A CLN8 nonsense mutation in the whole genome sequence of a mixed breed dog with neuronal ceroid lipofuscinosis and Australian Shepherd ancestry

Abstract

The neuronal ceroid lipofuscinoses (NCLs) are hereditary neurodegenerative diseases characterized by seizures and progressive cognitive decline, motor impairment, and vision loss accompanied by accumulation of autofluorescent lysosomal storage bodies in the central nervous system and elsewhere in the body. Mutations in at least 14 genes underlie the various forms of NCL. One of these genes, CLN8, encodes an intrinsic membrane protein of unknown function that appears to be localized primarily to the endoplasmic reticulum. Most CLN8 mutations in people result in a form of NCL with a late infantile onset and relatively rapid progression. A mixed breed dog with Australian Shepherd and Blue Heeler ancestry developed neurological signs characteristic of NCL starting at about 8months of age. The signs became progressively worse and the dog was euthanized at 21months of age due to seizures of increasing frequency and severity. Postmortem examination of the brain and retinas identified massive accumulations of intracellular autofluorescent inclusions characteristic of the NCLs. Whole genome sequencing of DNA from this dog identified a CLN8:c.585G>A transition that predicts a CLN8:p.Trp195* nonsense mutation. This mutation appears to be rare in both ancestral breeds. All of our 133 archived DNA samples from Blue Heelers, and 1481 of our 1488 archived Australian Shepherd DNA samples tested homozygous for the reference CLN8:c.585G allele. Four of the Australian Shepherd samples tested heterozygous and 3 tested homozygous for the mutant CLN8:c.585A allele. All 3 dogs homozygous for the A allele exhibited clinical signs of NCL and in 2 of them NCL was confirmed by postmortem evaluation of brain tissue. The occurrence of confirmed NCL in 3 of 4 CLN8:c.585A homozygous dogs, plus the occurrence of clinical signs consistent with NCL in the fourth homozygote strongly suggests that this rare truncating mutation causes NCL. Identification of this NCL-causing mutation provides the opportunity for identifying dogs that can be used to establish a canine model for the CLN8 disease (also known as late infantile variant or late infantile CLN8 disease).

Golden Retriever dogs with neuronal ceroid lipofuscinosis have a two-base-pair deletion and frameshift in CLN5

“…In other breeds, dogs with genetically defined NCL have developed many of the same neurological signs as these Golden Retrievers; however, the ages at onset of the disease signs have varied among the various canine NCL models. Compared to the affected Golden Retrievers, Dachshunds with mutations in PPT1 or TPP1, a Chinese Crested Dog with a mutation in MFSD8, and English Setters and a mixed breed dog with different CLN8 mutations all had diseases with earlier onsets of neurodegenerative signs [10,11,[14][15][16]. The onset of disease signs in Tibetan Terriers with NCL due to a mutation in ATP13A2 has occurred at a much later age (4 to 6 years) than that of the Golden Retrievers [35].…”

“…This was the third whole genome sequence we generated with DNA from a dog with a confirmed case of NCL. Earlier, we used this approach to identify a CLN8 nonsense mutation as the probable cause of NCL in a mixed-breed dog and to identify a single-base-pair deletion and frameshift in MFSD8 as the probable cause of NCL in a Chinese Crested Dog [14,16]. In the proband's whole genome sequence, a scan of the coding regions of the 13 canine orthologs of the genes known to harbor human NCL mutations identified only one plausible causal sequence variant: a 2 bp deletion and frame shift in CLN5.…”

“…Among the animal species, NCLs have most frequently been reported in dogs. Previous studies have identified 9 different mutations in the canine orthologs of 8 of the 13 genes associated with human NCL as shown in Table 1 [10][11][12][13][14][15][16][17][18][19][20][21][22]. Prior to the discovery of the causative mutations, NCL was common in three dog breeds: the Tibetan Terrier, the Border Collie, and the American Bulldog [23][24][25][26].…”

“…The identification of the mutations responsible for the NCL in each of these breeds [12,[17][18][19] has led to the development of DNA tests that revealed the identity of asymptomatic heterozygous mutation carriers and, thereby, has assisted dog breeders in their efforts to reduce the incidence of NCL in these breeds [24,25]. The NCLs in most of the other dog breeds appear to be rare [10,11,[13][14][15][16]. DNA-based screening to identify mutation carriers would not be cost-effective in these breeds except in breeding lines where dogs with the disease have already been identified.…”

Multifocal retinopathy in Dachshunds with CLN2 neuronal ceroid lipofuscinosis

“…Histologically the NCLs are characterized by accumulation of intracellular autofluorescent storage bodies in nervous tissues, including the retina, as well as in other tissues (Haltia, 2006). NCLs have been reported in numerous dog breeds, and of the canine NCLs, eight causative mutations have been identified in the canine orthologs of genes that contain NCL-causing mutations in human patients (Awano et al, 2006a; Awano et al, 2006b; Farias et al, 2011; Guo et al, 2014; Guo et al, 2015; Katz et al, 2011; Katz et al, 2005a; Melville et al, 2005; Sanders et al, 2010). A form of NCL that results from a null mutation in the TPP1 gene encoding the lysosomal storage enzyme tripeptidyl-peptidase-1 (TPP1) was discovered in miniature longhaired Dachshunds (Awano et al, 2006a).…”