The first gene-therapy for Tay-Sachs (TS) disease appears to have been a success. TS is caused by a deficiency of enzymes HexA and HexB, that normally break down complex fats in the brain. Children born with TS have a life expectancy of around 5 years.
This therapy has been 14 years in development and uses an Adeno-Associated Virus (AAV) injected into the Cerebrospinal Fluid (CSF) and directly into the thalamus to ensure widespread delivery to important regions of the brain. This study used a dose that resulted in delaying progression out of safety concerns whereas evidence in animal trials suggests higher doses would constitute a total cure.
While this therapy is likely a god-send for families with children afflicted by TS, the road to distribution and widespread use is unclear. Not least because these therapies are incredibly expensive, making it very unlikely that they will be developed for ultra-rare genetic diseases in the near future.