SEARCH

SEARCH BY CITATION

Abstract

  1. Top of page
  2. Abstract
  3. Natural History
  4. General Host Factors
  5. Immunosuppression
  6. Metabolic Factors
  7. Liver-Specific Factors
  8. Host Genetic Factors
  9. Viral Factors
  10. Conclusion
  11. References

Hepatitis C follows a variable course with some patients developing progressive liver fibrosis, cirrhosis and hepatocellular carcinoma, while others have minimal or no significant liver disease after decades of infection. Studies have identified both host and viral factors associated with disease progression. The importance of general factors such as age at infection, gender, immune status and alcohol consumption has long been recognized; however recently, polymorphisms in a wide array of genes have also been shown to be associated with progressive fibrosis. How specific viral proteins may contribute to disease progression has also been studied. This review highlights what is currently known about the factors associated with progressive liver injury in patients with hepatitis C. A greater understanding of the determinants of disease progression will hopefully lead to improved utilization of existing treatments and ultimately may aid in identification of new therapeutic targets. (Hepatology 2006;43;S194–S206.)

Although hepatitis C (HCV) infection has emerged as the most common cause of end-stage liver disease and the leading indication for liver transplantation in the United States, many infected patients live decades with minimal or no progression of disease.1 Natural history studies have identified numerous factors, some viral and others host-related, that are associated with progressive liver injury; however both demonstration of direct causal relationships and mechanistic understanding of such relationships are still largely lacking. This is particularly important given the current therapeutic options for HCV. While antiviral therapy for HCV has greatly improved over the past decade, treatment is costly, associated with numerous side effects and up to 50% of patients ultimately do not respond. Understanding which patients are likely to develop significant liver disease would allow more rational use of therapy and more importantly may provide clues to help optimize current treatments and potentially help identify new therapeutic targets.

Natural History

  1. Top of page
  2. Abstract
  3. Natural History
  4. General Host Factors
  5. Immunosuppression
  6. Metabolic Factors
  7. Liver-Specific Factors
  8. Host Genetic Factors
  9. Viral Factors
  10. Conclusion
  11. References

Chronic HCV infection is a very slowly progressive disease, making prospective evaluation of its natural history very difficult. Because acute HCV is usually subclinical, by the time most patients present to medical attention, they have usually been infected for years if not decades.2 By this time, disease is usually well established. In contrast, if patients are followed from the time of a known exposure, long periods of time are necessary to document any evidence of progressive liver injury. Consequently, our understanding of the natural history of HCV is based largely on evidence garnered from cross-sectional, retrospective-prospective, or short longitudinal studies very early or relatively late in the course of disease. How these two ends of the spectrum tie together is very difficult to ascertain but is of critical importance if we are to identify patients at risk for progressive liver disease early enough to potentially intervene.

Liver pathology in chronic HCV infection is characterized by portal and lobular necroinflammation.3 While this eventually leads to fibrosis and ultimately cirrhosis in a percentage of patients, the rate at which it occurs and the factors that influence the rate and degree of progression remain largely speculative. The rate of fibrosis progression has been estimated from serial liver biopsy studies. By dividing the change in fibrosis grade by the time interval between biopsies or from the time of known exposure, rates of progression can be estimated. However, this type of analysis has significant limitations. First, it is based on the presumption of a linear progression over time. While this may be the case, there is evidence that fibrosis may accelerate with time and the baseline rate of progression likely varies significantly between patient populations.4 In addition, numerical estimates of fibrosis progression are based on the assumption that the quantitative scores that have been used to grade fibrosis are also linear. Whether a biopsy scored as an F4 really has twice as much fibrosis as a biopsy scored as F2 is not clear. Other important potential limitations are sampling error, incorrect dating of HCV exposure and referral bias of only patients with significant liver disease.5

Despite the inherent limitations, serial biopsy data remain the best available tool to estimate progression of fibrosis. In the largest study, Poynard et al.4 reviewed 2235 HCV infected patients and estimated their rate of progression from no fibrosis (F0) to cirrhosis (F4) using either serial biopsies or a single biopsy and the date of infection. They found that fibrosis progression was fairly linear over time but was asymmetrically distributed, suggesting that patients do not all progress at the same rate. While the median time to cirrhosis for the whole group was 30 years, it was evident that there were slow, intermediate and rapid fibrosers among the cohort. Based on differing rates of progression, 33% would be expected to reach cirrhosis in less than 20 years while 31% would take 50 years or more to become cirrhotic. Numerous smaller studies of similar design have found fairly similar median rates of fibrosis progression, but have also confirmed that there is marked variability within and between patient populations.6, 7

