The role of MOF in the ionizing radiation response is conserved in Drosophila melanogaster

The Molecular Mechanisms in Senescent Cells Induced by Natural Aging and Ionizing Radiation

This review discusses the relationship between cellular senescence and radiation exposure. Given the wide range of ionizing radiation sources encountered by people in professional and medical spheres, as well as the influence of natural background radiation, the question of the effect of radiation on biological processes, particularly on aging processes, remains highly relevant. The parallel relationship between natural and radiation-induced cellular senescence reveals the common aspects underlying these processes. Based on recent scientific data, the key points of the effects of ionizing radiation on cellular processes associated with aging, such as genome instability, mitochondrial dysfunction, altered expression of miRNAs, epigenetic profile, and manifestation of the senescence-associated secretory phenotype (SASP), are discussed. Unraveling the molecular mechanisms of cellular senescence can make a valuable contribution to the understanding of the molecular genetic basis of age-associated diseases in the context of environmental exposure.

Heat-induced SIRT1-mediated H4K16ac deacetylation impairs resection and SMARCAD1 recruitment to double strand breaks

Hyperthermia inhibits DNA double-strand break (DSB) repair that utilizes homologous recombination (HR) pathway by a poorly defined mechanism(s); however, the mechanisms for this inhibition remain unclear. Here we report that hyperthermia decreases H4K16 acetylation (H4K16ac), an epigenetic modification essential for genome stability and transcription. Heat-induced reduction in H4K16ac was detected in humans, Drosophila, and yeast, indicating that this is a highly conserved response. The examination of histone deacetylase recruitment to chromatin after heat-shock identified SIRT1 as the major deacetylase subsequently enriched at gene-rich regions. Heat-induced SIRT1 recruitment was antagonized by chromatin remodeler SMARCAD1 depletion and, like hyperthermia, the depletion of the SMARCAD1 or combination of the two impaired DNA end resection and increased replication stress. Altered repair protein recruitment was associated with heat-shock-induced γ-H2AX chromatin changes and DSB repair processing. These results support a novel mechanism whereby hyperthermia impacts chromatin organization owing to H4K16ac deacetylation, negatively affecting the HR-dependent DSB repair.

Role of the Histone Acetyl Transferase MOF and the Histone Deacetylase Sirtuins in Regulation of H4K16ac During DNA Damage Repair and Metabolic Programming: Implications in Cancer and Aging

The accurate repair of genomic damage mediated by ionizing radiation (IR), chemo- or radiomimetic drugs, or other exogenous agents, is necessary for maintenance of genome integrity, preservation of cellular viability and prevention of oncogenic transformation. Eukaryotes have conserved mechanisms designed to perceive and repair the damaged DNA quite efficiently. Among the different types of DNA damage, double strand breaks (DSB) are the most detrimental. The cellular DNA DSB response is a hierarchical signaling network that integrates damage sensing and repair with chromatin structural changes that involve a range of pre-existing and induced covalent modifications. Recent studies have revealed that pre-existing histone modifications are important contributors within this signaling/repair network. This chapter discusses the role of a critical histone acetyl transferase (HAT) known as MOF (males absent on the first) and the histone deacetylases (HDACs) Sirtuins on histone H4K16 acetylation (H4K16ac) and DNA damage repair. We also discuss the role of this important histone modification in light of metabolic rewiring and its role in regulating human pathophysiologic states.

Role of histone acetyltransferases MOF and Tip60 in genome stability

The accurate repair of DNA damage specifically the chromosomal double-strand breaks (DSBs) arising from exposure to physical or chemical agents, such as ionizing radiation (IR) and radiomimetic drugs is critical in maintaining genomic integrity. The DNA DSB response and repair is facilitated by hierarchical signaling networks that orchestrate chromatin structural changes specifically histone modifications which impact cell-cycle checkpoints through enzymatic activities to repair the broken DNA ends. Various histone posttranslational modifications such as phosphorylation, acetylation, methylation and ubiquitylation have been shown to play a role in DNA damage repair. Recent studies have provided important insights into the role of histone-specific modifications in sensing DNA damage and facilitating the DNA repair. Histone modifications have been shown to determine the pathway choice for repair of DNA DSBs. This review will summarize the role of important histone acetyltransferases MOF and Tip60 mediated acetylation in repair of DNA DSBs in eukaryotic cells.

