The Chromatin Regulator BRPF3 Preferentially Activates the HBO1 Acetyltransferase but Is Dispensable for Mouse Development and Survival

HBO1, a MYSTerious KAT and its links to cancer

The histone acetyltransferase HBO1, also known as KAT7, is a major chromatin modifying enzyme responsible for H3 and H4 acetylation. It is found within two distinct tetrameric complexes, the JADE subunit-containing complex and BRPF subunit-containing complex. The HBO1-JADE complex acetylates lysine 5, 8 and 12 of histone H4, and the HBO1-BRPF complex acetylates lysine 14 of histone H3. HBO1 regulates gene transcription, DNA replication, DNA damage repair, and centromere function. It is involved in diverse signaling pathways and plays crucial roles in development and stem cell biology. Recent work has established a strong relationship of HBO1 with the histone methyltransferase MLL/KMT2A in acute myeloid leukemia. Here, we discuss functional and pathological links of HBO1 to cancer, highlighting the underlying mechanisms that may pave the way to the development of novel anti-cancer therapies.

Clinical features and underlying mechanisms of KAT6B disease in a Chinese boy

BACKGROUND

Lysine acetyltransferase 6B (KAT6B) encodes a highly conserved histone acetyltransferase that regulates the expression of multiple genes and is essential for human growth and development.

METHODS

We identified a novel frameshift variant c.3185del (p.leu1062Argfs*52) in a 5-year-old Chinese boy and further analyzed KAT6B expression and its interacting complexes and downstream products using real-time quantitative polymerase chain reaction (qPCR). Furthermore, we assessed its three-dimensional protein structure and compared the variant with other reported KAT6B variants.

RESULTS

The deletion changed the leucine at position 1062 into an arginine, resulting in translation termination after base 3340, which may have affected protein stability and protein-protein interactions. KAT6B mRNA expression levels in this case were substantially different from those of the parents and controls in the same age range. There were also significant differences in mRNA expression levels among affected children's parents. RUNX2 and NR5A1, downstream products of the gene, affect the corresponding clinical symptoms. The mRNA expression levels of the two in children were lower than those of their parents and controls in the same age range.

CONCLUSION

This deletion in KAT6B may affect protein function and cause corresponding clinical symptoms through interactions with key complexes and downstream products.

KAT7 promotes radioresistance through upregulating PI3K/AKT signaling in breast cancer

Chromatin-modifying enzymes are commonly altered in cancers, but the molecular mechanism by which they regulate cancers remains poorly understood. Herein, we demonstrated that Lysine acetyltransferase 7 (KAT7) was upregulated in breast cancer. KAT7 expression negatively correlated with the survival of breast cancer patients, and KAT7 silencing suppressed breast cancer radioresistance in vitro. Mechanistically, KAT7 activated Phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) transcription, leading to enhanced PI3K/AKT signaling and radioresistance. Overexpression of AKT or PIK3CA restored radioresistance suppression induced by KAT7 inhibition. Moreover, overexpression of KAT7, but not KAT7 acetyltransferase activity-deficient mutants promoted AKT phosphorylation at the Ser473 site, PIK3CA expression and radioresistance suppression due to KAT7 inhibition. In conclusion, KAT7 has huge prospects for clinical application as a new target for predicting radioresistance in breast cancer patients.

The MOZ-BRPF1 acetyltransferase complex in epigenetic crosstalk linked to gene regulation, development, and human diseases

Acetylation of lysine residues on histone tails is an important post-translational modification (PTM) that regulates chromatin dynamics to allow gene transcription as well as DNA replication and repair. Histone acetyltransferases (HATs) are often found in large multi-subunit complexes and can also modify specific lysine residues in non-histone substrates. Interestingly, the presence of various histone PTM recognizing domains (reader domains) in these complexes ensures their specific localization, enabling the epigenetic crosstalk and context-specific activity. In this review, we will cover the biochemical and functional properties of the MOZ-BRPF1 acetyltransferase complex, underlining its role in normal biological processes as well as in disease progression. We will discuss how epigenetic reader domains within the MOZ-BRPF1 complex affect its chromatin localization and the histone acetyltransferase specificity of the complex. We will also summarize how MOZ-BRPF1 is linked to development via controlling cell stemness and how mutations or changes in expression levels of MOZ/BRPF1 can lead to developmental disorders or cancer. As a last touch, we will review the latest drug candidates for these two proteins and discuss the therapeutic possibilities.

