Drosophila p53 is required to increase the levels of the dKDM4B demethylase after UV-induced DNA damage to demethylate histone H3 lysine 9

Epigenetic Regulation of Nucleotide Excision Repair

Genomic DNA is constantly attacked by a plethora of DNA damaging agents both from endogenous and exogenous sources. Nucleotide excision repair (NER) is the most versatile repair pathway that recognizes and removes a wide range of bulky and/or helix-distorting DNA lesions. Even though the molecular mechanism of NER is well studied through in vitro system, the NER process inside the cell is more complicated because the genomic DNA in eukaryotes is tightly packaged into chromosomes and compacted into a nucleus. Epigenetic modifications regulate gene activity and expression without changing the DNA sequence. The dynamics of epigenetic regulation play a crucial role during the in vivo NER process. In this review, we summarize recent advances in our understanding of the epigenetic regulation of NER.

Genomics of Adaptations in Ungulates

Ungulates are a group of hoofed animals that have long interacted with humans as essential sources of food, labor, clothing, and transportation. These consist of domesticated, feral, and wild species raised in a wide range of habitats and biomes. Given the diverse and extreme environments inhabited by ungulates, unique adaptive traits are fundamental for fitness. The documentation of genes that underlie their genomic signatures of selection is crucial in this regard. The increasing availability of advanced sequencing technologies has seen the rapid growth of ungulate genomic resources, which offers an exceptional opportunity to understand their adaptive evolution. Here, we summarize the current knowledge on evolutionary genetic signatures underlying the adaptations of ungulates to different habitats.

A chromatin scaffold for DNA damage recognition: how histone methyltransferases prime nucleosomes for repair of ultraviolet light-induced lesions

The excision of mutagenic DNA adducts by the nucleotide excision repair (NER) pathway is essential for genome stability, which is key to avoiding genetic diseases, premature aging, cancer and neurologic disorders. Due to the need to process an extraordinarily high damage density embedded in the nucleosome landscape of chromatin, NER activity provides a unique functional caliper to understand how histone modifiers modulate DNA damage responses. At least three distinct lysine methyltransferases (KMTs) targeting histones have been shown to facilitate the detection of ultraviolet (UV) light-induced DNA lesions in the difficult to access DNA wrapped around histones in nucleosomes. By methylating core histones, these KMTs generate docking sites for DNA damage recognition factors before the chromatin structure is ultimately relaxed and the offending lesions are effectively excised. In view of their function in priming nucleosomes for DNA repair, mutations of genes coding for these KMTs are expected to cause the accumulation of DNA damage promoting cancer and other chronic diseases. Research on the question of how KMTs modulate DNA repair might pave the way to the development of pharmacologic agents for novel therapeutic strategies.

Genome-wide Kdm4 histone demethylase transcriptional regulation in Drosophila

The histone lysine demethylase 4 (Kdm4/Jmjd2/Jhdm3) family is highly conserved across species and reverses di- and tri-methylation of histone H3 lysine 9 (H3K9) and lysine 36 (H3K36) at the N-terminal tail of the core histone H3 in various metazoan species including Drosophila, C.elegans, zebrafish, mice and humans. Previous studies have shown that the Kdm4 family plays a wide variety of important biological roles in different species, including development, oncogenesis and longevity by regulating transcription, DNA damage response and apoptosis. Only two functional Kdm4 family members have been identified in Drosophila, compared to five in mammals, thus providing a simple model system. Drosophila Kdm4 loss-of-function mutants do not survive past the early 2nd instar larvae stage and display a molting defect phenotype associated with deregulated ecdysone hormone receptor signaling. To further characterize and identify additional targets of Kdm4, we employed a genome-wide approach to investigate transcriptome alterations in Kdm4 mutants versus wild-type during early development. We found evidence of increased deregulated transcripts, presumably associated with a progressive accumulation of H3K9 and H3K36 methylation through development. Gene ontology analyses found significant enrichment of terms related to the ecdysteroid hormone signaling pathway important in development, as expected, and additionally previously unidentified potential targets that warrant further investigation. Since Kdm4 is highly conserved across species, our results may be applicable more widely to other organisms and our genome-wide dataset may serve as a useful resource for further studies.

KDM4B: A Nail for Every Hammer?

Epigenetic changes are well-established contributors to cancer progression and normal developmental processes. The reversible modification of histones plays a central role in regulating the nuclear processes of gene transcription, DNA replication, and DNA repair. The KDM4 family of Jumonj domain histone demethylases specifically target di- and tri-methylated lysine 9 on histone H3 (H3K9me3), removing a modification central to defining heterochromatin and gene repression. KDM4 enzymes are generally over-expressed in cancers, making them compelling targets for study and therapeutic inhibition. One of these family members, KDM4B, is especially interesting due to its regulation by multiple cellular stimuli, including DNA damage, steroid hormones, and hypoxia. In this review, we discuss what is known about the regulation of KDM4B in response to the cellular environment, and how this context-dependent expression may be translated into specific biological consequences in cancer and reproductive biology.

