An essential switch in subunit composition of a chromatin remodeling complex during neural development

A non-canonical SWI/SNF complex is a synthetic lethal target in cancers driven by BAF complex perturbation

Mammalian SWI/SNF chromatin remodeling complexes exist in three distinct, final-form assemblies: canonical BAF (cBAF), PBAF, and a newly-characterized non-canonical complex, ncBAF. However, their complex-specific targeting on chromatin, functions and roles in disease remain largely undefined. Here, we comprehensively mapped complex assemblies on chromatin and found that ncBAF complexes uniquely localize to CTCF sites and promoters. We identified ncBAF subunits as synthetic lethal targets specific to synovial sarcoma (SS) and malignant rhabdoid tumor (MRT), which share in common cBAF complex (SMARCB1 subunit) perturbation. Chemical and biological depletion of the BRD9 subunit of ncBAF rapidly attenuates SS and MRT cell proliferation. Notably, in cBAF-perturbed cancers, ncBAF complexes maintain gene expression at retained CTCF-promoter sites, and function in a manner distinct from fusion oncoprotein-bound complexes. Taken together, these findings unmask the unique chromatin targeting and function of ncBAF complexes and present new cancer-specific therapeutic targets.

ACTL6A expression promotes invasion, metastasis and epithelial mesenchymal transition of colon cancer

BACKGROUND

Metastasis is the main cause of death in patients with advanced stage colon cancer. Epithelial mesenchymal transition (EMT) plays an important role in invasion and metastasis. Actin-like 6A (ACTL6A) is vital for embryogenesis and differentiation and is also critical for metastasis and EMT in hepatocellular carcinoma, as observed in our previous study. In the present study, we further explored the role of ACTL6A in colon cancer metastasis.

METHODS

ACTL6A expression levels were analyzed in normal colon, colon adenoma and colon cancer specimens using public databases and tissue samples. ACTL6A expression and its association with clinicopathologic features of colon cancer patients were also analyzed. ACTL6A-overexpression and ACTL6A-knockdown colon cancer cells were used to perform cytological experiments to explore the potential biological function of ACTL6A in metastasis and EMT in colon cancer.

RESULTS

The data from both the Gene Expression Omnibus (GEO) and Oncomine databases showed that ACTL6A expression levels in colon adenoma and cancer were higher than those in normal colon samples. The ACTL6A expression level in fresh colon cancer specimens was also higher than that in the corresponding adjacent normal colon specimens. Patients with high ACTL6A expression directly correlated with advanced pT status, distant metastasis, poor differentiation and microvascular/perineural invasion. ACTL6A overexpression promoted migration and invasion of colon cancer cells, whereas ACTL6A knockdown exhibited the opposite effect in vitro. Moreover, we demonstrated that ACTL6A promoted EMT in colon cancer cells in vitro.

CONCLUSIONS

Our findings indicate that ACTL6A exhibits pro-tumor function and acts as an EMT activator in colon cancer. ACTL6A may serve as a potential therapeutic target for colon cancer.

Domain architecture of BAF250a reveals the ARID and ARM-repeat domains with implication in function and assembly of the BAF remodeling complex

BAF250a and BAF250b are subunits of the SWI/SNF chromatin-remodeling complex that recruit the complex to chromatin allowing transcriptional activation of several genes. Despite being the central subunits of the SWI/SNF complex, the structural and functional annotation of BAF250a/b remains poorly understood. BAF250a (nearly 2200 residues protein) harbors an N-terminal DNA binding ARID (~110 residues) and a C-terminal folded region (~250 residues) of unknown structure and function, recently annotated as BAF250_C. Using hydrophobic core analysis, fold prediction and comparative modeling, here we have defined a domain boundary and associate a β-catenin like ARM-repeat fold to the C-terminus of BAF250a that encompass BAF250_C. The N-terminal DNA-binding ARID is found in diverse domain combinations in proteins imparting unique functions. We used a comparative sequence analysis based approach to study the ARIDs from diverse domain contexts and identified conserved residue positions that are important to preserve its core structure. Supporting this, mutation of one such conserved residue valine, at position 1067, to glycine, resulted in destabilization, loss of structural integrity and DNA binding affinity of ARID. Additionally, we identified a set of conserved and surface-exposed residues unique to the ARID when it co-occurs with the ARM repeat containing BAF250_C in BAF250a. Several of these residues are found mutated in somatic cancers. We predict that these residues in BAF250a may play important roles in mediating protein-DNA and protein-protein interactions in the BAF complex.

In silico screening of cancer-associated mutations in the HSA domain of BRG1 and its role in affecting the Arp-HSA sub-complex of SWI/SNF

SWI/SNF (SWItch/Sucrose Non-Fermentable) complexes regulate the gene expression programs by remodeling the nucleosome architecture of the chromatin functional elements. These large multi-component complexes comprise eight or more subunits and are conserved from yeast to human. Noticeably, nuclear actin and actin-related proteins (Arps) are an integral part of these complexes and are known to directly interact with the helicase-SANT-associated (HSA) domain of ATPase subunit. Recently, SWI/SNF subunits are gaining importance because of the prevalence of cancer-causing mutations associated with them. The functional characterization of the mutations in the SWI/SNF subunits is important for understanding their role in tumorigenesis and identifying potential therapeutic strategies. To study the actin-related complex of human SWI/SNF and the cancer-associated mutations interfering Arp assembly with the ATPase subunit, we modelled the structure of the β-actin-BAF53A-HSA complex based on the yeast Arp-HSA complex (PDB ID: 4I6M). Seven deleterious mutations in the HSA domain of BRG1 were identified based on the functional screening of cancer-associated mutations in the COSMIC database. Detailed structural analysis of the six mutations (R466H, R469W, Y489C, K502N, R513Q and R521P) based on molecular dynamics (MD) simulations reveal the distinct effect of each mutation in destabilizing the structure of the Arp-HSA complex. Predominantly we could notice the long-range effect of the HSA mutations in influencing the dynamics of the Arp subunits.

The Nature of Actin-Family Proteins in Chromatin-Modifying Complexes

Actin is not only one of the most abundant proteins in eukaryotic cells, but also one of the most versatile. In addition to its familiar involvement in enabling contraction and establishing cellular motility and scaffolding in the cytosol, actin has well-documented roles in a variety of processes within the confines of the nucleus, such as transcriptional regulation and DNA repair. Interestingly, monomeric actin as well as actin-related proteins (Arps) are found as stoichiometric subunits of a variety of chromatin remodeling complexes and histone acetyltransferases, raising the question of precisely what roles they serve in these contexts. Actin and Arps are present in unique combinations in chromatin modifiers, helping to establish structural integrity of the complex and enabling a wide range of functions, such as recruiting the complex to nucleosomes to facilitate chromatin remodeling and promoting ATPase activity of the catalytic subunit. Actin and Arps are also thought to help modulate chromatin dynamics and maintain higher-order chromatin structure. Moreover, the presence of actin and Arps in several chromatin modifiers is necessary for promoting genomic integrity and an effective DNA damage response. In this review, we discuss the involvement of actin and Arps in these nuclear complexes that control chromatin remodeling and histone modifications, while also considering avenues for future study to further shed light on their functional importance.