Pre-existing H4K16ac levels in euchromatin drive DNA repair by homologous recombination in S-phase

The homologous recombination (HR) repair pathway maintains genetic integrity after DNA double-strand break (DSB) damage and is particularly crucial for maintaining fidelity of expressed genes. Histone H4 acetylation on lysine 16 (H4K16ac) is associated with transcription, but how pre-existing H4K16ac directly affects DSB repair is not known. To answer this question, we used CRISPR/Cas9 technology to introduce I-SceI sites, or repair pathway reporter cassettes, at defined locations within gene-rich (high H4K16ac/euchromatin) and gene-poor (low H4K16ac/heterochromatin) regions. The frequency of DSB repair by HR is higher in gene-rich regions. Interestingly, artificially targeting H4K16ac at specific locations using gRNA/dCas9-MOF increases HR frequency in euchromatin. Finally, inhibition/depletion of RNA polymerase II or Cockayne syndrome B protein leads to decreased recruitment of HR factors at DSBs. These results indicate that the pre-existing H4K16ac status at specific locations directly influences the repair of local DNA breaks, favoring HR in part through the transcription machinery.

The relationship between histone posttranslational modification and DNA damage signaling and repair

PURPOSE

The cellular response to DNA damage occurs in the context of an organized chromatin environment in order to maintain genome integrity. Immediately after DNA damage, an array of histone modifications are induced to relieve the physical constraints of the chromatin environment, mark the site as damaged, and function as a platform for the assembly of mediator and effector proteins of DNA damage response signaling pathway. Changes in chromatin structure in the vicinity of the DNA double-strand break (DSB) facilitates the efficient initiation of the DNA damage signaling cascade. Failure of induction of DNA damage responsive histone modifications may lead to genome instability and cancer. Here we will discuss our current understanding of the DNA damage responsive histone modifications and their role in DNA repair as well as their implications for genome stability. We further discuss recent studies which highlight not only how histone modification has involved in the signaling and remodeling at the DSB but also how it influences the DNA repair pathway choice.

CONCLUSIONS

Histone modifications pattern alter during the induction of DNA DSBs induction as well as during the repair and recovery phase of DNA damage response. It will be interesting to understand more precisely, how DSBs within chromatin are repaired by HR and NHEJ. The emergence of proteomic and genomic technologies in combination with advanced microscopy and imaging methods will help in better understanding the role of chromatin environment in the regulation of genome stability.

MOF Suppresses Replication Stress and Contributes to Resolution of Stalled Replication Forks

The human MOF (hMOF) protein belongs to the MYST family of histone acetyltransferases and plays a critical role in transcription and the DNA damage response. MOF is essential for cell proliferation; however, its role during replication and replicative stress is unknown. Here we demonstrate that cells depleted of MOF and under replicative stress induced by cisplatin, hydroxyurea, or camptothecin have reduced survival, a higher frequency of S-phase-specific chromosome damage, and increased R-loop formation. MOF depletion decreased replication fork speed and, when combined with replicative stress, also increased stalled replication forks as well as new origin firing. MOF interacted with PCNA, a key coordinator of replication and repair machinery at replication forks, and affected its ubiquitination and recruitment to the DNA damage site. Depletion of MOF, therefore, compromised the DNA damage repair response as evidenced by decreased Mre11, RPA70, Rad51, and PCNA focus formation, reduced DNA end resection, and decreased CHK1 phosphorylation in cells after exposure to hydroxyurea or cisplatin. These results support the argument that MOF plays an important role in suppressing replication stress induced by genotoxic agents at several stages during the DNA damage response.

