Researchers explore the safety and efficacy of permanently disrupting PCSK9 expression using in vivo CRISPR gene editing to induce lasting reductions of LDL cholesterol in the blood.
High blood cholesterol is a common risk factor for cardiovascular diseases. In 2017, elevated levels of LDL cholesterol claimed nearly four million lives, and is accountable for a third of deaths due to ischemic heart diseases and stroke. To reduce LDL levels in the blood, lipid-lowering therapies such as monoclonal antibodies, small interfering RNAs, and drugs that target the liver protein PCSK9 have been used.
Under normal circumstances, the PCSK9 protein regulates LDL levels by degrading LDL receptors, which in turn slows down the removal of LDL from the bloodstream. PCSK9 inhibitors work by interfering with the proper functioning of PCSK9. While PCSK9 inhibitors are effective, they require routine administration to maintain low lipid levels. As such, problems associated with patient non-adherence to treatment often arise and compromise the efficacy of these medications.
To extend the effects of lipid-lowering therapies and reduce the need for routine medications, Dr. Kiran Musunuru and colleagues have proposed a novel strategy to permanently decrease PCSK9 expression using therapeutic gene editing. With gene editing technologies, particularly CRISPR-Cas nucleases, scientists can make precise and stable modifications of single nucleotides in the genome, thus allowing them to correct causative mutations or inactivate disease-causing genes.
In this novel study, the researchers first formulated lipid nanoparticles containing ABE8.8 adenine base editors. These editors are enzymes that substitute specific adenine residues with guanine and consequently alter protein translation. Next, they added guide RNAs into the nanoparticle to help direct the base editor to specific splice sites in PCSK9-regulating genes.
Upon confirming that the CRISPR base editor could effectively and precisely disrupt the expression of PCSK9 in primary human hepatocytes, they proceeded to test with mice models in which they observed 70 per cent editing efficiency in the liver, the primary site of PCSK9 expression in the body.
Given the positive results, Musunuru and colleagues decided to take their investigation one step further and experimented on cynomolgus monkeys (Macaca fascicularis), from which they observed similar levels of editing efficiency. In the monkeys, the team observed an 81 per cent decrease of PCSK9 in the blood and a 65 per cent drop in LDL cholesterol.
In their current long-term study, they have successfully reached a stable 90 per cent reduction of circulating PCSK9 and a 60 per cent decrease in LDL even after eight months from the first dosage, proving the technology’s benefits to reduce LDL cholesterol and treat associated cardiovascular diseases in a “once-and-done” approach. When compared to conventional lipid-lowering therapies, their innovative approach showed similar or better results.
The team also reported minimal adverse health effects and off-target. While markers of liver damage temporarily increased during the treatment, it was noted that the markers appeared in response to the nanoparticles rather than the PCSK9 editing technology. Nevertheless, further studies will be required to validate the safety and risks of in vivo gene editing.
Representing the first efficient delivery of a CRISPR base editor to non-human primates, the results of this study strongly support the use of in vivo base editing as a precise and effective “one-time” strategy to target PCSK9 and reduce cholesterol levels. [APBN]
Source: Musunuru et al. (2021). In vivo CRISPR base editing of PCSK9 durably lowers cholesterol in primates. Nature, 593(7859), 429-434.