Hox genes are differentially expressed along the embryonic anteroposterior axis. We used chromatin immunoprecipitation to detect chromatin changes at the Hoxd4 locus during neurogenesis in P19 cells and embryonic day 8.0 (E8.0) and E10.5 mouse embryos. During Hoxd4 induction in both systems, we observed that histone modifications typical of transcriptionally active chromatin occurred first at the 3 neural enhancer and then at the promoter. Moreover, the sequential distribution of histone modifications between E8.0 and E10.5 was consistent with a spreading of open chromatin, starting with the enhancer, followed by successively more 5 intervening sequences, and culminating at the promoter. Neither RNA polymerase II (Pol II) nor CBP associated with the inactive gene. During Hoxd4 induction, CBP and RNA Pol II were recruited first to the enhancer and then to the promoter. Whereas the CBP association was transient, RNA Pol II remained associated with both regulatory regions. Histone modification and transcription factor recruitment occurred in posterior, Hox-expressing embryonic tissues, but never in anterior tissues, where such genes are inactive. Together, our observations demonstrate that the direction of histone modifications at Hoxd4 mirrors colinear gene activation across Hox clusters and that the establishment of anterior and posterior compartments is accompanied by the imposition of distinct chromatin states.Mammalian Hox genes encode a highly conserved family of homeodomain-containing transcription factors that play a fundamental role in specifying positional identity along embryonic axes (23,24,32). Thirty-nine Hox genes are organized into four clusters on different chromosomes, with each containing up to 11 genes (24). Since genes at a given locus are all transcribed in the same direction, one can assign 3Ј and 5Ј ends to each Hox cluster. Importantly, there is a correlation between the genomic location of a specific Hox gene within a cluster and its time and functional domain of expression during development, a phenomenon termed colinearity. Thus, genes located more 3Ј in each cluster are expressed in the embryo earlier and in a more anterior position than those located more 5Ј in the cluster (8,15,27). Altering normal Hox expression results in homeotic transformations and malformations (24, 48), establishing the biological relevance of this tightly controlled expression pattern. Given the integral role of chromatin remodeling in gene regulation, colinear Hox gene activation could be explained in part by a successive 3Ј to 5Ј conversion from closed to open chromatin along the length of a Hox cluster.
Correct patterning of the antero-posterior axis of the embryonic trunk is dependent on spatiotemporally restricted Hox gene expression. In this study, we identified components of the Hoxd4 P1 promoter directing expression in neurally differentiating retinoic acid-treated P19 cells. We mapped three nucleosomes that are subsequently remodeled into an open chromatin state upon retinoic acid-induced Hoxd4 transcription. These nucleosomes spanned the Hoxd4 transcriptional start site in addition to a GC-rich positive regulatory element located 3 to the initiation site. We further identified two major cis-acting regulatory elements. An autoregulatory element was shown to recruit HOXD4 and its cofactor PBX1 and to positively regulate Hoxd4 expression in differentiating P19 cells. Conversely, the Polycomb group (PcG) protein Ying-Yang 1 (YY1) binds to an internucleosomal linker and represses Hoxd4 transcription before and during transcriptional activation. Sequential chromatin immunoprecipitation studies revealed that the PcG protein MEL18 was co-recruited with YY1 only in undifferentiated P19 cells, suggesting a role for MEL18 in silencing Hoxd4 transcription in undifferentiated P19 cells. This study links for the first time local chromatin remodeling events that take place during transcriptional activation with the dynamics of transcription factor association and DNA accessibility at a Hox regulatory region.Hox gene transcriptional activation marks the onset of an intricate series of events leading to proper embryonic patterning in all animals. The products of Hox genes, homeodomain-containing HOX transcription factors, are essential in specifying antero-posterior positional identity, hindbrain development, limb formation, and numerous additional morphogenetic and organogenetic events (1, 2). Given their crucial role in embryonic development, the genes encoding HOX proteins are highly conserved throughout the animal kingdom, and their expression is tightly regulated (3). In mammals, 39 Hox genes are organized into four clusters, each located on a different chromosome (1). Comparison of the clusters reveals 13 possible gene positions, although none of the clusters retains a full complement of 13 genes. Hox genes occupying the same positions are termed paralogs, sharing high sequence identity and functional redundancy. One can assign a 3Ј and a 5Ј end to a cluster since all genes are transcribed in the same direction. A unique feature of Hox gene clusters is a process termed "colinearity," correlating both the timing of transcriptional activation and the anterior expression borders with the position of a particular Hox gene along a cluster (4). Therefore, genes located more 3Ј are expressed earlier and have a more anterior expression border than genes located more 5Ј along the cluster. This observation and several other studies have led to the hypothesis that a sequential opening of chromatin, starting at the 3Ј end of a cluster and moving successively 5Ј, leads to the release of silencing, first at the 3Ј end, and seque...
Much of our understanding of the pathology of acute leukemias is based on studies of 11q23 chromosomal translocations of the gene encoding the mixed lineage leukemia-1 (MLL1) histone H3 lysine 4 (H3K4) methyltransferase. Translocations of the MLL1 gene result in MLL1-fusion (MLL1F) proteins that replace the catalytic SET domain with 1 of more than 80 fusion partners that are thought to drive leukemogenesis through a gain-of-function mechanism. However, this mechanism does not explain how loss of the catalytic SET domain results in upregulated H3K4 trimethylation and target gene expression in MLL1F-leukemic stem cells. In this investigation, we introduced a homozygous loss-of-function point mutation in MLL1 in human induced pluripotent stem cells. The resulting cells mislocalized a different H3K4 trimethyltransferase, SETd1a, to MLL1 target gene promoters and recapitulated a phenotype commonly observed in MLL1F leukemic stem cells, with aberrant histone methylation and gene expression signatures characteristic of "mixed lineage" cells. These results suggest that loss of MLL1 histone methyltransferase activity, and the associated impact on the epigenome, are common molecular features underlying MLL1F pathology.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.