Abstract 1095: Exploiting altered methionine metabolism to overcome treatment resistance in glioblastoma

Glioblastoma (GBM) is the most aggressive adult brain tumor and is uniformly fatal due to resistance to standard therapies such as radiation (RT) and chemotherapy. Our group and others have identified altered metabolism as a key mediator of GBM RT resistance. Methionine is an essential sulfur-containing amino acid that cells use to synthesize antioxidants, polyamines and S-adenosyl methionine (SAM), which drives intracellular methylation reactions. Methionine uptake is dramatically elevated in GBM compared to normal brain, but what GBMs use this methionine for, and whether it governs GBM treatment resistance, is unknown. Here, we find that RT acutely increases the levels of numerous methionine-related metabolites in multiple RT-resistant GBM models. To interrogate metabolic pathway activity, we used 13C5 methionine stable isotope tracing to show that GBMs respond to RT by activating the conversion of methionine to SAM, which is dependent on signaling through the DNA damage response. We developed in vivo methionine stable isotope tracing techniques to confirm these findings in orthotopic PDX models of GBM. Blocking the conversion of methionine to SAM, through pharmacologic inhibition of methionine adenosyltransferase 2A (MAT2A), slowed the repair of RT-induced DNA damage and increased cell death in GBM models following RT. These effects were especially pronounced in GBM models lacking the methionine salvage enzyme methylthioadenosine phosphorylase (MTAP). Pharmacologic inhibition of MAT2A in flank and orthotopic in vivo GBM models depleted SAM levels and slowed tumor growth when combined with RT. Combining MAT2A inhibition with dietary methionine restriction and RT slowed GBM tumor growth even further. Together, our work demonstrates a new signaling link between DNA damage and methionine-driven SAM synthesis in GBM. Inhibiting SAM synthesis slows the repair of RT-induced DNA damage and augments RT efficacy. This therapeutic strategy may be especially effective in GBMs defective in methionine salvage and spare normal cortex in which methionine salvage is active.

Citation Format: Navyateja Korimerla, Kari-Wilder Romans, Peter Kalev, Ayesha Kothari, Nathan Qi, Charles Evans, Maureen Kachman, Marc L Hyer, Katya Marjon, Taryn Sleger, Daniel R Wahl. Exploiting altered methionine metabolism to overcome treatment resistance in glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1095.

Discovery of AG-270, a First-in-Class Oral MAT2A Inhibitor for the Treatment of Tumors with Homozygous MTAP Deletion

The metabolic enzyme methionine adenosyltransferase 2A (MAT2A) was recently implicated as a synthetic lethal target in cancers with deletion of the methylthioadenosine phosphorylase (MTAP) gene, which is adjacent to the CDKN2A tumor suppressor and codeleted with CDKN2A in approximately 15% of all cancers. Previous attempts to target MAT2A with small-molecule inhibitors identified cellular adaptations that blunted their efficacy. Here, we report the discovery of highly potent, selective, orally bioavailable MAT2A inhibitors that overcome these challenges. Fragment screening followed by iterative structure-guided design enabled >10 000-fold improvement in potency of a family of allosteric MAT2A inhibitors that are substrate noncompetitive and inhibit release of the product, S-adenosyl methionine (SAM), from the enzyme's active site. We demonstrate that potent MAT2A inhibitors substantially reduce SAM levels in cancer cells and selectively block proliferation of MTAP-null cells both in tissue culture and xenograft tumors. These data supported progressing AG-270 into current clinical studies (ClinicalTrials.gov NCT03435250).

Cancer Dependencies: PRMT5 and MAT2A in MTAP/p16-Deleted Cancers

Discovery of targeted therapies that selectively exploit the genetic inactivation of specific tumor suppressors remains a major challenge. This includes the prevalent deletion of the CDKN2A/ MTAP locus, which was first reported nearly 40 years ago. The more recent advent of RNA interference and functional genomic screening technologies led to the identification of hidden collateral lethalities occurring with passenger deletions of MTAP in cancer cells. In particular, small-molecule inhibition of the type II arginine methyltransferase PRMT5 and the S-adenosylmethionine-producing enzyme MAT2A each presents a precision medicine approach for the treatment of patients whose tumors have homozygous loss of MTAP. In this review, we highlight key aspects of MTAP, PRMT5, and MAT2A biology to provide a conceptual framework for developing novel therapeutic strategies in tumors with MTAP deletion and to summarize ongoing efforts to drug PRMT5 and MAT2A.

