Dissecting CTCF site function in a tense HoxD locus

In this issue of Genes & Development, Amândio and colleagues (pp. 1490–1509) dissect the function of a cluster of several CTCF binding sites in the HoxD cluster by iterative deletions in mice. They found additive functions for some, and intriguingly found that some sites function as insulators, while others function as anchors for enhancer–promoter interactions. These functions vary depending on developmental context. The work provides new insights into the roles played by CTCF in regulating developmental patterns and 3D chromatin organization.

Sequential in cis mutagenesis in vivo reveals various functions for CTCF sites at the mouse HoxD cluster

Mammalian Hox gene clusters contain a range of CTCF binding sites. In addition to their importance in organizing a TAD border, which isolates the most posterior genes from the rest of the cluster, the positions and orientations of these sites suggest that CTCF may be instrumental in the selection of various subsets of contiguous genes, which are targets of distinct remote enhancers located in the flanking regulatory landscapes. We examined this possibility by producing an allelic series of cumulative in cis mutations in these sites, up to the abrogation of CTCF binding in the five sites located on one side of the TAD border. In the most impactful alleles, the global chromatin architecture of the locus was modified, yet not drastically, illustrating that CTCF sites located on one side of a strong TAD border are sufficient to organize at least part of this insulation. Spatial colinearity in the expression of these genes along the major body axis was nevertheless maintained, despite abnormal expression boundaries. In contrast, strong effects were scored in the selection of target genes responding to particular enhancers, leading to the misregulation of Hoxd genes in specific structures. Altogether, while most enhancer–promoter interactions can occur in the absence of this series of CTCF sites, the binding of CTCF in the Hox cluster is required to properly transform a rather unprecise process into a highly discriminative mechanism of interactions, which is translated into various patterns of transcription accompanied by the distinctive chromatin topology found at this locus. Our allelic series also allowed us to reveal the distinct functional contributions for CTCF sites within this Hox cluster, some acting as insulator elements, others being necessary to anchor or stabilize enhancer–promoter interactions, and some doing both, whereas they all together contribute to the formation of a TAD border. This variety of tasks may explain the amazing evolutionary conservation in the distribution of these sites among paralogous Hox clusters or between various vertebrates.

Sequential in-cis mutagenesis in vivo reveals various functions for CTCF sites at the mouse HoxD cluster

Mammalian Hox gene clusters contain a range of CTCF binding sites. In addition to their importance in organizing a TAD border, which isolates the most posterior genes from the rest of the cluster, the positions and orientations of these sites suggest that CTCF may be instrumental in the selection of various subsets of contiguous genes, which are targets of distinct remote enhancers located in the flanking regulatory landscapes. We examined this possibility by producing an allelic series of cumulative in-cis mutations in these sites, up to the abrogation of CTCF binding in the five sites located on one side of the TAD border. In the most impactful alleles, the global chromatin architecture of the locus was modified, yet not drastically, illustrating that CTCF sites located on one side of a strong TAD border are sufficient to organize at least part of this insulation. Spatial colinearity in the expression of these genes along the major body axis was nevertheless maintained, despite abnormal expression boundaries. In contrast, strong effects were scored in the selection of target genes responding to particular enhancers, leading to the mis-regulation of Hoxd genes in specific structures. Altogether, while most enhancer-promoter interactions can occur in the absence of this series of CTCF sites, it seems that the binding of CTCF in the Hox cluster is required to properly transform a rather unprecise process into a highly discriminative mechanism of interactions, which is translated into various patterns of transcription accompanied by the distinctive chromatin topology found at this locus. Our allelic series also allowed us to reveal the distinct functional contributions for CTCF sites within this Hox cluster, some acting as insulator elements, others being necessary to anchor or stabilize enhancer-promoter interactions and some doing both, whereas all together contribute to the formation of a TAD border. This variety of tasks may explain the amazing evolutionary conservation in the distribution of these sites amongst paralogous Hox clusters or between various vertebrates.

Recurrent chromosomal translocations in sarcomas create a mega-complex that mislocalizes NuA4/TIP60 to Polycomb target loci

Chromosomal translocations frequently promote carcinogenesis by producing gain-of-function fusion proteins. Recent studies have identified highly recurrent chromosomal translocations in patients with Endometrial Stromal Sarcomas (ESS) and Ossifying FibroMyxoid Tumors (OFMT) leading to an in-frame fusion of PHF1 (PCL1) to six different subunits of the NuA4/TIP60 complex. While NuA4/TIP60 is a co-activator that acetylates chromatin and loads the H2A.Z histone variant, PHF1 is part of the Polycomb repressive complex 2 (PRC2) linked to transcriptional repression of key developmental genes through methylation of histone H3 on lysine 27. In this study, we characterize the fusion protein produced by the EPC1-PHF1 translocation. The chimeric protein assembles a mega-complex harboring both NuA4/TIP60 and PRC2 activities and leads to mislocalization of chromatin marks in the genome. These are linked to aberrant gene expression, in particular over an entire topologically-associated domain including part of the HOXD cluster. Furthermore, we show that JAZF1, implicated with PRC2 components in the most frequent translocations in ESS, is a potent transcription activator that physically associates with NuA4/TIP60. Altogether, these results indicate that most chromosomal translocations linked to these sarcomas employ the same molecular oncogenic mechanism through a physical merge of NuA4/TIP60 and PRC2 complexes leading to mislocalization of histone marks and aberrant polycomb target gene expression.

