New perspectives on epigenetic modifications and PARP inhibitor resistance in HR-deficient cancers.

The clinical treatment of DNA-repair defective tumours has been revolutionised by the use of poly(ADP) ribose polymerase (PARP) inhibitors. However, the efficacy of these compounds is hampered by resistance, which is attributed to numerous mechanisms including rewiring of the DNA damage response to favour pathways that repair PARP inhibitor-mediated damage. Here, we comment on recent findings by our group identifying the lysine methyltransferase SETD1A as a novel factor that conveys PARPi resistance. We discuss the implications, with a particular focus on epigenetic modifications and H3K4 methylation. We also deliberate on the mechanisms responsible, the consequences for the refinement of PARP inhibitor use in the clinic, and future possibilities to circumvent drug resistance in DNA-repair deficient cancers.

H3K4 methylation by SETD1A/BOD1L facilitates RIF1-dependent NHEJ.

The 53BP1-RIF1-shieldin pathway maintains genome stability by suppressing nucleolytic degradation of DNA ends at double-strand breaks (DSBs). Although RIF1 interacts with damaged chromatin via phospho-53BP1 and facilitates recruitment of the shieldin complex to DSBs, it is unclear whether other regulatory cues contribute to this response. Here, we implicate methylation of histone H3 at lysine 4 by SETD1A-BOD1L in the recruitment of RIF1 to DSBs. Compromising SETD1A or BOD1L expression or deregulating H3K4 methylation allows uncontrolled resection of DNA ends, impairs end-joining of dysfunctional telomeres, and abrogates class switch recombination. Moreover, defects in RIF1 localization to DSBs are evident in patient cells bearing loss-of-function mutations in SETD1A. Loss of SETD1A-dependent RIF1 recruitment in BRCA1-deficient cells restores homologous recombination and leads to resistance to poly(ADP-ribose)polymerase inhibition, reinforcing the clinical relevance of these observations. Mechanistically, RIF1 binds directly to methylated H3K4, facilitating its recruitment to, or stabilization at, DSBs.

MYBL2 amplification in breast cancer: Molecular mechanisms and therapeutic potential.

The MYBL2 gene, also known as B-MYB, is essential to regulate vital cellular processes including cell proliferation, differentiation and DNA repair. Changes in these pathways can facilitate cancer development and as such targeting these processes represent an effective method to treat multiple cancer types. Alterations in gene expression have been identified in cancer cells including changes in MYBL2, which appears to be of particular significance in breast cancer (BC) patients. Upregulation of MYBL2 in BC can occur via multiple mechanisms, including changes in regulation by micro RNAs, amplification of the 20q13 gene coding region and single nucleotide polymorphisms in the MYBL2 gene itself or associated genes. Evidence from multiple studies suggests MYBL2 expression could be used as a biomarker for disease severity in BC patients, which could identify those who require a more targeted treatment approach to prevent disease recurrence. In fact, high MYBL2 expression correlates with BC metastasis, worse relapse free survival and shorter overall survival, providing strong evidence that upregulation of MYBL2 functions contributes to more aggressive disease. This review summarises the significance of amplified MYBL2 expression to the development and pathogenesis of BC and suggests ways to target this multifunctional protein as an effective treatment to prevent disease recurrence.

On your marks, get SET(D1A): the race to protect stalled replication forks.

We recently identified that methylation of lysine 4 of histone H3 (H3K4) by SETD1A (SET domain containing 1A) maintains genome stability by protecting newly-replicated DNA from degradation. Mechanistically, SETD1A-dependent histone methylation regulates nucleosome mobilisation by FANCD2 (FA complementation group D2), a crucial step in maintaining genome integrity with important implications in chemo-sensitivity.

MYBL2 Supports DNA Double Strand Break Repair in Hematopoietic Stem Cells.

