Chromatin Readers in Health and Disease

Chromatin Readers in Health and Disease, Volume 35, a new release in the Translational Epigenetics series, gathers and makes actionable our current understanding of how chromatin readers regulate access to genetic information, and how their aberrant regulation can contribute to human pathologies. Chromatin readers discussed include 14-3-3 Dinshaw, ADD, Ankyrin, BAH, BET, BIR, BRCT, bromodomains and Kac readers, chromodomains and chromobarrel readers, citrullination readers, macrodomains and poly-ADP-ribose readers, MBT, PHD and double PHD, PWWP, SUMO (H4K12) readers, Tudor and TTD, UDR and ubiquitin, WD40, YEATS (crotonyl reader), MBD, SRA, and Methyl-RNA readers.In the book, more than a dozen leaders in the field examine a range of protein readers, their relationship to human disease, and the early therapeutics that act as chromatin signaling factors to treat cancers and Huntington's disease, among other disorders.

• Enables researchers and clinicians to understand chromatin signaling mechanisms that regulate gene expression through chromatin readers
• Highlights the role of chromatin readers in a variety of human pathologies, as well as early therapeutics that act on chromatin signaling
• Includes chapter contributions from international leaders in the field

• Language: English
• Publisher: Academic Press
• Release date: Sep 22, 2023
• ISBN: 9780323903141

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Preface

Olivier Binda, Department of Cellular and Molecular Medicine, Faculty of Medicine, University of Ottawa, Ottawa, ON, Canada

Summary

Access to genetic information is central to all cellular processes and biology in general, ranging from developmental biology all the way to pathological states. To fit a genome composed of millions of nucleotides within the nucleus of a eukaryotic cell on a micrometer scale, the genome needs to be packaged, such as the .zip data compression format for computer information, in a compact form called chromatin. However, this packaging feat restricts access to genetic information. Therefore, the cell needs tools to open or close the chromatin fiber to facilitate or restrict access according to developmental, metabolic, and general cellular needs. This cellular toolkit is composed of proteins called READERS, which decipher chromatin states and recruit enzymatic activities and transcription factors to regulate access to genetic information. In this textbook, you will learn about some of these READERS and how they behave like police officers directing traffic at intersections.

Chromatin

The human genome is composed of diploid cells of ~3 billion base pairs of DNA, which side by side would total approximately 1 m in length, that are compacted within a nucleus of diameter ~10 μm. This packaging feat is made possible by small basic proteins called histones. Genomic DNA is wrapped around a histone octamer composed of two copies of each histone H2A, H2B, H3, and H4, at a rate of ~146 base pairs of DNA per nucleosome. Although impressive, packaging genomic DNA into nucleosomes restricts access to genetic information and thus impairs DNA-templated transactions, such as replication during cell division, gene expression via transcription of genes required for particular cellular states, and repair of damage to the genome due to various genotoxic stresses. Thus, chromatin signaling pathways are required to allow or restrict access to genetic information during development and day-to-day cellular activities. These include enzymatic activities that mark chromatin for access, blockade, or poised states.

Histone modifiers

To circumvent issues related to the tight packaging of the genome into nucleosomes, histones have evolved in a way that the amino terminal portion of each histone protrudes outside the nucleosome structure and is thus readily available for posttranslational modifications (PTMs) by small chemical moieties such as acetyl, methyl, and phosphate, as well as protein modifications, such as SUMOylation and ubiquitinylation. These modifications can alter the structure of the nucleosome, but, most importantly, allow protein-protein interactions between histones and READER proteins.

Histone mark writers

Histone tails can be acetylated by histone acetyltransferases (HATs) [1] and methylated by arginine methyltransferases [2,3] and lysine methyltransferases [4,5], among a variety of PTMs [6,7]. These PTMs, more specifically known as histone marks (hereafter simply referred to as marks or histone marks), can physically alter the compaction of nucleosomes to enable chromatin compaction or relaxation. For example, acetylation of lysine alters the charge of histones making histone-DNA interactions more relaxed, thereby opening up the chromatin and allowing access to the underlying genetic information to transcription factors.

Histone mark erasers

Although histone methylation marks were long thought to be permanent, lysine methylation (Kme) and arginine methylation (Rme) marks can be removed by demethylases, such as LSD1 [8] and Jumonji domain demethylases [9].