HOXD-AS1 facilitates cell migration and invasion as an oncogenic lncRNA by competitively binding to miR-877-3p and upregulating FGF2 in human cervical cancer

Background Long non-coding RNAs (LncRNAs) are dysregulated in multiple human cancers and they are highly involved in tumor progression. Previous studies have identified the oncogenic lncRNA HOXD cluster antisense RNA 1 (HOXD-AS1) in human cancers, while its roles in cervical cancer (CC) remain unclear. Herein we intended to characterize the implication of HOXD-AS1 in CC. Methods qRT-PCR was applied to examine the relative expression of HOXD-AS1 in CC tissues, cell lines and transfected cells. Wound healing and transwell assays were applied to detect cell migration and invasion alteration. The targeting relationship between miRNA and mRNA/lncRNA was determined by dual luciferase reporter, qRT-PCR and western blot assays. Results HOXD-AS1 was overexpressed in CC tissues and cell lines. Its higher level predicted worse prognosis of CC patients. SiRNA mediated knockdown of HOXD-AS1 repressed CC cell migration and invasion, and its overexpression did the opposite. Mechanistically, HOXD-AS1 acted as a competing endogenous RNA (ceRNA) to sponge miR-877-3p and led to upregulation of FGF2, a target of miR-877-3p. Importantly, either miR-877-3p overexpression or FGF2 inhibition could abolish the migration and invasion promotion induced by HOXD-AS1. Conclusion HOXD-AS1 functions as a tumor-promoting lncRNA via the miR-877-3p/FGF2 axis in CC. HOXD-AS1 might be a promising therapeutic target as well as a novel prognostic biomarker for CC.

A complex regulatory landscape involved in the development of mammalian external genitals

Developmental genes are often controlled by large regulatory landscapes matching topologically associating domains (TADs). In various contexts, the associated chromatin backbone is modified by specific enhancer–enhancer and enhancer–promoter interactions. We used a TAD flanking the mouse HoxD cluster to study how these regulatory architectures are formed and deconstructed once their function achieved. We describe this TAD as a functional unit, with several regulatory sequences acting together to elicit a transcriptional response. With one exception, deletion of these sequences didn’t modify the transcriptional outcome, a result at odds with a conventional view of enhancer function. The deletion and inversion of a CTCF site located near these regulatory sequences did not affect transcription of the target gene. Slight modifications were nevertheless observed, in agreement with the loop extrusion model. We discuss these unexpected results considering both conventional and alternative explanations relying on the accumulation of poorly specific factors within the TAD backbone.

HOXD-AS1 confers cisplatin resistance in gastric cancer through epigenetically silencing PDCD4 via recruiting EZH2

Increasing evidence suggests that dysregulation of long non-coding RNAs (lncRNAs) is implicated in chemoresistance in cancers. However, the function and molecular mechanisms of lncRNAs in gastric cancer chemoresistance are still not well understood. In this study, we aimed to investigate the functional role and the underlying molecular mechanisms of lncRNA HOXD cluster antisense RNA 1 (HOXD-AS1) in cisplatin (DDP) resistance in gastric cancer. Our results revealed that HOXD-AS1 was upregulated in DDP-resistant gastric cancer tissues and cells. Patients with gastric cancer with high HOXD-AS1 expression levels had a poor prognosis. Knockdown of HOXD-AS1 facilitated the sensitivity of DDP-resistant gastric cancer cells to DDP. Additionally, HOXD-AS1 epigenetically silenced PDCD4 through binding to the histone methyltransferase enhancer of zeste homologue 2 (EZH2) on the promoter of PDCD4, thus increasing H3K27me3. More importantly, PDCD4 silencing counteracted HOXD-AS1 knockdown-mediated enhancement of DDP sensitivity in DDP-resistant gastric cancer cells. In summary, HOXD-AS1 led to DDP resistance in gastric cancer by epigenetically suppressing PDCD4 expression, providing a novel therapeutic strategy for patients with gastric cancer with chemoresistance.

Hox Temporal Collinearity: Misleading Fallacy or Essential Developmental Mechanism?

Kondo and collaborators recently reported the absence of Hox temporal collinearity in Xenopus tropicalis. They found none in the initiation of accumulation of  Hox transcripts (detected via RNA seq). And none in the initial expression sequence of primary unprorocessed transcripts (Identified by using qRT-PCR against introns or intron-exon boundaries).  Nor in the initial acquisition by Hox gene DNA of a mark for active chromatin.  These findings are in conflict with the idea that temporal collinearity has to do with the initiation of Hox gene transcription or with the opening of and a progression from repressed to active states in Hox chromatin.  But collinear acquisition of the same active chromatin mark has been shown by others in murine 5’ Hoxd cluster genes.The reason for this difference is unknown . This careful study thus indicated that the initiation phase of Hox expression shows no temporal collinearity in X. tropicalis. A previous study in X. laevis from the same group also showed   that the sequence of times for reaching (normalised) half maximal Hox expression showed no temporal collinearity. These conclusions are likely to be correct. These authors do however also conclude that “experimental evidence for the temporal collinearity hypothesis is not strong” There is however strong evidence that Hox temporal collinearity does occur in early vertebrate embryos.   Below. I present and discuss 3 lines of evidence to resolve the present conflict   I argue that Hox temporal collinearity actually does exist and that it is part of a central mechanism in early development.