1. ¹Department of Toxicogenetics, Leiden University Medical Center, Leiden, The Netherlands
2. ²Unit for Cancer Genetics, Institute of Molecular Pathology and Immunology of the University of Porto, Porto, Portugal
3. ³Division of Molecular Biology, The Netherlands Cancer Institute, Amsterdam, The Netherlands

Correspondence: Niels de Wind, Department of Toxicogenetics, Leiden University Medical Center, PO box 9600, 2300 RC Leiden, The Netherlands. E-mail: n.de_wind@lumc.nl

The first two authors contributed equally to this work.

Received 9 January 2009; Accepted 23 April 2009; Published online 26 May 2009.

Abstract

A substantial fraction of sporadic and inherited colorectal and endometrial cancers in humans is deficient in DNA mismatch repair (MMR). These cancers are characterized by length alterations in ubiquitous simple sequence repeats, a phenotype called microsatellite instability. Here we have exploited this phenotype by developing a novel approach for the highly selective gene therapy of MMR-deficient tumors. To achieve this selectivity, we mutated the VP22FCU1 suicide gene by inserting an out-of-frame microsatellite within its coding region. We show that in a significant fraction of microsatellite-instable (MSI) cells carrying the mutated suicide gene, full-length protein becomes expressed within a few cell doublings, presumably resulting from a reverting frameshift within the inserted microsatellite. Treatment of these cells with the innocuous prodrug 5-fluorocytosine (5-FC) induces strong cytotoxicity and we demonstrate that this owes to multiple bystander effects conferred by the suicide gene/prodrug combination. In a mouse model, MMR-deficient tumors that contained the out-of-frame VP22FCU1 gene displayed strong remission after treatment with 5-FC, without any obvious adverse systemic effects to the mouse. By virtue of its high selectivity and potency, this conditional enzyme/prodrug combination may hold promise for the treatment or prevention of MMR-deficient cancer in humans.