PTEN loss of function Authored: Orlic-Milacic, M, 2012-07-18 EC Number: 3.1.3.67 Edited: Matthews, L, 2012-08-03 One of the functions of PTEN is to act as a phosphoinositide phosphatase that catalyzes dephosphorylation of PIP3 into PIP2. PTEN thus reduces the amount of available PIP3, counteracting PI3K activity and downregulating AKT signaling. PTEN is frequently targeted by loss of function mutations in cancer and in familial cancer syndromes known as PTHS (PTEN hamartoma tumor syndromes, a collection of diseases including Cowden syndrome, Bannayan-Riley-Ruvalcaba syndrome, and Lhermitte-Duclos disease). Some PTEN loss-of-function variants are also found in autism spectrum disorder patients. For a recent review of PTEN involvement in cancer, please refer to Hollander et al. 2011. <br> <br>The majority of missense mutations that impair phosphoinositide phosphatase activity of PTEN cluster in exon 5 of PTEN gene and result in substitution of amino acid residues in the catalytic cleft of the phosphatase domain. Arginine at position 130 is the most frequently substituted PTEN residue in cancer. R130 of human PTEN is the last arginine residue in the conserved H-C-K/R-A-G-K-G-R sequence (corresponding to HCXXGXXR motif of protein tyrosine phosphatases) in the catalytic cleft of the PTEN phosphatase domain and is essential for catalysis (Barford et al. 1994, Lee et al. 1999). PTEN R130 substitution mutants show markedly decreased phosphoinositide phosphatase activity (Han et al. 2000, Koul et al. 2002) and are frequently found in endometrial carcinoma (Kong et al. 1997, Konopka et al. 2007). The cysteine residue at position 124 (C124) of human PTEN, in the conserved H-C-K/R-A-G-K-G-R sequence, 'attacks' the phosphate group of a substrate and forms a thio-phosphate intermediate during the dephosphorylation reaction (Guan and Dixon 1991, Barford et al. 1994, Lee et al. 1999). Therefore, substitution of this critical C124 abolishes PTEN phosphatase activity (Han et al. 2000, Koul et al. 2002). Substitution of histidine H123 in the conserved H-C-K/R-A-G-K-G-R sequence also impairs PTEN phosphatase activity (Lee et al. 1999).<br><br>Missense mutations also target amino acid residues in the N-terminal phosphatase domain that are outside the catalytic cleft. Substitution of histidine at position 93 affects the conserved WPD loop of the phosphatase domain of PTEN, and PTEN H93 mutants show low phosphoinositide phosphatase activity (Lee et al. 1999). Serine residue S170 and histidine residue H173 participate in the formation of hydrogen bonds between the N-terminal phosphatase domain of PTEN and the C-terminal membrane-binding C2 domain (Lee et al. 1999). H173, and to a lesser extent S170, are targeted by missense mutations in cancer, and substitution mutants have impaired phosphoinositide phosphatase activity (Han et al. 2000). <br><br>Missense mutations also occur in the C2 domain of PTEN. The C2 domain is implicated in membrane binding and localization of PTEN, but also in PTEN roles unrelated to its phosphoinositide phosphatase function (Raftopoulou et al. 2004). Since the roles of these C2 domain PTEN mutants in cancer have not been clarified, these variants will be annotated when more information becomes available. <br><br>Besides missense mutations, nonsense mutations that result in PTEN protein truncation are also frequently found in cancer samples. The three residues most frequently targeted by nonsense mutations are R130, R233 and R335. While R130* mutation directly affects the phosphatase domain of PTEN, R233* and R335* affect the C2 domain. It is has not yet been elucidated whether R233* and R335* affect PTEN membrane localization or impair PTEN function in some other way. <br><br>In cancer, PTEN is also frequently inactivated by genomic deletions and loss of heterozygosity (LOH) affecting chromosome band 10q23 or by epigenetic silencing (reviewed by Hollander et al. 2011). Pubmed10555148 Pubmed10866302 Pubmed11948419 Pubmed14976311 Pubmed1654322 Pubmed17219201 Pubmed21430697 Pubmed8128219 Pubmed9326929 Reactome Database ID Release 432317387 Reactome, http://www.reactome.org ReactomeREACT_147756 Reviewed: Thorpe, Lauren, 2012-08-13 Reviewed: Yuzugullu, Haluk, 2012-08-13 Reviewed: Zhao, Jean J, 2012-08-13