Genetic risk factors for VIPN in childhood acute lymphoblastic leukemia patients identified using whole-exome sequencing

AIM

To identify genetic markers associated with vincristine-induced peripheral neuropathy (VIPN) in childhood acute lymphoblastic leukemia.

PATIENTS & METHODS

Whole-exome sequencing data were combined with exome-wide association study to identify predicted-functional germline variants associated with high-grade VIPN. Genotyping was then performed for top-ranked signals (n = 237), followed by validation in independent replication group (n = 405).

RESULTS

Minor alleles of rs2781377/SYNE2 (p = 0.01) and rs10513762/MRPL47 (p = 0.01) showed increased risk, whereas that of rs3803357/BAHD1 had a protective effect (p = 0.007). Using a genetic model based on weighted genetic risk scores, an additive effect of combining these loci was observed (p = 0.003). The addition of rs1135989/ACTG1 further enhanced model performance (p = 0.0001).

CONCLUSION

Variants in SYNE2, MRPL47 and BAHD1 genes are putative new risk factors for VIPN in childhood acute lymphoblastic leukemia.

Elevated H3K79 homocysteinylation causes abnormal gene expression during neural development and subsequent neural tube defects

Neural tube defects (NTDs) are serious congenital malformations. Excessive maternal homocysteine (Hcy) increases the risk of NTDs, while its mechanism remains elusive. Here we report the role of histone homocysteinylation in neural tube closure (NTC). A total of 39 histone homocysteinylation sites are identified in samples from human embryonic brain tissue using mass spectrometry. Elevated levels of histone KHcy and H3K79Hcy are detected at increased cellular Hcy levels in human fetal brains. Using ChIP-seq and RNA-seq assays, we demonstrate that an increase in H3K79Hcy level down-regulates the expression of selected NTC-related genes including Cecr2, Smarca4, and Dnmt3b. In human NTDs brain tissues, decrease in expression of CECR2, SMARCA4, and DNMT3B is also detected along with high levels of Hcy and H3K79Hcy. Our results suggest that higher levels of Hcy contribute to the onset of NTDs through up-regulation of histone H3K79Hcy, leading to abnormal expressions of selected NTC-related genes.

Upgraded Methodology for the Development of Early Diagnosis of Parkinson's Disease Based on Searching Blood Markers in Patients and Experimental Models

Numerous attempts to develop an early diagnosis of Parkinson's disease (PD) by searching biomarkers in biological fluids were unsuccessful. The drawback of this methodology is searching markers in patients at the clinical stage without guarantee that they are also characteristic of either preclinical stage or prodromal stage (preclinical-prodromal stage). We attempted to upgrade this methodology by selecting only markers that are found both in patients and in PD animal models. HPLC and RT-PCR were used to estimate the concentration of amino acids, catecholamines/metabolites in plasma and gene expression in lymphocytes in 36 untreated early-stage PD patients and 52 controls, and in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice at modeling the clinical ("symptomatic") stage and preclinical-prodromal ("presymptomatic") stage of PD. It was shown that among 13 blood markers found in patients, 7 markers are characteristic of parkinsonian symptomatic mice and 3 markers of both symptomatic and presymptomatic mice. According to our suggestion, the detection of the same marker in patients and symptomatic animals indicates adequate reproduction of pathogenesis along the corresponding metabolic pathway, whereas the detection of the same marker in presymptomatic animals indicates its specificity for preclinical-prodromal stage. This means that the minority of markers found in patients-decreased concentration of L-3,4-dihydroxyphenylalanine (L-DOPA) and dihydroxyphenylacetic acid (DOPAC) and increased dopamine D3 receptor gene expression-are specific for preclinical-prodromal stage and are suitable for early diagnosis of PD. Thus, we upgraded a current methodology for development of early diagnosis of PD by searching blood markers not only in patients but also in parkinsonian animals.

MicroRNAs Overcome Cell Fate Barrier by Reducing EZH2-Controlled REST Stability during Neuronal Conversion of Human Adult Fibroblasts

The ability to convert human somatic cells efficiently to neurons facilitates the utility of patient-derived neurons for studying neurological disorders. As such, ectopic expression of neuronal microRNAs (miRNAs), miR-9/9^∗ and miR-124 (miR-9/9^∗-124) in adult human fibroblasts has been found to evoke extensive reconfigurations of the chromatin and direct the fate conversion to neurons. However, how miR-9/9^∗-124 break the cell fate barrier to activate the neuronal program remains to be defined. Here, we identified an anti-neurogenic function of EZH2 in fibroblasts that acts outside its role as a subunit of Polycomb Repressive Complex 2 to directly methylate and stabilize REST, a transcriptional repressor of neuronal genes. During neuronal conversion, miR-9/9^∗-124 induced the repression of the EZH2-REST axis by downregulating USP14, accounting for the opening of chromatin regions harboring REST binding sites. Our findings underscore the interplay between miRNAs and protein stability cascade underlying the activation of neuronal program.

Mechanistic Insights Into MicroRNA-Induced Neuronal Reprogramming of Human Adult Fibroblasts

The use of transcriptional factors as cell fate regulators are often the primary focus in the direct reprogramming of somatic cells into neurons. However, in human adult fibroblasts, deriving functionally mature neurons with high efficiency requires additional neurogenic factors such as microRNAs (miRNAs) to evoke a neuronal state permissive to transcription factors to exert their reprogramming activities. As such, increasing evidence suggests brain-enriched miRNAs, miR-9/9^∗ and miR-124, as potent neurogenic molecules through simultaneously targeting of anti-neurogenic effectors while allowing additional transcription factors to generate specific subtypes of human neurons. In this review, we will focus on methods that utilize neuronal miRNAs and provide mechanistic insights by which neuronal miRNAs, in synergism with brain-region specific transcription factors, drive the conversion of human fibroblasts into clinically relevant subtypes of neurons. Furthermore, we will provide insights into the age signature of directly converted neurons and how the converted human neurons can be utilized to model late-onset neurodegenerative disorders.

Human cellular models of medium spiny neuron development and Huntington disease

The loss of gamma-aminobutyric acid (GABA)-ergic medium spiny neurons (MSNs) in the striatum is the hallmark of Huntington disease (HD), an incurable neurodegenerative disorder characterized by progressive motor, psychiatric, and cognitive symptoms. Transplantation of MSNs or their precursors represents a promising treatment strategy for HD. In initial clinical trials in which HD patients received fetal neurografts directly into the striatum without a pretransplant cell-differentiation step, some patients exhibited temporary benefits. Meanwhile, major challenges related to graft overgrowth, insufficient survival of grafted cells, and limited availability of donated fetal tissue remain. Thus, the development of approaches that allow modeling of MSN differentiation and HD development in cell culture platforms may improve our understanding of HD and translate, ultimately, into HD treatment options. Here, recent advances in the in vitro differentiation of MSNs derived from fetal neural stem cells/progenitor cells (NSCs/NPCs), embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and induced NSCs (iNSCs) as well as advances in direct transdifferentiation are reviewed. Progress in non-allele specific and allele specific gene editing of HTT is presented as well. Cell characterization approaches involving phenotyping as well as in vitro and in vivo functional assays are also discussed.

