The non-structural 5A protein of hepatitis C virus exhibits genotypic differences in interferon antagonism q Ya-Hui Tsai 1 , Wan-Fen Kuang 2 , Tsai-Yi Lu 1 , Jia-Horng Kao 2 , Ming-Yang Lai 3 , Chun-Jen Liu 3 , Pei-Jer Chen 3 , Lih-Hwa Hwang 1,2, * 1 Graduate Institute of Microbiology, National Taiwan University College of Medicine, Taipei 100, Taiwan 2 Hepatitis Research Center, National Taiwan University Hospital, National Taiwan University College of Medicine, Taipei 100, Taiwan 3 Graduate Institute of Clinical Medicine, National Taiwan University College of Medicine, Taipei 100, Taiwan Background/ Aims: Patients infected with hepatitis C virus (HCV) genotype 2 or 3 usually respond better to interferon (IFN) treatment than those infected with genotype 1. In this study, we investigated whether the non-structural 5A protein (NS5A) of HCV genotypes 1 and 2 (1b-NS5A and 2a-NS5A, respectively) exerted differential counteractivities against IFN treatment. Methods: We compared the inhibitory effects of 1b-NS5As and 2a-NS5As on IFN activity. We also investigated the replication inhibition of HCV subgenomic replicons containing 1b-NS5A or 2a-NS5A in response to IFN treatment. Results: 1b-NS5As exerted more profound inhibitory effects on IFN activity than 2a-NS5As. The replication of the 2a-NS5A-containing replicons was more sensitive to IFN treatment than that of the 1b-NS5A-containing replicons. Deletion of the interferon sensitivity-determining region/ protein kinase R-binding domain (PKR-BD), the V3 domain, or the C-terminus region of NS5A significantly abrogated its anti-IFN activity. Domain swapping between 1b-NS5A and 2a-NS5A in the V3 domain and/or the C-terminus region resulted in a transfer of their anti-IFN activity. Conclusions: 1b-NS5As exert higher magnitudes of IFN antagonism than do 2a-NS5As. The V3 and the C-terminus regions are responsible for the differential anti-IFN effects. This phenomenon may partly explain the genotype-linked differences in the response of HCV to IFN treatment. Ó 2008 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved. Keywords: Hepatitis C virus; NS5A; Genotype; IFN antagonism 1. Introduction The PEGylated interferon (IFN)-a in combination with ribavirin is currently the optimal therapy for patients infected with the hepatitis C virus (HCV) [1]. However, the sustained virological response (SVR) rates to the combination therapy vary significantly among patients infected with different HCV genotypes. In gen- eral, they are about 75–80% for patients infected with HCV genotype 2 or 3, but only about 35–45% for those infected with HCV genotype 1 [2,3]. The underlying mechanisms through which different HCV genotypes have differing IFN responses are not clear, but may be related to variations in genome sequence, which are responsible for differences in viral protein structure and function. IFN-a, a type I interferon, functions as the frontline defense mechanism for the host against viral infection. 0168-8278/$34.00 Ó 2008 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jhep.2008.06.030 Received 9 January 2008; received in revised form 13 May 2008; accepted 26 June 2008; available online 16 September 2008 Associate Editor: J.G. McHutchison q The authors who have taken part in the research of this paper declared that they do not have a relationship with the manufacturers of the materials involved either in the past or present and they did not receive funding from the manufacturers to carry out their research. * Corresponding author. Tel.: +886 2 28267114; fax: +886 2 28212880. E-mail address: lhhwang@ym.edu.tw (L.-H. Hwang). www.elsevier.com/locate/jhep Journal of Hepatology 49 (2008) 899–907