Low efficiency of gene transfer is one of the major limitations of gene therapy. A
solution to this problem may be transmission; by modification of the transgene, the gene product can be secreted and internalized by the surrounding cells. Cancer gene
therapy using the
herpes simplex thymidine kinase (HSV-TK) suicide gene is a promising treatment, and TK has been used in clinical trials with some success. However, this kind of
therapy has limited efficacy due to the low level of gene transfer reached. A modified TK
protein, capable of migrating from the producing cell to neighboring cells, would result in a greater proportion of cells affected by the treatment. As a first step towards transmission, we constructed a secretory form of HSV-TK by including the Igkappa
leader peptide in the gene. An endoplasmatic reticulum export signal was added to the construct to further improve its secretion. Secretion and
protein production in
cancer cells, the enzymatic activity of the modified
proteins and the ability of the modified TK to sensitize
cancer cells to
ganciclovir were tested. Addition of the Igkappa leader resulted in high levels of secretion of HSV-TK, with up to 70% of the total amount of
protein secreted. Inclusion of an ER export signal did not further improve secretion. The
enzyme activity of the secreted TK however, was decreased when compared to native TK. This study is the first to report on secretion of TK, and provides a first step in a novel strategy to improve the efficiency of cancer gene
therapy. The loss of function in secreted TK however, may present a major hurdle in the development of a transmitted form of TK.