rtTA toxicity limits the usefulness of the SP-C-rtTA transgenic mouse

The doxycycline (DOX)-inducible gene expression systems allow tight temporal and spatial control of transgene expression, invaluable in studies of organ development and disease pathogenesis. Transgenic mice using the human Surfactant Protein C promoter to drive the expression of the reverse tetracycline transactivator (SP-C-rtTA) enabled functional analysis of essential gene function during lung development. Here we report that DOX-fed SP-C-rtTA mice during the period in which Type II cells differentiate results in cellular toxicity that may have confounded the interpretation of previous reports using this line. These effects included impaired alveologenesis, loss/reduction in expression of surfactant-associated proteins, and death. Severity was dependent on genetic background: outbred mice or those on a CD1 background are highly susceptible, whereas the C57BL/6 background appeared resistant by morphological criteria. However, quantitative analysis reveled that DOX-fed, SP-C-rtTA C57BL/6 pups had reduced surfactant mRNA accumulation that could contribute to synthetic lethality when combined with other genetic alterations. We conclude that the combination of genetic backgrounds, length of DOX exposure and the presence of the SP-C-rtTA transgene contributed more than previously appreciated to the similarities seen in the phenotypes reported by investigators using the SP-C-rtTA, (tetO)₇-Cre. These studies demonstrate the importance of using appropriate SP-C-rtTA only controls in all experiments.