Evaluation of three canine gamma-crystallins (CRYGB, CRYGC, and CRYGS) as candidates for hereditary cataracts in the dachshund

Abstract

Purpose: We analyzed the gamma-crystallin genes CRYGB, CRYGC, and CRYGS in the dog and tested single nucleotide polymorphisms (SNPs) for linkage and association with primary noncongenital cataract (CAT) in the dachshund, a popular dog breed. The crystallin genes may be involved in the pathogenesis of canine CAT as shown in humans and mice.

Methods: We sequenced all exons and their flanking intronic regions of the CRYGB, CRYGC, and CRYGS genes and in addition, the complete cDNA of these three genes using lens tissue from CAT-affected and unaffected dogs of several breeds. After examining BLASTN analyses, we compared the gene structure with the predicted genes in the current dog genome assembly and the orthologs of humans and mice.

Results: The search for SNPs within these crystallin genes revealed a total of five polymorphisms. As both CAT-affected and unaffected dogs shared identical haplotypes, there was no cosegregation of the SNP alleles with the affected animals. Expression did not differ among CAT-affected and unaffected dogs.

Conclusions: The polymorphisms reported for CRYGB, CRYGC, and CRYGS can be excluded as causative mutations for the CAT phenotype in the wire- and smooth-haired dachshund. The canine cataract gene orthologs described here may serve as a valuable resource for further studies in other dog breeds to develop a canine model. Many different dog breeds are affected by CAT. The use of the SNPs presented in this paper can facilitate the screening of more dog breeds.

Figure 1

γ-Crystallin protein alignment. Shown are the alignment of the canine CRYGB protein (175 amino acids), the canine CRYGC protein (174 amino acids), and the canine CRYGS protein (178 amino acids) derived from our sequenced cDNA with the known orthologous protein sequences. The sequences were derived from GenBank entries with the following accession numbers: NP_005201 (human CRYGB), NP_658906 (mouse CRYGB), NP_066269 (human CRYGC), NP_031801 (mouse CRYGC), NP_060011 (human CRYGS) and NP_034097 (mouse CRYGS). Residues identical to the dog are indicated by asterisks. The three exons are labeled by different colors. All exons included only complete triplets.

Figure 2

γ-Crystallin cDNA analysis. Bands of cDNA PCR products of the lens tissue of two dogs affected by CAT and an unaffected dog for each gene (CRYGB, 567 bp; CRYGC, 603 bp; and CRYGS, 645 bp) on an agarose gel. In the gel, band 1=mixed breed, unaffected; band 2=dachshund mix, affected; band 3=German shepherd, affected. The cDNAs of the other four dogs did not differ in sequence and product size.