A Missense Variant in the Bardet-Biedl Syndrome 2 Gene (BBS2) Leads to a Novel Syndromic Retinal Degeneration in the Shetland Sheepdog

Abstract

Canine progressive retinal atrophy (PRA) describes a group of hereditary diseases characterized by photoreceptor cell death in the retina, leading to visual impairment. Despite the identification of multiple PRA-causing variants, extensive heterogeneity of PRA is observed across and within dog breeds, with many still genetically unsolved. This study sought to elucidate the causal variant for a distinct form of PRA in the Shetland sheepdog, using a whole-genome sequencing approach. Filtering variants from a single PRA-affected Shetland sheepdog genome compared to 176 genomes of other breeds identified a single nucleotide variant in exon 11 of the Bardet-Biedl syndrome-2 gene (BBS2) (c.1222G>C; p.Ala408Pro). Genotyping 1386 canids of 155 dog breeds, 15 cross breeds and 8 wolves indicated the c.1222G>C variant was only segregated within Shetland sheepdogs. Out of 505 Shetland sheepdogs, seven were homozygous for the variant. Clinical history and photographs for three homozygotes indicated the presence of a novel phenotype. In addition to PRA, additional clinical features in homozygous dogs support the discovery of a novel syndromic PRA in the breed. The development and utilization of a diagnostic DNA test aim to prevent the mutation from becoming more prevalent in the breed.

Keywords: BBS; BBS2; PRA; canine; retinal degeneration; syndromic.

Figure 1

Photographs of PRA-affected proband SS1 in the initial litter show the uncharacteristic ‘wavy’ coat, upturned nose and dental defects (A–C).

Figure 2

IGV display of WGS reads aligned across the BBS2 c.1222G>C SNV region. Grey horizontal bars show paired-end sequencing reads aligned to the canine reference genome across the BBS2 exon 11 location. Above these sequencing reads is a histogram representing sequencing coverage across the region. Track (A) shows the WGS reads from the PRA-affected SS (proband SS1). The blue block shows a homozygous SNV change from the reference nucleotide ‘G’ to a ‘C’ at position chr2:59693737. Track (B) shows WGS data from a PRA non-affected control genome (Hungarian Vizsla dog) that is homozygous for the reference ‘G’ allele. BBS2 is expressed in retinal tissue as demonstrated by RNA-seq alignment from two control dogs: one Dalmatian dog (track (C)) and one Petit Basset Griffon Vendéen (PBGV) dog (track (D)), both of which are also homozygous for the reference allele at this position.

Figure 3

Sanger sequencing of the BBS2 c.1222G>C variant confirmed its presence in proband SS1 (track (A)). Additional dogs were also homozygous for the c.1222G>C variant (tracks (B,C)). SSs heterozygous for the variant and homozygous for the wild-type allele (G/G) are shown in tracks (D) and (E), respectively.