Chromatin opening ability of pioneer factor Pax7 depends on unique isoform and C-terminal domain.

Pioneer factors are transcription factors (TFs) that have the unique ability to recognise their target DNA sequences within closed chromatin. Whereas their interactions with cognate DNA is similar to other TFs, their ability to interact with chromatin remains poorly understood. Having previously defined the modalities of DNA interactions for the pioneer factor Pax7, we have now used natural isoforms of this pioneer as well as deletion and replacement mutants to investigate the Pax7 structural requirements for chromatin interaction and opening. We show that the GL+ natural isoform of Pax7 that has two extra amino acids within the DNA binding paired domain is unable to activate the melanotrope transcriptome and to fully activate a large subset of melanotrope-specific enhancers targeted for Pax7 pioneer action. This enhancer subset remains in the primed state rather than being fully activated, despite the GL+ isoform having similar intrinsic transcriptional activity as the GL- isoform. C-terminal deletions of Pax7 lead to the same loss of pioneer ability, with similar reduced recruitments of the cooperating TF Tpit and of the co-regulators Ash2 and BRG1. This suggests complex interrelations between the DNA binding and C-terminal domains of Pax7 that are crucial for its chromatin opening pioneer ability.

Regulatory integration of Hox factor activity with T-box factors in limb development.

In tetrapods, Tbx4, Tbx5 and Hox cluster genes are crucial for forelimb and hindlimb development and mutations in these genes are responsible for congenital limb defects. The molecular basis of their integrated mechanisms of action in the context of limb development remains poorly understood. We studied Tbx4 and Hoxc10 owing to their overlapping loss-of-function phenotypes and colocalized expression in mouse hindlimb buds. We report an extensive overlap between Tbx4 and Hoxc10 genome occupancy and their putative target genes. Tbx4 and Hoxc10 interact directly with each other, have the ability to bind to a previously unrecognized T-box-Hox composite DNA motif and show synergistic activity when acting on reporter genes. Pitx1, the master regulator for hindlimb specification, also shows extensive genomic colocalization with Tbx4 and Hoxc10. Genome occupancy by Tbx4 in hindlimb buds is similar to Tbx5 occupancy in forelimbs. By contrast, another Hox factor, Hoxd13, also interacts with Tbx4/Tbx5 but antagonizes Tbx4/Tbx5-dependent transcriptional activity. Collectively, the modulation of Tbx-dependent activity by Hox factors acting on common DNA targets may integrate different developmental processes for the balanced formation of proportionate limbs.

RNA sequencing reveals the role of splicing polymorphisms in regulating human gene expression.

Expression levels of many human genes are under the genetic control of expression quantitative trait loci (eQTLs). Despite technological advances, the precise molecular mechanisms underlying most eQTLs remain elusive. Here, we use deep mRNA sequencing of two CEU individuals to investigate those mechanisms, with particular focus on the role of splicing control loci (sQTLs). We identify a large number of genes that are differentially spliced between the two samples and associate many of those differences with nearby single nucleotide polymorphisms (SNPs). Subsequently, we investigate the potential effect of splicing SNPs on eQTL control in general. We find a significant enrichment of alternative splicing (AS) events within a set of highly confident eQTL targets discovered in previous studies, suggesting a role of AS in regulating overall gene expression levels. Next, we demonstrate high correlation between the levels of mature (exonic) and unprocessed (intronic) RNA, implying that ∼75% of eQTL target variance can be explained by control at the level of transcription, but that the remaining 25% may be regulated co- or post-transcriptionally. We focus on eQTL targets with discordant mRNA and pre-mRNA expression patterns and use four examples: USMG5, MMAB, MRPL43, and OAS1, to dissect the exact downstream effects of the associated genetic variants.

Sonic hedgehog accelerates DNA replication to cause replication stress promoting cancer initiation in medulloblastoma.

The mechanisms generating cancer-initiating mutations are not well understood. Sonic hedgehog (SHH) pathway activation is frequent in medulloblastoma (MB), with PTCH1 mutations being a common initiating event. Here we investigated the role of the developmental mitogen SHH in initiating carcinogenesis in the cells of origin: granule cell progenitors (GCPs). We delineate a molecular mechanism for tumor initiation in MB. Exposure of GCPs to Shh causes a distinct form of DNA replication stress, increasing both origin firing and fork velocity. Shh promotes DNA helicase loading and activation, with increased Cdc7-dependent origin firing. The S-phase duration is reduced and hyper-recombination occurs, causing copy number neutral loss of heterozygosity-a frequent event at the PTCH1/ptch1 locus. Moreover, Cdc7 inhibition to attenuate origin firing reduces recombination and preneoplastic tumor formation in mice. Therefore, tissue-specific replication stress induced by Shh promotes loss of heterozygosity, which in tumor-prone Ptch1+/- GCPs results in loss of this tumor suppressor-an early cancer-initiating event.