And yet, it moves: nuclear and chromatin dynamics of a heterochromatic double-strand break

Heterochromatin is mostly composed of repeated DNA sequences prone to aberrant recombination. How cells maintain the stability of these sequences during double-strand break (DSB) repair has been a long-standing mystery. Studies in Drosophila cells revealed that faithful homologous recombination repair of heterochromatic DSBs relies on the striking relocalization of repair sites to the nuclear periphery before Rad51 recruitment and repair progression. Here, we summarize our current understanding of this response, including the molecular mechanisms involved, and conserved pathways in mammalian cells. We will highlight important similarities with pathways identified in budding yeast for repair of other types of repeated sequences, including rDNA and short telomeres. We will also discuss the emerging role of chromatin composition and regulation in heterochromatin repair progression. Together, these discoveries challenged previous assumptions that repair sites are substantially static in multicellular eukaryotes, that heterochromatin is largely inert in the presence of DSBs, and that silencing and compaction in this domain are obstacles to repair.

This article is part of the themed issue ‘Chromatin modifiers and remodellers in DNA repair and signalling’.

Systematic genetic interaction studies identify histone demethylase Utx as potential target for ameliorating Huntington's disease

Huntington's disease (HD) is a dominantly inherited neurodegenerative disease caused by alterations in the huntingtin gene (htt). Transcriptional dysregulation is an early event in HD progression. Protein acetylation and methylation particularly on histones regulates chromatin structure thereby preventing or facilitating transcription. Although protein acetylation has been found to affect HD symptoms, little is known about the potential role of protein methylation in HD pathology. In recent years, a series of proteins have been described that are responsible for methylating and demethylating histones as well as other proteins. We carried out systematic genetic interaction studies testing lysine and arginine methylases and demethylases in a Drosophila melanogaster HD model. We found that modulating methylation enzymes that typically affect histone positions H3K4, H3K36 or H3K79 had varying effects on HD pathology while modulating ones that typically affect constitutive heterochromatin marks at H3K9 and H4K20 generally had limited impact on HD pathology. In contrast, modulating enzymes acting on the facultative heterochromatin mark at H3K27 had specific effects on HD pathology, with reduction of the demethylase Utx rescuing HTT-induced pathology while reducing Polycomb Repressive Complex2 core methylase components led to more aggressive pathology. Further exploration of the mechanism underlying the methylation-specific interactions suggest that these lysine and arginine methylases and demethylases are likely exerting their influence through non-histone targets. These results highlight a novel therapeutic approach for HD in the form of Utx inhibition.

JMJD5 cleaves monomethylated histone H3 N-tail under DNA damaging stress

The histone H3 N-terminal protein domain (N-tail) is regulated by multiple posttranslational modifications, including methylation, acetylation, phosphorylation, and by proteolytic cleavage. However, the mechanism underlying H3 N-tail proteolytic cleavage is largely elusive. Here, we report that JMJD5, a Jumonji C (JmjC) domain-containing protein, is a Cathepsin L-type protease that mediates histone H3 N-tail proteolytic cleavage under stress conditions that cause a DNA damage response. JMJD5 clips the H3 N-tail at the carboxyl side of monomethyl-lysine (Kme1) residues. In vitro H3 peptide digestion reveals that JMJD5 exclusively cleaves Kme1 H3 peptides, while little or no cleavage effect of JMJD5 on dimethyl-lysine (Kme2), trimethyl-lysine (Kme3), or unmethyl-lysine (Kme0) H3 peptides is observed. Although H3 Kme1 peptides of K4, K9, K27, and K36 can all be cleaved by JMJD5 in vitro, K9 of H3 is the major cleavage site in vivo, and H3.3 is the major H3 target of JMJD5 cleavage. Cleavage is enhanced at gene promoters bound and repressed by JMJD5 suggesting a role for H3 N-tail cleavage in gene expression regulation.

KDM4B/JMJD2B is a p53 target gene that modulates the amplitude of p53 response after DNA damage

The p53 tumor suppressor protein plays a critical role in orchestrating the genomic response to various stress signals by acting as a master transcriptional regulator. Differential gene activity is controlled by transcription factors but also dependent on the underlying chromatin structure, especially on covalent histone modifications. After screening different histone lysine methyltransferases and demethylases, we identified JMJD2B/KDM4B as a p53-inducible gene in response to DNA damage. p53 directly regulates JMJD2B gene expression by binding to a canonical p53-consensus motif in the JMJD2B promoter. JMJD2B induction attenuates the transcription of key p53 transcriptional targets including p21, PIG3 and PUMA, and this modulation is dependent on the catalytic capacity of JMJD2B. Conversely, JMJD2B silencing led to an enhancement of the DNA-damage driven induction of p21 and PIG3. These findings indicate that JMJD2B acts in an auto-regulatory loop by which p53, through JMJD2B activation, is able to influence its own transcriptional program. Functionally, exogenous expression of JMJD2B enhanced subcutaneous tumor growth of colon cancer cells in a p53-dependent manner, and genetic inhibition of JMJD2B impaired tumor growth in vivo. These studies provide new insights into the regulatory effect exerted by JMJD2B on tumor growth through the modulation of p53 target genes.

