Genetic prevalence and clinical relevance of canine Mendelian disease variants in over one million dogs.

Hundreds of genetic variants implicated in Mendelian disease have been characterized in dogs and commercial screening is being offered for most of them worldwide. There is typically limited information available regarding the broader population frequency of variants and uncertainty regarding their functional and clinical impact in ancestry backgrounds beyond the discovery breed. Genetic panel screening of disease-associated variants, commercially offered directly to the consumer or via a veterinary clinician, provides an opportunity to establish large-scale cohorts with phenotype data available to address open questions related to variant prevalence and relevance. We screened the largest canine cohort examined in a single study to date (1,054,293 representative dogs from our existing cohort of 3.5 million; a total of 811,628 mixed breed dogs and 242,665 purebreds from more than 150 countries) to examine the prevalence and distribution of a total of 250 genetic disease-associated variants in the general population. Electronic medical records from veterinary clinics were available for 43.5% of the genotyped dogs, enabling the clinical impact of variants to be investigated. We provide detailed frequencies for all tested variants across breeds and find that 57% of dogs carry at least one copy of a studied Mendelian disease-associated variant. Focusing on a subset of variants, we provide evidence of full penetrance for 10 variants, and plausible evidence for clinical significance of 22 variants, on diverse breed backgrounds. Specifically, we report that inherited hypocatalasia is a notable oral health condition, confirm that factor VII deficiency presents as subclinical bleeding propensity and verify two genetic causes of reduced leg length. We further assess genome-wide heterozygosity levels in over 100 breeds, and show that a reduction in genome-wide heterozygosity is associated with an increased Mendelian disease variant load. The accumulated knowledge represents a resource to guide discussions on genetic test relevance by breed.

Genetic epidemiology of blood type, disease and trait variants, and genome-wide genetic diversity in over 11,000 domestic cats.

In the largest DNA-based study of domestic cats to date, 11,036 individuals (10,419 pedigreed cats and 617 non-pedigreed cats) were genotyped via commercial panel testing elucidating the distribution and frequency of known disease, blood type, and physical trait associated genetic variants across cat breeds. This study provides allele frequencies for many disease-associated variants for the first time and provides updates on previously reported information with evidence suggesting that DNA testing has been effectively used to reduce disease associated variants within certain pedigreed cat populations over time. We identified 13 disease-associated variants in 47 breeds or breed types in which the variant had not previously been documented, highlighting the relevance of comprehensive genetic screening across breeds. Three disease-associated variants were discovered in non-pedigreed cats only. To investigate the causality of nine disease-associated variants in cats of different breed backgrounds our veterinarians conducted owner interviews, reviewed clinical records, and invited cats to have follow-up clinical examinations. Additionally, genetic variants determining blood types A, B and AB, which are relevant clinically and in cat breeding, were genotyped. Appearance-associated genetic variation in all cats is also discussed. Lastly, genome-wide SNP heterozygosity levels were calculated to obtain a comparable measure of the genetic diversity in different cat breeds. This study represents the first comprehensive exploration of informative Mendelian variants in felines by screening over 10,000 pedigreed cats. The results qualitatively contribute to the understanding of feline variant heritage and genetic diversity and demonstrate the clinical utility and importance of such information in supporting breeding programs and the research community. The work also highlights the crucial commitment of pedigreed cat breeders and registries in supporting the establishment of large genomic databases, that when combined with phenotype information can advance scientific understanding and provide insights that can be applied to improve the health and welfare of cats.

A new Finnish flavor of feline coat coloration, "salmiak," is associated with a 95-kb deletion downstream of the KIT gene.

Cats with a distinctive white hair pattern of unknown molecular cause have been discovered in the Finnish domestic cat population. Based on the unique appearance of these cats, we have named this phenotype salmiak ("salty licorice"). The use of a commercially available panel test to genotype four salmiak-colored cats revealed the absence of all known variants associated with white-haired phenotypic loci: full White (W), Spotting (Ws) and the Birman white Gloves associated (wg) allele of the KIT proto-oncogene (KIT) gene. Whole-genome sequencing on two salmiak-colored cats was conducted to search for candidate causal variants in the KIT gene. Despite a lack of coding variants, visual inspection of the short read alignments revealed a large ~95 kb deletion located ~65 kb downstream of the KIT gene in the salmiak cats. Additional PCR genotyping of 180 domestic cats and three salmiak-colored cats confirmed the homozygous derived variant genotype fully concordant with the salmiak phenotype. We suggest the newly identified variant be designated as wsal for "w salmiak".

