The MacroH2A1.1 - PARP1 Axis at the Intersection Between Stress Response and Metabolism

Identification of a novel DNA oxidative damage repair pathway, requiring the ubiquitination of the histone variant macroH2A1.1

BACKGROUND

The histone variant macroH2A (mH2A), the most deviant variant, is about threefold larger than the conventional histone H2A and consists of a histone H2A-like domain fused to a large Non-Histone Region responsible for recruiting PARP-1 to chromatin. The available data suggest that the histone variant mH2A participates in the regulation of transcription, maintenance of heterochromatin, NAD+ metabolism, and double-strand DNA repair.

RESULTS

Here, we describe a novel function of mH2A, namely its implication in DNA oxidative damage repair through PARP-1. The depletion of mH2A affected both repair and cell survival after the induction of oxidative lesions in DNA. PARP-1 formed a specific complex with mH2A nucleosomes in vivo. The mH2A nucleosome-associated PARP-1 is inactive. Upon oxidative damage, mH2A is ubiquitinated, PARP-1 is released from the mH2A nucleosomal complex, and is activated. The in vivo-induced ubiquitination of mH2A, in the absence of any oxidative damage, was sufficient for the release of PARP-1. However, no release of PARP-1 was observed upon treatment of the cells with either the DNA alkylating agent MMS or doxorubicin.

CONCLUSIONS

Our data identify a novel pathway for the repair of DNA oxidative lesions, requiring the ubiquitination of mH2A for the release of PARP-1 from chromatin and its activation.

Roles of Histone H2A Variants in Cancer Development, Prognosis, and Treatment

Histones are nuclear proteins essential for packaging genomic DNA and epigenetic gene regulation. Paralogs that can substitute core histones (H2A, H2B, H3, and H4), named histone variants, are constitutively expressed in a replication-independent manner throughout the cell cycle. With specific chaperones, they can be incorporated to chromatin to modify nucleosome stability by modulating interactions with nucleosomal DNA. This allows the regulation of essential fundamental cellular processes for instance, DNA damage repair, chromosomal segregation, and transcriptional regulation. Among all the histone families, histone H2A family has the largest number of histone variants reported to date. Each H2A variant has multiple functions apart from their primary role and some, even be further specialized to perform additional tasks in distinct lineages, such as testis specific shortH2A (sH2A). In the past decades, the discoveries of genetic alterations and mutations in genes encoding H2A variants in cancer had revealed variants' potentiality in driving carcinogenesis. In addition, there is growing evidence that H2A variants may act as novel prognostic indicators or biomarkers for both early cancer detection and therapeutic treatments. Nevertheless, no studies have ever concluded all identified variants in a single report. Here, in this review, we summarize the respective functions for all the 19 mammalian H2A variants and their roles in cancer biology whilst potentiality being used in clinical setting.

Histone and Histone Acetylation-Related Alterations of Gene Expression in Uninvolved Psoriatic Skin and Their Effects on Cell Proliferation, Differentiation, and Immune Responses

Psoriasis is a chronic immune-mediated skin disease in which the symptom-free, uninvolved skin carries alterations in gene expression, serving as a basis for lesion formation. Histones and histone acetylation-related processes are key regulators of gene expression, controlling cell proliferation and immune responses. Dysregulation of these processes is likely to play an important role in the pathogenesis of psoriasis. To gain a complete overview of these potential alterations, we performed a meta-analysis of a psoriatic uninvolved skin dataset containing differentially expressed transcripts from nearly 300 individuals and screened for histones and histone acetylation-related molecules. We identified altered expression of the replication-dependent histones HIST2H2AA3 and HIST2H4A and the replication-independent histones H2AFY, H2AFZ, and H3F3A/B. Eight histone chaperones were also identified. Among the histone acetyltransferases, ELP3 and KAT5 and members of the ATAC, NSL, and SAGA acetyltransferase complexes are affected in uninvolved skin. Histone deacetylation-related alterations were found to affect eight HDACs and members of the NCOR/SMRT, NURD, SIN3, and SHIP HDAC complexes. In this article, we discuss how histone and histone acetylation-related expression changes may affect proliferation and differentiation, as well as innate, macrophage-mediated, and T cell-mediated pro- and anti-inflammatory responses, which are known to play a central role in the development of psoriasis.

