Scientists from Milan, Italy, have designed a therapeutic approach to insert genes in hematopoietic stem cells using viruses and treat genetic diseases. Two studies, one on Wiskott-Aldrich syndrome and one on metachromatic dystrophy, published in print in the journal Science, show that this gene therapy approach can help treat several genetic diseases that have no cure other than bone marrow transplantation.
Genetic diseases are characterized by a missing or mutated gene which leads to a protein that cannot function properly. The finding of a suitable donor for bone marrow or stem cell transplantation is vital. But finding a match is a matter of chance, and rarely possible. For these patients with unmatched donors, gene therapy is the only solution.
Gene therapy is the insertion of a gene into cells that lack the normal functioning gene, and to achieve this, scientists use viruses. Viruses have the remarkable ability to enter a human cell and deposit their genome into the human DNA. Thus, the virus genome then uses the host machinery to produce viral proteins and construct new viruses — the perfect parasite. But thanks to virologists who have extensively studied some viruses, we know exactly what the viral genome contains. And scientists can remove the parts of the viral genome that cause diseases, and keep the ones that are useful in gene therapy. These modified viruses, as a result, can still enter the cell, they can still insert their genome into the host’s genome and activate its machinery to make proteins, but they can cause no harm anymore. And if the modified virus contains a human gene that is damaged in a patient, the patient’s cells will now be able to produce functional copies of the protein and alleviate the damage.
Scientists have this knowledge for decades now, but why it was not easy to treat patients based on this strategy previously? Because it is not yet possible to introduce the gene of interest at the desired position in the human genome. If the viral genome is inserted in a position where some other disease-related genes exist, it is very likely that they will also become activated, together with the therapeutic protein, and cause another disease, e.g. cancer. Sometimes the viral genome will be inserted in a hidden, inaccessible genomic region and it will not produce enough amounts of the therapeutic protein. Or it will be inserted in a hyperactive region and produce large, toxic amounts of the protein.
To overcome these problems, researchers try to control where the viral genome will go by modifying it and use viruses that, more or less, insert their genomes in controlled locations in the host genome and induce safe and desirable production of therapeutic protein. Both recent studies used lentiviruses modified to contain the genes of Wiskott-Aldrich syndrome (WAS) protein or the arylsulfatase A (ARSA) gene. Deficiency of the WAS protein cause WAS and the patients present deficient immune system and reduced ability to form blood clots. Deficiency of the ARSA protein leads to severe defects in the nervous system and cause Metachromatic leukodystrophy (MLD) or several other diseases associated with motor and cognitive impairment. None of these genetic diseases have a cure yet.
The new approach used by these researchers has been successful in mice few years ago, and now they report their findings in human patients. “The work really shows that this approach, with improvements in viral vectors, is coming to a stage where it can be thought of as a valid treatment for a number of diseases,” said Patrick Aubourg to The Scientist, pediatric neurologist and gene therapy expert at INSERM in Paris, France, who was not involved in either study.
The scientists removed hematopoietic stem cells (HSCs) (cells which reside in the bone marrow and produce all kinds of blood cells) from three children with WAS, or from nine with early onset MLD. They transferred lentiviruses containing either the WASP or the ARSA gene, respectively, and they put the HSCs back into the patients.
Remarkably, all three WAS patients presented normal levels of the WAS protein which corrected the white blood cell measurements and alleviated all damage. Three patients with ARSA gene transfer, produced normal amounts of the ARSA protein which blocked disease manifestation while the siblings of these patients, who were not treated by this gene therapy approach, presented symptoms at the expected age. Particularly one of the patients, at age of 39 months was able to walk or run with a little help and have normal cognitive skills while the untreated sibling was unable to support head and neck, required a wheel-chair and was incapable of any voluntary speech.
“At this stage the patient normally have brain damage or are dead, but instead they are alive and thriving.” said Dr. Luigi Naldini who was involved in both studies,
The authors acknowledge that longer follow-up of the treatment is required to fully assess these therapeutic approaches together with their safety. However, the results are already promising as it is the first time that patients remained asymptomatic for such a long time.