Histone Acetyltransferase Activity of MOF Is Required for MLL-AF9 Leukemogenesis

Chromatin-based mechanisms offer therapeutic targets in acute myeloid leukemia (AML) that are of great current interest. In this study, we conducted an RNAi-based screen to identify druggable chromatin regulator-based targets in leukemias marked by oncogenic rearrangements of the MLL gene. In this manner, we discovered the H4K16 histone acetyltransferase (HAT) MOF to be important for leukemia cell growth. Conditional deletion of Mof in a mouse model of MLL-AF9-driven leukemogenesis reduced tumor burden and prolonged host survival. RNA sequencing showed an expected downregulation of genes within DNA damage repair pathways that are controlled by MOF, as correlated with a significant increase in yH2AX nuclear foci in Mof-deficient MLL-AF9 tumor cells. In parallel, Mof loss also impaired global H4K16 acetylation in the tumor cell genome. Rescue experiments with catalytically inactive mutants of MOF showed that its enzymatic activity was required to maintain cancer pathogenicity. In support of the role of MOF in sustaining H4K16 acetylation, a small-molecule inhibitor of the HAT component MYST blocked the growth of both murine and human MLL-AF9 leukemia cell lines. Furthermore, Mof inactivation suppressed leukemia development in an NUP98-HOXA9-driven AML model. Taken together, our results establish that the HAT activity of MOF is required to sustain MLL-AF9 leukemia and may be important for multiple AML subtypes. Blocking this activity is sufficient to stimulate DNA damage, offering a rationale to pursue MOF inhibitors as a targeted approach to treat MLL-rearranged leukemias. Cancer Res; 77(7); 1753-62. ©2017 AACR.

Histone acetyltransferase activity of MOF is required for adult but not early fetal hematopoiesis in mice

K(lysine) acetyltransferase 8 (KAT8, also known as MOF) mediates the acetylation of histone H4 at lysine 16 (H4K16ac) and is crucial for murine embryogenesis. Lysine acetyltransferases have been shown to regulate various stages of normal hematopoiesis. However, the function of MOF in hematopoietic stem cell (HSC) development has not yet been elucidated. We set out to study the role of MOF in general hematopoiesis by using a Vav1-cre-induced conditional murine Mof knockout system and found that MOF is critical for hematopoietic cell maintenance and HSC engraftment capacity in adult hematopoiesis. Rescue experiments with a MOF histone acetyltransferase domain mutant illustrated the requirement for MOF acetyltransferase activity in the clonogenic capacity of HSCs and progenitors. In stark contrast, fetal steady-state hematopoiesis at embryonic day (E) 14.5 was not affected by homozygous Mof deletion despite dramatic loss of global H4K16ac. Hematopoietic defects start manifesting in late gestation at E17.5. The discovery that MOF and its H4K16ac activity are required for adult but not early and midgestational hematopoiesis supports the notion that multiple chromatin regulators may be crucial for hematopoiesis at varying stages of development. MOF is therefore a developmental-stage-specific chromatin regulator found to be essential for adult but not early fetal hematopoiesis.

A multifaceted role for MOF histone modifying factor in genome maintenance

MOF (males absent on the first) was initially identified as a dosage compensation factor in Drosophila that acetylates lysine 16 of histone H4 (H4K16ac) and increased gene transcription from the single copy male X-chromosome. In humans, however, the ortholog of Drosophila MOF has been shown to interact with a range of proteins that extend its potential significance well beyond transcription. For example, recent results indicate MOF is an upstream regulator of the ATM (ataxia-telangiectasia mutated) protein, the loss of which is responsible for ataxia telangiectasia (AT). ATM is a key regulatory kinase that interacts with and phosphorylates multiple substrates that influence critical, cell-cycle control and DNA damage repair pathways in addition to other pathways. Thus, directly or indirectly, MOF may be involved in a wide range of cellular functions. This review will focus on the contribution of MOF to cellular DNA repair and new results that are beginning to examine the in vivo physiological role of MOF.

