The evolution of lineage-specific regulatory activities in the human embryonic limb.

The evolution of human anatomical features likely involved changes in gene regulation during development. However, the nature and extent of human-specific developmental regulatory functions remain unknown. We obtained a genome-wide view of cis-regulatory evolution in human embryonic tissues by comparing the histone modification H3K27ac, which provides a quantitative readout of promoter and enhancer activity, during human, rhesus, and mouse limb development. Based on increased H3K27ac, we find that 13% of promoters and 11% of enhancers have gained activity on the human lineage since the human-rhesus divergence. These gains largely arose by modification of ancestral regulatory activities in the limb or potential co-option from other tissues and are likely to have heterogeneous genetic causes. Most enhancers that exhibit gain of activity in humans originated in mammals. Gains at promoters and enhancers in the human limb are associated with increased gene expression, suggesting they include molecular drivers of human morphological evolution.

Disrupting the three-dimensional regulatory topology of the Pitx1 locus results in overtly normal development.

Developmental gene expression patterns are orchestrated by thousands of distant-acting transcriptional enhancers. However, identifying enhancers essential for the expression of their target genes has proven challenging. Maps of long-range regulatory interactions may provide the means to identify enhancers crucial for developmental gene expression. To investigate this hypothesis, we used circular chromosome conformation capture coupled with interaction maps in the mouse limb to characterize the regulatory topology of Pitx1, which is essential for hindlimb development. We identified a robust hindlimb-specific interaction between Pitx1 and a putative hindlimb-specific enhancer. To interrogate the role of this interaction in Pitx1 regulation, we used genome editing to delete this enhancer in mouse. Although deletion of the enhancer completely disrupts the interaction, Pitx1 expression in the hindlimb is only mildly affected, without any detectable compensatory interactions between the Pitx1 promoter and potentially redundant enhancers. Pitx1 enhancer null mice did not exhibit any of the characteristic morphological defects of the Pitx1-/- mutant. Our results suggest that robust, tissue-specific physical interactions at essential developmental genes have limited predictive value for identifying enhancer mutations with strong loss-of-function phenotypes.

Origin and evolution of developmental enhancers in the mammalian neocortex.

Morphological innovations such as the mammalian neocortex may involve the evolution of novel regulatory sequences. However, de novo birth of regulatory elements active during morphogenesis has not been extensively studied in mammals. Here, we use H3K27ac-defined regulatory elements active during human and mouse corticogenesis to identify enhancers that were likely active in the ancient mammalian forebrain. We infer the phylogenetic origins of these enhancers and find that ∼20% arose in the mammalian stem lineage, coincident with the emergence of the neocortex. Implementing a permutation strategy that controls for the nonrandom variation in the ages of background genomic sequences, we find that mammal-specific enhancers are overrepresented near genes involved in cell migration, cell signaling, and axon guidance. Mammal-specific enhancers are also overrepresented in modules of coexpressed genes in the cortex that are associated with these pathways, notably ephrin and semaphorin signaling. Our results also provide insight into the mechanisms of regulatory innovation in mammals. We find that most neocortical enhancers did not originate by en bloc exaptation of transposons. Young neocortical enhancers exhibit smaller H3K27ac footprints and weaker evolutionary constraint in eutherian mammals than older neocortical enhancers. Based on these observations, we present a model of the enhancer life cycle in which neocortical enhancers initially emerge from genomic background as short, weakly constrained "proto-enhancers." Many proto-enhancers are likely lost, but some may serve as nucleation points for complex enhancers to evolve.

Transformation of a transposon into a derived prolactin promoter with function during human pregnancy.

Transposable elements (TEs) are known to provide DNA for host regulatory functions, but the mechanisms underlying the transformation of TEs into cis-regulatory elements are unclear. In humans two TEs--MER20 and MER39--contribute the enhancer/promoter for decidual prolactin (dPRL), which is dramatically induced during pregnancy. We show that evolution of the strong human dPRL promoter was a multistep process that took millions of years. First, MER39 inserted near MER20 in the primate/rodent ancestor, and then there were two phases of activity enhancement in primates. Through the mapping of causal nucleotide substitutions, we demonstrate that strong promoter activity in apes involves epistasis between transcription factor binding sites (TFBSs) ancestral to MER39 and derived sites. We propose a mode of molecular evolution that describes the process by which MER20/MER39 was transformed into a strong promoter, called "epistatic capture." Epistatic capture is the stabilization of a TFBS that is ancestral but variable in outgroup lineages, and is fixed in the ingroup because of epistatic interactions with derived TFBSs. Finally, we note that evolution of human promoter activity coincides with the emergence of a unique reproductive character in apes, highly invasive placentation. Because prolactin communicates with immune cells during pregnancy, which regulate fetal invasion into maternal tissues, we speculate that ape dPRL promoter activity evolved in response to increased invasiveness of ape fetal tissue.

