High-dose oncogenic PIK3CA drives constitutive cellular stemness through self-sustained TGFβ pathway activation

Abstract Oncogenic PIK3CA mutations activate phosphoinositide 3-kinase-alpha (PI3Kα) and are among the commonest somatic mutations in cancer. We recently demonstrated that the “hotspot” variant PIK3CAH1047R exerts striking allele dose-dependent effects on stemness in human pluripotent stem cells (hPSCs), and found multiple oncogenic PIK3CA copies in a substantial proportion of human cancers. This suggested that the consequences of oncogenic PI3K signaling may differ according to the strength of genetic PIK3CA activation. Here, to identify the stemness-promoting mechanism, we profiled isogenic wild-type, PIK3CAWT/H1047R and PIK3CAH1047R/H1047R iPSCs by high-depth transcriptomics, proteomics and reverse-phase protein arrays (RPPA). We report that the phenotypic switch in homozygous PIK3CAH1047R hPSCs occurs downstream of signaling “rewiring” towards self-sustained TGFβ pathway activation and increased NODAL expression, which was no longer reversible by pharmacological PI3Kα inhibition. Gene expression analysis of PIK3CA-associated human breast cancers in The Cancer Genome Atlas revealed increased expression of NODAL according to tumor stage and PIK3CAH1047R allele dosage. Together with the emerging link between NODAL re-expression and cancer aggressiveness, our data suggest that TGFβ pathway inhibitors warrant investigation in breast tumors stratified by PIK3CAH1047R allele dosage. One-sentence summary Biallelic genetic PI3Kα activation rewires signaling and induces constitutive stemness downstream from self-sustained TGFβ pathway activation.