After several decades of searching, a group of researchers finally discovered how the hepatitis A virus infects healthy human cells. Their recent study unveiled the receptor utilized by the virus: the ganglioside. The receptor shows how the pathogen replicates and spread disease.
The discovery of the mechanisms of the hepatitis A virus (HAV) was led by researchers at the University of North Carolina (UNC), a public research university in the US. Their findings unveiled the critical role of gangliosides in the viral mechanisms of HAV. The same molecules might explain the transmission of ancient hepatitis viruses between mammalian species. They published the results in Nature Microbiology.
Facts on Hepatitis Viral Disease
According to the World Health Organization, a specialized agency of the United Nations, HAV causes a viral liver disease characterized by mild to severe symptoms. Depending on relevant factors, HAV can result in the untimely death of an individual. Its transmission is more commonly recognized in contaminated food and water, and direct contact with an infected person. Compared to other types of hepatitis, most people infected by HAV usually recover fully and gain immunity against the pathogen. However, there are some that may die from fulminant hepatitis or acute liver failure due to hepatitis.
In the Global Hepatitis Report 2017, the agency revealed that hepatitis B and C are responsible for 96% of all hepatitis deaths in the world. But hepatitis, as a disease in general, remains a global health threat because of various complications, such as liver failure, cancer, and coinfection. In 2015, hepatitis led to 1.34 million deaths globally. That mortality was comparable to tuberculosis and apparently greater than the deaths caused by HIV. And based on available data, hepatitis mortality was observed with a steady increase, compared to the decreasing trend of HIV and tuberculosis.
As of 2015, the majority of the 1.34 million deaths were due to chronic liver disease and primary liver cancer. The former was linked to about 720,000 deaths due to liver cirrhosis, and the latter was correlated to about 470,000 deaths, induced by hepatocellular carcinoma. Meanwhile, an estimated 257 million people were living with chronic hepatitis B and 71 million people were living with chronic hepatitis C.
Statista, a German portal for statistics, highlighted the vaccine for hepatitis B. It was one of the most widespread vaccines in the world with 84% of the share of immunized one-year-old children in 2018. The hepatitis B vaccine was the fourth most common vaccine, outranked by tuberculosis vaccine at 89%, tetanus, diphtheria, and whooping cough vaccines at 86%, and polio vaccine at 85% among children one year of age.
The Gatekeeper to Hepatitis A Virus
At UNC, researchers found the gatekeeper that allowed HAV to infect human liver cells. For almost 50 years, the gatekeeper eluded medical experts, making treatment development difficult. While a vaccine has been available, no specific treatment could address every case of hepatitis-associated liver failure. Once the virus conquered the liver, the patient could die from liver failure in days or weeks after the infection.
In the study, researchers analyzed the uniqueness of HAV to learn more about its ability to infect host cells. Their analyses showed two variations of HAV inside a host: the nonenveloped HAV (nHAV) and the quasi-enveloped HAV (eHAV). Between the two, only the eHAV could be detected in the blood during an acute infection, while the nHAV could be detected in the feces of the host due to viral shedding. The presence of two distinct HAV variants in two different places was a puzzle for decades in HAV research.
To solve the puzzle, researchers compared HAV to other hepatitis viruses and to non-hepatitis viruses. Similar to any virus type, HAV would require receptors to enter and infect host cells. Although the receptors for hepatitis B and C were made clear before, the ones for HAV have been a mystery for years. Using the background on eHAV and nHAV, they were able to validate one thing and disprove another.
Early studies suggested the human protein TIM1 as a receptor for HAV. However, researchers in this study confirmed that cells with TIM1 could still be infected by the virus. Via the CRISPER-Cas9 gene-editing tool, they filtered 20,000 genes in cultured cells to isolated the gene required for HAV infection. Five specific genes were connected to HAV and all of them were required by the pathogen. When these genes were examined, they had a common denominator: the genes were involved in the synthesis of gangliosides, molecules that traffic chemicals across channels within cells.
"Discovering that gangliosides are essential receptors for HAV infection adds an interesting plot twist to the hepatitis A story. Gangliosides are structurally similar across mammalian species, unlike proteins, which helps explain cross-species transmission of ancient hepatoviruses. Understanding what helps a virus jump from one animal species to another is incredibly important, as evidenced so plainly by the current Covid-19 pandemic," said Dr. Stanley Lemon, the senior author of the study and professor of medicine and microbiology at UNC School of Medicine.
By composition, gangliosides are described as sugary fatty acid molecules with similar structures among mammals. The enzyme required to create gangliosides is the ceramide glucosyltransferase, but the gene responsible for the encoding of that enzyme is the UGCG gene. This gene is the principal culprit of the five genes highlighted by the gene-editing tool. In theory, disabling this gene should prevent HAV infection in human cells.
Researchers decided to test that theory by knocking the UGCG gene out in lab experiments. The results confirmed the prevention of HAV infection due to the impaired gatekeeper – the gangliosides. Next, they applied a chemical inhibitor of the enzyme in cultured liver cells. The inhibitor prevented eHAV and nHAV infection. Finally, they injected HAV RNA inside the cells to know if the gangliosides were the ultimate requirement. The insertion of the RNA within the cells resulted in viral replication, which suggested gangliosides as a prerequisite for replication, but not for making new viral particles.
Despite the early research in the link between gangliosides and HAV, the study findings showed a promising new avenue for preventing or treating hepatitis A. The inhibition of the molecules may become a potential focus in treatment development since a vaccine is already available. A novel treatment for HAV can be significant in saving people from HAV-related liver complications.