Distance running alters peripheral microRNAs implicated in metabolism, fluid balance, and myosin regulation in a sex-specific manner

Microribonucleic acids (miRNAs) mediate adaptive responses to exercise and may serve as biomarkers of exercise intensity/capacity. Expression of miRNAs is altered in skeletal muscle, plasma, and saliva following exertion. Women display unique physiologic responses to endurance exercise, and miRNAs respond to pathologic states in sex-specific patterns. However sex-specific miRNA responses to exercise remain unexplored. This study utilized high-throughput RNA sequencing to measure changes in salivary RNA expression among 25 collegiate runners following a single long-distance run. RNA concentrations in pre- and post-run saliva was assessed through alignment and quantification of 4,694 miRNAs and 27,687 mRNAs. Pair-wise Wilcoxon rank-sum test identified miRNAs with significant [false discovery rate (FDR) < 0.05] post-run changes. Associations between miRNA levels and predicted mRNA targets were explored with Pearson correlations. Differences in miRNA patterns between men ( n = 13) and women ( n = 12) were investigated with two-way analysis of variance. Results revealed 122 salivary miRNAs with post-run changes. The eight miRNAs with the largest changes were miR-3671, miR-5095 (downregulated); and miR-7154-3p, miR-200b-5p, miR-5582-3p, miR-6859-3p, miR-6751-5p, miR-4419a (upregulated). Predicted mRNA targets for these miRNAs represented 15 physiologic processes, including glycerophospholipid metabolism (FDR = 0.042), aldosterone-regulated sodium reabsorption (FDR = 0.049), and arrhythmogenic ventricular cardiomyopathy (FDR = 0.018). Twenty-six miRNA/mRNA pairs had associated changes in post-run levels. Three miRNAs (miR-4675, miR-6745, miR-6746-3p) demonstrated sex-specific responses to exercise. Numerous salivary miRNAs change in response to endurance running and target the expression of genes involved in metabolism, fluid balance, and musculoskeletal adaptations. A subset of miRNAs may differentiate the metabolic response to exercise in men and women.

The chromatin basis of neurodevelopmental disorders: Rethinking dysfunction along the molecular and temporal axes

The complexity of the human brain emerges from a long and finely tuned developmental process orchestrated by the crosstalk between genome and environment. Vis à vis other species, the human brain displays unique functional and morphological features that result from this extensive developmental process that is, unsurprisingly, highly vulnerable to both genetically and environmentally induced alterations. One of the most striking outcomes of the recent surge of sequencing-based studies on neurodevelopmental disorders (NDDs) is the emergence of chromatin regulation as one of the two domains most affected by causative mutations or Copy Number Variations besides synaptic function, whose involvement had been largely predicted for obvious reasons. These observations place chromatin dysfunction at the top of the molecular pathways hierarchy that ushers in a sizeable proportion of NDDs and that manifest themselves through synaptic dysfunction and recurrent systemic clinical manifestation. Here we undertake a conceptual investigation of chromatin dysfunction in NDDs with the aim of systematizing the available evidence in a new framework: first, we tease out the developmental vulnerabilities in human corticogenesis as a structuring entry point into the causation of NDDs; second, we provide a much needed clarification of the multiple meanings and explanatory frameworks revolving around "epigenetics", highlighting those that are most relevant for the analysis of these disorders; finally we go in-depth into paradigmatic examples of NDD-causing chromatin dysregulation, with a special focus on human experimental models and datasets.

Shaping Gene Expression by Landscaping Chromatin Architecture: Lessons from a Master

Since its discovery as a skeletal muscle-specific transcription factor able to reprogram somatic cells into differentiated myofibers, MyoD has provided an instructive model to understand how transcription factors regulate gene expression. Reciprocally, studies of other transcriptional regulators have provided testable hypotheses to further understand how MyoD activates transcription. Using MyoD as a reference, in this review, we discuss the similarities and differences in the regulatory mechanisms employed by tissue-specific transcription factors to access DNA and regulate gene expression by cooperatively shaping the chromatin landscape within the context of cellular differentiation.

Stability of Chromatin Remodeling Complex Subunits Is Determined by Their Phosphorylation Status

It was found that, in the differentiated cells of mouse brain, the level of core (Brg1 and BAF155) and specific (BRD7, BAF180, and PHF10) subunits of the chromatin-remodeling complex PBAF is reduced compared to the undifferentiated proliferating cells. Phosphorylation of PBAF complex subunits is required for maintaining their stability in differentiated brain cells.

MicroRNAs Induce a Permissive Chromatin Environment that Enables Neuronal Subtype-Specific Reprogramming of Adult Human Fibroblasts

Directed reprogramming of human fibroblasts into fully differentiated neurons requires massive changes in epigenetic and transcriptional states. Induction of a chromatin environment permissive for acquiring neuronal subtype identity is therefore a major barrier to fate conversion. Here we show that the brain-enriched miRNAs miR-9/9^∗ and miR-124 (miR-9/9^∗-124) trigger reconfiguration of chromatin accessibility, DNA methylation, and mRNA expression to induce a default neuronal state. miR-9/9^∗-124-induced neurons (miNs) are functionally excitable and uncommitted toward specific subtypes but possess open chromatin at neuronal subtype-specific loci, suggesting that such identity can be imparted by additional lineage-specific transcription factors. Consistently, we show that ISL1 and LHX3 selectively drive conversion to a highly homogeneous population of human spinal cord motor neurons. This study shows that modular synergism between miRNAs and neuronal subtype-specific transcription factors can drive lineage-specific neuronal reprogramming, providing a general platform for high-efficiency generation of distinct subtypes of human neurons.

Epigenetics and epitranscriptomics in temporal patterning of cortical neural progenitor competence

Yoon et al. review epigenetic and epitranscriptomic mechanisms that regulate the lineage specification of neural progenitor cells in the developing brain.

During embryonic brain development, neural progenitor/stem cells (NPCs) sequentially give rise to different subtypes of neurons and glia via a highly orchestrated process. To accomplish the ordered generation of distinct progenies, NPCs go through multistep transitions of their developmental competence. The molecular mechanisms driving precise temporal coordination of these transitions remains enigmatic. Epigenetic regulation, including changes in chromatin structures, DNA methylation, and histone modifications, has been extensively investigated in the context of cortical neurogenesis. Recent studies of chemical modifications on RNA, termed epitranscriptomics, have also revealed their critical roles in neural development. In this review, we discuss advances in understanding molecular regulation of the sequential lineage specification of NPCs in the embryonic mammalian brain with a focus on epigenetic and epitranscriptomic mechanisms. In particular, the discovery of lineage-specific gene transcripts undergoing rapid turnover in NPCs suggests that NPC developmental fate competence is determined much earlier, before the final cell division, and is more tightly controlled than previously appreciated. We discuss how multiple regulatory systems work in harmony to coordinate NPC behavior and summarize recent findings in the context of a model of epigenetic and transcriptional prepatterning to explain NPC developmental competence.

