Monoallelic Expression Of Autosomal Genes Research Articles

X-Chromosome Inactivation and Autosomal Random Monoallelic Expression as "Faux Amis".

X-chromosome inactivation (XCI) and random monoallelic expression of autosomal genes (RMAE) are two paradigms of gene expression regulation where, at the single cell level, genes can be expressed from either the maternal or paternal alleles. X-chromosome inactivation takes place in female marsupial and placental mammals, while RMAE has been described in mammals and also other species. Although the outcome of both processes results in random monoallelic expression and mosaicism at the cellular level, there are many important differences. We provide here a brief sketch of the history behind the discovery of XCI and RMAE. Moreover, we review some of the distinctive features of these two phenomena, with respect to when in development they are established, their roles in dosage compensation and cellular phenotypic diversity, and the molecular mechanisms underlying their initiation and stability.

Semicoordinated allelic-bursting shape dynamic random monoallelic expression in pregastrulation embryos

SummaryRecently, allele-specific single-cell RNA-seq analysis has demonstrated widespread dynamic random monoallelic expression of autosomal genes (aRME) in different cell types. However, the prevalence of dynamic aRME during pregastrulation remains unknown. Here, we show that dynamic aRME is widespread in different lineages of pregastrulation embryos. Additionally, the origin of dynamic aRME remains elusive. It is believed that independent transcriptional bursting from each allele leads to dynamic aRME. Here, we show that allelic burst is not perfectly independent; instead it happens in a semicoordinated fashion. Importantly, we show that semicoordinated allelic bursting of genes, particularly with low burst frequency, leads to frequent asynchronous allelic bursting, thereby contributing to dynamic aRME. Furthermore, we found that coordination of allelic bursting is lineage specific and genes regulating the development have a higher degree of coordination. Altogether, our study provides significant insights into the prevalence and origin of dynamic aRME and their developmental relevance during early development.

Autosomal Clonal Monoallelic Expression: Natural or Artifactual?

The prevalence of mitotically heritable clonal random monoallelic expression of autosomal genes (aRME) remains controversial. Specifically, presence of clonal aRME is well supported in vitro but remains elusive in vivo. Here, we provide critical insights into this matter and discuss whether prevalent clonal aRME is natural or artifactual.

CTCF-Mediated Genome Architecture Regulates the Dosage of Mitotically Stable Mono-allelic Expression of Autosomal Genes

The mechanisms that guide the clonally stable random mono-allelic expression of autosomal genes remain enigmatic. We show that (1) mono-allelically expressed (MAE) genes are assorted and insulated from bi-allelically expressed (BAE) genes through CTCF-mediated chromatin loops; (2) the cell-type-specific dynamics of mono-allelic expression coincides with the gain and loss of chromatin insulator sites; (3) dosage of MAE genes is more sensitive to the loss of chromatin insulation than that of BAE genes; and (4) inactive alleles of MAE genes are significantly more insulated than active alleles and are de-repressed upon CTCF depletion. This alludes to a topology wherein the inactive alleles of MAE genes are insulated from the spatial interference of transcriptional states from the neighboring bi-allelic domains via CTCF-mediated loops. We propose that CTCF functions as a typical insulator on inactive alleles, but facilitates transcription through enhancer-linking on active allele of MAE genes, indicating widespread allele-specific regulatory roles of CTCF.

Analysis of allelic expression patterns in clonal somatic cells by single-cell RNA-seq.

Cellular heterogeneity can emerge from the expression of only one parental allele. However, it has remained controversial whether, or to what degree, random monoallelic expression of autosomal genes (aRME) is mitotically inherited (clonal) or stochastic (dynamic) in somatic cells, particularly in vivo. Here, we used allele-sensitive single-cell RNA-seq on clonal primary mouse fibroblasts and in vivo human CD8+ T-cells to dissect clonal and dynamic monoallelic expression patterns. Dynamic aRME affected a considerable portion of the cells’ transcriptomes, with levels dependent on the cells’ transcriptional activity. Importantly, clonal aRME was detected but was surprisingly scarce (<1% of genes) and affected mainly the most low-expressed genes. Consequently, the overwhelming portion of aRME occurs transiently within individual cells and patterns of aRME are thus primarily scattered throughout somatic cell populations rather than, as previously hypothesized, confined to patches of clonally related cells.

