Research efforts in the laboratory of Aleem Siddiqui, PhD, professor of medicine at the University of California, San Diego, have led to the discovery of a mechanism by which the hepatitis C virus (HCV) damages the liver and lingers on for years, resulting in a chronic infection. The virus targets the powerhouse of the cell, the mitochondrion, which prevents the liver from mounting an immune attack to combat the infection.1
According to Henry Masur, MD, chief of Critical Care Medicine at the National Institutes of Health, 80% of individuals infections. Masur told Evidence-Based Immunology and Infectious Disease, “Of the 80% who develop a lifetime infection, 20% will develop liver cirrhosis or hepatocellular carcinoma. Identifying this population early on, using biomarkers, such as fibrosis, or imaging techniques, would help with controlling disease progression.” Masur, a past president of the Infectious Disease Society of America (IDSA), has helped develop the IDSA/AASLD (American Association for the Study of Liver Diseases) guidelines for testing, managing, and treating HCV.
Previous work from the group identified the genes induced following HCV infection, which results in mitochondrial destruction. HCV-infected hepatoma cells in the laboratory—and more importantly, in infected liver tissues of chronic HCV patients—demonstrated increased expression of 2 genes: Parkin and PINK1. Additionally, perinuclear clustering of the mitochondria and mitochondrial translocation of Parkin were observed. The outcome: mitophagy—a process of mitochondrial engulfment by the autophagosomes (Figure 1).
Why does the virus bring about these changes? The authors discovered that the process of Parkin-dependent mitophagy maintains persistent HCV infection.2
Building on their previous results, the group then asked: What prevents the infected cells with the injured mitochondria from undergoing apoptosis? The answer resulted in a publication in PNAS. The viral infection induced the expression of the genes Drp1 and Mff. Drp1 then translocated to the mitochondria, leading to mitochondrial fission and mitophagy. The Drp1-mediated asymmetric fragmentation resulted in the formation of 1 healthy and 1 damaged mitochondrion. The presence
of the healthy mitochondrion keeps the HCV-infected cell alive, while mitophagy of the damaged mitochondrion provides biochemical resources for continued viral replication and virulence. However, silencing the expression of Drp1 or Parkin resulted in hepatocyte apoptosis.1
A chronic liver infection that affects more than 3 million people in the United States,1 HCV can cause liver failure and even result in liver cancer. The symptoms following infection with the deadly virus, which claims 12,000 lives annually in the United States, can develop anywhere from 2 weeks to 6 months after exposure following acute infection, but can take up to 30 years to develop following chronic infection. The lack of—or delayed appearance of—symptoms makes the disease that much harder to
treat. Patients may experience nausea, vomiting, fever, fatigue, lack of appetite, abdominal pain, dark urine, gray stools, joint pain, and jaundice.3
Despite the absence of symptoms, the virus can be detected in the blood, and the CDC3 as well as the US Preventive Services Task Force (USPSTF)4 recommend that baby boomers (adults born between 1945 and 1965) get tested for HCV infection. “The HCV-infected population can be subdivided into 3 major groups,” said Masur. “The smallest group of these are homosexual males, a larger group is comprised of injectable drug users (IDUs) who share needles, and the largest group includes baby boomers who may have experimented with IDUs in the past.”
The Cost of Diagnosing and Treating HCV
The recent debate on the high cost of treating HCV has emerged, not just as a result of the price of Sovaldi (by Gilead) and Olysio (by Janssen),5 but also because of the sheer number of patients who are likely infected with the virus, as was discussed by the panel that was convened by The American Journal of Managed Care earlier this year.6 The challenge, according to the panel members, lies in who pays for the treatment of those who test positive. If diagnosed relatively early, employer-based commercial insurances such as UnitedHealthCare and Aetna would have to bear treatment costs; if diagnosed after retirement, Medicare would pay.
