Antitumor therapy based on cellular competition.

A major obstacle for the efficacy of cancer gene therapy is the need to transduce a high proportion of tumor cells with genes that directly or indirectly cause their death. During the formation of certain organs, cells compete among themselves to colonize the whole tissue. We reasoned that cell competition could be used to increase the proportion of cells that become transfected in a tumor. For this, a transgene that provides a selective advantage to the transfected cells should be used. If the same gene conferred a suicide mechanism the tumor could be eradicated after a period of selection. Bystander effect of transfected cells over neighboring nonmodified cells may eliminate tumors even with incomplete replacement of tumor cells. To test this strategy a competitive advantage was provided to colon cancer cells, using a gene encoding a fusion protein of dihydrofolate reductase (DHFR) and thymidine kinase (TK). DHFR confers resistance to methotrexate (MTX) and TK confers sensitivity to ganciclovir (GCV). Modified cells were also transduced with green fluorescent protein and parental cells with red fluorescent protein. In vitro and in vivo experiments were performed, using various proportions of modified cells and applying positive selection with MTX followed by negative selection with GCV. In vitro, cell competition was evident. Under MTX treatment, tumor cells transfected with the DHFR-TK fusion gene efficiently replaced the parental cells (from 0.1 to 90% in 35 days). After this positive selection period, negative selection with GCV eliminated the transfected cells. In vivo, positive selection was also achieved and resulted in a statistically significant therapeutic effect.