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There is now strong evidence that the virus AAV2, alongside an underlying genetic predisposition, played a key role in cases of acute hepatitis in children, according to a new study published in Nature – the first detailed research investigation into the worldwide outbreak.

The peer-reviewed study, which was led by researchers at the University of Glasgow in collaboration with the Royal Hospital for Children in Glasgow, Public Health Scotland (PHS) and ISARIC (International Severe Acute Respiratory and emerging Infections Consortium) WHO Clinical Characterisation Protocol UK (CCP-UK), found that the common virus AAV2 (adeno-associated virus 2) was present in a range of different samples taken from children with acute unexplained hepatitis. In contrast, AAV2 was not found to be commonly present in samples taken from children in the control groups. Researchers believe that AAV2 virus may have played a key role in the development of acute hepatitis in a small number of young children around the world.

The Glasgow team were the first in the world to identify the AAV2 connection with recent childhood hepatitis cases.

AAV2 was first identified by scientists in 1965 and infects up to 90% of the population, with most people being infected and developing immunity by the age of 18. The virus is not known to normally cause disease and require co-infection with certain viruses, such as herpes viruses and adenoviruses, the latter cause gastroenteritis as well as cold or flu-like symptoms.

Since April 2022, a number of young children worldwide have developed jaundice and acute severe hepatitis of unknown origin. While the outbreak has now largely subsided, the World Health Organisation estimates there have been at least 1010 probable cases in 35 countries. Children with the condition commonly had to be hospitalised for a number of days. In the UK the majority of the 270 cases were under the age of five years old, with many requiring admission to intensive care and 15 children requiring liver transplants as a result of their condition.

In July 2022 two UK pre-print studies, which were led independently and simultaneously, made the first link between AAV2 and acute hepatitis in children. One examined cases from Scotland, and was carried out by the MRC-University of Glasgow Centre for Virus Research (CVR) and the Royal Hospital for Children in Glasgow, in partnership with PHS and ISARIC CCP-UK. The second studied cases from across all four UK nations, led by Great Ormond Street Hospital and the UCL Great Ormond Street Institute of Child Health (UCL GOS ICH), in partnership with ISARIC and the UK Health Security Agency. At that time, researchers across the two teams found that AAV2 was present in 96% of cases of unknown hepatitis examined across both studies, and as a result they suggested it may offer the best explanation for the onset of hepatitis in affected children.

Now, with more patients, samples and controls, this updated peer-reviewed study – the first of its kind in the world – offers important new data which strongly suggests that infection with AAV2 and an underlying genetic association (the HLA DRB1*04:01 gene) were associated with the onset of acute hepatitis in children.

Research and patient genetics in detail

The study carried out a detailed investigation of 32 cases and 74 control subjects. Of the 32 children with acute hepatitis, 4 children needed to be treated in a specialist liver unit and one child required a liver transplant. After treatment all the children in the study recovered, though recovery did take a number of months even for the least unwell children.

Using a range of tests including next-generation sequencing, real-time PCR, liver and blood tests, the research team compared samples and were able to confirm the presence of AAV2 in the liver of all children with acute hepatitis. Moreover, a majority of the patients with acute hepatitis were found to have AAV2 antibodies present in their blood, indicating recent infection. Real-time PCR data revealed 81% of acute hepatitis patients had AAV2 in their blood, compared with only 7% of the control group.

The study also examined the genetics of patients with unknown hepatitis to find out whether any of the children may have been more susceptible to this type of acute hepatitis. Using detailed genomic testing of the patients, researchers were able to identify differences in the Human Leukocyte Antigen gene, that were not commonly found in the control groups of Scottish platelet donors or UK Biobank participants. The team believe this genetic variant may offer another part of the answer as to why some children have become seriously unwell.

A version of the AVV2 virus is currently used to deliver gene therapy. While hepatitis is a common phenomenon among patients receiving this treatment, it is usually prevented from becoming serious by treating pre-emptively with steroids before, and for several weeks after treatment.

Professor Emma Thomson, Clinical Professor and Consultant in Infectious Diseases at the MRC-University of Glasgow Centre for Virus Research (CVR) and senior author of this Nature study, said: “The presence of the AAV2 virus is associated with unexplained hepatitis in children. This virus replicates in the presence of another virus, usually an adenovirus. AAV2 may cause disease itself or it may be a useful biomarker of recent adenovirus infection which may be the main underlying pathogen, but which can be harder to detect.

“There are many unanswered questions and larger studies are urgently needed to investigate the role of AAV2 in paediatric hepatitis cases, particularly the role of the immune response in the disease process. We also need to understand more about seasonal circulation of AAV2, a virus that is not routinely monitored – it may be that a peak of adenovirus infection has coincided with a peak in AAV2 exposure, leading to an unusual manifestation of hepatitis in susceptible young children who carry the HLA DRB1*04:01 gene, a relatively common gene in Scotland.”

Dr Antonia Ho, Clinical Senior Lecturer and Consultant in Infectious Diseases at the MRC-University of Glasgow Centre for Virus Research, said: “With the inclusion of more cases, addition of control children that were recruited at the same time as cases, as well as antibody data demonstrating that most cases had evidence of AAV2 IgM antibodies (indicating recent infection) and liver staining that shows the presence of AAV2 in the liver, our updated peer-reviewed paper provides convincing evidence that AAV2, along with a genetic predisposition play an important role in the onset of acute hepatitis in children.”

The previous pre-print studies were able to rule out the likelihood of recent or prior SARS-CoV-2 infection as a direct cause for the acute hepatitis. In those, researchers found no SARS-CoV-2 in the liver of patients, while the team in Scotland found that only two thirds of the cases had antibodies to the infection, similar to SARS-CoV-2 antibody prevalence in Scottish children at the time.

The paper, ‘Adeno-associated virus 2 infection in children with non-A-E hepatitis’ , is published in Nature. The work was funded by Public Health Scotland, the National Institute for Health Research (NIHR) and the Medical Research Council.

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Enquiries: ali.howard@glasgow.ac.uk or elizabeth.mcmeekin@glasgow.ac.uk / 0141 330 6557 or 0141 330 4831

First published: 30 March 2023