Batool Akhtar-Zaidi

Epigenomic Enhancer Profiling Defines a Signature of Colon Cancer

Colorectal Cancer Signature The mutations and genome aberrations that characterize cancer result in often dramatically altered gene and protein expression patterns. It is these altered expression patterns that directly and indirectly drive progression of the disease. In human primary colorectal cancer cells, Akhtar-Zaidi et al. (p. 736, published online 12 April) analyzed the pattern of epigenetically modified chromatin at “enhancer” sequences that are known to be critical in the control of gene expression. An epigenetic enhancer signature was defined that was specifically associated with colorectal cancer cells. Methylation tags at long-distance gene regulatory elements provide a signature specific to cancer cells. Cancer is characterized by gene expression aberrations. Studies have largely focused on coding sequences and promoters, even though distal regulatory elements play a central role in controlling transcription patterns. We used the histone mark H3K4me1 to analyze gain and loss of enhancer activity genome-wide in primary colon cancer lines relative to normal colon crypts. We identified thousands of variant enhancer loci (VELs) that comprise a signature that is robustly predictive of the in vivo colon cancer transcriptome. Furthermore, VELs are enriched in haplotype blocks containing colon cancer genetic risk variants, implicating these genomic regions in colon cancer pathogenesis. We propose that reproducible changes in the epigenome at enhancer elements drive a specific transcriptional program to promote colon carcinogenesis.

CHD7 Targets Active Gene Enhancer Elements to Modulate ES Cell-Specific Gene Expression

CHD7 is one of nine members of the chromodomain helicase DNA–binding domain family of ATP–dependent chromatin remodeling enzymes found in mammalian cells. De novo mutation of CHD7 is a major cause of CHARGE syndrome, a genetic condition characterized by multiple congenital anomalies. To gain insights to the function of CHD7, we used the technique of chromatin immunoprecipitation followed by massively parallel DNA sequencing (ChIP–Seq) to map CHD7 sites in mouse ES cells. We identified 10,483 sites on chromatin bound by CHD7 at high confidence. Most of the CHD7 sites show features of gene enhancer elements. Specifically, CHD7 sites are predominantly located distal to transcription start sites, contain high levels of H3K4 mono-methylation, found within open chromatin that is hypersensitive to DNase I digestion, and correlate with ES cell-specific gene expression. Moreover, CHD7 co-localizes with P300, a known enhancer-binding protein and strong predictor of enhancer activity. Correlations with 18 other factors mapped by ChIP–seq in mouse ES cells indicate that CHD7 also co-localizes with ES cell master regulators OCT4, SOX2, and NANOG. Correlations between CHD7 sites and global gene expression profiles obtained from Chd7 +/+, Chd7 +/−, and Chd7 −/− ES cells indicate that CHD7 functions at enhancers as a transcriptional rheostat to modulate, or fine-tune the expression levels of ES–specific genes. CHD7 can modulate genes in either the positive or negative direction, although negative regulation appears to be the more direct effect of CHD7 binding. These data indicate that enhancer-binding proteins can limit gene expression and are not necessarily co-activators. Although ES cells are not likely to be affected in CHARGE syndrome, we propose that enhancer-mediated gene dysregulation contributes to disease pathogenesis and that the critical CHD7 target genes may be subject to positive or negative regulation.

The Developmental Potential of iPSCs Is Greatly Influenced by Reprogramming Factor Selection

Induced pluripotent stem cells (iPSCs) are commonly generated by transduction of Oct4, Sox2, Klf4, and Myc (OSKM) into cells. Although iPSCs are pluripotent, they frequently exhibit high variation in terms of quality, as measured in mice by chimera contribution and tetraploid complementation. Reliably high-quality iPSCs will be needed for future therapeutic applications. Here, we show that one major determinant of iPSC quality is the combination of reprogramming factors used. Based on tetraploid complementation, we found that ectopic expression of Sall4, Nanog, Esrrb, and Lin28 (SNEL) in mouse embryonic fibroblasts (MEFs) generated high-quality iPSCs more efficiently than other combinations of factors including OSKM. Although differentially methylated regions, transcript number of master regulators, establishment of specific superenhancers, and global aneuploidy were comparable between high- and low-quality lines, aberrant gene expression, trisomy of chromosome 8, and abnormal H2A.X deposition were distinguishing features that could potentially also be applicable to human.

