Large scale genomic reorganization of topological domains at the HoxD locus

CRISPR/Cas9 and FLP-FRT mediated regulatory dissection of the BX-C of Drosophila melanogaster

The homeotic genes or Hox define the anterior-posterior (AP) body axis formation in bilaterians and are often present on the chromosome in an order collinear to their function across the AP axis. However, there are many cases wherein the Hox are not collinear, but their expression pattern is conserved across the AP axis. The expression pattern of Hox is attributed to the cis-regulatory modules (CRMs) consisting of enhancers, initiators, or repressor elements that regulate the genes in a segment-specific manner. In the Drosophila melanogaster Hox complex, the bithorax complex (BX-C) and even the CRMs are organized in an order that is collinear to their function in the thoracic and abdominal segments. In the present study, the regulatorily inert regions were targeted using CRISPR/Cas9 to generate a series of transgenic lines with the insertion of FRT sequences. These FRT lines are repurposed to shuffle the CRMs associated with Abd-B to generate modular deletion, duplication, or inversion of multiple CRMs. The rearrangements yielded entirely novel phenotypes in the fly suggesting the requirement of such complex manipulations to address the significance of higher order arrangement of the CRMs. The functional map and the transgenic flies generated in this study are important resources to decipher the collective ability of multiple regulatory elements in the eukaryotic genome to function as complex modules.

Homeotic Genes: Clustering, Modularity, and Diversity

Hox genes code for transcription factors and are evolutionarily conserved. They regulate a plethora of downstream targets to define the anterior-posterior (AP) body axis of a developing bilaterian embryo. Early work suggested a possible role of clustering and ordering of Hox to regulate their expression in a spatially restricted manner along the AP axis. However, the recent availability of many genome assemblies for different organisms uncovered several examples that defy this constraint. With recent advancements in genomics, the current review discusses the arrangement of Hox in various organisms. Further, we revisit their discovery and regulation in Drosophila melanogaster. We also review their regulation in different arthropods and vertebrates, with a significant focus on Hox expression in the crustacean Parahyale hawaiensis. It is noteworthy that subtle changes in the levels of Hox gene expression can contribute to the development of novel features in an organism. We, therefore, delve into the distinct regulation of these genes during primary axis formation, segment identity, and extra-embryonic roles such as in the formation of hair follicles or misregulation leading to cancer. Toward the end of each section, we emphasize the possibilities of several experiments involving various organisms, owing to the advancements in the field of genomics and CRISPR-based genome engineering. Overall, we present a holistic view of the functioning of Hox in the animal world.

Co-Regulated Genes and Gene Clusters

There are many co-regulated genes in eukaryotic cells. The coordinated activation or repression of such genes occurs at specific stages of differentiation, or under the influence of external stimuli. As a rule, co-regulated genes are dispersed in the genome. However, there are also gene clusters, which contain paralogous genes that encode proteins with similar functions. In this aspect, they differ significantly from bacterial operons containing functionally linked genes that are not paralogs. In this review, we discuss the reasons for the existence of gene clusters in vertebrate cells and propose that clustering is necessary to ensure the possibility of selective activation of one of several similar genes.

Dbx2 regulation in limbs suggests interTAD sharing of enhancers

BACKGROUND

During tetrapod limb development, the HOXA13 and HOXD13 transcription factors are critical for the emergence and organization of the autopod, the most distal aspect where digits will develop. Since previous work had suggested that the Dbx2 gene is a target of these factors, we set up to analyze in detail this potential regulatory interaction.

RESULTS

We show that HOX13 proteins bind to mammalian-specific sequences at the vicinity of the Dbx2 locus that have enhancer activity in developing digits. However, the functional inactivation of the DBX2 protein did not elicit any particular phenotype related to Hox genes inactivation in digits, suggesting either redundant or compensatory mechanisms. We report that the neighboring Nell2 and Ano6 genes are also expressed in distal limb buds and are in part controlled by the same Dbx2 enhancers despite being localized into two different topologically associating domains (TADs) flanking the Dbx2 locus.