Bromodomain (BrD) Family Members as Regulators of Cancer Stemness-A Comprehensive Review

Epigenetic mechanisms involving DNA methylation and chromatin modifications have emerged as critical facilitators of cancer heterogeneity, substantially affecting cancer development and progression, modulating cell phenotypes, and enhancing or inhibiting cancer cell malignant properties. Not surprisingly, considering the importance of epigenetic regulators in normal stem cell maintenance, many chromatin-related proteins are essential to maintaining the cancer stem cell (CSC)-like state. With increased tumor-initiating capacities and self-renewal potential, CSCs promote tumor growth, provide therapy resistance, spread tumors, and facilitate tumor relapse after treatment. In this review, we characterized the epigenetic mechanisms that regulate the acquisition and maintenance of cancer stemness concerning selected epigenetic factors belonging to the Bromodomain (BrD) family of proteins. An increasing number of BrD proteins reinforce cancer stemness, supporting the maintenance of the cancer stem cell population in vitro and in vivo via the utilization of distinct mechanisms. As bromodomain possesses high druggable potential, specific BrD proteins might become novel therapeutic targets in cancers exhibiting de-differentiated tumor characteristics.

BRPF1-KAT6A/KAT6B Complex: Molecular Structure, Biological Function and Human Disease

The bromodomain and PHD finger-containing protein1 (BRPF1) is a member of family IV of the bromodomain-containing proteins that participate in the post-translational modification of histones. It functions in the form of a tetrameric complex with a monocytic leukemia zinc finger protein (MOZ or KAT6A), MOZ-related factor (MORF or KAT6B) or HAT bound to ORC1 (HBO1 or KAT7) and two small non-catalytic proteins, the inhibitor of growth 5 (ING5) or the paralog ING4 and MYST/Esa1-associated factor 6 (MEAF6). Mounting studies have demonstrated that all the four core subunits play crucial roles in different biological processes across diverse species, such as embryonic development, forebrain development, skeletal patterning and hematopoiesis. BRPF1, KAT6A and KAT6B mutations were identified as the cause of neurodevelopmental disorders, leukemia, medulloblastoma and other types of cancer, with germline mutations associated with neurodevelopmental disorders displaying intellectual disability, and somatic variants associated with leukemia, medulloblastoma and other cancers. In this paper, we depict the molecular structures and biological functions of the BRPF1-KAT6A/KAT6B complex, summarize the variants of the complex related to neurodevelopmental disorders and cancers and discuss future research directions and therapeutic potentials.

The histone lysine acetyltransferase HBO1 (KAT7) regulates hematopoietic stem cell quiescence and self-renewal

