Links between chromatin structure and regulation of alternative pre-mRNA splicing in mammalian cells

Intron removal is a necessary step for expression of most genes in higher eukaryotes, and alternative splice selection is a highly regulated mechanism that endows a single gene with the possibility to codify for multiple transcripts. Pre-mRNA splicing occurs largely co-transcriptionally, and its outcome is influenced by transcription elongation and chromatin structure. In this thesis we have used two different approaches to study novel links between chromatin structure and alternative splicing regulation. In the first approach, we identified genome-wide progesterone-regulated cassette exons and compared them with nucleosome density profiles, with the aim of finding correlations between changes in nucleosome positioning and changes in alternative splicing. We find that, even if all exons harbor a well-positioned exonic nucleosome, four different classes of nucleosome density profiles can be identified around alternative exons, which strongly correlate with the DNA sequence GC content. Transitions between these profiles occur upon hormone stimulation and can be correlated with alternative splicing changes, although changes in nucleosome profiles are also observed in non-regulated exons. In particular hormone-induced exon inclusion is more frequently linked to changes in nucleosome density than hormone-induced skipped exons, which tend to have low nucleosome density profiles even before hormone treatment. Peaks of nucleosome density before alternative exons tend to correlate with exon inclusion. In the second approach, we took advantage of ENCODE data of chromatin epigenetic signatures and RNA-Seq in multiple cell lines to evaluate functional enrichment of histone modifications over alternative exons. We find that three histone modifications (H3K4me3, H3K27ac and H3K9ac) co-occur in a subset of exons when they are highly included. These features are sufficient to predict differential inclusion levels in other cell lines. Moreover, they are enriched in exons characterized by the presence of DNase hypersensitive sites, promoter signatures and RNA Pol II accumulation. These observations suggest a functional role for 3-dimensional genome structure in the regulation of alternative splicing.La eliminación de intrones es un paso necesario para expresar la mayoría de los genes en eucariotas superiores. La selección alternativa del corte y empalme (pre-mRNA splicing) es un mecanismo altamente regulado que dota a un solo gen con la posibilidad de codificar para múltiples transcritos. El splicing del pre-mRNA se produce en gran parte de manera cotranscripcional y por esto resultado está influenciado por la elongación de la transcripción y la estructura de la cromatina. En esta tesis se han utilizado dos enfoques diferentes para estudiar nuevos vínculos entre la estructura de la cromatina y la regulación del splicing alternativo. En el primer enfoque hemos identificado, a nivel de todo el genoma, exons internos regulados por progesterona y los hemos comparados con los perfiles de densidad de nucleosomas, con el objetivo de encontrar correlaciones entre los cambios en el posicionamiento de nucleosomas y en el splicing alternativo. Hemos encontrado que, aunque todos los exones albergan un nucleosoma bien posicionado, se pueden identificar cuatro clases diferentes de perfiles de densidad de nucleosomas alrededor de exones alternativos, que se correlacionan fuertemente con el contenido G+C en la secuencia del ADN. Las transiciones entre estos perfiles se producen tras la estimulación con la hormona y se pueden correlacionar con los cambios en splicing alternativo, aunque se observan también cambios en los perfiles de nucleosomas en exones no regulados. La inclusión de exones inducida por la hormona está relacionada más a menudo con cambios en la densidad de nucleosomas que la exclusión. Estos exones excluidos tienden a tener perfiles de baja densidad nucleosomal incluso antes del tratamiento hormonal. Los picos de densidad de nucleosomas antes de exones alternativos tienden a correlacionarse con la exclusión de exones. En el segundo enfoque nos aprovechamos de los datos de ENCODE de marcas epigenéticas de la cromatina y de RNA-Seq en múltiples líneas celulares, para evaluar el enriquecimiento funcional de las modificaciones de histonas en exones alternativos. Encontramos que tres modificaciones de las histonas (H3K4me3, H3K27ac y H3K9ac) co-ocurren en un subconjunto de exones mas incluidos. Estas características son suficientes para predecir los niveles de inclusión diferenciales de estos exones entre líneas celulares. Además, estos exones se caracterizan también por la presencia de sitios hipersensibles a la DNasa, de marcas de promotores y la acumulación de ARN Pol II . Estas observaciones sugieren un papel funcional para la estructura en 3 dimensiones del genoma en la regulación del splicing alternativo

Role of six single nucleotide polymorphisms, risk factors in coronary disease, in OLR1 alternative splicing

