To investigate the factors influencing the bystander effect--a key
element in the efficacy of suicide gene therapy against
cancer--we compared the effect triggered by four extremely efficient gene/
prodrug combinations, i.e., VZVtk/BVDU, the
thymidine kinase of Varicella zoster virus associated with (E)-5-(2-bromovinyl)-2'-
deoxyuridine; VZVtk/BVaraU, the same
enzyme associated with (E)-5-(2-bromovinyl)-1-beta-D-
arabinofuranosyluracil; HSVtk/BVDU, the association of the Herpes simplex virus
thymidine kinase with BVDU; and the classical HSVtk/GCV (
ganciclovir) paradigm. The cells used, the human MDA-MB-435
breast cancer, and the rat 9L
glioblastoma lines were equally sensitive in vitro to these four associations. In both cell types, the combinations involving
pyrimidine analogues (BVDU, BVaraU) displayed a smaller bystander killing than the combination involving the
purine analogue (GCV). In addition, the bystander effect induced by all the tk/
prodrug systems was reduced in MDA-MB-435 cells in comparison to 9L cells; albeit, the viral
kinases were produced at a higher level in the
breast cancer cells. All systems induced apoptotic death in the two cell types, but the MDA-MB-435 cells, deprived of
connexin 43, were noncommunicating in striking contrast with the 9L cells. That functional gap junctions have to be increased in order to improve the
breast cancer cell response to suicide gene therapy was demonstrated by transducing the
Cx43 gene: this modification enhanced the bystander effect associated in vitro with GCV treatment and, by itself, decreased the tumorigenicity of the untreated cells. However, the noncommunicating MDA-MB-435 cells triggered a significant bystander effect both in vitro and in vivo with the HSVtk/GCV system, showing that communication through gap junctions is not the only mechanism involved.