The histone acetyltransferase HBO1 (MYST2, KAT7) is indispensable for postgastrulation development, histone H3 lysine 14 acetylation (H3K14Ac), and the expression of embryonic patterning genes. In this study, we report the role of HBO1 in regulating hematopoietic stem cell function in adult hematopoiesis. We used 2 complementary cre-recombinase transgenes to conditionally delete Hbo1 (Mx1-Cre and Rosa26-CreERT2). Hbo1-null mice became moribund due to hematopoietic failure with pancytopenia in the blood and bone marrow 2 to 6 weeks after Hbo1 deletion. Hbo1-deleted bone marrow cells failed to repopulate hemoablated recipients in competitive transplantation experiments. Hbo1 deletion caused a rapid loss of hematopoietic progenitors. The numbers of lineage-restricted progenitors for the erythroid, myeloid, B-, and T-cell lineages were reduced. Loss of HBO1 resulted in an abnormally high rate of recruitment of quiescent hematopoietic stem cells (HSCs) into the cell cycle. Cycling HSCs produced progenitors at the expense of self-renewal, which led to the exhaustion of the HSC pool. Mechanistically, genes important for HSC functions were downregulated in HSC-enriched cell populations after Hbo1 deletion, including genes essential for HSC quiescence and self-renewal, such as Mpl, Tek(Tie-2), Gfi1b, Egr1, Tal1(Scl), Gata2, Erg, Pbx1, Meis1, and Hox9, as well as genes important for multipotent progenitor cells and lineage-specific progenitor cells, such as Gata1. HBO1 was required for H3K14Ac through the genome and particularly at gene loci required for HSC quiescence and self-renewal. Our data indicate that HBO1 promotes the expression of a transcription factor network essential for HSC maintenance and self-renewal in adult hematopoiesis.

Hydrophobic cavity-directed azide-acetyllysine photochemistry for profiling non-histone interacting partners of bromodomain protein 1

Bromodomain containing protein 1 (BRD1) plays critical roles in chromatin acetylation, gene transcription, erythropoiesis, and brain development. BRD1 is also implicated in several human conditions and is a therapeutic target for cancer. Although, the bromodomain is known to bind acetylated histones, how the function of BRD1 is regulated via non-histone acetylation is unexplored. To identify the non-histone acetylome of BRD1, we develop an R585AzF variant carrying photo responsive 4-azido phenylalanine (AzF) via amber suppressor mutagenesis. We demonstrate biochemical integrity of the AzF-containing analogue and its ability to crosslink non-histone interacting partners present in human cells. Subsequent proteomic experiments led to the identification of the novel BRD1 interactome representing diverse signaling pathways. As a proof-of-concept demonstration, we validated acetylated PDIA1 protein as a bona fide binding partner of BRD1. Our work suggests that BRD1 interacts with additional acetyllysine motifs, beyond those characterized in histone proteins.

Bromodomain-containing protein 1 (BRD1) was engineered to carry photo-activatable amino acid for characterization of non-histone interacting partners.

The genetic architecture of DNA replication timing in human pluripotent stem cells

DNA replication follows a strict spatiotemporal program that intersects with chromatin structure but has a poorly understood genetic basis. To systematically identify genetic regulators of replication timing, we exploited inter-individual variation in human pluripotent stem cells from 349 individuals. We show that the human genome's replication program is broadly encoded in DNA and identify 1,617 cis-acting replication timing quantitative trait loci (rtQTLs) - sequence determinants of replication initiation. rtQTLs function individually, or in combinations of proximal and distal regulators, and are enriched at sites of histone H3 trimethylation of lysines 4, 9, and 36 together with histone hyperacetylation. H3 trimethylation marks are individually repressive yet synergistically associate with early replication. We identify pluripotency-related transcription factors and boundary elements as positive and negative regulators of replication timing, respectively. Taken together, human replication timing is controlled by a multi-layered mechanism with dozens of effectors working combinatorially and following principles analogous to transcription regulation.

Insights into the Molecular Mechanisms of Histone Code Recognition by the BRPF3 Bromodomain

Bromodomains are evolutionarily conserved reader modules that recognize acetylated lysine residues on the histone tails to facilitate gene transcription. The bromodomain and PHD finger containing protein 3 (BRPF3) is a scaffolding protein that forms a tetrameric complex with HBO1 histone acetyltransferase (HAT) and two other subunits, which is known to regulate the HAT activity and substrate specificity. However, its molecular mechanism, histone ligands, and biological functions remain unknown. Herein, we identify mono- (H4K5ac) and di- (H4K5acK12ac) acetylated histone peptides as novel interacting partners of the BRPF3 bromodomain. Consistent with this, pull-down assays on purified histones from human cells confirm the interaction of BRPF3 bromodomain with acetylated histone H4. Further, MD simulation studies highlight the binding mode of acetyllysine (Kac) and the stability of bromodomain-histone peptide complexes. Collectively, our findings provide a key insight into how histone targets of the BRPF3 bromodomain direct the recruitment of HBO1 complex to chromatin for downstream transcriptional regulation.