The evolution of menstruation: a new model for genetic assimilation: explaining molecular origins of maternal responses to fetal invasiveness.

Why do humans menstruate while most mammals do not? Here, we present our answer to this long-debated question, arguing that (i) menstruation occurs as a mechanistic consequence of hormone-induced differentiation of the endometrium (referred to as spontaneous decidualization, or SD); (ii) SD evolved because of maternal-fetal conflict; and (iii) SD evolved by genetic assimilation of the decidualization reaction, which is induced by the fetus in non-menstruating species. The idea that menstruation occurs as a consequence of SD has been proposed in the past, but here we present a novel hypothesis on how SD evolved. We argue that decidualization became genetically stabilized in menstruating lineages, allowing females to prepare for pregnancy without any signal from the fetus. We present three models for the evolution of SD by genetic assimilation, based on recent advances in our understanding of the mechanisms of endometrial differentiation and implantation. Testing these models will ultimately shed light on the evolutionary significance of menstruation, as well as on the etiology of human reproductive disorders like endometriosis and recurrent pregnancy loss.

Massively parallel disruption of enhancers active in human neural stem cells.

Changes in gene regulation have been linked to the expansion of the human cerebral cortex and to neurodevelopmental disorders, potentially by altering neural progenitor proliferation. However, the effects of genetic variation within regulatory elements on neural progenitors remain obscure. We use sgRNA-Cas9 screens in human neural stem cells (hNSCs) to disrupt 10,674 genes and 26,385 conserved regions in 2,227 enhancers active in the developing human cortex and determine effects on proliferation. Genes with proliferation phenotypes are associated with neurodevelopmental disorders and show biased expression in specific fetal human brain neural progenitor populations. Although enhancer disruptions overall have weaker effects than gene disruptions, we identify enhancer disruptions that severely alter hNSC self-renewal. Disruptions in human accelerated regions, implicated in human brain evolution, also alter proliferation. Integrating proliferation phenotypes with chromatin interactions reveals regulatory relationships between enhancers and their target genes contributing to neurogenesis and potentially to human cortical evolution.

Convergent evolution of endometrial prolactin expression in primates, mice, and elephants through the independent recruitment of transposable elements.

Prolactin (PRL) is a multifunctional signaling molecule best known for its role in regulating lactation in mammals. Systemic PRL is produced by the anterior pituitary, but extrapituitary PRL has also been detected in many tissues including the human endometrium. Prolactin is essential for pregnancy in rodents and one of the most dramatically induced genes in the endometrium during human pregnancy. The promoter for human endometrial Prl is located about 5.8 kb upstream of the pituitary promoter and is derived from a transposable element called MER39. Although it has been shown that prolactin is expressed in the pregnant endometrium of a few mammals other than humans, MER39 has been described as primate specific. Thus, in an effort to understand mechanisms of prolactin regulatory evolution, we sought to determine how uterine prolactin is transcribed in species that lack MER39. Using a variety of complementary strategies, including reverse transcriptase-polymerase chain reaction, 5' rapid amplification of cDNA ends, and whole-transcriptome sequencing, we show that endometrial Prl expression is not a shared character of all placental mammals, as it is not expressed in rabbits, pigs, dogs, or armadillos. We show that in primates, mice, and elephants, prolactin mRNA is transcribed in the pregnant endometrium from alternative promoters, different from the pituitary promoter and different from each other. Moreover, we demonstrate that the spider monkey promoter derives from the long terminal repeat (LTR) element MER39 as in humans, the mouse promoter derives from the LTR element MER77, and the elephant promoter derives from the lineage-specific LINE retrotransposon L1-2_LA. We also find surprising variation of transcriptional start sites within these transposable elements and of Prl splice variants, suggesting a high degree of flexibility in the promoter architecture even among closely related species. Finally, the three groups shown here to express endometrial prolactin-the higher primates, the rodents, and the elephant-represent three of the four lineages that showed adaptive evolution of the Prl gene in an earlier study (Wallis M. 2000. Episodic evolution of protein hormones: molecular evolution of pituitary prolactin. J Mol Evol. 50:465-473), which supports our findings and suggests that the selective forces responsible for accelerated Prl evolution were in the endometrium. This is the first reported case of convergent evolution of gene expression through the independent recruitment of different transposable elements, highlighting the importance of transposable elements in gene regulatory, and potentially adaptive, evolution.

