Drug-inducible systems allowing the control of gene expression in mammalian cells are invaluable tools for genetic research, and could also fulfill essential roles in gene- and cell-based therapy. Currently available systems, however, often have limited in vivo functionality because of leakiness, insufficient levels of induction, lack of tissue specificity or prohibitively complicated designs. Here we describe a lentiviral vector-based, conditional gene expression system for drug-controllable expression of polymerase (Pol) II promoter-driven transgenes or Pol III promoter-controlled sequences encoding small inhibitory hairpin RNAs (shRNAs). This system has great robustness and versatility, governing tightly controlled gene expression in cell lines, in embryonic or hematopoietic stem cells, in human tumors xenotransplanted into nude mice, in the brain of rats injected intraparenchymally with the vector, and in transgenic mice generated by infection of fertilized oocytes. These results open up promising perspectives for basic or translational research and for the development of gene-based therapeutics.

Mutations in Cu/Zn superoxide dismutase (encoded by SOD1), one of the causes of familial amyotrophic lateral sclerosis (ALS), lead to progressive death of motoneurons through a gain-of-function mechanism. RNA interference (RNAi) mediated by viral vectors allows for long-term reduction in gene expression and represents an attractive therapeutic approach for genetic diseases characterized by acquired toxic properties. We report that in SOD1(G93A) transgenic mice, a model for familial ALS, intraspinal injection of a lentiviral vector that produces RNAi-mediated silencing of SOD1 substantially retards both the onset and the progression rate of the disease.

The Krüppel-associated box (KRAB) domain is a transcriptional repression module responsible for the DNA binding-dependent gene silencing activity of hundreds of vertebrate zinc finger proteins. We previously exploited KRAB-mediated repression within the context of a tet repressor-KRAB fusion protein and of lentiviral vectors to create a method of external gene control. We demonstrated that with this system transcriptional silencing was fully reversible in cell culture as well as in vivo. Here we reveal that, in sharp contrast, KRAB-mediated repression results in irreversible gene silencing through promoter DNA methylation if it acts during the first few days of mouse development.The human genome encodes several hundreds of KRAB 4 -containing ZFPs, a vertebrate-specific family of DNA-binding factors that likely plays major roles in gene regulatory networks important for ontogenesis, differentiation, and cell growth (1-4). When tethered to DNA via their zinc finger motifs, KRAB-ZFPs recruit the KAP1 (KRAB-associated protein 1) corepressor (also known as TIF1␤, transcription intermediary factor 1␤; KRIP1, KRAB-interacting protein 1; or TRIM28, tripartite motif protein 28) via their KRAB domain. KAP1 is characterized by the presence of a RING finger, B-boxes, a coiledcoil region, a PHD finger, and a bromodomain (5-7). The first three of these motifs define the so-called RBCC or TRIM domain, which is both necessary and sufficient for homo-oligomerization and direct binding to KRAB. Upon recruitment to DNA loci, KAP1 functions as a scaffold for the formation of a multimolecular complex comprising HP1 (heterochromatin protein 1), histone deacetylases, and histone methyltransferases, which induces transcriptional repression through the formation of heterochromatin (8, 9). The primordial importance of this process from the earliest stage of mammalian development is supported by the embryonic lethality of the mouse KAP1 knock-out, which results in failure to gastrulate and death at day E5.5 (10). Considering the obvious importance of KRAB-mediated gene control, it is quite remarkable that only very few gene targets of the KRAB-containing ZFPs have so far been identified (11).We described previously a conditional gene regulation system based on the use of the tetracycline-controllable trans-repressor tTRKRAB, a chimeric protein built by fusing KRAB with the DNA binding domain of the Escherichia coli tetracycline repressor (tetR). Upon binding tetracycline operator (tetO) sequences, tTRKRAB induces transcriptional repression, which can silence RNA polymerase I, II, and III promoters situated within a radius of a few kilobases, thus allowing for doxycycline-regulated gene expression and knockdown (12-15) (Fig. 1A). Using lentiviral vectors as vehicles for this system, we demonstrated that KRAB-induced silencing is fully reversible in vitro and in vivo even after prolonged periods of repression in diverse contexts such as in a variety of established human and murine cell lines, in primary cells, including mouse ES cells, and ...

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