Bromodomain proteins: protectors against endogenous DNA damage and facilitators of genome integrity

Endogenous DNA damage is a major contributor to mutations, which are drivers of cancer development. Bromodomain (BRD) proteins are well-established participants in chromatin-based DNA damage response (DDR) pathways, which maintain genome integrity from cell-intrinsic and extrinsic DNA-damaging sources. BRD proteins are most well-studied as regulators of transcription, but emerging evidence has revealed their importance in other DNA-templated processes, including DNA repair and replication. How BRD proteins mechanistically protect cells from endogenous DNA damage through their participation in these pathways remains an active area of investigation. Here, we review several recent studies establishing BRD proteins as key influencers of endogenous DNA damage, including DNA–RNA hybrid (R-loops) formation during transcription and participation in replication stress responses. As endogenous DNA damage is known to contribute to several human diseases, including neurodegeneration, immunodeficiencies, cancer, and aging, the ability of BRD proteins to suppress DNA damage and mutations is likely to provide new insights into the involvement of BRD proteins in these diseases. Although many studies have focused on BRD proteins in transcription, evidence indicates that BRD proteins have emergent functions in DNA repair and genome stability and are participants in the etiology and treatment of diseases involving endogenous DNA damage.

Bromodomain (BRD) proteins, known to regulate gene expression, switching particular genes on and off, also play key roles in repairing DNA damage, and studying them may help identify treatments for various diseases, including cancer. DNA damage can occur during normal cellular metabolism, for example, during copying DNA and gene expression. DNA damage is implicated in tumor formation as well as in neurodegeneration, immunodeficiency, and aging. Seo Yun Lee and colleagues at The University of Texas at Austin, USA, have reviewed new results showing how BRD proteins function in repairing DNA damage. They report that when DNA is damaged during copying in BRD-deficient cells, tumors can result. They also report that defects in BRD proteins are often present in cancers. Studying how BRD proteins function in both healthy and diseased cells could help to identify new therapies.

BRPF3-HUWE1-mediated regulation of MYST2 is required for differentiation and cell-cycle progression in embryonic stem cells

Brpf-histone acetyltransferase (HAT) complexes have important roles in embryonic development and regulating differentiation in ESCs. Among Brpf family, Brpf3 is a scaffold protein of Myst2 histone acetyltransferase complex that plays crucial roles in gene regulation, DNA replication, development as well as maintaining pluripotency in embryonic stem cells (ESCs). However, its biological functions in ESCs are not elucidated. In this study, we find out that Brpf3 protein level is critical for Myst2 stability and E3 ligase Huwe1 functions as a novel negative regulator of Myst2 via ubiquitin-mediated degradation. Importantly, Brpf3 plays an antagonistic role in Huwe1-mediated degradation of Myst2, suggesting that protein-protein interaction between Brpf3 and Myst2 is required for retaining Myst2 stability. Further, Brpf3 overexpression causes the aberrant upregulation of Myst2 protein levels which in turn induces the dysregulated cell-cycle progression and also delay of early embryonic development processes such as embryoid-body formation and lineage commitment of mouse ESCs. The Brpf3 overexpression-induced phenotypes can be reverted by Huwe1 overexpression. Together, these results may provide novel insights into understanding the functions of Brpf3 in proper differentiation as well as cell-cycle progression of ESCs via regulation of Myst2 stability by obstructing Huwe1-mediated ubiquitination. In addition, we suggest that this is a useful report which sheds light on the function of an unknown gene in ESC field.

