Frequency of RPGRIP1 and MAP9 genetic modifiers of canine progressive retinal atrophy, in 132 breeds of dog.

Variants in RPGRIP1 and MAP9, termed RPGRIP1ins44 and MAP9del respectively, are both associated with a form of canine progressive retinal atrophy referred to as RPGRIP1-CRD and have both been demonstrated to modify the development and progression of this disease. In the current study both variants were genotyped in at least 50 dogs of 132 diverse breeds and the data reveal that both segregate in multiple breeds. Individually, each variant is common within largely non-overlapping subsets of breed, and there is a negative correlation between their frequencies within breeds that segregate both variants. The frequency of both variants exceeds 0.05 in a single breed only, the Miniature Longhaired Dachshund. These data indicate that both variants are likely to be ancient and predate the development and genetic isolation of modern dog breeds. That both variants are present individually at high frequency in multiple breeds is consistent with the hypothesis that homozygosity of either variant alone is not associated with a clinically relevant phenotype, whereas the negative correlation between the two variants is consistent with the application of selective pressure, from dog breeders, against homozygosity at both loci, probably due to the more severe phenotype associated with homozygosity at both loci.

Ocular melanosis in the adult Cairn Terrier population within the United Kingdom.

PURPOSE: Prospective investigation to determine the prevalence of ocular melanosis in adult Cairn Terriers within the United Kingdom using a previously established staging scheme. METHODS: Ophthalmic assessment was performed on adult Cairn Terriers, recruited from various geographic locations within the United Kingdom. Examination included gonioscopy, rebound tonometry, slit lamp biomicroscopy, and indirect ophthalmoscopy, performed by one examiner (AM). RESULTS: A total of 93 dogs were examined, including 52 females and 41 males, aged between 15 months and 16 years 4 months. Sixty of 93 dogs (64.5%) were >7 years of age. Nine of 93 dogs (9.6%) demonstrated changes consistent with ocular melanosis. Four of 9 (44.4%) had Stage 1 disease and 5 of 9 (55.6%), Stage 2. Stages 3 or 4 were not identified in any dogs. Mean intraocular pressures in affected and unaffected dogs were 14.7 mmHg (range 12-17 mmHg) and 12.8 mmHg (range 5-21 mmHg), respectively. Incomplete pupil dilation was noted in affected dogs following pharmacologic mydriasis. CONCLUSION: Ocular melanosis was identified in approximately 10% of examined dogs, over half were dogs of breeding age (<7 years of age). It is possible that Grade 1 disease could go undetected, prior to obvious scleral pigment accumulation (Grade 2 disease). It is therefore recommended that dogs undergoing pre-breeding screens have pre-dilation assessment of the anterior segment using slit lamp biomicroscopy with subsequent gonioscopy to clearly assess for circumferential thickening of the iris base that might otherwise go undetected. Additionally, regular reassessment of breeding dogs is advised as disease progression could be rapid.

A TNR Frameshift Variant in Weimaraner Dogs with an Exercise-Induced Paroxysmal Movement Disorder.

BACKGROUND: Some paroxysmal movement disorders remain without an identified genetic cause. OBJECTIVES: The aim was to identify the causal genetic variant for a paroxysmal dystonia-ataxia syndrome in Weimaraner dogs. METHODS: Clinical and diagnostic investigations were performed. Whole genome sequencing of one affected dog was used to identify private homozygous variants against 921 control genomes. RESULTS: Four Weimaraners were presented for episodes of abnormal gait. Results of examinations and diagnostic investigations were unremarkable. Whole genome sequencing revealed a private frameshift variant in the TNR (tenascin-R) gene in an affected dog, XM_038542431.1:c.831dupC, which is predicted to truncate more than 75% of the open read frame. Genotypes in a cohort of 4 affected and 70 unaffected Weimaraners showed perfect association with the disease phenotype. CONCLUSIONS: We report the association of a TNR variant with a paroxysmal dystonia-ataxia syndrome in Weimaraners. It might be relevant to include sequencing of this gene in diagnosing humans with unexplained paroxysmal movement disorders. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Characterisation of canine KCNIP4: A novel gene for cerebellar ataxia identified by whole-genome sequencing two affected Norwegian Buhund dogs.

