To the Editor:
We thank Leiro-Fernandez et al. for their valuable comments on our manuscript Citation[1]. We would, however, like to address some of their comments.
Leiro-Fernandez et al. did not agree with the definition of hepatotoxicity used in our study, which was an increase in serum aspartate aminotransferase level greater than two-times the upper normal limit. Different definitions can be used to define clinically relevant hepatotoxicity Citation[2,3]. We chose the definition in use by the TB Control clinic at the BC Centre for Disease control (BC, Canada), to be consistent with their policies and procedures. An increase in the aspartate aminotransferase of two-times the upper normal limit is clinically relevant to us, as we discontinue isoniazid therapy and monitor by liver function test more frequently when this threshold is exceeded. Using this same definition for our cases in this genetics case–control study is appropriate because one of our study‘s goals was to determine whether there is evidence that a genetic test can presymptomatically predict isoniazid-induced hepatotoxicity in the context of our clinic.
A total of 170 subjects (23 cases and 147 controls) were included in our study Citation[1]. In the original article (see Table 1 in Citation[1]), we specified the racial origin of the participants – Asian was the most common ethnicity (40.7%) followed by Caucasian (30.5%). We agree that there is a wide difference in NAT2 and CYP2E1 genotype distribution among Asian and Caucasian patients, which could explain differences in results between studies. Larger scale studies with homogenous populations would be useful from this perspective.
To genotype CYP2E1, we used a method based on that described by Stephens et al.Citation[4] to genotype the RsaI polymorphic site (rs2031920) approximately 1-kb upstream of the CYP2E1 gene by PCR-RFLP. A new set of PCR primers was designed to avoid a newer SNP annotated in the NCBI SNP database Citation[101] as overlapping with one of the Stephens et al. primers. Our primers were: forward: 5´-GCCACTGGAAAGGAAAGAGA-3´ and reverse: 5´-TTGTGTGTGTGGTTAGAATGAAGA-3´. Although the genotypes of the RsaI polymorphism are called c1 and c2, Leiro-Fernandez et al. are correct in saying that this variant corresponds to the CYP2E1*5 allele Citation[102], rather than the CYP2E1*1C allele, as we misstated in our paper.
Vuilleumier et al. genotyped both the RsaI (*5) and TaqI (*1B) polymorphisms of CYP2E1 and found that the *1A/*1A wild-type genotype was associated with isoniazid-induced hepatotoxicity compared with genotypes that contained *5 and/or *1B alleles Citation[5]. Huang et al. genotyped only the RsaI site, and also found that the homozygous wild-type genotype was associated with increased risk of drug-induced hepatotoxicity compared with genotypes containing the *5 allele Citation[6].
We thank Leiro-Fernandez et al. once again for their useful comments and for communicating their results regarding the CYP2E1*5 polymorphism, which are consistent with our negative results. It seems likely that the lack of consistency of results for these association studies of CYP2E1 variants with TB drug-induced hepatotoxicity in the literature is owing not only to ethnic differences, but also due to the small sample size of these studies, all of which genotyped fewer than 200 subjects. Large sample sizes are required to detect effects of modest size. These results underline the need for much larger single-drug, ethnic-specific studies of well-defined pharmacogenetic phenotypes.
Ethical conduct of research
The authors state that they have obtained appropriate institutional review board approval or have followed the principles outlined in the Declaration of Helsinki for all human or animal experimental investigations. In addition, for investigations involving human subjects, informed consent has been obtained from the participants involved.
Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.
References
- Yamada S , TangM, RichardsonK et al.: Genetic variations of NAT2 and CYP2E1 and isoniazid hepatotoxicity in a diverse population.Pharmacogenomics10(9) , 1433–1445 (2009).
- Benichou C : Criteria of drug-induced liver disorders. Report of an international consensus meeting.J. Hepatol.11(2) , 272–276 (1990).
- Saukkonen JJ , CohnDL, JasmerRM et al.: An official ATS statement: hepatotoxicity of antituberculosis therapy.Am. J. Respir. Crit. Care Med.174(8) , 935–952 (2006).
- Stephens EA , TaylorJA, KaplanN et al.: Ethnic variation in the CYP2E1 gene: polymorphism analysis of 695 African–Americans, European–Americans and Taiwanese.Pharmacogenetics4(4) , 185–192 (1994).
- Vuilleumier N , RossierMF, ChiappeA et al.: CYP2E1 genotype and isoniazid-induced hepatotoxicity in patients treated for latent tuberculosis.Eur. J. Clin. Pharmacol.62(6) , 423–429 (2006).
- Huang YS , ChernHD, SuWJ et al.: Cytochrome P450 2E1 genotype and the susceptibility to antituberculosis drug-induced hepatitis.Hepatology37(4) , 924–930 (2003).
▪ Websites
- NCBI SNP database www.ncbi.nlm.nih.gov/projects/SNP
- CYP2E1 allele nomenclature www.cypalleles.ki.se/cyp2e1.htm