The Functional Analysis of Histone Acetyltransferase MOF in Tumorigenesis

Changes in chromatin structure and heritably regulating the gene expression by epigenetic mechanisms, such as histone post-translational modification, are involved in most cellular biological processes. Thus, abnormal regulation of epigenetics is implicated in the occurrence of various diseases, including cancer. Human MOF (males absent on the first) is a member of the MYST (Moz-Ybf2/Sas3-Sas2-Tip60) family of histone acetyltransferases (HATs). As a catalytic subunit, MOF can form at least two distinct multiprotein complexes (MSL and NSL) in human cells. Both complexes can acetylate histone H4 at lysine 16 (H4K16); however, the NSL complex possesses broader substrate specificity and can also acetylate histone H4 at lysines 5 and 8 (H4K5 and H4K8), suggesting the complexity of the intracellular functions of MOF. Silencing of MOF in cells leads to genomic instability, inactivation of gene transcription, defective DNA damage repair and early embryonic lethality. Unbalanced MOF expression and its corresponding acetylation of H4K16 have been found in certain primary cancer tissues, including breast cancer, medulloblastoma, ovarian cancer, renal cell carcinoma, colorectal carcinoma, gastric cancer, as well as non-small cell lung cancer. In this review, we provide a brief overview of MOF and its corresponding histone acetylation, introduce recent research findings that link MOF functions to tumorigenesis and speculate on the potential role that may be relevant to tumorigenic pathways.

More complex transcriptional regulation and stress response by MOF

MOF (males absent on the first) was initially discovered as a dosage compensation factor that regulates the epigenetic acetylation of histone H4 lysine 16. In this issue, Sheikh et al. demonstrate that MOF expression is not required for normal kidney tissue function but is required for maintaining transcriptional regulation under conditions of stress. This work along with results from previous investigators highlights the importance of the cell lineage-chromatin modification interaction in determining transcriptional programs and physiological outcomes under normal and stress conditions.

MOF maintains transcriptional programs regulating cellular stress response

MOF (MYST1, KAT8) is the major H4K16 lysine acetyltransferase (KAT) in Drosophila and mammals and is essential for embryonic development. However, little is known regarding the role of MOF in specific cell lineages. Here we analyze the differential role of MOF in proliferating and terminally differentiated tissues at steady state and under stress conditions. In proliferating cells, MOF directly binds and maintains the expression of genes required for cell cycle progression. In contrast, MOF is dispensable for terminally differentiated, postmitotic glomerular podocytes under physiological conditions. However, in response to injury, MOF is absolutely critical for podocyte maintenance in vivo. Consistently, we detect defective nuclear, endoplasmic reticulum and Golgi structures, as well as presence of multivesicular bodies in vivo in podocytes lacking Mof following injury. Undertaking genome-wide expression analysis of podocytes, we uncover several MOF-regulated pathways required for stress response. We find that MOF, along with the members of the non-specific lethal but not the male-specific lethal complex, directly binds to genes encoding the lysosome, endocytosis and vacuole pathways, which are known regulators of podocyte maintenance. Thus, our work identifies MOF as a key regulator of cellular stress response in glomerular podocytes.

MOF phosphorylation by ATM regulates 53BP1-mediated double-strand break repair pathway choice

Cell-cycle phase is a critical determinant of the choice between DNA damage repair by nonhomologous end-joining (NHEJ) or homologous recombination (HR). Here, we report that double-strand breaks (DSBs) induce ATM-dependent MOF (a histone H4 acetyl-transferase) phosphorylation (p-T392-MOF) and that phosphorylated MOF colocalizes with γ-H2AX, ATM, and 53BP1 foci. Mutation of the phosphorylation site (MOF-T392A) impedes DNA repair in S and G2 phase but not G1 phase cells. Expression of MOF-T392A also blocks the reduction in DSB-associated 53BP1 seen in wild-type S/G2 phase cells, resulting in enhanced 53BP1 and reduced BRCA1 association. Decreased BRCA1 levels at DSB sites correlates with defective repairosome formation, reduced HR repair, and decreased cell survival following irradiation. These data support a model whereby ATM-mediated MOF-T392 phosphorylation modulates 53BP1 function to facilitate the subsequent recruitment of HR repair proteins, uncovering a regulatory role for MOF in DSB repair pathway choice during S/G2 phase.

