Locus specific epigenetic modalities of random allelic expression imbalance

Most autosomal genes are thought to be expressed from both alleles, with some notable exceptions, including imprinted genes and genes showing random monoallelic expression (RME). The extent and nature of RME has been the subject of debate. Here we investigate the expression of several candidate RME genes in F1 hybrid mouse cells before and after differentiation, to define how they become persistently, monoallelically expressed. Clonal monoallelic expression is not present in embryonic stem cells, but we observe high frequencies of monoallelism in neuronal progenitor cells by assessing expression status in more than 200 clones. We uncover unforeseen modes of allelic expression that appear to be gene-specific and epigenetically regulated. This non-canonical allelic regulation has important implications for development and disease, including autosomal dominant disorders and opens up therapeutic perspectives.

Allele-resolved single-cell multi-omics uncovers the dynamics and transcriptional kinetics of X-chromosome upregulation

X-chromosome inactivation (XCI) and upregulation (XCU) are the major opposing chromosome-wide modes of gene regulation that collectively achieve dosage compensation in mammals, but the regulatory link between the two remains elusive. Here, we use allele-resolved single-cell RNA-seq combined with chromatin accessibility profiling to finely dissect the separate effects of XCI and XCU on RNA levels during mouse development. We uncover that balanced X dosage is flexibly attained through expression tuning by XCU in a sex- and lineage-specific manner along varying degrees of XCI and across developmental and cellular states. Male blastomeres achieve XCU upon zygotic genome activation while females experience two distinct waves of XCU, upon imprinted- and random XCI, and ablation of Xist impedes female XCU. Contrary to widely established models of mammalian dosage compensation, naïve female embryonic cells carrying two active X chromosomes do not exhibit upregulation but express both alleles at basal level, yet collectively exceeding the RNA output of a single hyperactive allele. We show, in vivo and in vitro, that XCU is kinetically driven by X-specific modulation of transcriptional burst frequency, coinciding with increased compartmentalization of the hyperactive allele. Altogether, our data provide unprecedented insights into the dynamics of mammalian XCU, prompting a revised model of the chain in events of allelic regulation by XCU and XCI in unitedly achieving stable cellular levels of X-chromosome transcripts.

Locus specific epigenetic modalities of random allelic expression imbalance

ABSTRACTMost autosomal genes are thought to be expressed from both alleles, with some notable exceptions, including imprinted genes and genes showing random monoallelic expression (RME). The extent and nature of RME has been the subject of debate. Here we investigate the expression of several candidate RME genes in F1 hybrid mouse cells before and after differentiation, to define how they become persistently, monoallelically expressed. Clonal monoallelic expression was not observed in ESCs, but when we assessed expression status in more than 200 clones of neuronal progenitor cells, we observed high frequencies of monoallelism. We uncovered unforeseen modes of allelic expression that appear to be gene-specific and epigenetically regulated. This non-canonical allelic regulation has important implications for development and disease, including autosomal dominant disorders and opens up novel therapeutic perspectives.

CTCF Regulates Allelic Expression of Igf2 by Orchestrating a Promoter-Polycomb Repressive Complex 2 Intrachromosomal Loop

ABSTRACT CTCF is a zinc finger DNA-binding protein that regulates the epigenetic states of numerous target genes. Using allelic regulation of mouse insulin-like growth factor II (Igf2) as a model, we demonstrate that CTCF binds to the unmethylated maternal allele of the imprinting control region (ICR) in the Igf2/H19 imprinting domain and forms a long-range intrachromosomal loop to interact with the three clustered Igf2 promoters. Polycomb repressive complex 2 is recruited through the interaction of CTCF with Suz12, leading to allele-specific methylation at lysine 27 of histone H3 (H3-K27) and to suppression of the maternal Igf2 promoters. Targeted mutation or deletion of the maternal ICR abolishes this chromatin loop, decreases allelic H3-K27 methylation, and causes loss of Igf2 imprinting. RNA interference knockdown of Suz12 also leads to reactivation of the maternal Igf2 allele and biallelic Igf2 expression. CTCF and Suz12 are coprecipitated from nuclear extracts with antibodies specific for either protein, and they interact with each other in a two-hybrid system. These findings offer insight into general epigenetic mechanisms by which CTCF governs gene expression by orchestrating chromatin loop structures and by serving as a DNA-binding protein scaffold to recruit and bind polycomb repressive complexes.