A 6‐bp Deletion Variant in a Novel Canine Glutathione‐S‐Transferase Gene ( GSTT 5 ) Leads to Loss of Enzyme Function

Objectives Glutathione‐S‐transferases ( GST s) detoxify reactive xenobiotics, and defective GST gene polymorphisms increase cancer risk in humans. A low activity GST ‐theta variant was previously found in research beagles. The purpose of our study was to determine the molecular basis for this phenotype and its allele frequency in pet dogs. Methods Banked livers from 45 dogs of various breeds were screened for low GST ‐theta activity by the substrate 1,2‐dichloro‐4‐nitrobenzene ( DCNB ), and were genotyped for variants in a novel canine GST gene, GSTT 5 . Whole‐genome sequences from 266 dogs were genotyped at one discovered variant GSTT 5 locus. Results Canine livers ranged 190‐fold in GST ‐theta activities, and a GSTT 5 exon coding variant 385_390del GACCAG (Asp129_Gln130del) was significantly associated with low activity ( P  < 0.0001) and a marked decrease in hepatic protein expression ( P  = 0.0026). Recombinant expression of variant GSTT 5 led to a 92% decrease in V max for DCNB ( P  = 0.0095). The minor allele frequency ( MAF ) for 385_390del GACCAG was 0.144 in 45 dog livers, but was significantly higher in beagles (0.444) versus nonbeagles (0.007; P  = 0.0004). The homozygous genotype was significantly over‐represented in Pembroke Welsh corgis ( P  < 0.0001) based on available whole‐genome sequence data. Conclusions An Asp129_Gln130del variant in canine GSTT 5 is responsible for marked loss of GST ‐theta enzyme activity. This variant is significantly over‐represented in purpose‐bred laboratory beagles and in Pembroke Welsh corgis. Additional work will determine the prevalence of this variant among other purebred dogs, and will establish the substrate range of this polymorphic canine enzyme with respect to common environmental carcinogens.