Biology subjects

Lopez-Delisle, L.

Publications and source records attributed to Lopez-Delisle, L..

At least 19 recordsLinked to original sources

bioRxiv2025

Fine-tuning mechanical constraints uncouple patterning and gene expression in murine pseudo-embryos

The interplay between mechanical forces and genetic programs is fundamental to embryonic development, yet how these factors independently or jointly influence morphogenesis and cell fate decisions remains poorly understood. Here, we fine-tune the mechanical environment of murine gastruloids, three-dimensional in vitro models of early embryogenesis, by embedding them in bioinert hydrogels with precisely tunable stiffness and timing of application. This approach reveals that external constraints can selectively influence transcriptional profiles, patterning, or morphology, depending on the level and timing of mechanical modulation. Gastruloids embedded in ultra-soft hydrogels (< 30 Pa) elongate robustly, preserving both anteroposterior patterning and transcriptional profiles. In contrast, embedding at higher stiffness disrupts polarization while leaving gene expression largely unaffected. Conversely, earlier embedding significantly impacts transcriptional profiles independently of polarization defects, highlighting the uncoupling of patterning and transcription. These findings suggest that distinct cellular states respond differently to external constraints. Live imaging and cell tracking imply that impaired cell motility underlies polarization defects, underscoring the role of mechanical forces in shaping morphogenesis independently of transcriptional changes. By allowing precise control over external mechanical boundaries, our approach provides a powerful platform to dissect how physical and biochemical factors interact to orchestrate early embryonic development.

developmental biology↗

bioRxiv2024

WNT signaling coordinately controls mouse limb bud outgrowth and establishment of the digit-interdigit pattern

Self-organization, such as the emergence of a pattern from a homogenous state, is a fascinating property of biological systems. Early limb bud outgrowth and patterning in mice are controlled by a robust and self-regulatory signaling system, and initiation of the periodic digit-interdigit pattern appears under control of a self-regulatory Turing system. Previous studies established the requirement of WNT and BMP signaling for both early limb bud and digit-interdigit morphogenesis, but the molecular changes underlying the transition from early limb bud signaling to the digit-interdigit patterning system remained unknown. Here, we use small molecule inhibitors to rapidly but transiently block WNT signaling to identify the early transcriptional targets that are altered during disruption and recovery of limb bud and digit development. Together, this study highlights the overarching role of WNT signaling in controlling early limb bud outgrowth and patterning, and establishment of the periodic digit-interdigit pattern. Finally, the transient WNT signaling disruption approach reveals the plasticity and robustness of these self-organizing limb bud and digit patterning systems.

developmental biology↗

bioRxiv2024

Size-dependent temporal decoupling of morphogenesis and transcriptional programs in gastruloids

Understanding the interplay between cell fate specification and morphogenetic changes remains a central challenge in developmental biology. Gastruloids, self-organizing stem cell-based models of post-implantation mammalian development, provide a powerful platform to address this question. Here, we show that physical parameters, particularly system size, critically influence the timing and outcomes of morphogenetic processes. Larger gastruloids exhibit delayed symmetry breaking, increased multipolarity, and prolonged axial elongation, with morphogenesis driven by system size. Despite these variations, transcriptional programs and cell fate composition remain remarkably stable across a broad size range. Notably, extreme sizes show distinct transcriptional modules and clear shifts in gene expression patterns. Intriguingly, size perturbation experiments rescued the morphogenetic and pattern phenotypes observed in extreme sizes, demonstrating the remarkable adaptability of gastruloids to their effective system size. These findings establish gastruloids as versatile models for studying spatiotemporal dynamics in mammalian embryogenesis and reveal how physical constraints decouple transcriptional from morphogenetic programs.