A form of hereditary cerebellar ataxia has recently been described in the Norwegian Buhund dog breed. This study aimed to identify the genetic cause of the disease. Whole-genome sequencing of two Norwegian Buhund siblings diagnosed with progressive cerebellar ataxia was carried out, and sequences compared with 405 whole genome sequences of dogs of other breeds to filter benign common variants. Nine variants predicted to be deleterious segregated among the genomes in concordance with an autosomal recessive mode of inheritance, only one of which segregated within the breed when genotyped in additional Norwegian Buhunds. In total this variant was assessed in 802 whole genome sequences, and genotyped in an additional 505 unaffected dogs (including 146 Buhunds), and only four affected Norwegian Buhunds were homozygous for the variant. The variant identified, a T to C single nucleotide polymorphism (SNP) (NC_006585.3:g.88890674T>C), is predicted to cause a tryptophan to arginine substitution in a highly conserved region of the potassium voltage-gated channel interacting protein KCNIP4. This gene has not been implicated previously in hereditary ataxia in any species. Evaluation of KCNIP4 protein expression through western blot and immunohistochemical analysis using cerebellum tissue of affected and control dogs demonstrated that the mutation causes a dramatic reduction of KCNIP4 protein expression. The expression of alternative KCNIP4 transcripts within the canine cerebellum, and regional differences in KCNIP4 protein expression, were characterised through RT-PCR and immunohistochemistry respectively. The voltage-gated potassium channel protein KCND3 has previously been implicated in spinocerebellar ataxia, and our findings suggest that the Kv4 channel complex KCNIP accessory subunits also have an essential role in voltage-gated potassium channel function in the cerebellum and should be investigated as potential candidate genes for cerebellar ataxia in future studies in other species.

An ABCA4 loss-of-function mutation causes a canine form of Stargardt disease.

Autosomal recessive retinal degenerative diseases cause visual impairment and blindness in both humans and dogs. Currently, no standard treatment is available, but pioneering gene therapy-based canine models have been instrumental for clinical trials in humans. To study a novel form of retinal degeneration in Labrador retriever dogs with clinical signs indicating cone and rod degeneration, we used whole-genome sequencing of an affected sib-pair and their unaffected parents. A frameshift insertion in the ATP binding cassette subfamily A member 4 (ABCA4) gene (c.4176insC), leading to a premature stop codon in exon 28 (p.F1393Lfs*1395), was identified. In contrast to unaffected dogs, no full-length ABCA4 protein was detected in the retina of an affected dog. The ABCA4 gene encodes a membrane transporter protein localized in the outer segments of rod and cone photoreceptors. In humans, the ABCA4 gene is associated with Stargardt disease (STGD), an autosomal recessive retinal degeneration leading to central visual impairment. A hallmark of STGD is the accumulation of lipofuscin deposits in the retinal pigment epithelium (RPE). The discovery of a canine homozygous ABCA4 loss-of-function mutation may advance the development of dog as a large animal model for human STGD.

NME5 frameshift variant in Alaskan Malamutes with primary ciliary dyskinesia.

Primary ciliary dyskinesia (PCD) is a hereditary defect of motile cilia in humans and several domestic animal species. Typical clinical findings are chronic recurrent infections of the respiratory tract and fertility problems. We analyzed an Alaskan Malamute family, in which two out of six puppies were affected by PCD. The parents were unaffected suggesting autosomal recessive inheritance. Linkage and homozygosity mapping defined critical intervals comprising ~118 Mb. Whole genome sequencing of one case and comparison to 601 control genomes identified a disease associated frameshift variant, c.43delA, in the NME5 gene encoding a sparsely characterized protein associated with ciliary function. Nme5-/- knockout mice exhibit doming of the skull, hydrocephalus and sperm flagellar defects. The genotypes at NME5:c.43delA showed the expected co-segregation with the phenotype in the Alaskan Malamute family. An additional unrelated Alaskan Malamute with PCD and hydrocephalus that became available later in the study was also homozygous mutant at the NME5:c.43delA variant. The mutant allele was not present in more than 1000 control dogs from different breeds. Immunohistochemistry demonstrated absence of the NME5 protein from nasal epithelia of an affected dog. We therefore propose NME5:c.43delA as the most likely candidate causative variant for PCD in Alaskan Malamutes. These findings enable genetic testing to avoid the unintentional breeding of affected dogs in the future. Furthermore, the results of this study identify NME5 as a novel candidate gene for unsolved human PCD and/or hydrocephalus cases.

