The intriguing complexities of mammalian gene regulation: how to link enhancers to regulated genes. Are we there yet?

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

The noncoding genome and hearing loss

The age of sequencing has provided unprecedented insights into the human genome. The coding region of the genome comprises nearly 20,000 genes, of which approximately 4000 are associated with human disease. Beyond the protein-coding genome, which accounts for only 3% of the genome, lies a vast pool of regulatory elements in the form of promoters, enhancers, RNA species, and other intricate elements. These features undoubtably influence human health and disease, and as a result, a great deal of effort is currently being invested in deciphering their identity and mechanism. While a paucity of material has caused a lag in identifying these elements in the inner ear, the emergence of technologies for dealing with a minimal number of cells now has the field working overtime to catch up. Studies on microRNAs (miRNAs), long non-coding RNAs (lncRNAs), methylation, histone modifications, and more are ongoing. A number of microRNAs and other noncoding elements are known to be associated with hearing impairment and there is promise that regulatory elements will serve as future tools and targets of therapeutics and diagnostics. This review covers the current state of the field and considers future directions for the noncoding genome and implications for hearing loss.

Transcriptional repression shapes the identity and function of tissue macrophages

The changing extra‐ and intracellular microenvironment calls for rapid cell fate decisions that are precisely and primarily regulated at the transcriptional level. The cellular components of the immune system are excellent examples of how cells respond and adapt to different environmental stimuli. Innate immune cells such as macrophages are able to modulate their transcriptional programs and epigenetic regulatory networks through activation and repression of particular genes, allowing them to quickly respond to a rapidly changing environment. Tissue macrophages are essential components of different immune‐ and nonimmune cell‐mediated physiological mechanisms in mammals and are widely used models for investigating transcriptional regulatory mechanisms. Therefore, it is critical to unravel the distinct sets of transcription activators, repressors, and coregulators that play roles in determining tissue macrophage identity and functions during homeostasis, as well as in diseases affecting large human populations, such as metabolic syndromes, immune‐deficiencies, and tumor development. In this review, we will focus on transcriptional repressors that play roles in tissue macrophage development and function under physiological conditions.

Genotype-Phenotype Relationships in the Context of Transcriptional Adaptation and Genetic Robustness

Genetic manipulations with a robust and predictable outcome are critical to investigate gene function, as well as for therapeutic genome engineering. For many years, knockdown approaches and reagents including RNA interference and antisense oligonucleotides dominated functional studies; however, with the advent of precise genome editing technologies, CRISPR-based knockout systems have become the state-of-the-art tools for such studies. These technologies have helped decipher the role of thousands of genes in development and disease. Their use has also revealed how limited our understanding of genotype-phenotype relationships is. The recent discovery that certain mutations can trigger the transcriptional modulation of other genes, a phenomenon called transcriptional adaptation, has provided an additional explanation for the contradicting phenotypes observed in knockdown versus knockout models and increased awareness about the use of each of these approaches. In this review, we first cover the strengths and limitations of different gene perturbation strategies. Then we highlight the diverse ways in which the genotype-phenotype relationship can be discordant between these different strategies. Finally, we review the genetic robustness mechanisms that can lead to such discrepancies, paying special attention to the recently discovered phenomenon of transcriptional adaptation. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Does Developmental Variability in the Number of Midbrain Dopamine Neurons Affect Individual Risk for Sporadic Parkinson's Disease?

Parkinson’s disease (PD) is a slowly progressing neurodegenerative disorder that is coupled to both widespread protein aggregation and to loss of substantia nigra dopamine (DA) neurons, resulting in a wide variety of motor and non-motor signs and symptoms. Recent findings suggest that the PD process is triggered several years before there is sufficient degeneration of DA neurons to cause onset of overt motor symptoms. According to this concept, the number of DA neurons present in the substantia nigra at birth could influence the time from the molecular triggering event until the clinical diagnosis with lower number of neurons at birth increasing the risk to develop the disease. Conversely, the risk for diagnosis would be reduced if the number of DA neurons is high at birth. In this commentary, we discuss the genetic and epigenetic factors that might influence the number of nigral DA neurons that each individual is born with and how these may be linked to PD risk.

Brd4-bound enhancers drive cell-intrinsic sex differences in glioblastoma

Consistent sex differences in incidence and outcome have been reported in numerous cancers including brain tumors. GBM, the most common and aggressive primary brain tumor, occurs with higher incidence and shorter survival in males compared to females. Brd4 is essential for regulating transcriptome-wide gene expression and specifying cell identity, including that of GBM. We report that sex-biased Brd4 activity drives sex differences in GBM and renders male and female tumor cells differentially sensitive to BET inhibitors. The observed sex differences in BETi treatment strongly indicate that sex differences in disease biology translate into sex differences in therapeutic responses. This has critical implications for clinical use of BET inhibitors further affirming the importance of inclusion of sex as a biological variable.

