HCV genotype 3: a wolf in sheep’s clothing

“All truths are easy to understand once they are discovered; the point is to discover them”. Galileo Galilei

At present, the hepatitis C virus (HCV) is a major cause of morbidity and mortality affecting over 185 million people worldwide,[1] equivalent to a global prevalence of about 2.5% in 2015. Genotype 3 (HCV G3) is one of the seven recognized genotypes.[2] HCV G3 is the second most common genotype overall and is estimated to account for 54.3 million cases throughout the world (30.1%).[1] Although three-quarters of them occur in South Asia where it is endemic, the 3a subtype is an ‘epidemic subtype’ widely distributed geographically, probably associated with injecting drug use.[3]

In the past two years, the history of HCV infection has changed radically with the appearance of the new direct-acting antiviral agents (DAAs).[4] Although HCV G3 was one of those genotypes who achieved a better sustained viral response (SVR) using pegylated interferon and ribavirin (PEG-IFN/Rbv) therapy,[5] the current effectiveness of the new DAAs against HCV G3 leaves a lot to be desired compared with the results obtained with other genotypes [6]. This is a major problem as compared with other genotypes, HCV G3 is associated with faster progression of fibrosis,[7,8] a greater risk for hepatocellular carcinoma (HCC),[8–10] and a higher mortality.[11] Why is it so pathogenic and resistant to treatment? The reasons for this ‘aggressiveness’ are without doubt multiple, complex and not well known.

First, it is important to remember that the host immune response plays an important role in HCV G3 infection because of its potential to contribute to viral clearance. So, acutely HCV-infected patients are much more likely to spontaneously clear HCV if they are infected with HCV G3 than HCV G1.[12] Indeed, chronically infected HCV G3 patients had higher SVR rates after shorter treatment with PEG-IFN/Rbv therapy when compared with those with chronic HCV G1 infection.[13] One of the possible reasons could be that in monocytic cell and plasmacytoid dendritic cell lines and in macrophages differentiated from monocytes with macrophage colony-stimulating factor, HCV G3 induces greater interferon transcription than either genotype 1a or 1b.[14] However, this apparent benefit may backfire because of the increased rate of fibrosis progression of HCV G3, probably because of the higher non-parenchymal cell transcription of IFN genes following intracellular HCV G3 sensing.[14]

It has been reported previously that HCV G3 is associated with a significantly increased risk of developing cirrhosis and HCC compared with HCV G1 and association that is independent of the patients’ age, diabetes, body mass index, or antiviral treatment.[8] The high viremia observed in HCV G3-infected patients may be a marker of rapid disease damage, reflecting either the inability of the immune system to control the infection or the existence of some escape mechanisms in HCV G3 which prevents the immune system response from being effective.[15]

Secondly, another problem that is not well understood is the interaction between HCV and lipid metabolism.[16] So, HCV G3 selectively interferes with the late cholesterol synthesis pathway,[17] although this interference is resolved after the SVR. Other mechanisms that alter lipid metabolism are increased the novo lipogenesis and the inhibition of mitochondrial fatty acid degradation.[18] At what level of lipid metabolism does HCV G3 work? Is the damage the consequence of the virus or of its proteins in infected hepatocytes? Given that, in previous studies, the variables independently associated with SVR were high LDL levels,[19,20] low HDL levels [19] and statin use,[19] one might think that statins would be a useful option for such patients. Nonetheless, this is not actually the case with HCV G3. In one analysis of patients with HCV 1–3 genotypes who received combination therapy with PEG-IFN/Rbv, the significant impact of statin use was only observed among the HCV G1 patients.[20] Similar findings were reported by Kurincic et al. [21]

Steatosis is a common histologic finding in patients infected by HCV G3, independently of the presence of fibrosis, diabetes, hepatic inflammation, ongoing alcohol abuse, higher body mass index and older age [22]. Indeed, steatosis in HCV G3 infected patients is not the result of overexpression of genes involved in lipogenesis.[23] The higher rates of hepatic steatosis in HCV G3 patients, even in absence of other metabolic complications, suggest that some specific viral sequences may be involved in the etiology of steatosis.[18] In fact, after reaching SVR, hepatic steatosis in these patients had disappeared.[24,25] Another possible explanation for the high presence of steatosis could be that HCV G3 steatosis induces the liberation of proinflammatory chemokines that increase the recruitment of inflammatory cells to the liver.[14] In support of this idea, the depletion of liver Kupffer cells prevents the development of diet-induced hepatic steatosis and insulin resistance.[26]