Longitudinal follow-up studies of patients after a known exposure to HCV have suggested lower rates of fibrosis progression, however the results may be somewhat misleading. In a large study from Ireland, Kenny-Walsh and colleagues reported that only 2% of 376 women infected by HCV-contaminated anti-D immune globulin had developed cirrhosis at 17 years of follow-up.8 However, it is important to note that 51% of patients had fibrosis on biopsy and 15% had bridging fibrosis (F3). If patients with bridging fibrosis and cirrhosis (F3/F4) are considered together, 17% of the cohort had advanced fibrosis. All of the patients in this study were young women with no other risk factors for progressive disease. Hence, the rate of progression of fibrosis was actually quite in keeping with previous estimates from serial biopsy studies. Although another study of women infected with immune globulin from Germany reported bridging fibrosis or cirrhosis in only 3.4% after 20 years follow-up,9 most other studies of post-transfusion HCV suggest more rapid progression with rates of cirrhosis ranging from 8% to 24% over a mean follow-up of 8-14 years.10–14 What is clear from the available data is that although HCV may be a mild disease in certain populations, a significant proportion of patients develop progressive liver injury over time and the rate of progression varies greatly between cohorts.

Numerous studies have looked at factors associated with progression of fibrosis in HCV infection (Table 1). Multivariate analyses of data from cross-sectional studies consistently identify male gender, chronic alcohol use and older age at infection to be significantly associated with more rapid progression of disease.4 Other factors such as duration of infection, long-term immunosuppression and hepatitis B (HBV) co-infection have also been found to be associated with progression.5 Longitudinal data have shown that the presence of fibrosis and perhaps more importantly the degree of necroinflammation on initial biopsy predict future fibrosis progression.6, 7, 15 In keeping with the concept that greater inflammation leads to greater fibrosis, Ghany et al. found that patients with higher transaminase values were also more likely to progress.6 More recently, investigators have looked at other factors correlating with HCV progression, ranging from steatosis and insulin resistance to genetic polymorphisms in inflammatory, oxidative stress and even coagulation cascades. Viral factors such as quasispecies evolution and direct toxic effects of HCV proteins have also been evaluated. Numerous associations have been identified, however how these factors may promote hepatic fibrosis in some, but not all patients, remains poorly understood.

Table 1. Host and Viral Factors Affecting Fibrosis Progression in Chronic HCV Infection
Host Factors
 Demographics
  - Duration of Infection
  - Age at Infection
  - Gender
 Toxins
  - Alcohol Consumption
  - Smoking/Cannabis Use (?)
  - Iron Overload
 Immunosuppression
  - HIV Co-infection
  - Organ Transplantation
 Metabolic Factors
  - Steatosis
  - Insulin Resistance
 Liver Specific Factors
  - HBV/HAV Co-infection
  - ALT Elevation
  - Fibrosis
 Genetic Factors
  - Polymorphisms
Viral Factors
 Viral Proteins
  - Oxidative stress
  - Apoptosis
  - Steatosis
  - Cytokine Stimulation

General Host Factors

  1. Top of page
  2. Abstract
  3. Natural History
  4. General Host Factors
  5. Immunosuppression
  6. Metabolic Factors
  7. Liver-Specific Factors
  8. Host Genetic Factors
  9. Viral Factors
  10. Conclusion
  11. References

Age and Gender

While duration of infection has been shown to be a predictor of disease progression, somewhat surprisingly, the age at infection appears to be even more important. Multiple studies have shown that older age at infection independently correlates with more rapid development of fibrosis even when controlled for duration of infection.4, 7, 16, 17 Poynard et al. found a 3-fold higher rate of progression in patients infected above age 50 than in those infected between 21 and 30 years of age (0.333 vs. 0.105 fibrosis units per year, P <.0001) and identified the age of 40 as an important threshold for more rapid fibrosis.4

Most studies of HCV-infected children have documented a slow progression of disease, with children infected at an older age progressing faster than those infected at or near birth.18–21 Immune status and prior medical history are clearly important as highlighted by reports of low rates of progression 20 years after childhood cardiac surgery compared to moderate fibrosis or cirrhosis in 50% of HCV-infected survivors of pediatric malignancies after only 12 years of follow-up.18–22

How exactly age at infection affects the rate of progression is not clear. It has been suggested to be a consequence of age-related changes in immune function.23 There may also be intrinsic changes in the liver's ability to tolerate viral infection with age. After liver transplantation, fibrosis progression has been shown to be associated with increasing age of the donor organ, irrespective of the age of the recipient.24 Similarly, drug-induced liver injury and the severity of acute hepatitis A infection also worsen with age.25 Although organ senescence is not thought to be a major cause of hepatic dysfunction, there may be subtle changes in the ability of the aging liver to tolerate toxic insults.

