Background

Increases in serum gamma-glutamyltransferase (GGT) activity have been reported in Thoroughbred (TB) racehorses and associated with maladaptation to training but the underlying etiology remains unknown.

Hypothesis/objectives

Classify the etiology of high GGT syndrome in racing TBs by assessment of pancreatic enzymes, vitamin E concentrations, and both a candidate gene and whole genome association study. We hypothesized that a genetic variant resulting in antioxidant insufficiency or pancreatic dysfunction would be responsible for high GGT syndrome in TBs.

Animals

A total of 138 California racing TBs. Amylase: n = 31 affected (serum GGT activity ≥60 IU/L), n = 52 control (serum GGT activity <40 IU/L). Lipase: n = 19 affected, n = 35 control. Serum α-tocopherol concentrations: n = 32 affected, n = 46 control. Genome-wide association study (GWAS): 36 affected, 58 control. Whole genome sequencing: n = 5 affected, n = 5 control.

Methods

Biochemical and vitamin analytes were compared among cohorts. A GWAS was performed and a subset of TBs underwent whole genome sequencing to interrogate candidate genes and positional genetic regions.

Results

Serum lipase and amylase activity and α-tocopherol concentrations did not differ between groups. No genetic variants were identified in 2 candidate genes (UGT1A1 and GGT1) that associated with the phenotype. Four single nucleotide polymorphisms (SNPs) approached a suggestive association with the phenotype (P = 2.15 × 10^-5 ), defining a 100 kb region on chromosome 5 surrounding cluster of differentiation 1a (CD1A1), a transmembrane gene related to the major histocompatibility complex.

Conclusions and clinical importance

An underlying genetic etiology may exist for high GGT syndrome in racing TBs, similar to genetic disorders in humans.