Role of the tumour suppressor gene pten in sporadic colorectal cancer

Publication Type:
Thesis
Issue Date:
2005
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NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- Colorectal cancer (CRC) is the fourth most common malignancy in Western countries and represents the second leading cause of cancer related death after lung cancer in men and breast cancer in women. Despite strong candidacy for PTEN (Phosphatase and TENsin homologue deleted on chromosome 10) in the pathogenesis of colorectal cancer (CRC), initial reports of PTEN mutation screening in sporadic CRC found mutations to be uncommon. However, more recent analyses reported a 17-19% incidence of PTEN mutations exclusively in microsatellite unstable (MSI+) colorectal tumours. To verify the role of PTEN in sporadic colorectal cancer, a series of unselected primary sporadic colorectal tumours (n=41) were screened for genetic alterations of the gene. This analysis showed that structural PTEN abnormalities (mutation and/or deletion) were present in 17/41 (41%) tumours and affected both alleles in over half of the cases. All but one of the tumours with PTEN mutations were microsatellite stable (MSI-), suggesting that the PTEN gene may be involved in a distinct pathway of colorectal tumorigenesis that is separate from the pathway of mismatch repair deficiency. Subsequently, immunohistochemical analysis of PTEN expression showed that 29/41 (71%) tumours had altered (either reduced or absent) PTEN expression. All tumours with structural PTEN genetic abnormalities showed either a reduction or absence of PTEN expression, and, conversely, none of the tumours that retained PTEN expression harboured any detectable PTEN alterations. Additionally a subset of 12 (29%) tumours, that showed altered PTEN expression without the presence of any detectable PTEN genetic alteration(s), were also detected. It is possible that epigenetic monoallelic and biallelic silencing of PTEN occurs in these tumours. The presence of active, or functional PTEN is associated with low cellular levels of phosphorylated Akt/ PKB, a downstream target of PTEN which is involved in the induction of apoptosis. Loss of PTEN, or its lipid phosphatase activity, leads to an increase in the level of phosphorylated Akt and subsequent cell survival. The wild-type PTEN and each of the ten identified PTEN mutations were engineered into a mammalian expression system, and transiently transfected into the PTEN null (-/-) U87MG glioblastoma cell line, known to have high endogenous levels of phosphorylated Akt. Four of the 10 (40%) detected PTEN mutations were shown to alter PTEN function, shown by the inability of these mutants (K125E, K125X, D153N and 319X) to suppress endogenous phosphorylated Akt in U87MG cells like the wild type PTEN. Additionally these four PTEN mutants also showed an altered subcellular distribution compared to the subcellular distribution of wild type PTEN. While the wild type PTEN appeared to be equally distributed throughout the cytoplasm and nucleus the four above mentioned PTEN mutants appeared to localise predominantly to the cytoplasm. The demonstration of PTEN mutations in 19.5% of sporadic colorectal tumours, and deletions (LOH) in a similar proportion, with consequent reduction or abolition of PTEN expression in all cases, redefines the role of this important tumour suppressor gene in sporadic CRC, particularly MSI- tumours. Furthermore, based on these preliminary functional evaluations it would seem that 40% of the detected PTEN mutations (K125E, K125X, D153N and 319X) alter PTEN function and this has implications for the tumours in which these PTEN mutations were identified in. Tumours harbouring these PTEN mutations are expected to be lipid phosphatase deficient and as a consequence, have over-activation of the pro-proliferative PI3K/Akt pathway.
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