Polycomb subunits Ezh1 and Ezh2 regulate the Merkel cell differentiation program in skin stem cells

Identification of multiple transcription factor genes potentially involved in the development of electrosensory versus mechanosensory lateral line organs

In electroreceptive jawed vertebrates, embryonic lateral line placodes give rise to electrosensory ampullary organs as well as mechanosensory neuromasts. Previous reports of shared gene expression suggest that conserved mechanisms underlie electroreceptor and mechanosensory hair cell development and that electroreceptors evolved as a transcriptionally related "sister cell type" to hair cells. We previously identified only one transcription factor gene, Neurod4, as ampullary organ-restricted in the developing lateral line system of a chondrostean ray-finned fish, the Mississippi paddlefish (Polyodon spathula). The other 16 transcription factor genes we previously validated in paddlefish were expressed in both ampullary organs and neuromasts. Here, we used our published lateral line organ-enriched gene-set (arising from differential bulk RNA-seq in late-larval paddlefish), together with a candidate gene approach, to identify 25 transcription factor genes expressed in the developing lateral line system of a more experimentally tractable chondrostean, the sterlet (Acipenser ruthenus, a small sturgeon), and/or that of paddlefish. Thirteen are expressed in both ampullary organs and neuromasts, consistent with conservation of molecular mechanisms. Seven are electrosensory-restricted on the head (Irx5, Irx3, Insm1, Sp5, Satb2, Mafa and Rorc), and five are the first-reported mechanosensory-restricted transcription factor genes (Foxg1, Sox8, Isl1, Hmx2 and Rorb). However, as previously reported, Sox8 is expressed in ampullary organs as well as neuromasts in a catshark (Scyliorhinus canicula), suggesting the existence of lineage-specific differences between cartilaginous and ray-finned fishes. Overall, our results support the hypothesis that ampullary organs and neuromasts develop via largely conserved transcriptional mechanisms, and identify multiple transcription factors potentially involved in the formation of electrosensory versus mechanosensory lateral line organs.

Different expression patterns of p63 and p73 in Felis catus papillomavirus type 2-associated feline Merkel cell carcinomas and other epidermal carcinomas

Merkel cell carcinoma (MCC) is a cutaneous neuroendocrine tumor, and more than 90% of feline MCC cases test positive for Felis catus papillomavirus type 2 (FcaPV2). In the present study, basal cell markers p40, p63, and p73 and the stem cell marker SOX2 and cytokeratin 14 (CK14) were immunohistochemically examined in normal fetal, infant, and adult feline skin tissues. The expression of these proteins was examined in tumors positive for FcaPV2, including MCC, basal cell carcinoma (BCC), Bowenoid in situ carcinoma (BISC), and squamous cell carcinoma (SCC). Infant and adult feline skin tissues had mature Merkel cells, which were CK14-, CK18+, CK20+, SOX2+, synaptophysin+ and CD56+, while fetal skin tissue had no mature Merkel cells. MCC was immunopositive for p73, CK18, and SOX2 in 32/32 cases, and immunonegative for CK14 in 31/32 cases and for p40 and p63 in 32/32 cases. These results indicate that MCC exhibits different immunophenotypes from Merkel cells (p73-) and basal cells (p40+, p63+, and SOX2-). In contrast, all 3 BCCs, 1 BISC, and 2 SCCs were immunopositive for the basal cell markers p40, p63, and p73. The life cycle of papillomavirus is closely associated with the differentiation of infected basal cells, which requires the transcription factor p63. Changes in p63 expression in FcaPV2-positive MCC may be associated with unique cytokeratin expression patterns (CK14-, CK18+, and CK20+). Furthermore, SOX2 appears to be involved in Merkel cell differentiation in cats, similar to humans and mice.

SOX family transcription factors as therapeutic targets in wound healing: A comprehensive review

The SOX family plays a vital role in determining the fate of cells and has garnered attention in the fields of cancer research and regenerative medicine. It also shows promise in the study of wound healing, as it actively participates in the healing processes of various tissues such as skin, fractures, tendons, and the cornea. However, our understanding of the mechanisms behind the SOX family's involvement in wound healing is limited compared to its role in cancer. Gaining insight into its role, distribution, interaction with other factors, and modifications in traumatized tissues could provide valuable new knowledge about wound healing. Based on current research, SOX2, SOX7, and SOX9 are the most promising members of the SOX family for future interventions in wound healing. SOX2 and SOX9 promote the renewal of cells, while SOX7 enhances the microvascular environment. The SOX family holds significant potential for advancing wound healing research. This article provides a comprehensive review of the latest research advancements and therapeutic tools related to the SOX family in wound healing, as well as the potential benefits and challenges of targeting the SOX family for wound treatment.

Distinct Regulation of EZH2 and its Repressive H3K27me3 Mark in Polyomavirus-Positive and -Negative Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is an aggressive skin cancer for which Merkel cell polyomavirus integration and expression of viral oncogenes small T and Large T have been identified as major oncogenic determinants. Recently, a component of the PRC2 complex, the histone methyltransferase enhancer of zeste homolog 2 (EZH2) that induces H3K27 trimethylation as a repressive mark has been proposed as a potential therapeutic target in MCC. Because divergent results have been reported for the levels of EZH2 and trimethylation of lysine 27 on histone 3, we analyzed these factors in a large MCC cohort to identify the molecular determinants of EZH2 activity in MCC and to establish MCC cell lines' sensitivity to EZH2 inhibitors. Immunohistochemical expression of EZH2 was observed in 92% of MCC tumors (156 of 170), with higher expression levels in virus-positive than virus-negative tumors (P = 0.026). For the latter, we showed overexpression of EZHIP, a negative regulator of the PRC2 complex. In vitro, ectopic expression of the large T antigen in fibroblasts led to the induction of EZH2 expression, whereas the knockdown of T antigens in MCC cell lines resulted in decreased EZH2 expression. EZH2 inhibition led to selective cytotoxicity on virus-positive MCC cell lines. This study highlights the distinct mechanisms of EZH2 induction between virus-negative and -positive MCC.

Current and preclinical treatment options for Merkel cell carcinoma

INTRODUCTION

Merkel cell carcinoma (MCC) is a rare, highly aggressive form of skin cancer with neuroendocrine features. The origin of this cancer is still unclear, but research in the last 15 years has demonstrated that MCC arises via two distinct etiologic pathways, i.e. virus and UV-induced. Considering the high mortality rate and the limited therapeutic options available, this review aims to highlight the significance of MCC research and the need for advancement in MCC treatment.

AREAS COVERED

With the advent of the immune checkpoint inhibitor therapies, we now have treatment options providing a survival benefit for patients with advanced MCC. However, the issue of primary and acquired resistance to these therapies remains a significant concern. Therefore, ongoing efforts seeking additional therapeutic targets and approaches for MCC therapy are a necessity. Through a comprehensive literature search, we provide an overview on recent preclinical and clinical studies with respect to MCC therapy.

EXPERT OPINION

Currently, the only evidence-based therapy for MCC is immune checkpoint blockade with anti-PD-1/PD-L1 for advanced patients. Neoadjuvant, adjuvant and combined immune checkpoint blockade are promising treatment options.