Sex can be an important determinant of cancer phenotype, and exploring sex-biased tumor biology holds promise for identifying novel therapeutic targets and new approaches to cancer treatment. In an established isogenic murine model of glioblastoma (GBM), we discovered correlated transcriptome-wide sex differences in gene expression, H3K27ac marks, large Brd4-bound enhancer usage, and Brd4 localization to Myc and p53 genomic binding sites. These sex-biased gene expression patterns were also evident in human glioblastoma stem cells (GSCs). These observations led us to hypothesize that Brd4-bound enhancers might underlie sex differences in stem cell function and tumorigenicity in GBM. We found that male and female GBM cells exhibited sex-specific responses to pharmacological or genetic inhibition of Brd4. Brd4 knockdown or pharmacologic inhibition decreased male GBM cell clonogenicity and in vivo tumorigenesis while increasing both in female GBM cells. These results were validated in male and female patient-derived GBM cell lines. Furthermore, analysis of the Cancer Therapeutic Response Portal of human GBM samples segregated by sex revealed that male GBM cells are significantly more sensitive to BET (bromodomain and extraterminal) inhibitors than are female cells. Thus, Brd4 activity is revealed to drive sex differences in stem cell and tumorigenic phenotypes, which can be abrogated by sex-specific responses to BET inhibition. This has important implications for the clinical evaluation and use of BET inhibitors.

Optimisation of Tet-On inducible systems for Sleeping Beauty-based chimeric antigen receptor (CAR) applications

Regulated expression of genetic elements that either encode polypeptides or various types of functional RNA is a fundamental goal for gene therapy. Inducible expression may be preferred over constitutive promoters to allow clinician-based control of gene expression. Existing Tet-On systems represent one of the tightest rheostats for control of gene expression in mammals. However, basal expression in absence of tetracycline compromises the widespread application of Tet-controlled systems in gene therapy. We demonstrate that the order of P2A-linked genes of interest was critical for maximal response and tightness of a chimeric antigen receptor (CAR)-based construct. The introduction of G72V mutation in the activation region of the TetR component of the rtTA further improved the fold response. Although the G72V mutation resulted in a removal of a cryptic splice site within rtTA, additional removal of this splice site led to only a modest improvement in the fold-response. Selective removal of key promoter elements (namely the BRE, TATA box, DPE and the four predicted Inr) confirmed the suitability of the minimal CMV promoter and its downstream sequences for supporting inducible expression. The results demonstrate marked improvement of the rtTA based Tet-On system in Sleeping Beauty for applications such as CAR T cell therapy.

Enhancer Dysfunction in 3D Genome and Disease

Spatiotemporal patterns of gene expression depend on enhancer elements and other factors during individual development and disease progression. The rapid progress of high-throughput techniques has led to well-defined enhancer chromatin properties. Various genome-wide methods have revealed a large number of enhancers and the discovery of three-dimensional (3D) genome architecture showing the distant interacting mechanisms of enhancers that loop to target gene promoters. Whole genome sequencing projects directed at cancer have led to the discovery of substantial enhancer dysfunction in misregulating gene expression and in tumor initiation and progression. Results from genome-wide association studies (GWAS) combined with functional genomics analyses have elucidated the functional impacts of many cancer risk-associated variants that are enriched within the enhancer regions of chromatin. Risk variants dysregulate the expression of enhancer variant-associated genes via 3D genomic interactions. Moreover, these enhancer variants often alter the chromatin binding affinity for cancer-relevant transcription factors, which in turn leads to aberrant expression of the genes associated with cancer susceptibility. In this review, we investigate the extent to which these genetic regulatory circuits affect cancer predisposition and how the recent development of genome-editing methods have enabled the determination of the impacts of genomic variation and alteration on cancer phenotype, which will eventually lead to better management plans and treatment responses to human cancer in the clinic.

