Silencing the gene activity of the hepatitis B virus appears to prevent it from reproducing: the approach could affect latent forms of the infection.
Modifying the chemical labels attached to DNA without altering its original sequence: this, in short, is the goal of epigenetic editing, an evolution of gene therapy that we can imagine as a sort of molecular switch, capable of activating or silencing the expression of a gene without modifying the genome permanently.
Now this technique has been used to turn off the gene activity of the hepatitis B virus, which can remain latent in the nucleus of liver cells and which drug therapies are never able to completely inactivate.
In preclinical studies, that is, not yet on human patients, epigenetic editing has been used to add chemical groups to the genome of the hepatitis B virus in infected cells, thus silencing its gene activity and preventing it from producing new viral proteins.
In essence, the expression of viral genes was turned off, without however cutting the DNA double helix as is done with CRISPR/Cas9 molecular scissors, the genome editing method awarded the Nobel Prize for Chemistry in 2020 which, in some cases, can give rise to incorrect reorganizations of the chromosomes.
The therapy has shown promising results in animal models, leading to profound repression of proteins (antigens) that indicate the activity of the hepatitis B virus for over six months. The results of the work, conducted by the biotechnology company nChroma Bio of Boston, the San Raffaele Telethon Institute for Gene Therapy (SR-Tiget) and the National Molecular Genetics Institute “Romeo ed Enrica Invernizzi” of Milan, were published in the scientific journal Nature Biomedical Engineering.
The permanent reservoir of hepatitis B
Hepatitis B is an infection that affects the liver. It is caused by the HBV virus, which is transmitted through blood or other infected body fluids (therefore also sexually) and against which a widely effective vaccine is available.
In 5-10% of adult patients with hepatitis B, the disease becomes chronic, with possible very serious effects, such as liver failure (loss of liver function), cirrhosis (progressive scarring of the liver) or liver cancer. The WHO estimates that worldwide, 240 million people live with a chronic form of hepatitis B.
Current pharmacological therapies are able to suppress the activity of the hepatitis B virus but not eliminate it: patients with chronic forms are therefore forced to take antivirals for life. The pathogen is in fact able to integrate into the host’s genome and deceive the immune system so as not to be attacked.
One of the ways it does this is by producing free mini-chromosomes that sit in the nucleus of cells and act as genomic archives for the virus, helping it replicate again when antiviral drugs are stopped. Other pieces of the hepatitis B virus integrated into the DNA of cells also generate proteins that alter the response of immune cells to the pathogen; this form of supplementation causes molecular changes in liver cells that can cause cancer.
Disable everything!
The scientists used a genetic editor called CRMA-1001, capable of deactivating both persistent forms of the virus – both its permanent reservoirs and its integrations into DNA. The therapy contains an enzyme also used in the CRISPR technique, Cas9, which, however, is deprived of its “cutting” capacity; and also RNA strands that guide the enzyme to its specific targets on the viral DNA. The editor was administered as a single-dose treatment, within lipid nanoparticles, first into human liver cells, then into mice and macaques with hepatitis B.
In mice, a single injection of the therapy reduced both levels of viral DNA and proteins that indicate the activity of the hepatitis B virus: in 25 of 60 rodents, both values were undetectable for more than six months. The safety of increasing doses of the medicine was investigated in macaques, with minimal and temporary adverse effects.
No more drugs for life?
The long-term goal is to have a gene silencer that turns off the effect of the hepatitis virus on cells with a single administration, achieving a long-lasting functional cure of the infection and freeing patients from the need to take antivirals for life.
The results have given “green light” to the advancement of trials in human patients: trials are expected to begin in January in Hong Kong and New Zealand. However, it will be necessary to understand whether the therapy is able to completely eradicate the viral infection from the cells and for how long, and whether there really are no unwanted effects, even invisible ones: the equivalent of the off-target mutations of traditional CRISPR, but at the level of gene expression. And for this, it will be necessary to monitor study participants for a very long time.