Pioneer and nonpioneer factor cooperation drives lineage specific chromatin opening.

Pioneer transcription factors are characterized by having the unique property of enabling the opening of closed chromatin sites, for implementation of cell fates. We previously found that the pioneer Pax7 specifies melanotrope cells through deployment of an enhancer repertoire, which allows binding of Tpit, a nonpioneer factor that determines the related lineages of melanotropes and corticotropes. Here, we investigate the relation between these two factors in the pioneer mechanism. Cell-specific gene expression and chromatin landscapes are defined by scRNAseq and chromatin accessibility profiling. We find that in vivo deployment of the melanotrope enhancer repertoire and chromatin opening requires both Pax7 and Tpit. In cells, binding of heterochromatin targets by Pax7 is independent of Tpit but Pax7-dependent chromatin opening requires Tpit. The present work shows that pioneer core properties are limited to the ability to recognize heterochromatin targets and facilitate nonpioneer binding. Chromatin opening per se may be provided through cooperation with nonpioneer factors.

Gene expression and isoform variation analysis using Affymetrix Exon Arrays.

BACKGROUND: Alternative splicing and isoform level expression profiling is an emerging field of interest within genomics. Splicing sensitive microarrays, with probes targeted to individual exons or exon-junctions, are becoming increasingly popular as a tool capable of both expression profiling and finer scale isoform detection. Despite their intuitive appeal, relatively little is known about the performance of such tools, particularly in comparison with more traditional 3' targeted microarrays. Here, we use the well studied Microarray Quality Control (MAQC) dataset to benchmark the Affymetrix Exon Array, and compare it to two other popular platforms: Illumina, and Affymetrix U133. RESULTS: We show that at the gene expression level, the Exon Array performs comparably with the two 3' targeted platforms. However, the interplatform correlation of the results is slightly lower than between the two 3' arrays. We show that some of the discrepancies stem from the RNA amplification protocols, e.g. the Exon Array is able to detect expression of non-polyadenylated transcripts. More importantly, we show that many other differences result from the ability of the Exon Array to monitor more detailed isoform-level changes; several examples illustrate that changes detected by the 3' platforms are actually isoform variations, and that the nature of these variations can be resolved using Exon Array data. Finally, we show how the Exon Array can be used to detect alternative isoform differences, such as alternative splicing, transcript termination, and alternative promoter usage. We discuss the possible pitfalls and false positives resulting from isoform-level analysis. CONCLUSION: The Exon Array is a valuable tool that can be used to profile gene expression while providing important additional information regarding the types of gene isoforms that are expressed and variable. However, analysis of alternative splicing requires much more hands on effort and visualization of results in order to correctly interpret the data, and generally results in considerably higher false positive rates than expression analysis. One of the main sources of error in the MAQC dataset is variation in amplification efficiency across transcripts, most likely caused by joint effects of elevated GC content in the 5' ends of genes and reduced likelihood of random-primed first strand synthesis in the 3' ends of genes. These effects are currently not adequately corrected using existing statistical methods. We outline approaches to reduce such errors by filtering out potentially problematic data.

Fusion of TTYH1 with the C19MC microRNA cluster drives expression of a brain-specific DNMT3B isoform in the embryonal brain tumor ETMR.

Embryonal tumors with multilayered rosettes (ETMRs) are rare, deadly pediatric brain tumors characterized by high-level amplification of the microRNA cluster C19MC. We performed integrated genetic and epigenetic analyses of 12 ETMR samples and identified, in all cases, C19MC fusions to TTYH1 driving expression of the microRNAs. ETMR tumors, cell lines and xenografts showed a specific DNA methylation pattern distinct from those of other tumors and normal tissues. We detected extreme overexpression of a previously uncharacterized isoform of DNMT3B originating at an alternative promoter that is active only in the first weeks of neural tube development. Transcriptional and immunohistochemical analyses suggest that C19MC-dependent DNMT3B deregulation is mediated by RBL2, a known repressor of DNMT3B. Transfection with individual C19MC microRNAs resulted in DNMT3B upregulation and RBL2 downregulation in cultured cells. Our data suggest a potential oncogenic re-engagement of an early developmental program in ETMR via epigenetic alteration mediated by an embryonic, brain-specific DNMT3B isoform.

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

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.