EPISPOT: an epigenome-driven approach for detecting and interpreting hotspots in molecular QTL studies

We present EPISPOT, a fully joint framework which exploits large panels of epigenetic marks as variant-level information to enhance molecular quantitative trait locus (QTL) mapping. Thanks to a purpose-built Bayesian inferential algorithm, our approach effectively couples simultaneous QTL analysis of thousands of genetic variants and molecular traits genome-wide, and hypothesis-free selection of biologically interpretable marks which directly contribute to the QTL effects. This unified learning approach boosts statistical power and sheds light on the regulatory basis of the uncovered associations. EPISPOT is also tailored to the modelling of trans-acting genetic variants, including QTL hotspots, whose detection and functional interpretation are challenging with standard approaches. We illustrate the advantages of EPISPOT in simulations emulating real-data conditions and in an epigenome-driven monocyte expression QTL study which confirms known hotspots and reveals new ones, as well as plausible mechanisms of action. In particular, based on monocyte DNase-I sensitivity site annotations selected by the method from > 150 epigenetic annotations, we clarify the mediation effects and cell-type specificity of well-known master hotspots in the vicinity of the lyzosyme gene. EPISPOT is radically new in that it makes it possible to forgo the daunting and underpowered task of one-mark-at-a-time enrichment analyses for the prioritisation of QTL hits. Our method can be used to enhance the discovery and functional understanding of signals in QTL problems with all types of outcomes, be they transcriptomic, proteomic, lipidomic, metabolic or clinical.

Epigenetics of Cellular Memory: Insights from the Chromatin Accessibility Landscape of the Mitotic Genome

Normal development requires the coordination of cell cycle progression and gene expression to produce physiologically appropriate cell numbers of various lineages. The concomitant dysregulation of these two cellular programs is central to many malignant and non-malignant hematologic diseases, yet researchers still lack clear, general principles of how intrinsic properties of cell division could influence transcriptional regulation. Mitosis is a unique phase of the cell cycle that dramatically disrupts transcription: chromosomes condense to form microscopically recognizable structures, the nucleus is disassembled, RNA synthesis ceases, and the transcription machinery and many transcription factors are evicted from mitotic chromatin. How cells “remember” tissue-specific transcriptional programs through mitotic divisions remains largely unknown. Some transcription factors, including the erythroid master regulator, GATA1, and certain chromatin features are known to remain associated with DNA during mitosis. These molecular entities have been proposed to serve as mitotic “bookmarks” -- molecules that store gene regulatory information at individual loci through mitosis. However, we have limited knowledge of the composition, mechanism and function of mitotic bookmarks. In this context, chromatin structure deserves special consideration, as chromosome condensation during mitosis could potentially hinder transcription factor binding. To obtain the first genome-wide view of chromatin accessibility during mitosis, we mapped the DNase I sensitivity of the interphase versus mitotic genome in two maturation stages in a murine erythroblast cell line, G1E. Despite microscopic condensation of chromosomes during mitosis, we found that DNase I sensitivity is extensively preserved throughout the mappable genome, indicating that mitotic chromatin is not as condensed as commonly presumed. Individual genes and cis-regulatory elements can maintain all, part of, or none of its interphase accessibility during mitosis, demonstrating that accessibility of mitotic chromatin is locally specified. Promoters generally maintain accessibility during mitosis; moreover, promoters with the highest degree of accessibility preservation in mitosis in G1E cells tend to also be accessible across many murine tissues in interphase. In contrast to promoters, we found that enhancer accessibility is preferentially lost during mitosis, raising the possibility that memory of enhancer regulation may be altered during mitosis. Since enhancers play crucial roles in specifying tissue-specific gene expression patterns, we propose that this phase of the cell cycle may be especially susceptible to resetting of transcriptional programs. This hypothesis is supported by our preliminary results that revealed aberrant RNA polymerase II re-engagement with the genome and transcript production in early G1. Thus, mitosis could be a source of gene expression heterogeneity, with potential implications for cell fate transitions in proliferative cells, such as during stem cell lineage commitment, experimental reprogramming, and tumorigenesis. Disclosures No relevant conflicts of interest to declare.

Tracking chromatin states using controlled DNase I treatment and real-time PCR

A novel approach to DNase I-sensitivity analysis was applied to examining genes of the spermatogenic pathway, reflective of the substantial morphological and genomic changes that occur during this program of differentiation. A new real-time PCR-based strategy that considers the nuances of response to nuclease treatment was used to assess the nuclease susceptibility through differentiation. Data analysis was automated with the K-Lab PCR algorithm, facilitating the rapid analysis of multiple samples while eliminating the subjectivity usually associated with Ct analyses. The utility of this assay and analytical paradigm as applied to nuclease-sensitivity mapping is presented.