In response to DNA replication stress, DNA damage signaling kinases inhibit origin firing and promote the remodeling and stabilization of replication forks, leading to a systemic reduction in DNA synthesis that protects genomic integrity. Little is understood about the regulatory mechanisms of replication stress recovery, including the mechanisms involved in the restart of stalled replication forks. Here, we identify the oncogenic phosphatase PPM1D/WIP1 as a critical regulator of replication fork restart. Upon recovery from replication stress, PPM1D prevents excessive MRE11- and DNA2-dependent nucleolytic degradation of stalled forks. Loss of PPM1D function leads to defects in RAD51 recruitment to chromatin and impairs RAD51-dependent fork restart. Phosphoproteomic analysis reveals that PPM1D regulates a network of ATM substrates, several of which are phosphorylated at an S/T-Q-(E/D)n motif. Strikingly, inhibition of ATM suppresses the deleterious consequences of impaired PPM1D function at replication forks, enabling timely fork restart. The dominant effect of ATM hyper-signaling in suppressing fork restart occurs, in part, through the excessive engagement of 53BP1 and consequent RAD51 antagonization. These findings uncover a new mode of ATM signaling responding to fork stalling and highlights the need for PPM1D to restrain ATM signaling and enable proper fork restart.

• Publication type
• Journal Article MeSH
• Preprint MeSH

Cell cycle checkpoints, oncogene-induced senescence and programmed cell death represent intrinsic barriers to tumorigenesis. Protein phosphatase magnesium-dependent 1 (PPM1D) is a negative regulator of the tumour suppressor p53 and has been implicated in termination of the DNA damage response. Here, we addressed the consequences of increased PPM1D activity resulting from the gain-of-function truncating mutations in exon 6 of the PPM1D. We show that while control cells permanently exit the cell cycle and reside in senescence in the presence of DNA damage caused by ionising radiation or replication stress induced by the active RAS oncogene, RPE1-hTERT and BJ-hTERT cells carrying the truncated PPM1D continue proliferation in the presence of DNA damage, form micronuclei and accumulate genomic rearrangements revealed by karyotyping. Further, we show that increased PPM1D activity promotes cell growth in the soft agar and formation of tumours in xenograft models. Finally, expression profiling of the transformed clones revealed dysregulation of several oncogenic and tumour suppressor pathways. Our data support the oncogenic potential of PPM1D in the context of exposure to ionising radiation and oncogene-induced replication stress.

• MeSH
• Cell Death genetics   MeSH
• Humans MeSH
• Mice MeSH
• Cell Transformation, Neoplastic * genetics   MeSH
• Tumor Suppressor Protein p53 genetics   metabolism   MeSH
• DNA Damage * genetics   MeSH
• Cell Proliferation genetics   MeSH
• Protein Phosphatase 2C * genetics   metabolism   MeSH
• Phosphoprotein Phosphatases genetics   metabolism   MeSH
• Cellular Senescence * genetics   MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Tumor Suppressor Protein p53 MeSH
• PPM1D protein, human MeSH Browser
• Protein Phosphatase 2C * MeSH
• Phosphoprotein Phosphatases MeSH

Oncogene-induced replication stress has been recognized as a major cause of genome instability in cancer cells. Increased expression of cyclin E1 caused by amplification of the CCNE1 gene is a common cause of replication stress in various cancers. Protein phosphatase magnesium-dependent 1 delta (PPM1D) is a negative regulator of p53 and has been implicated in termination of the cell cycle checkpoint. Amplification of the PPM1D gene or frameshift mutations in its final exon promote tumorigenesis. Here, we show that PPM1D activity further increases the replication stress caused by overexpression of cyclin E1. In particular, we demonstrate that cells expressing a truncated mutant of PPM1D progress faster from G1 to S phase and fail to complete licensing of the replication origins. In addition, we show that transcription-replication collisions and replication fork slowing caused by CCNE1 overexpression are exaggerated in cells expressing the truncated PPM1D. Finally, replication speed and accumulation of focal DNA copy number alterations caused by induction of CCNE1 expression was rescued by pharmacological inhibition of PPM1D. We propose that increased activity of PPM1D suppresses the checkpoint function of p53 and thus promotes genome instability in cells expressing the CCNE1 oncogene.