Crosstalk between NSL histone acetyltransferase and MLL/SET complexes: NSL complex functions in promoting histone H3K4 di-methylation activity by MLL/SET complexes

hMOF (MYST1), a histone acetyltransferase (HAT), forms at least two distinct multiprotein complexes in human cells. The male specific lethal (MSL) HAT complex plays a key role in dosage compensation in Drosophila and is responsible for histone H4K16ac in vivo. We and others previously described a second hMOF-containing HAT complex, the non-specific lethal (NSL) HAT complex. The NSL complex has a broader substrate specificity, can acetylate H4 on K16, K5, and K8. The WD (tryptophan-aspartate) repeat domain 5 (WDR5) and host cell factor 1 (HCF1) are shared among members of the MLL/SET (mixed-lineage leukemia/set-domain containing) family of histone H3K4 methyltransferase complexes. The presence of these shared subunits raises the possibility that there are functional links between these complexes and the histone modifications they catalyze; however, the degree to which NSL and MLL/SET influence one another's activities remains unclear. Here, we present evidence from biochemical assays and knockdown/overexpression approaches arguing that the NSL HAT promotes histone H3K4me2 by MLL/SET complexes by an acetylation-dependent mechanism. In genomic experiments, we identified a set of genes including ANKRD2, that are affected by knockdown of both NSL and MLL/SET subunits, suggested they are co-regulated by NSL and MLL/SET complexes. In ChIP assays, we observe that depletion of the NSL subunits hMOF or NSL1 resulted in a significant reduction of both H4K16ac and H3K4me2 in the vicinity of the ANKRD2 transcriptional start site proximal region. However, depletion of RbBP5 (a core component of MLL/SET complexes) only reduced H3K4me2 marks, but not H4K16ac in the same region of ANKRD2, consistent with the idea that NSL acts upstream of MLL/SET to regulate H3K4me2 at certain promoters, suggesting coordination between NSL and MLL/SET complexes is involved in transcriptional regulation of certain genes. Taken together, our results suggest a crosstalk between the NSL and MLL/SET complexes in cells.

Genome-wide distribution of histone H4 Lysine 16 acetylation sites and their relationship to gene expression

Background

Histone post-translational modifications are critical determinants of chromatin structure and function, impacting multiple biological processes including DNA transcription, replication, and repair. The post-translational acetylation of histone H4 at lysine 16 (H4K16ac) was initially identified in association with dosage compensation of the Drosophila male X chromosome. However, in mammalian cells, H4K16ac is not associated with dosage compensation and the genomic distribution of H4K16ac is not precisely known. Therefore, we have mapped the genome-wide H4K16ac distribution in human cells.

Results

We performed H4K16ac chromatin immunoprecipitation from human embryonic kidney 293 (HEK293) cells followed by hybridization to whole-genome tiling arrays and identified 25,893 DNA regions (false discovery rate <0.005) with average length of 692 nucleotides. Interestingly, although a majority of H4K16ac sites localized within genes, only a relatively small fraction (~10%) was found near promoters, in contrast to the distribution of the acetyltransferase, MOF, responsible for acetylation at K16 of H4. Using differential gene expression profiling data, 73 genes (> ±1.5-fold) were identified as potential H4K16ac-regulated genes. Seventeen transcription factor-binding sites were significantly associated with H4K16ac occupancy (p < 0.0005). In addition, a consensus 12-nucleotide guanine-rich sequence motif was identified in more than 55% of the H4K16ac peaks.

Conclusions

The results suggest that H4K16 acetylation has a limited effect on transcription regulation in HEK293 cells, whereas H4K16ac has been demonstrated to have critical roles in regulating transcription in mouse embryonic stem cells. Thus, H4K16ac-dependent transcription regulation is likely a cell type specific process.

