Genetics of androgen disposition : implications for doping tests

Abstract

Anabolic androgenic steroids (AAS) are derivatives of testosterone. Doping with AAS is a severe challenge to the vision, moral and ethics in sports and has also become an increasing problem in society.

Testosterone abuse is conventionally assessed by the urinary testosterone glucuronide/ epitestosterone glucuronide (T/E) ratio, levels above 4.0 being considered suspicious. However, there is a large inter-individual variation in testosterone glucuronide and epitestosterone glucuronide excretion, which challenges the accuracy of the test.

There are reasons to believe that genetic variation is the single most important cause of variation in disposition of many androgenic compounds. Twin studies in men have demonstrated heritability estimates of 85% and 96% for production rates of testosterone and dihydrotestosterone, respectively. The primary aim of this thesis was to investigate the contribution of genetic components to inter-individual variation in androgen disposition.

We found that a deletion polymorphism in the UGT2B17 gene was strongly associated with the urinary testosterone glucuronide levels. All individuals homozygous for the deletion had negligible amounts of urinary testosterone glucuronide. It is a common polymorphism with an allele frequency of 29 % in Swedes and 78 % in Koreans.

After a single dose of 360 mg testosterone, 40 % of the subjects homozygous for the UGT2B17 deletion never reached the T/E cut off ratio of 4.0. We showed that the sensitivity and specificity of the T/E test could be markedly improved by using genotype-based cut off ratios.

A CYP17 T>C promoter polymorphism was associated with urinary epitestosterone glucuronide levels and consequently the T/E ratio. High natural T/E ratios due to low urinary levels of epitestosterone glucuronide may be partly explained by this polymorphism. Considering only individuals with a functional UGT2B17 enzyme, carriers of the CYP17 TT genotype exhibited 64 % higher T/E ratios than men with one or two C-alleles. The frequency of the TT genotype was 35 % in Caucasians.

Another polymorphism (E77G) in 17b-hydroxysteroid dehydrogenase type 5 (HSD17B5), an enzyme that catalyses the step between androstenedione and testosterone, was associated with serum testosterone levels in Caucasian men. Considering only individuals with a functional UGT2B17 enzyme, this polymorphism was also associated with urinary testosterone glucuronide levels. The polymorphism had an allele frequency of 4.8 % in Swedes but was completely absent in Koreans. We also found a novel functional promoter polymorphism in the CYP7B1 gene, which was associated with significantly higher promoter activity and had an allele frequency of 4 % in Swedes and 0.65 % in Koreans.

In summary, genetic variation has a large impact on androgen disposition. This variation is of the utmost importance for the interpretation of doping test results. The role of this variation for diseases in steroid target organs or for endocrine adverse reactions to drugs remains to be elucidated.