SOX2-Sensing: Insights into the Role of SOX2 in the Generation of Sensory Cell Types in Vertebrates

The SOX2 transcription factor is a key regulator of nervous system development, and its mutation in humans leads to a rare disease characterized by severe eye defects, cognitive defects, hearing defects, abnormalities of the CNS and motor control problems. SOX2 has an essential role in neural stem cell maintenance in specific regions of the brain, and it is one of the master genes required for the generation of induced pluripotent stem cells. Sox2 is expressed in sensory organs, and this review will illustrate how it regulates the differentiation of sensory cell types required for hearing, touching, tasting and smelling in vertebrates and, in particular, in mice.

Dermal appendage-dependent patterning of zebrafish atoh1a+ Merkel cells

Touch system function requires precise interactions between specialized skin cells and somatosensory axons, as exemplified by the vertebrate mechanosensory Merkel cell-neurite complex. Development and patterning of Merkel cells and associated neurites during skin organogenesis remain poorly understood, partly due to the in utero development of mammalian embryos. Here, we discover Merkel cells in the zebrafish epidermis and identify Atonal homolog 1a (Atoh1a) as a marker of zebrafish Merkel cells. We show that zebrafish Merkel cells derive from basal keratinocytes, express neurosecretory and mechanosensory machinery, extend actin-rich microvilli, and complex with somatosensory axons, all hallmarks of mammalian Merkel cells. Merkel cells populate all major adult skin compartments, with region-specific densities and distribution patterns. In vivo photoconversion reveals that Merkel cells undergo steady loss and replenishment during skin homeostasis. Merkel cells develop concomitant with dermal appendages along the trunk and loss of Ectodysplasin signaling, which prevents dermal appendage formation, reduces Merkel cell density by affecting cell differentiation. By contrast, altering dermal appendage morphology changes the distribution, but not density, of Merkel cells. Overall, our studies provide insights into touch system maturation during skin organogenesis and establish zebrafish as an experimentally accessible in vivo model for the study of Merkel cell biology.

Polycomb Repressive Complex 2 in Oncology

Dynamic regulation of the chromatin state by Polycomb Repressive Complex 2 (PRC2) provides an important mean for epigenetic gene control that can profoundly influence normal development and cell lineage specification. PRC2 and PRC2-induced methylation of histone H3 lysine 27 (H3K27) are critically involved in a wide range of DNA-templated processes, which at least include transcriptional repression and gene imprinting, organization of three-dimensional chromatin structure, DNA replication and DNA damage response and repair. PRC2-based genome regulation often goes wrong in diseases, notably cancer. This chapter discusses about different modes-of-action through which PRC2 and EZH2, a catalytic subunit of PRC2, mediate (epi)genomic and transcriptomic regulation. We will also discuss about how alteration or mutation of the PRC2 core or axillary component promotes oncogenesis, how post-translational modification regulates functionality of EZH2 and PRC2, and how PRC2 and other epigenetic pathways crosstalk. Lastly, we will briefly touch on advances in targeting EZH2 and PRC2 dependence as cancer therapeutics.

Current In Vitro and In Vivo Models to Study MCPyV-Associated MCC

Merkel cell polyomavirus (MCPyV) is the only human polyomavirus currently known to cause human cancer. MCPyV is believed to be an etiological factor in at least 80% of cases of the rare but aggressive skin malignancy Merkel cell carcinoma (MCC). In these MCPyV+ MCC tumors, clonal integration of the viral genome results in the continued expression of two viral proteins: the viral small T antigen (ST) and a truncated form of the viral large T antigen. The oncogenic potential of MCPyV and the functional properties of the viral T antigens that contribute to neoplasia are becoming increasingly well-characterized with the recent development of model systems that recapitulate the biology of MCPyV+ MCC. In this review, we summarize our understanding of MCPyV and its role in MCC, followed by the current state of both in vitro and in vivo model systems used to study MCPyV and its contribution to carcinogenesis. We also highlight the remaining challenges within the field and the major considerations related to the ongoing development of in vitro and in vivo models of MCPyV+ MCC.

Merkel Cell Carcinoma Sensitivity to EZH2 Inhibition Is Mediated by SIX1 Derepression

Polycomb repressive complex 2 has a critical role in the maintenance of bivalent promoters and is often perturbed in cancer, including neuroendocrine tumors. In this study, we investigated the susceptibility of Merkel cell carcinoma (MCC), a neuroendocrine carcinoma of the skin, to inhibitors of the Polycomb repressive complex 2 catalytic subunit EZH2. We show that a subset of MCC cell lines is sensitive to EZH2 inhibitor-induced cell viability loss. We find that inhibitor treatment of susceptible cells derepresses the Polycomb repressive complex 2 target SIX1, a transcription factor in the PAX-SIX-EYA-DACH network normally involved in inner ear hair cell development, and that PAX-SIX-EYA-DACH network transcription factors are critical contributors to EZH2 inhibitor-induced MCC cell viability loss. Furthermore, we show the EZH2 inhibitor tazemetostat slows the growth of MCC xenografts and derepresses SIX1 and its downstream inner ear transcriptional target MYO6 in vivo. We propose that EZH2 inhibition in MCC leads to SIX1 derepression with dysregulation of hearing-related transcriptional programs and growth inhibition. This study provides evidence that MCC tumors may be specifically susceptible to EZH2 inhibitors, while giving mechanistic insight into the transcriptional programs these inhibitors perturb in MCC, and potentially in other neuroendocrine cancers.

Merkel cell polyomavirus and associated Merkel cell carcinoma

Merkel cell polyomavirus (MCPyV) is a ubiquitous skin infection that can cause Merkel cell carcinoma (MCC), a highly lethal form of skin cancer with a nearly 50% mortality rate. Since the discovery of MCPyV in 2008, great advances have been made to improve our understanding of how the viral encoded oncoproteins contribute to MCC oncogenesis. However, our knowledge of the MCPyV infectious life cycle and its oncogenic mechanisms are still incomplete. The incidence of MCC has tripled over the past two decades, but effective treatments are lacking. Only recently have there been major victories in combatting metastatic MCC with the application of PD-1 immune checkpoint blockade. Still, these immune-based therapies are not ideal for patients with a medical need to maintain systemic immune suppression. As such, a better understanding of MCPyV's oncogenic mechanisms is needed in order to develop more effective and targeted therapies against virus-associated MCC. In this review, we discuss current areas of interest for MCPyV and MCC research and the progress made in elucidating both the natural host of MCPyV infection and the cell of origin for MCC. We also highlight the remaining gaps in our knowledge on the transcriptional regulation of MCPyV, which may be key to understanding and targeting viral oncogenesis for developing future therapies.