An evolutionary test of the isoform switching hypothesis of functional progesterone withdrawal for parturition: humans have a weaker repressive effect of PR-A than mice.

BACKGROUND: A decrease in maternal serum progesterone (P4) concentrations precedes the onset of labor in most placental mammals. Humans differ by maintaining high levels of P4 throughout birth. Parturition in humans probably includes mechanisms that undercut the pregnancy sustaining function of P4. One attractive hypothesis is the isoform switching hypothesis (ISH). ISH is supported by in vitro evidence that progesterone receptor isoform A (PR-A) inhibits PR-B and that the PR-A/PR-B ratio increases towards term. MATERIALS AND METHODS: Here, we test the hypothesis that isoform switching is an adaptation to high levels of P4 at term, predicting that, in humans, PR-A mediated repression of PR-B is stronger than in mouse. We use reporter assays with human and mouse PRs to detect species differences in the repressive effects of PR-A. RESULTS: We found that human PR-B is less sensitive to repression by human PR-A than mouse PR-B, contrary to our prediction. The difference between human and mouse PR-B sensitivity is most pronounced at PR-A/PR-B ratios typical for the preterm myometrium. CONCLUSIONS: Our results are inconsistent with the ISH. We speculate that, instead, the lower sensitivity of human PR-B to PR-A may be relevant for the maintenance of pregnancy at high progesterone levels and increasing PR-A concentrations towards term.

Modeling uniquely human gene regulatory function via targeted humanization of the mouse genome.

The evolution of uniquely human traits likely entailed changes in developmental gene regulation. Human Accelerated Regions (HARs), which include transcriptional enhancers harboring a significant excess of human-specific sequence changes, are leading candidates for driving gene regulatory modifications in human development. However, insight into whether HARs alter the level, distribution, and timing of endogenous gene expression remains limited. We examined the role of the HAR HACNS1 (HAR2) in human evolution by interrogating its molecular functions in a genetically humanized mouse model. We find that HACNS1 maintains its human-specific enhancer activity in the mouse embryo and modifies expression of Gbx2, which encodes a transcription factor, during limb development. Using single-cell RNA-sequencing, we demonstrate that Gbx2 is upregulated in the limb chondrogenic mesenchyme of HACNS1 homozygous embryos, supporting that HACNS1 alters gene expression in cell types involved in skeletal patterning. Our findings illustrate that humanized mouse models provide mechanistic insight into how HARs modified gene expression in human evolution.

Adaptive changes in the transcription factor HoxA-11 are essential for the evolution of pregnancy in mammals.

Evolutionary change in gene regulation can result from changes in cis-regulatory elements, leading to differences in the temporal and spatial expression of genes or in the coding region of transcription factors leading to novel functions or both. Although there is a growing body of evidence supporting the importance of cis-regulatory evolution, examples of protein-mediated evolution of novel developmental pathways have not been demonstrated. Here, we investigate the evolution of prolactin (PRL) expression in endometrial cells, which is essential for placentation/pregnancy in eutherian mammals and is a direct regulatory target of the transcription factor HoxA-11. Here, we show that (i) endometrial PRL expression is a derived feature of placental mammals, (ii) the PRL regulatory gene HoxA-11 experienced a period of strong positive selection in the stem-lineage of eutherian mammals, and (iii) only HoxA-11 proteins from placental mammals, including the reconstructed ancestral eutherian gene, are able to up-regulate PRL from the promoter used in endometrial cells. In contrast, HoxA-11 from the reconstructed therian ancestor, opossum, platypus, and chicken are unable to up-regulate PRL expression. These results demonstrate that the evolution of novel gene expression domains is not only mediated by the evolution of cis-regulatory elements but can also require evolutionary changes of transcription factor proteins themselves.

Forty keratin-associated beta-proteins (beta-keratins) form the hard layers of scales, claws, and adhesive pads in the green anole lizard, Anolis carolinensis.