A loss-of-function variant in canine GLRA1 associates with a neurological disorder resembling human hyperekplexia.

Hereditary hyperekplexia is a rare neuronal disorder characterized by an exaggerated startle response to sudden tactile or acoustic stimuli. In this study, we present a Miniature Australian Shepherd family showing clinical signs, which have genetic and phenotypic similarities with human hereditary hyperekplexia: episodes of muscle stiffness that could occasionally be triggered by acoustic stimuli. Whole genome sequence data analysis of two affected dogs revealed a 36-bp deletion spanning the exon-intron boundary in the glycine receptor alpha 1 (GLRA1) gene. Further validation in pedigree samples and an additional cohort of 127 Miniature Australian Shepherds, 45 Miniature American Shepherds and 74 Australian Shepherds demonstrated complete segregation of the variant with the disease, according to an autosomal recessive inheritance pattern. The protein encoded by GLRA1 is a subunit of the glycine receptor, which mediates postsynaptic inhibition in the brain stem and spinal cord. The canine GLRA1 deletion is located in the signal peptide and is predicted to cause exon skipping and subsequent premature stop codon resulting in a significant defect in glycine signaling. Variants in GLRA1 are known to cause hereditary hyperekplexia in humans; however, this is the first study to associate a variant in canine GLRA1 with the disorder, establishing a spontaneous large animal disease model for the human condition.

Correction: Frequency and distribution of 152 genetic disease variants in over 100,000 mixed breed and purebred dogs.

[This corrects the article DOI: 10.1371/journal.pgen.1007361.].

Frequency and distribution of 152 genetic disease variants in over 100,000 mixed breed and purebred dogs.

Knowledge on the genetic epidemiology of disorders in the dog population has implications for both veterinary medicine and sustainable breeding. Limited data on frequencies of genetic disease variants across breeds exists, and the disease heritage of mixed breed dogs remains poorly explored to date. Advances in genetic screening technologies now enable comprehensive investigations of the canine disease heritage, and generate health-related big data that can be turned into action. We pursued population screening of genetic variants implicated in Mendelian disorders in the largest canine study sample examined to date by examining over 83,000 mixed breed and 18,000 purebred dogs representing 330 breeds for 152 known variants using a custom-designed beadchip microarray. We further announce the creation of MyBreedData (www.mybreeddata.com), an online updated inherited disorder prevalence resource with its foundation in the generated data. We identified the most prevalent, and rare, disease susceptibility variants across the general dog population while providing the first extensive snapshot of the mixed breed disease heritage. Approximately two in five dogs carried at least one copy of a tested disease variant. Most disease variants are shared by both mixed breeds and purebreds, while breed- or line-specificity of others is strongly suggested. Mixed breed dogs were more likely to carry a common recessive disease, whereas purebreds were more likely to be genetically affected with one, providing DNA-based evidence for hybrid vigor. We discovered genetic presence of 22 disease variants in at least one additional breed in which they were previously undescribed. Some mutations likely manifest similarly independently of breed background; however, we emphasize the need for follow up investigations in each case and provide a suggested validation protocol for broader consideration. In conclusion, our study provides unique insight into genetic epidemiology of canine disease risk variants, and their relevance for veterinary medicine, breeding programs and animal welfare.

Defining the ABC of gene essentiality in streptococci.

BACKGROUND: Utilising next generation sequencing to interrogate saturated bacterial mutant libraries provides unprecedented information for the assignment of genome-wide gene essentiality. Exposure of saturated mutant libraries to specific conditions and subsequent sequencing can be exploited to uncover gene essentiality relevant to the condition. Here we present a barcoded transposon directed insertion-site sequencing (TraDIS) system to define an essential gene list for Streptococcus equi subsp. equi, the causative agent of strangles in horses, for the first time. The gene essentiality data for this group C Streptococcus was compared to that of group A and B streptococci. RESULTS: Six barcoded variants of pGh9:ISS1 were designed and used to generate mutant libraries containing between 33,000-66,000 unique mutants. TraDIS was performed on DNA extracted from each library and data were analysed separately and as a combined master pool. Gene essentiality determined that 19.5% of the S. equi genome was essential. Gene essentialities were compared to those of group A and group B streptococci, identifying concordances of 90.2% and 89.4%, respectively and an overall concordance of 83.7% between the three species. CONCLUSIONS: The use of barcoded pGh9:ISS1 to generate mutant libraries provides a highly useful tool for the assignment of gene function in S. equi and other streptococci. The shared essential gene set of group A, B and C streptococci provides further evidence of the close genetic relationships between these important pathogenic bacteria. Therefore, the ABC of gene essentiality reported here provides a solid foundation towards reporting the functional genome of streptococci.