Investigation of the allele frequency of the G>A intron 10 ADAMTS17 mutation causing primary lens luxation in the Portuguese Podengo breed.

OBJECTIVE: To establish the allele frequency of the PLL-causing G>A intron 10 ADAMTS17 mutation in the Portuguese Podengo population in the UK and investigate a possible correlation between the mutation and short stature. METHODS: Two groups of dogs (Group 1 and Group 2) were recruited for the purpose of the study. Group 1 (n = 40) consisted of dogs which were genotyped only and Group 2 (n = 42) consisted of dogs which were genotyped, underwent a full ophthalmological examination and also had their height measured at the withers. RESULTS: In Group 1, genotyping for the ADAMTS17:c.1473+1G>A mutation confirmed 1/40 homozygous for the mutated allele (-/-), 7/40 heterozygous for the mutated allele (+/-), and 32/40 homozygous for the wild-type allele (+/+) dogs. In Group 2, genotyping of the dogs confirmed 6/42 heterozygous for the mutated allele (+/-) and homozygous for the wild-type allele (+/+) dogs. In total, 1/82 (1.2%) dogs were confirmed to be homozygous for the mutated allele, 13/82 (15.8%) heterozygous for the mutated allele and 68/82 (83%) homozygous for the wild-type allele. The frequency of the mutated allele across both groups was calculated as 0.09. A statistically significant correlation between the mutation and short stature could not be established (p = .590). CONCLUSIONS: The frequency of the mutation calculated in this study (0.09) is high. Genetic testing should be considered for each dog prior to breeding with a view of selective breeding.

Improving the resolution of canine genome-wide association studies using genotype imputation: A study of two breeds.

Genotype imputation using a reference panel that combines high-density array data and publicly available whole genome sequence consortium variant data is potentially a cost-effective method to increase the density of extant lower-density array datasets. In this study, three datasets (two Border Collie; one Italian Spinone) generated using a legacy array (Illumina CanineHD, 173 662 SNPs) were utilised to assess the feasibility and accuracy of this approach and to gather additional evidence for the efficacy of canine genotype imputation. The cosmopolitan reference panels used to impute genotypes comprised dogs of 158 breeds, mixed breed dogs, wolves and Chinese indigenous dogs, as well as breed-specific individuals genotyped using the Axiom Canine HD array. The two Border Collie reference panels comprised 808 individuals including 79 Border Collies and 426 326 or 426 332 SNPs; and the Italian Spinone reference panel comprised 807 individuals including 38 Italian Spinoni and 476 313 SNPs. A high accuracy for imputation was observed, with the lowest accuracy observed for one of the Border Collie datasets (mean R2  = 0.94) and the highest for the Italian Spinone dataset (mean R2  = 0.97). This study's findings demonstrate that imputation of a legacy array study set using a reference panel comprising both breed-specific array data and multi-breed variant data derived from whole genomes is effective and accurate. The process of canine genotype imputation, using the valuable growing resource of publicly available canine genome variant datasets alongside breed-specific data, is described in detail to facilitate and encourage use of this technique in canine genetics.

MKLN1 splicing defect in dogs with lethal acrodermatitis.

Lethal acrodermatitis (LAD) is a genodermatosis with monogenic autosomal recessive inheritance in Bull Terriers and Miniature Bull Terriers. The LAD phenotype is characterized by poor growth, immune deficiency, and skin lesions, especially at the paws. Utilizing a combination of genome wide association study and haplotype analysis, we mapped the LAD locus to a critical interval of ~1.11 Mb on chromosome 14. Whole genome sequencing of an LAD affected dog revealed a splice region variant in the MKLN1 gene that was not present in 191 control genomes (chr14:5,731,405T>G or MKLN1:c.400+3A>C). This variant showed perfect association in a larger combined Bull Terrier/Miniature Bull Terrier cohort of 46 cases and 294 controls. The variant was absent from 462 genetically diverse control dogs of 62 other dog breeds. RT-PCR analysis of skin RNA from an affected and a control dog demonstrated skipping of exon 4 in the MKLN1 transcripts of the LAD affected dog, which leads to a shift in the MKLN1 reading frame. MKLN1 encodes the widely expressed intracellular protein muskelin 1, for which diverse functions in cell adhesion, morphology, spreading, and intracellular transport processes are discussed. While the pathogenesis of LAD remains unclear, our data facilitate genetic testing of Bull Terriers and Miniature Bull Terriers to prevent the unintentional production of LAD affected dogs. This study may provide a starting point to further clarify the elusive physiological role of muskelin 1 in vivo.