Despite a similar prevalence of HCV infection between genders, numerous reports have shown that men progress faster to cirrhosis and have an increased risk of hepatocellular carcinoma.4, 16, 26, 27 Even controlling for other factors such as alcohol consumption, obesity and iron overload, the risk of progression remains 2.5-fold higher in men than women.4 A recent French study suggests that this may be due to a protective effect of estrogen.28 Postmenopausal women and nulliparous women had increased rates of fibrosis progression, but use of hormone replacement therapy reduced the rate to that seen in premenopausal women. A rat model of hepatic fibrogenesis also showed that estrogen has a protective effect, however the precise mechanism remains unknown.29

A recent study from the Mayo clinic challenges the strength of the demographic associations identified with progressive fibrosis in hepatitis C. Using methods from population epidemiology, Kim et al. argue that the association with advanced fibrosis seen with older age at infection and male gender, may be an artifact of the simple reality that such patients are at higher risk of death from other causes.30 Their model suggests that up to half of the association between age at infection and fibrosis can be accounted for by considering competing risks of ‘natural’ mortality.

Alcohol and Smoking

Numerous studies have documented that alcohol consumption increases the rate of progression of fibrosis and mortality in HCV.31, 32 The threshold level above which alcohol hastens disease progression is unknown but estimates range from 30-80 g/day and the effects appear to be synergistic above 50 g/day of alcohol.33–35 The histological pattern in alcoholics infected with HCV is usually typical of that seen in HCV rather than alcoholic liver disease suggesting that alcohol worsens HCV pathology rather than simply causing secondary, additive damage of a different nature.36 A number of mechanisms have been proposed to account for the effects of alcohol on HCV progression, including immune dysfunction, oxidative stress and steatosis leading to apoptosis and even increased viral replication.37–39 Stimulation of proinflammatory and profibrotic cytokines may also be important.40

Although less well established than for alcohol, some evidence suggests that smoking may lead to more rapid progression of HCV infection. Hezode et al. showed that patients that smoked more than 15 cigarettes daily for many years had higher histological activity scores on biopsy, a finding found in numerous studies to portend more rapid fibrosis progression.6, 41 Recently, the same group found that daily cannabis use was found to be an independent predictor of rapid fibrosis progression.42 This result was somewhat surprising as previous data have shown that activation of cannabinoid receptors mediates an antifibrogenic effect on the liver.43

Iron

Mild iron overload is a common liver biopsy finding in patients with HCV infection. Because both iron overload and HCV lead to progressive fibrosis, several groups have examined whether iron may accelerate HCV-induced liver damage. In a small study of patients with hereditary hemochromatosis and HCV, patients with both conditions developed advanced fibrosis at a younger age with lower levels of hepatic iron than patients with either disease alone.44 Similarly, more advanced fibrosis was seen in heavily transfused thalassemic patients with HCV infection.45, 46

The effects have been less clear in modestly iron-overloaded patients. Different groups have examined C282Y and H63D heterozygotes infected with HCV, however despite similar methodologies, the results are somewhat discrepant. While two large studies reported that HFE mutations in HCV-infected patients were strongly associated with iron overload and advanced fibrosis/cirrhosis,47, 48 two other studies of similar patients found only modestly increased hepatic iron and no increase in fibrosis or histological activity,49, 50

Iron may affect HCV infection through a direct effect on the virus. Theurl et al. have shown that iron stimulates the expression of translation initiation factor 3 (eIF3), which is essential for HCV translation.51 On the other hand, iron has also been shown to suppress HCV replication in tissue culture.52 Iron loading also appears to affect treatment responses to antiviral therapy, and although some data suggest that pre-treatment phlebotomy may be useful,53 most have found only a marginal or no benefit.54, 55

Immunosuppression

  1. Top of page
  2. Abstract
  3. Natural History
  4. General Host Factors
  5. Immunosuppression
  6. Metabolic Factors
  7. Liver-Specific Factors
  8. Host Genetic Factors
  9. Viral Factors
  10. Conclusion
  11. References

In addition to affecting the likelihood of clearing HCV spontaneously or with antiviral therapy, the immune status of the patient also affects the progression of disease. HCV is generally considered to be either non- or only mildly cytopathic, with most of the liver injury resulting from the host immune response to the virus.56 However, understanding of this process remains incomplete as immunosuppressed patients have more rapidly progressive disease.