Systematic discovery of genetic modulation by Jumonji histone demethylases in Drosophila

Jumonji (JmjC) domain proteins influence gene expression and chromatin organization by way of histone demethylation, which provides a means to regulate the activity of genes across the genome. JmjC proteins have been associated with many human diseases including various cancers, developmental and neurological disorders, however, the shared biology and possible common contribution to organismal development and tissue homeostasis of all JmjC proteins remains unclear. Here, we systematically tested the function of all 13 Drosophila JmjC genes. Generation of molecularly defined null mutants revealed that loss of 8 out of 13 JmjC genes modify position effect variegation (PEV) phenotypes, consistent with their ascribed role in regulating chromatin organization. However, most JmjC genes do not critically regulate development, as 10 members are viable and fertile with no obvious developmental defects. Rather, we find that different JmjC mutants specifically alter the phenotypic outcomes in various sensitized genetic backgrounds. Our data demonstrate that, rather than controlling essential gene expression programs, Drosophila JmjC proteins generally act to “fine-tune” different biological processes.

Biological function and regulation of histone and non-histone lysine methylation in response to DNA damage

DNA damage response (DDR) signaling network is initiated to protect cells from various exogenous and endogenous damage resources. Timely and accurate regulation of DDR proteins is required for distinct DNA damage repair pathways. Post-translational modifications of histone and non-histone proteins play a vital role in the DDR factor foci formation and signaling pathway. Phosphorylation, ubiquitylation, SUMOylation, neddylation, poly(ADP-ribosyl)ation, acetylation, and methylation are all involved in the spatial-temporal regulation of DDR, among which phosphorylation and ubiquitylation are well studied. Studies in the past decade also revealed extensive roles of lysine methylation in response to DNA damage. Lysine methylation is finely regulated by plenty of lysine methyltransferases, lysine demethylases, and can be recognized by proteins with chromodomain, plant homeodomain, Tudor domain, malignant brain tumor domain, or proline-tryptophan-tryptophan-proline domain. In this review, we outline the dynamics and regulation of histone lysine methylation at canonical (H3K4, H3K9, H3K27, H3K36, H3K79, and H4K20) and non-canonical sites after DNA damage, and discuss their context-specific functions in DDR protein recruitment or extraction, chromatin environment establishment, and transcriptional regulation. We also present the emerging advances of lysine methylation in non-histone proteins during DDR.

Histone lysine demethylases in Drosophila melanogaster

Epigenetic regulation of chromatin structure is a fundamental process for eukaryotes. Regulators include DNA methylation, microRNAs and chromatin modifications. Within the chromatin modifiers, one class of enzymes that can functionally bind and modify chromatin, through the removal of methyl marks, is the histone lysine demethylases. Here, we summarize the current findings of the 13 known histone lysine demethylases in Drosophila melanogaster, and discuss the critical role of these histone-modifying enzymes in the maintenance of genomic functions. Additionally, as histone demethylase dysregulation has been identified in cancer, we discuss the advantages for using Drosophila as a model system to study tumorigenesis.

Histone demethylases in chromatin biology and beyond

Histone methylation plays fundamental roles in regulating chromatin‐based processes. With the discovery of histone demethylases over a decade ago, it is now clear that histone methylation is dynamically regulated to shape the epigenome and regulate important nuclear processes including transcription, cell cycle control and DNA repair. In addition, recent observations suggest that these enzymes could also have functions beyond their originally proposed role as histone demethylases. In this review, we focus on recent advances in our understanding of the molecular mechanisms that underpin the role of histone demethylases in a wide variety of normal cellular processes.

Overexpression of KDM4 lysine demethylases disrupts the integrity of the DNA mismatch repair pathway

The KDM4 family of lysine demethylases consists of five members, KDM4A, -B and -C that demethylate H3K9me2/3 and H3K36me2/3 marks, while KDM4D and -E demethylate only H3K9me2/3. Recent studies implicated KDM4 proteins in regulating genomic instability and carcinogenesis. Here, we describe a previously unrecognized pathway by which hyperactivity of KDM4 demethylases promotes genomic instability. We show that overexpression of KDM4A-C, but not KDM4D, disrupts MSH6 foci formation during S phase by demethylating its binding site, H3K36me3. Consequently, we demonstrate that cells overexpressing KDM4 members are defective in DNA mismatch repair (MMR), as evident by the instability of four microsatellite markers and the remarkable increase in the spontaneous mutations frequency at the HPRT locus. Furthermore, we show that the defective MMR in cells overexpressing KDM4C is mainly due to the increase in its demethylase activity and can be mended by KDM4C downregulation. Altogether, our data suggest that cells overexpressing KDM4A-C are defective in DNA MMR and this may contribute to genomic instability and tumorigenesis.