A LINE-1 insertion situated in the promoter of IMPG2 is associated with autosomal recessive progressive retinal atrophy in Lhasa Apso dogs.

BACKGROUND: Canine progressive retinal atrophies are a group of hereditary retinal degenerations in dogs characterised by depletion of photoreceptor cells in the retina, which ultimately leads to blindness. PRA in the Lhasa Apso (LA) dog has not previously been clinically characterised or described in the literature, but owners in the UK are advised to have their dog examined through the British Veterinary Association/ Kennel Club/ International Sheep Dog Society (BVA/KC/ISDS) eye scheme annually, and similar schemes that are in operation in other countries. After the exclusion of 25 previously reported canine retinal mutations in LA PRA-affected dogs, we sought to identify the genetic cause of PRA in this breed. RESULTS: Analysis of whole-exome sequencing data of three PRA-affected LA and three LA without signs of PRA did not identify any exonic or splice site variants, suggesting the causal variant was non-exonic. We subsequently undertook a genome-wide association study (GWAS), which identified a 1.3 Mb disease-associated region on canine chromosome 33, followed by whole-genome sequencing analysis that revealed a long interspersed element-1 (LINE-1) insertion upstream of the IMPG2 gene. IMPG2 has previously been implicated in human retinal disease; however, until now no canine PRAs have been associated with this gene. The identification of this PRA-associated variant has enabled the development of a DNA test for this form of PRA in the breed, here termed PRA4 to distinguish it from other forms of PRA described in other breeds. This test has been used to determine the genotypes of over 900 LA dogs. A large cohort of genotyped dogs was used to estimate the allele frequency as between 0.07-0.1 in the UK LA population. CONCLUSIONS: Through the use of GWAS and subsequent sequencing of a PRA case, we have identified a LINE-1 insertion in the retinal candidate gene IMPG2 that is associated with a form of PRA in the LA dog. Validation of this variant in 447 dogs of 123 breeds determined it was private to LA dogs. We envisage that, over time, the developed DNA test will offer breeders the opportunity to avoid producing dogs affected with this form of PRA.

A variant in OLFML3 is associated with pectinate ligament abnormality and primary closed-angle glaucoma in Border Collies from the United Kingdom.

PURPOSE: Canine primary closed-angle glaucoma (PCAG) is a complex disease caused by multiple genetic factors. A c.590G>A variant in OLFML3 was recently reported to be a candidate for pectinate ligament abnormality (PLA) and PCAG in the Border Collie. We investigated the association of this variant with PLA and PCAG in Border Collies from the United Kingdom. METHODS: The OLFML3 variant was genotyped in 106 Border Collies comprising 90 with normal eyes (controls) and 16 with PLA (n = 11) and/or PCAG (n = 5) (cases). Genotyping was performed in an additional 103 Border Collies to estimate variant frequency within the population. To investigate the association of the variant with disease in other breeds, genotyping was performed in 337 non-Border Collies with PLA and/or PCAG. RESULTS: Of the 90 controls, 71 were homozygous for the wild-type allele, two were homozygous for the variant, and 17 were heterozygous. Of the 16 cases, three were homozygous for the wild-type allele, 11 were homozygous for the variant, and two were heterozygous. The association of the variant allele with disease was significant (P = 1.1 x 10-9 ). We estimated the frequency of this variant to be 4.4% within the United Kingdom Border Collie population, and it was not identified in clinically affected dogs of any other breed. CONCLUSIONS: This study confirms the association of the OLFML3 variant with PLA and PCAG in Border Collies from the United Kingdom. DNA testing for the variant and selective breeding can reasonably be expected to result in a reduction of PLA and PCAG prevalence in the breed.

Primary closed angle glaucoma in the Basset Hound: Genetic investigations using genome-wide association and RNA sequencing strategies.

Purpose: To investigate the genetic basis of primary closed angle glaucoma (PCAG) in European Basset Hounds using genome-wide association and RNA sequencing strategies. Methods: DNA samples from 119 European Basset Hounds were genotyped on the 170 K SNP CanineHD BeadChip array (Illumina) comprising 37 with normal iridocorneal angles (controls), 57 with pectinate ligament abnormality (PLA cases), and 25 with PCAG (PCAG cases). Genome-wide association studies (GWASs) of the PLA and PCAG cases were conducted. Whole transcriptome sequences of iridocorneal angle tissues from five Basset Hounds with PCAG were compared with those from four dogs with normal eyes to investigate differences in gene expression between the affected and unaffected eyes in GWAS-associated loci. A variant in NEB, previously reported to be associated with PCAG in American Basset Hounds, was genotyped in cohorts of European Basset Hounds and non-Basset Hounds. Results: The GWASs revealed 1.4 and 0.2 Mb regions, on chromosomes 24 and 37, respectively, that are statistically associated with PCAG. The former locus has previously been associated with glaucoma in humans. Whole transcriptome analysis revealed differential gene expression of eight genes within these two loci. The NEB variant was not associated with PLA or PCAG in this set of European Basset Hounds. Conclusions: We identified two novel loci for canine PCAG. Further investigation is required to elucidate candidate variants that underlie canine PCAG.

Canine genome assembly correction facilitates identification of a MAP9 deletion as a potential age of onset modifier for RPGRIP1-associated canine retinal degeneration.

Retinal degeneration (RD) in the Miniature Long Haired Dachshund (MLHD) is a cone-rod dystrophy resulting in eventual blindness in affected individuals. In a previous study, a 44-nucleotide insertion (ins44) in exon 2 of RPGRIP1 was associated with RD. However, results on an extended population of MLHD revealed a variable RD onset age for ins44 homozygous dogs. Further investigations using a genome-wide association study comparing early onset and late onset RD cases identified an age of onset modifying locus for RD, approximately 30 Mb upstream of RPGRIP1 on chr15. In this investigation, target enriched sequencing identified a MAP9 deletion spanning approximately 22 kb associated with early RD onset. Identification of the deletion required correction to the CanFam3.1 genome build as canine MAP9 is part of a historic tandem duplication, resulting in incomplete assembly of this genome region. The deletion breakpoints were identified in MAP9 intron 10 and in a downstream partial MAP9 pseudogene. The fusion of these two genes, which we have called MAP9 EORD (microtubule-associated protein, early onset retinal degeneration), is in frame and is expressed at the RNA level, with the 3' region containing several predicted deleterious variants. We speculate that MAP9 associates with α-tubulin in the basal body of the cilium. RPGRIP1 is also known to locate to the cilium, where it is closely associated with RPGR. RPGRIP1 mutations also cause redistribution of α-tubulin away from the ciliary region in photoreceptors. Hence, a MAP9 partial deficit is a particularly attractive candidate to synergise with a partial RPGRIP1 deficit to cause a more serious disease.

Genome sequencing reveals a splice donor site mutation in the SNX14 gene associated with a novel cerebellar cortical degeneration in the Hungarian Vizsla dog breed.

BACKGROUND: Cerebellar cortical degeneration (CCD) is an increasingly recognised neurodegenerative disease process affecting many dog breeds. Typical presentation consists of a progressive cerebellar ataxia, with a variable age at onset and rate of progression between different breeds. Cerebellar histopathological findings typically consist of primary Purkinje neuronal degeneration and loss, with variable secondary depletion of the granular and molecular cell layers. Causative genes have been identified associated with CCD in several breeds, allowing screening for selective breeding to reduce the prevalence of these conditions. There have been no previous reports of CCD in Hungarian Vizslas. RESULTS: Two full-sibling Hungarian Vizsla puppies from a litter of nine presented with a history of progressive ataxia, starting around three months of age. Clinical signs included marked hypermetric and dysmetric ataxia, truncal sway, intention tremors and absent menace responses, with positional horizontal nystagmus in one dog. Routine diagnostic investigations were unremarkable, and magnetic resonance imaging performed in one dog revealed mild craniodorsal cerebellar sulci widening, supportive of cerebellar atrophy. Owners of both dogs elected for euthanasia shortly after the onset of signs. Histopathological examination revealed primary Purkinje neuron loss consistent with CCD. Whole genome sequencing was used to successfully identify a disease-associated splice donor site variant in the sorting nexin 14 gene (SNX14) as a strong causative candidate. An altered SNX14 splicing pattern for a CCD case was demonstrated by RNA analysis, and no SNX14 protein could be detected in CCD case cerebellum by western blotting. SNX14 is involved in maintaining normal neuronal excitability and synaptic transmission, and a mutation has recently been found to cause autosomal recessive cerebellar ataxia and intellectual disability syndrome in humans. Genetic screening of 133 unaffected Hungarian Vizslas revealed the presence of three heterozygotes, supporting the presence of carriers in the wider population. CONCLUSIONS: This is the first report of CCD in Hungarian Vizsla dogs and identifies a highly associated splice donor site mutation in SNX14, with an autosomal recessive mode of inheritance suspected.