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.

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.

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.

Exonic Short Interspersed Nuclear Element Insertion in FAM161A Is Associated with Autosomal Recessive Progressive Retinal Atrophy in the English Shepherd.

Progressive retinal atrophies (PRAs) are a genetically heterogeneous group of inherited eye diseases that affect over 100 breeds of dog. The initial clinical sign is visual impairment in scotopic conditions, as a consequence of rod photoreceptor cell degeneration. Photopic vision degeneration then follows, due to progression of the disease to the cone photoreceptors, and ultimately results in complete blindness. Two full-sibling English Shepherds were diagnosed with PRA at approximately 5 years old and tested clear of all published PRA genetic variants. This study sought to identify the novel PRA-associated variant segregating in the breed. We utilised a combined approach of whole genome sequencing of the probands and homozygosity mapping of four cases and 22 controls and identified a short interspersed nuclear element within an alternatively spliced exon in FAM161A. The XP_005626197.1 c.17929_ins210 variant was homozygous in six PRA cases and heterozygous or absent in control dogs, consistent with a recessive mode of inheritance. The insertion is predicted to extend exon 4 by 39 aberrant amino acids followed by an early termination stop codon. PRA is intractable to treatment, so the development of a genetic screening test, based on the associated variant, is significant, because it provides dog breeders/owners with a means of reducing the frequency of the disease variant within this breed as well as minimising the risk of breeding puppies that will develop this blinding disease.

Association of FGF4L1 Retrogene Insertion with Prolapsed Gland of the Nictitans (Cherry Eye) in Dogs.

Cherry eye is the common name for prolapse of the nictitans gland, a tear-producing gland situated under the third eyelid of dogs. Cherry eye is characterized by a red fleshy protuberance in the corner of the eye, resembling a cherry. This protrusion is a displacement of the normal gland of the third eyelid, thought to be caused by a defect in the connective tissue that secures the gland in place. Options for treatment may include anti-inflammatory medications in mild cases, but surgical replacement of the gland is usually indicated. Cherry eye is most often seen in dogs under the age of two years, with certain breeds having a higher incidence, suggesting a potential genetic association. Integration of panel genetic testing into routine clinical practice allows for the generation of large numbers of genotyped individuals paired with clinical records and enables the investigation of common disorders using a genome-wide association study (GWAS) approach at scale. In this investigation, several thousand cases and controls for cherry eye in both purebred dogs and mixed breeds are used for a large-scale GWAS, revealing a single peak of genome-wide significance on canine chromosome 18, directly at the location of the previously identified FGF4 insertion known to cause chondrodysplasia in several breeds.

Genome-wide mRNA sequencing of a single canine cerebellar cortical degeneration case leads to the identification of a disease associated SPTBN2 mutation.

BACKGROUND: Neonatal cerebellar cortical degeneration is a neurodegenerative disease described in several canine breeds including the Beagle. Affected Beagles are unable to ambulate normally from the onset of walking and the main pathological findings include Purkinje cell loss with swollen dendritic processes. Previous reports suggest an autosomal recessive mode of inheritance. The development of massively parallel sequencing techniques has presented the opportunity to investigate individual clinical cases using genome-wide sequencing approaches. We used genome-wide mRNA sequencing (mRNA-seq) of cerebellum tissue from a single Beagle with neonatal cerebellar cortical degeneration as a method of candidate gene sequencing, with the aim of identifying the causal mutation. RESULTS: A four-week old Beagle dog presented with progressive signs of cerebellar ataxia and the owner elected euthanasia. Histopathology revealed findings consistent with cerebellar cortical degeneration. Genome-wide mRNA sequencing (mRNA-seq) of RNA from cerebellum tissue was used as a method of candidate gene sequencing. After analysis of the canine orthologues of human spinocerebellar ataxia associated genes, we identified a homozygous 8 bp deletion in the ß-III spectrin gene, SPTBN2, associated with spinocerebellar type 5 in humans. Genotype analysis of the sire, dam, ten clinically unaffected siblings, and an affected sibling from a previous litter, showed the mutation to fully segregate with the disorder. Previous studies have shown that ß-III spectrin is critical for Purkinje cell development, and the absence of this protein can lead to cell damage through excitotoxicity, consistent with the observed Purkinje cell loss, degeneration of dendritic processes and associated neurological dysfunction in this Beagle. CONCLUSIONS: An 8 bp deletion in the SPTBN2 gene encoding ß-III spectrin is associated with neonatal cerebellar cortical degeneration in Beagle dogs. This study shows that mRNA-seq is a feasible method of screening candidate genes for mutations associated with rare diseases when a suitable tissue resource is available.