Nucleotide excision repair efficiency in quiescent human fibroblasts is modulated by circadian clock

The efficiency of Nucleotide Excision Repair (NER)process is crucial for maintaining genomic integrity because in many organisms, including humans, it represents the only system able to repair a wide range of DNA damage. The aim of the work was to investigate whether the efficiency of the repair of photoproducts induced by UV-light is affected by the circadian phase at which irradiation occurred. NER activity has been analyzed in human quiescent fibroblasts (in the absence of the cell cycle effect), in which circadian rhythmicity has been synchronized with a pulse of dexamethasone. Our results demonstrate that both DNA damage induction and repair efficiency are strictly dependent on the phase of the circadian rhythm at which the cells are UV-exposed. Furthermore, the differences observed between fibroblasts irradiated at different circadian times (CTs) are abolished when the clock is obliterated. In addition, we observe that chromatin structure is regulated by circadian rhythmicity. Maximal chromatin relaxation occurred at the same CT when photoproduct formation and removal were highest. Our data suggest that the circadian clock regulates both the DNA sensitivity to UV damage and the efficiency of NER by controlling chromatin condensation mainly through histone acetylation.

Epigenetic modulation of metabolic decisions

In the recent years there has been a tremendous increase in our understanding of chromatin, transcription and the importance of metabolites in their regulation. This review highlights what is currently sparse information that suggest existence of a refined system integrating metabolic and chromatin control. We indicate possible regulatory modes, such as feed forward amplification, that may help effect and stabilize long-lasting phenotypic decisions within and even across generations using adipogenesis as the primary context.

The emerging role of lysine demethylases in DNA damage response: dissecting the recruitment mode of KDM4D/JMJD2D to DNA damage sites

KDM4D is a lysine demethylase that removes tri- and di- methylated residues from H3K9 and is involved in transcriptional regulation and carcinogenesis. We recently showed that KDM4D is recruited to DNA damage sites in a PARP1-dependent manner and facilitates double-strand break repair in human cells. Moreover, we demonstrated that KDM4D is an RNA binding protein and mapped its RNA-binding motifs. Interestingly, KDM4D-RNA interaction is essential for its localization on chromatin and subsequently for efficient demethylation of its histone substrate H3K9me3. Here, we provide new data that shed mechanistic insights into KDM4D accumulation at DNA damage sites. We show for the first time that KDM4D binds poly(ADP-ribose) (PAR) in vitro via its C-terminal region. In addition, we demonstrate that KDM4D-RNA interaction is required for KDM4D accumulation at DNA breakage sites. Finally, we discuss the recruitment mode and the biological functions of additional lysine demethylases including KDM4B, KDM5B, JMJD1C, and LSD1 in DNA damage response.

Characterization of the Drosophila group ortholog to the amino-terminus of the alpha-thalassemia and mental retardation X-Linked (ATRX) vertebrate protein

The human ATRX gene encodes hATRX, a chromatin-remodeling protein harboring an helicase/ATPase and ADD domains. The ADD domain has two zinc fingers that bind to histone tails and mediate hATRX binding to chromatin. dAtrx, the putative ATRX homolog in Drosophila melanogaster, has a conserved helicase/ATPase domain but lacks the ADD domain. A bioinformatic search of the Drosophila genome using the human ADD sequence allowed us to identify the CG8290 annotated gene, which encodes three ADD harboring- isoforms generated by alternative splicing. This Drosophila ADD domain is highly similar in structure and in the amino acids which mediate the histone tail contacts to the ADD domain of hATRX as shown by 3D modeling. Very recently the CG8290 annotated gene has been named dadd1. We show through pull-down and CoIP assays that the products of the dadd1 gene interact physically with dAtrx_L and HP1a and all of them mainly co-localize in the chromocenter, although euchromatic localization can also be observed through the chromosome arms. We confirm through ChIP analyses that these proteins are present in vivo in the same heterochromatic regions. The three isoforms are expressed throughout development. Flies carrying transheterozygous combinations of the dadd1 and atrx alleles are semi-viable and have different phenotypes including the appearance of melanotic masses. Interestingly, the dAdd1-b and c isoforms have extra domains, such as MADF, which suggest newly acquired functions of these proteins. These results strongly support that, in Drosophila, the atrx gene diverged and that the dadd1-encoded proteins participate with dAtrx in some cellular functions such as heterochromatin maintenance.

Chromatin modifications and DNA repair: beyond double-strand breaks

DNA repair must take place in the context of chromatin, and chromatin modifications and DNA repair are intimately linked. The study of double-strand break repair has revealed numerous histone modifications that occur after induction of a DSB, and modification of the repair factors themselves can also occur. In some cases the function of the modification is at least partially understood, but in many cases it is not yet clear. Although DSB repair is a crucial activity for cell survival, DSBs account for only a small percentage of the DNA lesions that occur over the lifetime of a cell. Repair of single-strand gaps, nicks, stalled forks, alternative DNA structures, and base lesions must also occur in a chromatin context. There is increasing evidence that these repair pathways are also regulated by histone modifications and chromatin remodeling. In this review, we will summarize the current state of knowledge of chromatin modifications that occur during non-DSB repair, highlighting similarities and differences to DSB repair as well as remaining questions.