Investigation of SLA4A3 as a candidate gene for human retinal disease.

SLC4A3 has been shown to cause retinal degeneration in a genetically engineered knockout mouse, and in a naturally occurring form of canine progressive retinal atrophy considered to be the equivalent of retinitis pigmentosa in humans (RP). This study was undertaken to investigate if SLC4A3 coding variants were implicated in human retinal degeneration. SLC4A3 exons were amplified and sequenced in 200 patients with autosomal recessive retinal degeneration who had no known molecular diagnosis for their condition, which included 197 unrelated individuals with suspected RP and three individuals with other forms of retinal disease. Three rare variants were identified that were predicted to be potentially pathogenic, however each variant was heterozygous in a single patient and therefore not considered disease-causing in isolation. Of these three variants, SNP-3 was the rarest, with an allele frequency of 7.06 x 10(-5) (>46,000 exomes from the ExAC database). In conclusion, no compound heterozygous or homozygous potentially pathogenic variants were identified that would account for recessive RP or retinal degeneration in this cohort, however the possibility remains that the rare variants identified could be acting with as yet undiscovered mutations in introns or regulatory regions. SLC4A3 remains an excellent candidate gene for human retinal degeneration, and with the advent of whole exome and whole genome sequencing of cohorts of molecularly unsolved patients with syndromic and non-syndromic forms of retinal degeneration, SLC4A3 may yet be implicated in human disease.

A Carbohydrate Sulfotransferase-6 (CHST6) gene mutation is associated with Macular Corneal Dystrophy in Labrador Retrievers.

PURPOSE: To locate and identify variants associated with macular corneal dystrophy (MCD) in Labrador Retriever (LR) dogs, in the candidate gene carbohydrate sulfotransferase-6 (CHST6). METHODS: The single coding exon of canine CHST6 was sequenced in one affected LR with MCD and one control LR clinically clear of ocular disease. A further 71 control LR with unknown clinical status were sequenced for the putative causal variant in CHST6. A TaqMan SNP genotyping assay was developed and used to screen an additional 84 dogs (five affected LR and 79 clinically clear LR). Finally, the variant was screened in a third cohort of 89 unrelated LR with unknown clinical status to estimate its allele frequency in the population of LR in the United Kingdom. RESULTS: A single nucleotide polymorphism (SNP) was identified within the coding exon of CHST6, resulting in a missense mutation (c.814C>A, p.R272S). All six LR affected with MCD were homozygous for the mutant allele, while 140/151 control LR were homozygous for the wild-type allele and 11/151 were heterozygous for the mutation, indicating an association with MCD (P < 10-5 ). The mutant allele was present in the unrelated LR cohort at a frequency of 0.017, suggesting carrier and affection rates of 3.3% and 0.028%, respectively. CONCLUSIONS: A missense mutation in the CHST6 gene is strongly associated with autosomal recessive MCD in the LR.

Spinocerebellar ataxia in the Italian Spinone dog is associated with an intronic GAA repeat expansion in ITPR1.