T-cell-specific deletion of Mof blocks their differentiation and results in genomic instability in mice

Ataxia telangiectasia patients develop lymphoid malignancies of both B- and T-cell origin. Similarly, ataxia telangiectasia mutated (Atm)-deficient mice exhibit severe defects in T-cell maturation and eventually develop thymomas. The function of ATM is known to be influenced by the mammalian orthologue of the Drosophila MOF (males absent on the first) gene. Here, we report the effect of T-cell-specific ablation of the mouse Mof (Mof) gene on leucocyte trafficking and survival. Conditional Mof(Flox/Flox) (Mof (F/F)) mice expressing Cre recombinase under control of the T-cell-specific Lck proximal promoter (Mof(F/F)/Lck-Cre(+)) display a marked reduction in thymus size compared with Mof(F/F)/Lck-Cre(-) mice. In contrast, the spleen size of Mof(F/F)/Lck-Cre(+) mice was increased compared with control Mof(F/F)/Lck-Cre(-) mice. The thymus of Mof(F/F)/Lck-Cre(+) mice contained significantly reduced T cells, whereas thymic B cells were elevated. Within the T-cell population, CD4(+)CD8(+) double-positive T-cell levels were reduced, whereas the immature CD4(-)CD8(-) double-negative (DN) population was elevated. Defective T-cell differentiation is also evident as an increased DN3 (CD44(-)CD25(+)) population, the cell stage during which T-cell receptor rearrangement takes place. The differentiation defect in T cells and reduced thymus size were not rescued in a p53-deficient background. Splenic B-cell distributions were similar between Mof(F/F)/Lck-Cre(+) and Mof(F/F)/Lck-Cre(-) mice except for an elevation of the κ light-chain population, suggestive of an abnormal clonal expansion. T cells from Mof(F/F)/Lck-Cre(+) mice did not respond to phytohaemagglutinin (PHA) stimulation, whereas LPS-stimulated B cells from Mof(F/F)/Lck-Cre(+) mice demonstrated spontaneous genomic instability. Mice with T-cell-specific loss of MOF had shorter lifespans and decreased survival following irradiation than did Mof(F/F)/Lck-Cre(-) mice. These observations suggest that Mof plays a critical role in T-cell differentiation and that depletion of Mof in T cells reduces T-cell numbers and, by an undefined mechanism, induces genomic instability in B cells through bystander mechanism. As a result, these mice have a shorter lifespan and reduced survival after irradiation.

Histone modifications and DNA double-strand break repair after exposure to ionizing radiations

Ionizing radiation exposure induces highly lethal DNA double-strand breaks (DSBs) in all phases of the cell cycle. After DSBs are detected by the cellular machinery, these breaks are repaired by either of two mechanisms: (1) nonhomologous end joining (NHEJ), which re-ligates the broken ends of the DNA and (2) homologous recombination (HR), that makes use of an undamaged identical DNA sequence as a template to maintain the fidelity of DNA repair. DNA DSB repair must occur within the context of the natural cellular DNA structure. Among the major factors influencing DNA organization are specific histone and nonhistone proteins that form chromatin. The overall chromatin structure regulates DNA damage responses since chromatin status can impede DNA damage site access by repair proteins. During the process of DNA DSB repair, several chromatin alterations are required to sense damage and facilitate accessibility of the repair machinery. The DNA DSB response is also facilitated by hierarchical signaling networks that orchestrate chromatin structural changes that may coordinate cell-cycle checkpoints involving multiple enzymatic activities to repair broken DNA ends. During DNA damage sensing and repair, histones undergo posttranslational modifications (PTMs) including phosphorylation, acetylation, methylation and ubiquitylation. Such histone modifications represent a histone code that directs the recruitment of proteins involved in DNA damage sensing and repair processes. In this review, we summarize histone modifications that occur during DNA DSB repair processes.