Investigation of the RB1-SOX2 axis constitutes a tool for viral status determination and diagnosis in Merkel cell carcinoma

MCC (Merkel cell carcinoma) is an aggressive neuroendocrine cutaneous neoplasm. Integration of the Merkel cell polyomavirus (MCPyV) is observed in about 80% of the cases, while the remaining 20% are related to UV exposure. Both MCPyV-positive and -negative MCCs-albeit by different mechanisms-are associated with RB1 inactivation leading to overexpression of SOX2, a major contributor to MCC biology. Moreover, although controversial, loss of RB1 expression seems to be restricted to MCPyV-negative cases.The aim of the present study was to assess the performances of RB1 loss and SOX2 expression detected by immunohistochemistry to determine MCPyV status and to diagnose MCC, respectively.Overall, 196 MCC tumors, 233 non-neuroendocrine skin neoplasms and 70 extra-cutaneous neuroendocrine carcinomas (NEC) were included. SOX2 and RB1 expressions were assessed by immunohistochemistry in a tissue micro-array. Diagnostic performances were determined using the likelihood ratio (LHR).RB1 expression loss was evidenced in 27% of the MCC cases, 12% of non-neuroendocrine skin tumors and 63% of extra-cutaneous NEC. Importantly, among MCC cases, RB1 loss was detected in all MCPyV(-) MCCs, while MCPyV( +) cases were consistently RB1-positive (p < 0.001). SOX2 diffuse expression was observed in 92% of the MCC cases and almost never observed in non-neuroendocrine skin epithelial neoplasms (2%, p < 0.0001, LHR +  = 59). Furthermore, SOX2 diffuse staining was more frequently observed in MCCs than in extra-cutaneous NECs (30%, p < 0.001, LHR +  = 3.1).These results confirm RB1 as a robust predictor of MCC viral status and further suggest SOX2 to be a relevant diagnostic marker of MCC.

EZH2 modulates retinoic acid signaling to ensure myotube formation during development

Sequential differentiation of pre-somitic progenitors into myocytes and subsequently into myotubes and myofibers is essential for the myogenic differentiation program (MDP) crucial for muscle development. Signaling factors involved in MDP are Polycomb Repressive Complex 2 (PRC2) targets in various developmental contexts. PRC2 is active in the developing myotomes during MDP, but how it regulates MDP is unclear. Here, we found that myocyte differentiation to myotubes requires Enhancer of Zeste 2 (EZH2), the catalytic component of PRC2. We observed elevated retinoic-acid (RA) signaling in the prospective myocytes in the Ezh2 mutants (E8.5-Mus^Ezh2 ), and its inhibition can partially rescue the myocyte differentiation defect. Together, our data demonstrate a new role for PRC2-EZH2 during myocyte differentiation into myotubes by modulating RA signaling.

Dermal EZH2 orchestrates dermal differentiation and epidermal proliferation during murine skin development

Skin development and patterning is dependent on factors that regulate the stepwise differentiation of dermal fibroblasts concomitant with dermal-epidermal reciprocal signaling, two processes that are poorly understood. Here we show that dermal EZH2, the methyltransferase enzyme of the epigenetic Polycomb Repressive Complex 2 (PRC2), is a new coordinator of both these processes. Dermal EZH2 activity is present during dermal fibroblast differentiation and is required for spatially restricting Wnt/β-catenin signaling to reinforce dermal fibroblast cell fate. Later in development, dermal EZH2 regulates the expression of reticular dermal markers and initiation of secondary hair follicles. Embryos lacking dermal Ezh2 have elevated epidermal proliferation and differentiation that can be rescued by small molecule inhibition of retinoic acid (RA) signaling. Together, our study reveals that dermal EZH2 is acting like a rheostat to control the levels of Wnt/β-catenin and RA signaling to impact fibroblast differentiation cell autonomously and epidermal keratinocyte development non-cell autonomously, respectively.

From Merkel Cell Polyomavirus Infection to Merkel Cell Carcinoma Oncogenesis

Merkel cell polyomavirus (MCPyV) infection causes near-ubiquitous, asymptomatic infection in the skin, but occasionally leads to an aggressive skin cancer called Merkel cell carcinoma (MCC). Epidemiological evidence suggests that poorly controlled MCPyV infection may be a precursor to MCPyV-associated MCC. Clearer understanding of host responses that normally control MCPyV infection could inform prophylactic measures in at-risk groups. Similarly, the presence of MCPyV in most MCCs could imbue them with vulnerabilities that-if better characterized-could yield targeted intervention solutions for metastatic MCC cases. In this review, we discuss recent developments in elucidating the interplay between host cells and MCPyV within the context of viral infection and MCC oncogenesis. We also propose a model in which insufficient restriction of MCPyV infection in aging and chronically UV-damaged skin causes unbridled viral replication that licenses MCC tumorigenesis.

Characterizing DNA methylation signatures and their potential functional roles in Merkel cell carcinoma

BACKGROUND

Merkel cell carcinoma (MCC) is a rare but aggressive skin cancer with limited treatment possibilities. Merkel cell tumors display with neuroendocrine features and Merkel cell polyomavirus (MCPyV) infection in the majority (80%) of patients. Although loss of histone H3 lysine 27 trimethylation (H3K27me3) has been shown during MCC tumorigenesis, epigenetic dysregulation has largely been overlooked.

METHODS

We conducted global DNA methylation profiling of clinically annotated MCC primary tumors, metastatic skin tumors, metastatic lymph node tumors, paired normal tissues, and two human MCC cell lines using the Illumina Infinium EPIC DNA methylation BeadArray platform.

RESULTS

Significant differential DNA methylation patterns across the genome are revealed between the four tissue types, as well as based on MCPyV status. Furthermore, 964 genes directly regulated by promoter or gene body DNA methylation were identified with high enrichment in neuro-related pathways. Finally, our findings suggest that loss of H3K27me3 occupancy in MCC is attributed to KDM6B and EZHIP overexpression as a consequence of promoter DNA hypomethylation.

CONCLUSIONS

We have demonstrated specific DNA methylation patterns for primary MCC tumors, metastatic MCCs, and adjacent-normal tissues. We have also identified DNA methylation markers that not only show potential diagnostic or prognostic utility in MCC management, but also correlate with MCC tumorigenesis, MCPyV expression, neuroendocrine features, and H3K27me3 status. The identification of DNA methylation alterations in MCC supports the need for further studies to understand the clinical implications of epigenetic dysregulation and potential therapeutic targets in MCC.

Epigenetic regulation and signalling pathways in Merkel cell development

Merkel cells are specialized epithelial cells connected to afferent nerve endings responsible for light-touch sensations, formed at specific locations in touch-sensitive regions of the mammalian skin. Although Merkel cells are descendants of the epidermal lineage, little is known about the mechanisms responsible for the development of these unique mechanosensory cells. Recent studies have highlighted that the Polycomb group (PcG) of proteins play a significant role in spatiotemporal regulation of Merkel cell formation. In addition, several of the major signalling pathways involved in skin development have been shown to regulate Merkel cell development as well. Here, we summarize the current understandings of the role of developmental regulators in Merkel cell formation, including the interplay between the epigenetic machinery and key signalling pathways, and the lineage-specific transcription factors involved in the regulation of Merkel cell development.

Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is a rare and extremely aggressive neuroendocrine carcinoma of the skin, with increasing incidence worldwide. This review intends to propose a comprehensive evaluation of MCC epidemiology, clinical features, pathogenetic mechanisms, diagnosis, and therapies. A section is dedicated to immunological aspects and another to the involvement of angiogenesis and angiogenic growth factors in MCC progression, proposing novel diagnostic and therapeutic approaches. Advanced MCC tumors have been treated with immune checkpoint inhibitors with effective results. Therefore, the state of art of this immunotherapy is also examined, reporting on the most recent clinical trials in the field. We conclude by underlining the achievements in the understanding of MCC pathology and indicating the present needs for effective diagnosis and therapeutic management of the disease.