Acetylation marks are deposited by HAT enzymes [1] and have long been known to be reversible PTMs by histone deacetylases (HDACs) [10]. There are three classes of HDACs: class I, class II, and the NAD-dependent class III deacetylases called sirtuins (SIRT1-7) [11].

Histone variants

The human genome contains several copies of genes that encode each core histone and also contains genes that encode for histone variants. For example, core histone H3 has variants, such as H3.2, H3.3, H3.X, and H3.Y, while H2A has variants such as H2AX, H2AZ, and macroH2A. These variants usually differ only slightly by a few amino acid residues. However, seemingly small, these alterations can have a huge impact on chromatin signaling. For example, histone variants H3.1–3 contain a lysine residue at position 79, while variants H3.X and H3.Y contain a serine at the same position (Fig. 1).

Fig. 1

Fig. 1 Sequence alignment of core histone H3 and the H3 variants H3.2, H3.3, H3.X, and H3.Y. Amino acid residues 1–83 are shown to highlight similarities and differences. Conserved residues are denoted by a *.

Histone mark readers

Acetyl mark READERS of the BROMODOMAIN family were probably the first histone mark READERS that were identified. In humans, there are hundreds of BROMODOMAIN-containing proteins. Several are actually being targeted chemically for cancer therapies [12].

In recent years, the grand family of histone mark READERS has expanded with the identification of several protein domains that function in recognizing histone marks. It now includes ADD [13], Ankyrin [14], BAH [15], BET [16], BIR [17], BRCT [18], BROMO [19], CHROMO [20], Macrodomain [21], MBT [22], PHD [23–25], PWWP [26], SPIN repeats [27], TUDOR [28], UDR [29], WD40 [30], and YEATS [31,32], as well as methyl DNA (MBD) [33] and methyl RNA (YTH) [34] READER domains. The list of READERS is likely to keep expanding; however, it is noteworthy to mention that not all proteins containing a conserved READER domain can associate with modified histones; some READERS interact with unmodified histones and some READERS simply lack structural features, such as an aromatic cage, to allow interactions with histones.

Some proteins, not necessarily considered as READERS, nevertheless are able to differentiate between core histones and variant histones. For example, DAXX is preferentially associated with the histone variant H3.3 [35]. Similarly, the HIRA histone chaperone complex subunit ASF1 is associated with the histone variant H3.3 through small differences between H3.3 and the canonical histone H3 [36,37].

In this textbook, we cover several READERS including ADD domains, BAH domains, BRCT domains, BROMO domains, CHROMO domains, MBT repeats, PHD and double PHD fingers, PWWP, SPIN repeats, WD-40 repeats, YEATS, and DNA methylation READERS. However, TUDOR and tandem TUDOR domains, R-loop READERS, and multivalent READERS should have been covered, but prominent contributors were not available due to other commitments. Thus, here is a short overview of some neglected READERS to provide a few references for further reading and piquer l’intérêt of the reader.

TUDOR domains as methyl arginine and methyl lysine READERS

Among the so-called royal family of histone mark READERS, the peculiar TUDOR domain, as its name suggests, is promiscuous and can recognize both methyl arginine and methyl lysine residues. More seriously, TUDOR domains have been involved in DNA damage repair through binding of 53BP1 to the histone mark of H4K20me2 [28] and RNA metabolism through SMN binding to methyl arginine-modified snRNP proteins [38].

Multivalent READERS

Single READER domains can recognize modified histones (or nonhistone proteins), but several chromatin-associated proteins actually possess several READER domains of the same type (e.g., MLL proteins contain 3–5 PHD domains) or a combination of READER domains (e.g., SETDB1 and 53BP1). These are called multivalent READERS.

The H3K9 lysine methyltransferase SETDB1 shows 3 TUDOR domains at the amino terminus. The triple TUDOR of SETDB1 binds dual H3K9meK14ac marks to silence LINE elements in the genome [39]. ZMYND8 contains a PHD-BROMO cassette that is associated with the H3K4me1K14ac mark to regulate transcription [40]. Another example of a multivalent reader is 53BP1, which recognizes H4K20me2 through a TUDOR domain and the ubiquitin mark of H2AK15ub via a UDR domain at double-strand break DNA damaged sites [29].

R-loops and methyl RNA readers

R-loops are RNA:DNA hybrids that form during transcription and can lead to DNA damage and genome instability [41]. The N⁶-methyladenosine (m⁶A) modification marks the R-loops and is recognized by the YTH domain-containing protein YTHDF2 [42] and the fourth K homology (KH) domain of IMP1 [43].

