PIK3CA mutation in gastric cancer and the role of microsatellite instability status in mutations of exons 9 and 20 of the PIK3CA gene

Background. A better understanding of molecular gastric cancer (GC) entities may help in tailored treatments of that neoplasm. The PIK3CA mutation is one of the most important in many cancers. Objectives. We performed a comparison of clinical and pathological data of the PIK3CA mutation in GC patients. Material and methods. The analysis was done on 472 patients operated on in one center. Polymerase chain reaction (PCR) was used for the screening of PIK3CA (exon 9 and 20). For microsatellite instability (MSI) we used 5 quasi-monomorphic mononucleotide repeats − BAT-26, BAT-25, NR-24, NR-21, and NR-27. The clinical and pathological data was analyzed. Results. PIK3CA mutation was observed in 10 out of 472 GC patients (2.1%). Nine out of 10 were MSI (9 of 111 MSI patients – 8.1%). Half of the 10 patients had mutations in exon 9 and the other half in exon 20. A majority of patients with the PIK3CA mutation had MSI (p < 0.001). The 5-year survival of MSI patients with the PIK3CA mutation was 40% and without the mutation, 70.4% (p = 0.309). For patients with the mutation in exon 9, the 5-year survival was 0%, and for those with the mutation in exon 20, 80% (p = 0.031). The Cox proportional hazards regression analysis did not show that PIK3CA is statistically correlated with a worse overall survival. Conclusions. PIK3CA mutation in GC is a rare finding. It is strongly associated with the MSI molecular subgroup, presenting a worse outcome than other MSI patients. A completely different outcome is associated with the mutation in exon 9 compared to the mutation in exon 20, with the latter being more favorable.