Histone modifications are key epigenetic regulatory features that have important roles in many cellular events. Lysine methylations mark various sites on the tail and globular domains of histones and their levels are precisely balanced by the action of methyltransferases (‘writers’) and demethylases (‘erasers’). In addition, distinct effector proteins (‘readers’) recognize specific methyl-lysines in a manner that depends on the neighboring amino-acid sequence and methylation state. Misregulation of histone lysine methylation has been implicated in several cancers and developmental defects. Therefore, histone lysine methylation has been considered a potential therapeutic target, and clinical trials of several inhibitors of this process have shown promising results. A more detailed understanding of histone lysine methylation is necessary for elucidating complex biological processes and, ultimately, for developing and improving disease treatments. This review summarizes enzymes responsible for histone lysine methylation and demethylation and how histone lysine methylation contributes to various biological processes. Elucidating how enzymes add and subtract methyl groups to the proteins that package DNA could lead to new disease treatments. In a review article, Jaehoon Kim and colleagues from the Korea Advanced Institute of Science and Technology in Daejeon, South Korea, summarize the mechanisms by which histone proteins, which assemble to form spool-like complexes around which DNA wraps into more compact units, are modified and how misregulation of this process can lead to cancer, developmental defects and other health problems. The authors focus on how methyl groups are added to and removed from residues of the amino acid lysine within histone proteins. They liken certain enzymes to writers, erasers and readers of histone lysine methylation, and make the case that these enzymes should be investigated as potential therapeutic targets.

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Writing, erasing and reading histone lysine methylations

Genomic instability is a key hallmark of cancer that arises owing to defects in the DNA damage response (DDR) and/or increased replication stress. These alterations promote the clonal evolution of cancer cells via the accumulation of driver aberrations, including gene copy-number changes, rearrangements and mutations; however, these same defects also create vulnerabilities that are relatively specific to cancer cells, which could potentially be exploited to increase the therapeutic index of anticancer treatments and thereby improve patient outcomes. The discovery that BRCA-mutant cancer cells are exquisitely sensitive to inhibition of poly(ADP-ribose) polymerase has ushered in a new era of research on biomarker-driven synthetic lethal treatment strategies for different cancers. The therapeutic landscape of antitumour agents targeting the DDR has rapidly expanded to include inhibitors of other key mediators of DNA repair and replication, such as ATM, ATR, CHK1 and CHK2, DNA-PK and WEE1. Efforts to optimize these therapies are ongoing across a range of cancers, involving the development of predictive biomarker assays of responsiveness (beyond BRCA mutations), assessment of the mechanisms underlying intrinsic and acquired resistance, and evaluation of rational, tolerable combinations with standard-of-care treatments (such as chemotherapeutics and radiation), novel molecularly targeted agents and immune-checkpoint inhibitors. In this Review, we discuss the current status of anticancer therapies targeting the DDR.

State-of-the-art strategies for targeting the DNA damage response in cancer

Therapeutic advances in oncology have not fully translated to the treatment of metastatic disease, which remains largely incurable. Metastatic subclones can emerge both early and late in the life of the primary tumor. A better understanding of the genetic evolution of metastatic disease has the potential to reveal differences in the therapeutic vulnerabilities of primary and metastatic tumors, shed light on the temporal patterns of and routes to metastatic colonization, and provide insight into the biology of the metastatic process. Here we review recent comparative studies of primary and metastatic tumors, including data suggesting that macroevolutionary shifts (the onset of chromosomal instability) contribute to the evolution of metastatic disease. We also discuss the practical challenges associated with these studies and how they might be overcome.

http://science.sciencemag.org/content/sci/352/6282/169.full.pdf

Metastasis as an evolutionary process

https://www.cell.com/cancer-cell/pdf/S1535-6108(15)00346-3.pdf

Inhibiting WEE1 Selectively Kills Histone H3K36me3-Deficient Cancers by dNTP Starvation.

Human cancers commonly have mutations in epigenetic regulatory genes, and several small molecules that target epigenetic regulators are in clinical trials. Here, Pfister and Ashworth discuss the biological complexity of epigenetic regulation in cancer and provide an overview of inhibitors that target gain-of-function mutations, as well as synthetic lethal approaches to target loss-of-function mutations in epigenetic regulators.

Marked for death: targeting epigenetic changes in cancer.

