Optic nerve hypoplasia in miniature poodle dogs: A preliminary genetic and candidate gene association study

Abstract

Objective: To conduct a genetic and candidate gene association study with samples from phenotype-ascertained dogs to identify putative disease-associated gene/mutation for optic nerve hypoplasia (ONH) in the miniature poodle.

Animals studied: A total of 43 miniature poodles from the United States and Europe, nine affected bilaterally with ONH, were included in the study. Pedigree information was recorded.

Procedures: A pedigree including all animals studied was assembled. Twenty-one genes typically expressed in ganglion cells or that are associated with ocular malformations and have a critical function in eye and neural retina development were selected. Exons and exon-intron boundaries of eight genes were sequenced in four ONH cases and four controls. Furthermore, cases and controls were genotyped with the Illumina CanineHD BeadChip to obtain genotypes for 13 additional candidate genes for haplotype association.

Results: The assembled pedigree connected all ONH-affected dogs to a possible common founder. Identified variants and haplotypes of the tested candidate genes did not segregate with the phenotype using Identity by Descent approach assuming autosomal recessive inheritance with variable but yet unknown penetrance.

Conclusions: Pedigree analysis did not reveal the inheritance pattern. There is no evidence of association of the evaluated candidate genes with ONH; therefore, the screened candidate genes can provisionally be ruled out as causally associated with the disease.

Keywords: candidate genes; dog; micropapilla; miniature poodle; optic nerve hypoplasia.

Figure 1. Retinal changes in ONH-affected dog.

Fundus photographs of two 8-week old littermate miniature poodles that are normal (A) or affected with ONH (B) and were included in the pedigree and candidate gene analyses; OD-right eye; OS-left eye. Whereas the normal disc is round, pink and has a distinct vascular pattern on the surface at 8 weeks of age (A), the hypoplastic discs (white arrows) are markedly reduced in size. Note that the ONH-affected OS (B) has a pre-retinal hemorrhage on the retinal surface (asterisk). This is a frequent finding in puppies that arises from bleeding from the regressing hyaloid vasculature, and disappears with aging.

Figure 2. Pedigree connecting Optic Nerve Hypoplasia and Micropapilla affected animals.

All sampled animals are related to a single founder male born more than 40 years ago. For simplicity, the pedigree only shows prior generations tracing back to that founder. No examination records were available for animals with a dot in the center of the symbol. Parents of affected dogs either had an unknown status, were normal or diagnosed with Mp. Dogs sequenced by Sanger sequencing and the Illumina CanineHD BeadChip that were used for candidate gene sequencing and haplotype analysis are marked with an oblique arrow. The control dog that was only sequenced by Sanger sequencing is marked with an asterisk.

Figure 3. Haplotype analyses of candidate genes in ONH affected dogs and controls.

Haplotypes for the candidate genes studied for ONH affected and control dogs. Note that haplotype analysis was only carried out in normal control and ONH affected dogs, and not in dogs with Mp. Haplotypes for candidate genes sequenced by Sanger sequencing (A), and haplotypes for candidate genes genotyped with Illumina CanineHD BeadChip (B1, B2). Phenotypic status was assigned as detailed in Materials and Methods. Haplotypes for genes with informative SNPs are shown in animals used for association study. Haplotypes of HESX1 and TUBA8 are not shown as all animals shared the same homozygous wildtype haplotype. SNP IDs for SNPs located in dbSNP are given next to the variant base pair position.

Figure 3. Haplotype analyses of candidate genes in ONH affected dogs and controls.

Haplotypes for the candidate genes studied for ONH affected and control dogs. Note that haplotype analysis was only carried out in normal control and ONH affected dogs, and not in dogs with Mp. Haplotypes for candidate genes sequenced by Sanger sequencing (A), and haplotypes for candidate genes genotyped with Illumina CanineHD BeadChip (B1, B2). Phenotypic status was assigned as detailed in Materials and Methods. Haplotypes for genes with informative SNPs are shown in animals used for association study. Haplotypes of HESX1 and TUBA8 are not shown as all animals shared the same homozygous wildtype haplotype. SNP IDs for SNPs located in dbSNP are given next to the variant base pair position.

Figure 3. Haplotype analyses of candidate genes in ONH affected dogs and controls.

Haplotypes for the candidate genes studied for ONH affected and control dogs. Note that haplotype analysis was only carried out in normal control and ONH affected dogs, and not in dogs with Mp. Haplotypes for candidate genes sequenced by Sanger sequencing (A), and haplotypes for candidate genes genotyped with Illumina CanineHD BeadChip (B1, B2). Phenotypic status was assigned as detailed in Materials and Methods. Haplotypes for genes with informative SNPs are shown in animals used for association study. Haplotypes of HESX1 and TUBA8 are not shown as all animals shared the same homozygous wildtype haplotype. SNP IDs for SNPs located in dbSNP are given next to the variant base pair position.