KDM4C (GASC1) lysine demethylase is associated with mitotic chromatin and regulates chromosome segregation during mitosis

Various types of human cancers exhibit amplification or deletion of KDM4A-D members, which selectively demethylate H3K9 and H3K36, thus implicating their activity in promoting carcinogenesis. On this basis, it was hypothesized that dysregulated expression of KDM4A-D family promotes chromosomal instabilities by largely unknown mechanisms. Here, we show that unlike KDM4A-B, KDM4C is associated with chromatin during mitosis. This association is accompanied by a decrease in the mitotic levels of H3K9me3. We also show that the C-terminal region, containing the Tudor domains of KDM4C, is essential for its association with mitotic chromatin. More specifically, we show that R919 residue on the proximal Tudor domain of KDM4C is critical for its association with chromatin during mitosis. Interestingly, we demonstrate that depletion or overexpression of KDM4C, but not KDM4B, leads to over 3-fold increase in the frequency of abnormal mitotic cells showing either misaligned chromosomes at metaphase, anaphase-telophase lagging chromosomes or anaphase-telophase bridges. Furthermore, overexpression of KDM4C demethylase-dead mutant has no detectable effect on mitotic chromosome segregation. Altogether, our findings implicate KDM4C demethylase activity in regulating the fidelity of mitotic chromosome segregation by a yet unknown mechanism.

PARP1-dependent recruitment of KDM4D histone demethylase to DNA damage sites promotes double-strand break repair

Members of the lysine (K)-specific demethylase 4 (KDM4) A-D family of histone demethylases are dysregulated in several types of cancer. Here, we reveal a previously unrecognized role of KDM4D in the DNA damage response (DDR). We show that the C-terminal region of KDM4D mediates its rapid recruitment to DNA damage sites. Interestingly, this recruitment is independent of the DDR sensor ataxia telangiectasia mutated (ATM), but dependent on poly (ADP-ribose) polymerase 1 (PARP1), which ADP ribosylates KDM4D after damage. We demonstrate that KDM4D is required for efficient phosphorylation of a subset of ATM substrates. We note that KDM4D depletion impairs the DNA damage-induced association of ATM with chromatin, explaining its effect on ATM substrate phosphorylation. Consistent with an upstream role in DDR, KDM4D knockdown disrupts the damage-induced recombinase Rad51 and tumor protein P53 binding protein foci formation. Consequently, the integrity of homology-directed repair and nonhomologous end joining of DNA breaks is impaired in KDM4D-deficient cells. Altogether, our findings implicate KDM4D in DDR, furthering the links between the cancer-relevant networks of epigenetic regulation and genome stability.

Jumonji domain-containing protein 2B silencing induces DNA damage response via STAT3 pathway in colorectal cancer

Background:

Jumonji domain-containing protein 2B (JMJD2B), directly targeted by hypoxia-inducible factor 1α, maintains the histone methylation balance important for the transcriptional activation of many oncogenes. Jumonji domain-containing protein 2B has been implicated in colorectal cancer (CRC) progression; however, the mechanism remains unclear.

Methods:

Immunofluorescence and western blotting detected phosphorylated histone H2AX, characteristic of double-strand breaks, and comet assay was used to investigate DNA damage, in CRC cells after JMJD2B small interfering RNA (siRNA) transfection. We assessed the resulting in vitro responses, that is, cell cycle progression, apoptosis, and senescence coupled with JMJD2B silencing-induced DNA damage, studying the regulatory role of signal transducers and activators of transcription 3 (STAT3). The JMJD2B silencing anti-cancer effect was determined using an in vivo CRC xenograft model.

Results:

Jumonji domain-containing protein 2B knockdown induced DNA damage via ataxia telangiectasia-mutated (ATM) and ATM and Rad3-related pathway activation, resulting in cell cycle arrest, apoptosis, and senescence in both normoxia and hypoxia. Signal transducers and activators of transcription 3 suppression by JMJD2B silencing enhanced DNA damage. Intratumoural injection of JMJD2B siRNA suppressed tumour growth in vivo and activated the DNA damage response (DDR).

Conclusions:

Jumonji domain-containing protein 2B has an essential role in cancer cell survival and tumour growth via DDR mediation, which STAT3 partially regulates, suggesting that JMJD2B is a potential anti-cancer target.

Effects of sulforaphane and 3,3'-diindolylmethane on genome-wide promoter methylation in normal prostate epithelial cells and prostate cancer cells

