Background:PIK3CA has been identified as an oncogene in multiple forms of human malignancies including glioblastoma multiforme, gastric adenocarcinoma head and neck squamous cell, cervical, and ovarian cancers. To date, however, colorectal cancer (CRC) has been evaluated for somatic PIK3CA mutations in only one published series and to be involved nearly one third of CRC. Data suggest that the gene may be involved in the metamorphosis of dysplastic cells to invasive cancer, not as an initiating event in carcinogenesis. Methods: Paired normal/tumor tissue, and in some cases blood, was collected from 197 patients with colorectal cancer. Patients were selected from a number of ongoing studies specifically to enrich for cases with defective mismatch repair. Tumor DNA was extracted from microdissected frozen or paraffin-embedded tissue sections by a standard phenol/chloroform procedure or with a DNA extraction kit.The corresponding normal control DNA for each patient was derived from adjacent normal mucosa or blood leukocytes. For blood specimens, DNA was extracted using the Puregene nucleic acid isolation kit. Microsatellite instability testing (MSI) was analyzed with 6 dinucleotide microsatellite markers (D5S346, MYCL, D18S55, D17S250, D10S197, and ACTC) and one mononucleotide repeat (BAT 26).Tumors were classified as MSH-H if >30% markers demonstrated instability, MSH-L if <30% demonstrated MSI, and MSS if no marker exhibited MSI. Mutational screening and direct sequencing of PIK3CA gene was done using PCR primers for amplifying exons 9 and 20, the two mutational hotspots. Results: Among the 197 CRC samples, a total of 49 cases showed exonic variations. By comparing with corresponding normal tissues, 26 of 49 tumors had a polymorphic T to G transversion at base pair position 1779. Thus, from the analysis of the two major hotspots, exon 9 and 20, there were a total of 24 tumors with PIK3CA mutation (12.2 %). Of these mutations, 8 were in exon 9 (33.3%); 14 in exon 20(58.3%); and 2 in both exons (8.3%). We found that significantly more tumors with regional or distant metastases (16/95) had PIK3CA mutations compared to earlier stage tumors (p =0.049). There were no statistically significant differences according to age (p=0.32), gender (p=0.22), ploidy (p=0.19), tumor location (p=0.42), or MSI status (p=0.39) and BRAF mutation status (p=0.70). Discussion: PIK3CA mutations were detected in 12.2% of CRC. The only clinical feature associated with somatic PIK3CA mutations was that of advanced stage disease with either regional or distant metastasis. This finding offers support for the involvement of PIK3CA mutation in the activation of a tumor's metastatic potential.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]