HIV Co-Infection

HIV-HCV co-infection is a common occurrence because of shared modes of transmission. Up to 30% of HIV-infected individuals in the United States are co-infected with HCV and this combination is likely much higher in developing countries where the prevalence of both diseases is increased.57 Prior to the era of highly active antiretroviral therapy (HAART), most patients died of AIDS before developing significant liver disease. This pattern has turned full circle. Reports from both the United States and Europe have shown that in HIV+ patients on HAART, the most common cause of death is end-stage liver disease, predominantly due to HCV co-infection.58–60 A meta-analysis of 8 studies of co-infection showed a 2-fold increased rate of progression to cirrhosis and a 6-fold increased risk of decompensated liver disease over HCV mono-infection.61 Factors identified to correlate with rapid progression include low CD4+ cell count (<200 cells/mm3), older age at HCV infection and significant alcohol consumption (<50 g/day).62

The increased liver-related mortality has raised concerns that HAART may actually have a long-term negative effect on HCV progression. Although no mechanism has been demonstrated, possibilities include immune reconstitution leading to greater inflammation or a direct hepatotoxic effect of the antiretroviral agents.63 Reassuring data has emerged from two recent studies, suggesting that co-infected patients on HAART actually have slower progression of hepatic fibrosis, approaching that seen in HCV mono-infection.63, 64

Transplantation

The effect of immunosuppression on HCV has also been examined in the setting of organ transplantation. Patients with both renal and bone marrow transplants have been shown to have more aggressive HCV with accelerated fibrosis and progression to cirrhosis.65, 66 HCV recurs universally following liver transplantation and in many patients disease progression is markedly accelerated with up to 30% developing cirrhosis within 5 years.67 Numerous factors including degree and type of immunosuppression, donor age, pre and post transplant viral load, HCV genotype and potentially living-donor transplantation have been shown to correlate with disease progression.68–72 However, with so many variables affecting outcome, post-transplant HCV recurrence serves as only a very limited model for understanding fibrosis progression in HCV in the non-transplant setting.

Metabolic Factors

  1. Top of page
  2. Abstract
  3. Natural History
  4. General Host Factors
  5. Immunosuppression
  6. Metabolic Factors
  7. Liver-Specific Factors
  8. Host Genetic Factors
  9. Viral Factors
  10. Conclusion
  11. References

Hepatic steatosis has long been recognized as a common histological feature of liver biopsies from HCV-infected individuals. Although non-alcoholic fatty liver disease (NAFLD) is very prevalent in the general population, most studies suggest that the concomitant finding of both diseases is greater than would be expected by chance.73

Whether HCV has a direct steatogenic effect remains controversial. In genotype 3 infection the presence of steatosis correlates with intrahepatic viral replication, is commonly present in non-obese patients and resolves with successful viral clearance, implying it is a direct consequence of the virus itself.74 Accumulating evidence suggests that this may be due to interference with hepatocyte lipid handling and induction of oxidative stress by HCV core protein.73, 75 In contrast, in genotype non-3 infections, steatosis is highly correlated with obesity and insulin resistance and is unresponsive to antiviral therapy.76 However, HCV may still promote steatosis by furthering insulin resistance or through direct viral effects on cellular processes.

Regardless of how HCV may cause steatosis, what is becoming increasingly clear is that the presence of fat in the liver, particularly in genotype non-3 infections, has a negative impact on HCV progression. Multiple studies have shown that steatosis and body mass index are independent predictors of advanced fibrosis in HCV infection.76–82 The mechanism by which steatosis worsens fibrosis is not known; however its strong association with the metabolic syndrome and type II diabetes mellitus (DM) may offer some clues. Hui and colleagues used the homeostasis model assessment (HOMA) index to evaluate insulin resistance and showed that it was an independent predictor of both degree and rate of progression of fibrosis in HCV infection.83 Similarly, high fasting glucose and insulin levels have also been shown to be associated with progressive disease.81, 84 Stellate cell activation and up-regulation of connective tissue growth factor by hyperinsulinemia and hyperglycemia have been proposed as potential mechanisms for the accelerated fibrosis observed.83, 85, 86

