Hepatitis C Virus (HCV)

What is Hepatitis C Virus (HCV)?

Hepatitis C virus (HCV) is the causative agent that causes hepatitis C. Hepatitis C virus is an RNA virus and is a relatively common hepatotropic virus. After hepatitis C virus infection, 70% to 80% of patients will develop chronic viral hepatitis, which is a category B infectious disease. It is mainly transmitted through blood. According to the World Health Organization (WHO), about 71 million people worldwide were infected with chronic hepatitis C in 2015, and a large number of chronically infected people will develop cirrhosis or liver cancer, and about 399,000 people die from hepatitis C and related cirrhosis and liver cancer every year.

Hepatitis C Virus genotype

Hepatitis C virus is a single-stranded positive-stranded RNA virus, due to the lack of proofreading function of RNA polymerase (RdRPs) in the replication process, coupled with the characteristics of high-frequency replication (up to 1012 copies of viral particles can be replicated per day), the virus gene is very prone to mutation, resulting in the existence of multiple genetically related but different quasi-species in the infected person, and different virus genotypes are produced among different infected people.

The hepatitis C virus genome is made up of approximately 9600 nucleotides and encodes an open reading frame of 3010 amino acids. The HCV genome is a positively polarized, single-stranded RNA that is divided into the Flaviviridae and Hepatoviridae genera. It is estimated that about 71 million people are currently infected (WHO, 2017). The 5'- and 3'-untranslated regions (UTRs) are essential for viral replication and translation. Translation of HCV proteins begins at the ribosome entry site inside the 5'-UTR and produces a single polyprotein. This multiprotein is processed by cellular proteases to produce structural proteins that form viral particles (core and envelope glycoproteins E1 and E2) or by viral proteases to produce non-structural (NS) proteins (p7, NS2, NS3, NS4A, NS4B, NS5A, and NS5B).

Fig.1 Genomic structure of Hepatitis C Virus. (Tsukiyama-Kohara Kyoko, 2017)

In 1993, Simmonds et al. established a naming convention that is now widely recognized and applied internationally based on the genetic homology and evolutionary distance differences of different strains in the NS5 region, and divided HCV into types 1-6 (represented by Arabic numerals, 1, 2, 3, etc.) and several subtypes (lowercase English letters, a, b, c, etc.) according to the order of discovery.

Up to now, HCV has been divided into at least 8 genotypes and more than 90 genotypes, and HCV genotypes/subtypes have obvious geographical distribution differences, among which types 1, 3 and 2 are widely distributed worldwide, type 4 is mostly distributed in Central Africa and Central Asian countries, type 5 is mostly found in South Africa countries, type 6 is more common in East Asia and Southeast Asia, and types 7 and 8 are only found in some individuals.

Fig.2 Map of the worldwide distribution of hepatitis C virus genotypes. (Tsukiyama-Kohara Kyoko, 2017)

Hepatitis C Virus test

HCV antibody testing

HCV antibody detection technology is the earliest HCV detection technology that has been developed, researched and applied for the longest time. HCV antibodies are produced in response to the body's immune cells in response to hepatitis C virus infection. Antibodies circulate in the bloodstream and are often detected. Hepatitis C antibodies do not have any protective effect on the body, and a positive hepatitis C antibody does not necessarily mean that you have hepatitis C. The diagnosis of hepatitis C is based on the presence or absence of hepatitis C virus in the blood, so HCV antibodies are the main indicator of whether HCV has been infected. HCV antibody testing is suitable for screening high-risk groups and can also be used for primary screening of patients with HCV infection.

HCV-RNA nucleic acid testing

HCV-RNA is a marker of HCV replication, and HCV-RNA can be detected within 14 days of HCV infection, which can be used as the main reference for judging the efficacy of hepatitis C infection and hepatitis C treatment. Early diagnosis of HCV infection relies more on sensitive quantitative HCV-RNA testing. There are two methods for HCV RNA detection, both qualitative and quantitative, and the basic principle is reverse transcription polymerase chain reaction (RT-PCR). Qualitative PCR is more sensitive than quantitative PCR. Therefore, qualitative PCR is mainly used for the diagnosis of acute and chronic hepatitis C, while quantitative PCR is used for efficacy monitoring.

Treatment of hepatitis C virus infection

Until 2011, the standard treatment for hepatitis C virus (HCV) infection was a combination of pegylated interferon (PegIFN) and ribavirin (RBV). However, this interferon-based treatment has a low overall cure rate of only 54% to 56% and has significant toxic side effects, resulting in approximately 14% of patients discontinuing treatment halfway through because they cannot tolerate the adverse effects of treatment. Based on these limitations, scientists are constantly seeking more effective and better tolerated treatments.

With the deepening understanding of the HCV genome and its replication cycle, breakthroughs have been made in the development of direct-acting antiviral drugs (DAAs). The HCV genome is approximately 9.6 thousand base pairs and encodes three structural proteins (Core, E1, and E2) and seven non-structural proteins (NS2, NS3, NS4A, NS4B, NS5A, and NS5B). Among them, the non-structural proteins of HCV play an indispensable role in the replication process of the virus. Therefore, inhibitors targeting these non-structural proteins have become the focus of the development of DAAs.

For example, Boceprevir, Telaprevir, and Paritaprevir are NS3/4A inhibitors that inhibit viral replication by blocking the activity of NS3/4A proteases. Ombitasvir, Ledipasvir, Elbasvir, and Velpatasvir are NS5A inhibitors that block multiple stages of viral replication by interfering with the function of the NS5A protein. Sofosbuvir and Dasabuvir are NS5B inhibitors, with the former being directly embedded in the RNA strand of the virus as a nucleoside polymerase inhibitor, while the latter acting as a non-nucleoside polymerase inhibitor inhibiting its function by altering its structure.

At present, the combination of two or more DAAs has become the cornerstone of the treatment of HCV infection. The combination of these drugs not only significantly improves cure rates, but also reduces treatment time and side effects. For example, the combination of Sofosbuvir and Ledipasvir has been shown to cure more than 90% of HCV-infected patients within 12 weeks. In addition, other combinations such as fluorescein with α interferon, the protease inhibitor Telaprevir, the NS5A inhibitor BMS-790052, or the NS5B polymerase inhibitor PSI-7977 have also shown potent synergistic antiviral effects.

Reference

• Tsukiyama-Kohara, Kyoko, and Michinori Kohara. Hepatitis C virus: viral quasispecies and genotypes. International journal of molecular sciences 19.1 (2017): 23.

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