Evaluation of canine heat shock transcription factor 4 (HSF4) as a candidate gene for primary cataracts in the Dachshund and the Entlebucher Mountain dog

Allele and genotype frequencies for primary hereditary cataract, multifocal retinopathy 1, and degenerative myelopathy in Pyrenean Mountain dog from Italy

Pyrenean Mountain Dog (PMD) is an ancient dog breed firstly described in XIV century in the Pyrenees Region and nowadays diffused both in Europe and in the US. Hereditary Cataract (HC), defined as the inherited opacity of the lens, involves clinical signs ranging from reduced vision to glaucoma. A molecular basis of HC was firstly described in Staffordshire Bull Terriers and then reported in multiple canine breeds. The HC-associated variation is a single nucleotide deletion in HSF4 gene that introduces a premature stop codon (c.962del, p.Ala321*). Multifocal Retinopathy 1 (MR) is an ocular disorder characterized by multiple areas of retinal degeneration, caused in various dog breeds (including PMD) by a single nucleotide variant (SNV) in BEST1 gene that generates a premature stop codon (c.73G>A, p.Arg25*). Degenerative Myelopathy (DM) is an adult-onset, progressive neurodegenerative disease and it is associated to a SNV in SOD1 gene causing a change in aminoacidic sequence of the protein (c.118G>A, p.Glu40Lys). This causative variant has been described in various dog breeds, including PMD. Aim of this study was to determine the allele frequencies for the abovementioned three genetic diseases in the Italian breeding PMD population. The survey found no dogs carrying the allele (deletion) associated with HC, while three dogs (6 %) were heterozygous (G/A) for the MR-associated variant, and seven dogs (13 %) were heterozygous (G/A) for the DM-associated alteration, indicating that the variant alleles frequency were 0  %, 3 %, and 7 %, respectively. Appropriate mating management is suggested for the prevention of genetic diseases spreading in the PMD population.

Cataracts in Havanese: genome wide association study reveals two loci associated with posterior polar cataract

BACKGROUND

Cataract is considered an important health issue in Havanese, and studies indicate a breed predisposition. Possible consequences of cataracts include lens induced uveitis, reduced eyesight, and blindness in severe cases. Reducing the prevalence of cataracts could therefore improve health and welfare significantly. The most frequently diagnosed forms of cataract in Havanese are cortical- and anterior suture line cataract, but cases of posterior polar cataract are also regularly reported. Out of the three, posterior polar- and cortical cataracts are considered the most clinically relevant.

RESULTS

We performed a genome wide association study that included 57 controls and 27 + 23 + 7 cases of cortical-, anterior suture line- and posterior polar cataract, respectively. An association analysis using a mixed linear model, revealed two SNPs on CFA20 (BICF2S23632983, p = 7.2e-09) and CFA21 (BICF2G630640490, p = 3.3e-09), that were significantly associated with posterior polar cataract, both of which are linked to relevant candidate genes. The results suggest that the two variants are linked to alleles with large effects on posterior polar cataract formation, possibly in a dominant fashion, and identifies regions that should be subject to further sequencing. Promising regions on CFA4 and CF30 were also identified in the association analysis of cortical cataract. The top SNPs on each chromosome, chr4_12164500 (p = 4.3e-06) and chr30_28836339 (p = 5.6e-06), are located within, or in immediate proximity to, potential cataract candidate genes. The study shows that age at examination is strongly associated with sensitivity of cataract screening. Havanese in Norway are on average 3.4 years old when eye examinations are performed: an age where most dogs that are genetically at risk have not yet developed clinically observable changes. Increasing the average age of breeding animals could increase accuracy of selection, leading to improved health.

CONCLUSIONS

The study identified two loci, on CFA20 and CFA21, that were significantly associated with posterior polar cataract in Havanese. SNPs that showed putative association with cortical cataracts, were observed on CFA4 and CFA30. All the top SNPs are located in close proximity to cataract candidate genes. The study also show that sensitivity of cataract screening is highly dependent on age at examination.

FYCO1 Frameshift Deletion in Wirehaired Pointing Griffon Dogs with Juvenile Cataract

Different breed-specific inherited cataracts have been described in dogs. In this study, we investigated an inbred family of Wirehaired Pointing Griffon dogs in which three offspring were affected by juvenile cataract. The pedigree suggested monogenic autosomal recessive inheritance of the trait. Whole-genome sequencing of an affected dog revealed 12 protein-changing variants that were not present in 566 control genomes, of which two were located in functional candidate genes, FYCO1 and CRYGB. Targeted genotyping of both variants in the investigated family excluded CRYGB and revealed perfect co-segregation of the FYCO1 variant with the juvenile cataract phenotype. This variant, FYCO1:c.2024delG, represents a 1 bp frameshift deletion predicted to truncate ~50% of the open reading frame p.(Ser675Thrfs*5). FYCO1 encodes the FYVE and coiled-coil domain autophagy adaptor 1, a known regulator of lens autophagy, which is required for the normal homeostasis in the eye. In humans, at least 37 pathogenic variants in FYCO1 have been shown to cause autosomal recessive cataract. Fcyo1^−/− knockout mice also develop cataracts. Together with the current knowledge on FYCO1 variants and their functional impact in humans and mice, our data strongly suggest FYCO1:c.2024delG as a candidate causative variant for the observed juvenile cataract in Wirehaired Pointing Griffon dogs. To the best of our knowledge, this study represents the first report of a FYCO1-related cataract in domestic animals.

The genetics of eye disorders in the dog

Inherited forms of eye disease are arguably the best described and best characterized of all inherited diseases in the dog, at both the clinical and molecular level and at the time of writing 29 different mutations have been documented in the scientific literature that are associated with an inherited ocular disorder in the dog. The dog has already played an important role in the identification of genes that are important for ocular development and function as well as emerging therapies for inherited blindness in humans. Similarities in disease phenotype and eye structure and function between dog and man, together with the increasingly sophisticated genetic tools that are available for the dog, mean that the dog is likely to play an ever increasing role in both our understanding of the normal functioning of the eye and in our ability to treat inherited eye disorders. This review summarises the mutations that have been associated with inherited eye disorders in the dog.