CONCLUSIONS

We conclude that Hoxa13 and Hoxd genes cooperatively activate Dbx2 expression in developing digits through binding to mammalian specific regulatory sequences in the Dbx2 neighborhood. Furthermore, these enhancers can overcome TAD boundaries in either direction to co-regulate a set of genes located in distinct chromatin domains.

Challenges and Opportunities of Genomic Approaches in Therapeutics Development

The magnitude of all therapeutic responses is significantly determined by genome structure, variation, and functional interactions. This determination occurs at many levels which are discussed in the current review. Well-established examples of structural variation between individuals are known to dictate an individual's response to numerous drugs, as clearly illustrated by warfarin. The exponential rate of genomic-based interrogation is coupled with an expanding repertoire of genomic technologies and applications. This is leading to an ever more sophisticated appreciation of how structural variation, regulation of transcription and genomic structure, both individually and collectively, define cell therapeutic responses.

The relationship between genome structure and function

Precise patterns of gene expression in metazoans are controlled by three classes of regulatory elements: promoters, enhancers and boundary elements. During differentiation and development, these elements form specific interactions in dynamic higher-order chromatin structures. However, the relationship between genome structure and its function in gene regulation is not completely understood. Here we review recent progress in this field and discuss whether genome structure plays an instructive role in regulating gene expression or is a reflection of the activity of the regulatory elements of the genome.

How chromosome topologies get their shape: views from proximity ligation and microscopy methods

The 3D organization of our genome is an important determinant for the transcriptional output of a gene in (patho)physiological contexts. The spatial organization of linear chromosomes within nucleus is dominantly inferred using two distinct approaches, chromosome conformation capture (3C) and DNA fluorescent in situ hybridization (DNA-FISH). While 3C and its derivatives score genomic interaction frequencies based on proximity ligation events, DNA-FISH methods measure physical distances between genomic loci. Despite these approaches probe different characteristics of chromosomal topologies, they provide a coherent picture of how chromosomes are organized in higher-order structures encompassing chromosome territories, compartments, and topologically associating domains. Yet, at the finer topological level of promoter-enhancer communication, the imaging-centered and the 3C methods give more divergent and sometimes seemingly paradoxical results. Here, we compare and contrast observations made applying visual DNA-FISH and molecular 3C approaches. We emphasize that the 3C approach, due to its inherently competitive ligation step, measures only 'relative' proximities. A 3C interaction enriched between loci, therefore does not necessarily translates into a decrease in absolute spatial distance. Hence, we advocate caution when modeling chromosome conformations.

The regulatory landscapes of developmental genes

Regulatory landscapes have been defined in vertebrates as large DNA segments containing diverse enhancer sequences that produce coherent gene transcription. These genomic platforms integrate multiple cellular signals and hence can trigger pleiotropic expression of developmental genes. Identifying and evaluating how these chromatin regions operate may be difficult as the underlying regulatory mechanisms can be as unique as the genes they control. In this brief article and accompanying poster, we discuss some of the ways in which regulatory landscapes operate, illustrating these mechanisms using genes important for vertebrate development as examples. We also highlight some of the techniques available to researchers for analysing regulatory landscapes.

Prioritization of genes driving congenital phenotypes of patients with de novo genomic structural variants

BACKGROUND

Genomic structural variants (SVs) can affect many genes and regulatory elements. Therefore, the molecular mechanisms driving the phenotypes of patients carrying de novo SVs are frequently unknown.

METHODS

We applied a combination of systematic experimental and bioinformatic methods to improve the molecular diagnosis of 39 patients with multiple congenital abnormalities and/or intellectual disability harboring apparent de novo SVs, most with an inconclusive diagnosis after regular genetic testing.

RESULTS

In 7 of these cases (18%), whole-genome sequencing analysis revealed disease-relevant complexities of the SVs missed in routine microarray-based analyses. We developed a computational tool to predict the effects on genes directly affected by SVs and on genes indirectly affected likely due to the changes in chromatin organization and impact on regulatory mechanisms. By combining these functional predictions with extensive phenotype information, candidate driver genes were identified in 16/39 (41%) patients. In 8 cases, evidence was found for the involvement of multiple candidate drivers contributing to different parts of the phenotypes. Subsequently, we applied this computational method to two cohorts containing a total of 379 patients with previously detected and classified de novo SVs and identified candidate driver genes in 189 cases (50%), including 40 cases whose SVs were previously not classified as pathogenic. Pathogenic position effects were predicted in 28% of all studied cases with balanced SVs and in 11% of the cases with copy number variants.