The observation that the fibrosis seen in patients with HCV and steatosis may be similar to that found in NAFLD, raises the possibility that the fibrogenic mechanisms are related in the two diseases.87, 88 In NAFLD the prevailing hypothesis is that in order for steatosis to lead to inflammation and fibrosis, a second “hit” is required.89 Oxidative stress leading to lipid peroxidation, as well as a variety of cytokines have been proposed as potential candidates for the second “hit”.90 Lipid peroxidation has been documented in patients with HCV and correlates with degree of fibrosis.91 The stimulus for lipid peroxidation in HCV is not known but as in NAFLD, increased fatty acid oxidation secondary to steatosis-induced hepatic insulin resistance has been proposed.89, 92 The antiviral inflammatory response may play a significant role as a source of free radicals and proinflammatory and profibrotic cytokines.92 In addition, steatosis-associated apoptosis in HCV has been shown to correlate with increased fibrosis.93 Kupffer cell engulfment of apoptotic bodies leads to local TGF-β release, a strong stimulus for stellate cell activation.94 A direct viral effect may also contribute, as HCV core and nonstructural proteins have been shown to induce oxidative stress within infected cells.95

Interventions targeting steatosis, such as weight loss and exercise have shown preliminary benefits in HCV progression but more specific therapies are clearly needed.96 Whether specific agents such as the peroxisome proliferator activated receptor (PPAR) gamma ligands, shown to be effective in NAFLD, may be helpful in HCV-related steatosis remains to be seen.

Liver-Specific Factors

  1. Top of page
  2. Abstract
  3. Natural History
  4. General Host Factors
  5. Immunosuppression
  6. Metabolic Factors
  7. Liver-Specific Factors
  8. Host Genetic Factors
  9. Viral Factors
  10. Conclusion
  11. References

ALT

Alanine aminotransferase (ALT) has been used as a specific indicator of hepatic inflammation, but whether it correlates with hepatic disease progression remains somewhat controversial. Although in general, patients with a persistently normal ALT have mild inflammation and fibrosis on liver biopsy, this is not a universal finding. In the largest study to date, 691 patients with a normal ALT for at least 6 months were followed and 159 were biopsied.97 Only 17% had normal histology, however most had mild disease and only 1% had cirrhosis. However, other groups have seen less optimistic results. In a British study none of 91 patients with normal ALT had normal histology and 16% had stage 3 or 4 fibrosis.98 More concerning are reports of fibrosis progression in 23% to 33% of patients with persistently normal ALT over 3-6 years of follow-up.7, 99 Fortunately, patients with normal ALT respond equally well as those with elevated ALT to interferon-based antiviral therapy.100–102 Therefore, although the US Preventative Services Task Force102 concluded that HCV screening is not recommended even for high-risk groups, the data in patients with normal ALT would argue to the contrary. Such patients may have significant liver pathology and respond to antiviral therapy; therefore, as articulated in a rebuttal paper by Alter et al., they should be screened for HCV infection.103

Co-Infection With Other Hepatitis Virus

Although accurate estimates of HBV and HCV co-infection are lacking, in areas where both viruses are endemic, it is likely to be very high. In most instances, one virus tends to dominate and, thus, it is unusual to find high levels of both HBV and HCV viremia.104 However, it does appear that co-infection may lead to more progressive liver disease.105

In addition to evident co-infection, occult HBV infection has been documented in patients with HCV. Such patients have HBV DNA detectable by PCR despite being HBsAg and possibly anti-HBc negative. The initial report from Cacciola and colleagues documented that occult HBV was common among patients with HCV (33%) and was associated with an increased risk of cirrhosis (33% vs. 19% P <.05).106 More recent reports from Italy, Taiwan and the United States have found no evidence that occult HBV hastens progression of HCV or worsens response to therapy.107–109 It does, however, appear to promote hepatocarcinogenesis.110, 111

An initial report of 7 cases of fulminant hepatitis following HAV infection in 17 patients with HCV was very alarming.112 Although a subsequent report of 166 cases of acute HAV in patients with chronic HCV reported no mortality, guidelines still recommend routine HAV vaccination for all patients infected with HCV.113

Hepatitis G (GBV-C), SEN and TT virus have all infrequently been reported to cause mild acute hepatitis. To date, long-term studies indicate no risk of progressive liver injury with chronic infection by any of these agents. Despite a high co-infection rate with HCV, there is no evidence that these viruses affect the natural history or response to therapy.113–115

Host Genetic Factors

  1. Top of page
  2. Abstract
  3. Natural History
  4. General Host Factors
  5. Immunosuppression
  6. Metabolic Factors
  7. Liver-Specific Factors
  8. Host Genetic Factors
  9. Viral Factors
  10. Conclusion
  11. References

Genetic Polymorphisms

The markedly varying rates of disease progression in HCV infection led investigators to look for host factors that might predict disease course. After examining broad demographic characteristics, it was natural to proceed to genetic studies to determine if specific polymorphisms were associated with disease outcome. To date, numerous studies have been performed examining a wide variety of candidate genes (Table 2). Although some important associations have been identified, the results have been hampered by discrepant findings, small sample sizes and unclear biological significance.

