A diphtheria toxin resistance marker for in vitro and in vivo selection of stably transduced human cells

We developed a selectable marker rendering human cells resistant to Diphtheria Toxin (DT). The marker (DT(R)) consists of a primary microRNA sequence engineered to downregulate the ubiquitous DPH2 gene, a key enzyme for the biosynthesis of the DT target diphthamide. DT(R) expression in human cells invariably rendered them resistant to DT in vitro, without altering basal cell growth. DT(R)-based selection efficiency and stability were comparable to those of established drug-resistance markers. As mice are insensitive to DT, DT(R)-based selection can be also applied in vivo. Direct injection of a GFP-DT(R) lentiviral vector into human cancer cell-line xenografts and patient-derived tumorgrafts implanted in mice, followed by systemic DT administration, yielded tumors entirely composed of permanently transduced cells and detectable by imaging systems. This approach enabled high-efficiency in vivo selection of xenografted human tumor tissues expressing ectopic transgenes, a hitherto unmet need for functional and morphological studies in laboratory animals.