CONCLUSIONS

These results demonstrate an integrated computational and experimental approach to predict driver genes based on analyses of WGS data with phenotype association and chromatin organization datasets. These analyses nominate new pathogenic loci and have strong potential to improve the molecular diagnosis of patients with de novo SVs.

Tales from topographic oceans: topologically associated domains and cancer

The 3D organization of the genome within the cell nucleus has come into sharp focus over the last decade. This has largely arisen because of the application of genomic approaches that have revealed numerous levels of genomic and chromatin interactions, including topologically associated domains (TADs). The current review examines how these domains were identified, are organized, how their boundaries arise and are regulated, and how genes within TADs are coordinately regulated. There are many examples of the disruption to TAD structure in cancer and the altered regulation, structure and function of TADs are discussed in the context of hormone responsive cancers, including breast, prostate and ovarian cancer. Finally, some aspects of the statistical insight and computational skills required to interrogate TAD organization are considered and future directions discussed.

Functional Consequences of Chromosomal Rearrangements on Gene Expression: Not So Deleterious After All?

Chromosomes are folded and organized into topologically associating domains (TADs) which provide a framework for the interaction of enhancers with the promoter of their target gene(s). Structural rearrangements observed during evolution or in disease contexts suggest that changes in genome organization strongly affect gene expression and can have drastic phenotypic effects. In this review, I will discuss how recent genomic engineering experiments reveal a more contrasted picture, suggesting that TADs are important but not always essential for gene expression regulation.

Developmentally regulated Shh expression is robust to TAD perturbations

Topologically associating domains (TADs) have been proposed to both guide and constrain enhancer activity. Shh is located within a TAD known to contain all its enhancers. To investigate the importance of chromatin conformation and TAD integrity on developmental gene regulation, we have manipulated the Shh TAD - creating internal deletions, deleting CTCF sites, and deleting and inverting sequences at TAD boundaries. Chromosome conformation capture and fluorescence in situ hybridisation assays were used to investigate the changes in chromatin conformation that result from these manipulations. Our data suggest that these substantial alterations in TAD structure have no readily detectable effect on Shh expression patterns or levels of Shh expression during development - except where enhancers are deleted - and result in no detectable phenotypes. Only in the case of a larger deletion at one TAD boundary could ectopic influence of the Shh limb enhancer be detected on a gene (Mnx1) in the neighbouring TAD. Our data suggests that, contrary to expectations, the developmental regulation of Shh expression is remarkably robust to TAD perturbations.

Feather Evolution from Precocial to Altricial Birds

Birds are the most abundant terrestrial vertebrates and their diversity is greatly shaped by the feathers. How avian evolution is linked to feather evolution has long been a fascinating question. Numerous excellent studies have shed light on this complex relationship by investigating feather diversity and its underlying molecular mechanisms. However, most have focused on adult domestic birds, and the contribution of feather diversity to environmental adaptation has not been well-studied. In this review, we described bird diversity using the traditional concept of the altricial-precocial spectrum in bird hatchlings. We combined the spectrum with a recently published avian phylogeny to profile the spectrum evolution. We then focused on the discrete diagnostic character of the spectrum, the natal down, and propose a hypothesis for the precocial-to-altricial evolution. For the underlying molecular mechanisms in feather diversity and bird evolution, we reviewed the literature and constructed the known mechanisms for feather tract definition and natal down development. Finally, we suggested some future directions for research on altricial-precocial divergence, which may expand our understanding of the relationship between natal down diversity and bird evolution.