Congenital keratoconjunctivitis sicca and ichthyosiform dermatosis in Cavalier King Charles spaniel dogs. Part II: candidate gene study.

PURPOSE: To identify causative mutation(s) for congenital keratoconjunctivitis sicca and ichthyosiform dermatosis (CKCSID) in Cavalier King Charles spaniel (CKCS) dogs using a candidate gene approach. METHODS: DNA samples from 21 cases/parents were collected. Canine candidate genes (CCGs) for similar inherited human diseases were chosen. Twenty-eight candidate genes were identified by searching the Pubmed OMIM database (http://www.ncbi.nlm.nih.gov/omim). Canine orthologues of human candidate genes were identified using the Ensembl orthologue prediction facility (http://www.ensembl.org/index.html). Two microsatellites flanking each candidate gene were selected, and primers to amplify each microsatellite were designed using the Whitehead Institute primer design website (http://frodo.wi.mit.edu/primer3/). The microsatellites associated with all 28 CCGs were genotyped on a panel of 21 DNA samples from CKCS dogs (13 affected and eight carriers). Genotyping data was analyzed to identify markers homozygous in affected dogs and heterozygous in carriers (homozygosity mapping). RESULTS: None of the microsatellites associated with 25 of the CCGs displayed an association with CKCSID in the 21 DNA samples tested. Three CCGs associated microsatellites were monomorphic across all samples tested. CONCLUSIONS: Twenty-five CCGs were excluded as cause of CKCSID. Three CCGs could not be excluded from involvement in the inheritance of CKCSID.

Large scale across-breed genome-wide association study reveals a variant in HMGA2 associated with inguinal cryptorchidism risk in dogs.

Cryptorchidism is the most common congenital sex development disorder in dogs. Despite this, little progress has been made in understanding its genetic background. Extensive genetic testing of dogs through consumer and veterinary channels using a high-density SNP genotyping microarray coupled with links to clinical records presents the opportunity for a large-scale genome-wide association study to elucidate the molecular risk factors associated with cryptorchidism in dogs. Using an inter-breed genome-wide association study approach, a significant statistical association on canine chromosome 10 was identified, with the top SNP pinpointing a variant of HMGA2 previously associated with adult weight variance. In further analysis we show that incidence of cryptorchidism is skewed towards smaller dogs in concordance with the identified variant's previous association with adult weight. This study represents the first putative variant to be associated with cryptorchidism in dogs.

Whole Genome Sequencing of Giant Schnauzer Dogs with Progressive Retinal Atrophy Establishes NECAP1 as a Novel Candidate Gene for Retinal Degeneration.

Canine progressive retinal atrophies (PRA) are genetically heterogeneous diseases characterized by retinal degeneration and subsequent blindness. PRAs are untreatable and affect multiple dog breeds, significantly impacting welfare. Three out of seven Giant Schnauzer (GS) littermates presented with PRA around four years of age. We sought to identify the causal variant to improve our understanding of the aetiology of this form of PRA and to enable development of a DNA test. Whole genome sequencing of two PRA-affected full-siblings and both unaffected parents was performed. Variants were filtered based on those segregating appropriately for an autosomal recessive disorder and predicted to be deleterious. Successive filtering against 568 canine genomes identified a single nucleotide variant in the gene encoding NECAP endocytosis associated 1 (NECAP1): c.544G>A (p.Gly182Arg). Five thousand one hundred and thirty canids of 175 breeds, 10 cross-breeds and 3 wolves were genotyped for c.544G>A. Only the three PRA-affected GS were homozygous (allele frequency in GS, excluding proband family = 0.015). In addition, we identified heterozygotes belonging to Spitz and Dachshund varieties, demonstrating c.544G>A segregates in other breeds of German origin. This study, in parallel with the known retinal expression and role of NECAP1 in clathrin mediated endocytosis (CME) in synapses, presents NECAP1 as a novel candidate gene for retinal degeneration in dogs and other species.