Targeting epigenetic modulation of cholesterol synthesis as a therapeutic strategy for head and neck squamous cell carcinoma

The histone methyltransferase EZH2 silences gene expression via H3 lysine 27 trimethylation and has been recognized as an important antitumour therapeutic target. However, the clinical application of existing EZH2 inhibitors is not satisfactory for the treatment of solid tumours. To discover novel strategies against head and neck squamous cell carcinoma (HNSCC), we performed genomics, metabolomics and RNA omics studies in HNSCC cells treated with EZH2 inhibitors. It was found that EZH2 inhibitors strongly induced the expression of genes in cholesterol synthesis. Through extensive drug screening we found that inhibition of squalene epoxidase (a key enzyme of endogenous cholesterol synthesis) synergistically increased the squalene content and enhanced the sensitivity of HNSCC cells to EZH2 inhibitors. Our findings provide an experimental and theoretical basis for the development of new combinations of EZH2 inhibitors to treat HNSCC.

Polycomb complexes redundantly maintain epidermal stem cell identity during development

In this study, Cohen et al. sought to understand the functional contribution of PRC1 and PRC2, which largely overlap in their genomic binding and cooperate to establish repressive chromatin domains demarcated by H2AK119ub and H3K27me3, to gene repression. By using the developing murine epidermis as a paradigm, they uncovered a previously unappreciated functional redundancy between Polycomb complexes, and their findings show how PRC1 and PRC2 function as two independent counterparts, providing a repressive safety net that protects and preserves lineage identity.

Polycomb repressive complex 1 (PRC1) and PRC2 are critical epigenetic developmental regulators. PRC1 and PRC2 largely overlap in their genomic binding and cooperate to establish repressive chromatin domains demarcated by H2AK119ub and H3K27me3. However, the functional contribution of each complex to gene repression has been a subject of debate, and understanding of its physiological significance requires further studies. Here, using the developing murine epidermis as a paradigm, we uncovered a previously unappreciated functional redundancy between Polycomb complexes. Coablation of PRC1 and PRC2 in embryonic epidermal progenitors resulted in severe defects in epidermal stratification, a phenotype not observed in the single PRC1-null or PRC2-null epidermis. Molecular dissection indicated a loss of epidermal identity that was coupled to a strong derepression of nonlineage transcription factors, otherwise repressed by either PRC1 or PRC2 in the absence of its counterpart. Ectopic expression of subsets of PRC1/2-repressed nonepidermal transcription factors in wild-type epidermal stem cells was sufficient to suppress epidermal identity genes, highlighting the importance of functional redundancy between PRC1 and PRC2. Altogether, our studies show how PRC1 and PRC2 function as two independent counterparts, thereby providing a repressive safety net that protects and preserves lineage identity.

Overview of the molecular determinants contributing to the expression of Psoriasis and Psoriatic Arthritis phenotypes

Psoriasis and psoriatic arthritis are multifactorial chronic disorders whose etiopathogenesis essentially derives from the alteration of several signalling pathways and the co-occurrence of genetic, epigenetic and non-genetic susceptibility factors that altogether affect the functional and structural property of the skin. Although shared and differential susceptibility genes and molecular pathways are known to contribute to the onset of pathological phenotypes, further research is needed to dissect the molecular causes of psoriatic disease and its progression towards Psoriatic Arthritis. This review will therefore be addressed to explore differences and similarities in the etiopathogenesis and progression of both disorders, with a particular focus on genes involved in the maintenance of the skin structure and integrity (keratins and collagens), modulation of patterns of recognition (through Toll-like receptors and dectin-1) and immuno-inflammatory response (by NLRP3-dependent inflammasome) to microbial pathogens. In addition, special emphasis will be given to the contribution of epigenetic elements (methylation pattern, non-coding RNAs, chromatin modifiers and 3D genome organization) to the etiopathogenesis and progression of psoriasis and psoriatic arthritis. The evidence discussed in this review highlights how the knowledge of patients' clinical and (epi)genomic make-up could be helpful for improving the available therapeutic strategies for psoriasis and psoriatic arthritis treatment.

Molecular Pathogenesis of Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is an aggressive neuroendocrine carcinoma of the skin with two distinct etiologies. Clonal integration of Merkel cell polyomavirus DNA into the tumor genome with persistent expression of viral T antigens causes at least 60% of all MCC. UV damage leading to highly mutated genomes causes a nonviral form of MCC. Despite these distinct etiologies, both forms of MCC are similar in presentation, prognosis, and response to therapy. At least three oncogenic transcriptional programs feature prominently in both forms of MCC driven by the virus or by mutation. Both forms of MCC have a high proliferative growth rate with increased levels of cell cycle-dependent genes due to inactivation of the tumor suppressors RB and p53, a strong MYC signature due to MYCL activation by the virus or gene amplification, and an attenuated neuroendocrine differentiation program driven by the ATOH1 transcription factor.

Targeting the epigenetic addiction of Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare but very aggressive neuroendocrine cancer of the skin, with very limited therapeutic options. Although immunotherapy is effective in some cases, there is an unmet need for new therapeutic approaches in MCCs. In this issue of EMBO Molecular Medicine, Leiendecker et al identify a selective vulnerability of MCC for inhibitors of the lysine-specific histone demethylase 1A (LSD1). LSD1 inhibitors promote differentiation of tumor cells toward normal Merkel cell fate, impairing tumor cell growth in vivo, and opening new avenues for the treatment of patients with MCC.

LSD1 inhibition induces differentiation and cell death in Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a highly aggressive, neuroendocrine skin cancer that lacks actionable mutations, which could be utilized for targeted therapies. Epigenetic regulators governing cell identity may represent unexplored therapeutic entry points. Here, we targeted epigenetic regulators in a pharmacological screen and discovered that the lysine‐specific histone demethylase 1A (LSD1/KDM1A) is required for MCC growth in vitro and in vivo. We show that LSD1 inhibition in MCC disrupts the LSD1‐CoREST complex leading to displacement and degradation of HMG20B (BRAF35), a poorly characterized complex member that is essential for MCC proliferation. Inhibition of LSD1 causes derepression of transcriptional master regulators of the neuronal lineage, activates a gene expression signature resembling normal Merkel cells, and induces cell cycle arrest and cell death. Our study unveils the importance of LSD1 for maintaining cellular plasticity and proliferation in MCC. There is also growing evidence that cancer cells exploit cellular plasticity and dedifferentiation programs to evade destruction by the immune system. The combination of LSD1 inhibitors with checkpoint inhibitors may thus represent a promising treatment strategy for MCC patients.

Epigenetic Regulation of Spermatogonial Stem Cell Homeostasis: From DNA Methylation to Histone Modification

Spermatogonial stem cells(SSCs)are the ultimate germline stem cells with the potential of self-renewal and differentiation, and a dynamic balance of SSCs play an essential role in spermatogenesis. During the gene expression process, genomic DNA and nuclear protein, working together, contribute to SSC homeostasis. Recently, emerging studies have shown that epigenome-related molecules such as chromatin modifiers play an important role in SSC homeostasis through regulating target gene expression. Here, we focus on two types of epigenetic events, including DNA methylation and histone modification, and summarize their function in SSC homeostasis. Understanding the molecular mechanism during SSC homeostasis will promote the recognition of epigenetic biomarkers in male infertility, and bring light into therapies of infertile patients.Graphical Abstract.