Spinocerebellar ataxia in the Italian Spinone dog breed is characterised by a progressive gait abnormality that manifests from approximately 4 months of age. The disorder shows an autosomal recessive mode of inheritance, and affected individuals are usually euthanized by one year of age on welfare grounds due to an inability to ambulate. Using a homozygosity mapping technique with six cases and six controls, we mapped the disease locus to chromosome 20 of the canine genome. Linkage analysis across an extended pedigree confirmed the association, with microsatellite C20.374 achieving a maximal LOD score of 4.41. All five genes within the disease-associated interval were exon resequenced, although no exonic candidate mutations were identified. A targeted resequencing approach was therefore adopted to sequence the entire disease-associated interval. Analysis of the sequencing data revealed a GAA repeat expansion in intron 35 of ITPR1, which was homozygous in all cases and heterozygous in obligate carriers. Partial impairment of cerebellar ITPR1 expression in affected dogs was demonstrated by immunohistochemistry. Given the association of ITPR1 mutations with spinocerebellar ataxia (SCA) type 15 (also designated SCA16) in humans and that an intronic GAA repeat expansion has been shown to cause Friedreich ataxia, the repeat expansion is an excellent candidate for the cause of spinocerebellar ataxia in the Italian Spinone. This finding represents the first naturally occurring pathogenic intronic GAA repeat expansion in a non-human species and a novel mechanism for ITPR1 associated spinocerebellar ataxia.

A novel locus on canine chromosome 13 is associated with cataract in the Australian Shepherd breed of domestic dog.

Hereditary cataract is a common ocular disorder in the purebred dog population and is a leading cause of visual impairment and blindness in dogs. Despite this, little is known to date about the genetics underlying this condition. We have used a genome-wide association study and targeted resequencing approach to identify a novel locus for cataracts in the Australian Shepherd breed of dog, using dogs that are clear of an HSF4 mutation, previously identified as the major susceptibility locus in this breed. Cataract cases were defined as dogs with bilateral posterior cataracts, or bilateral nuclear cataracts. Controls were at least 8 years of age with no evidence of cataracts or other ocular abnormality. Using 15 bilateral posterior polar cataract cases and 68 controls, we identified a genome-wide statistical association for cataracts in the Australian Shepherd on canine chromosome 13 at 46.4 Mb (P value: 1.5 × 10(-7)). We sequenced the 14.16 Mb associated region in ten Australian Shepherds to search for possible causal variants underlying the association signal and conducted additional fine-mapping of the region by genotyping 28 intronic variants that segregated correctly in our ten sequenced dogs. From this analysis, the strongest associated variants were located in intron 5 of the SCFD2 gene. Further study will require analysis of additional cases and controls and ocular tissue from dogs affected with bilateral cataracts that are free of the HSF4 mutation.

Parallel mapping and simultaneous sequencing reveals deletions in BCAN and FAM83H associated with discrete inherited disorders in a domestic dog breed.

The domestic dog (Canis familiaris) segregates more naturally-occurring diseases and phenotypic variation than any other species and has become established as an unparalled model with which to study the genetics of inherited traits. We used a genome-wide association study (GWAS) and targeted resequencing of DNA from just five dogs to simultaneously map and identify mutations for two distinct inherited disorders that both affect a single breed, the Cavalier King Charles Spaniel. We investigated episodic falling (EF), a paroxysmal exertion-induced dyskinesia, alongside the phenotypically distinct condition congenital keratoconjunctivitis sicca and ichthyosiform dermatosis (CKCSID), commonly known as dry eye curly coat syndrome. EF is characterised by episodes of exercise-induced muscular hypertonicity and abnormal posturing, usually occurring after exercise or periods of excitement. CKCSID is a congenital disorder that manifests as a rough coat present at birth, with keratoconjunctivitis sicca apparent on eyelid opening at 10-14 days, followed by hyperkeratinisation of footpads and distortion of nails that develops over the next few months. We undertook a GWAS with 31 EF cases, 23 CKCSID cases, and a common set of 38 controls and identified statistically associated signals for EF and CKCSID on chromosome 7 (P(raw) 1.9×10(-14); P(genome) = 1.0×10(-5)) and chromosome 13 (P(raw) 1.2×10(-17); P(genome) = 1.0×10(-5)), respectively. We resequenced both the EF and CKCSID disease-associated regions in just five dogs and identified a 15,724 bp deletion spanning three exons of BCAN associated with EF and a single base-pair exonic deletion in FAM83H associated with CKCSID. Neither BCAN or FAM83H have been associated with equivalent disease phenotypes in any other species, thus demonstrating the ability to use the domestic dog to study the genetic basis of more than one disease simultaneously in a single breed and to identify multiple novel candidate genes in parallel.