US researchers have used a combined stem cell and gene therapy approach in a mouse model of Sanfilippo type B. Their findings indicate the approach can be used to deliver NAGLU, the enzyme deficient in Sanfilippo type B, to the brain to treat the brain disease.
The team, led by A/Prof Michelina Iacovino of the University of California Los Angeles, used a human form of NAGLU linked to a protein called IGFII, which can help NAGLU enter cells. As a result, the NAGLU enzyme created by one cell in the brain is more likely to also enter other brain cells. This process, called ‘cross-correction’, increases the chances of the enzyme reaching as many brain cells as possible.
The NAGLU-IGFII gene was placed into previously-developed neuronal stem cells (NSCs) derived from mice with Sanfilippo type B. NSCs are a type of stem cell capable of both renewing itself and transforming into more specialised brain cells.
Compared to NSCs from mice without Sanfilippo, the type B NSCs treated with NAGLU-IGFII generated four times more NAGLU inside the cells and 20 times more NAGLU outside the cells.
When they treated Sanfilippo type B NSCs in the dish with the NAGLU-rich fluid collected from gene-corrected cell cultures, there was a 3.5 times increase in NAGLU activity in the type B cells. This showed that the NAGLU-IGFII could cross-correct other cells. Next, they wanted to confirm this was also possible in the brains of living mice.
Twelve newborn mice with Sanfilippo type B were treated with an injection of these NAGLU-IGFII NSCs and then evaluated at 9 months of age to determine the long-term effect of the treatment.
The team found that the injected NSCs could differentiate into more specialised brain cells, including neurons and glial cells that play an essential role in the brain. They also found that the NSCs travel to four other areas of the brain, however the amount of engraftment was highly variable between the mice.
Despite this, all 12 mice had NAGLU-IGFII enzyme activity of at least 10% that of their healthy siblings with one healthy copy of NAGLU. It is thought that only 1-5% of normal enzyme levels can correct storage accumulation in Sanfilippo and other diseases, so 10% could make a great difference.
The team found that the inflammation normally seen in the brains of Sanfilippo type B mice was greatly reduced and the size of the lysosomes in cells, which are usually greatly enlarged, was also reduced to the equivalent size seen in unaffected mice.
The team also identified two potential brain biomarkers that could be used to show the extent of neuron loss and damage in Sanfilippo syndrome, with the proteins Map2 and Synaptophysin restored to normal levels following the treatment.
While more research is needed before a clinical trial could be commenced, this study highlights the potential of combined neural stem cell and gene therapy approaches for Sanfilippo type B, particularly with the use of a modified NAGLU enzyme.
The work was supported by multiple grants including one from Cure Sanfilippo Foundation (USA).
Currently, there is one clinical trial focused on patients with Sanfilippo type B - Allievex’s enzyme replacement therapy trial, which delivers NAGLU to the brain via a port implanted under the scalp. Orchard Therapeutics is currently running a Sanfilippo type A clinical trial involving gene therapy-corrected hematopoietic stem cells, which are injected intravenously and can cross the blood-brain barrier (latest information here).