Lysine acetyltransferase 8 is involved in cerebral development and syndromic intellectual disability

Epigenetic integrity is critical for many eukaryotic cellular processes. An important question is how different epigenetic regulators control development and influence disease. Lysine acetyltransferase 8 (KAT8) is critical for acetylation of histone H4 at lysine 16 (H4K16), an evolutionarily conserved epigenetic mark. It is unclear what roles KAT8 plays in cerebral development and human disease. Here, we report that cerebrum-specific knockout mice displayed cerebral hypoplasia in the neocortex and hippocampus, along with improper neural stem and progenitor cell (NSPC) development. Mutant cerebrocortical neuroepithelia exhibited faulty proliferation, aberrant neurogenesis, massive apoptosis, and scant H4K16 propionylation. Mutant NSPCs formed poor neurospheres, and pharmacological KAT8 inhibition abolished neurosphere formation. Moreover, we describe KAT8 variants in 9 patients with intellectual disability, seizures, autism, dysmorphisms, and other anomalies. The variants altered chromobarrel and catalytic domains of KAT8, thereby impairing nucleosomal H4K16 acetylation. Valproate was effective for treating epilepsy in at least 2 of the individuals. This study uncovers a critical role of KAT8 in cerebral and NSPC development, identifies 9 individuals with KAT8 variants, and links deficient H4K16 acylation directly to intellectual disability, epilepsy, and other developmental anomalies.

Deficient histone H3 propionylation by BRPF1-KAT6 complexes in neurodevelopmental disorders and cancer

Lysine acetyltransferase 6A (KAT6A) and its paralog KAT6B form stoichiometric complexes with bromodomain- and PHD finger-containing protein 1 (BRPF1) for acetylation of histone H3 at lysine 23 (H3K23). We report that these complexes also catalyze H3K23 propionylation in vitro and in vivo. Immunofluorescence microscopy and ATAC-See revealed the association of this modification with active chromatin. Brpf1 deletion obliterates the acylation in mouse embryos and fibroblasts. Moreover, we identify BRPF1 variants in 12 previously unidentified cases of syndromic intellectual disability and demonstrate that these cases and known BRPF1 variants impair H3K23 propionylation. Cardiac anomalies are present in a subset of the cases. H3K23 acylation is also impaired by cancer-derived somatic BRPF1 mutations. Valproate, vorinostat, propionate and butyrate promote H3K23 acylation. These results reveal the dual functionality of BRPF1-KAT6 complexes, shed light on mechanisms underlying related developmental disorders and various cancers, and suggest mutation-based therapy for medical conditions with deficient histone acylation.

ATAT1 regulates forebrain development and stress-induced tubulin hyperacetylation

α-Tubulin acetyltransferase 1 (ATAT1) catalyzes acetylation of α-tubulin at lysine 40 in various organisms ranging from Tetrahymena to humans. Despite the importance in mammals suggested by studies of cultured cells, the mouse Atat1 gene is non-essential for survival, raising an intriguing question about its real functions in vivo. To address this question, we systematically analyzed a mouse strain lacking the gene. The analyses revealed that starting at postnatal day 5, the mutant mice display enlarged lateral ventricles in the forebrain, resembling ventricular dilation in human patients with ventriculomegaly. In the mice, ventricular dilation is due to hypoplasia in the septum and striatum. Behavioral tests of the mice uncovered deficits in motor coordination. Birth-dating experiments revealed that neuronal migration to the mutant septum and striatum is impaired during brain development. In the mutant embryonic fibroblasts, we found mild defects in cell proliferation and primary cilium formation. Notably, in these cells, ATAT1 is indispensable for tubulin hyperacetylation in response to high salt, high glucose, and hydrogen peroxide-induced oxidative stress. We investigated the role of ATAT1 in the hematopoietic system using multicolor flow cytometry and found that this system remains normal in the mutant mice. Although tubulin acetylation was undetectable in a majority of mutant tissues, residual levels were detected in the heart, skeletal muscle, trachea, oviduct, thymus and spleen. This study thus not only establishes the importance of ATAT1 in regulating mouse forebrain development and governing tubulin hyperacetylation during stress responses, but also suggests the existence of an additional α-tubulin acetyltransferase.

