Canine Hereditary Ataxia in Old English Sheepdogs and Gordon Setters Is Associated with a Defect in the Autophagy Gene Encoding RAB24

Abstract

Old English Sheepdogs and Gordon Setters suffer from a juvenile onset, autosomal recessive form of canine hereditary ataxia primarily affecting the Purkinje neuron of the cerebellar cortex. The clinical and histological characteristics are analogous to hereditary ataxias in humans. Linkage and genome-wide association studies on a cohort of related Old English Sheepdogs identified a region on CFA4 strongly associated with the disease phenotype. Targeted sequence capture and next generation sequencing of the region identified an A to C single nucleotide polymorphism (SNP) located at position 113 in exon 1 of an autophagy gene, RAB24, that segregated with the phenotype. Genotyping of six additional breeds of dogs affected with hereditary ataxia identified the same polymorphism in affected Gordon Setters that segregated perfectly with phenotype. The other breeds tested did not have the polymorphism. Genome-wide SNP genotyping of Gordon Setters identified a 1.9 MB region with an identical haplotype to affected Old English Sheepdogs. Histopathology, immunohistochemistry and ultrastructural evaluation of the brains of affected dogs from both breeds identified dramatic Purkinje neuron loss with axonal spheroids, accumulation of autophagosomes, ubiquitin positive inclusions and a diffuse increase in cytoplasmic neuronal ubiquitin staining. These findings recapitulate the changes reported in mice with induced neuron-specific autophagy defects. Taken together, our results suggest that a defect in RAB24, a gene associated with autophagy, is highly associated with and may contribute to canine hereditary ataxia in Old English Sheepdogs and Gordon Setters. This finding suggests that detailed investigation of autophagy pathways should be undertaken in human hereditary ataxia.

RAB24 facilitates clearance of autophagic compartments during basal conditions

“…autophagic vacuoles. 35 This study is in agreement with our findings that nucleotide binding is important for the recruitment of RAB24 to autophagic compartments and that RAB24 is needed for autolysosome clearance.…”

“…24 Recent results from Agler et al reveal that a mutation in RAB24 is associated with a canine ataxia, a hereditary neurodegenerative disease. 35 This mutation results in one amino acid change in the putative switch I region in RAB24 suggestive of an effect on nucleotide binding. Affected dogs exhibit Purkinje neuron loss and axonal spheroids throughout the granular layer cells, containing late Figure 8.…”

Emerging nexus between RAB GTPases, autophagy and neurodegeneration

“…Supporting this finding was the genetic study of canine heredity ataxia that ascribed mutations in RAB24 to be a cause of the disease. 10 The authors also described Purkinje neurons harboring a number of uncleared late autophagic vesicles. 10 Thus, RAB24-mediated autophagy induction might protect neurons against cell death.…”

Canine Hereditary Ataxia

“…Most of the familial syndromes reported thus far are inherited by an autosomal recessive mode including the Old English Sheepdog (OES), 11,12 Gordon Setter, 12,13 Scottish Terrier, 14 Finnish Hound, 15 Rhodesian Ridgeback, 16 Beagle, 17 and Australian Kelpie. 18 An X-linked disorder has been reported in the English Pointer.…”

“…Detailed examination in OES and Gordon setters reveals large intracytoplasmic ubiquitin inclusions in the cerebellar cortex and axonal spheroids in the cerebellar white matter that contain autophagy vacuoles. 12,26,27 Although these changes are typical of a neurodegenerative process, not all breeds show the same findings. For example, increased numbers of polyglucosan bodies are found in the molecular layer in the Scottish Terrier.…”