PIK3CA, the gene coding for the p110α subunit of phosphoinositide 3-kinase (PI3K), is frequently mutated in in human tumours. These mutations constitutively activate PI3K increasing its signalling pathway and are involved in regulating several cancer hallmarks including proliferation, growth, survival and migration. The PI3K pathway has also been linked to genomic instability in cancer but the role of PIK3CA mutations in promoting DNA damage/repair associated with tumourigenesis is still not clear. This could have implications for understanding the biological consequences and clinical significance of PIK3CA mutations.
In order to study the role of PIK3CA mutation in promoting DNA damage/repair, we have used our novel mouse model with a conditional knock-in of the common H1047R mutation into the endogenous PIK3CA gene.
Two cell types were used, immortalized mouse mammary epithelial cells (iMMECs) and mouse embryonic fibroblasts (MEFs), both with corresponding mutant and isogenic wild type control cell lines. DNA damage was assessed by measuring the levels of γH2AX, a histone that gets recruited to chromatin when double-strand breaks are detected. The rate of DNA repair was assessed after irradiating the cells with 2 Gy ionizing radiation to induce DNA damage, and the formation of γH2AX foci was monitored over time using immunofluorescence and confocal microscopy.
In the absence of irradiation the level of DNA damage was not significantly different between wild type and mutant cells. However, our results show that mutant PIK3CA cells repair radiation-induced DNA damage faster than the wild type cells. One possible explanation could be that the increase of PI3K activity resulting from PIK3CA mutation is involved in the upregulation of DNA damage response enzymes such as ATM, ATR or DNA-PK as has been previously reported. ATM and ATR are involved in homologous recombination and DNA-PK initiates non-homologous recombination. This increase in DNA repair activity could act as a mechanism to compensate for the increased oxidative stress due to the metabolic reprogramming as well as the genomic instability observed in PIK3CA mutant cells. However, further experiments are needed to confirm these findings.