Impact of genome architecture on the functional activation and repression of Hox regulatory landscapes

BACKGROUND

The spatial organization of the mammalian genome relies upon the formation of chromatin domains of various scales. At the level of gene regulation in cis, collections of enhancer sequences define large regulatory landscapes that usually match with the presence of topologically associating domains (TADs). These domains often contain ranges of enhancers displaying similar or related tissue specificity, suggesting that in some cases, such domains may act as coherent regulatory units, with a global on or off state. By using the HoxD gene cluster, which specifies the topology of the developing limbs via highly orchestrated regulation of gene expression, as a paradigm, we investigated how the arrangement of regulatory domains determines their activity and function.

RESULTS

Proximal and distal cells in the developing limb express different levels of Hoxd genes, regulated by flanking 3' and 5' TADs, respectively. We characterized the effect of large genomic rearrangements affecting these two TADs, including their fusion into a single chromatin domain. We show that, within a single hybrid TAD, the activation of both proximal and distal limb enhancers globally occurred as when both TADs are intact. However, the activity of the 3' TAD in distal cells is generally increased in the fused TAD, when compared to wild type where it is silenced. Also, target gene activity in distal cells depends on whether or not these genes had previously responded to proximal enhancers, which determines the presence or absence of H3K27me3 marks. We also show that the polycomb repressive complex 2 is mainly recruited at the Hox gene cluster and can extend its coverage to far-cis regulatory sequences as long as confined to the neighboring TAD structure.

CONCLUSIONS

We conclude that antagonistic limb proximal and distal enhancers can exert their specific effects when positioned into the same TAD and in the absence of their genuine target genes. We also conclude that removing these target genes reduced the coverage of a regulatory landscape by chromatin marks associated with silencing, which correlates with its prolonged activity in time.

Causal Gene Regulatory Network Modeling and Genomics: Second-Generation Challenges

Gene regulatory network modeling has played a major role in advancing the understanding of developmental systems, by crystallizing structures of relevant extant information, by formally posing hypothetical functional relationships between network elements, and by offering clear predictive tests to improve understanding of the mechanisms driving developmental progression. Both ordinary differential equation (ODE)-based and Boolean models have also been highly successful in explaining dynamics within subcircuits of more complex processes. In a very small number of cases, gene regulatory network models of much more global scope have been proposed that successfully predict the dynamics of the processes establishing most of an embryonic body plan. Can such successes be expanded to very different developmental systems, including post-embryonic mammalian systems? This perspective discusses several problems that must be solved in more quantitative and predictive theoretical terms, to make this possible. These problems include: the effects of cellular history on chromatin state and how these affect gene accessibility; the dose dependence of activities of many transcription factors (a problem for Boolean models); stochasticity of some transcriptional outputs (a problem for simple ODE models); response timing delays due to epigenetic remodeling requirements; functionally different kinds of repression; and the regulatory syntax that governs responses of genes with multiple enhancers.

Heterogeneous combinatorial expression of Hoxd genes in single cells during limb development

BACKGROUND

Global analyses of gene expression during development reveal specific transcription patterns associated with the emergence of various cell types, tissues, and organs. These heterogeneous patterns are instrumental to ensure the proper formation of the different parts of our body, as shown by the phenotypic effects generated by functional genetic approaches. However, variations at the cellular level can be observed within each structure or organ. In the developing mammalian limbs, expression of Hox genes from the HoxD cluster is differentially controlled in space and time, in cells that will pattern the digits and the forearms. While the Hoxd genes broadly share a common regulatory landscape and large-scale analyses have suggested a homogenous Hox gene transcriptional program, it has not previously been clear whether Hoxd genes are expressed together at the same levels in the same cells.

RESULTS

We report a high degree of heterogeneity in the expression of the Hoxd11 and Hoxd13 genes. We analyzed single-limb bud cell transcriptomes and show that Hox genes are expressed in specific combinations that appear to match particular cell types. In cells giving rise to digits, we find that the expression of the five relevant Hoxd genes (Hoxd9 to Hoxd13) is unbalanced, despite their control by known global enhancers. We also report that specific combinatorial expression follows a pseudo-time sequence, which is established based on the transcriptional diversity of limb progenitors.