SETD2-Dependent Histone H3K36 Trimethylation Is Required for Homologous Recombination Repair and Genome Stability

DNA double-strand breaks (DSBs) are toxic lesions, which if improperly repaired can result in cell death or genomic instability. DSB repair is usually facilitated by the classical non-homologous end joining (C-NHEJ), or homologous recombination (HR) pathways. However, a mutagenic alternative NHEJ pathway, microhomology-mediated end joining (MMEJ), can also be deployed. While MMEJ is suppressed by C-NHEJ, the relationship between HR and MMEJ is less clear. Here, we describe a role for HR genes in suppressing MMEJ in human cells. By monitoring DSB mis-repair using a sensitive HPRT assay, we found that depletion of HR proteins, including BRCA2, BRCA1 or RPA, resulted in a distinct mutational signature associated with significant increases in break-induced mutation frequencies, deletion lengths and the annealing of short regions of microhomology (2-6 bp) across the break-site. This signature was dependent on CtIP, MRE11, POLQ and PARP, and thus indicative of MMEJ. In contrast to CtIP or MRE11, depletion of BRCA1 resulted in increased partial resection and MMEJ, thus revealing a functional distinction between these early acting HR factors. Together these findings indicate that HR factors suppress mutagenic MMEJ following DSB resection.

A role for human homologous recombination factors in suppressing microhomology-mediated end joining.

Understanding the mechanisms of chromosomal double-strand break repair (DSBR) provides insight into genome instability, oncogenesis and genome engineering, including disease gene correction. Research into DSBR exploits rare-cutting endonucleases to cleave exogenous reporter constructs integrated into the genome. Multiple reporter constructs have been developed to detect various DSBR pathways. Here, using a single endogenous reporter gene, the X-chromosomal disease gene encoding hypoxanthine phosphoribosyltransferase (HPRT), we monitor the relative utilization of three DSBR pathways following cleavage by I-SceI or CRISPR/Cas9 nucleases. For I-SceI, our estimated frequencies of accurate or mutagenic non-homologous end-joining and gene correction by homologous recombination are 4.1, 1.5 and 0.16%, respectively. Unexpectedly, I-SceI and Cas9 induced markedly different DSBR profiles. Also, using an I-SceI-sensitive HPRT minigene, we show that gene correction is more efficient when using long double-stranded DNA than single- or double-stranded oligonucleotides. Finally, using both endogenous HPRT and exogenous reporters, we validate novel cell cycle phase-specific I-SceI derivatives for investigating cell cycle variations in DSBR. The results obtained using these novel approaches provide new insights into template design for gene correction and the relationships between multiple DSBR pathways at a single endogenous disease gene.

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Use of the HPRT gene to study nuclease-induced DNA double-strand break repair

CRISPR/Cas9, base editors and prime editors comprise the contemporary genome editing toolbox. Many studies have optimized the use of CRISPR/Cas9, as the original CRISPR genome editing system, in substituting single nucleotides by homology directed repair (HDR), although this remains challenging. Studies describing modifications that improve editing efficiency fall short of isolating clonal cell lines or have not been validated for challenging loci or cell models. We present data from 95 transfections using a colony forming and an immortalized cell line comparing the effect on editing efficiency of donor template modifications, concentration of components, HDR enhancing agents and cold shock. We found that in silico predictions of guide RNA efficiency correlated poorly withactivity in cells. Using NGS and ddPCR we detected editing efficiencies of 5–12% in the transfected populations which fell to 1% on clonal cell line isolation. Our data demonstrate the variability of CRISPR efficiency by cell model, target locus and other factors. Successful genome editing requires a comparison of systems and modifications to develop the optimal protocol for the cell model and locus. We describe the steps in this process in a flowchart for those embarking on genome editing using any system and incorporate validated HDR-boosting modifications for those using CRISPR/Cas9.

/pdf/optimizing-crispr-cas9-editing-of-repetitive-single-3o0zgklm.pdf

Sara Ahrabi

Papers

SETD2-Dependent Histone H3K36 Trimethylation Is Required for Homologous Recombination Repair and Genome Stability

A role for human homologous recombination factors in suppressing microhomology-mediated end joining.

Use of the HPRT gene to study nuclease-induced DNA double-strand break repair

Optimizing CRISPR/Cas9 Editing of Repetitive Single Nucleotide Variants