Myelodysplastic syndromes (MDS) are a heterogeneous group of diseases characterized by blood cytopenias that occur as a result of somatic mutations in hematopoietic stem cells (HSC). MDS leads to ineffective hematopoiesis, and as many as 30% of patients progress to acute myeloid leukemia (AML). The mechanisms by which mutations accumulate in HSC during aging remain poorly understood. Here we identify a novel role for MYBL2 in DNA double-strand break (DSB) repair in HSC. In patients with MDS, low MYBL2 levels associated with and preceded transcriptional deregulation of DNA repair genes. Stem/progenitor cells from these patients display dysfunctional DSB repair kinetics after exposure to ionizing radiation (IR). Haploinsufficiency of Mybl2 in mice also led to a defect in the repair of DSBs induced by IR in HSC and was characterized by unsustained phosphorylation of the ATM substrate KAP1 and telomere fragility. Our study identifies MYBL2 as a crucial regulator of DSB repair and identifies MYBL2 expression levels as a potential biomarker to predict cellular response to genotoxic treatments in MDS and to identify patients with defects in DNA repair. Such patients with worse prognosis may require a different therapeutic regimen to prevent progression to AML.Significance: These findings suggest MYBL2 levels may be used as a biological biomarker to determine the DNA repair capacity of hematopoietic stem cells from patients with MDS and as a clinical biomarker to inform decisions regarding patient selection for treatments that target DNA repair.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/20/5767/F1.large.jpg Cancer Res; 78(20); 5767-79. ©2018 AACR.

Differential expression of CD148 on leukocyte subsets in inflammatory arthritis.

INTRODUCTION: Monocytic cells play a central role in the aetiology of rheumatoid arthritis, and manipulation of the activation of these cells is an approach currently under investigation to discover new therapies for this and associated diseases. CD148 is a transmembrane tyrosine phosphatase that is highly expressed in monocytes and macrophages and, since this family of molecules plays an important role in the regulation of cell activity, CD148 is a potential target for the manipulation of macrophage activation. For any molecule to be considered a therapeutic target, it is important for it to be increased in activity or expression during disease. METHODS: We have investigated the expression of CD148 in two murine models of arthritis and in joints from rheumatoid arthritis (RA) patients using real-time PCR, immunohistochemistry, and studied the effects of proinflammatory stimuli on CD148 activity using biochemical assays. RESULTS: We report that CD148 mRNA is upregulated in diseased joints of mice with collagen-induced arthritis. Furthermore, we report that in mice CD148 protein is highly expressed in infiltrating monocytes of diseased joints, with a small fraction of T cells also expressing CD148. In human arthritic joints both T cells and monocytes expressed high levels of CD148, however, we show differential expression of CD148 in T cells and monocytes from normal human peripheral blood compared to peripheral blood from RA and both normal and RA synovial fluid. Finally, we show that synovial fluid from rheumatoid arthritis patients suppresses CD148 phosphatase activity. CONCLUSIONS: CD148 is upregulated in macrophages and T cells in human RA samples, and its activity is enhanced by treatment with tumour necrosis factor alpha (TNFα), and reduced by synovial fluid or oxidising conditions. A greater understanding of the role of CD148 in chronic inflammation may lead to alternative therapeutic approaches to these diseases.

Does oxidative inactivation of CD45 phosphatase in rheumatoid arthritis underlie immune hyporesponsiveness?

The protein tyrosine phosphatase (PTP) CD45 is critical in regulating the earliest steps in T-cell-receptor signaling but, similar to all PTPs, it is susceptible to oxidative inactivation. Given the widely reported effects of oxidant damage associated with rheumatoid arthritis (RA), we examined whether CD45 phosphatase activity was altered in CD4(+) T cells from RA patients and related this to CD4(+) T-cell function and redox status. CD45 phosphatase specific activity in T cells from RA peripheral blood (PB) and synovial fluid was 56% and 59% lower than in healthy control (HC) PB, respectively. In contrast, CD45 activity in T cells from disease controls (DSC) was not significantly different from HC. Both reduced glutathione (GSH) (p<0.001) and oxidized glutathione (GSSG) (p<0.05) were significantly lower in RA PB T cells compared with HC PB T cells. Treatment of RA PB T cells with the GSH precursor N-acetyl cysteine increased CD45 phosphatase activity and proliferation, while it decreased Lck kinase phosphorylation, which is regulated by CD45. Our observations lead to the hypothesis that the largely reversible oxidative inactivation of the CD45 phosphatase may underlie the decreased signaling efficiency and functional responsiveness which are characteristic of RA PB CD4(+) T cells.