developmental biology↗

bioRxiv2024

CTCF-DEPENDENT INSULATION OF Hoxb13 AND THE HETEROCHRONIC CONTROL OF TAIL LENGTH

In mammals, tail length is controlled by several genetic determinants, amongst which Hox13 genes located at the posterior extremities of Hox clusters, whose main function are to terminate the extension of the body axis. In this view, the precise timing in the transcriptional activation of these genes may impact upon body length. Unlike other Hox clusters, HoxB lacks all posterior genes between Hoxb9 and Hoxb13, two genes separated by a ca. 70 kb large DNA segment containing an unusually high number of CTCF sites, suggesting it isolates Hoxb13 from the rest of the cluster, thereby delaying its negative impact on trunk extension. We deleted the spacer DNA to induce a potential heterochronic gain of function of Hoxb13 at physiological concentration and observed a shortening of the tail as well as other abnormal phenotypes, which were all rescued by inactivating Hoxb13 in-cis with the deletion. A comparable gain of function was observed in mutant ES cells grown as pseudo-embryos in vitro, which allowed us to examine in details the importance of both the number and the orientation of CTCF sites in the insulating activity of the DNA spacer. A short cassette containing all the CTCF sites was sufficient to insulate Hoxb13 from the rest of HoxB and additional modifications of this CTCF cassette showed that two CTCF sites in convergent orientations are already capable of importantly delaying Hoxb13 activation in these conditions. We discuss the relative importance of genomic distance versus number and orientation of CTCF sites in preventing Hoxb13 to be activated too early during trunk extension and hence to modulate tail length.

developmental biology↗

bioRxiv2024

EVOLUTIONARY CO-OPTION OF AN ANCESTRAL CLOACAL REGULATORY LANDSCAPE DURING THE EMERGENCE OF DIGITS AND GENITALS

The transition from fins to limbs has been a rich source of discussion for more than a century. One open and important issue is understanding how the mechanisms that pattern digits arose during vertebrate evolution. In this context, the analysis of Hox gene expression and functions to infer evolutionary scenarios has been a productive approach to explain the changes in organ formation, particularly in limbs. In tetrapods, the transcription of Hoxd genes in developing digits depends on a well-characterized set of enhancers forming a large regulatory landscape1,2. This control system has a syntenic counterpart in zebrafish, even though they lack bona fide digits, suggestive of deep homology3 between distal fin and limb developmental mechanisms. We tested the global function of this landscape to assess ancestry and source of limb and fin variation. In contrast to results in mice, we show here that the deletion of the homologous control region in zebrafish has a limited effect on the transcription of hoxd genes during fin development. However, it fully abrogates hoxd expression within the developing cloaca, an ancestral structure related to the mammalian urogenital sinus. We show that similar to the limb, Hoxd gene function in the urogenital sinus of the mouse also depends on enhancers located in this same genomic domain. Thus, we conclude that the current regulation underlying Hoxd gene expression in distal limbs was co-opted in tetrapods from a preexisting cloacal program. The orthologous chromatin domain in fishes may illustrate a rudimentary or partial step in this evolutionary co-option.

evolutionary biology↗

bioRxiv2023

Cadherins modulate the self-organizing potential of gastruloids

Gastruloids have recently emerged as an efficient four-dimensional model for studying some aspects of post-implantation embryonic patterning. They undergo gastrulation-like processes leading to the self-organization into highly reproducible biological objects. Here, we sought to uncover the molecular and cellular mechanism underlying this remarkable property. We report that self-organization competence is associated with a cell-specific coordination of a Cadherin switch. We find that N-Cadherin hinders gastruloids morphogenetic competence, for its inactivation leads to the formation of trunk-like structures in absence of extra-cellular matrix analogues. In contrast, E-Cadherin repression by Snai1 is critical for self-organization: Snai1 establishes a cell-specific repressive pace by triggering the repression of a pluripotency-associated transcription program and its chromatin landscape, thus allowing a proper transition from E-to N-Cadherin to occur. Altogether, this work establishes a molecular mechanism that integrates the exit from pluripotency and the pace of cell differentiation, leading to the observed self-organizing potential of gastruloids.

