Characterisation of pathogenic PTEN variants that retain AKT suppressive activity

PTEN is a major tumour suppressor and a phosphoinositide lipid phosphatase that suppresses activation of the PI3K-AKT signalling pathway. In addition to its lipid phosphatase activity, PTEN displays phosphatase activity towards protein substrates in vitro, a function that also appears to be required for embryonic development and tumour suppression. Here, we characterise a group of PTEN mutants that retain apparently normal lipid phosphatase activity in vitro and suppress the canonical PI3K-AKT signalling network in cells but are impaired in their ability to suppress glioma cell invasion and support normal epithelial lumen formation. Most of these mutations have been identified in human cancers and many are located in the unstructured C2 domain D-Loop and the C-terminal PDZ-binding motif, although their detailed effects on protein function remain poorly characterised. To investigate the impact of these mutations on tumour suppression, we generated knock-in mice expressing PTEN R308C, a variant that has been identified in at least seven independent cancers. Pten^R308C/R308C mice develop normally and neither Pten^R308C/R308C nor Pten^+/R308C mouse embryonic fibroblasts (MEFs) show the elevated AKT phosphorylation observed with Pten^-/- MEFs. However, compared with Pten^+/+ mice, both Pten^+/R308C and Pten^R308C/R308C mice exhibited significantly reduced overall survival with approximately 40% being culled due to illness and/or the presence of palpable tumours. Our findings suggest that, in addition to its lipid phosphatase activity and ability to suppress AKT signalling, other functions of PTEN and/or uncharacterised mechanisms regulating its lipid phosphatase activity are also required for normal tumour suppression.