Abstract 111: PIK3R1 gene alterations, insights on its role in lung cancer
Authors: Bonastre Ester , Pros Eva , Cespedes Montserrat Sanchez 
Published: August 2012

Abstract
Gene abnormalities are the cause of cancer and efforts are still needed to identify all the genes that promote carcinogenesis, including those altered at low frequencies. The occurrence of homozygous deletions (HD) is one mechanism for tumor suppressor gene inactivation. We scrutinized the SNP arrays at the Sanger Database to search for HD in lung cancer cell lines. Several candidate tumor suppressors, including PIK3R1, were found to be altered by HD. PIK3R1 codifies for p85α, a regulatory subunit of the PI3K, and mutations have been found in different tumor types. The PI3K pathway is highly relevant in cancer since it contains well known oncogens and tumor suppressors. Despite that, the functional significance of PIK3R1 mutations is uncertain, because activating and inactivating alterations have been described in tumors. Here, we aimed to clarify the role of this gene in cancer. We amplified by PCR each individual exon of the gene and confirmed the presence of a HD at the NCI-H2171 cell line. Mutational analysis of PIK3R1 was then extended to a panel of 86 lung cancer cell lines. We identified several changes including an heterozygous nonsense mutation, c.211G&gt;T (p.G71*), an heterozygous missense mutation, c.211G&gt;T (p.D13H) and several intronic changes outside the canonical splice sites. Intriguingly, the levels of p85α protein in the cell line carrying the p.G71* mutation were almost zero, indicating the presence of a second hit. To search for the presence of intragenic small deletions, undetectable by single exon amplification, we studied the cDNA of PIK3R1. No additional alterations were detected in this cell line nor in the other cells carrying PIK3R1 heterozygous missense mutations. Next, we used cancer cell lines that have no p85α protein due to HD to determine whether the lack of p85α impinges the AKT-mTOR pathway. To this end, we tested the levels of pAKT-T308, pAKT-S473 and pS6-S235/236, upon different cell culture conditions (i.e. +/−IGF) in cancer cells carrying alterations at distinct components of the PI3K pathway. We found that cells with alterations at ERBB2 and PTEN had constitutive high levels of pAKT and pS6, in all the conditions tested. Surprisingly, some cells lacking p85α protein due to HD at PIK3R1, readily regulated the activation/inactivation of the pathway, accordingly to the cell culture conditions. This indicates that the inactivation of the p85α protein in cancer does not compromise the appropriate modulation of AKT/mTOR signalling, possibly due to a compensation effect by other regulatory subunits. In conclusion, PIK3R1 is mutated in lung cancer, albeit at low frequencies. Furthermore, PIK3R1 alterations contribute to oncogenesis by mechanisms that require further investigation.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 111. doi:1538-7445.AM2012-111