An intronic LINE-1 insertion in MERTK is strongly associated with retinopathy in Swedish Vallhund dogs.

The domestic dog segregates a significant number of inherited progressive retinal diseases, several of which mirror human retinal diseases and which are collectively termed progressive retinal atrophy (PRA). In 2014, a novel form of PRA was reported in the Swedish Vallhund breed, and the disease was mapped to canine chromosome 17. The causal mutation was not identified, but expression analyses of the retinas of affected Vallhunds demonstrated a 6-fold increased expression of the MERTK gene compared to unaffected dogs. Using 24 retinopathy cases and 97 controls with no clinical signs of retinopathy, we replicated the chromosome 17 association in Swedish Vallhunds from the UK and aimed to elucidate the causal variant underlying this association using whole genome sequencing (WGS) of an affected dog. This revealed a 6-8 kb insertion in intron 1 of MERTK that was not present in WGS of 49 dogs of other breeds. Sequencing and BLASTN analysis of the inserted segment was consistent with the insertion comprising a full-length intact LINE-1 retroelement. Testing of the LINE-1 insertion for association with retinopathy in the UK set of 24 cases and 97 controls revealed a strong statistical association (P-value 6.0 x 10-11) that was subsequently replicated in the original Finnish study set (49 cases and 89 controls (P-value 4.3 x 10-19). In a pooled analysis of both studies (73 cases and 186 controls), the LINE-1 insertion was associated with a ~20-fold increased risk of retinopathy (odds ratio 23.41, 95% confidence intervals 10.99-49.86, P-value 1.3 x 10-27). Our study adds further support for regulatory disruption of MERTK in Swedish Vallhund retinopathy; however, further work is required to establish a functional overexpression model. Future work to characterise the mechanism by which this intronic mutation disrupts gene regulation will further improve the understanding of MERTK biology and its role in retinal function.

Correction: Two Independent Mutations in ADAMTS17 Are Associated with Primary Open Angle Glaucoma in the Basset Hound and Basset Fauve de Bretagne Breeds of Dog.

[This corrects the article DOI: 10.1371/journal.pone.0140436.].

Paroxysmal Dyskinesia in Norwich Terrier Dogs.

BACKGROUND: Episodic muscular hypertonicity in Norwich terrier dogs was first reported in a brief communication in 1984. Since then, the condition has remained poorly characterized. OBJECTIVES: The aims of this study were to characterize the phenomenology, clinical course, and family history of paroxysmal dyskinesia in the Norwich terrier and to estimate its prevalence in the United Kingdom. METHODS: The owners of Norwich terrier dogs born since January 1, 2000 were invited to complete a specifically designed questionnaire aimed at identifying affected and unaffected dogs and investigating the clinical characteristics of this paroxysmal dyskinesia. Pedigrees were collected and reviewed. RESULTS: The questionnaire was returned for 198 Norwich terrier dogs. Of these, 26 (13%) were classified as affected by paroxysmal dyskinesia after revision of the questionnaires and after obtaining videos of the episodes, veterinary medical records, and telephone interviews with the owners. All dogs were neurologically normal between episodes. No significant abnormalities were detected on diagnostic investigations. Mean age at the first episode was 3 years. The episodes were characterized by sustained muscular hypertonicity in the pelvic limbs, lumbar region, and thoracic limbs, impairing posture and locomotion without loss of consciousness. Episode frequency varied both between and within individuals. Stress, anxiety, excitement, and variation in daily routine were recognized as episode triggers in 13 dogs. Episode duration generally was from 2 to 5 minutes (range, from < 2 to 30 minutes). The majority of affected dogs were related. CONCLUSIONS: Paroxysmal dyskinesia segregates in an extended pedigree of Norwich terrier dogs and thus is potentially an inherited disorder in this breed.