The GM2 gangliosidoses, Tay-Sachs disease (TSD) and Sandhoff disease (SD), are progressive neurodegenerative disorders that are caused by a mutation in the enzyme β-N-acetylhexosaminidase (Hex). Due to the recent emergence of novel experimental treatments, biomarker development has become particularly relevant in GM2 gangliosidosis as an objective means to measure therapeutic efficacy. Here we describe blood, cerebrospinal fluid (CSF), magnetic resonance imaging (MRI), and electrodiagnostic methods for evaluating disease progression in the feline SD model and application of these approaches to assess AAV-mediated gene therapy. SD cats were treated by intracranial injections of the thalami combined with either the deep cerebellar nuclei or a single lateral ventricle using AAVrh8 vectors encoding feline Hex. Significantly altered in untreated SD cats, blood and CSF based biomarkers were largely normalized after AAV gene therapy. Also reduced after treatment were expansion of the lysosomal compartment in peripheral blood mononuclear cells and elevated activity of secondary lysosomal enzymes. MRI changes characteristic of the gangliosidoses were documented in SD cats and normalized after AAV gene therapy. The minimally invasive biomarkers reported herein should be useful to assess disease progression of untreated SD patients and those in future clinical trials.

Sandhoff disease (SD) is a fatal neurodegenerative disease caused by a mutation in the enzyme β-N-acetylhexosaminidase. Children with infantile onset SD develop seizures, loss of motor tone and swallowing problems, eventually reaching a vegetative state with death typically by 4years of age. Other symptoms include vertebral gibbus and cardiac abnormalities strikingly similar to those of the mucopolysaccharidoses. Isolated fibroblasts from SD patients have impaired catabolism of glycosaminoglycans (GAGs). To evaluate mucopolysaccharidosis-like features of the feline SD model, we utilized radiography, MRI, echocardiography, histopathology and GAG quantification of both central nervous system and peripheral tissues/fluids. The feline SD model exhibits cardiac valvular and structural abnormalities, skeletal changes and spinal cord compression that are consistent with accumulation of GAGs, but are much less prominent than the severe neurologic disease that defines the humane endpoint (4.5±0.5months). Sixteen weeks after intracranial AAV gene therapy, GAG storage was cleared in the SD cat cerebral cortex and liver, but not in the heart, lung, skeletal muscle, kidney, spleen, pancreas, small intestine, skin, or urine. GAG storage worsens with time and therefore may become a significant source of pathology in humans whose lives are substantially lengthened by gene therapy or other novel treatments for the primary, neurologic disease.

Sandhoff disease (SD) is an autosomal recessive lysosomal storage disease caused by defects in the β-subunit of β-N-acetylhexosaminidase (Hex), the enzyme that catabolizes GM2 ganglioside. Hex deficiency causes neuronal storage of GM2 and related glycoconjugates, resulting in progressive neurodegeneration and death, typically in infancy. No effective treatment exists for human patients. Adeno-associated virus (AAV) gene therapy led to improved clinical outcome and survival of SD cats treated before the onset of disease symptoms. Most human patients are diagnosed after clinical disease onset, so it is imperative to test AAV-gene therapy in symptomatic SD cats to provide a realistic indication of therapeutic benefits that can be expected in humans. In this study, AAVrh8 vectors injected into the thalamus and deep cerebellar nuclei of symptomatic SD cats resulted in widespread central nervous system enzyme distribution, although a substantial burden of storage material remained. Cats treated in the early symptomatic phase showed delayed disease progression and a significant survival increase versus untreated cats. Treatment was less effective when administered later in the disease course, although therapeutic benefit was still possible. Results are encouraging for the treatment of human patients and provide support for the development AAV-gene therapy for human SD.