Categories: Nervous system phene

Links to MONDO diseases: No links.

Mendelian trait/disorder: yes

Mode of inheritance: Mitochondrial

Considered a defect: yes

Key variant known: yes

Year key variant first reported: 2006

Species-specific name: canine spongiform leukoencephalomyelopathy; leucodystrophy; Shetland Sheepdog leukodystrophy

Inheritance: Li et al. (2006): "Three litters of Australian cattle dog puppies were born to the same bitch with two different sires, with both male and female pups being affected ... . The sires did not produce any affected pups when mated to other females, .... ... A second family of dogs of a different breed (Shetland sheepdogs) was also studied (Wood and Patterson, 2001). In this family, three litters with affected pups were born to the same bitch and two different sires. A total of nineteen pups were born, twelve being affected. Six pups from the first litter, two from the second litter, and four from the third litter developed a progressive neurologic disease. Both sexes were affected." Pedigree information suggested a maternal inheritance which was later confirmed as mitochondrial inheritance. As the mutation may only occur in some of the copies of mtDNA (heteroplasmy) not all puppies were affected and disease presented with variable levels of severity.

Molecular basis: Li et al. (2006) showed that this disorder is due to a missense variant in the mitochondrial gene for cytochrome b. The variant is a G>A transition as position 14,474 of the mitochondrial genome (NC_002008) predicted to result is a p.V98M substitution on the protein level. This is the first report in domesticated animals of a naturally-occurring base substitution in a mitochondrial gene, leading to an inherited disorder

Genetic engineering: Unknown
Have human generated variants been created, e.g. through genetic engineering and gene editing

Clinical features: Li et al. (2006): "most affected [Australian cattle] dogs showed a moderate to severe generalized whole body tremor at 3–4 weeks of age. However, one affected pup in the third litter did not develop obvious neurological signs until 9 weeks of age. The tremor and dysmetric gait were typical of cerebellar involvement. All pups were initially able to ambulate but became progressively more ataxic over the next several weeks. Interestingly, each pup had a slightly different phenotypic expression of its disease, along with variability in the rapidity of disease progression. With time, the pups failed to grow, showed spasticity in all four limbs, and had an inability to ambulate with eventual lateral recumbency. ... Vision and audition remained intact as did nociception. Electrophysiological testing of CNS function showed abnormal brainstem auditory evoked responses with conduction delay, which was also seen in studies of conduction in the spinal cord ... . ... Affected [Shetland sheepdog] pups were smaller than normal littermates. The neurologic defects were variable, ranging from mild to severe and consisted of tremor, ataxia, paresis, rigidity and spasticity, inability to ambulate, dysphagia, and seizures. Clinical signs developed between 7 days and 3 weeks with some pups dying by 7 weeks of age. All laboratory blood testing was normal, although a CT scan of one affected pup showed dilation of the lateral and 4th ventricles and diffuse hypomyelination of the white matter (Wood and Patterson, 2001)."

Pathology: Li et al. (2006): "In both breeds, widespread vacuolation of myelin was found in subcortical white matter, cerebellum, brain stem, and spinal cord ... , while no pathological changes were found in lung, heart, small intestines, jejunum, spleen, liver, and kidney. Axons surrounded by vacuolated myelin appeared intact, and occasional scattered demyelinated axons were noted ... . However, some axons appeared to be degenerating ... , and in severely affected areas, axonal loss and gliosis were seen ... . The degree of vacuolation varied from dog to dog in the spinal cord and also varied in its degree and tract location along the cord in the same animal. Occasional areas of normal myelin were seen ... ."

Breeds: Australian Cattle Dog (Dog) (VBO_0200088), Shetland Sheepdog (Dog) (VBO_0201217).
Breeds in which the phene has been documented. For breeds in which a likely causal variant has been documented, see the variant table below

Associated gene: