PIK3CA Mutations and PTEN Loss Correlate with Similar Prognostic Factors and Are Not Mutually Exclusive in Breast Cancer

Purpose: The phosphatidylinositol 3′-kinase/Akt pathway is frequently altered in breast cancer. PTEN, a phosphatase that opposes the effect of phosphatidylinositol 3′-kinase, can be mutated or lost, whereas the PIK3CA gene is mutated. These have been proposed as alternative mechanisms, and their clinicalpathology significance is under discussion. In this study, we aimed to explore whether PIK3CA mutations and PTEN loss are mutually exclusive mechanisms, correlate with other known clinicopathologic markers, or have clinical implication in breast cancer. Experimental Design: Exons 9 and 20 of the PIK3CA gene were analyzed in 270 breast tumors, and mutations were detected by single-stranded conformational analysis followed by sequencing. The expression of PTEN was evaluated by immunohistochemistry in 201 tumors. Results: PIK3CA mutations were found in 24% of the tumors and associated with estrogen receptor+ status, small size, negative HER2 status, high Akt1, and high cyclin D1 protein expression. PTEN was negative in 37% of the cases and PTEN loss was associated with PIK3CA mutations (P = 0.0024). Tumors presenting PTEN loss or both alterations were often estrogen receptor+, small in size, and HER2−. PIK3CA mutations predicted for longer local recurrence-free survival. Moreover, PTEN loss by itself or combined with mutated PIK3CA tended to confer radiosensitivity. In addition, the patients with high S-phase fraction had longer recurrence-free survival if they carried mutations in the PIK3CA gene and/or had lost PTEN, whereas the same alterations were associated with shorter recurrence-free survival among patients with low S-phase fraction. Conclusions: PIK3CA mutations and PTEN loss were not mutually exclusive events and associated with similar prognostic factors.

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