Enzyme therapy directly into the brain tested for Sanfilippo type D
Results from mouse study support efforts to bring therapy to the clinic
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Delivering a lab-made version of the missing enzyme in Sanfilippo syndrome type D directly into the brain reduced toxic heparan sulfate buildup, a key Sanfilippo event, and eased signs of inflammation in a mouse model of the disease, a new study reports.
These benefits were observed even when the treatment was given on a schedule designed to mimic a real-world regimen in patients, “demonstrating preclinical efficacy for [Sanfilippo syndrome type D]” and supporting further investigation to bring the therapy to the clinic, the researchers noted.
According to the team, these new data “[allow] us to better understand [Sanfilippo type D] pathogenesis,” or disease development, which previously had “not been robustly achieved.”
“Collectively, the data presented herein represent a meaningful step [toward] the development of a treatment for [Sanfilippo type D],” the scientists wrote.
The study, “Recombinant human alpha-N-acetylglucosamine-6-sulfatase delivered to Sanfilippo D mice with repeated intracerebroventricular injections corrects CNS pathology,” was published in the journal PLOS One by U.S. researchers.
Sanfilippo syndrome is caused by mutations that impair the body’s ability to break down the complex sugar molecule heparan sulfate. As a result, heparan sulfate accumulates to toxic levels inside lysosomes — the cell’s recycling centers — throughout the body. This leads to damage primarily to the central nervous system, or CNS, comprised of the brain and spinal cord, and subsequent Sanfilippo symptoms.
Type D is rarest form of Sanfilippo, with no approved treatments
In Sanfilippo syndrome type D, which is the rarest type, this buildup results from mutations in the GNS gene, which prevents cells from producing an enzyme known as N-acetylglucosamine-6-sulfatase. That enzyme is necessary to remove specific sulfate groups from heparan sulfate, allowing the molecule to be properly broken down and cleared from cells.
“Currently, there are few studies looking at the cellular [disease mechanisms] of [Sanfilippo syndrome type D] and there are no approved treatments for [this disease type],” the researchers wrote.
To address these gaps, the team first set out to better understand how Sanfilippo syndrome type D affects the brain at the cellular level. The researchers then tested whether supplying the missing enzyme could help reverse these changes.
In the lab, the team used 15-week-old Sanfilippo D mice that completely lack the GNS enzyme and develop the same toxic buildup of heparan sulfate seen in people with the disease. Because type D can appear later in childhood, studying adult mice helps better reflect the timing of a diagnosis in humans. Healthy mice served as controls.
An analysis of brain tissue revealed 56 proteins present at different levels between Sanfilippo D and healthy mice. The Sanfilippo D mice showed clear signs of disrupted lysosomal function, problems in the cellular systems responsible for breaking down waste, and increased activity of immune-related pathways — all features consistent with the known biology of Sanfilippo syndrome.
The team then tested whether supplying a functional version of the missing enzyme, an approach known as enzyme replacement therapy or ERT, could reverse these abnormalities.
In mice, ERT delivered directly into fluid surrounding the brain and spinal cord
In the mice, a small device was implanted to deliver a lab-made version of the human GNS enzyme, called rhGNS, directly into the cerebrospinal fluid (CSF), the liquid that surrounds the brain and spinal cord. This delivery method bypasses the blood-brain barrier, a protective membrane that normally prevents large molecules such as enzymes from entering the CNS.
Adult mice received four doses of rhGNS over a period of two weeks, at doses of 3, 30, or 200 micrograms. A separate group received only a placebo solution. The mice tolerated surgery well and survived to the end of the study without early death or weight loss.
After treatment, rhGNS-treated mice showed significant increases in GNS enzyme activity in brain tissue, confirming that the ERT was active once delivered to the CNS.
These changes were accompanied by significant, dose-dependent reductions in heparan sulfate in both brain tissue and CSF, with the highest dose bringing levels close to those seen in healthy mice. The therapy also helped correct some of the altered protein patterns seen in untreated Sanfilippo D mice.
Additional experiments modeled treatment that could be used in patients
To better model a long-term ERT regimen that could be used in patients, the researchers conducted a second experiment in which 8-week-old mice received a single dose of rhGNS every 14 days for 12 weeks, or approximately three months. This prolonged regimen was well tolerated, with no premature deaths or significant weight loss.
As in the shorter study, rhGNS produced dose-dependent and sustained reductions in heparan sulfate in both brain tissue and CSF, again bringing levels at the highest dose close to those of healthy mice.
The researchers also examined glial inflammation, or gliosis, an inflammatory response driven by overactive microglia, the brain’s immune cells, and astrocytes, its support cells.
“Gliosis is a significant driver of pathology in the [Sanfilippo syndrome type D] mice and has been noted in [Sanfilippo syndrome type D] patient autopsies as well,” the researchers wrote.
In the somatosensory cortex and the striatum, two brain regions commonly affected in the disease, rhGNS led to dose-dependent reductions in markers of activated microglia and astrocytes. At the highest dose, gliosis dropped to levels seen in healthy mice.
This preclinical evidence … [supports] advancing research … to higher animal or human trials to bring ERT for [Sanfilippo syndrome type D] to the clinic.
According to the team, this study “provides … evidence that significant cellular disruptions in the brains of [Sanfilippo syndrome type D] mice exist and that rhGNS corrects this pathology when delivered directly to the CNS in adult animals.”
The findings will help in the development of new treatments for this rare disease type, the team noted.
“This preclinical evidence along with expert consensus that CSF [heparan sulfate] is a sufficient biomarker to use as a clinical endpoint support advancing research on rhGNS to higher animal or human trials to bring ERT for [Sanfilippo syndrome type D] to the clinic,” the researchers concluded.
