Reply to “IL28B polymorphism and hepatitis C: A genetic marker of peginterferon-α sensitivity devoid from classical interferon-α side effects?”
Authors: A.J. Thompson, P.J. Clark, A.J. Muir, M.S. Sulkowski
Journal: Journal of Hepatology (2012)

Genome-wide association study of interferon-related cytopenia in chronic hepatitis C patientsJournal of HepatologyVol. 56Issue 2PreviewInterferon-alfa (IFN)-related cytopenias are common and may be dose-limiting. We performed a genome wide association study on a well-characterized genotype 1 HCV cohort to identify genetic determinants of peginterferon-α (pegIFN)-related thrombocytopenia, neutropenia, and leukopenia. Full-Text PDF IL28B polymorphism and hepatitis C: A genetic marker of peginterferon-α sensitivity devoid from classical interferon-α side effects?Journal of HepatologyVol. 56Issue 3PreviewSingle nucleotide polymorphism (SNP) in the region of the interleukin-28B (IL28B) gene has recently been associated with spontaneous and treatment-induced clearance of hepatitis C virus infection [1–3]. The SNPs are located near the IL28B gene of chromosome 19, implicating a role for its gene product, interferon-λ3, in the immune response to hepatitis C virus. The mechanisms by which IL28B polymorphism effects the sensitivity to exogenous peginterferon-α therapy is unclear at present. Full-Text PDF Open Access We appreciate the interest of Orlent et al. in our recent study [[1]Thompson A.J. Clark P.J. Singh A. Ge D. Fellay J. Zhu M. et al.Genome-wide association study of interferon-related cytopenia in chronic hepatitis C patients.J Hepatol. 2012; 56: 313-319Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar] and would like to comment on the issues raised in their letter. (i) The primary endpoint we selected for all genome-wide analyses of peginterferon (pegIFN)-induced cytopenia was quantitative reduction in cell counts, as a continuous variable, at week 4. We chose the continuous variable as the primary endpoint for two reasons. Firstly, this method maximizes the statistical power of the dataset. Secondly, it avoids the pitfalls of defining pre-specified thresholds for cytopenia, where timing and the chosen level of the threshold are arbitrary. We chose the week 4 timepoint to minimize confounding by dose adjustment/adherence. It is true that week 4 preceded the nadir for pegIFN-related thrombocytopenia and neutropenia in this study, which occurred between week 8 and 12. However, the rate of decline was most profound in the first 4 weeks, with significant reductions in both platelet counts and neutrophil counts by week 4 [[1]Thompson A.J. Clark P.J. Singh A. Ge D. Fellay J. Zhu M. et al.Genome-wide association study of interferon-related cytopenia in chronic hepatitis C patients.J Hepatol. 2012; 56: 313-319Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar]. We have subsequently tested for association between the IL28B polymorphism rs12979860 and quantitative reduction of platelet/neutrophil counts at week 12. No significant association was observed. (ii) Orlent et al. suggested that inclusion of patients treated with low-dose pegIFN-α-2b in our analysis may have confounded the results. We included these patients to maximize the sample size. We did adjust for pegIFN dose however, as stated in the methodology [[1]Thompson A.J. Clark P.J. Singh A. Ge D. Fellay J. Zhu M. et al.Genome-wide association study of interferon-related cytopenia in chronic hepatitis C patients.J Hepatol. 2012; 56: 313-319Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar], by coding a binary variable for full vs. low dose pegIFN. This variable was included in all statistical models. PegIFN dose was significantly associated with both thrombocytopenia and neutropenia as expected. IL28B polymorphism was not significantly associated with pegIFN-cytopenia when analysis was limited to patients receiving full-dose pegIFN. (iii) Orlent et al. also raised the possibility of inclusion bias. It is true that we limited the analysis of week 4 cytopenia to patients who were at least 80% adherent to study treatment to this timepoint. This decision was taken to refine the clinical phenotype. However, it should be noted that only 26 patients were excluded from the analyses of platelet/neutrophil counts on the basis of adherence at the week 4 timepoint [[1]Thompson A.J. Clark P.J. Singh A. Ge D. Fellay J. Zhu M. et al.Genome-wide association study of interferon-related cytopenia in chronic hepatitis C patients.J Hepatol. 2012; 56: 313-319Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar]. Inclusion of these 26 patients into the analysis does not change the results of the analyses. The majority of the patients who consented to genetic testing, but were not included in this analysis, were excluded on the basis of genotyping quality control or missing data points (only patients with complete datasets for all relevant variables could be included in the analyses). There was no significant difference in the median reductions of platelet counts or neutrophil counts comparing patients who were included in the final analysis vs. patients who consented to genetic testing but were not included in the final analyses (patients of European American, African American and Hispanic ethnicity considered separately). We therefore feel that inclusion bias within this study was unlikely. We have recently analyzed the overall IDEAL cohort for patient factors that might influence consent to the performance of genetic testing and the data will be presented at AASLD 2011 [[2]Jazwinski A. Clark P.J. Thompson A.J. Gordon S.C. Lawitz E. McCone J. et al.Predictors of consent to pharmacogenomics testing in the IDEAL study.Hepatology. 2011; 54 (393): 554AGoogle Scholar]. There were no differences in treatment outcomes or overall adverse events (including cytopenia) between patients who did, or did not consent, to participation in the pharmacogenomics substudy. The data suggest that IL28B polymorphism does not influence the occurrence or severity of IFN-related cytopenia. However, we acknowledge that the methodology of genome-wide association studies has some limitations, including the statistical requirement to correct for multiple testing. As a consequence, small effect sizes may not be detected. Therefore, we cannot exclude the possibility that there may be a small contribution (odds ratio <1.5) of common genetic variants to IFN-related cytopenias. The clinical importance of such a contribution would be expected to be modest. Drs. Thompson, Muir and Sulkowski report having received research and grant support and receiving consulting fees or acted in an advisory capacity for Schering-Plough (now Merck, Inc). Dr. Thompson is co-inventor of a patent application based on the ITPA finding. Dr. Clark has no conflicts to report.