Mutations of the Catalytic Subunit of Class IA PI3K Are Constitutively Active and Confer Factor-Independent Growth to Early Hematopoietic Cells.

Background: Activating mutations of the catalytic subunit of class IA phosphoinositide 3-kinase alpha (PIK3CA) are clustered in small hot-spot regions of the PIK3CA gene, including exon 9 within the helical domain and exon 11 within the kinase domain. They have been linked to several human neoplasias, including colorectal, breast and hepatocellular cancers. In acute leukemias, PIK3CA mutations have not been investigated in larger chorts and so far only been observed in a few patients. Since the PI3K/Akt/GSK3beta pathway is an important signaling cascade of receptor tyrosine kinases (e.g. Flt3R, Kit) which are frequently activated in acute leukemias we investigated the functional activity of PIK3CA mutants.

Materials and methods: We transfected early hematopoietic cells (Ba/F3 cell line) with PI3KCA exon 9 and 11 mutations and investigated the cells in an in vitro factor-independent growth assay and pharmacologic inhibition experiments.

Results: We demonstrate that mutations in the helical or kinase domain of PIK3CA lead to the constitutive activation of PI3Kalpha in Ba/F3 cells, inducing factor-independent growth of the IL3-dependent cells. The frequency of IL3-independent Ba/F3 cells after tranfection with exon 9 and 11 PIK3CA mutants was equivalent to the frequency confered by PIK3CA mutants containing the membrane localization signal of either src or ras. Proliferation and survival of the cells were inhibited by the PI3K inhibitors LY294002 and Quercitin or an inhibitor of the PI3K downstream target Akt. Inhibition occurred in a dose- and time-dependent manner and could be reverted by addition of IL-3. One of the major targets of PI3K/Akt signaling is GSK3beta which becomes inactivated after Akt-mediated phosphorylation. By using a GSK3beta-specific inhibitor or LiCl we could show that the inactivation of GSK3beta alone did not result in factor-independent growth of Ba/F3 cells. However, GSK3beta inhibition led to a delay in the induction of cell death after IL3-withdrawal.

Conclusion: Activating mutations of PIK3CA, associated with several human neoplasias and acute leukemia are functionally active in hematopoietic cells, confer factor independency and respond to PI3K/Akt inhibition.