ZNF143 provides sequence specificity to secure chromatin interactions at gene promoters

Chromatin interactions connect distal regulatory elements to target gene promoters guiding stimulus- and lineage-specific transcription. Few factors securing chromatin interactions have so far been identified. Here, by integrating chromatin interaction maps with the large collection of transcription factor-binding profiles provided by the ENCODE project, we demonstrate that the zinc-finger protein ZNF143 preferentially occupies anchors of chromatin interactions connecting promoters with distal regulatory elements. It binds directly to promoters and associates with lineage-specific chromatin interactions and gene expression. Silencing ZNF143 or modulating its DNA-binding affinity using single-nucleotide polymorphisms (SNPs) as a surrogate of site-directed mutagenesis reveals the sequence dependency of chromatin interactions at gene promoters. We also find that chromatin interactions alone do not regulate gene expression. Together, our results identify ZNF143 as a novel chromatin-looping factor that contributes to the architectural foundation of the genome by providing sequence specificity at promoters connected with distal regulatory elements. Chromatin interactions can connect distal regulatory elements to promoters via protein factors, but few such factors have been identified. Here, the authors show that zinc-finger protein ZNF143 is a sequence-specific chromatin-looping factor that connects promoters with distal regulatory elements.

β-catenin has sequential roles in the survival and specification of ventral dermis

The dermis promotes the development and maintains the functional components of skin, such as hair follicles, sweat glands, nerves and blood vessels. The dermis is also crucial for wound healing and homeostasis of the skin. The dermis originates from the somites, the lateral plate mesoderm and the cranial neural crest. Despite the importance of the dermis in the structural and functional integrity of the skin, genetic analysis of dermal development in different parts of the embryo is incomplete. The signaling requirements for ventral dermal cell development have not been established in either the chick or the mammalian embryo. We have shown previously that Wnt signaling specifies the dorsal dermis from the somites. In this study, we demonstrate that Wnt/β-catenin signaling is necessary for the survival of early ventral dermal progenitors. In addition, we show that, at later stages, Wnt/β-catenin signaling is sufficient for ventral dermal cell specification. Consistent with the different origins of dorsal and ventral dermal cells, our results demonstrate both conserved and divergent roles ofβ -catenin/Wnt signaling in dermal development.

H3K4me3 inversely correlates with DNA methylation at a large class of non-CpG-island-containing start sites

BackgroundIn addition to mutations, epigenetic silencing of genes has been recognized as a fundamental mechanism that promotes human carcinogenesis. To date, characterization of epigenetic gene silencing has largely focused on genes in which silencing is mediated by hypermethylation of promoter-associated CpG islands, associated with loss of the H3K4me3 chromatin mark. Far less is known about promoters lacking CpG-islands or genes that are repressed by alternative mechanisms.MethodsWe performed integrative ChIP-chip, DNase-seq, and global gene expression analyses in colon cancer cells and normal colon mucosa to characterize chromatin features of both CpG-rich and CpG-poor promoters of genes that undergo silencing in colon cancer.ResultsEpigenetically repressed genes in colon cancer separate into two classes based on retention or loss of H3K4me3 at transcription start sites. Quantitatively, of transcriptionally repressed genes that lose H3K4me3 in colon cancer (K4-dependent genes), a large fraction actually lacks CpG islands. Nonetheless, similar to CpG-island containing genes, cytosines located near the start sites of K4-dependent genes become DNA hypermethylated, and repressed K4-dependent genes can be reactivated with 5-azacytidine. Moreover, we also show that when the H3K4me3 mark is retained, silencing of CpG island-associated genes can proceed through an alternative mechanism in which repressive chromatin marks are recruited.ConclusionsH3K4me3 equally protects from DNA methylation at both CpG-island and non-CpG island start sites in colon cancer. Moreover, the results suggest that CpG-rich genes repressed by loss of H3K4me3 and DNA methylation represent special instances of a more general epigenetic mechanism of gene silencing, one in which gene silencing is mediated by loss of H3K4me3 and methylation of non-CpG island promoter-associated cytosines.

Publisher Correction: ZNF143 provides sequence specificity to secure chromatin interactions at gene promoters

This corrects the article DOI: 10.1038/ncomms7186.