developmental biology↗

bioRxiv2023

Chondrogenic Enhancer Landscape of Limb and Axial Skeleton Development

The precisely orchestrated differentiation of chondrocytes during skeleton development is a critical determinant of human height and body shape and disruptions of this process can cause severe skeletal abnormalities. The ultimate size and shape of each of over 200 bones depends on the intricate spatiotemporal regulation of chondrogenic and chondrocyte differentiation genes, but the genomic architecture coordinating these events remains poorly defined. Here we provide a comprehensive map of transcriptional enhancers specifically active in chondrocytes and show that they provide a mechanistic framework through which noncoding genetic variants can influence human stature. We isolated limb and trunk fetal chondrocytes from mice with a Col2a1 fluorescent regulatory sensor and used RNA-seq to identify 780 genes that are specifically expressed during chondrogenesis. To create cell type-specific enhancer maps, we performed ATAC-seq to map open chromatin regions and ChIP-seq for H3K27ac, an enhancer-associated histone modification, and identified 2704 putative chondrogenic enhancer regions. Most (74%) of these enhancers showed pan-chondrogenic activity, with smaller populations being restricted to limb (18%) or trunk (8%) chondrocytes only. We found that chondrogenic enhancers are enriched for the binding of several chondrogenic transcription factors including SOX9. Finally, we find that genetic variation overlapping chondrogenic enhancers explains a higher fraction of the heritability of human adult height than the one overlapping non-chondrogenic enhancers. This data provides a comprehensive mapping of the chondrogenic enhancer repertoire, paving the way to interpreting the role of non-coding sequence polymorphisms in phenotypic variation and bone diseases.

genomics↗

bioRxiv2022

Sequential And Directional Insulation By Conserved CTCF Sites Underlies The Hox Timer In Pseudo-Embryos

During development, Hox genes are activated in a time sequence following their relative positions on their clusters, leading to the proper identities of structures along the rostral to caudal axis. To understand the mechanism operating this Hox timer, we used ES-cells derived stembryos and show that the core of the process involves the start of transcription at the 3 part of the cluster, following Wnt signaling, and the concomitant loading of cohesin complexes on the transcribed DNA segments, i.e., with an asymmetric distribution along the gene cluster. Chromatin extrusion then occurs with successively more posterior CTCF sites acting as transient insulators, thus generating a progressive time-delay in the activation of more 5-located genes due to long-range contacts with a flanking TAD. Mutant stembryos support this model and reveal that the iterated presence of evolutionary conserved and regularly spaced intergenic CTCF sites control the precision and the pace of this temporal mechanism.

developmental biology↗

bioRxiv2022

CONTEXT-DEPENDENT ENHANCER FUNCTION REVEALED BYTARGETED INTER-TAD RELOCATION

The expression of genes with a key function during development is frequently controlled by large regulatory landscapes containing multiple enhancer elements. These landscapes often match Topologically Associating Domains (TADs) and sometimes integrate range of similar enhancers, thus leading to TADs having a global regulatory specificity. To assess the relative functional importance of enhancer sequences versus the regulatory domain they are included in, we set out to transfer one particular enhancer sequence from its native domain into a TAD with a closely related, yet different functional specificity. We used Hoxd genes and their biphasic regulation during limb development as a paradigm, since they are first activated in proximal limb cells by enhancers located in one TAD, which is then silenced at the time when the neighboring TAD starts to activate its enhancers in distal limb cells. We introduced a strong distal limb enhancer into the proximal limb TAD and found that its new context strongly suppresses its distal specificity, even though it continues to be bound by HOX13 transcription factors, which normally are responsible for this activity. Using local genetic alterations and chromatin conformation measurements, we see that the enhancer is capable of interacting with target genes, with a pattern comparable to its adoptive neighborhood of enhancers. Its activity in distal limb cells can be rescued only when a large portion of the surrounding environment is removed. These results indicate that, at least in some cases, the functioning of enhancer elements is subordinated to the local chromatin context, which can exert a dominant control over its activity.