Therapeutic Properties of Ayahuasca Components in Ischemia/Reperfusion Injury of the Eye

Ischemic eye diseases are major causes of vision impairment. Thus, potential retinoprotective effects of N'N-dimethyltryptamine (DMT) were investigated. To inhibit its rapid breakdown by monoamine-oxidase A (MAO-A) enzyme, DMT was co-administered with harmaline, a β-carboline in the Amazonian Ayahuasca brew. Using ligation, 60 min of ischemia was provoked in eyes of rats, followed by 7 days of reperfusion whilst animals received harmaline alone, DMT + harmaline, or vehicle treatment. After 1 week of reperfusion, electroretinographical (ERG) measurements, histological analysis, and Western blot were performed. Harmaline alone exhibited retinoprotection in ischemia-reperfusion (I/R) which was, surprisingly, counterbalanced by DMT in case of co-administration. As both MAO-A inhibition and DMT increase serotoninergic tone synergistically, communicated to be anti-ischemic, thus, involvement of other pathways was investigated. Based on our experiments, DMT and harmaline exert opposite effects on important ocular proteins such as PARP1, NFκB, MMP9, or HSP70, each having a critical role in a different mechanism of eye-ischemia-related pathologies, e.g., cell death, inflammation, tissue destruction, and oxidative stress. Since DMT is proclaimed to be a promising drug candidate, its potentially undesirable effect on eye-ischemia should be further investigated. Meanwhile, this experiment revealed the potential therapeutic effect of MAO-A inhibitor harmaline in I/R-related eye diseases.

Evolution, structure and function of divergent macroH2A1 splice isoforms

The replacement of replication-coupled histones with non-canonical histone variants provides chromatin with additional properties and contributes to the plasticity of the epigenome. MacroH2A histone variants are counterparts of the replication-coupled histone H2A. They are characterized by a unique tripartite structure, consisting of a histone fold, an unstructured linker, and a globular macrodomain. MacroH2A1.1 and macroH2A1.2 are the result of alternative splicing of the MACROH2A1 gene and can have opposing biological functions. Here, we discuss the structural differences between the macrodomains of the two isoforms, resulting in differential ligand binding. We further discuss how this modulates gene regulation by the two isoforms, in cases resulting in opposing role of macroH2A1.1 and macroH2A1.2 in development and differentiation. Finally, we share recent insight in the evolution of macroH2As. Taken together, in this review, we aim to discuss in unprecedented detail distinct properties and functions of the fascinating macroH2A1 splice isoforms.

Histone Variant macroH2A1.1 Enhances Nonhomologous End Joining-dependent DNA Double-strand-break Repair and Reprogramming Efficiency of Human iPSCs

DNA damage repair (DDR) is a safeguard for genome integrity maintenance. Increasing DDR efficiency could increase the yield of induced pluripotent stem cells (iPSC) upon reprogramming from somatic cells. The epigenetic mechanisms governing DDR during iPSC reprogramming are not completely understood. Our goal was to evaluate the splicing isoforms of histone variant macroH2A1, macroH2A1.1, and macroH2A1.2, as potential regulators of DDR during iPSC reprogramming. GFP-Trap one-step isolation of mtagGFP-macroH2A1.1 or mtagGFP-macroH2A1.2 fusion proteins from overexpressing human cell lines, followed by liquid chromatography-tandem mass spectrometry analysis, uncovered macroH2A1.1 exclusive interaction with Poly-ADP Ribose Polymerase 1 (PARP1) and X-ray cross-complementing protein 1 (XRCC1). MacroH2A1.1 overexpression in U2OS-GFP reporter cells enhanced specifically nonhomologous end joining (NHEJ) repair pathway, while macroH2A1.1 knock-out (KO) mice showed an impaired DDR capacity. The exclusive interaction of macroH2A1.1, but not macroH2A1.2, with PARP1/XRCC1, was confirmed in human umbilical vein endothelial cells (HUVEC) undergoing reprogramming into iPSC through episomal vectors. In HUVEC, macroH2A1.1 overexpression activated transcriptional programs that enhanced DDR and reprogramming. Consistently, macroH2A1.1 but not macroH2A1.2 overexpression improved iPSC reprogramming. We propose the macroH2A1 splicing isoform macroH2A1.1 as a promising epigenetic target to improve iPSC genome stability and therapeutic potential.

