A new platform for regulating the expression of therapeutic genes, which will have major implications for gene and cell therapy, was presented at the 10th Annual Meeting of the American Society of Gene Therapy (ASGT) in Seattle.
Carefully regulating gene expression is crucial to the success of gene and cell therapy. For many applications, gene transfer is being employed to engineer cells for therapeutic factor production, expansion, selection of a desired cell type and even fail-safe suicide. These applications often require precise regulation in order to ensure gene expression in the correct tissue and prevent it in unwanted cell types.
Now, a team of scientists led by Dr. Luigi Naldini at the San Raffaele Telethon Institute for Gene Therapy (HSR-TIGET) in Milan have developed a new platform. Dr. Naldini and colleagues provided a striking demonstration that their new design can enable delivered genes to become highly responsive to a cell's identity. This is particularly relevant for the emerging field of stem cell gene therapy, in which genes are delivered into a cell that can give rise to many distinct cell types.
To achieve their goal, the TIGET scientists came up with a strategy to take advantage of a recently discovered network of gene regulation mediated by small RNA molecules, known as microRNA.
MicroRNAs downregulate the expression of specific genes in cells where the gene is not needed, and thereby have an important influence over the identity of the cell. More than 350 mammalian microRNAs have been identified, with many being present only in some specific tissues and cell types.
Dr. Naldini's group showed that the addition of microRNA binding sites into their gene delivery vectors results in gene regulation dictated by the cell's own cognate microRNA. Simply put, they could engineer their gene to be turned off in cells where the microRNA is present. As a proof-of-principle, the researchers used a lentiviral vector to introduce a gene encoding a fluorescent protein into human embryonic stem cells. Their gene contained target sites for a microRNA specific of neuronal cells. While undifferentiated, the embryonic stem cells expressed high levels of the fluorescent gene, however, as soon as the cells turned into neurons, the engineered gene was turned off in response to the presence of the microRNA in the neurons. So versatile is their approach, that they could even demonstrate the reverse. By changing the microRNA binding site in their vector to one recognized by an embryonic stem cell-specific microRNA, their vector was kept off in immature embryonic stem cells, but began to express itself as the cell turned into more defined tissues. This new platform now provides a means for scientists to achieve highly tailored gene expression patterns when delivering recombinant genetic material into cells.
“We had been looking for ways to regulate transgenes better, when it hit us, nature had already solved the problem for us,” says Dr. Brian Brown, one of the studies authors. “Now, we can utilize a cell's own regulatory network to control a gene we introduce”.
Dr. Naldini's group has already begun to successfully exploit microRNA regulation for achieving stable long-term correction of hemophilia in the mouse model and for improving the safety of hematopoietic stem cell gene therapy.
The American Society of Gene Therapy is a professional non-profit medical and scientific organization dedicated to the understanding, development and application of gene and related cell and nucleic acid therapies and the promotion of professional and public education in the field. For more information, visit