Epigenetic changes, including aberrant DNA methylation, result in altered gene expression and play an important role in carcinogenesis. Phytochemicals such as sulforaphane (SFN) and 3,3'-diindolylmethane (DIM) are promising chemopreventive agents for the treatment of prostate cancer. Both have been shown to induce re-expression of genes, including tumor suppressor genes silenced in cancer cells, via modulation of epigenetic marks including DNA methylation. However, it remained unclear the effects SFN and DIM on DNA methylation at a genomic scale. The goal of this study was to determine the genome-wide effects of SFN and DIM on promoter methylation in normal prostate epithelial cells and prostate cancer cells. Both SFN and DIM treatment decreased DNA methyltransferase expression in normal prostate epithelial cells (PrEC), and androgen-dependent (LnCAP) and androgen-independent (PC3) prostate cancer cells. The effects of SFN and DIM on promoter methylation profiles in normal PrEC, LnCAP and PC3 prostate cancer cells were determined using methyl-DNA immunoprecipitation followed by genome-wide DNA methylation array. We showed widespread changes in promoter methylation patterns, including both increased and decreased methylation, in all three prostate cell lines in response to SFN or DIM treatments. In particular, SFN and DIM altered promoter methylation in distinct sets of genes in PrEC, LnCAP, and PC3 cells, but shared similar gene targets within a single cell line. We further showed that SFN and DIM reversed many of the cancer-associated methylation alterations, including aberrantly methylated genes that are dysregulated or are highly involved in cancer progression. Overall, our data suggested that both SFN and DIM are epigenetic modulators that have broad and complex effects on DNA methylation profiles in both normal and cancerous prostate epithelial cells. Results from our study may provide new insights into the epigenetic mechanisms by which SFN and DIM exert their cancer chemopreventive effects.

Drosophila Kdm4 demethylases in histone H3 lysine 9 demethylation and ecdysteroid signaling

The dynamic regulation of chromatin structure by histone post-translational modification is an essential regulatory mechanism that controls global gene transcription. The Kdm4 family of H3K9me2,3 and H3K36me2,3 dual specific histone demethylases has been implicated in development and tumorigenesis. Here we show that DrosophilaKdm4A and Kdm4B are together essential for mediating ecdysteroid hormone signaling during larval development. Loss of Kdm4 genes leads to globally elevated levels of the heterochromatin marker H3K9me2,3 and impedes transcriptional activation of ecdysone response genes, resulting in developmental arrest. We further show that Kdm4A interacts with the Ecdysone Receptor (EcR) and colocalizes with EcR at its target gene promoter. Our studies suggest that Kdm4A may function as a transcriptional co-activator by removing the repressive histone mark H3K9me2,3 from cognate promoters.

KDM4/JMJD2 histone demethylases: epigenetic regulators in cancer cells

Lysine methylation is one of the most prominent histone posttranslational modifications that regulate chromatin structure. Changes in histone lysine methylation status have been observed during cancer formation, which is thought to be a consequence of the dysregulation of histone lysine methyltransferases or the opposing demethylases. KDM4/JMJD2 proteins are demethylases that target histone H3 on lysines 9 and 36 and histone H1.4 on lysine 26. This protein family consists of three ~130-kDa proteins (KDM4A-C) and KDM4D/JMJD2D, which is half the size, lacks the double PHD and Tudor domains that are epigenome readers and present in the other KDM4 proteins, and has a different substrate specificity. Various studies have shown that KDM4A/JMJD2A, KDM4B/JMJD2B, and/or KDM4C/JMJD2C are overexpressed in breast, colorectal, lung, prostate, and other tumors and are required for efficient cancer cell growth. In part, this may be due to their ability to modulate transcription factors such as the androgen and estrogen receptor. Thus, KDM4 proteins present themselves as novel potential drug targets. Accordingly, multiple attempts are under way to develop KDM4 inhibitors, which could complement the existing arsenal of epigenetic drugs that are currently limited to DNA methyltransferases and histone deacetylases.

p53 promotes repair of heterochromatin DNA by regulating JMJD2b and SUV39H1 expression

Constitutive heterochromatin is important for maintaining chromosome stability but also delays the repair of DNA double strand breaks (DSB). DSB repair in complex mammalian genomes involves a fast phase (2–6 hrs) where most of the breaks are rapidly repaired, and a slow phase (up to 24 hrs) where the remaining damages in heterochromatin are repaired. We found that p53 deficiency delays the slow phase DNA repair after ionizing irradiation. P53 deficiency prevents down regulation of histone H3K9 trimethylation at pericentric heterochromatin after DNA damage. Moreover, p53 directly induces expression of the H3 K9 demethylase JMJD2b through promoter binding. P53 activation also indirectly down regulates expression of the H3 K9 methytransferase SUV39H1. Depletion of JMJD2b or sustained expression of SUV39H1 delays the repair of heterochromatin DNA and reduces clonogenic survival after ionizing irradiation. The results suggest that by regulating JMJD2b and SUV39H1 expression, p53 not only controls transcription but also promotes heterochromatin relaxation to accelerate a rate-limiting step in the repair of complex genomes.

The genetic depletion or the triptolide inhibition of TFIIH in p53 deficient cells induce a JNK-dependent cell death in Drosophila

TFIIH participates in transcription, nucleotide excision repair and the control of the cell cycle. In this work, we demonstrate that the Dmp52 subunit of TFIIH in Drosophila physically interacts with the fly p53 homologue, Dp53. The depletion of Dmp52 in the wing disc generates chromosome fragility, increases apoptosis and produces wings with a reduced number of cells; cellular proliferation, however, is not affected. Interestingly, instead of suppressing the apoptotic phenotype, the depletion of Dp53 in Dmp52-depleted wing disc cells increases apoptosis and the number of cells that suffer from chromosome fragility. The apoptosis induced by the depletion of Dmp52 alone is partially dependent on the JNK pathway. In contrast, the enhanced apoptosis caused by the simultaneous depletion of Dp53 and Dmp52 is absolutely JNK-dependent. In this study, we also show that the anti-proliferative drug triptolide, which inhibits the ATPase activity of the XPB subunit of TFIIH, phenocopies the JNK-dependent massive apoptotic phenotype of Dp53-depleted wing disc cells; this observation suggests that the mechanism by which triptolide induces apoptosis in p53-deficient cancer cells involves the activation of the JNK death pathway.