ATP-Dependent Chromatin Remodeling During Cortical Neurogenesis

The generation of individual neurons (neurogenesis) during cortical development occurs in discrete steps that are subtly regulated and orchestrated to ensure normal histogenesis and function of the cortex. Notably, various gene expression programs are known to critically drive many facets of neurogenesis with a high level of specificity during brain development. Typically, precise regulation of gene expression patterns ensures that key events like proliferation and differentiation of neural progenitors, specification of neuronal subtypes, as well as migration and maturation of neurons in the developing cortex occur properly. ATP-dependent chromatin remodeling complexes regulate gene expression through utilization of energy from ATP hydrolysis to reorganize chromatin structure. These chromatin remodeling complexes are characteristically multimeric, with some capable of adopting functionally distinct conformations via subunit reconstitution to perform specific roles in major aspects of cortical neurogenesis. In this review, we highlight the functions of such chromatin remodelers during cortical development. We also bring together various proposed mechanisms by which ATP-dependent chromatin remodelers function individually or in concert, to specifically modulate vital steps in cortical neurogenesis.

Transcriptional and Epigenetic Control of Mammalian Olfactory Epithelium Development

The postnatal mammalian olfactory epithelium (OE) represents a major aspect of the peripheral olfactory system. It is a pseudostratified tissue that originates from the olfactory placode and is composed of diverse cells, some of which are specialized receptor neurons capable of transducing odorant stimuli to afford the perception of smell (olfaction). The OE is known to offer a tractable miniature model for studying the systematic generation of neurons and glia that typify neural tissue development. During OE development, stem/progenitor cells that will become olfactory sensory neurons and/or non-neuronal cell types display fine spatiotemporal expression of neuronal and non-neuronal genes that ensures their proper proliferation, differentiation, survival, and regeneration. Many factors, including transcription and epigenetic factors, have been identified as key regulators of the expression of such requisite genes to permit normal OE morphogenesis. Typically, specific interactive regulatory networks established between transcription and epigenetic factors/cofactors orchestrate histogenesis in the embryonic and adult OE. Hence, investigation of these regulatory networks critical for OE development promises to disclose strategies that may be employed in manipulating the stepwise transition of olfactory precursor cells to become fully differentiated and functional neuronal and non-neuronal cell types. Such strategies potentially offer formidable means of replacing injured or degenerated neural cells as therapeutics for nervous system perturbations. This review recapitulates the developmental cellular diversity of the olfactory neuroepithelium and discusses findings on how the precise and cooperative molecular control by transcriptional and epigenetic machinery is indispensable for OE ontogeny.

Disruption of mammalian SWI/SNF and polycomb complexes in human sarcomas: mechanisms and therapeutic opportunities

Soft-tissue sarcomas are increasingly characterized and subclassified by genetic abnormalities that represent underlying drivers of their pathology. Hallmark tumor suppressor gene mutations and pathognomonic gene fusions collectively account for approximately one-third of all sarcomas. These genetic abnormalities most often result in global transcriptional misregulation via disruption of protein regulatory complexes which govern chromatin architecture. Specifically, alterations to mammalian SWI/SNF (mSWI/SNF or BAF) ATP-dependent chromatin remodeling complexes and polycomb repressive complexes cause disease-specific changes in chromatin architecture and gene expression across a number of sarcoma subtypes. Understanding the functions of chromatin regulatory complexes and the mechanisms underpinning their roles in oncogenesis will be required for the design and development of new therapeutic strategies in sarcomas. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Nervous system development and disease: A focus on trithorax related proteins and chromatin remodelers

The nervous system comprises many different cell types including neurons, glia, macrophages, and immune cells, each of which is defined by specific patterns of gene expression, morphology, function, and anatomical location. Establishment of these complex and highly regulated cell fates requires spatial and temporal coordination of gene transcription. Open chromatin (euchromatin) allows transcription factors to interact with gene promoters and activate lineage specific genes, whereas closed chromatin (heterochromatin) remains inaccessible to transcriptional activation. Changes in the genome-wide distribution of euchromatin accompany transcriptional plasticity that allows the diversity of mature cell fates to be generated during development. In the past 20years, many new genes and gene families have been identified to participate in regulation of chromatin accessibility. These genes include chromatin remodelers that interact with Trithorax group (TrxG) and Polycomb group (PcG) proteins to activate or repress transcription, respectively. Here we review the role of TrxG proteins in neurodevelopment and disease.

Mutations in the BAF-Complex Subunit DPF2 Are Associated with Coffin-Siris Syndrome

Variants affecting the function of different subunits of the BAF chromatin-remodelling complex lead to various neurodevelopmental syndromes, including Coffin-Siris syndrome. Furthermore, variants in proteins containing PHD fingers, motifs recognizing specific histone tail modifications, have been associated with several neurological and developmental-delay disorders. Here, we report eight heterozygous de novo variants (one frameshift, two splice site, and five missense) in the gene encoding the BAF complex subunit double plant homeodomain finger 2 (DPF2). Affected individuals share common clinical features described in individuals with Coffin-Siris syndrome, including coarse facial features, global developmental delay, intellectual disability, speech impairment, and hypoplasia of fingernails and toenails. All variants occur within the highly conserved PHD1 and PHD2 motifs. Moreover, missense variants are situated close to zinc binding sites and are predicted to disrupt these sites. Pull-down assays of recombinant proteins and histone peptides revealed that a subset of the identified missense variants abolish or impaire DPF2 binding to unmodified and modified H3 histone tails. These results suggest an impairment of PHD finger structural integrity and cohesion and most likely an aberrant recognition of histone modifications. Furthermore, the overexpression of these variants in HEK293 and COS7 cell lines was associated with the formation of nuclear aggregates and the recruitment of both wild-type DPF2 and BRG1 to these aggregates. Expression analysis of truncating variants found in the affected individuals indicated that the aberrant transcripts escape nonsense-mediated decay. Altogether, we provide compelling evidence that de novo variants in DPF2 cause Coffin-Siris syndrome and propose a dominant-negative mechanism of pathogenicity.

Role of Forkhead Box O (FOXO) transcription factor in aging and diseases

Fork head box O (FOXO) transcription factor is a key player in an evolutionarily conserved pathway. The mammalian FOXO family consists of FOXO1, 3, 4 and 6, are highly similar in their structure, function and regulation. To maintain optimum body function, the organisms have developed complex mechanisms for homeostasis. Importantly, it is well known that when these mechanisms dysregulate it results in the development of age-related disease. FOXO proteins are involved in a diverse cellular function and also have clinical significance including cell cycle arrest, cell differentiation, tumour suppression, DNA repair, longevity, diabetic complications, immunity, wound healing, regulation of metabolism and thus treatment of several types of diseases. By the combinations of post-translational modifications FOXO's serve as a 'molecular code' to sense external stimuli and recruit it as to specific regions of the genome and provide an integrated cellular response to changing physiological conditions. Akt/Protein kinase B a signaling pathway as a main regulator of FOXO to perform a diverse function in organisms. The present review summarizes the molecular and clinical aspects of FOXO transcription factor. And also elaborate the interaction of FOXO with the nucleosome remodelling complex to target genes, which is essential to cellular homeostasis.

