NCL7 in Chihuahuas and Chinese Crested Dogs

The neuronal ceroid lipofuscinoses (NCL)  are a group of heterogeneous inherited neurodegenerative diseases  occurring in various kinds of mammals, incl. dogs, cats and humans.  In some animals, the causal mutation leading to NCL has not been found and described yet. In connection with NCL in animal and humans, mutations have been identified in six different genes CLN 1,2,3,4,5,6 and 8 (Daly et al. 1998, Gupta et al. 2001). Each mutation in the given gene causes a unique form of NCL. The NCL occurs in several dog breed, such as Border Collie, English setter, American bulldog, Dachshund, Polish lowland sheepdog or Tibetan terrier.  The NCL of type 1 applies to Dachshunds.

The characteristic feature of the NCL disease is excessive accumulation of waste compounds of lipid character (ceroid and lipofuscin) in the cells of the nervous system.  The presence of a high content of lipofuscin and its increasing pressure affects and destroys the nerve cells in the cortex and the cerebellum and the retina cells.

Symptoms of this disease seem to be highly variable even within a single breed. Affected dogs usually exhibit at least four of the following progressive neurological clinical signs: vision loss, behavioural changes, cerebellar ataxia, tremors, impaired cognitive and motor function, sleep disturbances and convulsions. NCL includes neurological symptoms such as disorientation, anxiety and aggression, seizures and eating difficulties. Sudden loss of vision is common. The onset and clinical course of the disease are highly variable and individual. The rate of neurodegeneration increases with age, and all affected individuals develop psychiatric abnormalities and convulsions. Changes in gait and posture may also be observed - stumbling gait, leg stiffness. As NCL is not curable, it results in the premature death of the affected individual.

The cause of NCL7 in Chihuahuas and Chinese Crested Dogs is the c.846del mutation in the MFSD8 gene. The mutation causes a frameshift and insertion of premature stop codon resulting to truncation of the protein. Symptoms of NCL7 in these breeds mainly include blindness, anxiety, and cognitive impairment.

The mode of inheritance of the mutation is autosomal recessive. This means that only individuals who inherit the mutated gene from both parents will develop the disease. Carriers of the mutated gene are clinically healthy but pass the mutation on to their offspring. In the case of a mating between two heterozygous individuals, theoretically 25% of the offspring will be completely healthy, 50% of the offspring will be carriers and 25% of the offspring will inherit the mutated gene from both parents and will therefore be affected by the disease.

The genetic test can clearly reveal the genotype of the animal and is a useful tool for breeders to prevent unintentional breeding of affected puppies.

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References:

Guo, J., O'Brien, D.P., Mhlanga-Mutangadura, T., Olby, N.J., Taylor, J.F., Schnabel, R.D., Katz, M.L., Johnson, G.S. : A rare homozygous MFSD8 single-base-pair deletion and frameshift in the whole genome sequence of a Chinese Crested dog with neuronal ceroid lipofuscinosis. BMC Vet Res 10:960, 2015. Pubmed reference: 25551667