The first fly model of Sanfilippo syndrome type A has been developed and characterized, providing new data on genes involved in disease development.
The study, “Neuronal-specific impairment of heparan sulfate degradation in Drosophila reveals pathogenic mechanisms for Mucopolysaccharidosis type IIIA,” appeared in the journal Experimental Neurology.
Sanfilippo syndrome, or mucopolysaccharidosis type III (MPS III), is a group of five lysosomal disorders characterized by specific deficits in enzymes required for the breakdown of heparan sulfate, a complex sugar chain attached to a protein core, in cellular organelles called lysosomes.
The inability to degrade heparan sulfate and the associated storage of glycolipids — molecules made of sugar and fat portions — leads to intracellular alterations and, eventually, disease.
Patients with Sanfilippo syndrome usually start showing symptoms as toddlers. These include learning difficulties, aggressive or hyperactive behavior, and sleep impairment. The mental capacities of patients progressively worsen, and later they have general and localized seizures.
Despite the development of diverse animal models, the link between the accumulation of heparan sulfate and the mental and motor decline in patients with Sanfilippo syndrome remains to be identified. Understanding this correlation is essential to develop therapeutic approaches for the disease.
Fly (Drosophila) models of several lysosomal disorders are established. Humans and flies share common pathways of heparan sulfate biosynthesis. In addition, the biochemical structure of heparan sulfate has been characterized in flies, which showed similar features in mammals.
As in humans, the enzyme SGSH — whose deficiency characterizes the most common subtype of Sanfilippo syndrome, type A — is essential to degrading heparan sulfate in flies.
The research team used previously characterized lab assays to assess nervous system dysfunction and screened multiple Drosophila fly mutants in search of cellular and molecular disease mechanisms in Sanfilippo syndrome type A.
Investigators identified increased levels of heparan sulfate in flies with genetically decreased (knockdown) levels of the SGSH gene.
“This assay forms the basis for clinical diagnosis in [Sanfilippo syndrome] patients, where all cells of the body have impaired SGSH function,” the investigators wrote.
As these patients primarily have neurological symptoms, the scientists then performed specific SGSH knockdown in neurons. The results showed a higher abundance or production of markers of disrupted lysosomal function in the brain and salivary glands. This phenotype was associated with progressive impairment in the flies’ climbing ability, which is indicative of neurological dysfunction.
The data also revealed proteins and pathways that are associated with decreased climbing ability, including proteins related to autophagy (Atg1 and Atg18, involved in the cellular degradation of molecules in lysosomes) and the heat shock protein HSPA1, which stabilizes other proteins.
Increased production of two antioxidant enzymes (Sod1 and Sod2) was also linked with the worsened climbing phenotype in SGSH fly mutants.
“These results support maintenance of oxidative status as an important regulator of dysfunction in neuronal cells with impaired ability for heparan sulfate degradation,” the researchers wrote.
The study also revealed that reducing the biosynthesis of heparan sulfate by knocking-down the levels of two specific enzymes worsens the fly phenotype, “an important observation given that substrate inhibition is being evaluated clinically as a treatment for [Sanfilippo syndrome type A],” the researchers wrote.