Merkel Cell Polyomavirus T Antigens Induce Merkel Cell-Like Differentiation in GLI1-Expressing Epithelial Cells

Merkel cell carcinoma (MCC) is an aggressive skin cancer frequently caused by the Merkel cell polyomavirus (MCPyV). It is still under discussion, in which cells viral integration and MCC development occurs. Recently, we demonstrated that a virus-positive MCC derived from a trichoblastoma, an epithelial neoplasia bearing Merkel cell (MC) differentiation potential. Accordingly, we hypothesized that MC progenitors may represent an origin of MCPyV-positive MCC. To sustain this hypothesis, phenotypic comparison of trichoblastomas and physiologic human MC progenitors was conducted revealing GLI family zinc finger 1 (GLI1), Keratin 17 (KRT 17), and SRY-box transcription factor 9 (SOX9) expressions in both subsets. Furthermore, GLI1 expression in keratinocytes induced transcription of the MC marker SOX2 supporting a role of GLI1 in human MC differentiation. To assess a possible contribution of the MCPyV T antigens (TA) to the development of an MC-like phenotype, human keratinocytes were transduced with TA. While this led only to induction of KRT8, an early MC marker, combined GLI1 and TA expression gave rise to a more advanced MC phenotype with SOX2, KRT8, and KRT20 expression. Finally, we demonstrated MCPyV-large T antigens’ capacity to inhibit the degradation of the MC master regulator Atonal bHLH transcription factor 1 (ATOH1). In conclusion, our report suggests that MCPyV TA contribute to the acquisition of an MC-like phenotype in epithelial cells.

High-Content Analysis of Cell Migration Dynamics within a Micropatterned Screening Platform

Cell migration is a fundamental biological process that is dynamically regulated by complex interactions between the microenvironment and intrinsic gene expression programs. Here, a high-throughput cell migration assay is developed using micropatterned and dynamically adhesive polymer brush substrates, which support highly precise and consistent control over cell-matrix interactions within a 96-well cell culture plate format. This system is combined with automated imaging and quantitation of both cell motility and organization of the F-actin cytoskeleton for high-content analysis of cell migration phenotypes. Using this platform to screen a library of 147 epigenetic inhibitors identifies a set of EZH2-specific compounds that promote cytoskeletal remodeling and accelerates keratinocyte migration through derepression of an epithelial to mesenchymal transition-like gene expression program. Together, these studies establish the high-throughput, micropatterned assay as a powerful tool for discovery of novel therapeutic targets and for dissecting complex gene-environment interactions involved in wound repair.

Atonal bHLH transcription factor 1 is an important factor for maintaining the balance of cell proliferation and differentiation in tumorigenesis

Establishing the link between cellular processes and oncogenesis may aid the elucidation of targeted and effective therapies against tumor cell proliferation and metastasis. Previous studies have investigated the mechanisms involved in maintaining the balance between cell proliferation, differentiation and migration. There is increased interest in determining the conditions that allow cancer stem cells to differentiate as well as the identification of molecules that may serve as novel drug targets. Furthermore, the study of various genes, including transcription factors, which serve a crucial role in cellular processes, may present a promising direction for future therapy. The present review described the role of the transcription factor atonal bHLH transcription factor 1 (ATOH1) in signaling pathways in tumorigenesis, particularly in cerebellar tumor medulloblastoma and colorectal cancer, where ATOH1 serves as an oncogene or tumor suppressor, respectively. Additionally, the present review summarized the associated therapeutic interventions for these two types of tumors and discussed novel clinical targets and approaches.

Decreased Sox2 Messenger RNA Expression in Basal Cell Carcinoma

Background

Sox2, zeb1, and p21 have been implicated in aggressive behavior of squamous cell carcinoma (SCC) and melanoma. However, their expression level in basal cell carcinoma (BCC) has not been elucidated. We hypothesized BCC, contrary to SCC, and melanoma, could be a suitable model to study mechanisms which attenuate tumor metastasis. The aim of this study was to examine the messenger RNA (mRNA) expression levels of sox2, zeb1, and p21 in BCC.

Materials and Methods

Twenty-seven nonmetastatic BCC and twelve normal skin samples were evaluated using real-time reverse transcriptase polymerase chain reaction.

Results

The stemness marker sox2 demonstrated marked down-regulation, but zeb1 and p21 showed no significant change.

Conclusions

Here, we report a negative association between sox2 mRNA expression level and nonmetastatic BCC, thus, providing a likely explanation for the fact that normal skin is more reliant on sox2 than BCC. BCC may be using decreased sox2 mRNA to remain incognito from metastatic potential.

Merkel cells of human oral mucosa express the pluripotent stem cell transcription factor Sox2

Merkel cells are neuroendocrine cells associated to a neural sensitive ending and localized primarily in the epidermis, although they are also found in oral mucosa. Sox2 or SRY-box2 is a key transcription factor important in the maintenance of embryonic neural crest stem cell pluripotency. Sox2 has been described in Merkel cells of skin and in Merkel cell carcinomas, but not specifically in oral Merkel cells. The aims of the present study were to analyze the density of Merkel cells in human oral mucosa and to study the expression of Sox2 in these cells. For these purposes, immunohistochemical analyses for Sox2 and CK20 (the best marker for Merkel cells) were automatically performed on sections of normal human oral mucosa. Double immunofluorescence for Sox2 and CK20 was also performed. To analyze the density of Merkel cells, CK20 positive cells were counted in each sample and the length of the epithelial apical edge was measured (cells/mm). Merkel cells, demonstrated by CK20 immunoreactivity, were found in 95% of oral mucosa specimens studied (n=21). Mean density of Merkel cells in oral mucosa was 1.71±2.34 cells/mm. Sox2 immunoreactivity was found in the nuclei of scattered cells located at the basal layer. Serial sections immunostained for Sox2 and CK20 showed that Sox2-positive cells of oral mucosa coexpressed CK20, confirming that they were Merkel cells. Immunofluorescence for Sox2 and CK20 showed colocalization of both markers, demonstrating that virtually all oral Merkel cells expressed Sox2. This transcription factor could play a role in Merkel cell maturation and maintenance.

Activity of PRC1 and Histone H2AK119 Monoubiquitination: Revising Popular Misconceptions

Polycomb group proteins are evolutionary conserved chromatin-modifying complexes, essential for the regulation of developmental and cell-identity genes. Polycomb-mediated transcriptional regulation is provided by two multi-protein complexes known as Polycomb repressive complex 1 (PRC1) and 2 (PRC2). Recent studies positioned PRC1 as a foremost executer of Polycomb-mediated transcriptional control. Mammalian PRC1 complexes can form multiple sub-complexes that vary in their core and accessory subunit composition, leading to fascinating and diverse transcriptional regulatory mechanisms employed by PRC1 complexes. These mechanisms include PRC1-catalytic activity toward monoubiquitination of histone H2AK119, a well-established hallmark of PRC1 complexes, whose importance has been long debated. In this review, the central roles that PRC1-catalytic activity plays in transcriptional repression are emphasized and the recent evidence supporting a role for PRC1 complexes in gene activation is discussed.

Chromatin Immunoprecipitation of Low Number of FACS-Purified Epidermal Cells

Chromatin immunoprecipitation followed by sequencing (ChIP-seq) is a method designed to detect interactions between chromatin and the proteins bound to it. This method has been widely used for characterizing epigenetic landscapes in many cell types; however, a limiting factor has been the requirement of a high number of cells. Here, we describe a protocol for ChIP in epidermal cells from a newborn mouse, purified by fluorescence-activated cell sorting (FACS). This protocol has been optimized specifically for prefixed, low cell numbers, resulting in enough immunoprecipitated DNA suitable for genome-wide analysis.