Phosphorylation within Intrinsic Disordered Region Discriminates Histone Variant macroH2A1 Splicing Isoforms-macroH2A1.1 and macroH2A1.2

Simple Summary

MacroH2A1, a histone H2A variant, is present as two alternative splicing isoforms, macroH2A1.1 and macroH2A1.2, which are finely regulated through several mechanisms, including post-translational modifications (PTM). In this article, the authors provide the PTM pattern of macroH2A1.1 and macroH2A1.2 in the same experimental setting through mass spec analysis. They report a different phosphorylation level in their intrinsically disordered linker region, which can be responsible for their different biological role, as computational analysis shows.

Abstract

Background: Gene expression in eukaryotic cells can be governed by histone variants, which replace replication-coupled histones, conferring unique chromatin properties. MacroH2A1 is a histone H2A variant containing a domain highly similar to H2A and a large non-histone (macro) domain. MacroH2A1, in turn, is present in two alternatively exon-spliced isoforms: macroH2A1.1 and macroH2A1.2, which regulate cell plasticity and proliferation in a remarkably distinct manner. The N-terminal and the C-terminal tails of H2A histones stem from the nucleosome core structure and can be target sites for several post-translational modifications (PTMs). MacroH2A1.1 and macroH2A1.2 isoforms differ only in a few amino acids and their ability to bind NAD-derived metabolites, a property allegedly conferring their different functions in vivo. Some of the modifications on the macroH2A1 variant have been identified, such as phosphorylation (T129, S138) and methylation (K18, K123, K239). However, no study to our knowledge has analyzed extensively, and in parallel, the PTM pattern of macroH2A1.1 and macroH2A1.2 in the same experimental setting, which could facilitate the understanding of their distinct biological functions in health and disease. Methods: We used a mass spectrometry-based approach to identify the sites for phosphorylation, acetylation, and methylation in green fluorescent protein (GFP)-tagged macroH2A1.1 and macroH2A1.2 expressed in human hepatoma cells. The impact of selected PTMs on macroH2A1.1 and macroH2A1.2 structure and function are demonstrated using computational analyses. Results: We identified K7 as a new acetylation site in both macroH2A1 isoforms. Quantitative comparison of histone marks between the two isoforms revealed significant differences in the levels of phosphorylated T129 and S170. Our computational analysis provided evidence that the phosphorylation status in the intrinsically disordered linker region in macroH2A1 isoforms might represent a key regulatory element contributing to their distinct biological responses. Conclusions: Taken together, our results report different PTMs on the two macroH2A1 splicing isoforms as responsible for their distinct features and distribution in the cell.

Genomic instability and metabolism in cancer

Genomic instability and metabolic reprogramming are among the key hallmarks discriminating cancer cells from normal cells. The two phenomena contribute to the robust and evasive nature of cancer, particularly when cancer cells are exposed to chemotherapeutic agents. Genomic instability is defined as the increased frequency of mutations within the genome, while metabolic reprogramming is the alteration of metabolic pathways that cancer cells undergo to adapt to increased bioenergetic demand. An underlying source of these mutations is the aggregate product of damage to the DNA, and a defective repair pathway, both resulting in the expansion of genomic lesions prior to uncontrolled proliferation and survival of cancer cells. Exploitation of DNA damage and the subsequent DNA damage response (DDR) have aided in defining therapeutic approaches in cancer. Studies have demonstrated that targeting metabolic reprograming yields increased sensitivity to chemo- and radiotherapies. In the past decade, it has been shown that these two key features are interrelated. Metabolism impacts DNA damage and DDR via regulation of metabolite pools. Conversely, DDR affects the response of metabolic pathways to therapeutic agents. Because of the interplay between genomic instability and metabolic reprogramming, we have compiled findings which more selectively highlight the dialog between metabolism and DDR, with a particular focus on glucose metabolism and double-strand break (DSB) repair pathways. Decoding this dialog will provide significant clues for developing combination cancer therapies.