Trending methods

Next-generation sequencing technologies opened the gate to a flood of data and allowed chromatin immunoprecipitation (ChIP) experiments to be assessed genome-wide, determining where the histone marks, transcription factors, and other chromatin-bound proteins are on the genome. Recent variations on ChIPseq, called CUT&RUN and CUT&TAG, allow researchers to avoid tedious steps and save precious time.

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Section 1

Histone mark readers

Chapter 1: ADD domains—A regulatory hub in chromatin biology and disease

Albert Jeltsch; Michel Choudalakis; Michael Dukatz; Stefan Kunert    Department of Biochemistry, Institute of Biochemistry and Technical Biochemistry, University of Stuttgart, Stuttgart, Germany

Abstract

ADD domains have been identified in the DNMT3A and DNMT3B DNA methyltransferases, the DNMT3L regulator of DNMT3 enzymes, and the ATRX SNF2 family chromatin remodeler. The domain folds into a GATA-like zinc finger, a PHD domain, and a carboxy-terminal alpha-helix. It binds to histone H3 amino-terminal tail peptides, if they are unmethylated at K4. This interaction contributes to the subnuclear localization of DNMT3 proteins and to the global anticorrelation of DNA methylation and H3K4me2/3. Due to the presence of an additional binding pocket for trimethyllysine, the ATRX ADD domain binds to H3 tail peptides unmodified at K4 and trimethylated at K9, and this property contributes to the heterochromatic localization of the ATRX protein. ADD domains are a hub for protein-protein interactions, and they are involved in the allosteric regulation of DNMT3A activity. Mutations in the ADD domain have been associated with the alpha-thalassemia mental retardation X-linked syndrome in the case of the ATRX protein and with cancer and the Tatton-Brown-Rahman overgrowth syndrome in the case of DNMT3A.

Keywords

ADD domain; DNMT3A; DNMT3B; DNMT3L; ATRX; Chromatin; Histone H3; Peptide binding; H3K4me3; H3K9me3

Acknowledgments

Work in the authors’ laboratory has been supported by Deutsche Forschungsgemeinschaft JE252/26-1 and JE252/10-3.

The ADD (ATRX-DNMT3-DNMT3L) protein domain was first identified as a Cys-rich domain in the ATRX protein [1]. It comprises about 140 amino acid residues and shares homology with plant homeodomains (PHDs) but contains an additional Cys2-Cys2 motif on the N-terminal side of the PHD domain [1,2]. In 1998, DNMT3A and DNMT3B methyltransferases were discovered and shown to contain a highly related Cys-rich region [3,4], and DNMT3L, discovered in 2000, also contains this domain (Fig. 1.1) [5]. Based on this, the ADD acronym for ATRX-DNMT3-DNMT3L was coined.

Fig. 1.1

Fig. 1.1 Alignment of the ADD domains of the human (hs) and mouse (mm) DNMT3A, DNMT3B, DNMT3L, and ATRX proteins. Residues interacting with the N-terminal end of the H3 peptide, the K4 ε-amino group, and the trimethylamine of K9me3 are colored in blue , red , and green , respectively. Amino acid numbers are indicated on the right. No Permission Required.

DNMT3A and DNMT3B are essential DNA methyltransferases with roles in the generation of DNA methylation patterns during mammalian development, germline differentiation, and preservation of high DNA methylation levels at repeats [6–8]. DNMT3L is a regulator of DNMT3 enzymes which increases their activity and has an important role in the generation of imprints in germline development [6]. DNMT3A is often mutated in hematological cancers, such as acute myeloid leukemia (AML) [9], and DNMT3A mutations were also observed in the Tatton-Brown-Rahman developmental overgrowth syndrome (OMIM 615879) [10]. In these diseases, mutations appear in the catalytic part of DNMT3A but also within the ADD domain. ATRX is a member of the SNF2 family of ATP-dependent chromatin remodelers [11] that together with its cofactor DAXX is essential for the deposition of the H3.3 histone variant at telomeres and pericentric heterochromatin [12]. Mutations in ATRX, mainly in the ADD domain and SNF2 part, are found in X chromosome-linked α-thalassemia mental retardation syndrome (ATRX syndrome, OMIM 301040) characterized by a variety of clinical symptoms including facial, skeletal and urogenital abnormalities, mental retardation, and mild α-thalassemia [2,11].