Conversion of Sox2-dependent Merkel cell carcinoma to a differentiated neuron-like phenotype by T antigen inhibition

Normal cells can be transformed into cancer cells by viral oncogenes. Reversion of a viral human cancer cell, however, into a differentiated cell by viral oncogene inhibition has not been described. Merkel cell carcinoma (MCC) is a neuroendocrine cancer caused by Merkel cell polyomavirus (MCV) that encodes a T antigen oncogene. When MCV⁺ MCC cells with T antigen knockdown are cocultured with keratinocytes, the MCC phenotype converts to a differentiated neuronal phenotype and loses Merkel cell factor Sox2 and Atoh1 expression. MCV large T activates Sox2 and Atoh1 by its ability to inhibit retinoblastoma. Sox2 inhibition similarly induced this phenotypic conversion of MCC. These findings suggest that MCV induces cancer by dysregulating embryonic Merkel cell differentiation pathways.

Viral cancers show oncogene addiction to viral oncoproteins, which are required for survival and proliferation of the dedifferentiated cancer cell. Human Merkel cell carcinomas (MCCs) that harbor a clonally integrated Merkel cell polyomavirus (MCV) genome have low mutation burden and require viral T antigen expression for tumor growth. Here, we showed that MCV⁺ MCC cells cocultured with keratinocytes undergo neuron-like differentiation with neurite outgrowth, secretory vesicle accumulation, and the generation of sodium-dependent action potentials, hallmarks of a neuronal cell lineage. Cocultured keratinocytes are essential for induction of the neuronal phenotype. Keratinocyte-conditioned medium was insufficient to induce this phenotype. Single-cell RNA sequencing revealed that T antigen knockdown inhibited cell cycle gene expression and reduced expression of key Merkel cell lineage/MCC marker genes, including HES6, SOX2, ATOH1, and KRT20. Of these, T antigen knockdown directly inhibited Sox2 and Atoh1 expression. MCV large T up-regulated Sox2 through its retinoblastoma protein-inhibition domain, which in turn activated Atoh1 expression. The knockdown of Sox2 in MCV⁺ MCCs mimicked T antigen knockdown by inducing MCC cell growth arrest and neuron-like differentiation. These results show Sox2-dependent conversion of an undifferentiated, aggressive cancer cell to a differentiated neuron-like phenotype and suggest that the ontology of MCC arises from a neuronal cell precursor.

Telomere dysfunction impairs epidermal stem cell specification and differentiation by disrupting BMP/pSmad/P63 signaling

Telomere shortening is associated with aging and age-associated diseases. Additionally, telomere dysfunction resulting from telomerase gene mutation can lead to premature aging, such as apparent skin atrophy and hair loss. However, the molecular signaling linking telomere dysfunction to skin atrophy remains elusive. Here we show that dysfunctional telomere disrupts BMP/pSmad/P63 signaling, impairing epidermal stem cell specification and differentiation of skin and hair follicles. We find that telomere shortening mediated by Terc loss up-regulates Follistatin (Fst), inhibiting pSmad signaling and down-regulating P63 and epidermal keratins in an ESC differentiation model as well as in adult development of telomere-shortened mice. Mechanistically, short telomeres disrupt PRC2/H3K27me3-mediated repression of Fst. Our findings reveal that skin atrophy due to telomere dysfunction is caused by a previously unappreciated link with Fst and BMP signaling that could be explored in the development of therapies.

Jarid1b promotes epidermal differentiation by mediating the repression of Ship1 and activation of the AKT/Ovol1 pathway

Objectives

Terminally differentiated stratified squamous epithelial cells play an important role in barrier protection of the skin. The integrity of epidermal cells is maintained by tight regulation of proliferation and differentiation. The aim of this study was to investigate the role of epigenetic regulator H3K4me3 and its demethylase Jarid1b in the control of epithelial cell differentiation.

Materials and methods

RT‐qPCR, Western blotting and IHC were used to detect mRNA and protein levels. We analysed cell proliferation by CCK8 assay and cell migration by wound healing assay. ChIP was used to measure H3K4me3 enrichment. A chamber graft model was established for epidermal development.

Results

Our studies showed that H3K4me3 was decreased during epidermal differentiation. The H3K4me3 demethylase Jarid1b positively controlled epidermal cell differentiation in vitro and in vivo. Mechanistically, we found that Jarid1b substantially increased the expression of mesenchymal‐epithelial transition (MET)‐related genes, among which Ovol1 positively regulated differentiation gene expression. In addition, Ovol1 expression was repressed by PI3K‐AKT pathway inhibitors and overexpression (O/E) of the PI3K‐AKT pathway suppressor Ship1. Knockdown (KD) of Ship1 activated downstream PI3K‐AKT pathway and enhanced Ovol1 expression in HaCaT. Importantly, we found that Jarid1b negatively regulated Ship1 expression, but not that of Pten, by directly binding to its promoter to modulate H3K4me3 enrichment.

Conclusion

Our results identify an essential role of Jarid1b in the regulation of the Ship1/AKT/Ovol1 pathway to promote epithelial cell differentiation.

Loss of PRC1 activity in different stem cell compartments activates a common transcriptional program with cell type-dependent outcomes

Polycomb repressive complexes are evolutionarily conserved complexes that maintain transcriptional repression during development and differentiation to establish and preserve cell identity. We recently described the fundamental role of PRC1 in preserving intestinal stem cell identity through the inhibition of non-lineage-specific transcription factors. To further elucidate the role of PRC1 in adult stem cell maintenance, we now investigated its role in LGR5⁺ hair follicle stem cells during regeneration. We show that PRC1 depletion severely affects hair regeneration and, different from intestinal stem cells, derepression of its targets induces the ectopic activation of an epidermal-specific program. Our data support a general role of PRC1 in preserving stem cell identity that is shared between different compartments. However, the final outcome of the ectopic activation of non-lineage-specific transcription factors observed upon loss of PRC1 is largely context-dependent and likely related to the transcription factors repertoire and specific epigenetic landscape of different cellular compartments.

Dissection of Merkel cell formation in hairy and glabrous skin reveals a common requirement for FGFR2-mediated signalling

Merkel cells are mechanosensory cells involved in tactile discrimination. Merkel cells have been primarily studied in the murine back skin, where they are found in specialized structures called touch domes located around primary hair follicles. Yet, little is known about the morphogenesis of Merkel cells in areas of the skin devoid of hair, such as the glabrous paw skin. Here, we describe Merkel cell formation in the glabrous paw skin during embryogenesis. We first found in the glabrous paw skin that Merkel cells were specified at E15.5, 24 hours later, compared to in the back skin. Additionally, by performing lineage-tracing experiments, we found that unlike in the back skin, SOX9(+) cells do not give rise to Merkel cells in the glabrous paw skin. Finally, we compared the transcriptomes of Merkel cells in the back and the glabrous paw skin and showed that they are similar. Genetic and transcriptome studies showed that the formation of Merkel cells in both regions was controlled by similar regulators. Among them was FGFR2, an upstream factor of MAPK signalling that was reported to have a critical function in Merkel cell formation in the back skin. Here, we showed that FGFR2 is also required for Merkel cell development in the glabrous paw skin. Taken together, our results demonstrate that Merkel cells in the murine back skin and glabrous paw skin are similar, and even though their formation is controlled by a common genetic programme, their precursor cells might differ.

