New Pathogenic Mechanisms in Krabbe Disease and their Correction via Adeno-Associated Virus Gene Therapy

Krabbe’s disease (KD, also known as Globoid Cell Leukodystrophy), is an infrequent lysosomal storage disease caused by mutations to the gene for the lysosomal enzyme galactosylceramidase (GALC), resulting in toxic accumulation of the lipid psychosine. Symptoms typically present around 6 months of age as progressive demyelination of the central and peripheral nervous system, along with muscle atrophy and neurosensory deficits, leading to an early decline if untreated. This three-part study first investigates the physiological impact of recently identified, psychosine-induced intraneuronal alpha-synuclein protein aggregates within the KD mouse model (twitcher). Knock-out of the alpha-synuclein gene (SNCA) resulted in modest improvements in survival and behavioral deficits, confirming a pathophysiological role of these aggregates in KD. In the second part, post-mortem brain tissue was analyzed to determine whether psychosine induced alpha-synuclein pathology could play a role in certain Parkinson’s disease (PD) patients. PD was correlated with increased psychosine levels and mutations in the GALC gene, suggesting that psychosine metabolism indeed may be a contributor in some PD patients and might represent a new biomarker for this disease. Last, the only current therapy available to KD patients is hematopoietic stem cell transplantation, a procedure that is mildly beneficial only if initiated before the onset of symptoms and yet patients still experience significant developmental delays and shortened lifespan. In this study’s third aim, a global gene therapy protocol was optimized and its long-term effects in the twitcher mouse examined. While the therapy resulted in over 1000% survival increases and initially a nearly complete metabolic correction, long-term treated mice developed a novel and unexpected phenotype. Rather than diffuse sclerosis of the CNS, the brains of treated twitcher displayed a multi-focal pattern of demyelination, associated with neuroinflammatory cells, blood vessel extravasation, and lysosomal accumulation. However, AAV gene therapy did prevent the formation of thioflavin-S positive protein aggregates throughout the animals’ lifespans. Together, this three-part study provides new evidence regarding alpha-synuclein’s role in KD, the influence of psychosine in adult alpha-synucleinopathies, and the longterm consequences of using AAV gene therapy to treat KD.