Differentiation-specific enhancer activity in transduced keratinocytes: a model for epidermal gene therapy.

HaCaT cells, a spontaneously immortalised, nontumorigenic keratinocyte line, were used as a more amenable model than primary keratinocytes for ex vivo-mediated gene transfer. These cells were transduced with retroviral vectors containing the factor IX cDNA under the control of a cytomegaloviral (CMV) promoter/enhancer alone or as hybrids with either the human papilloma virus-16 (HPV-16), keratin 14 (hK14) or keratin 5 (hK5) regulatory elements. Unlike primary keratinocytes, HaCaT cells tolerated transduction and G418 selection well. The HPV-16 and hK5 hybrid constructs were disproportionately more active in primary keratinocytes than in the basal-like HaCaT cells. After skin grafting to athymic mice, transduced HaCaT cells differentiated to form a stratified epidermis that remained viable for at least 99 days in some mice. Factor IX in plasma of mice grafted with vectors containing the HPV-16 and hK5 elements was two- to three-fold higher than with vectors containing the CMV promoter alone. These results are consistent with the expected up-regulation in differentiated suprabasal cells by the HPV-16 and hK5 elements. Enhancers may be useful in specifically targeting the differentiated layer of the epidermis or achieving higher levels of gene expression after transplantation.