Characterization of a low expression haplotype in canine glutathione S‐transferase ( GSTT1 ) and its prevalence in golden retrievers

Glutathione S‐transferase‐theta ( GSTT1 ) is a carcinogen detoxification enzyme, and low activity variants are associated with lymphoma in humans. We recently found a variant in the 3′ untranslated region ( UTR ) of canine GSTT1 , *101_102insT, which was predicted to change miRNA binding and was found in 5 of 17 golden retriever ( GR ) dogs with lymphoma but none of 14 healthy GRs . The aim of this study was to determine whether this variant led to decreased GSTT1 expression and was a discernible risk factor for lymphoma within the GR breed. On resequencing, *101_102insT appeared to be in complete linkage disequilibrium with 3 additional 3′ UTR variants, leading to the inferred haplotype * 3T >C; *101_102insT; * 190C >A; * 203T >C. In canine livers that were heterozygous for this variant haplotype, GSTT1 protein expression was significantly lower compared to the reference haplotype (densitometry .40 vs .64, P  = .022), and GSTT1 transcript levels by qPCR were also significantly lower (fold difference .52, P  = .012), without evidence of substantial allelic expression imbalance. The variant haplotype led to >50% decrease in expression in vitro (.31 ± .07 vs .64 ± .19; P  = .019). We found no significant difference in minor allele frequencies between 71 GR dogs with lymphoma ( MAF .162) and 33 healthy age‐matched controls ( MAF .136, P  = .69). Our results indicate that the variant GSTT1 3′ UTR haplotype containing *101_102insT reduces gene expression, which could lead to impaired carcinogen detoxification, but was not a detectable risk factor for lymphoma in GR dogs.