The oncogenic potential of Jumonji D2 (JMJD2/KDM4) histone demethylase overexpression

The Jumonji D2 proteins (JMJD2/KDM4) function to demethylate di- and trimethylated (me2/3) histone 3 lysine 9 (H3K9me2/3) and H3K36me3. Knockout mouse models for Kdm4b and Kdm4d have not resulted in gross abnormalities, while mouse models for Kdm4a and Kdm4c have not been reported. However, the KDM4 subfamily of demethylases are overexpressed in several tumor types. Overexpression of KDM4 proteins alters transcription and chromatin remodeling, driving cellular proliferation, anchorage-independent growth, invasion, and migration. Increased proliferation occurs through KDM4-mediated modification of cell cycle timing, as well as through increased numbers of replication forks. Recent evidence also suggests that KDM4C overexpression contributes to the maintenance of a pluripotent state. Together these data suggest that overexpression of KDM4 proteins induces numerous oncogenic effects.

Gene regulation by the lysine demethylase KDM4A in Drosophila

Lysine methylation of histones is associated with both transcriptionally active chromatin and with silent chromatin, depending on what residue is modified. Histone methyltransferases and demethylases ensure that histone methylations are dynamic and can vary depending on cell cycle- or developmental stage. KDM4A demethylates H3K36me3, a modification enriched in the 3' end of active genes. The genomic targets and the role of KDM4 proteins in development remain largely unknown. We therefore generated KDM4A mutant Drosophila, and identified 99 mis-regulated genes in first instar larvae. Around half of these genes were down-regulated and the other half up-regulated in dKDM4A mutants. Although heterochromatin protein 1a (HP1a) can stimulate dKDM4A demethylase activity in vitro, we find that they antagonize each other in control of dKDM4A-regulated genes. Appropriate expression levels for some dKDM4A-regulated genes rely on the demethylase activity of dKDM4A, whereas others do not. Surprisingly, although highly expressed, many demethylase-dependent and independent genes are devoid of H3K36me3 in wild-type as well as in dKDM4A mutant larvae, suggesting that some of the most strongly affected genes in dKDM4A mutant animals are not regulated by H3K36 methylation. By contrast, dKDM4A over-expression results in a global decrease in H3K36me3 levels and male lethality, which might be caused by impaired dosage compensation. Our results show that a modest increase in global H3K36me3 levels is compatible with viability, fertility, and the expression of most genes, whereas decreased H3K36me3 levels are detrimental in males.

Implication of posttranslational histone modifications in nucleotide excision repair

Histones are highly alkaline proteins that package and order the DNA into chromatin in eukaryotic cells. Nucleotide excision repair (NER) is a conserved multistep reaction that removes a wide range of generally bulky and/or helix-distorting DNA lesions. Although the core biochemical mechanism of NER is relatively well known, how cells detect and repair lesions in diverse chromatin environments is still under intensive research. As with all DNA-related processes, the NER machinery must deal with the presence of organized chromatin and the physical obstacles it presents. A huge catalogue of posttranslational histone modifications has been documented. Although a comprehensive understanding of most of these modifications is still lacking, they are believed to be important regulatory elements for many biological processes, including DNA replication and repair, transcription and cell cycle control. Some of these modifications, including acetylation, methylation, phosphorylation and ubiquitination on the four core histones (H2A, H2B, H3 and H4) or the histone H2A variant H2AX, have been found to be implicated in different stages of the NER process. This review will summarize our recent understanding in this area.

Physical and functional interactions between Drosophila homologue of Swc6/p18Hamlet subunit of the SWR1/SRCAP chromatin-remodeling complex with the DNA repair/transcription factor TFIIH

The multisubunit DNA repair and transcription factor TFIIH maintains an intricate cross-talk with different factors to achieve its functions. The p8 subunit of TFIIH maintains the basal levels of the complex by interacting with the p52 subunit. Here, we report that in Drosophila, the homolog of the p8 subunit (Dmp8) is encoded in a bicistronic transcript with the homolog of the Swc6/p18(Hamlet) subunit (Dmp18) of the SWR1/SRCAP chromatin remodeling complex. The SWR1 and SRCAP complexes catalyze the exchange of the canonical histone H2A with the H2AZ histone variant. In eukaryotic cells, bicistronic transcripts are not common, and in some cases, the two encoded proteins are functionally related. We found that Dmp18 physically interacts with the Dmp52 subunit of TFIIH and co-localizes with TFIIH in the chromatin. We also demonstrated that Dmp18 genetically interacts with Dmp8, suggesting that a cross-talk might exist between TFIIH and a component of a chromatin remodeler complex involved in histone exchange. Interestingly, our results also show that when the level of one of the two proteins is decreased and the other maintained, a specific defect in the fly is observed, suggesting that the organization of these two genes in a bicistronic locus has been selected during evolution to allow co-regulation of both genes.