Over the years, several studies have assessed the cost of treating patients with this debilitating disease. One analysis published in 2011 estimated that the total annual costs (direct and indirect) of HCV in the United States were $5.46 billion in 1997, while direct medical costs were predicted at $10.7 billion for the decade beginning 2010.7
The estimated costs in 2009 dollars are depicted in Figure 2, and were made, of course, prior to the approvals of Sovaldi and Olysio. The argument being presented by pharmaceutical companies as well as by some healthcare analysts is that the initial high cost of treatment can eliminate the later costs associated with treating a subsequent chronic infection, which can result in liver cirrhosis or necessitate a transplant.
The Enduring Problem of Persistent Viral Infection
Resuming the discussion of the mechanisms triggered by HCV that result in viral persistence, an improved understanding of the signaling pathways that are disrupted could generate novel disease targets.
One question whose answer still eludes scientists is whether viral clearance in some patients, those who seem to generate a much more robust immune response, indicates protection against reinfection. If it does, then a new approach to vaccine development could be attempted with the objective of protecting against reinfection rather than preventing a primary infection.
According to Masur, liver cirrhosis or fibrosis is a major problem in patients with advanced disease. “In today’s era of promising but expensive medications, it is essential to prioritize treatment. Of course all HCV patients will benefit from treatment with sofosbuvir, but do you start treatment with this expensive medication as soon as the infection is detected or after monitoring the patient for a while?” In his opinion, when it comes to prioritizing, the patients with the most advanced disease
(those with vasculitis, liver-related disease, kidney failure, or any other associated metabolic comorbidities) would have the most immediate benefit from treatment.
Spontaneous viral clearance, mediated by the host’s immune system, has been observed in nearly 25% of patients infected with the virus, usually within the first 6 months of infection.8 A few studies, conducted in chimpanzees9,10 as well as in humans,11 have indicated that adaptive immunity can protect against the development of a chronic infection, not necessarily sterilize against new infections. The human study, conducted in active IDUs who were free of the virus for at least 60 days, monitored the patients for a reinfection. The study observed protection from reinfection in 50% of the patients. Of the patients who were reinfected, spontaneous viral clearance was observed in 83% of the patients, with a significant reduction in the duration and maximum level of viral load during the reinfection, generation of broadened cellular immune responses as well as cross-reactive humoral responses.11
Does this indicate that protective immunity against HCV is possible? “Not likely,” says Masur. “Protective immunity has not yet been looked at in the HCV-susceptible population.” He believes it could be a very useful, though expensive, strategy among homosexuals and IDUs to reduce the communitywide burden of disease.
A more recent study, conducted among 188 IDUs in Australia, also yielded mixed results. Corroborating the observations of the above-mentioned study, the time to spontaneous clearance was shorter in patients with a confirmed reinfection as well as those with a possible reinfection, compared to clearance following the primary infection. However, 50% of confirmed reinfections and 41% of confirmed/possible reinfections did not spontaneously clear, hinting at partial acquired immunity against HCV.12
How does one reconcile the results of these studies? In addition to the fact that the virus and its infection patterns are very complex, likely due to adaptations as well as the presence of different genotypes, certain technical factors could also contribute. For example, fluctuations in the viral load from the primary infection may be classified as reinfection, an issue that could be avoided by adequate genetic testing of the virus.
Research results, such as those provided by Siddiqui’s group, can identify potential targets and components that could be used in the development of a “vaccine” for HCV, especially to prevent Chronic, not acute, HCV infection is associated with HCV-related morbidity and mortality,8 which in turn would inflate the cost of care. This approach, together with the new approved treatments (see Table, SP443), can definitely help control the prevalence of hepatitis C.
According to Masur, while the field is continually evolving, an optimum treatment strategy would be a multidrug pill that would target the several different HCV genotypes. He believes that the current advances in HCV treatment are a major scientific breakthrough, one of the most significant, potentially, of the century. “Great things are under way for the scientific community—a miraculous occurrence,” he says. To capitalize on this “immense opportunity, [we] need collaboration and partnership between the
pharmaceutical industry, the insurance providers, and the government.”