence in breast cancer patients. Thus TRA2β represent a potential target for therapeutics development. In summary, we gained new insights into the biological functions of SR proteins and identified novel oncogenic SF-regulated splicing events involved in tumor initiation and metastasis. Citation Format: Olga Anczukow, Shipra Das, Kuan-Ting Lin, Jie Wu, Martin Akerman, Senthil K. Muthuswamy, Adrian R. Krainer. Nonredundant functions of splicing factors in breast-cancer initiation and metastasis. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr A50.

Abstract A078: Differential functions of splicing factors in breast cancer initiation and metastasis

Cancer cells display aberrant alternative splicing profiles, leading to the production of isoforms that can increase cell proliferation and migration, resistance to apoptosis, or alter cell metabolism. Recently, recurrent somatic mutations in components of the splicing machinery have been identified in human tumors, raising a new interest in the field and suggesting that alterations in splicing factors are a new hallmark of cancer. Splicing factors elicit changes in splicing in a concentration-dependent manner. Thus, changes in the expression of these proteins, as reported in different types of cancers, can affect the splicing of multiple genes and are likely involved in splicing deregulation in cancer, even in the absence of mutations. Changes in the expression of splicing factors have been reported in various types of cancers, including breast. We demonstrated previously that the splicing factor SRSF1 is upregulated in human breast cancers and can transform mammary epithelial cells in vivo and in vitro. SRSF1 is a prototypical member of the SR protein family, composed of 12 members sharing structural similarities. However, little is known about differences and redundancies in their splicing targets and thus in their specific biological functions. We are now investigating whether additional splicing factors also promote mammary epithelial cell transformation by using models that mimic the correct biological context in which tumors arise. We first identified splicing factors overexpressed at the transcript and protein levels in a large collection of human breast tumors and cancer cell lines. We then compared the ability of 10 selected splicing factors to transform human mammary epithelial MCF-10A cells grown in organotypic 3-D culture. These cells undergo a 16-day differentiation program, forming growth-arrested, hollow acinar structures with polarized architecture, similar to the terminal units of mammary ducts. Various breast cancer oncogenes are known to disrupt acinar growth and/or architecture. We assessed how splicing-factor overexpression affects differentially cell proliferation and apoptosis, as well as cell migration and invasion. Interestingly, only certain splicing factors are oncogenic in this context, suggesting functional differences. Furthermore, specific splicing factors increase cell migration and invasion, without promoting transformation. We are now characterizing the splicing targets relevant for specific splicing-factor-mediated transformation by next-generation RNA-sequencing. Furthermore, we are investigating which splicing factors are necessary or sufficient to promote metastatic properties of human breast cancer cell lines in vitro and in vivo. Finally, we have identified splicing factors that cooperate specifically with the MYC oncogene and are co-expressed with MYC in human breast tumors. In conclusion, by identifying oncogenic splicing factors, their targets and their regulators, we hope to establish novel targets for targeted cancer therapies. Citation Format: Olga Anczukow, Kuan-Ting Lin, Shipra Das, Jie Wu, Martin Akerman, Senthil K. Muthuswamy, Adrian R. Krainer. Differential functions of splicing factors in breast cancer initiation and metastasis. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr A078.

Abstract B53: Differential functions of splicing factors in breast-cancer initiation and metastasis

Cancer cells often display aberrant profiles of alternative splicing, leading to the production of isoforms that can stimulate cell proliferation and migration, increase resistance to apoptosis, or alter cell metabolism. Recently, recurrent somatic mutations in components of the splicing machinery have been identified in human tumors, raising a new interest in the field and suggesting that alterations in splicing factors are a new hallmark of cancer. Splicing factors elicit changes in splicing in a concentration-dependent manner. Thus, changes in the expression of these proteins, as reported in different types of cancers, can affect the splicing of multiple genes and are likely involved in splicing deregulation in cancer, even in the absence of mutations. We previously demonstrated that the splicing factor SRSF19formerly SF2/ASF9 is often upregulated in human breast cancers and can transform mammary epithelial cells in vivo and in vitro. SRSF1 is a prototypical member of the SR protein family, composed of 12 members sharing structural similarities. However, little is known about differences and redundancies in their splicing targets and thus in their specific biological functions. We are now investigating wherever additional splicing factors can promote mammary epithelial cell transformation by using models that mimic the correct biological context in which tumors arise. We have mined data from a large collection of human breast tumors and cancer cell lines to identify splicing factors overexpressed at the transcript and protein levels. We then compared the ability of 10 selected splicing factors to transform human mammary epithelial MCF-10A cells grown in organotypic 3-D culture. Under these conditions, the cells undergo a 16-day differentiation program, forming growth-arrested, hollow acinar structures with polarized architecture, similar to the terminal units of mammary ducts. Various oncogenes associated with breast cancer are known to disrupt acinar growth and/or architecture. We assessed how splicing-factor overexpression affects cell proliferation and apoptosis, as well as cell migration and invasion. Interestingly, only certain splicing factors are oncogenic in this context, suggesting functional differences. Furthermore, specific splicing factors increase cell migration and invasion but are unable to promote transformation. We have demonstrated specificity in splicing-factor mediated transformation, and we are now investigating what are the relevant splicing targets for transformation by several of these oncogenic splicing factors. Furthermore, we are investigating which splicing factors are necessary or sufficient to promote cell invasion and metastatic potential in human breast cancer cell lines in vitro and in vivo. Finally, we have identified splicing factors that cooperate specifically with the MYC oncogene and are significantly co-expressed with MYC in human breast tumors. By identifying oncogenic splicing factors that can promote breast cancer, and their downstream effectors, we hope to establish candidate targets for therapeutics development based on the modulation of oncogenic splicing events and their regulators. Citation Format: Olga Anczukow, Kuan-Ting Lin, Martin Akerman, Shipra Das, Senthil K. Muthuswamy, Adrian Krainer. Differential functions of splicing factors in breast-cancer initiation and metastasis. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr B53.