Hypercholesterolemia: a strategy to turn off the defective gene without changing the DNA

After gene editing, i.e. the targeted modification of a gene's DNA sequence, comes epigenetic editing: the ability of modulating the level of activation of a gene without intervening in its sequence. This research area has become vastly active in recent years and now an article in the journal "Nature" offers the first proof of its long-term effectiveness in switching off a gene in vivo, in a model organism.

The work was signed by the team of Angelo Lombardo, Associate Professor of Histology at UniSR and head of the Epigenetic Regulation and Targeted Genome Modification laboratory at the San Raffaele Telethon Institute for Gene Therapy (SR-Tiget) in Milan.

The gene is called PCSK9 and is involved in the regulation of blood cholesterol levels. Some mutated variants of this gene cause familial hypercholesterolemia: a rare genetic condition characterized by a high risk of serious cardio- and cerebro-vascular diseases, such as heart attack and stroke, even at a young age.

In some patients with the disease, the gene is more active than normal, resulting in less effective liver cells in 'catching' the so-called 'bad' cholesterol, LDL. The consequence is an increase in blood cholesterol levels, which in turn is responsible for increased cardiovascular risk,

Lombardo explains.

A number of innovative therapies have already made to clinic research that aim to inactivate this gene in patients with familial hypercholesterolemia, including a gene editing platform that acts on the DNA sequence, and others are in an advanced phase of testing. For various reasons, however, PCSK9 also represents an excellent target for the newest epigenetic silencing technology.

To understand what it is, it is best to start from the concept of epigenetics: a set of mechanisms that regulates the state of expression of genes, i.e. whether they are turned on or off, without intervening in the DNA sequence. For example, it may involve the addition or elimination of particular chemical groups to the DNA molecule, such as to make it more or less accessible to the cellular machinery that initiates the process responsible for protein synthesis. Epigenetic silencing therefore means the possibility of turning off the expression of a target gene by intervening precisely on these mechanisms.

It is a sort of molecular switch that prevents the conversion of the information contained in the target gene into the corresponding protein,

clarifies Lombardo, one of the world pioneers of this technology.

Already in 2019, the researcher had founded, thanks to the Sofinnova-Telethon fund, a start-up, EpsilenBio, dedicated precisely to the development of an epigenetic silencing platform for the treatment of various diseases, acquired two years later by the American Chroma Medicine, of which Lombardo became co-founder.

The approach immediately gave excellent results in in vitro experiments, in cell lines, but it still lacked in vivo proof: a fundamental step in moving from the lab bench to the patient's bedside. And this is exactly the evidence obtained by Lombardo's group for the PCSK9 gene.

First, the researchers developed molecules (called "editors" in jargon) programmed to recognize and switch off this gene, adding particular chemical groups to its sequence. The second step was to encapsulate the editors in lipid nanoparticles, similar to those used for mRNA-based Covid vaccines, which were finally administered in mouse models. “We have effectively confirmed that in the treated experimental models PCSK9 is switched off in a stable and long-term way” underlines Martino Alfredo Cappelluti, first author of the study.

This positive result now opens up various interesting perspectives, starting from the development of drugs based on epigenetic silencing for hypercholesterolemia, both familial and acquired, i.e. not caused by mutations in single genes and decidedly more common.

Compared to other innovative treatments directed against PCSK9 - comments Lombardo -, this approach could have numerous advantages, as it is a therapy to be carried out only once in a lifetime, which does not modify the DNA sequence (with all the risks that this could entail) and with potentially reversible effects. Furthermore, the demonstration of efficacy obtained constitutes a very solid basis for developing epigenetic silencing strategies directed against the liver for other diseases, such as hepatitis B, but also at other organs, such as the central nervous system.