Infrequent occurrence of PIK3CA mutations in chronic lymphocytic leukemia

The phosphoinositide 3-kinase (PI3K) pathway is central to many different cell signalling pathways associated with cell growth, cell survival, cell movement and apoptosis [1]. PI3Ks are lipid kinases involved in phosphorylation of the inositol ring of phosphatidylinositol, which is a necessary component in signal transduction [2]. Class I PI3Ks are heterodimeric molecules composed of a catalytic subunit (p110a), also known as PIK3CA, together with a regulatory subunit. In recent years, several mutational and functional analyses of PIK3CA have shown that this catalytic subunit may function as an activating oncogene. PIK3CA mutations have frequently been detected in several cancer types, such as breast (8–27%), gastric (6–25%) and colon (11–32%) cancer [3,4]. Less is known about PIK3CA mutations in hematological malignancies, although screening studies in acute myeloid leukemia, acute lymphoblastic leukemia and non-Hodgkin lymphomas (subtypes not specified) have revealed a mutational frequency of 51% [4,5]. In contrast, a recent study on diffuse large B-cell lymphoma detected mutations in 17 out of 215 cases (8%) [6]. AKT is located downstream of PI3K and also serves as an important molecule in cellular signalling. Since several biological events including apoptosis are dependent on the PI3K-AKT signalling pathway, this is of particular interest in chronic lymphocytic leukemia (CLL), a disease where defects in apoptosis are considered to be a main feature. PI3K has been shown to be constitutively activated in CLL and furthermore linked to the expression of PKC isoforms [7,8]. The AKT signalling pathway has also been studied in CLL whereby stronger activation of AKT is associated with progressive disease [9]. Previous studies in solid tumors have identified a point mutation (E17) that leads to the activation of AKT1, however, a similar study in CLL and ALL could not detect any such mutation [10,11]. To this end, we sought to investigate any potential association between PIK3CA mutational status and the constitutive activation of PI3K in CLL and have performed mutational analysis of PIK3CA in 61 Swedish CLL cases (median age of diagnosis 64 years and male:female ratio 1.7:1). Previous sequencing results showed that 27/61 cases (44%) carried unmutated IGHV genes whilst 34/61 (56%) had less than 98% identity and were therefore classified as mutated. Furthermore, FISH analysis on 38/61 cases revealed 18 cases with 13q deletion, 8 with 11q deletion, 7 with 17p deletion (including two cases classified as ‘borderline’, 10–15%), one trisomy 12 case and five cases carrying no genetic aberration. PCR amplification using a proof-reading enzyme (AccuPrime Pfx, Invitrogen, USA) and subsequent sequence analysis of PIK3CA was performed to target two hotspot regions, Exon 9 and Exon 20, which have been reported to contain a high mutational load in various cancer forms. A comparison was made between the sequences and the PIK3CA cDNA sequence (GeneBank accession number MN_006218.2). Overall, only two mutations were detected in Exon 20, i.e. the kinase domain of PIK3CA (corresponding to the catalytic domain), whereas no mutations were observed in Exon 9. The first mutation was found at