Conditional suppression of cellular genes: lentivirus vector-mediated drug-inducible RNA interference

RNA interference has emerged as a powerful technique to downregulate the expression of specific genes in cells and in animals, thus opening new perspectives in fields ranging from developmental genetics to molec-ular therapeutics. Here, we describe a method that significantly expands the potential of RNA interference by permitting the conditional suppression of genes in mammalian cells. Within a lentivirus vector background, we subjected the polymerase III promoter-dependent production of small interfering RNAs to doxycycline-con-trollable transcriptional repression. The resulting system can achieve the highly efficient and completely drug-inducible knockdown of cellular genes. As lentivirus vectors can stably transduce a wide variety of targets both in vitro and in vivo and can be used to generate transgenic animals, the present system should have broad applications. The externally controllable expression of exogenous cDNAs can be readily obtained in cells or in animals owing to tech-niques pioneered more than a decade ago (2, 8). Recently, it was demonstrated that the knockdown of endogenous genes could be achieved by RNA interference, and plasmid- or viral vector-based delivery systems for the stable expression of smal