Purpose: Activation of phosphatidylinositol 3′-kinase pathway is implicated in the pathogenesis of mantle cell lymphoma (MCL). The genetic change in phosphatidylinositol 3′-kinase catalytic subunit α (PIK3CA) in MCL has not been identified. Experimental Design: Thirty-five primary MCL cases and 2 MCL cell lines (GRANTA-519 and Rec-1) were used to investigate somatic mutation and gene copy number of PIK3-CA. Gene copy number was determined using quantitative real-time PCR and fluorescence in situ hybridization. We used quantitative real-time reverse transcription-PCR to measure PIK3CA transcription levels. Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) and phoshorylated AKT protein levels were analyzed using Western blotting and immunohistochemistry. Flow cytometry was used to assess apoptosis after treatment of MCL cell lines and one control cell line with LY294002, a specific inhibitor of PI3KCA. Results: Fifteen of 22 (68%) MCL cases and the MCL cell lines harbored a gain (≥3) of PIK3CA gene copy number. In addition, cases with increased PIK3CA gene copy number had elevated PIK3CA mRNA levels. Furthermore, amplification of PIK3CA correlated with the status of AKT phosphorylation in 7 of 12 (58%) primary MCL cases. Inhibition of PIK3CA induced increased apoptosis in the MCL cell lines. PTEN protein expression was present in all 14 primary MCL cases and cell lines by Western blotting, whereas 5 of 33 (15%) cases tested by immunohistochemistry had loss of PTEN expression. Conclusions: We conclude that a gain of gene copy number of PIK3CA is frequent genetic alteration that contributes to MCL progression. PIK3CA is a promising therapeutic target in MCL. (Clin Cancer Res 2009;15(18):5724-32) Mantle cell lymphoma (MCL) is now recognized as a distinct clinicopathologic subtype of B-cell non-Hodgkin's lymphoma with well-characterized morphology, a distinct immunophenotypic profile, specific karyotypic abnormality, and even a unique underlying molecular alteration (1). MCL comprises approximately 3% to 10% of non-Hodgkin's lymphoma and is characterized by an aggressive clinical course and poor prognosis with a median survival of 3 to 5 years (2, 3). Although front therapy induces a high rate of complete remission, relapse is inevitable and new treatments are needed for relapsed MCL (4).The genetic hallmark of MCL is the chromosomal translocation t(11;14)(q13;q32), which results in deregulated aberrant expression of cyclin D1 (5-7). However, the concept that MCL represents a distinct clinicopathologic and molecularly defined entity does not preclude the existence of additional distinctive disease subsets that could define disease heterogeneity. The

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