MAT2A Inhibition Blocks the Growth of MTAP-Deleted Cancer Cells by Reducing PRMT5-Dependent mRNA Splicing and Inducing DNA Damage

The methylthioadenosine phosphorylase (MTAP) gene is located adjacent to the cyclin-dependent kinase inhibitor 2A (CDKN2A) tumor-suppressor gene and is co-deleted with CDKN2A in approximately 15% of all cancers. This co-deletion leads to aggressive tumors with poor prognosis that lack effective, molecularly targeted therapies. The metabolic enzyme methionine adenosyltransferase 2α (MAT2A) was identified as a synthetic lethal target in MTAP-deleted cancers. We report the characterization of potent MAT2A inhibitors that substantially reduce levels of S-adenosylmethionine (SAM) and demonstrate antiproliferative activity in MTAP-deleted cancer cells and tumors. Using RNA sequencing and proteomics, we demonstrate that MAT2A inhibition is mechanistically linked to reduced protein arginine methyltransferase 5 (PRMT5) activity and splicing perturbations. We further show that DNA damage and mitotic defects ensue upon MAT2A inhibition in HCT116 MTAP-/- cells, providing a rationale for combining the MAT2A clinical candidate AG-270 with antimitotic taxanes.

Abstract 3090: The MAT2A inhibitor, AG-270, combines with both taxanes and gemcitabine to yield enhanced anti-tumor activity in patient-derived xenograft models

MAT2A (methionine adenosyltransferase 2 alpha) is a critical enzyme within the methionine salvage pathway responsible for generating the universal methyl group donor, S-adenosyl methionine (SAM). We have developed a first-in-class small molecule inhibitor of MAT2A, AG-270, currently in a phase 1 clinical study (ClinicalTrials.gov NCT03435250) for the treatment of patients with solid tumors or lymphomas with MTAP (methylthioadenosine phosphorylase) deletion. The MTAP gene is deleted in approximately 15% of all human cancers, including non-small cell lung cancer (NSCLC; ~15-25%), pancreatic (~25%) and esophageal (~30%) cancer, and glioblastoma (~50%). To prioritize candidate combination partners for AG-270, a cell-based in vitro screening approach was employed using MTAP-null cell lines, in which AG-270 was combined with standard-of-care (SOC) agents as well as agents targeting pathways with hypothesized mechanistic links to MAT2A. Some of the best performing enhancers from this screen included paclitaxel (and docetaxel, using orthogonal screens) and gemcitabine. To assess the robustness of these combination findings in clinically relevant in vivo models, a series of patient-derived xenograft (PDX) experiments was undertaken to evaluate tolerability and efficacy in mice. Results demonstrated that AG-270, when combined with taxanes (paclitaxel/docetaxel) or gemcitabine, was well tolerated using SOC plasma exposures less than or equal to those achieved in patients. Importantly, combining AG-270 with taxanes and gemcitabine yielded additive-to-synergistic anti-tumor activity, with the docetaxel combination yielding 50% complete tumor regressions (CRs) in 2-3 PDX models. To study the mechanism of action, MAT2A was inhibited in vitro within HCT-116 MTAP −/− and wild-type cells, and we observed RNA splicing changes (via detained introns) altering genes involved in cell cycle regulation and DNA damage response, with a more pronounced effect found in the MTAP −/− genetic setting. Moreover, detained introns involving these same two pathways were modulated in MTAP −/− NSCLC PDX models treated with AG-270. Taken together, these data suggest AG-270 complements the known mechanism of action of taxanes and gemcitabine, and leads to enhanced DNA damage and inhibition of cellular proliferation. This work has helped identify a therapeutic strategy of combining AG-270 with taxanes and gemcitabine, which is currently being explored in an ongoing phase 1 clinical trial (NCT03435250).

