**6.3 Arylamine N-acetyltransferase 2 polymorphism**

Arylamine N-acetyltransferase 2 (NAT2) is a phase II xenobiotic detoxification enzyme metabolizing such chemicals as arylamines, aromatic and heterocyclic amines and hydrazines via N-acetylation and O-acetylation. In other words, it transfers the acetyl group from acetyl-coenzyme A to the nitrogen or oxygen of the substrate. It takes part in the metabolism of such drugs as sulfadimidine, sulfamethazine, isoniazid, nitrazepam, dapsone or caffeine [63]. Therefore, it determines human susceptibility to cancer and the side effects of drugs [64].

Biological significance of NAT2 could be demonstrated through the association between its polymorphisms with susceptibility and different types of cancer—lung, colon or bladder cancer [65, 66].

NAT2 is expressed in the male reproductive organs (genital ducts, testicular tissues, exocrine and prostate glands), promoting protective effect against the environmental chemicals that may lead to male urogenital diseases [67].

NAT2 is a 290 amino acid protein, encoded by intronless 9.9kb gene, located on the 8p22 chromosome and consists of 3 exons. Scientists identified the acetylation polymorphism more than 60 years ago in tuberculosis patients, who reacted to isoniazid toxicity in different ways [68]. NAT2 is characterized by a high number of polymorphic genes, comprising more than 66 alleles [69]. Most of these polymorphisms are synonymous and do not always cause variations in enzyme activity [70]. The most common and important variants of SNPs are rs1799929 (481C > T that does not cause leucine alteration at amino acid 161, L161 L) marked as NAT2\*11A and rs1799930 (590G > A that leads to the charged arginine replacement by polar glutamine at the codon 197 (R197Q )) marked as NAT2\*6 [71]. NAT2\*6 corresponds to a slow acetylator phenotype, while the NAT2\*11A to a rapid one that may differ from the NAT2\*6 by a threefold incensement in the metabolic rate [72]. These two extreme acetylator phenotypes are considered as risk factors for disease development after the subjection to arylamines and other NAT2 targets. For instance, NAT1 and NAT2 slow acetylator phenotypes were shown to be predisposing factors for prostate cancer [73], while the rapid NAT2 phenotypes had higher frequencies in contact-allergic patients [74]. It is considered that the existence of canonical isoniazid slow acetylator phenotype is caused by the reduction in NAT2 protein [75]. Among the other reasons, there are low levels of expression, instability or reduced catalytic activities [76].

There are substantial interethnic variations of the slow acetylator phenotype. Thus, it can be found in 40–70% of Caucasian and African people. At the same time, its frequency ranges from 10 to 30% in Japanese, Chinese, Korean and Thai people [77].

There is a lack of evidence to confirm the role of NAT2 polymorphism in the development of male infertility. However, recently it was proved that gene-environment interactions play a very important role in the case of NAT2 polymorphism and determine whether it will predispose male infertility. Thus, several data have showed that rs1799929 and rs1799929 SNPs themselves were not associated with the increased risks of idiopathic male infertility [78]. However, if the subject was exposed to cigarette smoking (OR = 1.71, 95% CI: 1.02–2.87, P = 0.042), alcohol abuse (OR = 2.14, 95% CI: 1.08–4.27, P = 0.029) and low fruit/vegetable intake (OR = 1.68, 95% CI: 1.01–2.79, P = 0.04), the risk of male infertility significantly increased in the case of slow acetylator phenotype rs1799930.

Contrariwise, rapid acetylator phenotype was found to cause higher DNAfragmentation levels after 2 days of meat diet [79]. Considering that the level of DNA fragmentation is significantly higher in infertile men [80], this study also proves that NAT2 polymorphism is involved in the process of impaired reproductive development in men.

The latest study, conducted on Vietnam males, revealed that idiopathic male infertility is associated with both the rs1799930 (OR = 3.10, 95%; CI: 1.92–5.01) and the rs1799929 (OR = 3.74, 95%; CI: 2.26–6.18) alleles. The current research also shows that GSTP1 and NAT2 have a synergetic effect—they cause the biggest risk of infertility only when both polymorphisms are present. Namely, the 481C > T rs1799929 (NAT2) and the 341C > T or 341 T > T rs1138272 (GSTP1) cause the 17-fold increase in the risk of idiopathic male infertility (OR = 17.24, 95%; CI: 7.30–40.74, P = 0.0001) [55].

Thereby, the NAT2 involvement in the process of male infertility development is highly probable, but more data are needed to confirm its role in mediating the impaired male reproduction.