The cellular basis of mechanosensory Merkel-cell innervation during development

Touch sensation is initiated by mechanosensory neurons that innervate distinct skin structures; however, little is known about how these neurons are patterned during mammalian skin development. We explored the cellular basis of touch-receptor patterning in mouse touch domes, which contain mechanosensory Merkel cell-neurite complexes and abut primary hair follicles. At embryonic stage 16.5 (E16.5), touch domes emerge as patches of Merkel cells and keratinocytes clustered with a previously unsuspected population of Bmp4-expressing dermal cells. Epidermal Noggin overexpression at E14.5 disrupted touch-dome formation but not hair-follicle specification, demonstrating a temporally distinct requirement for BMP signaling in placode-derived structures. Surprisingly, two neuronal populations preferentially targeted touch domes during development but only one persisted in mature touch domes. Finally, Keratin-17-expressing keratinocytes but not Merkel cells were necessary to establish innervation patterns during development. These findings identify key cell types and signaling pathways required for targeting Merkel-cell afferents to discrete mechanosensory compartments.

De Novo PITX1 Expression Controls Bi-Stable Transcriptional Circuits to Govern Self-Renewal and Differentiation in Squamous Cell Carcinoma

Basal tumor propagating cells (TPCs) control squamous cell carcinoma (SCC) growth by self-renewing and differentiating into supra-basal SCC cells, which lack proliferative potential. While transcription factors such as SOX2 and KLF4 can drive these behaviors, their molecular roles and regulatory interactions with each other have remained elusive. Here, we show that PITX1 is specifically expressed in TPCs, where it co-localizes with SOX2 and TRP63 and determines cell fate in mouse and human SCC. Combining gene targeting with chromatin immunoprecipitation sequencing (ChIP-seq) and transcriptomic analyses reveals that PITX1 cooperates with SOX2 and TRP63 to sustain an SCC-specific transcriptional feed-forward circuit that maintains TPC-renewal, while inhibiting KLF4 expression and preventing KLF4-dependent differentiation. Conversely, KLF4 represses PITX1, SOX2, and TRP63 expression to prevent TPC expansion. This bi-stable, multi-input network reveals a molecular framework that explains self-renewal, aberrant differentiation, and SCC growth in mice and humans, providing clues for developing differentiation-inducing therapeutic strategies.

The Effect of Overexpression of the Enhancer of Zeste Homolog 1 (EZH1) Gene on Aristolochic Acid-Induced Injury in HK-2 Human Kidney Proximal Tubule Cells In Vitro

Background

Acute kidney injury (AKI) involves the renal tubular epithelium. The enhancer of zeste homolog 1 (EZH1) gene has a role in cell development and differentiation. This study aimed to investigate the effect of overexpression of the EZH1 gene on aristolochic acid-induced injury in HK-2 human kidney proximal tubule epithelial cells in vitro.

Material/Methods

The HK-2 cells were cultured and treated with aristolochic acid and the effects of aristolochic acid-injury were evaluated using a cell counting kit-8 (CCK-8) assay. Overexpression of EZH1 used gene plasmid transfection into HK-2 cells. The cell apoptosis rate and levels of intracellular reactive oxygen species (ROS) were measured using flow cytometry. Quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot were performed to determine the expressions of inflammatory cytokines including interleukin (IL)-1β, IL-6, tumor necrosis factor-α (TNF-α), apoptosis-related genes, and the downstream target genes of NF-κB signaling pathway, including NFKBIA, CXCL8, and cyclin D1.

Results

Aristolochic acid inhibited HK-2 cell viability, induced cell apoptosis, increased the levels of ROS and inflammatory cytokines, including IL-1β, IL-6, TNF-α, and activated the NF-κB pathway. Overexpression the EZH1 gene inhibited HK-2 cell apoptosis, reduced ROS levels, and down-regulated the expressions of IL-1β, IL-6, TNF-α, Bax and Cyt C mRNA and protein, and increased the expressions of Bcl-2 and NFKBIA, CXCL8 and cyclin D1, indicating that overexpression of EZH1 suppressed NF-κB signaling in aristolochic acid-injured HK-2 cells.

Conclusions

Overexpression of EZH1 reduced HK-2 cell injury induced by aristolochic acid in vitro by inhibition of NF-κB signaling.

FGF signalling controls the specification of hair placode-derived SOX9 positive progenitors to Merkel cells

Merkel cells are innervated mechanosensory cells responsible for light-touch sensations. In murine dorsal skin, Merkel cells are located in touch domes and found in the epidermis around primary hairs. While it has been shown that Merkel cells are skin epithelial cells, the progenitor cell population that gives rise to these cells is unknown. Here, we show that during embryogenesis, SOX9-positive (+) cells inside hair follicles, which were previously known to give rise to hair follicle stem cells (HFSCs) and cells of the hair follicle lineage, can also give rise to Merkel Cells. Interestingly, while SOX9 is critical for HFSC specification, it is dispensable for Merkel cell formation. Conversely, FGFR2 is required for Merkel cell formation but is dispensable for HFSCs. Together, our studies uncover SOX9(+) cells as precursors of Merkel cells and show the requirement for FGFR2-mediated epithelial signalling in Merkel cell specification.

Merkel cells are mechanoreceptors located in the epidermis whose developmental origin is unclear. Here the authors show that Merkel cells originate from SOX9 positive cells inside hair follicles and that FGFR2-mediated epithelial signalling is required for their specification.

EZH2 deletion promotes spermatogonial differentiation and apoptosis

Spermatogenesis is crucial for male fertility and is therefore tightly controlled by a variety of epigenetic regulators. However, the function of enhancer of zeste homolog 2 (EZH2) in spermatogenesis and the molecular mechanisms underlying its activity remain poorly defined. Here, we demonstrate that deleting EZH2 promoted spermatogonial differentiation and apoptosis. EZH2 is expressed in spermatogonia, spermatocytes and round and elongated spermatids from stage 9 to 11 but not in leptotene and zygotene spermatocytes. Knocking down Ezh2 in vitro using a lentivirus impaired self-renewal in spermatogonial stem cells (SSC_s), and the conditional knockout of Ezh2 in spermatogonial progenitors promoted precocious spermatogonial differentiation. EZH2 functions to balance self-renewal and differentiation in spermatogonia by suppressing NEUROG3 and KIT via a direct interaction that is independent of its histone methyltransferase activity. Moreover, deleting Ezh2 enhanced the activation of CASP3 in spermatids, resulting in reduced spermatozoa production. Collectively, these data demonstrate that EZH2 plays a nonclassical role in the regulation of spermatogonial differentiation and apoptosis in murine spermatogenesis.

Histone modifiers: Dynamic regulators of the cutaneous transcriptome

By regulating the accessibility of the genome, epigenetic regulators such as histone proteins and the chromatin-modifying enzymes that act upon them control gene expression. Proper regulation of this "histone code" allows for the precise control of transcriptional networks that are essential for establishing and maintaining cell fate and identity, disruption of which may drive carcinogenesis. How these dynamic epigenetic regulators contribute to both skin homeostasis and disease is only beginning to be understood. Here we provide an update of the current understanding of histone modifiers in the skin. Indeed, as one of the most innovative and rapidly expanding areas in all of medicine, it is clear that epigenome-targeting therapies hold great promise for the treatment of dermatological diseases in the coming years.