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Citation Format: Marc L. Hyer, Peter Kalev, Mark Fletcher, Chi-Chao Chen, Elia Aguado-Fraile, Everton Mandley, Sheila Newhouse, Max Lein, Raj Nagaraja, Yesim Tuncay, Josh Murtie, Kevin M. Marks, Katya Marjon. The MAT2A inhibitor, AG-270, combines with both taxanes and gemcitabine to yield enhanced anti-tumor activity in patient-derived xenograft models [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3090.

Abstract 3510: IDH1-R132H tumor cells are not robustly sensitive to PARP inhibition in a 2-HG dependent manner

Mutations in the metabolic enzymes isocitrate dehydrogenase (IDH) 1 or 2 arise in a variety of malignancies and lead to the production of the oncometabolite (D)-2-hydroxyglutarate (2-HG). The recent approval by the FDA of mutant IDH1 and IDH2 (mIDH1/2) inhibitors in patients with mIDH1/2 relapsed/refractory acute myeloid leukemia (AML) underscores the clinical benefit of blocking production of 2-HG. Parallel investigations have indicated that IDH1/2 mutation leads to a ‘BRCAness’ phenotype and preferential sensitivity to PARP inhibition, ascribed to increased basal DNA damage and reduced capability of DNA damage repair. Significantly, these effects, and sensitivity to PARP inhibition, have been proposed to be 2-HG-dependent and thus potentially antagonistic with inhibition of mIDH1/2. In an effort to better understand the potential use of PARP inhibitors in mIDH cancers, we examined the effect of IDH mutations on the induction of DNA damage and regulation of DNA repair in IDH1-R132H mutant and wild type cell lines. We analyzed the levels of DNA damage by visualization of γH2AX by immunofluorescence and western blot and found, in agreement with previous work, that the presence of mutations in IDH1 correlates with higher basal DNA damage levels. However, in contrast to previous observations, we found that the reduction of 2-HG by treatment with mIDH1 inhibitors in vitro does not reverse this phenotype. Additionally, we observed that the presence of an IDH1 mutation provides reduction in homologous recombination efficiency, though not to the same extent as a cell lacking BRCA2. Furthermore, we failed to detect a heightened sensitivity to PARP inhibition alone in mIDH1 cells in cell-based assays as compared with the reduced viability observed for cells lacking BRCA2 when treated with PARP inhibitors. Similarly, we failed to observe any tumor growth inhibition in mIDH1 mouse xenografts treated with a PARP inhibitor. To account for this disconnect we examined large, publicly-available data sets to look for a ‘BRCAness’ genomic signature in AML, low grade glioma, and cholangiocarcinoma - all tumor types with a prevalence of IDH1 mutations. While we found a very minor fraction (<3%) of low-grade glioma carrying a mutational signature associated with DNA mismatch repair activity, we failed to find a ‘BRCAness’ signature in any of the three primary mIDH1 indications. Taken together, our data demonstrate that, while mIDH1 cells have increased basal levels of DNA damage, the reduction of 2-HG by selective mIDH1 inhibitors does not reduce the basal DNA repair deficiency in mIDH1 cells. Furthermore, the increase in DNA damage found in mIDH1 cells does not lead to heightened sensitivity to PARP inhibition in vivo.

Citation Format: Elia Aguado-Fraile, Christine Hudson, Taryn Sleger, Sebastien Ronseaux, Rohini Narayanaswamy, Sung Choe, Bin Wu, Peter Kalev, Brandon Nicolay. IDH1-R132H tumor cells are not robustly sensitive to PARP inhibition in a 2-HG dependent manner [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3510.