Structure of the ATRX-ADD domain

The structure of the ADD domain from ATRX was solved by NMR in 2007 [2]. It was shown to comprise three subdomains (Fig. 1.2). The first subdomain contains the four extra Cys residues of the Cys2-Cys2 motif, which bind a zinc ion and form a GATA-like zinc finger structure. As predicted, the second subdomain binds two zinc ions with eight Cys residues and forms a PHD finger that is packed against the GATA-like Zn-finger. As an additional element at the C-terminal part of the domain, a long α-helix is formed that starts from the PHD finger and docks via hydrophobic contacts onto the N-terminal GATA finger.

Fig. 1.2

Fig. 1.2 Superposition of the structures of the ADD domain of DNMT3A, DNMT3L, and ATRX. The structures of the ADD domains of DNMT3A (4U7T), DNMT3L (2PVC), and ATRX (4W5A) are shown in ribbon representation. The GATA-like zinc finger, PHD finger, and α-helix are colored in blue , green , and red , respectively. No Permission Required.

Histone tail interaction and structures of DNMT3-ADD domains

Strikingly, the following years have provided a steady flow of new information regarding the chromatin interactions of ADD domains. In a pioneering study in 2007, Ooi et al. discovered that the ADD domain of DNMT3L binds to the N-terminal part of the histone H3 tail and that methylation on lysine 4 (H3K4) interferes with this binding [13]. The structure of the complex of DNMT3L with the K4 unmethylated N-terminal tail of H3 showed that the histone peptide was mainly bound by the PHD finger part of the ADD domain (Fig. 1.3).

Fig. 1.3

Fig. 1.3 Structures of the DNMT3A (4U7T) and ATRX (3QL9) ADD domains with bound H3 peptides. The proteins are shown as surface and the peptide as stick. The GATA-like zinc finger, PHD finger, and C-terminal α-helix are colored in blue , green , and red , respectively, and the rest of the protein is tan. In the DNMT3A ADD, the backbone CO of A575 and E578 and the side chains of D531 and D529 are shown under the surface. The binding regions of the N-terminal amino group, K4-ε-amino group, and (for ATRX) K9me3 are highlighted. In the ATRX ADD, the residues D233, E218, D212, I209, and Y203 are shown under the surface. H-bonds are marked with cyan-colored dashed lines . No Permission Required.

In 2009 and 2010, this finding was further generalized in structural and biochemical work showing the specific binding of H3 tail peptides unmodified at K4 also for the ADD domains of DNMT3A and DNMT3B [14]. A crystal structure of the DNMT3A ADD domain covalently connected to an H3 peptide sequence was solved showing similar interactions as observed in the DNMT3L complex [15]. The structural analyses of the ADD domains of DNMT3L and DNMT3A [13,15] revealed that the histone peptide is bound as an additional strand via main-chain contacts to the central antiparallel two-strand β-sheet of the PHD subdomain. The specificity of this interaction is achieved by two aspartic acid residues positioned between the GATA-like zinc finger and PHD finger forming side-chain H-bonding interactions to the K4 ε-amino group, thereby excluding K4me2 and K4me3. Moreover, the N-terminal amino group of the H3 peptide is tightly anchored by the main-chain H-bonds in the PHD subdomain.

The biochemical study of Zhang et al. (2010) investigated the binding of the DNMT3A and DNMT3B ADD domains to peptide arrays containing a large number of different histone tail peptides with various posttranslational modifications in different combinations [14]. This experimental approach allowed to determine the effect of many modifications of the H3 tail on peptide binding, revealing that dimethylation, trimethylation, and acetylation of K4, phosphorylation of S10 and T11, and N-terminal acetylation significantly weaken the affinity to the protein domain. Moreover, in vitro DNA methylation studies with DNMT3A and reconstituted nucleosomes demonstrated that the ADD domain-mediated histone tail interaction enhanced the methylation of nucleosomal DNA. The obstruction of chromatin binding of the ADD domains of DNMT3A, DNMT3B, and DNMT3L by higher methylation states of H3K4 is one of the most striking effects connecting DNA methylation to the absence of H3K4me2/3. It readily explains the strong and robust anticorrelation of these two chromatin marks at CpG islands and gene promoters observed in numerous epigenome studies [8,16].