developmental biology↗

bioRxiv2021

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.

developmental biology↗

bioRxiv2021

baredSC: Bayesian Approach to Retrieve Expression Distribution of Single-Cell

The number of studies using single-cell RNA sequencing (scRNA-seq) is constantly growing. This powerful technique provides a sampling of the whole transcriptome of a cell. However, the commonly used droplet-based method often produces very sparse samples. Sparsity can be a major hurdle when studying the distribution of the expression of a specific gene or the correlation between the expressions of two genes. We show that the main technical noise associated with these scRNA-seq experiments is due to the sampling (i.e. Poisson noise). We developed a new tool named baredSC, for Bayesian Approach to Retrieve Expression Distribution of Single-Cell, which infers the intrinsic expression distribution in single-cell data using a Gaussian mixture model (GMM). baredSC can be used to obtain the distribution in one dimension for individual genes and in two dimensions for pairs of genes, in particular to estimate the correlation in the two genes expressions. We apply baredSC to simulated scRNA-seq data and show that the algorithm is able to uncover the expression distribution used to simulate the data, even in multi-modal cases with very sparse data. We also apply baredSC to two real biological data sets. First, we use it to measure the anti-correlation between Hoxd13 and Hoxa11, two genes with known genetic interaction in embryonic limb. Then, we study the expression of Pitx1 in embryonic hindlimb, for which a trimodal distribution has been identified through flow cytometry. While other methods to analyze scRNA-seq are too sensitive to sampling noise, baredSC reveals this trimodal distribution.

bioinformatics↗

bioRxiv2021

CONTEXT-INDEPENDENT FUNCTION OF A CHROMATIN BOUNDARY IN VIVO

Mammalian genomes are partitioned into sub-megabase to megabase-sized units of preferential interactions called topologically associating domains or TADs, which are likely important for the proper implementation of gene regulatory processes. These domains provide structural scaffolds for distant cis regulatory elements to interact with their target genes within the three-dimensional nuclear space and architectural proteins such as CTCF as well as the cohesin complex participate in the formation of the boundaries between them. However, the importance of the genomic context in providing a given DNA sequence the capacity to act as a boundary element remains to be fully investigated. To address this question, we randomly relocated a topological boundary functionally associated with the mouse HoxD gene cluster and show that it can indeed act similarly outside its initial genomic context. In particular, the relocated DNA segment recruited the required architectural proteins and induced a significant depletion of contacts between genomic regions located across the integration site. The host chromatin landscape was re-organized, with the splitting of the TAD wherein the boundary had integrated. These results provide evidence that topological boundaries can function independently of their site of origin, under physiological conditions during mouse development. AUTHOR SUMMARYDuring development, enhancer sequences tightly regulate the spatio-temporal expression of target genes often located hundreds of kilobases away. This complex process is made possible by the folding of chromatin into domains, which are separated from one another by specific genomic regions referred to as boundaries. In order to understand whether such boundary sequences require their particular genomic contexts to achieve their isolating effect, we analyzed the impact of introducing one such boundary, taken from the HoxD gene cluster, into a distinct topological domain. We show that this ectopic boundary splits the host domain into two sub-domains and affects the expression levels of a neighboring gene. We conclude that this sequence can work independently from its genomic context and thus carries all the information necessary to act as a boundary element.