Developing a sense of touch

The sensation of touch is mediated by mechanosensory neurons that are embedded in skin and relay signals from the periphery to the central nervous system. During embryogenesis, axons elongate from these neurons to make contact with the developing skin. Concurrently, the epithelium of skin transforms from a homogeneous tissue into a heterogeneous organ that is made up of distinct layers and microdomains. Throughout this process, each neuronal terminal must form connections with an appropriate skin region to serve its function. This Review presents current knowledge of the development of the sensory microdomains in mammalian skin and the mechanosensory neurons that innervate them.

p63 Transcription Factor Regulates Nuclear Shape and Expression of Nuclear Envelope-Associated Genes in Epidermal Keratinocytes

The maintenance of a proper nuclear architecture and three-dimensional organization of the genes, enhancer elements, and transcription machinery plays an essential role in tissue development and regeneration. Here we show that in the developing skin, epidermal progenitor cells of mice lacking p63 transcription factor display alterations in the nuclear shape accompanied by a marked decrease in expression of several nuclear envelope-associated components (Lamin B1, Lamin A/C, Sun1, Nesprin-3, Plectin) compared with controls. Furthermore, chromatin immunoprecipitation-quantitative PCR assay showed enrichment of p63 on Sun1, Syne3, and Plec promoters, suggesting them as p63 targets. Alterations in the nuclei shape and expression of nuclear envelope-associated proteins were accompanied by altered distribution patterns of the repressive histone marks trimethylation on lysine 27 of histone H3, trimethylation on lysine 9 of histone H3, and heterochromatin protein 1-alpha in p63-null keratinocytes. These changes were also accompanied by downregulation of the transcriptional activity and relocation of the keratinocyte-specific gene loci away from the sites of active transcription toward the heterochromatin-enriched repressive nuclear compartments in p63-null cells. These data demonstrate functional links between the nuclear envelope organization, chromatin architecture, and gene expression in keratinocytes and suggest nuclear envelope-associated genes as important targets mediating p63-regulated gene expression program in the epidermis.

The Sox2: GFP+/- knock-in mouse model does not faithfully recapitulate Sox2 expression in skin

Hair follicle heterogeneity may be regulated by distinct dermal papillae (DP) that represent mesenchymal lineages, which can be defined by Sox2 expression. However, it was recently shown that GFP expression in the Sox2: GFP+/- mouse model occurs in the DPs of all hair follicle types, challenging the idea that hair follicle heterogeneity can be defined by DP heterogeneity. Here, we investigated whether the knock-in mouse model faithfully expresses GFP when compared to endogenous Sox2 expression. The results reveal that GFP expression is aberrant in both the infundibulum of hair follicles and in the DPs. Consequently, we provide an explanation for the aberrant expression of the knock-in gene based on the original cloning strategy for the mouse model in the context of a newly identified regulatory element associated within the coding region of Sox2.

Novel orally bioavailable EZH1/2 dual inhibitors with greater antitumor efficacy than an EZH2 selective inhibitor

Polycomb repressive complex 2 (PRC2) methylates histone H3 lysine 27 and represses gene expression to regulate cell proliferation and differentiation. Enhancer of zeste homolog 2 (EZH2) or its close homolog EZH1 functions as a catalytic subunit of PRC2, so there are two PRC2 complexes containing either EZH2 or EZH1. Tumorigenic functions of EZH2 and its synthetic lethality with some subunits of SWItch/Sucrose Non-Fermentable (SWI/SNF) chromatin remodeling complexes have been observed. However, little is known about the function of EZH1 in tumorigenesis. Herein, we developed novel, orally bioavailable EZH1/2 dual inhibitors that strongly and selectively inhibited methyltransferase activity of both EZH2 and EZH1. EZH1/2 dual inhibitors suppressed trimethylation of histone H3 lysine 27 in cells more than EZH2 selective inhibitors. They also showed greater antitumor efficacy than EZH2 selective inhibitor in vitro and in vivo against diffuse large B-cell lymphoma cells harboring gain-of-function mutation in EZH2. A hematological cancer panel assay indicated that EZH1/2 dual inhibitor has efficacy against some lymphomas, multiple myeloma, and leukemia with fusion genes such as MLL-AF9, MLL-AF4, and AML1-ETO. A solid cancer panel assay demonstrated that some cancer cell lines are sensitive to EZH1/2 dual inhibitor in vitro and in vivo. No clear correlation was detected between sensitivity to EZH1/2 dual inhibitor and SWI/SNF mutations, with a few exceptions. Severe toxicity was not seen in rats treated with EZH1/2 dual inhibitor for 14 days at drug levels higher than those used in the antitumor study. Our results indicate the possibility of EZH1/2 dual inhibitors for clinical applications.

Repressing the Keratinocyte Genome: How the Polycomb Complex Subunits Operate in Concert to Control Skin and Hair Follicle Development

The Polycomb group proteins are transcriptional repressors that are critically important in the control of stem cell activity and maintenance of the identity of differentiated cells. Polycomb proteins interact with each other to form chromatin-associated repressive complexes (Polycomb repressive complexes 1 and 2) leading to chromatin compaction and gene silencing. However, the roles of the distinct components of the Polycomb repressive complex 2 in the control of skin development and keratinocyte differentiation remain obscure. Dauber et al. demonstrate the conditional ablations of three essential Polycomb repressive complex 2 subunits (EED, Suz12, or Ezh1/2) in the epidermal progenitors result in quite similar skin phenotypes including premature acquisition of a functional epidermal barrier, formation of ectopic Merkel cells, and defective postnatal hair follicle development. The reported data demonstrate that in skin epithelia, EED, Suz12, and Ezh1/2 function largely as subunits of the Polycomb repressive complex 2, which is important in the context of data demonstrating their independent activities in other cell types. The report provides an important platform for further analyses of the role of distinct Polycomb components in the control of gene expression programs in the disorders of epidermal differentiation, such as psoriasis and epidermal cancer.

The Role of p16INK4a Pathway in Human Epidermal Stem Cell Self-Renewal, Aging and Cancer

The epidermis is a self-renewing tissue. The balance between proliferation and differentiation processes is tightly regulated to ensure the maintenance of the stem cell (SC) population in the epidermis during life. Aging and cancer may be considered related endpoints of accumulating damages within epidermal self-renewing compartment. p16^INK4a is a potent inhibitor of the G1/S-phase transition of the cell cycle. p16^INK4a governs the processes of SC self-renewal in several tissues and its deregulation may result in aging or tumor development. Keratinocytes are equipped with several epigenetic enzymes and transcription factors that shape the gene expression signatures of different epidermal layers and allow dynamic and coordinated expression changes to finely balance keratinocyte self-renewal and differentiation. These factors converge their activity in the basal layer to repress p16^INK4a expression, protecting cells from senescence, and preserving epidermal homeostasis and regeneration. Several stress stimuli may activate p16^INK4a expression that orchestrates cell cycle exit and senescence response. In the present review, we discuss the role of p16^INK4a regulators in human epidermal SC self-renewal, aging and cancer.