Table 2. Genetic Polymorphisms Affecting HCV Progression
GenePolymorphismAssociationReference
Human Leukocyte Antigen (HLA)DRB1Reduced inflammation116–118
 DRB1*11Spontaneous HCV clearance120
Transporter Associated with Antigen Processing (TAP)TAP2*0103Normal ALT in Japanese patients123
Solute Family Carrier 11 Member 1 (prev Nramp1)SCL11A1 genotype 2/2 (promoter)Mild fibrosis124
Chemokine Receptor 5 (CCR5)CCR5Δ32Reduced inflammation, mild fibrosis, spontaneous HCV clearance125–7
RANTES−403 in promoterMild inflammation125, 127
Monocyte Chemotactic Protein-1 (MCP-1)G/A or G/G genotypeIncreased inflammation, advanced fibrosis128
Monocyte Chemotactic Protein-2 (MCP-2)Q46KAdvanced fibrosis125
Interleukin-10 (IL-10)ATA or ACC homozygosityRapid fibrosis131
Tumor Necrosis Factor βA/A allele (promoter region)Advanced fibrosis134
Angiotensinogen (AT)A/A allele in AT-6 promoter regionHigh AT genotype, advanced fibrosis132
Transforming Growth Factor β1 (TGF-β1)A/A allele at codon 25High TGF-β1 genotype, advanced fibrosis132
Microsomal Epoxide HydrolaseH/H allele homozygositySlow metabolizer phenotype, advanced fibrosis135
Factor VFactor V LeidenRapid fibrosis progression136

HLA and Related Polymorphisms

Because of the important effect of the immune system on the outcome of HCV infection, the effect of human leukocyte antigen (HLA) polymorphisms on disease progression, clearance and response to antiviral therapy has been studied. The most consistent finding has been the association of MHC class II haplotypes, specifically of DRB1, with less severe disease. DRB1*1104, DRB1*1101 and DRB1*11 have all been shown to be more common in patients with persistently normal ALT and less inflammation on liver biopsy.116–120 Interestingly, DRB1*11 has also been shown to be associated with spontaneous clearance of HCV.121 No specific alleles have been found to be strongly associated with advanced fibrosis.122 The finding that certain genetic markers associated with spontaneous clearance are also associated with milder disease is certainly intriguing and could have important implications for therapeutic or protective vaccine design; however, it is important to note that these studies were generally small and most were not well controlled for confounding factors such as alcohol consumption, age and gender.

Transporter associated with antigen processing (TAP) is involved in antigen presentation, making it potentially relevant in infectious disease outcome. The frequency of the TAP2*0103 allele was found to be increased in HCV-infected patients with normal ALT but, the functional significance of this marker is unknown.123 A Spanish group recently reported that polymorphisms in the solute carrier family 11 member 1 (SLC11A1, formerly Nramp1) protein, known to affect susceptibility and outcome in some infectious and autoimmune diseases, were associated with mild fibrosis in HCV infection.124

Cytokine Polymorphisms

Numerous cytokine and chemokine genes have been examined for polymorphisms predicting more severe liver disease. With the recognition that a 32 base-pair deletion in chemokine receptor 5 (CCR5) was protective against HIV infection, much interest has focused on this cytokine system in other diseases. In HCV, 3 large studies have demonstrated that mutations or polymorphisms affecting this chemokine pathway are associated with milder hepatic inflammation, likely due to altered leukocyte trafficking.125–127 Two studies also showed that a polymorphism in RANTES, an important CCR5 ligand, resulted in milder histological activity.125, 127 Hellier et al. also found that the CCR5Δ32 deletion was associated with more advanced fibrosis, but this was not seen in other studies.125

Monocyte chemotactic protein-1 (MCP-1) is a potent chemokine for monocytes and macrophages and is secreted by hepatic stellate cells to regulate leukocyte trafficking. A polymorphism resulting in a G rather than an A at position −2518 results in increased production of MCP-1. Muhlbauer et al. found that HCV-infected patients with genotype G/A or G/G were more likely to have advanced fibrosis and severe inflammation on liver biopsy than those with genotype A/A.128 Furthermore, they showed that nuclear extracts from hepatic stellate cells of genotype G/G had greater binding affinity to the MCP-1 promoter than those of genotype A/A, a possible explanation for the differences seen. However, it is noteworthy that three other groups did not confirm the association of this MCP-1 polymorphism with advanced fibrosis.125, 129, 130 The function of MCP-2 is less well understood and a polymorphism in this gene resulting in an amino acid change (Q46K) was shown in one study to be associated with more severe hepatic fibrosis in HCV infection.125