Skeletal Aging and Osteoporosis: Mechanisms and Therapeutics

Bone is a dynamic organ maintained by tightly regulated mechanisms. With old age, bone homeostasis, which is maintained by an intricate balance between bone formation and bone resorption, undergoes deregulation. Oxidative stress-induced DNA damage, cellular apoptosis, and cellular senescence are all responsible for this tissue dysfunction and the imbalance in the bone homeostasis. These cellular mechanisms have become a target for therapeutics to treat age-related osteoporosis. Genetic mouse models have shown the importance of senescent cell clearance in alleviating age-related osteoporosis. Furthermore, we and others have shown that targeting cellular senescence pharmacologically was an effective tool to alleviate age- and radiation-induced osteoporosis. Senescent cells also have an altered secretome known as the senescence associated secretory phenotype (SASP), which may have autocrine, paracrine, or endocrine function. The current review discusses the current and potential pathways which lead to a senescence profile in an aged skeleton and how bone homeostasis is affected during age-related osteoporosis. The review has also discussed existing therapeutics for the treatment of osteoporosis and rationalizes for novel therapeutic options based on cellular senescence and the SASP as an underlying pathogenesis of an aging bone.

Redox and Epigenetics in Human Pluripotent Stem Cells Differentiation

Significance: Since their discovery, induced pluripotent stem cells (iPSCs) had generated considerable interest in the scientific community for their great potential in regenerative medicine, disease modeling, and cell-based therapeutic approach, due to their unique characteristics of self-renewal and pluripotency. Recent Advances: Technological advances in iPSC genome-wide epigenetic profiling led to the elucidation of the epigenetic control of cellular identity during nuclear reprogramming. Moreover, iPSC physiology and metabolism are tightly regulated by oxidation-reduction events that mainly occur during the respiratory chain. In theory, iPSC-derived differentiated cells would be ideal for stem cell transplantation as autologous cells from donors, as the risks of rejection are minimal. Critical Issues: However, iPSCs experience high oxidative stress that, in turn, confers a high risk of increased genomic instability, which is most often linked to DNA repair deficiencies. Genomic instability has to be assessed before iPSCs can be used in therapeutic designs. Future Directions: This review will particularly focus on the links between redox balance and epigenetic modifications-in particular based on the histone variant macroH2A1-that determine DNA damage response in iPSCs and derived differentiated cells, and that might be exploited to decrease the teratogenic potential on iPSC transplantation. Antioxid. Redox Signal. 34, 335-349.

Circulating histone signature of human lean metabolic-associated fatty liver disease (MAFLD)

Background

Although metabolic associate fatty liver disease (MAFLD) is associated with obesity, it can also occur in lean patients. MAFLD is more aggressive in lean patients compared to obese patients, with a higher risk of mortality. Specific biomarkers to diagnose differentially lean or overweight MAFLD are missing. Histones and nucleosomes are released in the bloodstream upon cell death. Here, we propose a new, fast, imaging and epigenetics based approach to investigate the severity of steatosis in lean MAFLD patients.

Results

A total of 53 non-obese patients with histologically confirmed diagnosis of MAFLD were recruited. Twenty patients displayed steatosis grade 1 (0–33%), 24 patients with steatosis grade 2 (34–66%) and 9 patients with steatosis grade 3 (67–100%). The levels of circulating nucleosomes were assayed using enzyme-linked immunosorbent assay, while individual histones or histone dimers were assayed in serum samples by means of a new advanced flow cytometry ImageStream(X)-adapted method. Circulating nucleosome levels associated poorly with MAFLD in the absence of obesity. We implemented successfully a multi-channel flow methodology on ImageStream(X), to image single histone staining (H2A, H2B, H3, H4, macroH2A1.1 and macroH2A1.2). We report here a significant depletion of the levels of histone variants macroH2A1.1 and macroH2A1.2 in the serum of lean MAFLD patients, either individually or in complex with H2B.

Conclusions

In summary, we identified a new circulating histone signature able to discriminate the severity of steatosis in individuals with lean MAFLD, using a rapid and non-invasive ImageStream(X)-based imaging technology.

The taming of PARP1 and its impact on NAD+ metabolism

Background

Poly-ADP-ribose polymerases (PARPs) are key mediators of cellular stress response. They are intimately linked to cellular metabolism through the consumption of NAD⁺. PARP1/ARTD1 in the nucleus is the major NAD⁺ consuming activity and plays a key role in maintaining genomic integrity.

Scope of review

In this review, we discuss how different organelles are linked through NAD⁺ metabolism and how PARP1 activation in the nucleus can impact the function of distant organelles. We discuss how differentiated cells tame PARP1 function by upregulating an endogenous inhibitor, the histone variant macroH2A1.1.

Major conclusions

The presence of macroH2A1.1, particularly in differentiated cells, raises the threshold for the activation of PARP1 with consequences for DNA repair, gene transcription, and NAD⁺ homeostasis.