genetics↗

bioRxiv2021

MESOMELIC DYSPLASIAS ASSOCIATED WITH THE HOXD LOCUS ARE CAUSED BY REGULATORY REALLOCATIONS

Some human families display severe shortening and bending of the radius and ulna, a condition referred to as mesomelic dysplasia. Many of these families contain chromosomal rearrangements at 2q31, where the human HOXD locus maps. In mice, the dominant X-ray-induced Ulnaless inversion of the HoxD gene cluster produces a similar phenotype suggesting that the same mechanism is responsible for this pathology in humans and mice. Amongst the proposed explanations, the various alterations to the genomic structure of HOXD could expose Hoxd13 to proximal limb enhancers, leading to its deleterious gain-of-expression in the embryonic forelimb. To assess this hypothesis, we used an engineered 1Mb large inversion including the HoxD gene cluster, in order to position Hoxd13 within a chromatin domain rich in proximal limb enhancers. We show that these enhancers contact and activate Hoxd13 in proximal cells, concomitant to the formation of a mesomelic dysplasia phenotype. A secondary mutation in the coding frame of the HOXD13 protein in-cis with the inversion completely rescued the limb alterations, demonstrating that ectopic HOXD13 is indeed the unique cause of this bone anomaly. Single cell expression analysis and evaluation of HOXD13 binding sites in cells from this ectopic expression domain suggests that the phenotype arises primarily by acting through genes normally controlled by HOXD13 in distal limb cells. Altogether, these results provide a conceptual and mechanistic framework to understand and unify the molecular origins of human mesomelic dysplasia associated with 2q31.

developmental biology↗

bioRxiv2020

HOX13-MEDIATED DBX2 REGULATION IN LIMBS SUGGESTS INTER-TAD SHARING OF ENHANCERS

BackgroundDuring 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. ResultsWe show that HOX13 proteins bind to eutherian-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. ConclusionsWe conclude that Hoxa13 and Hoxd genes cooperatively activate Dbx2 expression in developing digits through binding to eutherian specific regulators elements 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. Bullet pointsO_LIHoxa13 and Hoxd genes cooperatively regulate Dbx2 expression in developing digits via eutherian specific enhancers. C_LIO_LIDbx2 is expressed in different digit joint precursors but its function there is not essential. C_LIO_LIDbx2 enhancers also control the expression of the Nell2 and Ano6 genes, which are located in different TADs, thus overcoming the boundary effect. C_LIO_LIDbx2 chromatin architecture and enhancers evolved in the mammalian lineage. C_LI Grant Sponsor and NumberSwiss National Research Foundation #310030B_138662. European Research Council grants RegulHox #588029

developmental biology↗

bioRxiv2020

SPECIFIC ECTODERMAL ENHANCERS CONTROL THE EXPRESSION OF HoxC GENES IN DEVELOPING MAMMALIAN INTEGUMENTS

Vertebrate Hox genes are key players in the establishment of structures during the development of the main body axis. Subsequently, they play important roles either in organizing secondary axial structures such as the appendages, or during homeostasis in postnatal stages and adulthood. Here we set up to analyze their elusive function in the ectodermal compartment, using the mouse limb bud as a model. We report that the HoxC gene cluster was globally co-opted to be transcribed in the distal limb ectoderm, where it is activated following the rule of temporal colinearity. These ectodermal cells subsequently produce various keratinized organs such as nails or claws. Accordingly, deletion of the HoxC cluster led to mice lacking nails (anonychia) and also hairs (alopecia), a condition stronger than the previously reported loss of function of Hoxc13, which is the causative gene of the ectodermal dysplasia 9 (ECTD9) in human patients. We further identified two ectodermal, mammalian-specific enhancers located upstream of the HoxC gene cluster, which act synergistically to regulate Hoxc gene expression in the hair and nail ectodermal organs. Deletion of these regulatory elements alone or in combination revealed a strong quantitative component in the regulation of Hoxc genes in the ectoderm, suggesting that these two enhancers may have evolved along with mammals to provide the level of HOXC proteins necessary for the full development of hairs and nails. Significance StatementIn this study, we report a unique and necessary function for the HoxC gene cluster in the development of some ectodermal organs, as illustrated both by the hair and nail phenotype displayed by mice lacking the Hoxc13 function and by the congenital anonychia (absence of nails) in full HoxC cluster mutants. We show that Hoxc genes are activated in a colinear manner in the embryonic limb ectoderm and are subsequently transcribed in developing nails and hairs. We identify two mammalian-specific enhancers located upstream of the HoxC cluster with and exclusive ectodermal specificity. Individual or combined enhancer deletions suggest that they act in combination to raise the transcription level of several Hoxc genes during hairs and nails development.

