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 G₁ 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. Cancer Res; 72(24); 6414–24. ©2012 AACR.

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. Cancer Res; 74(6); 1739–51. ©2014 AACR.

Background The EVI1 gene (3q26) codes for a zinc finger transcription factor with important roles in both mammalian development and leukemogenesis. Over-expression of EVI1 through either 3q26 rearrangements, MLL fusions, or other unknown mechanisms confers a poor prognosis in acute myeloid leukemia. Design and Methods We analyzed the prevalence and prognostic impact of EVI1 over-expression in a series of 476 patients with acute myeloid leukemia, and investigated the epigenetic modifications of the EVI1 locus which could be involved in the transcriptional regulation of this gene. Results Our data provide further evidence that EVI1 over-expression is a poor prognostic marker in acute myeloid leukemia patients less than 65 years old. Moreover, we found that patients with no basal expression of EVI1 had a better prognosis than patients with expression/over-expression (P=0.036). We also showed that cell lines with over-expression of EVI1 had no DNA methylation in the promoter region of the EVI1 locus, and had marks of active histone modifications: H3 and H4 acetylation, and trimethylation of histone H3 lysine 4. Conversely, cell lines with no expression of EVI1 have DNA hypermethylation and are marked by repressive trimethylation of histone H3 lysine 27 at the EVI1 promoter. Conclusions Our results identify EVI1 over-expression as a poor prognostic marker in a large, independent cohort of acute myeloid leukemia patients less than 65 years old, and show that the total absence of EVI1 expression has a prognostic impact on the outcome of such patients. Furthermore, we demonstrated for the first time that an aberrant epigenetic pattern involving DNA methylation, H3 and H4 acetylation, and trimethylation of histone H3 lysine 4 and histone H3 lysine 27 might play a role in the transcriptional regulation of EVI1 in acute myeloid leukemia. This study opens new avenues for a better understanding of the regulation of EVI1 expression at a transcriptional level.

Risk of developing myelodysplastic syndromes (MDS) is significantly increased in both multiple myeloma (MM) and MGUS, suggesting that is therapy independent. However, the incidence and sequelae of dysplastic hematopoiesis at diagnosis are unknown. Here, we used multidimensional flow cytometry (MFC) to prospectively screen for presence of MDS-associated phenotypic alterations (MDS-PA) in the bone marrow of 285 MM patients enrolled in the PETHEMA/GEM2012MENOS65 trial (NCT01916252), and investigated the clinical significance of monocytic MDS-PA in a larger series of 1,252 patients enrolled in four PETHEMA/GEM protocols. At diagnosis, 33/285 (11.6%) cases displayed MDS-PA. Bulk- and single-cell targeted sequencing of MDS recurrently mutated genes in CD34+ progenitors (and dysplastic lineages) from 67 patients unveiled clonal hematopoiesis in 13/26 (50%) cases with MDS-PA versus 9/41 (22%) without MDS-PA; TET2 and NRAS were the most frequently mutated genes. Dynamics of MDS-PA at diagnosis and after autologous transplant were evaluated in 86/285 patients, and showed that in most cases (69/86, 80%) MDS-PA either persisted or remained absent in patients with or without MDS-PA at diagnosis, respectively. Noteworthy, MDS-associated mutations unfrequently emerged after high-dose therapy. Based on MFC profiling, we found that patients with MDS-PA have altered hematopoiesis and Treg distribution in the tumor microenvironment. Importantly, presence of monocytic MDS-PA at diagnosis anticipated greater risk of hematological toxicity and was independently associated with inferior progression-free (HR:1.5, P=.02) and overall survival (HR:1.7, P=.01). This study unveils the biological and clinical significance of dysplastic hematopoiesis in newly-diagnosed MM, which can be screened with moderate sensitivity using cost-effective MFC.