Interleukin-10 (IL-10) is a major regulator of cellular immune responses, largely acting to suppress inflammatory cytokine secretion. Although homozygosity for 2 IL-10 haplotypes (ATA and ACC) was reported to be associated with more rapid fibrosis, other groups failed to show this association.131, 132 Polymorphisms in the TNFα and β promoters and TGF-β and angiotensinogen genes have also been reported to be associated with more advanced fibrosis.132–134

Other Polymorphisms

A study from Italy looked at liver disease severity in patients with differing alleles for the microsomal epoxide hydrolase gene, an important antioxidant enzyme.135 They found that patients with cirrhosis or HCC were more likely to have the slow metabolizer phenotype of this enzyme. HCV core and possibly non-structural proteins induce oxidative stress within liver cells and the ability of the individual to respond may significantly affect the resultant cell damage.

Because coagulation pathway activation has been shown in models of hepatic fibrosis, Wright et al. hypothesized that mutations may affect HCV disease progression. They examined polymorphisms in coagulation factors II and V and found that patients with the factor V Leiden mutation (A560G) were more likely to progress to cirrhosis.136

To help clarify the role of the various polymorphisms studied, Richardson and colleagues recently examined the effect on hepatic fibrosis of a total of 9 genes with previously identified polymorphisms.137 They confirmed the association of 6 of the polymorphisms with more rapid fibrosis but more importantly showed that an increasing number of polymorphisms in a given individual strongly increased the association with disease progression. Any of the individual polymorphisms had only modest effects, but combined they were strongly predictive of progressive disease. As high throughput genetic testing becomes more widely available, screening a panel of known susceptibility genes might be a feasible approach to identify patients at risk of rapid disease progression. Hopefully an understanding of the functional effects of these various polymorphisms will also lead to new therapeutic strategies.

Viral Factors

  1. Top of page
  2. Abstract
  3. Natural History
  4. General Host Factors
  5. Immunosuppression
  6. Metabolic Factors
  7. Liver-Specific Factors
  8. Host Genetic Factors
  9. Viral Factors
  10. Conclusion
  11. References

HCV has marked genetic diversity and exists as quasispecies in infected hosts because of high mutational rates. Such genetic variability may translate to functional differences among HCV isolates. Many studies have examined the role of viral factors on outcome of HCV infection.

Genotype and Viral Load

Although important differences are seen between genotypes in treatment response to interferon-based therapy, no effect of genotype has been convincingly shown on severity of liver disease. Initial reports of more severe disease in genotype 1b were not substantiated. Similarly, viral load affects only treatment response in genotype 1 infection but does not otherwise influence the natural history of disease.

Role of HCV in Disease Progression

The development of fibrosis in chronic HCV infection is thought to be due to chronic hepatic inflammation leading to hepatocyte death and regeneration. Infiltrating lymphocytes clear HCV by killing virus-infected hepatocytes through stimulation of Fas ligand and perforin-dependent apoptosis pathways. In addition, cytokine release, particularly IFN-γ and IL-2, results in non-cytopathic intracellular viral clearance. Cell death leads to cytokine release, including transforming growth factor beta (TGF-β), a strong stimulus for stellate cell activation. This promotes deposition of extracellualr matrix and ultimately progressive fibrosis. The stimulation of fibrosis is not unique to HCV infection but rather a common result of any chronic inflammatory state in the liver.

Although chronic inflammation may be the most important fibrogenic stimulus, HCV itself may further promote the process. Accumulating evidence suggests that HCV proteins interact with various host cell-signaling cascades, which may result in fibrosis due to increased apoptosis or other mechanisms.