developmental biology↗

bioRxiv2019

A Complex Regulatory Landscape Involved In The Development Of External Genitals

In vertebrates, developmental genes are often controlled by large regulatory landscapes matching the dimensions of topologically associating domains (TADs). In various ontogenic contexts, the associated constitutive chromatin backbone is modified by fine-tuned specific variations in enhancer-enhancer and enhancer-promoter interaction profiles. In this work, we take one of the TADs flanking the HoxD gene cluster as a paradigm to address the question of how these complex regulatory architectures are formed and how they are de-constructed once their function has been achieved. We suggest that this TAD can be considered as a coherent functional unit in itself, with several regulatory sequences acting together to elicit a transcriptional response. With one notable exception, the deletion of each of these sequences in isolation did not produce any substantial modification in the global transcriptional outcome of the system, a result at odds with a conventional view of long-range enhancer function. Likewise, both the deletion and inversion of a supposedly critical CTCF site located in a region rich in such sequences did not affect transcription of the target gene. In the latter case, however, slight modifications were observed in interaction profiles in vivo in agreement with the loop extrusion model, despite no apparent functional consequences. We discuss these unexpected results by considering both conventional explanations and an alternative possibility whereby a rather unspecific accumulation of particular factors within the TAD backbone may have a global impact upon transcription.

developmental biology↗

bioRxiv2019

IMPACT OF GENOME ARCHITECTURE UPON THE FUNCTIONAL ACTIVATION AND REPRESSION OF Hox REGULATORY LANDSCAPES

BackgroundThe 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 are largely determined by bound CTCF molecules and 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.\n\nResultsBy using the HoxD gene cluster as a paradigm, we investigated the effect of large genomic rearrangements affecting the two TADs flanking this locus, 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 initially positioned in either TADs globally occurred as when both TADs are intact. We also show that the timely implementation of distal limb enhancers depends on whether or not target genes had previously responded to proximal enhancers, due to the presence or absence of H3K27me3 marks.\n\nConclusionsFrom this work, we conclude that antagonistic limb proximal and distal enhancers can exert their specificities 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 and thus prolonged its activity in time. Since Polycomb group proteins are mainly recruited at the Hox gene cluster, our results suggest that Polycomb Repressive Complex 2 (PRC2) can extend its coverage to far-cis regulatory sequences as long as confined to the neighboring TAD structure.

developmental biology↗

bioRxiv2019

Constrained Transcriptional Polarity In The Organization Of Mammalian Hox Gene Clusters

In many animal species with a bilateral symmetry, Hox genes are clustered either at one or at several genomic loci. This organization has a functional relevance, as the transcriptional control applied to each gene depends upon its relative position within the gene cluster. It was previously noted that vertebrate Hox clusters display a much higher level of genomic organization than their invertebrate counterparts. The former are always more compact than the latter, they are generally devoid of repeats and of interspersed genes, and all genes are transcribed by the same DNA strand, suggesting that particular factors constrained these clusters towards a tighter structure during the evolution of the vertebrate lineage. Here we investigate the importance of uniform transcriptional orientation by engineering several alleles within the HoxD cluster such as to invert one or several transcription unit(s), with or without a neighboring CTCF site. We observe that the association between the tight structure of mammalian Hox clusters and their regulation makes inversions likely detrimental to the proper implementation of this complex genetic system. We propose that the consolidation of Hox clusters in vertebrates, including transcriptional polarity, evolved in conjunction with the emergence of global gene regulation via the flanking regulatory landscapes, to optimize a coordinated response of selected subsets of target genes in cis.

developmental biology↗