A recurrent activating PLCG1 mutation in cardiac angiosarcomas increases apoptosis resistance and invasiveness of endothelial cells.

Primary cardiac angiosarcomas are rare tumors with unfavorable prognosis. Pathogenic driver mutations are largely unknown. We therefore analyzed a collection of cases for genomic aberrations using SNP arrays and targeted next-generation sequencing (tNGS) of oncogenes and tumor-suppressor genes. Recurrent gains of chromosome 1q and a small region of chromosome 4 encompassing KDR and KIT were identified by SNP array analysis. Repeatedly mutated genes identified by tNGS were KDR with different nonsynonymous mutations, MLL2 with different nonsense mutations, and PLCG1 with a recurrent nonsynonymous mutation (R707Q) in the highly conserved autoinhibitory SH2 domain in three of 10 cases. PLCγ1 is usually activated by Y783 phosphorylation and activates protein kinase C and Ca(2+)-dependent second messengers, with effects on cellular proliferation, migration, and invasiveness. Ectopic expression of the PLCγ1-R707Q mutant in endothelial cells revealed reduced PLCγ1-Y783 phosphorylation with concomitant increased c-RAF/MEK/ERK1/2 phosphorylation, increased IP3 amounts, and increased Ca(2+)-dependent calcineurin activation compared with ectopic expressed PLCγ1-wild-type. Furthermore, cofilin, whose activation is associated with actin skeleton reorganization, showed decreased phosphorylation, and thus activation after expression of PLCγ1-R707Q compared with PLCγ1-wild-type. At the cellular level, expression of PLCγ1-R707Q in endothelial cells had no influence on proliferation rate, but increased apoptosis resistance and migration and invasiveness in in vitro assays. Together, these findings indicate that the PLCγ1-R707Q mutation causes constitutive activation of PLCγ1 and may represent an alternative way of activation of KDR/PLCγ1 signaling besides KDR activation in angiosarcomas, with implications for VEGF/KDR targeted therapies.