Interpretive Handbook

Arylamine N-acetyltransferase type 2 (NAT2) is a highly polymorphic phase 2 metabolic enzyme that conjugates hydrazine derivatives and aromatic amine drugs with acetyl-groups. NAT2 also is involved in the acetylation and activation of some procarcinogens.(1)

Individuals acetylate drugs at different rates by NAT2, and are described as having slow, intermediate, or fast acetylator phenotypes. A gradient exists in which the prevalence of slow acetylator phenotypes increases with decreasing distance to the equator. Near the equator, up to 80% of individuals may be slow

acetylators, while in some more northern countries, as few as 10% of the population may have the slow acetylator phenotype.

A number of drugs are metabolized by NAT2 including procainamide, dapsone, nitrazepam, hydralazine, zonisamide, and isoniazid. Isoniazid is used to treat and prevent tuberculosis, and is still used as a primary treatment agent. Adverse reactions with isoniazid, which include nausea, drug-induced hepatitis, peripheral neuropathy, and sideroblastic anemia, are associated more often with a slow NAT2 acetylator phenotype. These individuals may require a lower dose to avoid adverse reactions.

The NAT2 gene contains a single intronless exon of 870 base pairs and encodes 290 amino acids. NAT2 is highly polymorphic and contains 16 known single nucleotide polymorphisms (SNPs) and 1 single base pair deletion. These polymorphisms are combined into 36 known haplotype alleles. Each individual haplotype is predictive of either a fast or slow acetylator phenotype. Individuals with 2 fast haplotypes are predicted to be extensive (normal) metabolizers, while those with 1 fast and 1 slow haplotype are intermediate metabolizers, and those with 2 slow haplotypes are poor metabolizers.(2,3) Studies with patients who have different acetylator haplotypes have correlated the ratio of plasma N-acetylisoniazid/isoniazid drug concentrations with haplotypes, with slow and intermediate acetylators having lower ratios than fast acetylators.(4)  

Identifying patients who may require isoniazid dosing adjustments

The wild-type (normal) genotype for NAT2 is *4. This is the most commonly occurring allele in some, but not all, ethnic groups.(5)

Individuals are classified as being slow, intermediate, or fast acetylators depending on their diplotypes. Slow acetylators have 2 slow haplotypes, fast acetylators have 2 fast haplotypes, and intermediate acetylators have 1 of each.

Slow acetylators receiving isoniazid therapy should be monitored for signs of toxicity.

Dose reductions may be considered for both slow and intermediate acetylators. However, it should be verified that the reduced isoniazid dose produces serum levels within the therapeutic range.

This test sequences the entire NAT2 gene. All variants, including novel variants not listed, should be detected. However, novel variants not described in the literature may be of unknown significance.

Mutations in the primer binding regions can affect the testing and, ultimately, the genotyping interpretation made.

Drug-drug interactions and drug or metabolite inhibition must be considered when dealing with heterozygous individuals. Drug or metabolite inhibition can reduce residual functional NAT2 catalytic activity. Acetaminophen is a significant inhibitor of NAT2.

Patients may develop isoniazid toxicity problems if liver and kidney function are impaired, even in the absence of slow or intermediate acetylator status.

In patients who have received a recent blood transfusion or undergone an allogeneic hematopoietic cell or bone marrow transplantation, genotyping using DNA obtained from leukocytes may not provide useful information. For patients who have received a transfusion, wait 4 to 6 weeks until transfused cells have left the circulation before testing. For patients who have undergone allogeneic hematopoietic cell, bone marrow transplantation, or liver transplant, call the laboratory for instructions.

An interpretive report will be provided.

1. Meyer U: Polymorphism of human acetyltransferases. Environ Health Perspect 1994;102:213-216

2. Sabbagh A, Darlu P: Inferring haplotypes at the NAT2 locus: the computational approach. BMC Genetics 2005;6:30

3. Leff M, Fretland A, Doll M, and Heins D: Novel human N-acetyltransferase 2 alleles that differ in mechanism for slow acetylator phenotype. J Biol Chem 1999;274:34519-34522

4. Chen B, Li J-H, Xu Y-M, et al: The influence of NAT2 genotypes on the plasma concentration of isoniazid and acetylisoniazid in Chinese pulmonary tuberculosis patients. Clin Chim Acta 2006;365:104-108

5. Lin H, Han C, Lin B, Hardy S: Ethnic distribution of slow acetylator mutations in the polymorphic N-acetyltransferase (NAT2) gene. Pharmacogenetics 1994;4:125-134