56. Intracerebroventricular and Intravenous AAV Gene Therapy in Canine Globoid Cell Leukodystrophy

Abstract

Globoid cell leukodystrophy (GLD), or Krabbe disease, results from a deficiency in the hydrolytic enzyme galactosylceramidase (GALC), which is responsible for degrading the central and peripheral nervous system (CNS, PNS) myelin lipids galactosylceramide and galactosylsphingosine (psychosine). Symptoms in infants include irritability and stiffness with disease progressing rapidly to a vegetative state and death typically resulting by 2 years of age. GLD is naturally occurring in dogs and signs include ataxia, tremors, pelvic limb paralysis, loss of hearing, and blindness, with disease progression warranting euthanasia at 15.9 ± 4.6 weeks. In GLD dogs, GALC concentrations are decreased in leukocytes, CSF, and tissues and psychosine concentrations are elevated in serum and CSF. Conduction velocities in peripheral nerves of GLD dogs are < 50% of normal and G-ratios of the sciatic nerve are significantly higher than normal. Brain stem auditory evoked response testing exhibits a loss of waveform integrity and increased central conduction time consistent with auditory system demyelination. MRI of the brain shows T2-weighted hyperintensity of the white matter, widened sulci, and enlarged ventricles and MRS identified decreases in N-acetylaspartate and increases in choline. Diffusion tensor imaging established decreased fractional anisotropy and increased radial diffusivity in multiple white matter tracts including the corpus callosum and posterior internal capsule. Histologically, GLD dogs revealed severe loss of myelin, a decrease in oligodendrocytes, astrogliosis, and microgliosis. Combination gene therapy has been evaluated in GLD affected dogs using AAVrh10 encoding canine GALC (AAVrh10-cGALC). Two GLD dogs received a low dose of AAVrh10-cGALC, 1.2E12, by combination intravenous (IV) and intracerebroventricular (ICV) injection routes and displayed a modest increase in survival to 17.9 and 22.1 weeks of age. Two additional GLD dogs were treated with an increased dose of AAVrh10-cGALC, 1.9E13, and survival was further increased to 30.3 and 43.1 weeks of age. Both low and high dose combination IV and ICV therapy delayed the onset of neurological signs and prevented the onset of tremors, one of the debilitating neurologic signs in untreated GLD dogs. High dose AAVrh10-cGALC, but not low dose, resulted in complete normalization of pelvic and thoracic limb NCV and near normal sensory NCV. Combination therapy of either dose had negligible effect on the auditory system, as treated animals showed little to no improvement in distance between waveform peaks or hearing threshold over untreated animals. After high dose treatment with AAVrh10-cGALC, GALC activity reached near normal levels in the cerebellum and sciatic nerve, with levels decreasing in more distal brain regions. GALC levels in the liver and heart were intermediate between affected untreated GLD and normal control dogs. Interestingly, the highest GALC activity was found in the quadriceps muscle. GLD dogs treated with high dose AAV had CSF psychosine concentrations lower than untreated and low dose AAV-treated GLD dogs, despite being substantially older. Efficacy of hematopoietic stem cell transplant (HSCT) alone and in combination with IV AAVrh10-cGALC is currently being evaluated in GLD dogs. Ongoing studies suggest that addition of IV infusion of AAVrh10-cGALC substantially increases survival as compared to HSCT alone.