The role of DOK7 and its variants in colorectal cancer

The downstream of tyrosine kinase (Dok) protein family has seven members, Dok 1-7. The precise role of these proteins is not entirely clear although some authors have suggested a potential tumour suppressor role. The aims of this clinically oriented PhD were to determine the expression profile and role of DOK7 in human colorectal cancer, to determine whether DOK7 expression is associated with tumour pathology and clinical outcome data, to determine the functional role of DOK7 in colorectal cancer cells and to evaluate DOK7 coordinated cell signalling in colorectal cancer. Initial work revealed that DOK7 mRNA expression is significantly reduced in colorectal cancer tissue compared with normal colorectal tissue and is associated with overall survival in patients with colorectal cancer. DOK7 is variably expressed in colorectal cancer cell lines with high expression in HRT-18 cells and low expression in HT-115 and RKO cells. Difficulties in obtaining DOK7 knockdown and expression subsequently revealed that DOK7 has several splice variants coding for different isoforms. Further work investigating DOK7 variants 1-3 (DOK7V1-3) revealed that DOK7V1 mRNA expression is significantly reduced in colorectal cancer tissue compared with normal colorectal tissue and is associated with disease-free status and length of diseasefree survival. In contrast, DOK7V2 and DOK7V3 mRNA expression is significantly increased in colorectal cancer tissue compared with normal colorectal tissue. Functional assays using a DOK7V2 overexpression model did not show a significant difference in colorectal cancer cell growth, adhesion or invasion in RKO DOK7V2 overexpression cells compared to the control cell line. However, antibody based protein microarray revealed that DOK7V2 overexpression is associated with significant changes in the expression and phosphorylation (activation) of numerous proteins in RKO cells. Cell signalling pathways potentially implicated include PI-3K-Akt, mTOR, MAPK, Jak/STAT and angiogenesis signalling pathways. Our findings suggest that DOK7 plays an important role in colorectal carcinogenesis and that DOK7 isoforms may ply different and possibly opposing roles. Protein microarray data provides a number of potential avenues for further work to elucidate the interaction of Dok7 with other cell signalling proteins and may help identify future potential therapeutic targets