Corrected Neural Stem Cells Ease Signs of Sanfilippo B in Mouse Study
Findings may support future clinical studies of the approach in patients
Brain implantation of genetically corrected stem cells restored NAGLU enzyme activity and reduced signs of Sanfilippo syndrome type B in mice, a study demonstrates.
These findings may support further clinical studies using the approach in people diagnosed with Sanfilippo B.
In addition, researchers revealed novel features of the condition that can be used to assess the effectiveness of potential treatments.
The mouse study, “Brain transplantation of genetically corrected Sanfilippo type B Neural Stem Cells induces partial cross-correction of the disease,” was published in the journal Molecular Therapy – Methods & Clinical Development.
Experimental treatments being investigated to restore NAGLU activity
Sanfilippo syndrome type B, one of the four types of Sanfilippo, is a genetic condition caused by deficiency of N-acetylglucosaminidase (NAGLU) — an enzyme that normally degrades a large sugar molecule called heparan sulfate. Because of NAGLU deficiency, heparan sulfate builds up to toxic levels, causing tissue damage, especially in the brain.
Various potential treatments are currently being investigated to restore NAGLU activity. This includes enzyme replacement therapy, which involves providing the missing enzyme from an external source, and gene therapy to deliver a healthy version of the gene to patients’ cells.
An alternative approach is to use a patient’s own induced pluripotent stem cells (iPSCs), a type of stem cell that can become almost any cell type, to create neural stem cells (NSCs). These NSCs can then be corrected with gene therapy and implanted back into the same patient’s brain to provide a long-term supply of the deficient enzyme.
Researchers at UCLA tested this approach in a Sanfilippo B mouse model, which partially restored NAGLU activity and eased signs of brain disease.
In this report, the same team expanded on this work using a NAGLU enzyme attached to another protein — insulin-like growth factor 2 (IGFII) — designed to help NAGLU get into cells and enhance enzyme activity even more.
First, the researchers reprogramed embryonic fibroblasts (a cell type commonly found in connective tissue) from a Sanfilippo mouse model into iPSCs, which were then corrected to produce NAGLU-IGFII and transformed into NSCs. Finally, corrected NSCs were transplanted into the brains of Sanfilippo mice at birth, and the long-term effects (nine months) were evaluated.
Initial experiments confirmed the engraftment of NSCs into the brain of Sanfilippo mice, which became different types of brain cells, including nerve cells (neurons), astrocytes, and oligodendrocytes.
NAGLU-IGFII activity increased in brains of transplanted mice to 10% of normal
Consistently, the team detected an increase in NAGLU-IGFII enzymatic activity in the brains of all transplanted mice to 10% of normal levels. Researchers noted that 1%–5% of normal enzyme activity is sufficient to correct heparan sulfate accumulation. Overall, enzyme activity correlated with the level of NSC engraftment into the brain.
In the brain, the activation of immune cells called microglia, and astrocytes, which help regulate immune responses, is a characteristic feature of Sanfilippo type B.
The researchers found a 72-fold increase in microglia activation in Sanfilippo mice compared with unaffected mice. In contrast, brain transplant of NAGLU-IGFII was associated with a 1.84-fold reduction in microglial activation. A similar trend was found for astrocyte activation, with a 2.2-fold increase in affected mice and restoration to normal levels in transplanted mice.
Due to the NAGLU deficiency, heparan sulfate builds up in lysosomes, cell compartments that digest and recycle various molecules. As expected, Lamp1, a protein marker for lysosomal storage accumulation, was 1.7-fold higher in Sanfilippo mice, but treatment with NAGLU-IGFII reduced Lamp1 to levels found in unaffected mice. This finding was most pronounced closer to sites of transplant.
In patients, Sanfilippo ultimately results in pronounced neuronal loss and brain atrophy (shrinkage). Map2, a protein marker for neuronal loss, was significantly reduced in the hippocampus of Sanfilippo type B mice, a brain region important in learning and memory. After NAGLU-IGFII treatment, Map2 was restored to levels similar to those found in unaffected mice.
Synaptophysin is an abundant membrane protein found in neurons, and its accumulation is a marker for nerve fiber damage due to inflammation. In Sanfilippo mice, synaptophysin was increased three-fold, but treatment reduced these levels comparable to unaffected controls.
Further, large synaptophysin aggregates co-localized with Lamp1, “suggesting a lysosomal-mediated synaptophysin accumulation and mechanism of [nerve cell communication] failure,” the team wrote. “Alternatively, these aggregates may end up in the lysosome and fail to be broken down.”
“We have shown that the successful engraftment of [NSCs] expressing a modified NAGLU protein can be achieved,” the researchers concluded, and is “capable of partly preventing the accumulation of storage material and glial activation in 9 month post-treated [Sanfilippo type B mice].”
“We have also identified novel features of neuronal [disease] in the form of Map2 immunoreactivity and synaptophysin accumulation that can be used to more rigorously assess the efficacy of potential treatments,” they added.