Rare cancer-specific mutations in PIK3CA show gain of function

Abstract

Fifteen rare cancer-derived mutants of PIK3CA, the gene coding for the catalytic subunit p110␣ of phosphatidylinositol 3-kinase (PI3K), were examined for their biological and biochemical properties. Fourteen of these mutants show a gain of function: they induce rapamycin-sensitive oncogenic transformation of chicken embryo fibroblasts, constitutively activate Akt and TOR-mediated signaling, and show enhanced lipid kinase activity. Mapping of these mutants on a partial structural model of p110␣ suggests three groups of mutants, defined by their location in distinct functional domains of the protein. We hypothesize that each of these three groups induces a gain of PI3K function by a different molecular mechanism. Mutants in the C2 domain increase the positive surface charge of this domain and therefore may enhance the recruitment of p110␣ to cellular membranes. Mutants in the helical domain map to a contiguous surface of the protein and may affect the interaction with other protein(s). Mutants in the kinase domain are located near the hinge of the activation loop. They may alter the position and mobility of the activation loop. Arbitrarily introduced mutations that have no detectable phenotype map either to the interior of the protein or are positioned on a surface region that lies opposite to the exposed surfaces containing gain-of-function mutants. Engineered mutants that exchange acidic or neutral residues for basic residues on the critical surfaces show a gain of function. molecular model ͉ phosphatidylinositol 3-kinase ͉ lipid membrane ͉ protein-protein interaction ͉ activation loop T he catalytic subunit p110␣ of class I phosphatidylinositol 3-kinase (PI3K) is frequently mutated in cancer (1-5). The incidence of these cancer-specific point mutations varies with the tissue of origin and is particularly high in mammary and colorectal cancers (1, 2, 6 -12). A conspicuous feature of the p110␣ mutations is their specific location in the gene of p110␣: almost 80% of these mutations map to one of three hot spots in the p110␣ coding sequence (1). Two of the hot spot mutations are located in the helical domain of p110␣ at residue positions E542K and E545K, and the third hot spot (residue H1047R) is located in the C-terminal portion of the kinase domain. Besides these frequently occurring hot spot mutations, numerous rare, cancer-specific mutations have also been identified in p110␣ (1,3,7,8,10,11). These rare mutations are widely distributed over the entire coding sequence of p110␣. It has been assumed that the prevalence of hot spot mutations ref lects the strong selective growth advantage that these mutants provide to the cell. This suggestion is supported by the fact that all three hot spot mutations confer in vitro and in vivo oncogenicity on p110␣ (13-18). A corollary of this interpretation is that the rare mutants would not offer a comparable growth advantage and may therefore show either no phenotype or a loss of function. Consequently, they may be of no relevance to the oncogenic process. We studied 15 of the...

Oncogenic signaling of class I PI3K isoforms

“…Both N345K and C420R mutations cause the protein to exhibit gain of function and oncogenicity (Gymnopoulos et al, 2007). The oncogenicity of these C2 domain mutants may be facilitated by their more efficient localization to the plasma membrane (Gymnopoulos et al, 2007). Our findings showing that positive charges unique to the C2 domain of p110d are required for normal function support an essential role of this domain in the recruitment to the plasma membrane.…”

“…Several cancerspecific mutations in the C2 domain of p110a result in a change to a positive charge, that is, N345K, C378R, E418K, C420R and E453K (Campbell et al, 2004;Samuels et al, 2004;Saal et al, 2005). Both N345K and C420R mutations cause the protein to exhibit gain of function and oncogenicity (Gymnopoulos et al, 2007). The oncogenicity of these C2 domain mutants may be facilitated by their more efficient localization to the plasma membrane (Gymnopoulos et al, 2007).…”

PIK3CA mutations in the kinase domain (exon 20) of uterine endometrial adenocarcinomas are associated with adverse prognostic parameters

“…It was initially thought that rare mutations would not confer as much growth advantage as hot spot mutations; however, recent data indicate that rare PIK3CA mutations in those domains may also result in gain of lipid kinase activity. 39 Only the helical (exon 9) and kinase (exon 20) domains of PIK3CA have been analyzed in this study. Recently, the frequency and distribution of PIK3CA mutations were reviewed in a total of 2.334 tumor samples corresponding to colon, breast, liver, brain, stomach, lung, and ovary.…”

“…The distinct association of different PIK3CA mutations with histologic grade and depth of myometrial invasion is consistent with the recent suggestion that different categories of mutants, as defined by their structural and functional domains, would increase PI3K function by different mechanisms. 39 Mutations in the kinase domain of exon 20 are close to the hinge region of the catalytic loop and would lead to its activation; in contrast, mutations in the helical domain of exon 9 are clustered on an exposed PIK3CA in endometrial adenocarcinoma L Catasus et al surface patch of the protein and would change its ability to interact with other regulatory proteins, which may vary for each tissue. 39 Thus, the different mechanisms by which the two categories of PIK3CA mutations induce a gain of function may explain their different impact on tumor grade and myometrial invasion, as found in our study.…”

“…39 Mutations in the kinase domain of exon 20 are close to the hinge region of the catalytic loop and would lead to its activation; in contrast, mutations in the helical domain of exon 9 are clustered on an exposed PIK3CA in endometrial adenocarcinoma L Catasus et al surface patch of the protein and would change its ability to interact with other regulatory proteins, which may vary for each tissue. 39 Thus, the different mechanisms by which the two categories of PIK3CA mutations induce a gain of function may explain their different impact on tumor grade and myometrial invasion, as found in our study. Three tumors in our series carried more than one mutation, each in different functional domains of exons 9 and 20.…”