Letter to the Editor

To the Editor:

We read with interest the study by Yamada et al. regarding the genetic susceptibility to isoniazid drug-induced liver injury [1]. Currently, there is an interest in studying the possible relationship between genetic polymorphisms of the main isoniazid metabolizing phase I enzymes (NAT2 and CYP2E1) and the risk of drug-related hepatotoxicity. Our group is very interested in this field of investigation and its relevant clinical applications. In the study by Yamada et al., NAT2 and CYP2E1 polymorphisms were studied in an ethnically heterogeneous cohort of patients with latent TB owing to infection undertreatment with isoniazid monotherapy [1]. They found a negative association between NAT2 and CYP2E1 polymorphisms. We would like to comment on the study‘s methods that could have possibly affected the results, and to provide data from our experience in the identification of CYP2E1 polymorphisms.

We do not agree that the definition of hepatotoxicity utilized in the paper, an increase in serum aspartate aminotransferase level greater than two-times the upper normal limit, is the most appropriate [2]. Elevations of less than three-times the upper normal limit should not be considered clinically relevant and must be seen as a physiological adaptive response, based on the finding that up to 20% of individuals treated with isoniazid alone may experience a transient, asymptomatic transaminase elevation that primarily represents hepatic adaptation [3]. This variation in the definition could be considered a selection bias and might potentially affect the results. Second, given that the frequency of hepatotoxicity is less than 1% when secondary to isoniazid monotherapy, and even lower in patients without hepatotoxicity risk factors [3] as the authors mention in the discussion section, the sample size is not sufficient to avoid false-negative and false-positive associations. However, the results should be interpreted with caution until replicated in a larger sample. The authors also did not specify the racial origin of the participants, although it is known that there is a wide difference in NAT2 and CYP2E1 genotype distribution among Asian and Caucasian patients [4]. This could explain the discrepancies in the results compared with other studies. Finally, according to currently recommended nomenclature, CYP2E1[11,101] RFLP RsaI identifies the CYP2E1*5 allele. The variant CYP2E1*1C is characterized by 6 repeats in the 5´ flanking region, thus, the authors need to clarify the methodology used to detect the polymorphic repetitive sequence. In the discussion section it is stated that the CYP2E1*1C polymorphism has been associated with anti-TB drug-induced hepatotoxicity in studies by Vuilleumier et al.[5] and Huang et al.[6]. However, in these works CYP2E1*1B[5] and *5[6] polymorphisms were studied.

We would like to provide data from our experience in the identification of CYP2E1 polymorphisms. We performed a nested case–control study following the methodology previously described [7]. A total of 48 patients were diagnosed with anti-TB drug-induced hepatotoxicity, and 66 patients did not exhibit this complication. As a part of the study we analyzed CYP2E1*1A and *5 polymorphisms using PCR-RFLP in 41 cases and 58 controls. The CYP2E1*1A wild-type allele was present in either homozygous or heterozygous patterns in all the patients, but the CYP2E1*5 allele was present only in a heterozygous pattern in 12.1% of the patients. Among the 99 patients, the CYP2E1*1A/*1A genotype was present in 87.8% of cases and 87.9% of controls (p = 0.98), the *1A/*5 genotype in 12.2% of cases and 12.1% of controls (p = 0.98), and the *5/*5 homozygous genotype was not found in any patient (data not published). Therefore, our investigation failed to find a significant association.

The study by Huang et al. in a Chinese population demonstrated that the CYP2E1*1A/*1A genotype increased the risk of anti-TB drug-induced hepatotoxicity with an odds ratio (OR) of 2.52 (95% CI: 1.26–5.05) [6]. In this study, the total frequency of the *1A/*5 and *5/*5 genotypes in the control population was 44.9%, a much higher frequency than that obtained in our study. Vuilleumier et al. observed that the wild-type CYP2E1*1A allele was associated with elevated levels of anti-TB drug-induced liver enzymes in a highly ethnically heterogeneous population (45% Caucasians) [5]. In our study, although we recognize the need for a larger sample size, the low frequency of the CYP2E1*5 allele found did not warrant the replication of this investigation on a larger scale for Caucasian subjects.

In conclusion, we completely agree with the authors that this type of clinic-based investigation is necessary in order to clarify the role of genetic risk factors for anti-TB drug-induced hepatotoxicty.

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.