• Keywords
• PPM1D phosphatase, cancer, cell cycle, cyclin E1, replication stress,
• MeSH
• Cyclin E genetics   metabolism   MeSH
• Humans MeSH
• Tumor Suppressor Protein p53 * genetics   metabolism   MeSH
• Neoplasms * MeSH
• Genomic Instability MeSH
• Protein Phosphatase 2C genetics   metabolism   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Cyclin E MeSH
• Tumor Suppressor Protein p53 * MeSH
• PPM1D protein, human MeSH Browser
• Protein Phosphatase 2C MeSH

Protein phosphatase magnesium-dependent 1 delta (PPM1D) terminates the cell cycle checkpoint by dephosphorylating the tumour suppressor protein p53. By targeting additional substrates at chromatin, PPM1D contributes to the control of DNA damage response and DNA repair. Using proximity biotinylation followed by proteomic analysis, we identified a novel interaction between PPM1D and the shelterin complex that protects telomeric DNA. In addition, confocal microscopy revealed that endogenous PPM1D localises at telomeres. Further, we found that ATR phosphorylated TRF2 at S410 after induction of DNA double strand breaks at telomeres and this modification increased after inhibition or loss of PPM1D. TRF2 phosphorylation stimulated its interaction with TIN2 both in vitro and at telomeres. Conversely, induced expression of PPM1D impaired localisation of TIN2 and TPP1 at telomeres. Finally, recruitment of the DNA repair factor 53BP1 to the telomeric breaks was strongly reduced after inhibition of PPM1D and was rescued by the expression of TRF2-S410A mutant. Our results suggest that TRF2 phosphorylation promotes the association of TIN2 within the shelterin complex and regulates DNA repair at telomeres.

• MeSH
• Phosphorylation MeSH
• Humans MeSH
• DNA Damage MeSH
• Telomeric Repeat Binding Protein 2 * MeSH
• Telomere-Binding Proteins * metabolism   MeSH
• Proteomics MeSH
• Shelterin Complex * MeSH
• Telomere metabolism   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Telomeric Repeat Binding Protein 2 * MeSH
• Telomere-Binding Proteins * MeSH
• Shelterin Complex * MeSH
• TINF2 protein, human MeSH Browser

Genome integrity is protected by the cell-cycle checkpoints that prevent cell proliferation in the presence of DNA damage and allow time for DNA repair. The transient checkpoint arrest together with cellular senescence represent an intrinsic barrier to tumorigenesis. Tumor suppressor p53 is an integral part of the checkpoints and its inactivating mutations promote cancer growth. Protein phosphatase magnesium-dependent 1 (PPM1D) is a negative regulator of p53. Although its loss impairs recovery from the G2 checkpoint and promotes induction of senescence, amplification of the PPM1D locus or gain-of-function truncating mutations of PPM1D occur in various cancers. Here we used a transgenic mouse model carrying a truncating mutation in exon 6 of PPM1D (Ppm1dT). As with human cell lines, we found that the truncated PPM1D was present at high levels in the mouse thymus. Truncated PPM1D did not affect differentiation of T-cells in the thymus but it impaired their response to ionizing radiation (IR). Thymocytes in Ppm1dT/+ mice did not arrest in the checkpoint and continued to proliferate despite the presence of DNA damage. In addition, we observed a decreased level of apoptosis in the thymi of Ppm1dT/+ mice. Moreover, the frequency of the IR-induced T-cell lymphomas increased in Ppm1dT/+Trp53+/- mice resulting in decreased survival. We conclude that truncated PPM1D partially suppresses the p53 pathway in the mouse thymus and potentiates tumor formation under the condition of a partial loss of p53 function.