It is important to bear in mind that there is a significant correlation between the steatosis score and the titer of intrahepatic HCV RNA in patients with HCV G3, providing virological and some clinical evidence that steatosis is the morphological expression of a viral cytopathic effect in patients infected with this genotype.[27] This finding has important implications, such as lower SVR rates or higher relapse rates after HCV treatment.[28,29] Is steatosis a marker of rapid progression or bad prognosis in HCV G3 infected patients?

Non-alcoholic fatty liver disease (steatosis/steatohepatitis) is similarly recognized as the hepatic manifestation of metabolic syndrome (MS). HCV virus genotype 3 infection increases the risk of insulin resistance and diabetes, probably because of the direct effect of the virus on intracellular insulin signaling.[30] This situation not only increases the cardiovascular risk but also reduces the likelihood of achieving a SVR.[31] Another common manifestation of MS is obesity, a problem that also increases the expression of some inflammatory cytokines and activates several signaling pathways involved in the pathogenesis of insulin resistance.[32] The inflammation may also contribute to the pathogenesis of liver damage.[33] Obesity has also been correlated with a lower SVR rate.[34] Once again, this opens up an interesting way to research the mechanisms involving MS and SVR in these patients. Are insulin resistance and/or obesity indicators of the the existence of an established liver damage, even though we are unable to diagnose it? Is there some symbiotic relationship between the adipocytes and HCV G3 that reduces the efficacy of the DAAs? In view of the higher rates of SVR using the new DAAs, is the presence of MS still important in chronic HCV infection?.[35] Probably not, but there is as yet no concrete answer for this question and so the controversy about steatosis and HCV remains. Valenti et al. also reported that the rs738409 genotype, a polymorphism that influences liver fat without affecting insulin resistance and body composition, was associated with severe hepatic steatosis in patients infected with a non-3 HCV genotype, and also with fibrosis stage and cirrhosis (OR = 1.47; p = 0.002).[36] Similarly, Cai et al. [37] reported that rs738409 was associated with an increased risk of steatosis in patients infected with a non-3 HCV genotype. These results suggest distinct pathogenic mechanisms in the 3 and non-3 genotypes.

Moving on to the third point, and so concluding this topic, it is necessary to understand the clinical implications of the different HCV G3 subtypes (in other words, immunity, inflammation, prognosis, response to DAAs). This is something we already known for HCV G1a and 1b.[38] At least 10 HCV G3 subtypes have been described so far.[39] Are some of these HCV G3 subtypes able to evade the immune response? Can we expect the same SVR for different subtypes? The correct identification of HCV G3 subtypes would probably be necessary because they are crucial in clinical trials evaluating the new DAAs. No data have so far stratified the response of HCV G3 to the new DAAs, which could be an essential issue that requires further investigation.

In summary, given the aggressiveness of HCV G3, it is increasingly necessary to initiate antiviral treatment as soon as possible in all patients, including those with steatosis and/or MS. In these patients, even those with SVR, continued surveillance is necessary, paying careful attention to patients with cirrhosis. There is no doubt that better knowledge of HCV G3 should be a priority for us all.

Financial & competing interests disclosure

JR Blanco has carried out consulting work for Abbvie, Bristol-Myers Squibb, Gilead Sciences, Janssen, Merck, and ViiV Healthcare; has received compensation for lectures from Abbvie, Bristol-Myers Squibb, Gilead Sciences, Janssen, Merck, and ViiV Healthcare, as well as grants and payments for the development of educational presentations for Gilead Sciences and Bristol-Myers Squibb. A Rivero-Juarez is the recipient of a Postdoctoral Perfection Grant from Fundación Progreso y Salud, Consejería de Salud y Políticas Sociales, Junta de Andalucia (0024-RH-2013). He has received compensation for lectures from Bristol-Myers Squibb, Janssen, Merck, and ViiV Healthcare.The authors have no other 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 apart from those disclosed.