Epigenetic regulation of genomic integrity

Inefficient and inaccurate repair of DNA damage is the principal cause of DNA mutations, chromosomal aberrations, and carcinogenesis. Numerous multiple-step DNA repair pathways exist whose deployment depends on the nature of the DNA lesion. Common to all eukaryotic DNA repair pathways is the need to unravel the compacted chromatin structure to facilitate access of the repair machinery to the DNA and restoration of the original chromatin state afterward. Accordingly, our cells utilize a plethora of coordinated mechanisms to locally open up the chromatin structure to reveal the underlying DNA sequence and to orchestrate the efficient and accurate repair of DNA lesions. Here we review changes to the chromatin structure that are intrinsic to the DNA damage response and the available mechanistic insight into how these chromatin changes facilitate distinct stages of the DNA damage repair pathways to maintain genomic stability.

Chromatin structure following UV-induced DNA damage-repair or death?

In eukaryotes, DNA is compacted into a complex structure known as chromatin. The unravelling of DNA is a crucial step in DNA repair, replication, transcription and recombination as this allows access to DNA for these processes. Failure to package DNA into the nucleosome, the individual unit of chromatin, can lead to genomic instability, driving a cell into apoptosis, senescence, or cellular proliferation. Ultraviolet (UV) radiation damage causes destabilisation of chromatin integrity. UV irradiation induces DNA damage such as photolesions and subjects the chromatin to substantial rearrangements, causing the arrest of transcription forks and cell cycle arrest. Highly conserved processes known as nucleotide and base excision repair (NER and BER) then begin to repair these lesions. However, if DNA repair fails, the cell may be forced into apoptosis. The modification of various histones as well as nucleosome remodelling via ATP-dependent chromatin remodelling complexes are required not only to repair these UV-induced DNA lesions, but also for apoptosis signalling. Histone modifications and nucleosome remodelling in response to UV also lead to the recruitment of various repair and pro-apoptotic proteins. Thus, the way in which a cell responds to UV irradiation via these modifications is important in determining its fate. Failure of these DNA damage response steps can lead to cellular proliferation and oncogenic development, causing skin cancer, hence these chromatin changes are critical for a proper response to UV-induced injury.

Chromatin response to DNA double-strand break damage

Manipulation of chromatin, in which genomic DNA is packaged, is a fundamental requirement for all DNA-based metabolic processes in eukayotic cells. Histone variant incorporation, histone post-translational modifications, and ATP-dependent chromatin remodeling are three major strategies for chromatin manipulation, and are relatively well characterized in transcriptional regulation. Emerging lines of evidence indicate that histone variants (H2AX and H2A.Z), histone post-translational modifications (acetylation, phosphorylation, methylation and ubiquitination) and chromatin-remodeling complexes (INO80, SWR1, SWI/SNF, RSC and NuRD) are important and direct players in the DNA double-strand break (DSB) response as well. New studies also reveal that incorporation of histone variants into nucleosomes, histone modifications and ATP-dependent chromatin remodeling are specifically and intimately connected during the DSB damage response. This article summarizes the recent advances in our understanding of the relationship between chromatin modifications and the DSB damage response.

Open, repair and close again: chromatin dynamics and the response to UV-induced DNA damage

Due to its link with human pathologies, including cancer, the mechanism of Nucleotide Excision Repair (NER) has been extensively studied. Most of the pathway and players have been defined using in vitro reconstitution experiments. However, in vivo, the NER machinery must deal with the presence of organized chromatin, which in some regions, such as heterochromatin, is highly condensed but still susceptible to DNA damage. A series of events involving different chromatin-remodeling factors and histone-modifying enzymes target chromatin regions that contain DNA lesions. CPDs change the structure of the nucleosome, allowing access to factors that can recognize the lesion. Next, DDB1-DDB2 protein complexes, which mono-ubiquitinate histones H2A, H3, and H4, recognize nucleosomes containing DNA lesions. The ubiquitinated nucleosome facilitates the recruitment of ATP-dependent chromatin-remodeling factors and the XPC-HR23B-Centrin 2 complex to the target region. Different ATP-dependent chromatin-remodeling factors, such as SWI/SNF and INO80, have been identified as having roles in the UV irradiation response prior to the action of the NER machinery. Subsequently, remodeling of the nucleosome allows enzymatic reactions by histone-modifying factors that may acetylate, methylate or demethylate specific histone residues. Intriguingly, some of these histone modifications are dependent on p53. These histone modifications and the remodeling of the nucleosome allow the entrance of TFIIH, XPC and other NER factors that remove the damaged strand; then, gap-filling DNA synthesis and ligation reactions are carried out after excision of the oligonucleotide with the lesion. Finally, after DNA repair, the initial chromatin structure has to be reestablished. Therefore, factors that modulate chromatin dynamics contribute to the NER mechanism, and they are significant in the future design of treatments for human pathologies related to genome instability and the appearance of drug-resistant tumors.