Random monoallelic expression of autosomal genes: stochastic transcription and allele-level regulation.

Random monoallelic expression (RME) of genes represents a striking example of how stochastic molecular processes can result in cellular heterogeneity. Recent transcriptome-wide studies have revealed both mitotically stable and cell-to-cell dynamic forms of autosomal RME, with the latter presumably resulting from burst-like stochastic transcription. Here, we discuss the distinguishing features of these two forms of RME and revisit literature on their nature, pervasiveness and regulation. Finally, we explore how RME may contribute to phenotypic variation, including the incomplete penetrance and variable expressivity often seen in genetic disease.

A monoallelic-to-biallelic T-cell transcriptional switch regulates GATA3 abundance.

Protein abundance must be precisely regulated throughout life, and nowhere is the stringency of this requirement more evident than during T-cell development: A twofold increase in the abundance of transcription factor GATA3 results in thymic lymphoma, while reduced GATA3 leads to diminished T-cell production. GATA3 haploinsufficiency also causes human HDR (hypoparathyroidism, deafness, and renal dysplasia) syndrome, often accompanied by immunodeficiency. Here we show that loss of one Gata3 allele leads to diminished expansion (and compromised development) of immature T cells as well as aberrant induction of myeloid transcription factor PU.1. This effect is at least in part mediated transcriptionally: We discovered that Gata3 is monoallelically expressed in a parent of origin-independent manner in hematopoietic stem cells and early T-cell progenitors. Curiously, half of the developing cells switch to biallelic Gata3 transcription abruptly at midthymopoiesis. We show that the monoallelic-to-biallelic transcriptional switch is stably maintained and therefore is not a stochastic phenomenon. This unique mechanism, if adopted by other regulatory genes, may provide new biological insights into the rather prevalent phenomenon of monoallelic expression of autosomal genes as well as into the variably penetrant pathophysiological spectrum of phenotypes observed in many human syndromes that are due to haploinsufficiency of the affected gene.

Allelic expression analysis in the brain suggests a role for heterogeneous insults affecting epigenetic processes in autism spectrum disorders

Monoallelic expression, including genomic imprinting, X-chromosome inactivation and random monoallelic expression of autosomal genes are epigenetic phenomena. Genes that are expressed in a monoallelic way may be more vulnerable to genetic or epigenetic mutations. Thus, comprehensive exploration of monoallelic expression in human brains may shed light on complex brain disorders. Autism-related disorders are known to be associated with imprinted genes on chromosome 15. However, it is not clear whether other imprinted regions or other types of monoallelic expression are associated with autism spectrum disorder (ASD). Here, we performed a genome-wide survey of allele expression imbalance (AEI) in the human brain using single-nucleotide polymorphisms (SNPs), in 18 individuals with ASD and 15 controls. Individuals with ASD had the most extreme number of monoallelic expressed SNPs in both the autosomes and the X chromosome. In two cases that were studied in detail, the monoallelic expression was confined to specific brain region or cell type. Using these data, we were also able to define the allelic expression status of known imprinted genes in the human brain and to identify abnormal imprinting in an individual with ASD. Lastly, we developed an analysis of individual-level expression, focusing on the difference of each individual from the mean. We found that individuals with ASD had more genes that were up- or down-regulated in an individual-specific manner. We also identified pathways perturbed in specific individuals. These results underline the heterogeneity in gene regulation in ASD, at the level of both allelic and total expression.

Optimizing the detection of nascent transcripts by RNA fluorescence in situ hybridization.

An unusual feature of the mammalian genome is the number of genes exhibiting monoallelic expression. Recently random monoallelic expression of autosomal genes has been reported for olfactory and Ly-49 NK receptor genes, as well as for Il-2, Il-4 and Pax5. RNA fluorescence in situ hybridization (FISH) has been exploited to monitor allelic expression by visualizing the number of sites of transcription in individual nuclei. However, the sensitivity of this technique is difficult to determine for a given gene. We show that by combining DNA and RNA FISH it is possible to control for the hybridization efficiency and the accessibility and visibility of fluorescent probes within the nucleus.