Lysine Acetylation Goes Global: From Epigenetics to Metabolism and Therapeutics

Post-translational acetylation of lysine residues has emerged as a key regulatory mechanism in all eukaryotic organisms. Originally discovered in 1963 as a unique modification of histones, acetylation marks are now found on thousands of nonhistone proteins located in virtually every cellular compartment. Here we summarize key findings in the field of protein acetylation over the past 20 years with a focus on recent discoveries in nuclear, cytoplasmic, and mitochondrial compartments. Collectively, these findings have elevated protein acetylation as a major post-translational modification, underscoring its physiological relevance in gene regulation, cell signaling, metabolism, and disease.

Histone acetyltransferase 7 (KAT7)-dependent intragenic histone acetylation regulates endothelial cell gene regulation

Although the functional role of chromatin marks at promoters in mediating cell-restricted gene expression has been well characterized, the role of intragenic chromatin marks is not well understood, especially in endothelial cell (EC) gene expression. Here, we characterized the histone H3 and H4 acetylation profiles of 19 genes with EC-enriched expression via locus-wide chromatin immunoprecipitation followed by ultra-high-resolution (5 bp) tiling array analysis in ECs versus non-ECs throughout their genomic loci. Importantly, these genes exhibit differential EC enrichment of H3 and H4 acetylation in their promoter in ECs versus non-ECs. Interestingly, VEGFR-2 and VEGFR-1 show EC-enriched acetylation across broad intragenic regions and are up-regulated in non-ECs by histone deacetylase inhibition. It is unclear which histone acetyltransferases (KATs) are key to EC physiology. Depletion of KAT7 reduced VEGFR-2 expression and disrupted angiogenic potential. Microarray analysis of KAT7-depleted ECs identified 263 differentially regulated genes, many of which are key for growth and angiogenic potential. KAT7 inhibition in zebrafish embryos disrupted vessel formation and caused loss of circulatory integrity, especially hemorrhage, all of which were rescued with human KAT7. Notably, perturbed EC-enriched gene expression, especially the VEGFR-2 homologs, contributed to these vascular defects. Mechanistically, KAT7 participates in VEGFR-2 transcription by mediating RNA polymerase II binding, H3 lysine 14, and H4 acetylation in its intragenic region. Collectively, our findings support the importance of differential histone acetylation at both promoter and intragenic regions of EC genes and reveal a previously underappreciated role of KAT7 and intragenic histone acetylation in regulating VEGFR-2 and endothelial function.

Structural and mechanistic insights into regulation of HBO1 histone acetyltransferase activity by BRPF2

HBO1, a member of the MYST family of histone acetyltransferases (HATs), is required for global acetylation of histone H3K14 and embryonic development. It functions as a catalytic subunit in multisubunit complexes comprising a BRPF1/2/3 or JADE1/2/3 scaffold protein, and two accessory proteins. BRPF2 has been shown to be important for the HAT activity of HBO1 toward H3K14. Here we demonstrated that BRPF2 can regulate the HAT activity of HBO1 toward free H3 and H4, and nucleosomal H3. Particularly, a short N-terminal region of BRPF2 is sufficient for binding to HBO1 and can potentiate its activity toward H3K14. The crystal structure of the HBO1 MYST domain in complex with this segment of BRPF2 together with the biochemical and cell biological data revealed the key residues responsible for the HBO1–BRPF2 interaction. Our structural and functional data together indicate that the N-terminal region of BRPF2 plays an important role in the binding of HBO1 and a minor role in the binding of nucleosomes, which provide new mechanistic insights into the regulation of the HAT activity of HBO1 by BRPF2.