Abstract 2714: Targeting MAT2A in CDKN2A/MTAP-deleted cancers

While deletions of the p16/CDKN2A tumor suppressor were first discovered more than 30 years ago, therapeutics that selectively target such tumors have proven elusive. Recent work utilizing functional genomics has identified a synthetic lethal vulnerability that arises due to co-deletion of the adjacent metabolic gene, methylthioadenosine phosphorylase (MTAP). Loss of MTAP in these tumors leads to an accumulation of MTAP substrate 5'-methylthioadenosine (MTA), which partially inhibits the arginine methyltransferase PRMT5 and sensitizes tumors to shRNA-mediated depletion of PRMT5 and the upstream metabolic enzyme, methionine adenosyltransferase 2 alpha (MAT2A). To investigate the therapeutic potential of this finding, we utilized a biophysical binding screen followed by iterative structure-guided design to make the first highly potent, selective, and orally bioavailable inhibitors of MAT2A. MAT2A inhibitor treatment leads to potent inhibition of the growth of HCT116 MTAP-/- cells while sparing isogenic HCT116 MTAP+/+ cells. Tumor xenograft studies similarly demonstrated MTAP-selective growth inhibition in HCT116 MTAP-/- tumors compared to isogenic HCT116 MTAP+/+ tumors. Further, MTAP-deletion correlated with MAT2A inhibitor efficacy across a panel of >300 cell lines in vitro, and MAT2A inhibitor treatment was efficacious in a variety of MTAP-deleted patient-derived xenografts in vivo. Having demonstrated that potent MAT2A inhibitors selectively block the proliferation of MTAP-deleted cells and tumors, we sought to investigate the mechanism by which these effects arise. Using methylation proteomics we noted that MAT2A inhibitor treatment leads to selective inhibition of PRMT5 methylation activity in MTAP-deleted cancers in vitro and in vivo. RNA-seq analyses revealed that MAT2A inhibition leads to substantial defects in RNA splicing in MTAP-deleted cancers, consistent with published findings that PRMT5-mediated methylation of splicing complex proteins is critical for their function. MAT2A inhibitor treatment led to a substantial increase in detained introns, which were enriched in genes involved in cell cycle regulation and DNA damage response, thus implicating dysregulated splicing in the antiproliferative effects of MAT2A inhibition in MTAP-deleted cancer cells. Furthermore, we demonstrated substantial drug-drug synergy between MAT2A inhibitors and select agents inhibiting cell cycle progression or DNA repair. Importantly we validated key combination findings in vivo, including demonstration of synergy with the MAT2A inhibitor AG-270 and anti-mitotic taxanes. AG-270 is the first MAT2A inhibitor to enter clinical development and is under investigation in a Phase I trial that is currently enrolling patients with MTAP-deleted solid tumors (NCT03435250). Our findings suggest clinically-applicable combination strategies which may further enhance the efficacy of AG-270 in malignancies with this genetic lesion.

Citation Format: Katya Marjon, Peter Kalev, Marc Hyer, Mark Fletcher, Peili Zhang, Elia Aguado-Fraile, Everton Mandley, Zenon Konteatis, Jeremy Travins, Kevin Marks. Targeting MAT2A in CDKN2A/MTAP-deleted cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2714.

EWS/FLI Confers Tumor Cell Synthetic Lethality to CDK12 Inhibition in Ewing Sarcoma

Many cancer types are driven by oncogenic transcription factors that have been difficult to drug. Transcriptional inhibitors, however, may offer inroads into targeting these cancers. Through chemical genomics screening, we identified that Ewing sarcoma is a disease with preferential sensitivity to THZ1, a covalent small-molecule CDK7/12/13 inhibitor. The selective CDK12/13 inhibitor, THZ531, impairs DNA damage repair in an EWS/FLI-dependent manner, supporting a synthetic lethal relationship between response to THZ1/THZ531 and EWS/FLI expression. The combination of these molecules with PARP inhibitors showed striking synergy in cell viability and DNA damage assays in vitro and in multiple models of Ewing sarcoma, including a PDX, in vivo without hematopoietic toxicity.