Using bioinformatic methods we have detected the genes of 40 keratin-associated beta-proteins (KAbetaPs) (beta-keratins) from the first available draft genome sequence of a reptile, the lizard Anolis carolinensis (Broad Institute, Boston). All genes are clustered in a single but not yet identified chromosomal locus, and contain a single intron of variable length. 5'-RACE and RT-PCR analyses using RNA from different epidermal regions show tissue-specific expression of different transcripts. These results were confirmed from the analysis of the A. carolinensis EST libraries (Broad Institute). Most deduced proteins are 12-16 kDa with a pI of 7.5-8.5. Two genes encoding putative proteins of 40 and 45 kDa are also present. Despite variability in amino acid sequences, four main subfamilies can be described. The largest subfamily includes proteins high in glycine, a small subfamily contains proteins high in cysteine, a third large subfamily contains proteins high in cysteine and glycine, and the fourth, smallest subfamily comprises proteins low in cysteine and glycine. An inner region of high amino acid identity is the most constant characteristic of these proteins and maps to a region with two to three close beta-folds in the proteins. This beta-fold region is responsible for the formation of filaments of the corneous material in all types of scales in this species. Phylogenetic analysis shows that A. carolinensis KAbetaPs are more similar to those of other lepidosaurians (snake, lizard, and gecko lizard) than to those of archosaurians (chick and crocodile) and turtles.

Beta-keratins of the crocodilian epidermis: composition, structure, and phylogenetic relationships.

Nucleotide and deduced amino acid sequences of three beta-keratins of Nile crocodile scales are presented. Using 5'- and 3'-RACE analysis, two cDNA sequences of 1 kb (Cr-gptrp-1) and 1.5 kb (Cr-gptrp-2) were determined, corresponding to 17.4 and 19.3 kDa proteins, respectively, and a pI of 8.0. In genomic DNA amplifications, we determined that the 5'-UTR of Cr-gptrp-2 contains an intron of 621 nucleotides. In addition, we isolated a third gene (Cr-gptrp-3) in genomic DNA amplifications that exhibits seven amino acid differences with Cr-gptrp-2. Genomic organization of the sequenced crocodilian beta-keratin genes is similar to avian beta-keratin genes. Deduced proteins are rich in glycine, proline, serine, and tyrosine, and contain cysteines toward the N- and C-terminal regions, likely for the formation of disulfide bonds. Prediction of the secondary structure suggests that the central core box of 20 amino acids contains two beta-strands and has 75-90% identity with chick beta-keratins. Toward the C-terminus, numerous glycine-glycine-tyrosine and glycine-glycine-leucine repeats are present, which may contribute to making crocodile scales hard. In situ hybridization shows expression of beta-keratin genes in differentiating beta-cells of epidermal transitional layers. Phylogenetic analysis of the available archosaurian and lepidosaurian beta-keratins suggests that feather keratins diversified early from nonfeather keratins, deep in archosaur evolution. However, only the complete knowledge of all crocodilian beta-keratins will confirm whether feather keratins have an origin independent of those in bird scales, which preceded the split between birds and crocodiles.

Beta-keratins of turtle shell are glycine-proline-tyrosine rich proteins similar to those of crocodilians and birds.

This study presents, for the first time, sequences of five beta-keratin cDNAs from turtle epidermis obtained by means of 5'- and 3'-rapid amplification of cDNA ends (RACE) analyses. The deduced amino acid sequences correspond to distinct glycine-proline-serine-tyrosine rich proteins containing 122-174 amino acids. In situ hybridization shows that beta-keratin mRNAs are expressed in cells of the differentiating beta-layers of the shell scutes. Southern blotting analysis reveals that turtle beta-keratins belong to a well-conserved multigene family. This result was confirmed by the amplification and sequencing of 13 genomic fragments corresponding to beta-keratin genes. Like snake, crocodile and avian beta-keratin genes, turtle beta-keratins contain an intron that interrupts the 5'-untranslated region. The length of the intron is variable, ranging from 0.35 to 1.00 kb. One of the sequences obtained from genomic amplifications corresponds to one of the five sequences obtained from cDNA cloning; thus, sequences of a total of 17 turtle beta-keratins were determined in the present study. The predicted molecular weight of the 17 different deduced proteins range from 11.9 to 17.0 kDa with a predicted isoelectric point of 6.8-8.4; therefore, they are neutral to basic proteins. A central region rich in proline and with beta-strand conformation shows high conservation with other reptilian and avian beta-keratins, and it is likely involved in their polymerization. Glycine repeat regions, often containing tyrosine, are localized toward the C-terminus. Phylogenetic analysis shows that turtle beta-keratins are more similar to crocodilian and avian beta-keratins than to those of lizards and snakes.