• Keywords
• cancer, cell-cycle checkpoint, protein phosphatase, tumor suppressor p53,
• MeSH
• Apoptosis * MeSH
• Cell Cycle MeSH
• Radiation, Ionizing MeSH
• Lymphoma metabolism   MeSH
• Mice, Inbred C57BL MeSH
• Mice MeSH
• Tumor Suppressor Protein p53 metabolism   MeSH
• Neoplasms, Radiation-Induced metabolism   MeSH
• DNA Repair MeSH
• DNA Damage MeSH
• Cell Proliferation MeSH
• Protein Phosphatase 2C physiology   MeSH
• Thymocytes cytology   metabolism   MeSH
• Thymus Gland * cytology   metabolism   MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Tumor Suppressor Protein p53 MeSH
• Ppm1d protein, mouse MeSH Browser
• Protein Phosphatase 2C MeSH
• Trp53 protein, mouse MeSH Browser

The most common histological subtypes of cutaneous melanoma include superficial spreading and nodular melanoma. However, the spectrum of somatic mutations developed in those lesions and all potential druggable targets have not yet been fully elucidated. We present the results of a sequence capture NGS analysis of 114 primary nodular and superficial spreading melanomas identifying driver mutations using biostatistical, immunohistochemical and/or functional approach. The spectrum and frequency of pathogenic or likely pathogenic variants were identified across 54 evaluated genes, including 59 novel mutations, and the newly identified TP53 loss-of-function mutations p.(L194P) and p.(R280K). Frequently mutated genes most commonly affected the MAPK pathway, followed by chromatin remodeling, and cell cycle regulation. Frequent aberrations were also detected in the genes coding for proteins involved in DNA repair and the regulation and modification of cellular tight junctions. Furthermore, relatively frequent mutations were described in KDR and MET, which represent potential clinically important targets. Those results suggest that with the development of new therapeutic possibilities, not only BRAF testing, but complex molecular testing of cutaneous melanoma may become an integral part of the decision process concerning the treatment of patients with melanoma.

• MeSH
• Cell Cycle genetics   MeSH
• Adult MeSH
• Gene Frequency genetics   MeSH
• Genetic Predisposition to Disease genetics   MeSH
• Middle Aged MeSH
• Humans MeSH
• Melanoma, Cutaneous Malignant MeSH
• MAP Kinase Signaling System genetics   MeSH
• Melanoma genetics   pathology   MeSH
• Young Adult MeSH
• Loss of Function Mutation genetics   MeSH
• Biomarkers, Tumor genetics   MeSH
• Tumor Suppressor Protein p53 genetics   MeSH
• Skin Neoplasms genetics   pathology   MeSH
• DNA Repair genetics   MeSH
• Proto-Oncogene Proteins B-raf genetics   MeSH
• Chromatin Assembly and Disassembly genetics   MeSH
• Aged, 80 and over MeSH
• Aged MeSH
• Tight Junctions genetics   MeSH
• High-Throughput Nucleotide Sequencing MeSH
• Check Tag
• Adult MeSH
• Middle Aged MeSH
• Humans MeSH
• Young Adult MeSH
• Male MeSH
• Aged, 80 and over MeSH
• Aged MeSH
• Female MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Biomarkers, Tumor MeSH
• Tumor Suppressor Protein p53 MeSH
• Proto-Oncogene Proteins B-raf MeSH
• TP53 protein, human MeSH Browser

Protein phosphatase magnesium-dependent 1 delta (PPM1D) terminates cell response to genotoxic stress by negatively regulating the tumor suppressor p53 and other targets at chromatin. Mutations in the exon 6 of the PPM1D result in production of a highly stable, C-terminally truncated PPM1D. These gain-of-function PPM1D mutations are present in various human cancers but their role in tumorigenesis remains unresolved. Here we show that truncated PPM1D impairs activation of the cell cycle checkpoints in human non-transformed RPE cells and allows proliferation in the presence of DNA damage. Next, we developed a mouse model by introducing a truncating mutation in the PPM1D locus and tested contribution of the oncogenic PPM1DT allele to colon tumorigenesis. We found that p53 pathway was suppressed in colon stem cells harboring PPM1DT resulting in proliferation advantage under genotoxic stress condition. In addition, truncated PPM1D promoted tumor growth in the colon in Apcmin mice and diminished survival. Moreover, tumor organoids derived from colon of the ApcminPpm1dT/+ mice were less sensitive to 5-fluorouracil when compared to ApcminPpm1d+/+and the sensitivity to 5-fluorouracil was restored by inhibition of PPM1D. Finally, we screened colorectal cancer patients and identified recurrent somatic PPM1D mutations in a fraction of colon adenocarcinomas that are p53 proficient and show defects in mismatch DNA repair. In summary, we provide the first in vivo evidence that truncated PPM1D can promote tumor growth and modulate sensitivity to chemotherapy.