In 2015, results were reported that validated the importance of the DNMT3L ADD chromatin interaction in vivo [17]. A mouse strain was constructed containing a critical D124A point mutation in the ADD domain of DNMT3L. Genomic data showed a reduction in DNA methylation in the male germline which was particularly pronounced at non-CpG sites methylation, of which is known to depend on DNMT3A in germ cells. This effect led to the reactivation of retrotransposons accompanied by defects in spermatogenesis. In the same year, DNMT3A variants with mutations in the ADD domain were prepared that allowed the mutants to bind to H3 tails containing K4 methylation or T6 phosphorylation [18]. Expression of these DNMT3A mutants in ES cells generated altered DNA methylation patterns and perturbed the cellular differentiation program by causing defects in the lineage commitment and chromosomal instability. This demonstrated that the ADD domain is essential for regulating DNMT3A activity at chromatin regions depending on their histone PTM pattern.

Histone tail interaction of the ATRX-ADD domain

In 2011, novel findings regarding the chromatin interaction of the ADD domain of ATRX were published. Peptide array binding experiments revealed the specific interaction of this protein domain with K4 unmethylated and K9 trimethylated H3 tail peptides [19]. The binding of H3K9me3 is unique for the ADD domain of ATRX. It could be connected to tyrosine 203 in the GATA-like zinc finger that is also specific for the ATRX ADD domain. Crystal structure analyses revealed binding of the H3 peptide to the ATRX-ADD domain in a similar orientation as in the DNMT3A and DNMT3L ADD domains with a conserved interaction at the unmodified K4 [20,21] (Figs. 1.1 and 1.3). Strikingly, the structures revealed the existence of a hydrophobic binding pocket composed of Y203 and I209 for the methylated K9 side chain in the zinc finger part of the domain [20,21]. Different from the DNMT3-ADD structures, in the case of ATRX, the interaction with the N-terminus of the H3 peptide was mediated through an aspartic acid side chain. In all these three biochemical and structural studies, the H3K9me3-specific chromatin interaction of the ATRX-ADD domain was shown to be involved in the heterochromatic localization of ATRX [19–21]. Later, it was shown that ATRX can bind to K9me3/S10ph-modified H3 tails, which are enriched in heterochromatic repeat sequences of stimulated neurons [22]. The K9me3/S10ph double modification on H3 tails is not bound by many other H3K9me3 interactors, including HP1α [23], and the ability of ATRX to interact with these sequences was connected with a specific conformation of the bound peptide, which orients the S10 toward the solvent and does not bring it in contact with the protein, such that a phosphorylation is tolerated in the ATRX-ADD complex with H3 peptides [22].

Allosteric regulation of DNMT3A activity by the ADD domain

The ADD domain of DNMT3A has been shown not only to interact with histone tails, but also to be involved in the allosteric regulation of DNMT3A activity [6,8]. Biochemical methylation studies of reconstituted modified nucleosomes provided the first evidence that the binding of the H3 tail peptide leads to a stimulation of the catalytic activity of DNMT3A [14]. Li et al. (2011) showed that this stimulation is due to a direct allosteric mechanism and they identified an interface between the DNMT3A catalytic domain and the ADD domain that is essential for this effect [24].

The mechanism of this regulatory process was understood with the discovery of additional complex structures of DNMT3A/3L C-terminal fragments. The DNMT3A protein fragments in these new structures included the ADD domain and the catalytic domain of DNMT3A, and the structural data were complemented by detailed enzymatic activity assays [25]. Structures in the absence or presence of the H3 tail peptide showed that the ADD domain interacts with the catalytic domain at two distinct sites, creating two alternative arrangements of these two domains. In the absence of the H3 tail peptide, the ADD domain was bound to the catalytic domain at an autoinhibitory site, thereby blocking the access of the DNA to the active center and leading to inhibition of catalysis. In contrast, in the presence of the H3 tail peptide, ADD binding to the catalytic domain occurred at the so-called allosteric site, which does not compromise the catalytic activity. This peptide-dependent conformational switch can be explained by the observation that residues which are involved in the autoinhibitory-binding interface of the ADD domain are also key residues for the binding of the H3 peptide. Because of this, H3 tail peptide binding and the autoinhibitory interaction of the ADD domain with the catalytic domain are mutually exclusive. It will be interesting to find out if the ATRX-ADD domain has similar allosteric roles to regulate the biological activities of the ATRX protein.