•

Beyond DNA repair, PARP1 is essential for metabolic homeostasis.

•

Epigenetic mechanisms prevent metabolic disorders through PARP1 taming.

•

Beyond cancer, the development of PARP1 inhibitors offers diverse clinical potential.

Role of PARP-catalyzed ADP-ribosylation in the Crosstalk Between DNA Strand Breaks and Epigenetic Regulation

Covalent linkage of ADP-ribose units to proteins catalyzed by poly(ADP-ribose) polymerases (PARPs) plays important signaling functions in a plethora of cellular processes including DNA damage response, chromatin organization, and gene transcription. Poly- and mono-ADP-ribosylation of target macromolecules are often responsible both for the initiation and for coordination of these processes in mammalian cells. Currently, the number of cellular targets for ADP-ribosylation is rapidly expanding, and the molecular mechanisms underlying the broad substrate specificity of PARPs present enormous interest. In this review, the roles of PARP-mediated modifications of protein and nucleic acids, the readers of ADP-ribosylated structures, and the origin and function of programmed DNA strand breaks in PARP activation, transcription regulation, and DNA demethylation are discussed.

Anticancer innovative therapy: Highlights from the ninth annual meeting

The Ninth Annual Conference of "Anticancer Innovative Therapy", organized by Fondazione IRCCS Istituto Nazionale dei Tumori di Milano (Fondazione IRCCS INT) and hosted by Hotel Michelangelo, was held in Milan on 25 January 2019. Cutting-edge science was presented in two main scientific sessions: i) pre-clinical evidences and new targets, and ii) clinical translation. The Keynote lecture entitled "Cancer stem cells (CSCs): metabolic strategies for their identification and eradication" presented by M. Lisanti, was one of the highlights of the conference. One key concept of the meeting was how the continuous advances in our knowledge about molecular mechanisms in various fields of research (cancer metabolism reprogramming, epigenetic regulation, transformation/invasiveness, and immunology, among others) are driving cancer research towards more effective personalized antineoplastic strategies. Specifically, recent preclinical data on the following topics were discussed: 1. Polycomb group proteins in cancer; 2. A d16HER2 splice variant is a flag of HER2 addiction across HER2-positive cancers; 3. Studying chromatin as a nexus between translational and basic research; 4. Metabolomic analysis in cancer patients; 5. CDK4-6 cyclin inhibitors: clinical activity and future perspectives as immunotherapy adjuvant; and 6. Cancer stem cells (CSCs): metabolic strategies for their identification and eradication. In terms of clinical translation, several novel approaches were presented: 1. Developing CAR-T cell therapies: an update of preclinical and clinical development at University of North Carolina; 2. Vγ9Vδ2 T-cell activation and immune suppression in multiple myeloma; 3. Predictive biomarkers for real-world immunotherapy: the cancer immunogram model in the clinical arena; and 4. Mechanisms of resistance to immune checkpoint blockade in solid tumors. Overall, the pre-clinical and clinical findings presented could pave the way to identify novel actionable therapeutic targets to significantly enhance the care of persons with cancer.

Deficiency and haploinsufficiency of histone macroH2A1.1 in mice recapitulate hematopoietic defects of human myelodysplastic syndrome

Background

Epigenetic regulation is important in hematopoiesis, but the involvement of histone variants is poorly understood. Myelodysplastic syndromes (MDS) are heterogeneous clonal hematopoietic stem cell (HSC) disorders characterized by ineffective hematopoiesis. MacroH2A1.1 is a histone H2A variant that negatively correlates with the self-renewal capacity of embryonic, adult, and cancer stem cells. MacroH2A1.1 is a target of the frequent U2AF1 S34F mutation in MDS. The role of macroH2A1.1 in hematopoiesis is unclear.

Results

MacroH2A1.1 mRNA levels are significantly decreased in patients with low-risk MDS presenting with chromosomal 5q deletion and myeloid cytopenias and tend to be decreased in MDS patients carrying the U2AF1 S34F mutation. Using an innovative mouse allele lacking the macroH2A1.1 alternatively spliced exon, we investigated whether macroH2A1.1 regulates HSC homeostasis and differentiation. The lack of macroH2A1.1 decreased while macroH2A1.1 haploinsufficiency increased HSC frequency upon irradiation. Moreover, bone marrow transplantation experiments showed that both deficiency and haploinsufficiency of macroH2A1.1 resulted in enhanced HSC differentiation along the myeloid lineage. Finally, RNA-sequencing analysis implicated macroH2A1.1-mediated regulation of ribosomal gene expression in HSC homeostasis.