Identification of human silencers by correlating cross-tissue epigenetic profiles and gene expression

Compared to enhancers, silencers are notably difficult to identify and validate experimentally. In search for human silencers, we utilized H3K27me3-DNase I hypersensitive site (DHS) peaks with tissue specificity negatively correlated with the expression of nearby genes across 25 diverse cell lines. These regions are predicted to be silencers since they are physically linked, using Hi-C loops, or associated, using expression quantitative trait loci (eQTL) results, with a decrease in gene expression much more frequently than general H3K27me3-DHSs. Also, these regions are enriched for the binding sites of transcriptional repressors (such as CTCF, MECOM, SMAD4, and SNAI3) and depleted of the binding sites of transcriptional activators. Using sequence signatures of these regions, we constructed a computational model and predicted approximately 10,000 additional silencers per cell line and demonstrated that the majority of genes linked to these silencers are expressed at a decreased level. Furthermore, single nucleotide polymorphisms (SNPs) in predicted silencers are significantly associated with disease phenotypes. Finally, our results show that silencers commonly interact with enhancers to affect the transcriptional dynamics of tissue-specific genes and to facilitate fine-tuning of transcription in the human genome.

Networks of mRNA Processing and Alternative Splicing Regulation in Health and Disease

mRNA processing events introduce an intricate layer of complexity into gene expression processes, supporting a tremendous level of diversification of the genome's coding and regulatory potential, particularly in vertebrate species. The recent development of massive parallel sequencing methods and their adaptation to the identification and quantification of different RNA species and the dynamics of mRNA metabolism and processing has generated an unprecedented view over the regulatory networks that are established at this level, which contribute to sustain developmental, tissue specific or disease specific gene expression programs. In this chapter, we provide an overview of the recent evolution of transcriptome profiling methods and the surprising insights that have emerged in recent years regarding distinct mRNA processing events - from the 5' end to the 3' end of the molecule.

Extensive and functional overlap of the STAT6 and RXR cistromes in the active enhancer repertoire of human CD14+ monocyte derived differentiating macrophages

Macrophages are able to differentiate into classically polarized (M1) or alternatively polarized (M2) states upon encountering pro-inflammatory cytokines such as interferon (IFN) γ or anti-inflammatory cytokines such as interleukin (IL) -4/IL-13, respectively. Moreover, macrophages are known to regulate lipid metabolism via multiple members of the nuclear hormone receptor family, including the retinoid X receptors (RXR). It has been also documented that cytokines are able to modulate macrophage responses to lipid signals but the nature of these interactions and the underlying mechanisms of these processes especially at the level of the chromatinized genome are not well understood. Previous work from our laboratory suggested that STAT6 is a facilitator of nuclear receptor mediated transcriptional activity acting at the genome level. This prompted us to investigate genome-wide DNA binding events and the development of cistromes in human CD14+ monocyte-derived macrophages upon exposure to IL-4. We determined the impact of IL-4 on the PU.1, RXR and STAT6 cistromes within the active enhancer regions marked by H3K27-acetylation using chromatin immunoprecipitation followed by deep sequencing and integrated bioinformatics analyses. We found that about 2/3rd of the IL-4 induced STAT6 peaks co-localized with RXR peaks. These STAT6/RXR co-peaks differed at least in part from the non-overlapping RXR peaks regarding the most enriched de novo transcription factor binding motifs. Interestingly, RXR-binding was not regulated at the STAT6/RXR co-bound enhancers following IL-4 stimulation, but differential enhancer interactions were observed between the IL-4/STAT6 and RXR signaling pathways acting in a gene selective manner. Our results suggest that there is a novel, so far uncharacterized cistromic crosstalk between RXR and STAT6 that is likely to contribute to the formation of the active enhancer repertoire, transcriptome and differential signal-specific gene regulation of polarized macrophages.

Deletion of a Long-Range Dlx5 Enhancer Disrupts Inner Ear Development in Mice

Distal enhancers are thought to play important roles in the spatiotemporal regulation of gene expression during embryonic development, but few predicted enhancer elements have been shown to affect transcription of their endogenous genes or to alter phenotypes when disrupted. Here, we demonstrate that a 123.6-kb deletion within the mouse Slc25a13 gene is associated with reduced transcription of Dlx5, a gene located 660 kb away. Mice homozygous for the Slc25a13 deletion mutation [named hyperspin (hspn)] have malformed inner ears and are deaf with balance defects, whereas previously reported Slc25a13 knockout mice showed no phenotypic abnormalities. Inner ears of Slc25a13^hspn/hspn mice have malformations similar to those of Dlx5^-/- embryos, and Dlx5 expression is severely reduced in the otocyst but not the branchial arches of Slc25a13^hspn/hspn embryos, indicating that the Slc25a13^hspn deletion affects otic-specific enhancers of Dlx5 In addition, transheterozygous Slc25a13^+/hspn Dlx5^+/- mice exhibit noncomplementation with inner ear dysmorphologies similar to those of Slc25a13^hspn/hspn and Dlx5^-/-embryos, verifying a cis-acting effect of the Slc25a13^hspn deletion on Dlx5 expression. CRISPR/Cas9-mediated deletions of putative enhancer elements located within the Slc25a13^hspn deleted region failed to phenocopy the defects of Slc25a13^hspn/hspn mice, suggesting the possibility of multiple enhancers with redundant functions. Our findings in mice suggest that analogous enhancer elements in the human SLC25A13 gene may regulate DLX5 expression and underlie the hearing loss that is associated with split-hand/-foot malformation 1 syndrome. Slc25a13^hspn/hspn mice provide a new animal model for studying long-range enhancer effects on Dlx5 expression in the developing inner ear.

