Redesigned Escherichia coli cytosine deaminase: a new facet of suicide gene therapy

Background: The Escherichia coli cytosine deaminase (CD)/5-fluorocytosine (5-FC) approach emerges as a potential aid for suicide gene therapy in the field of modern cancer treatment. However, the poor binding affinity of CD towards 5-FC compared to the natural substrate cytosine limits its application for successful suicide gene therapy. Redesigning a bacterial mutant CD with site-directed mutagenesis showed higher potency compare to wild-type CD (wtCD) in vitro. In the present study, we conducted a comparative analysis of F186W mutant and wtCD in a human lung cancer cell line (A549).

Methods and results: A comparative investigation was initiated with cell viability analyses by MTT and trypan blue dye exclusion assays on A549 cells transfected with wtCD and F186W genes. The mode of cell death was confirmed by acridine Orange/ethidium Bromide dual staining. Furthermore, flow cytometric assessments were performed by cell cycle analysis and caspase 3 assay. The experimental results showed a drug dependent decrease in cell viability; interestingly, mutant (F186W) reached IC50 at a much lower concentration of prodrug (5-FC) than wtCD. Cell cycle analysis showed that G1 arrest of a larger population of 5-FC treated F186W transfected cells, in contrast to that of wtCD under similar conditions. The caspase 3 assay revealed progression and execution of apoptosis.

Conclusions: We report a novel bacterial CD mutant that provided a superior alternate to the wtCD suicide gene. The F186W mutant required a much lower dose of 5-FC to reach its IC50 , thus minimizing the systemic side effects of large doses of 5-FC as required for wtCD.