Drosophila MOF controls Checkpoint protein2 and regulates genomic stability during early embryogenesis

Background

In Drosophila embryos, checkpoints maintain genome stability by delaying cell cycle progression that allows time for damage repair or to complete DNA synthesis. Drosophila MOF, a member of MYST histone acetyl transferase is an essential component of male X hyperactivation process. Until recently its involvement in G2/M cell cycle arrest and defects in ionizing radiation induced DNA damage pathways was not well established.

Results

Drosophila MOF is highly expressed during early embryogenesis. In the present study we show that haplo-insufficiency of maternal MOF leads to spontaneous mitotic defects like mitotic asynchrony, mitotic catastrophe and chromatid bridges in the syncytial embryos. Such abnormal nuclei are eliminated and digested in the yolk tissues by nuclear fall out mechanism. MOF negatively regulates Drosophila checkpoint kinase 2 tumor suppressor homologue. In response to DNA damage the checkpoint gene Chk2 (Drosophila mnk) is activated in the mof mutants, there by causing centrosomal inactivation suggesting its role in response to genotoxic stress. A drastic decrease in the fall out nuclei in the syncytial embryos derived from mof¹/+; mnk^p6/+ females further confirms the role of DNA damage response gene Chk2 to ensure the removal of abnormal nuclei from the embryonic precursor pool and maintain genome stability. The fact that mof mutants undergo DNA damage has been further elucidated by the increased number of single and double stranded DNA breaks.

Conclusion

mof mutants exhibited genomic instability as evidenced by the occurance of frequent mitotic bridges in anaphase, asynchronous nuclear divisions, disruption of cytoskeleton, inactivation of centrosomes finally leading to DNA damage. Our findings are consistent to what has been reported earlier in mammals that; reduced levels of MOF resulted in increased genomic instability while total loss resulted in lethality. The study can be further extended using Drosophila as model system and carry out the interaction of MOF with the known components of the DNA damage pathway.

Chromatin modifications and the DNA damage response to ionizing radiation

In order to survive, cells have evolved highly effective repair mechanisms to deal with the potentially lethal DNA damage produced by exposure to endogenous as well as exogenous agents. Ionizing radiation exposure induces highly lethal DNA damage, especially DNA double-strand breaks (DSBs), that is sensed by the cellular machinery and then subsequently repaired by either of two different DSB repair mechanisms: (1) non-homologous end joining, which re-ligates the broken ends of the DNA and (2) homologous recombination, that employs an undamaged identical DNA sequence as a template, to maintain the fidelity of DNA repair. Repair of DSBs must occur within the natural context of the cellular DNA which, along with specific proteins, is organized to form chromatin, the overall structure of which can impede DNA damage site access by repair proteins. The chromatin complex is a dynamic structure and is known to change as required for ongoing cellular processes such as gene transcription or DNA replication. Similarly, during the process of DNA damage sensing and repair, chromatin needs to undergo several changes in order to facilitate accessibility of the repair machinery. Cells utilize several factors to modify the chromatin in order to locally open up the structure to reveal the underlying DNA sequence but post-translational modification of the histone components is one of the primary mechanisms. In this review, we will summarize chromatin modifications by the respective chromatin modifying factors that occur during the DNA damage response.

Dosage compensation in Drosophila melanogaster: epigenetic fine-tuning of chromosome-wide transcription

Dosage compensation is an epigenetic mechanism that normalizes gene expression from unequal copy numbers of sex chromosomes. Different organisms have evolved alternative molecular solutions to this task. In Drosophila melanogaster, transcription of the single male X chromosome is upregulated by twofold in a process orchestrated by the dosage compensation complex. Despite this conceptual simplicity, dosage compensation involves multiple coordinated steps to recognize and activate the entire X chromosome. We are only beginning to understand the intriguing interplay between multiple levels of local and long-range chromatin regulation required for the fine-tuned transcriptional activation of a heterogeneous gene population. This Review highlights the known facts and open questions of dosage compensation in D. melanogaster.