Protein-protein interaction of ADD domains

In addition to their abilities of H3 tail interaction and domain/domain interactions within their host proteins, ADD domains turned out to represent major hubs of protein/protein interaction. Over the years, many proteins with important roles in chromatin biology were found to interact with DNMT3A through the ADD domain (reviewed in Ref. [8]), including protein lysine methyltransferases (e.g., SUV39H1, SETDB1, and EZH2), reading domain proteins (e.g., HP1β, Mbd3, and MeCP2), transcription factors (e.g., PU.1 and RP58) and chromatin remodeling factors (e.g., hSNF2 and SMARCA4). In the context of the emerging evidence showing DNMT3 regulation by domain rearrangements of the ADD domain, the numerous additional interactors could have a direct impact not only on the targeting but also on the activity of the DNMT3A and DNMT3B methyltransferases. This concept was further elaborated in the case of the interaction of the TRD (transcription repression domain) domain of MECP2 with the ADD domain of DNMT3A and DNMT3B [26]. MECP2 binding stabilized the autoinhibitory conformation of DNMT3A causing an inhibition of DNMT3A activity. However, this effect was relieved by the binding of K4-unmodified H3 tail peptides to the ADD domain. The interaction of the ADD and TRD domains was recently mapped to the region D529-531 at the K4 binding pocket of ADD and K219 and K223 in TRD, which are located in a small folded part of the otherwise largely disordered protein [27]. These data indicate that the ADD domain functions as an integration device that regulates DNMT3A activity in response to interactions with different ligands. In order to understand the regulation of DNMT3 enzymes in cells, further efforts delineating the molecular mechanism of the ADD domain interactions with other factors are required. In particular, direct effects of the interactors on the catalytic activity via allosteric mechanisms need to be investigated.

Disease connections of ADD domains

The important mechanistic roles of the ADD domains in DNMT3A and ATRX are reflected by recurrent observations indicating that mutations in these domains have key roles in the development of different diseases. DNMT3A is among the most frequently mutated proteins in AML but also in other cancers, and G543 in the ADD domain is the second most abundant hot spot of somatic mutations (https://cancer.sanger.ac.uk/cosmic, retrieved in July 2020) [28], besides residues in the catalytic domain. Moreover, the M548K and C549R germline mutations in the ADD domain of DNMT3A were linked with the Tatton-Brown-Rahman syndrome, an overgrowth syndrome with intellectual disability [10]. As described above, the ADD domain is involved in chromatin targeting of DNMT3 enzymes, suggesting that interference with this process could be one disease mechanism of ADD mutants in DNMT3. Indeed, it has been shown that disruption of chromatin targeting of DNMT3A by a mutation in the PWWP domain leads to aberrant methylation of bivalent chromatin in mice [29], and global DNA methylation changes have also been reported in human cell lines and patients with mutations in the DNMT3A PWWP domain [30,31]. Whether these effects caused by mutations in the PWWP domain can provide a precedence case for ADD mutations remains to be investigated. Moreover, the interaction of the ADD domain of DNMT3A with its catalytic domain also has an important role in the regulation of DNMT3A’s catalytic activity, suggesting that the pathologic role of ADD mutations could even go beyond a simple targeting effect. In the case of the ATRX protein, the ADD domain is a major hot spot of germline mutations observed in the X-linked mental retardation associated with ATRX syndrome [2,11]. The ADD domain has been shown to be involved in chromatin targeting of ATRX, suggesting a potential mechanism of the disease association. However, the molecular understanding of the pathologic mechanism of the ATRX protein in general and the ADD domain in particular in the ATRX syndrome awaits further studies. Based on the important disease connections, further research unraveling details of the biochemical functions of the different ADD domain is necessary, and it may also assist in the development of more efficient, targeted treatments for these illnesses.

Conclusions

ADD domains are a member of the growing list of epigenome reading domains. They were identified in the DNMT3 enzymes and the ATRX chromatin remodeler. The domain binds to histone H3 amino-terminal tail peptides, if they are unmethylated at K4. Interestingly, the specificity of these two types of ADD domains differs, because the ATRX ADD domain also reads H3K9me3, illustrating that even related reading domains can differ in their histone tail interaction, an important caveat to be considered on a general basis. The histone tail interaction of the ADD domains has been shown to be involved in chromatin targeting of the host proteins and the occurrence of mutations in these domains in different diseases suggests a role of the domain in pathologies as well. However, further work will be required to delineate the disease-associated roles of these domains in more mechanistic details.

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