Failure of wild-type p53 gene therapy in human cancer cells expressing a mutant p53 protein

The introduction of exogenous wild-type p53 into human cancer cells bearing p53 mutation does not necessarily result in inhibition of tumor growth. We have demonstrated this in MDA-MB468 breast cancer cells which are hemizygo us for p53 mutation and also in KM12SM colorectal carcinoma cells which are heterozygous for p53 mutation. The wtp53 transfectants decreased three- to four-fold the number of colonies compared with controls. Most wtp53-expres sing cells died by apoptosis at early passages, but some cells were able to form colonies and their proliferation rate was similar to control transfec tants. This reversion was observed in three of the six MDA-MB-468 clones se lected. When MDA-wtp53 transfectants were implanted orthotopically in nude mice only one clone showed prolonged tumor latency. No differences were fou nd in either tumor proliferation or apoptosis in tumors. Integration and ex pression of exogenous wtp53 was assessed in early and late passages in vitr o, and in tumors growing in vivo. Consistently, rye found mutations in the exogenous wtp53 gene of MDA-MB468 transfectants. Excision of the exogenous gene was an alternative to abrogate the wtp53 function that was extremely e fficient in KM12 cells, although they maintained resistance to geneticin. T hese results were corroborated by the functional assay in yeast. In conclus ion, wtp53 is inactivated in these cancer cells by different mechanisms. Th e presence of mutated p53 may confer genome instability and mutator ability . which allows cells to escape the effects of the exogenous wtp53 and contr ibutes to the failure of wtp53 gene therapy.