Myst2/Kat7 histone acetyltransferase interaction proteomics reveals tumour-suppressor Niam as a novel binding partner in embryonic stem cells

MYST histone acetyltransferases have crucial functions in transcription, replication and DNA repair and are hence implicated in development and cancer. Here we characterise Myst2/Kat7/Hbo1 protein interactions in mouse embryonic stem cells by affinity purification coupled to mass spectrometry. This study confirms that in embryonic stem cells Myst2 is part of H3 and H4 histone acetylation complexes similar to those described in somatic cells. We identify a novel Myst2-associated protein, the tumour suppressor protein Niam (Nuclear Interactor of ARF and Mdm2). Human NIAM is involved in chromosome segregation, p53 regulation and cell proliferation in somatic cells, but its role in embryonic stem cells is unknown. We describe the first Niam embryonic stem cell interactome, which includes proteins with roles in DNA replication and repair, transcription, splicing and ribosome biogenesis. Many of Myst2 and Niam binding partners are required for correct embryonic development, implicating Myst2 and Niam in the cooperative regulation of this process and suggesting a novel role for Niam in embryonic biology. The data provides a useful resource for exploring Myst2 and Niam essential cellular functions and should contribute to deeper understanding of organism early development and survival as well as cancer. Data are available via ProteomeXchange with identifier PXD005987.

Biological function and histone recognition of family IV bromodomain-containing proteins

Bromodomain proteins function as epigenetic readers that recognize acetylated histone tails to facilitate the transcription of target genes. There are approximately 60 known human bromodomains, which are divided into eight sub-families based on structural conservation. The bromodomain-containing proteins in family IV include seven members (BRPF1, BRPF2, BRPF3, BRD7, BRD9, ATAD2, and ATAD2b). The bromodomains of each of these proteins recognize and bind acetyllysine residues on histone tails protruding from the nucleosome. However, the histone marks recognized by each bromodomain protein can be very different. The BRPF1 subunit of the MOZ histone acetyltransferase (HAT) recognizes acetylated histones H2AK5ac, H4K12ac, H3K14ac, H4K8ac, and H4K5ac. While the bromodomain of BRD7, a member of the SWI/SNF complex, was shown to preferentially recognize acetylated histones H3K9ac, H3K14ac, H4K8ac, H4K12ac, and H4K16ac. The bromodomains of BRPF2 and BRPF3 have similar sequences, and function as part of the HBO1 HAT complex, but there is limited data on which histone ligands they bind. Similarly, there is little known about the histone targets of the BRD9 and ATAD2b bromodomain proteins. Interestingly, the ATAD2 bromodomain was recently shown to preferentially bind to the di-acetylated H4K5acK12ac mark found in newly synthesized histones following DNA replication. However, despite the physiological importance of the family IV bromodomains, little is known about how they function at the molecular or atomic level. In this review, we summarize our understanding of how family IV bromodomains recognize and select for acetyllysine marks and discuss the importance of acetylated histone recognition for their biological functions.

Benzoisoquinolinediones as Potent and Selective Inhibitors of BRPF2 and TAF1/TAF1L Bromodomains

Bromodomains (BD) are readers of lysine acetylation marks present in numerous proteins associated with chromatin. Here we describe a dual inhibitor of the bromodomain and PHD finger (BRPF) family member BRPF2 and the TATA box binding protein-associated factors TAF1 and TAF1L. These proteins are found in large chromatin complexes and play important roles in transcription regulation. The substituted benzoisoquinolinedione series was identified by high-throughput screening, and subsequent structure–activity relationship optimization allowed generation of low nanomolar BRPF2 BD inhibitors with strong selectivity against BRPF1 and BRPF3 BDs. In addition, a strong inhibition of TAF1/TAF1L BD2 was measured for most derivatives. The best compound of the series was BAY-299, which is a very potent, dual inhibitor with an IC₅₀ of 67 nM for BRPF2 BD, 8 nM for TAF1 BD2, and 106 nM for TAF1L BD2. Importantly, no activity was measured for BRD4 BDs. Furthermore, cellular activity was evidenced using a BRPF2– or TAF1–histone H3.3 or H4 interaction assay.