BAFfling pathologies: Alterations of BAF complexes in cancer

To activate or repress specific genes, chromatin is constantly modified by chromatin-remodeling complexes. Among these complexes, the SWItch/Sucrose Non-Fermenting (SWI/SNF) complex, also referred to as BRG1-Associated Factor (BAF) complex, moves the nucleosome along chromatin using energy provided by ATP hydrolysis. In mammalian organisms, the SWI/SNF complex is composed of 10-15 subunits, depending on cell type, and a defect in one of these subunits can have dramatic consequences. In this review we will focus on the alterations identified in the SWI/SNF (BAF) complex subunits that lead to cancerous pathologies. While SMARCB1 was the first mutated subunit to be reported in a majority of malignant rhabdoid tumors, the advent of next-generation sequencing allowed the discovery of mutations in various SWI/SNF subunits within a broad spectrum of cancers. In most cases, the mutation leads to a loss of expression or to a truncated subunit unable to perform its function. Even though it is now commonly acknowledged that approximately 20% of all cancers present a mutation in a SWI/SNF subunit, some cancers are associated to a specific alteration of a SWI/SNF subunit, which acts either as tumor suppressor genes or as oncogenes, and therefore constitute diagnostic or prognostic biomarkers. Consistently, therapeutic strategies targeting SWI/SNF subunits or the genes affected downstream have been revealed to treat cancers.

New aspects of glioblastoma multiforme revealed by similarities between neural and glioblastoma stem cells

Neural stem cells (NSCs) undergo self-renewal and generate neurons and glial cells under the influence of specific signals from surrounding environments. Glioblastoma multiforme (GBM) is a highly lethal brain tumor arising from NSCs or glial precursor cells owing to dysregulation of transcriptional and epigenetic networks that control self-renewal and differentiation of NSCs. Highly tumorigenic glioblastoma stem cells (GSCs) constitute a small subpopulation of GBM cells, which share several characteristic similarities with NSCs. GSCs exist atop a stem cell hierarchy and generate heterogeneous populations that participate in tumor propagation, drug resistance, and relapse. During multimodal treatment, GSCs de-differentiate and convert into cells with malignant characteristics, and thus play critical roles in tumor propagation. In contrast, differentiation therapy that induces GBM cells or GSCs to differentiate into a neuronal or glial lineage is expected to inhibit their proliferation. Since stem cell differentiation is specified by the cells' epigenetic status, understanding their stemness and the epigenomic situation in the ancestor, NSCs, is important and expected to be helpful for developing treatment modalities for GBM. Here, we review the current findings regarding the epigenetic regulatory mechanisms of NSC fate in the developing brain, as well as those of GBM and GSCs. Furthermore, considering the similarities between NSCs and GSCs, we also discuss potential new strategies for GBM treatment.

Epigenetic regulation of neuroblastoma development

In recent years, technological advances have enabled a detailed landscaping of the epigenome and the mechanisms of epigenetic regulation that drive normal cell function, development and cancer. Rather than merely a structural entity to support genome compaction, we now look at chromatin as a very dynamic and essential constellation that is actively participating in the tight orchestration of transcriptional regulation as well as DNA replication and repair. The unique feature of chromatin flexibility enabling fast switches towards more or less restricted epigenetic cellular states is, not surprisingly, intimately connected to cancer development and treatment resistance, and the central role of epigenetic alterations in cancer is illustrated by the finding that up to 50% of all mutations across cancer entities affect proteins controlling the chromatin status. We summarize recent insights into epigenetic rewiring underlying neuroblastoma (NB) tumor formation ranging from changes in DNA methylation patterns and mutations in epigenetic regulators to global effects on transcriptional regulatory circuits that involve key players in NB oncogenesis. Insights into the disruption of the homeostatic epigenetic balance contributing to developmental arrest of sympathetic progenitor cells and subsequent NB oncogenesis are rapidly growing and will be exploited towards the development of novel therapeutic strategies to increase current survival rates of patients with high-risk NB.

Mitochondrially produced ATP affects stem cell pluripotency via Actl6a-mediated histone acetylation

ATP is mainly generated by glycolysis in pluripotent stem cells (PSCs) and is consumed to maintain cell viability. Differences in mitochondrial activity among induced (i)PSCs with different degrees of pluripotency are poorly understood. In this study, by comparing gene expression and mitochondrial activity among iPSCs with different degrees of pluripotency, we found that mitochondrial complex I gene expression, complex I activity, and cellular ATP levels were much higher in fully pluripotent stem cell lines than in partially pluripotent stem cell lines. Actin-like protein 6a (Actl6a), a component of ATP-dependent chromatin remodeling and histone acetylation complexes, was more highly expressed in fully pluripotent stem cell lines. ATP promoted Actl6a expression and histone acetylation. Actl6a knockdown reduced the pluripotency of embryonic stem cells (ESCs), and this reduction could not be rescued by the addition of ATP. Furthermore, inhibiting ATP formation by treatment with rotenone reduced the pluripotency of ESCs. These data suggest that the abundance of mitochondrially produced ATP affects stem cell pluripotency via Actl6a-mediated histone acetylation.-Zhang, Y., Cui, P., Li, Y., Feng, G., Tong, M., Guo, L., Li, T., Liu, L., Li, W., Zhou, Q. Mitochondrially produced ATP affects stem cell pluripotency via Actl6a-mediated histone acetylation.

Regulation of Plant Growth and Development: A Review From a Chromatin Remodeling Perspective

In eukaryotes, genetic material is packaged into a dynamic but stable nucleoprotein structure called chromatin. Post-translational modification of chromatin domains affects the expression of underlying genes and subsequently the identity of cells by conveying epigenetic information from mother to daughter cells. SWI/SNF chromatin remodelers are ATP-dependent complexes that modulate core histone protein polypeptides, incorporate variant histone species and modify nucleotides in DNA strands within the nucleosome. The present review discusses the SWI/SNF chromatin remodeler family, its classification and recent advancements. We also address the involvement of SWI/SNF remodelers in regulating vital plant growth and development processes such as meristem establishment and maintenance, cell differentiation, organ initiation, flower morphogenesis and flowering time regulation. Moreover, the role of chromatin remodelers in key phytohormone signaling pathways is also reviewed. The information provided in this review may prompt further debate and investigations aimed at understanding plant-specific epigenetic regulation mediated by chromatin remodeling under continuously varying plant growth conditions and global climate change.

Molecular and functional variation in iPSC-derived sensory neurons

Induced pluripotent stem cells (iPSCs), and cells derived from them, have become key tools for modeling biological processes, particularly in cell types that are difficult to obtain from living donors. Here we present a map of regulatory variants in iPSC-derived neurons, based on 123 differentiations of iPSCs to a sensory neuronal fate. Gene expression was more variable across cultures than in primary dorsal root ganglion, particularly for genes related to nervous system development. Using single-cell RNA-sequencing, we found that the number of neuronal versus contaminating cells was influenced by iPSC culture conditions before differentiation. Despite high differentiation-induced variability, our allele-specific method detected thousands of quantitative trait loci (QTLs) that influenced gene expression, chromatin accessibility, and RNA splicing. On the basis of these detected QTLs, we estimate that recall-by-genotype studies that use iPSC-derived cells will require cells from at least 20-80 individuals to detect the effects of regulatory variants with moderately large effect sizes.

Co-regulation of transcription by BRG1 and BRM, two mutually exclusive SWI/SNF ATPase subunits

BACKGROUND

SWI/SNF is a large heterogeneous multi-subunit chromatin remodeling complex. It consists of multiple sets of mutually exclusive components. Understanding how loss of one sibling of a mutually exclusive pair affects the occupancy and function of the remaining complex is needed to understand how mutations in a particular subunit might affect tumor formation. Recently, we showed that the members of the ARID family of SWI/SNF subunits (ARID1A, ARID1B and ARID2) had complex transcriptional relationships including both antagonism and cooperativity. However, it remains unknown how loss of the catalytic subunit(s) affects the binding and genome-wide occupancy of the remainder complex and how changes in occupancy affect transcriptional output.

