Characterization of β-glucuronidase for Enzyme Replacement Therapy in DYT6 Dystonia

BACKGROUND: Dystonia is a debilitating disorder defined by sustained involuntary twisting movements. The current symptomatic treatments for dystonia offer only modest efficacy but numerous side effects. Dominantly inherited, loss of function mutations in the THAP1 transcription factor cause DYT6 dystonia (DYT-THAP1). THAP1 modulates the development of oligodendrocyte progenitor cells (OPC) by regulating the catabolism of glycosaminoglycans (GAGs), a crucial component of the extracellular matrix. The loss of THAP1 within OPCs directly reduces GAG-catabolic lysosomal enzyme β-glucuronidase (GusB) causing the accumulation of GAGs that inhibit their own maturation to myelinating cells. The result is severe dysmyelination during early CNS maturation and impaired neurodevelopment. Genetic overexpression of GusB rescues the maturation deficits and CNS myelination in THAP1 deficient mice raising the critical question of whether β-glucuronidase enzyme replacement could restore myelination in THAP1 null mice. OBJECTIVE: Characterization of β-glucuronidase for enzyme replacement therapy (ERT) in DYT6 dystonia models. METHOD: To establish a symptomatic model of DYT6 dystonia, we utilized a Cre/LoxP based Thap1 conditional knockout mouse model ("THAP1-NCKO") and a GusB transgenic mouse line ("GusB-TG"). We evaluated the enzymatic activity and biodistribution of GusB in the CNS using biochemical and histochemical assays. RESULTS: We determined that THAP1-NCKO mice had lower GusB activity than their control counterparts. Interestingly, the activity of GusB is higher in adult (P60) mice compared to juvenile (P30) mice. Visualization of GusB activity showed that distribution of GusB was highest in white matter tracts. We showed that mice with THAP1- related deficits experienced a significant reduction in GusB activity within white matter tracts but not in other surveyed GusB positive brain areas. CONCLUSIONS: Age differentially affects CNS GusB enzymatic activity in a murine model of DYT6 dystonia. GusB enzyme exhibits a distinct biodistribution that varies regionally. White matter tracts experience more severe defects with THAP1 loss. Our results provide insights into the specific locations where GusB activity is deficient and highlight the importance of a "critical period" in which genetic insults have long lasting neurodevelopmental implications.