Targeted genome editing in haematopoietic stem cells
Prof Luigi Naldini - Universita Vita-Salute San Raffaele, Milan, Italy
We are really seeing an important expansion of gene therapy trials, in particular concerning haematopoietic stem cells and T-cells actually. In haematopoietic stem cells, which is what we have been mostly developing for the last fifteen years, this is aiming at treating several genetic diseases of the blood and the immune system. I would say a key development has been a platform based on lentiviral vectors which are more efficient and also safer than earlier generation vectors. So patients are showing a very high level of genetic engineering of the haematopoiesis associated with safe outcomes, so no indication of abnormal clonal expansion, and very significant benefit. We have three trials ongoing in our institute using lentiviral vector gene therapy; two of them are very advanced and they are one in Wiskott-Aldrich syndrome which is a combined immunodeficiency and platelet deficiency and the other is in metachromatic leukodystrophy which is a histology disease. In both cases patients are treated with their own stem cells which are harvested, modified with a vector, reinfused after conditioning. As I said, we see up to 80% of the haematopoiesis post-treatment genetically modified. This is as long as six years now for the first treated patients. We have eight patients treated in one trial and twenty in the other already, all of them are showing a very high level of marking with no sign, as I said, of altered proliferation and, importantly, very significant benefit.
In the case of Wiskott-Aldrich essentially, as you know, this disease can be treated by allogeneic transplant so the patients who were enrolled in the trial were the ones for which there was no matching donor available. The patients essentially have a clinical benefit as after a successful transplant so they have been essentially very well recovered from all the signs of the disease. They did not essentially experience the risk of an allogenic transplant because they were using their own cells.
In the case of metachromatic leukodystrophy this is more interesting because it’s a neurodegenerative disease so why would you treat with a haematopoietic stem cell therapy. Because there is a rationale of some of the stem cells to home in the brain during the transplant procedure to engraft as microglia progenitors and they can repopulate in part the local population like glial macrophages. This can help to clear the storage that’s why many storage diseases like mucopolysaccharidosis, Hurler, are also undergoing normally a stem cell transplant for treatment. But in the case of leukodystrophy this is unfortunately very unsatisfactory so it’s not routinely employed. We did in the mice testing which, because of gene therapy, we can overexpress the therapeutic enzyme into the stem cell so when the stem cells home in the brain they also release some more enzyme which may cross correct the other cells in the brain. What we are seeing is that as long as we treated patients very early in the disease we have prevented disease development today. So essentially the patients are doing very well while they would probably be unfortunately dead now because of the disease. So it’s quite a significant benefit that stem cell therapy is giving.
So that is making a case that there are in principle important strategies, both where stem cell transplant is working but then you could not have a properly matching donor and you can use own stem cell for the patient. Or in cases where there’s no obvious treatment with normal stem cells and by gene therapy we can make these cells overexpress for instance increasing gene dosage and they become effective. While this gene therapy is being tested in the clinic what we are doing in the lab is now using a gene editing approach which is the new twist in the genetic engineering approach. Gene therapy, as I discuss in the clinic, is using vectors which still semi-randomly integrate in the genome. There’s always some risk where this vector goes to integrate, even we are not seeing, as I said, adverse events today. Editing in cells means very precise genetic changes so we essentially direct either the insertion to a specific locus which is safe or we rewrite the gene to correct the mutation, that’s why we call it editing so we essentially rewrite the letters. This is using an artificial enzyme, the most famous one being CRISPR now, but we have been working with zinc finger nucleases and before. This is a way by which we can actually make a true precision medicine. It’s early days, it’s not yet in the clinic with haematopoietic stem cells, it is in the clinic with T-cells now so we’ll probably be soon seeing the first clinical testing. It may be the new approach.