A rare homozygous MFSD8 single-base-pair deletion and frameshift in the whole genome sequence of a Chinese Crested dog with neuronal ceroid lipofuscinosis

Juyuan Guo, Dennis P O’Brien, Tendai Mhlanga-Mutangadura, Natasha J Olby, Jeremy F Taylor, Robert D Schnabel, Martin L Katz, Gary S Johnson

BackgroundThe neuronal ceroid lipofuscinoses are heritable lysosomal storage diseases characterized by progressive neurological impairment and the accumulation of autofluorescent storage granules in neurons and other cell types. Various forms of human neuronal ceroid lipofuscinosis have been attributed to mutations in at least 13 different genes. So far, mutations in the canine orthologs of 7 of these genes have been identified in DNA from dogs with neuronal ceroid lipofuscinosis. The identification of new causal mutations could lead to the establishment of canine models to investigate the pathogenesis of the corresponding human neuronal ceroid lipofuscinoses and to evaluate and optimize therapeutic interventions for these fatal human diseases.Case presentationWe obtained blood and formalin-fixed paraffin-embedded brain sections from a rescue dog that was reported to be a young adult Chinese Crested. The dog was euthanized at approximately 19 months of age as a consequence of progressive neurological decline that included blindness, anxiety, and cognitive impairment. A diagnosis of neuronal ceroid lipofuscinosis was made based on neurological signs, magnetic resonance imaging of the brain, and fluorescence microscopic and electron microscopic examination of brain sections. We isolated DNA from the blood and used it to generate a whole genome sequence with 33-fold average coverage. Among the 7.2 million potential sequence variants revealed by aligning the sequence reads to the canine genome reference sequence was a homozygous single base pair deletion in the canine ortholog of one of 13 known human NCL genes: MFSD8:c.843delT. MFSD8:c.843delT is predicted to cause a frame shift and premature stop codon resulting in a truncated protein, MFSD8:p.F282Lfs13*, missing its 239 C-terminal amino acids. The MFSD8:c.843delT allele is absent from the whole genome sequences of 101 healthy canids or dogs with other diseases. The genotyping of archived DNA from 1478 Chinese Cresteds did not identify any additional MFSD8:c.843delT homozygotes and found only one heterozygote.ConclusionWe conclude that the neurodegenerative disease of the Chinese Crested rescue dog was neuronal ceroid lipofuscinosis and that homozygosity for the MFSD8:c.843delT sequence variant was very likely to be the molecular-genetic cause of the disease.