The OLR1 gene encodes the oxidized low-density lipoprotein receptor (LOX-1), which is responsible for the cellular uptake of oxidized LDL (Ox-LDL), foam cell formation in atheroma plaques and atherosclerotic plaque rupture. Alternative splicing (AS) of OLR1 exon 5 generates two protein isoforms with antagonistic functions in Ox-LDL uptake. Previous work identified six single nucleotide polymorphisms (SNPs) in linkage disequilibrium that influence the inclusion levels of OLR1 exon 5 and correlate with the risk of cardiovascular disease. Here we use minigenes to recapitulate the effects of two allelic series (Low- and High-Risk) on OLR1 AS and identify one SNP in intron 4 (rs3736234) as the main contributor to the differences in exon 5 inclusion, while the other SNPs in the allelic series attenuate the drastic effects of this key SNP. Bioinformatic, proteomic, mutational and functional high-throughput analyses allowed us to define regulatory sequence motifs and identify SR protein family members (SRSF1, SRSF2) and HMGA1 as factors involved in the regulation of OLR1 AS. Our results suggest that antagonism between SRSF1 and SRSF2/HMGA1, and differential recognition of their regulatory motifs depending on the identity of the rs3736234 polymorphism, influence OLR1 exon 5 inclusion and the efficiency of Ox-LDL uptake, with potential implications for atherosclerosis and coronary disease

Role of six single nucleotide polymorphisms, risk factors in coronary disease, in OLR1 alternative splicing

The OLR1 gene encodes the oxidized low-density lipoprotein receptor (LOX-1), which is responsible for the cellular uptake of oxidized LDL (Ox-LDL), foam cell formation in atheroma plaques and atherosclerotic plaque rupture. Alternative splicing (AS) of OLR1 exon 5 generates two protein isoforms with antagonistic functions in Ox-LDL uptake. Previous work identified six single nucleotide polymorphisms (SNPs) in linkage disequilibrium that influence the inclusion levels of OLR1 exon 5 and correlate with the risk of cardiovascular disease. Here we use minigenes to recapitulate the effects of two allelic series (Low- and High-Risk) on OLR1 AS and identify one SNP in intron 4 (rs3736234) as the main contributor to the differences in exon 5 inclusion, while the other SNPs in the allelic series attenuate the drastic effects of this key SNP. Bioinformatic, proteomic, mutational and functional high-throughput analyses allowed us to define regulatory sequence motifs and identify SR protein family members (SRSF1, SRSF2) and HMGA1 as factors involved in the regulation of OLR1 AS. Our results suggest that antagonism between SRSF1 and SRSF2/HMGA1, and differential recognition of their regulatory motifs depending on the identity of the rs3736234 polymorphism, influence OLR1 exon 5 inclusion and the efficiency of Ox-LDL uptake, with potential implications for atherosclerosis and coronary disease

Erratum to: Promoter-like epigenetic signatures in exons displaying cell type-specific splicing

After the publication of this work [1] an error was noticed in Fig. 7. The panel‘h’ is missing from Fig. 7. Please see the corrected figure below. The publisher apologises for this error

Promoter-like epigenetic signatures in exons displaying cell type-specific splicing

Background. Pre-mRNA splicing occurs mainly co-transcriptionally, and both nucleosome density and histone modifications have been proposed to play a role in splice site recognition and regulation. However, the extent and mechanisms behind this interplay remain poorly understood./nResults. We use transcriptomic and epigenomic data generated by the ENCODE project to investigate the association between chromatin structure and alternative splicing. We find a strong and significant positive association between H3K9ac, H3K27ac, H3K4me3, epigenetic marks characteristic of active promoters, and exon inclusion in a small but well-defined class of exons, representing approximately 4 % of all regulated exons. These exons are systematically maintained at comparatively low levels of inclusion across cell types, but their inclusion is significantly enhanced in particular cell types when in physical proximity to active promoters./nConclusion. Histone modifications and other chromatin features that activate transcription can be co-opted to participate in the regulation of the splicing of exons that are in physical proximity to promoter regions.We acknowledge support of the Spanish Ministry of Economy and Competitiveness, ‘Centro de Excelencia Severo Ochoa 2013-2017’, SEV-2012-0208. JC was supported by a SFRH/BD/33535/2008 from the Portuguese Foundation to Science and Technology. CI was supported by a La Caixa predoctoral fellowship. Work in JV’s lab was supported by Fundación Botín, by Banco de Santander through its Santander Universities Global Division and by Consolider RNAREG, MINECO, and AGAUR. We thank Anshul Kundaje, Ben Brown, Michael Snyder, Thomas Gingeras, and Alberto Kornblihtt for useful discussions and access to data, and Romina Garrido for administrative assistance

Relationship between nucleosome positioning and progesterone-induced alternative splicing in breast cancer cells

Splicing of mRNA precursors can occur cotranscriptionally and it has been proposed that chromatin structure influences splice site recognition and regulation. Here we have systematically explored potential links between nucleosome positioning and alternative splicing regulation upon progesterone stimulation of breast cancer cells. We confirm preferential nucleosome positioning in exons and report four distinct profiles of nucleosome density around alternatively spliced exons, with RNA polymerase II accumulation closely following nucleosome positioning. Hormone stimulation induces switches between profile classes, correlating with a subset of alternative splicing changes. Hormone-induced exon inclusion often correlates with higher nucleosome occupancy at the exon or the preceding intronic region and with higher RNA polymerase II accumulation. In contrast, exons skipped upon hormone stimulation display low nucleosome densities even before hormone treatment, suggesting that chromatin structure primes alternative splicing regulation. Skipped exons frequently harbor binding sites for hnRNP AB, a hormone-induced splicing regulator whose knock down prevents some hormone-induced skipping events. Collectively, our results argue that a variety of chromatin architecture mechanisms can influence alternative splicing decisions

Relationship between nucleosome positioning and progesterone-induced alternative splicing in breast cancer cells

Splicing of mRNA precursors can occur cotranscriptionally and it has been proposed that chromatin structure influences splice site recognition and regulation. Here we have systematically explored potential links between nucleosome positioning and alternative splicing regulation upon progesterone stimulation of breast cancer cells. We confirm preferential nucleosome positioning in exons and report four distinct profiles of nucleosome density around alternatively spliced exons, with RNA polymerase II accumulation closely following nucleosome positioning. Hormone stimulation induces switches between profile classes, correlating with a subset of alternative splicing changes. Hormone-induced exon inclusion often correlates with higher nucleosome occupancy at the exon or the preceding intronic region and with higher RNA polymerase II accumulation. In contrast, exons skipped upon hormone stimulation display low nucleosome densities even before hormone treatment, suggesting that chromatin structure primes alternative splicing regulation. Skipped exons frequently harbor binding sites for hnRNP AB, a hormone-induced splicing regulator whose knock down prevents some hormone-induced skipping events. Collectively, our results argue that a variety of chromatin architecture mechanisms can influence alternative splicing decisions

Co-option of the piRNA pathway for germline-specific alternative splicing of C. elegans TOR

Includes supplementary materials for the online appendix.Many eukaryotic genes contain embedded antisense transcripts and repetitive sequences of unknown function. We report that male germline-specific expression of an antisense transcript contained in an intron of C. elegans Target of Rapamycin (TOR, let-363) is associated with (1) accumulation of endo-small interfering RNAs (siRNAs) against an embedded Helitron transposon and (2) activation of an alternative 3′ splice site of TOR. The germline-specific Argonaute proteins PRG-1 and CSR-1, which participate in self/nonself RNA recognition, antagonistically regulate the generation of these endo-siRNAs, TOR mRNA levels, and 3′ splice-site selection. Supply of exogenous double-stranded RNA against the region of sense/antisense overlap reverses changes in TOR expression and splicing and suppresses the progressive multigenerational sterility phenotype of prg-1 mutants. We propose that recognition of a “nonself” intronic transposon by endo-siRNAs/the piRNA system provides physiological regulation of expression and alternative splicing of a host gene that, in turn, contributes to the maintenance of germline function across generations.We thank A. Fire, S. Ahmed, and E. Miska for sharing information and helpful discussions, members of our laboratories for critical feedback, the Caenorhabditis Genetics Center (CGC) for providing strains, Matthew Ragle for collection of sperm samples, Alan Zahler for assistance and critical feedback, and Montserrat Porta de la Riva for technical assistance. S.B.-S. was supported by an EMBO postdoctoral fellowship. A.T.L. is a Taub Fellow, supported by the Taub Foundation. J.C. is a Miguel Servet Junior Investigator. Work in J.V.’s lab is supported by Fundación Botín, Consolider RNAREG and Ministerio de Economía y Competitividad (MINECO). Work in B.L.’s lab is supported by the European Research Council (ERC), MINECO Plan Nacional grants BFU2008-00365 and BFU2011-26206, ERASysBio+ ERANET project EUI2009-04059 GRAPPLE, the EMBO Young Investigator Program, EU Framework 7 project 277899 4DCellFate, the AXA Research Fund, and the EMBL/CRG Systems Biology Program. Support from the CRG Genomics Facility is also acknowledged