N-acetyltransferase 2 (NAT2) gene polymorphism as a predisposing factor for phenytoin intoxication in tuberculous meningitis or tuberculoma patients having seizures - A pilot study

Abstract

Background & objectives: Simultaneous administration of phenytoin and isoniazid (INH) in tuberculous meningitis (TBM) or tuberculoma patients with seizures results in higher plasma phenytoin level and thus phenytoin intoxication. N-acetyltransferase 2 (NAT2) enzyme catalyses two acetylation reactions in INH metabolism and NAT2 gene polymorphism leads to slow and rapid acetylators. The present study was aimed to evaluate the effect of allelic variants of N-acetyltransferase 2 (NAT2) gene as a predisposing factor for phenytoin toxicity in patients with TBM or tuberculoma having seizures, and taking INH and phenytoin simultaneously.

Methods: Sixty patients with TBM or tuberculoma with seizures and taking INH and phenytoin simultaneously for a minimum period of seven days were included in study. Plasma phenytoin was measured by high performance liquid chromatography. NAT2 gene polymorphism was studied using restriction fragment length polymorphism and allele specific PCR.

Results: The patients were grouped into those having phenytoin intoxication and those with normal phenytoin level, and also classified as rapid or slow acetylators by NAT2 genotyping. Genotypic analysis showed that of the seven SNPs (single nucleotide polymorphisms) of NAT2 gene studied, six mutations were found to be associated with phenytoin intoxication. For rs1041983 (C282T), rs1799929 (C481T), rs1799931 (G857A), rs1799930 (G590A), rs1208 (A803G) and rs1801280 (T341C) allelic variants, the proportion of homozygous mutant was higher in phenytoin intoxicated group than in phenytoin non-intoxicated group.

Interpretation & conclusions: Homozygous mutant allele of NAT2 gene at 481site may act as a predisposing factor for phenytoin intoxication among TBM or tuberculoma patients having seizures.

Fig. 1

Electrophoresis band pattern by KpnI and BamHI digestion. Lane 1: 1kb DNA ladder, lane 2: KpnI digestion (wild), lane 3: KpnI digestion (mutant), lane 4: BamHI digestion (wild), lane 5: BamHI digestion (wild).

Fig. 2

Electrophoresis band pattern by MspI and FokI digestion. Lane 1: 50bp DNA ladder, lane 2: 100bp DNA ladder, lanes 3-5: MspI digestion (wild), lane 6: FokI digestion (wild), lane 7: FokI digestion (mutant), lane 8: FokI digestion (heterozygous).

Fig. 3

Electrophoresis band pattern by TaqI digestion. Lane 1: 25-300bp DNA ladder, lanes 2-4: TaqI digestion (wild), lanes 5-7: TaqI digestion (heterozygous), lanes 8-10: TaqI digestion (wild).

Fig. 4

Electrophoresis band pattern by DdeI digestion. Lane 1: 25-300bp DNA ladder, lanes 2-10: DdeI digestion (mutant).

Fig. 5

Electrophoresis band pattern from allele-specific PCR for T341C site. Lane 1:100bp DNA ladder, lanes 2 and 3: heterozygous, lanes 4 and 5: wild, lanes 6 and 7: mutant.