The Transcriptomic Evolution of Mammalian Pregnancy: Gene Expression Innovations in Endometrial Stromal Fibroblasts.

The endometrial stromal fibroblast (ESF) is a cell type present in the uterine lining of therian mammals. In the stem lineage of eutherian mammals, ESF acquired the ability to differentiate into decidual cells in order to allow embryo implantation. We call the latter cell type "neo-ESF" in contrast to "paleo-ESF" which is homologous to eutherian ESF but is not able to decidualize. In this study, we compare the transcriptomes of ESF from six therian species: Opossum (Monodelphis domestica; paleo-ESF), mink, rat, rabbit, human (all neo-ESF), and cow (secondarily nondecidualizing neo-ESF). We find evidence for strong stabilizing selection on transcriptome composition suggesting that the expression of approximately 5,600 genes is maintained by natural selection. The evolution of neo-ESF from paleo-ESF involved the following gene expression changes: Loss of expression of genes related to inflammation and immune response, lower expression of genes opposing tissue invasion, increased markers for proliferation as well as the recruitment of FOXM1, a key gene transiently expressed during decidualization. Signaling pathways also evolve rapidly and continue to evolve within eutherian lineages. In the bovine lineage, where invasiveness and decidualization were secondarily lost, we see a re-expression of genes found in opossum, most prominently WISP2, and a loss of gene expression related to angiogenesis. The data from this and previous studies support a scenario, where the proinflammatory paleo-ESF was reprogrammed to express anti-inflammatory genes in response to the inflammatory stimulus coming from the implanting conceptus and thus paving the way for extended, trans-cyclic gestation.

Evolutionary genomics. Evolutionary changes in promoter and enhancer activity during human corticogenesis.

Human higher cognition is attributed to the evolutionary expansion and elaboration of the human cerebral cortex. However, the genetic mechanisms contributing to these developmental changes are poorly understood. We used comparative epigenetic profiling of human, rhesus macaque, and mouse corticogenesis to identify promoters and enhancers that have gained activity in humans. These gains are significantly enriched in modules of coexpressed genes in the cortex that function in neuronal proliferation, migration, and cortical-map organization. Gain-enriched modules also showed correlated gene expression patterns and similar transcription factor binding site enrichments in promoters and enhancers, suggesting that they are connected by common regulatory mechanisms. Our results reveal coordinated patterns of potential regulatory changes associated with conserved developmental processes during corticogenesis, providing insight into human cortical evolution.

Ancient transposable elements transformed the uterine regulatory landscape and transcriptome during the evolution of mammalian pregnancy.

A major challenge in biology is determining how evolutionarily novel characters originate; however, mechanistic explanations for the origin of new characters are almost completely unknown. The evolution of pregnancy is an excellent system in which to study the origin of novelties because mammals preserve stages in the transition from egg laying to live birth. To determine the molecular bases of this transition, we characterized the pregnant/gravid uterine transcriptome from tetrapods to trace the evolutionary history of uterine gene expression. We show that thousands of genes evolved endometrial expression during the origins of mammalian pregnancy, including genes that mediate maternal-fetal communication and immunotolerance. Furthermore, thousands of cis-regulatory elements that mediate decidualization and cell-type identity in decidualized stromal cells are derived from ancient mammalian transposable elements (TEs). Our results indicate that one of the defining mammalian novelties evolved from DNA sequences derived from ancient mammalian TEs co-opted into hormone-responsive regulatory elements distributed throughout the genome.

Transposable element recruitments in the mammalian placenta: impacts and mechanisms.

Transposable elements (TEs) are mobile DNA elements found at high frequency in mammalian genomes. Although these elements are generally perceived as genomic parasites, they have the potential to influence host genome function in many beneficial ways. This article discusses the role TEs have played in the evolution of the placenta and pregnancy in viviparous mammals. Using examples from our own research and the literature, we argue that frequent recruitment of TEs, in particular of retroelements, has facilitated the extreme diversification of tissues at the maternal-fetal interface. We also discuss the mechanisms by which TEs have been recruited for functions during pregnancy. We argue that retroelements are pre-adapted to becoming cis-regulatory elements for host genomes because they need to utilize host regulatory signals for their own life cycle. However, although TEs contain some of the signals necessary for host functions upon insertion, they often require modification before acquiring a biological role in a host tissue. We discuss the process by which one TE was transformed into a promoter for prolactin expression in the endometrium, describing a model for TE domestication called 'epistatic capture'.