Conclusions

Together, our findings suggest a new epigenetic process contributing to hematopoiesis regulation. By combining clinical data with a discrete mutant mouse model and in vitro studies of human and mouse cells, we identify macroH2A1.1 as a key player in the cellular and molecular features of MDS. These data justify the exploration of macroH2A1.1 and associated proteins as therapeutic targets in hematological malignancies.

Electronic supplementary material

The online version of this article (10.1186/s13148-019-0724-z) contains supplementary material, which is available to authorized users.

Histone variant MacroH2A1 is downregulated in prostate cancer and influences malignant cell phenotype

BACKGROUND

Prostate cancer (PCa), a major cause of cancer-related morbidity and mortality worldwide and mostly asymptomatic at earliest stages, is characterized by disruption of genetic and epigenetic balance. A better understanding of how those mechanisms orchestrate disease might improve diagnostic and prognostic tools, allowing for improvements in treatment efficacy. Replacement of canonical histones, an epigenetic mechanism, is highly conserved among species and altered expression of histones variants (e.g., MacroH2A1) has been associated with tumorigenesis. H2AFY gene encodes two isoforms of H2A histone variant MacroH2A1: MacroH2A1.1 and MacroH2A1.2. Specifically, MacroH2A1.1 isoform inhibits cell proliferation and promotes cellular differentiation. Because the contribution of this histone variant to carcinogenesis has been reported in several cancer types, but not for PCa, we aimed to investigate the contribution of MacroH2A1 for prostate carcinogenesis.

METHODS

MacroH2A1, MacroH2A1.1 and MacroH2A1.2 isoforms and the corresponding splicing regulators transcript levels were evaluated by RT-qPCR, in a tissue cohort composed by PCa, prostatic intraepithelial neoplasia (PIN) and normal prostate cases. Knockdown for MacroH2A1 and MacroH2A1.1 was performed through lentiviral transduction in DU145 cells, and MacroH2A1.1 overexpression was achieved in LNCaP cells by plasmid transfection, followed by functional assays. Biological and/or experimental replicates were performed when necessary, and specific statistical tests were applied to perform data analysis.

RESULTS

MacroH2A1.1 transcript levels were downregulated in PIN and primary PCa compared to normal prostate tissues. The same was found for QKI, a MacroH2A1.1's splicing regulator. Moreover, lower MacroH2A1.1 and QKI expression levels associated with less differentiated tumors (Gleason score ≥ 7). Interestingly, MacroH2A1.1, but more impressively DDX17 (AUC = 0.93; p < 0.0001) and QKI (AUC = 0.94; p < 0.0001), accurately discriminated cancerous from noncancerous prostate tissues. Furthermore, in PCa cell lines, total MacroH2A1 knockdown augmented malignant features, whereas MacroH2A1.1 overexpression impressively attenuated the malignant phenotype.

CONCLUSIONS

Overall, our data, derived from primary PCa tissues and cell lines, anticipate a tumor suppressive role for MacroH2A1, particularly for the MacroH2A1.1 isoform, in prostate carcinogenesis.

Histone variant macroH2A: from chromatin deposition to molecular function

The eukaryotic genome is regulated in the context of chromatin. Specialized histones, known as histone variants, incorporate into chromatin to replace their canonical counterparts and represent an important layer of regulation to diversify the structural characteristics and functional outputs of chromatin. MacroH2A is an unusual histone variant with a bulky C-terminal non-histone domain that distinguishes it from all other histones. It is a critical player in stabilizing differentiated cell identity by posing as a barrier to somatic cell reprogramming toward pluripotency and acts as a tumor suppressor in a wide range of cancers. MacroH2A histones are generally regarded as repressive variants that are enriched at the inactive X chromosome (Xi) and broad domains across autosomal chromatin. Recent studies have shed light on to how macroH2A influences transcriptional outputs within distinct genomic contexts and revealed new intriguing molecular functions of macroH2A variants beyond transcriptional regulation. Furthermore, the mechanisms of its mysterious chromatin deposition are beginning to be unraveled, facilitating our understanding of its complex regulation of genome function.