RESULTS

We addressed this gap by depleting BRG1 and BRM, the two ATPase subunits in SWI/SNF, and characterizing the changes to chromatin occupancy of the remaining subunit and related this to transcription changes induced by loss of the ATPase subunits. We show that depletion of one subunit frequently leads to loss of the remaining subunit. This could cause either positive or negative changes in gene expression. At a subset of sites, the sibling subunit is either retained or gained. Additionally, we show genome-wide that BRG1 and BRM have both cooperative and antagonistic interactions with respect to transcription. Importantly, at genes where BRG1 and BRM antagonize one another we observe a nearly complete rescue of gene expression changes in the combined BRG/BRM double knockdown.

CONCLUSION

This series of experiments demonstrate that mutually exclusive SWI/SNF complexes have heterogeneous functional relationships and highlight the importance of considering the role of the remaining SWI/SNF complexes following loss or depletion of a single subunit.

The role of ISWI chromatin remodeling complexes in brain development and neurodevelopmental disorders

The mammalian ISWI (Imitation Switch) genes SMARCA1 and SMARCA5 encode the ATP-dependent chromatin remodeling proteins SNF2L and SNF2H. The ISWI proteins interact with BAZ (bromodomain adjacent to PHD zinc finger) domain containing proteins to generate eight distinct remodeling complexes. ISWI complex-mediated nucleosome positioning within genes and gene regulatory elements is proving important for the transition from a committed progenitor state to a differentiated cell state. Genetic studies have implicated the involvement of many ATP-dependent chromatin remodeling proteins in neurodevelopmental disorders (NDDs), including SMARCA1. Here we review the characterization of mice inactivated for ISWI and their interacting proteins, as it pertains to brain development and disease. A better understanding of chromatin dynamics during neural development is a prerequisite to understanding disease pathologies and the development of therapeutics for these complex disorders.

The SWI/SNF subunit Bcl7a contributes to motor coordination and Purkinje cell function

Chromatin remodelers have emerged as prominent regulators of epigenetic processes and potential drivers of various human pathologies. The multi-subunit chromatin-remodeling SWI/SNF complex determines gene expression programs and, consequently, contributes to the differentiation, maturation and plasticity of neurons. Here, we investigate the elusive biological function of Bcl7a and Bcl7b, two newly identified subunits of the SWI/SNF complex that are highly expressed throughout the brain. We generated ubiquitous and neuron-specific Bcl7a and Bcl7b single and double knockout mice. We provide evidence that Bcl7b is dispensable for animal survival as well as behavioral plasticity. Conversely, ubiquitous Bcl7a knockout results in perinatal lethality, while genetic deletion of Bcl7a in postmitotic neurons elicits motor abnormalities and affects dendritic branching of Purkinje cells, with no obvious synergistic relationship with Bcl7b. Collectively, our findings reveal novel insights into the cellular processes linked to BCL7-containing SWI/SNF complexes and their unrecognized roles in the brain.

ARID1A, a component of SWI/SNF chromatin remodeling complexes, is required for porcine embryo development

Mammalian embryos undergo dramatic epigenetic remodeling that can have a profound impact on both gene transcription and overall embryo developmental competence. Members of the SWI/SNF (Switch/Sucrose non-fermentable) family of chromatin-remodeling complexes reposition nucleosomes and alter transcription factor accessibility. These large, multi-protein complexes possess an SNF2-type ATPase (either SMARCA4 or SMARCA2) as their core catalytic subunit, and are directed to specific loci by associated subunits. Little is known about the identity of specific SWI/SNF complexes that serve regulatory roles during cleavage development. ARID1A, one of the SWI/SNF complex subunits, can affect histone methylation in somatic cells; here, we determined the developmental requirements of ARID1A in porcine oocytes and embryos. We found ARID1A transcript levels were significantly reduced in 4-cell porcine embryos as compared to germinal vesicle-stage oocytes, suggesting that ARID1A would be required for porcine cleavage-stage development. Indeed, injecting in vitro-matured and fertilized porcine oocytes with double-stranded interfering RNAs that target ARID1A, and evaluating their phenotype after seven days, revealed that the depletion of ARID1A results in significantly fewer cells than their respective control groups (p < 0.001).

Gfap and Osmr regulation by BRG1 and STAT3 via interchromosomal gene clustering in astrocytes

Gene clustering is relevant in the regulation of gene expression. However, the mechanisms of gene clustering remain to be elucidated. Using a glial differentiation system, we found that the clustering of Gfap, an astrocyte-pecific gene, with Osmr enhances transcription of both genes. BRG1 and the JAK-STAT pathway are central to the clustering.

Long-range chromatin interactions between gene loci in the cell nucleus are important for many biological processes, including transcriptional regulation. Previously, we demonstrated that several genes specifically cluster with the astrocyte-specific gene for glial fibrillary acidic protein (Gfap) during astrocyte differentiation; however, the molecular mechanisms for gene clustering remain largely unknown. Here we show that brahma-related gene 1 (BRG1), an ATP-dependent chromatin remodeling factor, and the transcription factor STAT3 are required for Gfap and oncostatin M receptor (Osmr) clustering and enhanced expression through recruitment to STAT3 recognition sequences and that gene clustering occurs prior to transcriptional up-regulation. BRG1 knockdown and JAK-STAT signaling inhibition impaired clustering, leading to transcriptional down-regulation of both genes. BRG1 and STAT3 were recruited to the same Gfap fragment; JAK-STAT signaling inhibition impaired BRG1 recruitment. Our results suggest that BRG1 and STAT3 coordinately regulate gene clustering and up-regulate Gfap and Osmr transcription.

Transcriptional and Post-Transcriptional Mechanisms of the Development of Neocortical Lamination

The neocortex is a laminated brain structure that is the seat of higher cognitive capacity and responses, long-term memory, sensory and emotional functions, and voluntary motor behavior. Proper lamination requires that progenitor cells give rise to a neuron, that the immature neuron can migrate away from its mother cell and past other cells, and finally that the immature neuron can take its place and adopt a mature identity characterized by connectivity and gene expression; thus lamination proceeds through three steps: genesis, migration, and maturation. Each neocortical layer contains pyramidal neurons that share specific morphological and molecular characteristics that stem from their prenatal birth date. Transcription factors are dynamic proteins because of the cohort of downstream factors that they regulate. RNA-binding proteins are no less dynamic, and play important roles in every step of mRNA processing. Indeed, recent screens have uncovered post-transcriptional mechanisms as being integral regulatory mechanisms to neocortical development. Here, we summarize major aspects of neocortical laminar development, emphasizing transcriptional and post-transcriptional mechanisms, with the aim of spurring increased understanding and study of its intricacies.

