Functional characterization of PIK3R1 aberrations in gynecological cancers

Abstract

Cancer development is initiated by genomic aberrations such as copy number variations and small nucleotide mutations. The genomic alterations can induce oncogenic signaling pathways which promote tumorigenesis and create therapeutic vulnerabilities. Blockade of these cancer-addicted pathways with the incorporation of genomic aberrations as biomarkers can be an effective treatment approach.

PIK3R1 encodes the regulatory subunit p85α of the class IA phosphatidylinositol 3- kinase (PI3K). p85α is a key component in regulating the activation of PI3K through the catalytic subunit p110 and the pathway negative regulator PTEN. Crucially, PIK3R1 genomic aberration is highly frequent across cancer lineages, underscoring the importance and the urgent need of investigating the strategy to target PIK3R1 aberrations for cancer patients. The aims of this thesis are to functionally characterize PIK3R1 driver aberrations and the resultant cancer signaling, and to determine the corresponding targeted therapeutics for cancer patients with such PIK3R1 aberrations.

In particular, PIK3R1 copy number loss can be found in multiple cancer types. Ovarian cancer has the most frequent PIK3R1 gene deletion, rendering it a good model to investigate the functional consequences driven by PIK3R1 deletion. Through a series of phenotypic assays, I demonstrated that PIK3R1 downregulation enhances the ovarian cancer tumorigenicity in vitro and in vivo. Strikingly, unveiled by proteomics, AKT and STAT3 were found to be activated in PIK3R1-depleted ovarian cancer cells. In-depth mechanistic studies further demonstrated that the activation of AKT and STAT3 is mediated by a docking protein Gab2. Remarkably, PIK3R1 loss confers cell sensitivity to AKT and STAT3 inhibitors in 3D spheroid and xenograft models. Combinatorial inhibition of AKT and STAT3 in vitro and in vivo achieves synergistic anti-tumor effect than the single reagent.

Small nucleotide mutations of PIK3R1 represent another type of frequent aberrations in cancers, particularly in endometrial cancer. Using a cell survival-based driver mutation-screening approach, several cancer patient-derived driver mutations within the nSH2 domain of PIK3R1 were distinguished from non-functional passenger mutations. Concordantly, driver mutation-expressing stable endometrial cancer cells and CRISPR/Cas9-edited mutation knock-in cells exhibited stronger elevation of in vitro and in vivo oncogenic abilities than wild-type cells. Further studies revealed that these driver mutations enhance PI3K catalytic activities which in turn activate downstream AKT signaling. Intriguingly, tyrosine receptor kinases including HER family, Met and IGF-1R are activated by the nSH2 domain driver mutations. This activation of upstream receptor signal represents potential activating mechanism of the downstream signaling. Accordingly, these driver mutations render the cancer cells more sensitive to inhibitors of AKT and HER family.

Collectively, this study systemically characterized multiple PIK3R1 driver aberrations in ovarian and endometrial cancers. The in-depth investigation provides mechanistic insight into the potential therapeutic strategies. The aberrations characterized in this study might be useful for patient stratification in genome-informed precision cancer medicine.