Abstract 1118: Synthetic lethality of CDK12 inhibition in tumors with EWS/FLI rearrangements

THZ1 is a potent, covalent inhibitor of the transcriptional CDKs, CDK7/12/13. Chemical genomic profiling of THZ1 across >1,000 diverse cancer cell lines revealed that EWS/FLI- rearranged Ewing sarcoma cells were remarkably sensitive to this molecule. We demonstrated that THZ1 inhibits the phosphorylation of the C-terminal domain of RNA Polymerase II, decreased colony formation capacity, and induced apoptosis in a dose-dependent manner in Ewing sarcoma cell lines. Using selective CDK7 and CDK12/13 inhibitors, we revealed that the primary target of THZ1 in Ewing sarcoma is CDK12/13. Genetic suppression of CDK12, but not CDK13, induced strong anti-viability effects, confirming CDK12 as the primary target. Treatment of Ewing sarcoma cell lines with THZ531, a novel CDK12/13 selective inhibitor, preferentially repressed genes involved in DNA damage repair. Additionally, suppression of EWS/FLI rendered Ewing sarcoma cells resistant to THZ531 and partially rescued the anti-viability effects of CDK12 knockdown. These results suggest that EWS/FLI imparts vulnerability to DNA damage repair inhibition and implicate a synthetic lethal relationship between the tumor-specific expression of EWS/FLI and CDK12 inhibition. Furthermore, we demonstrated that CDK12 and PARP inhibitors are highly synergistic in vitro, inducing widespread yH2AX foci formation. Interestingly, THZ531 impairs the ability of the PARP inhibitor, olaparib, to induce RAD51 foci formation, suggesting that THZ531 specifically causes a defect in homologous recombination repair. Moreover, we observed striking synergy of THZ1 and olaparib in two mouse models of Ewing sarcoma with limited toxicity observed. These findings have important translational significance as clinical trials with PARP inhibitors as single agents in Ewing sarcoma failed to demonstrate efficacy, highlighting the need to identify combination therapies that will enhance the activity of PARP inhibition. We anticipate that CDK12 and PARP inhibitor combinations will be of therapeutic interest in other ETS-rearranged tumors, as well as tumors with defects in DNA repair.

Citation Format: Amanda L. Balboni, Bjorn Stolte, Amy Saur Conway, Gabriela Alexe, Emily Jue Wang, Nicholas Kwiatkowski, Tinghu Zhang, Brian J. Abraham, Peter Kalev, Dipanjan Chowdhury, Cyril H. Benes, Richard A. Young, Nathanael S. Gray, Kimberly Stegmaier. Synthetic lethality of CDK12 inhibition in tumors with EWS/FLI rearrangements [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1118. doi:10.1158/1538-7445.AM2017-1118

p53 induces formation of NEAT1 lncRNA-containing paraspeckles that modulate replication stress response and chemosensitivity

In a search for mediators of the p53 tumor suppressor pathway, which induces pleiotropic and often antagonistic cellular responses, we identified the long noncoding RNA (lncRNA) NEAT1. NEAT1 is an essential architectural component of paraspeckle nuclear bodies, whose pathophysiological relevance remains unclear. Activation of p53, pharmacologically or by oncogene-induced replication stress, stimulated the formation of paraspeckles in mouse and human cells. Silencing Neat1 expression in mice, which prevents paraspeckle formation, sensitized preneoplastic cells to DNA-damage-induced cell death and impaired skin tumorigenesis. We provide mechanistic evidence that NEAT1 promotes ATR signaling in response to replication stress and is thereby engaged in a negative feedback loop that attenuates oncogene-dependent activation of p53. NEAT1 targeting in established human cancer cell lines induced synthetic lethality with genotoxic chemotherapeutics, including PARP inhibitors, and nongenotoxic activation of p53. This study establishes a key genetic link between NEAT1 paraspeckles, p53 biology and tumorigenesis and identifies NEAT1 as a promising target to enhance sensitivity of cancer cells to both chemotherapy and p53 reactivation therapy.