CONCLUSIONS

Our observations reveal the existence of distinct combinations of Hoxd genes at the single-cell level during limb development. In addition, we document that the increasing combinatorial expression of Hoxd genes in this developing structure is associated with specific transcriptional signatures and that these signatures illustrate a temporal progression in the differentiation of these cells.

Control of directionality of chromatin folding for the inter- and intra-domain contacts at the Tfap2c-Bmp7 locus

BACKGROUND

Contact domains of chromatin serve as a fundamental unit to regulate action of enhancers for target genes. Looping between a pair of CCCTC-binding factor (CTCF)-binding sites in convergent orientations underlies the formation of contact domains, while those in divergent orientations establish domain boundaries. However, every CTCF site is not necessarily engaged in loop or boundary structures, leaving functions of CTCF in varied genomic contexts still elusive. The locus containing Tfap2c and Bmp7 encompasses two contact domains separated by a region between the two genes, termed transition zone (TZ), characterized by two arrays of CTCF sites in divergent configuration. In this study, we created deletion and inversion alleles of these and other regions across the locus and investigated how they impinge on the conformation.

RESULTS

Deletion of the whole two CTCF arrays with the CRISPR/Cas9 system resulted in impairment of blocking of chromatin contacts by the TZ, as assessed by the circular chromatin conformation capture assay (4C-seq). Deletion and inversion of either of the two arrays similarly, but less pronouncedly, led to reduction in the blocking activity. Thus, the divergent configuration provides the TZ with the strong boundary activity. Uniquely, we show the TZ harbors a 50-kb region within one of the two arrays that contacts broadly with the both flanking intervals, regardless of the presence or orientation of the other CTCF array. Further, we show the boundary CTCF array has little impact on intra-domain folding; instead, locally associating CTCF sites greatly affect it.

CONCLUSIONS

Our results show that the TZ not only separates the two domains, but also bears a wide interval that shows isotropic behavior of chromatin folding, indicating a potentially complex nature of actual boundaries in the genome. We also show that CTCF-binding sites inside a domain greatly contribute to the intra-domain folding of chromatin. Thus, the study reveals diverse and context-dependent roles of CTCF in organizing chromatin conformation at different levels.

Enhancer talk

Enhancers are short noncoding segments of DNA (100-1000 bp) that control the temporal and spatial activity of genes in an orientation-independent manner. They can be separated from their target genes by large distances and are thus known as distal regulatory elements. One consequence of the variability in the distance separating enhancers and their target promoters is that it is difficult to determine which elements are involved in the regulation of a particular gene. Moreover, enhancers can be found in clusters in which multiple regulatory elements control expression of the same target gene. However, little is known about how the individual elements contribute to gene expression. Here, we describe how chromatin conformation promotes and constraints enhancer activity. Further, we discuss enhancer clusters and what is known about the contribution of individual elements to the regulation of target genes. Finally, we examine the reliability of different methods used to identify enhancers.

The HoxD cluster is a dynamic and resilient TAD boundary controlling the segregation of antagonistic regulatory landscapes

The mammalian HoxD cluster lies between two topologically associating domains (TADs) matching distinct enhancer-rich regulatory landscapes. During limb development, the telomeric TAD controls the early transcription of Hoxd genes in forearm cells, whereas the centromeric TAD subsequently regulates more posterior Hoxd genes in digit cells. Therefore, the TAD boundary prevents the terminal Hoxd13 gene from responding to forearm enhancers, thereby allowing proper limb patterning. To assess the nature and function of this CTCF-rich DNA region in embryos, we compared chromatin interaction profiles between proximal and distal limb bud cells isolated from mutant stocks where various parts of this boundary region were removed. The resulting progressive release in boundary effect triggered inter-TAD contacts, favored by the activity of the newly accessed enhancers. However, the boundary was highly resilient, and only a 400-kb deletion, including the whole-gene cluster, was eventually able to merge the neighboring TADs into a single structure. In this unified TAD, both proximal and distal limb enhancers nevertheless continued to work independently over a targeted transgenic reporter construct. We propose that the whole HoxD cluster is a dynamic TAD border and that the exact boundary position varies depending on both the transcriptional status and the developmental context.