Sangamo Therapeutics announced on July 13th that it had received fast-track approval from the FDA for its genome editing program for MPS I and MPS II.
This is excellent news. If the trials are successful, patients will have a permanent, stable production of circulating enzyme via the liver. It may even be possible to do away with intravenous infusions of ERT altogether. I had briefly covered the technology involved in an earlier post.
However, a few caveats are worth noting.
The current trials are aimed at patients with the attenuated forms of the disease, that is to say, patients who do not have CNS involvement. Since conventional ERT does not cross the blood-brain barrier, it is unclear whether enzyme produced by gene editing will fare any better.
In MPS II, the deficient enzyme, iduronate sulfatase, requires activation by FGE (formylglycine generating enzyme). This is controlled by the gene SUMF1. Since this gene functions normally in MPS II, FGE is already available in the cells. So one must assume that no further supplies of FGE will be required to activate the iduronate sulfatase that is produced by gene editing.
Ultimately, this approach should be seen primarily as a means of permanently replacing conventional ERT. Treatment of the neurological forms will almost certainly require other approaches. Many of these, such as fusion proteins and gene therapy, have shown early promise, but more work needs to be done.