FOXC1 modulates MYOC secretion through regulation of the exocytic proteins RAB3GAP1, RAB3GAP2 and SNAP25

The neurodegenerative disease glaucoma is one of the leading causes of blindness in the world. Glaucoma is characterized by progressive visual field loss caused by retinal ganglion cell (RGC) death. Both surgical glaucoma treatments and medications are available, however, they only halt glaucoma progression and are unable to reverse damage. Furthermore, many patients do not respond well to treatments. It is therefore important to better understand the mechanisms involved in glaucoma pathogenesis. Patients with Axenfeld-Rieger syndrome (ARS) offer important insight into glaucoma progression. ARS patients are at 50% risk of developing early onset glaucoma and respond poorly to treatments, even when surgical treatments are combined with medications. Mutations in the transcription factor FOXC1 cause ARS. Alterations in FOXC1 levels cause ocular malformations and disrupt stress response in ocular tissues, thereby contributing to glaucoma progression. In this study, using biochemical and molecular techniques, we show that FOXC1 regulates the expression of RAB3GAP1, RAB3GAP2 and SNAP25, three genes with central roles in both exocytosis and endocytosis, responsible for extracellular trafficking. FOXC1 positively regulates RAB3GAP1 and RAB3GAP2, while either increase or decrease in FOXC1 levels beyond its normal range results in decreased SNAP25. In addition, we found that FOXC1 regulation of RAB3GAP1, RAB3GAP2 and SNAP25 affects secretion of Myocilin (MYOC), a protein associated with juvenile onset glaucoma and steroid-induced glaucoma. The present work reveals that FOXC1 is an important regulator of exocytosis and establishes a new link between FOXC1 and MYOC-associated glaucoma.

Investigation of a possible extended risk haplotype in the IL23R region associated with ankylosing spondylitis

The IL23R region on chromosome 1 exhibits complex associations with ankylosing spondylitis (AS). We used publicly available epigenomic information and historical genetic association data to identify a putative regulatory element (PRE) in the intergenic region between IL23R and IL12RB2, which includes two single-nucleotide polymorphisms (SNPs) independently associated with AS—rs924080 (P=2 × 10⁻³) and rs11578380 (P=2 × 10⁻⁴). In luciferase reporter assays, this PRE showed silencer activity (P<0.001). Haplotype and conditional analysis of 4230 historical AS cases and 9700 controls revealed a possible AS-associated extended haplotype, including the PRE and risk variants at three SNPs (rs11209026, rs11209032 and rs924080), but excluding the rs11578380 risk variant. However, the rs924080 association was absent after conditioning on the primary association with rs11209032, which, in contrast, was robust to conditioning on all other AS-associated SNPs in this region (P<2 × 10⁻⁸). The role of this putative silencer on some IL23R extended haplotypes therefore remains unclear.

PPARs in the central nervous system: roles in neurodegeneration and neuroinflammation

Over 25 years have passed since peroxisome proliferators-activated receptors (PPARs), were first described. Like other members of the nuclear receptors superfamily, PPARs have been defined as critical sensors and master regulators of cellular metabolism. Recognized as ligand-activated transcription factors, they are involved in lipid, glucose and amino acid metabolism, taking part in different cellular processes, including cellular differentiation and apoptosis, inflammatory modulation and attenuation of acute and chronic neurological damage in vivo and in vitro. Interestingly, PPAR activation can simultaneously reprogram the immune response, stimulate metabolic and mitochondrial functions, promote axonal growth, induce progenitor cells to differentiate into myelinating oligodendrocytes, and improve brain clearance of toxic molecules such as β-amyloid peptide. Although the molecular mechanisms and cross-talk with different molecular pathways are still the focus of intense research, PPARs are considered potential therapeutic targets for several neuropathological conditions, including degenerative disorders such as Alzheimer's, Parkinson's and Huntington's disease. This review considers recent advances regarding PPARs, as well as new PPAR agonists. We focus on the mechanisms behind the neuroprotective effects exerted by PPARs and summarise the roles of PPARs in different pathologies of the central nervous system, especially those associated with degenerative and inflammatory mechanisms.