Abstract 2441: CDK12/13 inhibition cooperates with the Ewing sarcoma oncoprotein EWS/FLI to attenuate homologous recombination repair in Ewing sarcoma cells

A therapeutic challenge in pediatric oncology is the paucity of readily “druggable” genetic events in many of the childhood malignancies. These tumors are frequently defined by sentinel abnormalities involving transcription factors in an otherwise quiet genomic landscape. One approach to treating these tumors would involve direct targeting of the aberrant transcription factor; however, this is a drug discovery challenge. A second approach would be to identify synthetic lethal relationships in the context of the aberrant transcription factor. THZ1, a covalent and potent inhibitor of CDK7, CDK12, and CDK13, kinases involved in transcriptional regulation, recently emerged as a targeted strategy to impair aberrant transcription. Extensive profiling of THZ1 against a diverse panel of >1,000 cancer cell lines revealed that the pediatric solid tumor, Ewing sarcoma, was exceptionally sensitive to this compound. We found that the anti-proliferative effects of THZ1 in Ewing sarcoma can be attributed primarily to CDK12/13 inhibition. Treatment of Ewing sarcoma cells with THZ531, a covalent and selective CDK12/13 inhibitor, decreased the phosphorylation of the C-terminal domain of RNA polymerase II, induced apoptosis, and markedly decreased colony formation capacity of Ewing sarcoma cell lines. In contrast, treatment with a selective CDK7 inhibitor had minimal effect. EWS/FLI is the transcription factor fusion protein that typically drives tumor establishment and maintenance in Ewing sarcoma tumors. Based on prior reports of this compound class inhibiting a small subset of highly expressed genes critical to tumor maintenance, we expected that these inhibitors would disrupt oncogenic EWS/FLI-driven transcription as the mechanism of inducing cell death. Surprisingly, however, global gene expression profiling revealed that THZ531 did not selectively repress EWS/FLI or EWS/FLI target genes. Rather, we observed that THZ531 preferentially repressed genes involved in DNA damage repair. Consistent with this finding, we found that THZ531 induced defects in DNA damage repair and highly synergized with DNA damaging agents that induce lesions repaired by homologous recombination (HR). Furthermore, we found that suppression of EWS/FLI attenuated sensitivity to THZ531 and the PARP inhibitor olaparib and abrogated synergy observed with this drug combination. Thus, we conclude that EWS/FLI establishes tumor cell synthetic lethality to CDK12/13 inhibitors by imparting sensitivity to DNA repair defects. This work establishes a novel mechanism of action of CDK12/13 inhibitors and gives further credence to the role of EWS/FLI in DNA damage response. Ongoing work is dedicated to the in vivo testing of THZ1 alone, and in combination with olaparib, as a novel targeted therapy for the treatment of Ewing sarcoma.

Citation Format: Amanda Balboni, Björn Stolte, Peter Kalev, Nicholas Kwiatkowski, Tinghu Zhang, Brian Abraham, Gabriela Alexe, Dipanjan Chowdhury, Richard A. Young, Nathanael S. Gray, Kimberly Stegmaier. CDK12/13 inhibition cooperates with the Ewing sarcoma oncoprotein EWS/FLI to attenuate homologous recombination repair in Ewing sarcoma cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2441.

Dephosphorylation enables the recruitment of 53BP1 to double-strand DNA breaks

Excluding 53BP1 from chromatin is required to attenuate the DNA damage response during mitosis, yet the functional relevance and regulation of this exclusion are unclear. Here we show that 53BP1 is phosphorylated during mitosis on two residues, T1609 and S1618, located in its well-conserved ubiquitination-dependent recruitment (UDR) motif. Phosphorylating these sites blocks the interaction of the UDR motif with mononuclesomes containing ubiquitinated histone H2A and impedes binding of 53BP1 to mitotic chromatin. Ectopic recruitment of 53BP1-T1609A/S1618A to mitotic DNA lesions was associated with significant mitotic defects that could be reversed by inhibiting nonhomologous end-joining. We also reveal that protein phosphatase complex PP4C/R3β dephosphorylates T1609 and S1618 to allow the recruitment of 53BP1 to chromatin in G1 phase. Our results identify key sites of 53BP1 phosphorylation during mitosis, identify the counteracting phosphatase complex that restores the potential for DDR during interphase, and establish the physiological importance of this regulation.

p16INK4a impairs homologous recombination-mediated DNA repair in human papillomavirus-positive head and neck tumors