The BAF45D Protein Is Preferentially Expressed in Adult Neurogenic Zones and in Neurons and May Be Required for Retinoid Acid Induced PAX6 Expression

Adult neurogenesis is important for the development of regenerative therapies for human diseases of the central nervous system (CNS) through the recruitment of adult neural stem cells (NSCs). NSCs are characterized by the capacity to generate neurons, astrocytes, and oligodendrocytes. To identify key factors involved in manipulating the adult NSC neurogenic fate thus has crucial implications for the clinical application. Here, we report that BAF45D is expressed in the subgranular zone (SGZ) of the dentate gyrus, the subventricular zone (SVZ) of the lateral ventricle, and the central canal (CC) of the adult spinal cord. Coexpression of BAF45D with glial fibrillary acidic protein (GFAP), a radial glial like cell marker protein, was identified in the SGZ, the SVZ and the adult spinal cord CC. Quantitative analysis data indicate that BAF45D is preferentially expressed in the neurogenic zone of the LV and the neurons of the adult CNS. Furthermore, during the neuroectoderm differentiation of H9 cells, BAF45D is required for the expression of PAX6, a neuroectoderm determinant that is also known to regulate the self-renewal and neuronal fate specification of adult neural stem/progenitor cells. Together, our results may shed new light on the expression of BAF45D in the adult neurogenic zones and the contribution of BAF45D to early neural development.

Baf53a is involved in survival of mouse ES cells, which can be compensated by Baf53b

The human Baf (Brg1/Brm associated factor) complex, also known as the mammalian SWI/SNF chromatin-remodeling complex, is involved in a variety of cellular processes. The pluripotency and self-renewal abilities are major characteristics of embryonic stem (ES) cells and are regulated by the ES cell-specific BAF (esBAF) complex. Baf53a is one of the subunits of the esBAF complex. Here, we found that Baf53a was expressed in undifferentiated ES cells and that it interacted with Oct3/4. Analyses of tetracycline-inducible Baf53a conditional knockout ES cells revealed that the undifferentiated markers, including Nanog and Oct3/4, were expressed in Baf53a-deficient ES cells; however, growth of the cells was repressed, and expression of p53, p21, and cleaved Caspase 3 was increased. Cell death of Baf53a-deficient ES cells was rescued by overexpression of Baf53a, but not by the Baf53a M3 mutant (E388A/R389A/R390A). Interestingly, Baf53b, a homologue of Baf53a, rescued cell death of Baf53a-deficient ES cells. Baf53a-deficient ES cells overexpressing exogenous Baf53a or Baf53b remained in the undifferentiated state, proliferated, and repressed expression of p21. In summary, our findings suggest that Baf53a is involved in the survival of ES cells by regulating p53 and Caspase3, and that Baf53b is able to compensate for this functional aspect of Baf53a.

DPFF-1 transcription factor deficiency causes the aberrant activation of MPK-1 and meiotic defects in the Caenorhabditis elegans germline

The d4 family of transcription factors consists of three members in mammals. DPF1/neuro-d4 is expressed mainly in neurons and the peripheral nervous system, and is important for brain development. DPF2/requiem/ubi-d4 is expressed ubiquitously and presumably functions as an apoptotic factor, especially during the deprivation of trophic factors. DPF3/cer-d4 is expressed in neurons and in the heart, and is important for heart development and function in zebrafish. In Drosophila, there is only one member, dd4, whose function is still unknown, but it is expressed in many tissues and is particularly abundant in the brain of developing embryos and in adults. Here, we present DPFF-1, the only member of this family of proteins in the nematode C. elegans. DPFF-1 is similar to its mammalian homolog DPF2/requiem/ubi-d4 because it is ubiquitously expressed during embryogenesis and in adult tissues, and because it is important for the induction of germ cell apoptosis during stress. Here, we show that dpff-1 null mutant animals produce less progeny than wild-type nematodes, presumably due to meiotic defects. Gonads of dpff-1 deficient animals showed more germ cells in pachytene and overexpressed the P-MPK-1 signal. Additionally, these animals presented higher levels of p53-induced germ cell apoptosis than wild-type animals. Furthermore, we observed that dpff-1 deficient animals are more sensitive to heat shock. This is the first report showing that the d4 family of transcription factors could be involved in meiosis and stress protection.

BAF53a is a potential prognostic biomarker and promotes invasion and epithelial-mesenchymal transition of glioma cells

Increasing evidence indicates that BAF53a is crucial for embryonic development and maintenance of stemness, and may be associated with epithelial-mesenchymal transition (EMT), which suggests its involvement in cancer progression. However, the role of BAF53a in glioma remains unknown. In the present study, BAF53a was found to be highly expressed in glioma tissues and was associated with poor overall survival (OS) and progression-free survival (PFS) in glioma patients. A multivariate Cox regression analysis revealed that BAF53a might be an independent prognostic factor for OS and PFS in glioma patients. Further functional analysis indicated that BAF53a overexpression could promote proliferation and increase the motility and invasion of U87 glioma cells, whereas BAF53a knockdown had the opposite effect. In addition, BAF53a expression was associated with the levels of E‑cadherin and vimentin expression in glioma tissues. This was further confirmed in U87 cells expressing different levels of BAF53a; BAF53a overexpression was concomitant with decreased E‑cadherin and increased vimentin expression, whereas BAF53a knockdown showed the opposite pattern of expression. Taken together, these results suggest that BAF53a may be a novel prognostic factor for glioma patients, and that BAF53 may facilitate glioma progression by promoting proliferation, invasion, and associate with EMT. Therefore, BAF53a could be a potential promising biomarker and a target for the treatment of glioma.

SWI/SNF Infobase-An exclusive information portal for SWI/SNF remodeling complex subunits

Chromatin remodeling complexes facilitate the access of condensed genomic DNA during transcription, replication, and repair, by altering the histone-DNA contacts in the nucleosome structures. SWI/SNF (SWItch/Sucrose Non-Fermentable) family of ATP dependent chromatin remodeling complexes have been documented for their tumour suppressor function. Recent studies have reported the high frequency of cancer causing mutations in this protein family. There exist multiple subunits for this complex and can form context-dependent sub-complexes. The cataloguing of individual subunits of this complex is essential for understanding their specific functions and their mechanism of action during chromatin remodeling. This would also facilitate further studies to characterize cancer causing mutations in SWI/SNF subunits. In the current study, a database containing information on the subunits of SWI/SNF-α (BRG1/BRM-Associated Factors (BAF)) and SWI/SNF-β (Polybromo-Associated BAF (PBAF)) sub classes of SWI/SNF family has been curated and catalogued. The database hosts information on 27 distinct SWI/SNF subunits from 20 organisms spanning a wide evolutionary range of eukaryotes. A non-redundant set of 522 genes coding for SWI/SNF subunits have been documented in the database. A detailed annotation on each subunit, including basic protein/gene information, protein sequence, functional domains, homologs and missense mutations of human proteins have been provided with a user-friendly graphical interface. The SWI/SNF Infobase presented here, would be a first of its kind exclusive information portal on SWI/SNF complex subunits and would be a valuable resource for the research community working on chromatin remodeling. The database is available at http://scbt.sastra.edu/swisnfdb/index.php.

