Truncated PPM1D impairs stem cell response to genotoxic stress and promotes growth of APC-deficient tumors in the mouse colon

Abstract

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.

Truncated PPM1D Prevents Apoptosis in the Murine Thymus and Promotes Ionizing Radiation-Induced Lymphoma

“…1/2016) and were performed in C57Bl/6 mice. The mouse strain carrying a Ppm1d T allele with a premature stop codon in exon 6 of the Ppm1d was described previously [ 35 ]. Trp53 tm1Tyj/J mouse strain was obtained from the Jackson Laboratory (stock #002101) and was described previously [ 45 ].…”

“…Cancer cell lines (including U2OS and HCT116 cells) carrying heterozygous truncating mutations in PPM1D show G1 checkpoint override upon exposure to the mild level of IR [ 31 ]. Similarly, when we introduced truncating mutations in exon 6 of the PPM1D in human non-transformed retinal pigment epithelial (RPE1) cell lines using CRISPR/Cas9 technology, we observed decreased ability to induce the G1 checkpoint after exposure to IR [ 35 ]. However, whether the truncating PPM1D mutations contribute to tumorigenesis remains an open question.…”

“…However, whether the truncating PPM1D mutations contribute to tumorigenesis remains an open question. To experimentally address this, we have recently generated a mouse model in which we introduced a frame-shift mutation in the exon 6 of PPM1D using the Transcription activator-like effector nuclease (TALEN) technology [ 35 ]. We have found that the truncated PPM1D T allele protected intestinal stem cells from apoptosis by suppressing the p53 pathway [ 35 ].…”

“…To experimentally address this, we have recently generated a mouse model in which we introduced a frame-shift mutation in the exon 6 of PPM1D using the Transcription activator-like effector nuclease (TALEN) technology [ 35 ]. We have found that the truncated PPM1D T allele protected intestinal stem cells from apoptosis by suppressing the p53 pathway [ 35 ]. In addition, PPM1D T/+ mice showed a higher amount of intestinal polyps and increased frequency of colon adenocarcinoma induced by constitutively active Wnt signaling in APC min background [ 35 ].…”

“…We have found that the truncated PPM1D T allele protected intestinal stem cells from apoptosis by suppressing the p53 pathway [ 35 ]. In addition, PPM1D T/+ mice showed a higher amount of intestinal polyps and increased frequency of colon adenocarcinoma induced by constitutively active Wnt signaling in APC min background [ 35 ]. Although the PPM1D T allele alone did not induce the formation of colon tumors, it significantly potentiated the phenotype and reduced the survival of APC min mice [ 35 ].…”