PIK3CA gene mutations in endometrial carcinoma. Correlation with PTEN and K-RAS alterations

Summary

Alterations in the phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway are common in endometrial carcinoma. Inactivation of the tumor suppressor gene PTEN leads to a constitutively active PI3K pathway, which plays a role in the early steps of endometrial tumorigenesis. Other alterations in the PI3K/AKT pathway are mutations in the PIK3CA gene, which encode the p110α catalytic subunit of PI3K. PIK3CA mutations cluster to the helical (exon 9) and the kinase (exon 20) domains of the gene. In endometrial carcinomas, PIK3CA mutations have been found to coexist frequently with PTEN mutations, but it is not clear whether they occur in cells with monoallelic or biallelic inactivation of PTEN. In the present study we have evaluated PIK3CA mutational status in a series of 33 endometrial carcinomas, previously screened for microsatellite instability and mutations in PTEN, K-RAS, and CTNNB-1. The tumors were also evaluated for loss of heterozygosity on 10q23 and hypermethylation of the promoter region of PTEN/psiPTEN to assess the monoallelic or biallelic inactivation status of PTEN. PIK3CA mutations were detected in 8 (24%) of the 33 cases. Seven mutations were located in exon 20 and 1 in exon 9. PTEN alterations were found in 19 cases (57%). Biallelic inactivation of PTEN was demonstrated in 11 tumors, whereas 8 tumors exhibited alteration in only 1 of the 2 alleles. PIK3CA mutations coexisted with monoallelic alterations of PTEN in 4 cases (2 mutations and 2 allelic imbalances), with biallelic PTEN inactivation in 1 case (mutation and promoter methylation), and 3 tumors showed PIK3CA mutations in association with wild-type PTEN. PIK3CA mutations did not correlate with microsatellite instability or mutations in CTNNB-1. However, PIK3CA and K-RAS mutations (8 cases) were mutually exclusive alterations. In summary, the results confirm that PIK3CA mutations are frequent in endometrial carcinoma and support the hypothesis that PIK3CA mutations may have an additive effect to PTEN monoallelic inactivation in endometrial carcinoma.

Introduction

The phosphatidylinositol 3-kinase (PI3K)/AKT pathway plays a key role in the regulation of cellular homeostasis [1], [2]. Activation of cell-surface receptors recruits PI3K, which phosphorylates the phosphatidylinositol 4,5-bisphosphate substrate to generate phosphatidylinositol 3,4,5-trisphosphate, recognized by the protein kinase AKT and its regulator PDK1. In the cell membrane, AKT is phosphorylated in serine and threonine residues. Activated AKT modulates the expression of several genes involved in suppression of apoptosis and cell cycle progression. Moreover, there is a signaling cross talk between the PI3K/AKT pathway and the RAS-MEKK signaling pathway.

The PI3K/AKT pathway is frequently activated in endometrial carcinomas [3], often resulting from mutations in the tumor suppressor gene PTEN. PTEN antagonizes the PI3K/AKT pathway by dephosphorylating phosphatidylinositol 3,4,5-trisphosphate, leading to a decreased translocation of AKT to cellular membranes, and subsequent down-regulation of AKT phosphorylation and activation [4], [5], [6], [7]. PTEN alterations have been found in only 1 of the 2 alleles in some tumors. This has raised the question whether PTEN haploinsufficiency plays a role in endometrial tumorigenesis.

PI3K is a heterodimeric enzyme consisting of a catalytic subunit (p110) and a regulatory subunit (p85) [8]. The PI3K family comprises 8 members divided into several classes according to their sequence homology and substrate preference. The cloning of a number of catalytic subunits of PI3K has led to the classification of this multigene family into 3 groups (classes I, II, III). Class I members contain 4 different catalytic subunits (p110α, p110β, p110δ, and p110γ). There are 8 isoforms of p85 encoded by 3 genes [9]. The PIK3CA gene, located on chromosome 3q26.32, codes for the p110α catalytic subunit of PI3K [10]. A high frequency of mutations in the PIK3CA gene has been reported recently in various human cancers [11], including colon [12], breast [13], ovary [14], [15], and stomach [16]. Mutations are predominantly located in the helical (exon 9) and kinase (exon 20) domains. A recent study has described mutations in the PIK3CA gene in endometrial carcinomas for the first time [17]. In this series, PIK3CA mutations occurred in 36% of cases and coexisted frequently with PTEN mutations. The authors, however, did not evaluate the biallelic/monoallelic status of PTEN. These results indicate that PIK3CA mutations contribute to activate the PI3K/AKT pathway in endometrial carcinomas.

In the present study, we have assessed the frequency of PIK3CA mutations in a series of 33 endometrial carcinomas. We have correlated the presence of PIK3CA mutations with alterations of PTEN (mutations, loss of heterozygosity [LOH], promoter hypermethylation) and also with microsatellite instability, and mutations in the K-RAS and CTNNB-1 genes. One of the goals of the study was to assess a possible additive effect of PIK3CA mutations with monoallelic inactivation of PTEN.