The core protein has received the most attention regarding its interactions with host-cell processes. Core expression in vitro and in transgenic mice has been shown to induce oxidative stress through interference with mitochondrial electron transport leading to increased production of reactive oxygen species (ROS) and lipid peroxidation.95, 138, 139 Impairment of antioxidant pathways has also been demonstrated, with HCV-infected livers and core-expressing cell lines showing markedly reduced levels of heme oxygenase-1 (HO-1).140 In addition to directly damaging hepatocytes, oxidative stress promotes stellate cell activation, leading to increased fibrosis.141 The core protein has also been implicated in hepatic steatosis by interfering with hepatic assembly and secretion of very-low-density lipoprotein (VLDL).142

HCV core protein's interaction with mitochondria has also been shown to promote apoptosis. Both TRAIL and Fas ligand-mediated pathways have been implicated in cell culture and transgenic mice systems.138, 143, 144 Bantel et al. recently reported that activated caspase, a marker of apoptosis, was elevated in patients infected with HCV and correlated with the degree of fibrosis seen on liver biopsy.145 Direct viral-induced apoptosis as opposed to inflammatory-mediated cell death, is an attractive hypothesis to explain the significant fibrosis seen in a proportion of patients with HCV despite persistently normal ALT levels.

Similar to core protein, the HCV non-structural proteins have been shown to affect oxidative stress and apoptosis, however the mechanisms involved may differ. While core protein inhibited heme oxygenase 1 activity, expression of NS5A and NS3 also resulted in oxidative stress but stimulated, rather than impaired, antioxidant pathways.139

Unlike core protein, hepatic expression of NS5A in transgenic mice inhibited tumor necrosis factor (TNF)-stimulated apoptosis early in the signal transduction pathway.146 Whereas stimulation of apoptosis by HCV offers an explanation for non-inflammatory hepatic damage, apoptosis inhibition may account for HCV-associated hepatocarcinogenesis.

In addition to affecting cell death, HCV proteins may directly stimulate fibrogenesis. Patients with advanced fibrosis have been shown to have higher levels of cyclooxygenase 2 (COX 2) and matrix metalloproteinases 2 and 9 (MMP 2 and 9) in their livers, all of which have been previously implicated in liver fibrogenesis in animal models. In cell culture, expression of NS5A and/or core protein induced COX 2 and MMP 9.146 Microarray studies have also demonstrated that expression of NS5A in Huh-7 cells leads to up-regulation of genes involved in fibrogenesis such as those in the TGF-β superfamily.147 Confirming the importance of this finding, core and non-structural proteins expressed by an adenovirus vector were shown to directly stimulate TGF-β production from activated stellate cells.145 Thus, HCV may set up a progressive cycle of worsening hepatic fibrosis at least partially independent of immune activation and inflammation.

HCV has also been shown to interfere with liver regeneration and other cellular pathways that may contribute to fibrosis.149, 150 Further clarification of the effects of HCV proteins on cellular pathways will be helpful in understanding the mechanisms leading to progressive liver injury. Ultimately this may be important for recognizing susceptible patients and potentially for targeting specific pathways therapeutically.

Conclusion

  1. Top of page
  2. Abstract
  3. Natural History
  4. General Host Factors
  5. Immunosuppression
  6. Metabolic Factors
  7. Liver-Specific Factors
  8. Host Genetic Factors
  9. Viral Factors
  10. Conclusion
  11. References

Hepatitis C is an enormous public health problem worldwide. Although millions of people are infected, many will live full productive lives with no consequence from their disease while others will progress over years to decades to end-stage liver disease or death. This marked variability in natural history complicates everything from treatment decisions to resource allocation. Numerous host and some viral factors affect the progression of hepatitis C but unfortunately to date an understanding of the complex relationships between these factors is lacking and thus the ability to predict the course in a given individual remains very limited (Fig. 1). Identification of genetic polymorphisms associated with aggressive disease will hopefully allow for a better understanding of the mechanisms involved in disease progression and potentially offer new therapeutic targets, some of which may not involve the virus at all. The recent establishment of an HCV cell culture system will hopefully provide insights into specific viral factors that may be important for disease progression as well. Ultimately, as understanding of the factors relevant to disease progression improves, new therapies and rational use of those therapies will hopefully become a reality.

thumbnail image

Figure 1. Mechanisms promoting fibrosis. Viral replication leads to infiltration of HCV-specific and non-specific inflammatory cells. Cytokine production enhances viral clearance but also promotes hepatic fibrosis directly, through activation of stellate cells, and indirectly by promoting apoptosis. Viral proteins cause oxidative stress and consequent apoptosis by increased formation of reactive oxygen species and interference with antioxidant pathways. HCV also promotes steatosis, another profibrotic stimulus. Genetic polymorphisms in a variety of pathways as well as environmental factors such as iron overload and alcohol also contribute to hepatic fibrogenesis.

Download figure to PowerPoint

References

  1. Top of page
  2. Abstract
  3. Natural History
  4. General Host Factors
  5. Immunosuppression
  6. Metabolic Factors
  7. Liver-Specific Factors
  8. Host Genetic Factors
  9. Viral Factors
  10. Conclusion
  11. References