The p16INK4a protein is a principal cyclin-dependent kinase inhibitor that decelerates the cell cycle. Abnormally high levels of p16INK4a are commonly observed in human papillomavirus (HPV)-positive head and neck squamous cell carcinomas (HNSCC). We and others found that p16INK4a overexpression is associated with improved therapy response and survival of patients with HNSCC treated with radiotherapy. However, the functional role of p16INK4a in HNSCC remains unexplored. Our results implicate p16INK4a in regulation of homologous recombination-mediated DNA damage response independently from its role in control of the cell cycle. We found that expression of p16INK4a dramatically affects radiation sensitivity of HNSCC cells. p16INK4a overexpression impairs the recruitment of RAD51 to the site of DNA damage in HPV-positive cells by downregulating of cyclin D1 protein expression. Consistent with the in vitro findings, immunostaining of HNSCC patient samples revealed that high levels p16INK4a expression significantly correlated with decreased cyclin D1 expression. In summary, these findings reveal an unexpected function of p16INK4a in homologous recombination-mediated DNA repair response and imply p16INK4a status as an independent marker to predict response of patients with HNSCC to radiotherapy.

Loss of PPP2R2A inhibits homologous recombination DNA repair and predicts tumor sensitivity to PARP inhibition

Reversible phosphorylation plays a critical role in DNA repair. Here, we report the results of a loss-of-function screen that identifies the PP2A heterotrimeric serine/threonine phosphatases PPP2R2A, PPP2R2D, PPP2R5A, and PPP2R3C in double-strand break (DSB) repair. In particular, we found that PPP2R2A-containing complexes directly dephosphorylated ATM at S367, S1893, and S1981 to regulate its retention at DSB sites. Increased ATM phosphorylation triggered by PPP2R2A attenuation dramatically upregulated the activity of the downstream effector kinase CHK2, resulting in G(1) to S-phase cell-cycle arrest and downregulation of BRCA1 and RAD51. In tumor cells, blocking PPP2R2A thereby impaired the high-fidelity homologous recombination repair pathway and sensitized cells to small-molecule inhibitors of PARP. We found that PPP2R2A was commonly downregulated in non-small cell lung carcinomas, suggesting that PPP2R2A status may serve as a marker to predict therapeutic efficacy to PARP inhibition. In summary, our results deepen understanding of the role of PP2A family phosphatases in DNA repair and suggest PPP2R2A as a marker for PARP inhibitor responses in clinic.

Protein phosphatase 2A as a potential target for anticancer therapy

The kinase oncogenes are well-characterized drivers of cancer development, and several targeted therapies focused on both specific and selectively nonselective kinase inhibitors have now been approved for clinical use. In contrast, much less is known about the role of protein phosphatases, although modulation of their activities might form the foundation for an effective anti-cancer approach. The serine-threonine protein phosphatase 2A (PP2A) is implicated in the regulation of numerous signaling pathways and may function as a tumor suppressor. Recently pharmacological modulation of PP2A activity has been showed to have a potent anti-tumor activity in both in vitro and in vivo cancer models. These studies implicate PP2A as a promising therapeutic target for the treatment of cancer.

The complexity of double-strand break ends is a factor in the repair pathway choice

The repair of double-strand breaks in mammalian cells is carried out by two pathways: homologous recombination and nonhomologous end joining. The factors that regulate the mechanism through which a specific repair pathway is activated are still not clearly defined. To study whether the complexity of the double-strand break ends is a factor that determines the choice of the repair pathway, we examined the involvement of homologous recombination by the formation of Rad51 foci in human HeLa cells treated with bleomycin and ionizing radiation. The quantity of double-strand breaks was determined by gel electrophoresis and the formation of gamma-H2AX foci. Two hours after treatment with low doses of the agents that induced similar quantities of double-strand breaks that could be repaired effectively by the cells, Rad51 foci were observed only in the irradiated cells. Rad51 foci appeared in bleomycin-treated cells after prolonged exposure to the drug when the cells were arrested in the G2 phase of the cell cycle. Since bleomycin produces double-strand breaks that are less complex than the breaks induced by ionizing radiation, these results indicate that the complexity of the break ends is a factor in the choice of repair pathway and that homologous recombination is recruited in the repair of breaks with more complex multiply damaged ends during the late S and G2 phases of the cell cycle.