• MeSH
• Chromatin drug effects   MeSH
• Exons genetics   MeSH
• Fluorouracil pharmacology   MeSH
• Carcinogenesis drug effects   MeSH
• Cell Cycle Checkpoints genetics   MeSH
• Humans MeSH
• Mutation genetics   MeSH
• Mice MeSH
• Tumor Suppressor Protein p53 genetics   MeSH
• Colonic Neoplasms drug therapy   genetics   pathology   MeSH
• DNA Repair drug effects   MeSH
• DNA Damage drug effects   MeSH
• Cell Proliferation drug effects   MeSH
• Adenomatous Polyposis Coli Protein genetics   MeSH
• Protein Phosphatase 2C genetics   MeSH
• Gene Expression Regulation, Neoplastic drug effects   MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• adenomatous polyposis coli protein, mouse MeSH Browser
• Chromatin MeSH
• Fluorouracil MeSH
• Tumor Suppressor Protein p53 MeSH
• PPM1D protein, human MeSH Browser
• Adenomatous Polyposis Coli Protein MeSH
• Protein Phosphatase 2C MeSH
• Trp53 protein, mouse MeSH Browser

Genotoxic stress triggers a combined action of DNA repair and cell cycle checkpoint pathways. Protein phosphatase 2C delta (referred to as WIP1) is involved in timely inactivation of DNA damage response by suppressing function of p53 and other targets at chromatin. Here we show that WIP1 promotes DNA repair through homologous recombination. Loss or inhibition of WIP1 delayed disappearance of the ionizing radiation-induced 53BP1 foci in S/G2 cells and promoted cell death. We identify breast cancer associated protein 1 (BRCA1) as interactor and substrate of WIP1 and demonstrate that WIP1 activity is needed for correct dynamics of BRCA1 recruitment to chromatin flanking the DNA lesion. In addition, WIP1 dephosphorylates 53BP1 at Threonine 543 that was previously implicated in mediating interaction with RIF1. Finally, we report that inhibition of WIP1 allowed accumulation of DNA damage in S/G2 cells and increased sensitivity of cancer cells to a poly-(ADP-ribose) polymerase inhibitor olaparib. We propose that inhibition of WIP1 may increase sensitivity of BRCA1-proficient cancer cells to olaparib.

• Keywords
• DNA repair, PARP inhibitor, chemotherapy, genotoxic stress, olaparib, phosphatase,
• MeSH
• Tumor Suppressor p53-Binding Protein 1 metabolism   MeSH
• Apoptosis drug effects   MeSH
• Drug Resistance, Neoplasm drug effects   MeSH
• Chromatin metabolism   MeSH
• Phthalazines pharmacology   MeSH
• HEK293 Cells MeSH
• Homologous Recombination genetics   MeSH
• G2 Phase Cell Cycle Checkpoints MeSH
• S Phase Cell Cycle Checkpoints MeSH
• Humans MeSH
• Cell Line, Tumor MeSH
• Breast Neoplasms metabolism   MeSH
• DNA Repair genetics   physiology   MeSH
• Poly(ADP-ribose) Polymerase Inhibitors pharmacology   MeSH
• Piperazines pharmacology   MeSH
• DNA Damage genetics   physiology   MeSH
• Cell Proliferation drug effects   MeSH
• BRCA1 Protein metabolism   MeSH
• Protein Phosphatase 2C antagonists & inhibitors   genetics   metabolism   MeSH
• Antineoplastic Agents pharmacology   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Tumor Suppressor p53-Binding Protein 1 MeSH
• BRCA1 protein, human MeSH Browser
• Chromatin MeSH
• Phthalazines MeSH
• olaparib MeSH Browser
• Poly(ADP-ribose) Polymerase Inhibitors MeSH
• Piperazines MeSH
• PPM1D protein, human MeSH Browser
• BRCA1 Protein MeSH
• Protein Phosphatase 2C MeSH
• Antineoplastic Agents MeSH
• TP53BP1 protein, human MeSH Browser