Epigenetic Regulation of Adult Myogenesis

Skeletal muscle regeneration is an efficient stem cell-based repair system that ensures healthy musculature. For this repair system to function continuously throughout life, muscle stem cells must contribute to the process of myofiber repair as well as repopulation of the stem cell niche. The decision made by the muscle stem cells to commit to the muscle repair or to remain a stem cell depends upon patterns of gene expression, a process regulated at the epigenetic level. Indeed, it is well accepted that dynamic changes in epigenetic landscapes to control DNA accessibility and expression is a critical component during myogenesis for the effective repair of damaged muscle. Changes in the epigenetic landscape are governed by various posttranslational histone tail modifications, nucleosome repositioning, and DNA methylation events which collectively allow the control of changes in transcription networks during transitions of satellite cells from a dormant quiescent state toward terminal differentiation. This chapter focuses upon the specific epigenetic changes that occur during muscle stem cell-mediated regeneration to ensure myofiber repair and continuity of the stem cell compartment. Furthermore, we explore open questions in the field that are expected to be important areas of exploration as we move toward a more thorough understanding of the epigenetic mechanism regulating muscle regeneration.

Relating protein functional diversity to cell type number identifies genes that determine dynamic aspects of chromatin organisation as potential contributors to organismal complexity

Organismal complexity broadly relates to the number of different cell types within an organism and generally increases across a phylogeny. Whilst gene expression will underpin organismal complexity, it has long been clear that a simple count of gene number is not a sufficient explanation. In this paper, we use open-access information from the Ensembl databases to quantify the functional diversity of human genes that are broadly involved in transcription. Functional diversity is described in terms of the numbers of paralogues, protein isoforms and structural domains for each gene. The change in functional diversity is then calculated for up to nine orthologues from the nematode worm to human and correlated to the change in cell-type number. Those with the highest correlation are subject to gene ontology term enrichment and interaction analyses. We found that a range of genes that encode proteins associated with dynamic changes to chromatin are good candidates to contribute to organismal complexity.

SMARCB1 is required for widespread BAF complex-mediated activation of enhancers and bivalent promoters

Perturbations to mammalian SWI/SNF (BAF) complexes contribute to over 20% of human cancers, with driving roles first identified in malignant rhabdoid tumor (MRT), an aggressive pediatric cancer characterized by biallelic inactivation of the core BAF complex subunit SMARCB1 (BAF47). However, the mechanism by which this alteration contributes to tumorigenesis remains poorly understood. We find that BAF47 loss destabilizes BAF complexes on chromatin, absent significant changes in intra-complex integrity. Rescue of BAF47 in BAF47-deficient sarcoma cell lines results in increased genome-wide BAF complex occupancy, facilitating widespread enhancer activation and opposition of polycomb-mediated repression at bivalent promoters. We demonstrate differential regulation by BAF and PBAF complexes at enhancers and promoters, respectively, suggesting distinct functions of each complex which are perturbed upon BAF47 loss. Our results demonstrate collaborative mechanisms of mSWI/SNF-mediated gene activation, identifying functions that are coopted or abated to drive human cancers and developmental disorders.

Tumor suppression via inhibition of SWI/SNF complex-dependent NF-κB activation

The transcription factor NF-κB is constitutively activated in many epithelial tumors but few NF-κB inhibitors are suitable for cancer therapy because of its broad biological effects. We previously reported that the d4-family proteins (DPF1, DPF2, DPF3a/b) function as adaptor proteins linking NF-κB with the SWI/SNF complex. Here, using epithelial tumor cell lines, A549 and HeLaS3, we demonstrate that exogenous expression of the highly-conserved N-terminal 84-amino acid region (designated "CT1") of either DPF2 or DPF3a/b has stronger inhibitory effects on anchorage-independent growth than the single knockdown of any d4-family protein. This indicates that CT1 can function as an efficient dominant-negative mutant of the entire d4-family proteins. By in situ proximity ligation assay, CT1 was found to retain full adaptor function, indicating that the C-terminal region of d4-family proteins lacking in CT1 would include essential domains for SWI/SNF-dependent NF-κB activation. Microarray analysis revealed that CT1 suppresses only a portion of the NF-κB target genes, including representative SWI/SNF-dependent genes. Among these genes, IL6 was shown to strongly contribute to anchorage-independent growth. Finally, exogenous CT1 expression efficiently suppressed tumor formation in a mouse xenograft model, suggesting that the d4-family proteins are promising cancer therapy targets.

ATP-dependent chromatin remodeling during mammalian development

Precise gene expression ensures proper stem and progenitor cell differentiation, lineage commitment and organogenesis during mammalian development. ATP-dependent chromatin-remodeling complexes utilize the energy from ATP hydrolysis to reorganize chromatin and, hence, regulate gene expression. These complexes contain diverse subunits that together provide a multitude of functions, from early embryogenesis through cell differentiation and development into various adult tissues. Here, we review the functions of chromatin remodelers and their different subunits during mammalian development. We discuss the mechanisms by which chromatin remodelers function and highlight their specificities during mammalian cell differentiation and organogenesis.

Cellular senescence regulated by SWI/SNF complex subunits through p53/p21 and p16/pRB pathway

SWI/SNF complex is an evolutionarily well-conserved chromatin-remodeling complex, which is implicated in the nucleosomes removing or sliding, impacting on the DNA repair, replication and genes expression regulation. The SWI/SNF complex consists up to 12 protein subunits. The catalytic subunits are BRG1 or BRM, which are exclusive ATPase subunits. BRG1 has been reported to play an important role in cellular senescence. However, The function of non-catalytic subunits involved in cellular senescence is rarely investigated. Therefore, we focused on the senescence regulation roles of SWI/SNF non-catalytic subunits in cellular senescent model induced by H2O2. H2O2 treatment was used to induce cellular senescence models in vitro. Screening the candidate subunits involved in this process by comparing the expression levels of SWI/SNF subunits with/without H2O2 treatment. Over-expression and knockdown the candidate subunits were utilized to investigate the functions and mechanism of the subunits involved in senescence regulation. The expressions of BAF57, BAF60a and SNF5 were changed significantly after H2O2 treatment. Overexpression of the three subunits separately induced cell growth arrest in both HaCaT and GLL19 cells, while knockdown of the subunits separately eased the senescence induced by H2O2 treatment. Results further showed that BAF57, BAF60a and SNF5 regulated cellular senescence via both p53/p21 and p16/pRB pathways, and the three subunits all had a directly interaction with p53. These results indicated that BAF57, BAF60a and SNF5 might act as novel pro-senescence factors in both normal and tumor human skin cells. Therefore, inhibiting expression of the three factors might delay the cellular senescence process.

Chromatin Remodeling BAF (SWI/SNF) Complexes in Neural Development and Disorders

The ATP-dependent BRG1/BRM associated factor (BAF) chromatin remodeling complexes are crucial in regulating gene expression by controlling chromatin dynamics. Over the last decade, it has become increasingly clear that during neural development in mammals, distinct ontogenetic stage-specific BAF complexes derived from combinatorial assembly of their subunits are formed in neural progenitors and post-mitotic neural cells. Proper functioning of the BAF complexes plays critical roles in neural development, including the establishment and maintenance of neural fates and functionality. Indeed, recent human exome sequencing and genome-wide association studies have revealed that mutations in BAF complex subunits are linked to neurodevelopmental disorders such as Coffin-Siris syndrome, Nicolaides-Baraitser syndrome, Kleefstra's syndrome spectrum, Hirschsprung's disease, autism spectrum disorder, and schizophrenia. In this review, we focus on the latest insights into the functions of BAF complexes during neural development and the plausible mechanistic basis of how mutations in known BAF subunits are associated with certain neurodevelopmental disorders.