Mutations in the Chromatin Regulator Gene BRPF1 Cause Syndromic Intellectual Disability and Deficient Histone Acetylation

Identification of over 500 epigenetic regulators in humans raises an interesting question regarding how chromatin dysregulation contributes to different diseases. Bromodomain and PHD finger-containing protein 1 (BRPF1) is a multivalent chromatin regulator possessing three histone-binding domains, one non-specific DNA-binding module, and several motifs for interacting with and activating three lysine acetyltransferases. Genetic analyses of fish brpf1 and mouse Brpf1 have uncovered an important role in skeletal, hematopoietic, and brain development, but it remains unclear how BRPF1 is linked to human development and disease. Here, we describe an intellectual disability disorder in ten individuals with inherited or de novo monoallelic BRPF1 mutations. Symptoms include infantile hypotonia, global developmental delay, intellectual disability, expressive language impairment, and facial dysmorphisms. Central nervous system and spinal abnormalities are also seen in some individuals. These clinical features overlap with but are not identical to those reported for persons with KAT6A or KAT6B mutations, suggesting that BRPF1 targets these two acetyltransferases and additional partners in humans. Functional assays showed that the resulting BRPF1 variants are pathogenic and impair acetylation of histone H3 at lysine 23, an abundant but poorly characterized epigenetic mark. We also found a similar deficiency in different lines of Brpf1-knockout mice. These data indicate that aberrations in the chromatin regulator gene BRPF1 cause histone H3 acetylation deficiency and a previously unrecognized intellectual disability syndrome.

BRPF1 is essential for development of fetal hematopoietic stem cells

Hematopoietic stem cells (HSCs) serve as a life-long reservoir for all blood cell types and are clinically useful for a variety of HSC transplantation-based therapies. Understanding the role of chromatin organization and regulation in HSC homeostasis may provide important insights into HSC development. Bromodomain- and PHD finger-containing protein 1 (BRPF1) is a multivalent chromatin regulator that possesses 4 nucleosome-binding domains and activates 3 lysine acetyltransferases (KAT6A, KAT6B, and KAT7), suggesting that this protein has the potential to stimulate crosstalk between different chromatin modifications. Here, we investigated the function of BRPF1 in hematopoiesis by selectively deleting its gene in murine blood cells. Brpf1-deficient pups experienced early lethality due to acute bone marrow failure and aplastic anemia. The mutant bone marrow and fetal liver exhibited severe deficiency in HSCs and hematopoietic progenitors, along with elevated reactive oxygen species, senescence, and apoptosis. BRPF1 deficiency also reduced the expression of multipotency genes, including Slamf1, Mecom, Hoxa9, Hlf, Gfi1, Egr, and Gata3. Furthermore, BRPF1 was required for acetylation of histone H3 at lysine 23, a highly abundant but not well-characterized epigenetic mark. These results identify an essential role of the multivalent chromatin regulator BRPF1 in definitive hematopoiesis and illuminate a potentially new avenue for studying epigenetic networks that govern HSC ontogeny.

Development of small molecule inhibitors of BRPF1 and TRIM24 bromodomains

The entry of small molecule inhibitors of the bromodomain and extra C-terminal domain (BET) family of bromodomains into the clinic has demonstrated the therapeutic potential for this class of epigenetic acetyl-lysine reader proteins. Within the past two years, the development of potent inhibitors for the bromodomain and PHD finger containing protein (BRPF) family and the tripartite motif containing protein 24 (TRIM24) have been reported and are the subject of this review. Both proteins contain other domains with diverse functions and can also be part of a complex of proteins which have implications in epigenetic signaling and disease. These new small molecule tools will be useful for unraveling the biological contribution of the bromodomain and enable pharmacological validation of these proteins.