More CRISPR In Human Subjects
If you follow the progress of gene editing therapies in human disease, Tuesday was an interesting day. Verve Therapeutics (NASDAQ:VERV) has started a trial using CRISPR base editing technology to modify the PCSK9 gene in people with a disease called heterozygous familial hypercholesterolemia (HeFH).
I’ve written about PCSK9 (proprotein convertase subtilisin/kexin type 9) a number of times before on the blog. It’s one of the more compelling single gene/protein targets for lowering LDL cholesterol, since there are a few natural-mutation “human knockouts” out there who have extremely low cholesterol levels, very good cardiovascular profiles, and no apparent side effects. Loss-of-function mutations short of full abrogation seem to be beneficial as well. Those reports have led over the years to a lot of effort from biopharma – that linked post goes over some of these. There are antibodies on the market targeting the protein, and several companies have tried to come up with small molecules targeting it as well (although these tend to be not so small, since PCSK9 doesn’t have any sort of traditional small-molecule binding site). For some recent references on these efforts, see this paper. A siRNA therapy targeting the gene has recently been approved.
Familial hypercholesterolemia, for its part, is pretty much what the name implies: an inherited genetic disorder that leads to high levels of cholesterol, specifically high LDL levels. That’s famously associated with notably poor cardiovascular outcomes with aging, but in HeFH patients, the heart attacks start showing up in the 20s and 30s. There are a lot of mutations that can lead to that situation, mostly mutations in the LDL receptor protein. Maybe one percent of cases are actually due to gain-of-function mutations in PCSK9 itself. But regardless of the reason for high LDL, taking out PCSK9 function should increase expression of LDL receptors and thus lead to lower circulating levels of LDL itself. You would expect the absolute LDL levels not to drop to what you see in the regular “human knockout” patients, because in the case of HeFH, you’re expressing more of a receptor protein that’s intrinsically less functional, but on the relative scale, whacking PCSK9 should still lead to substantial LDL lowering.
That whacking (technical term there) is being done in Verve’s case by using CRISPR base-editing technology to make a specific A-to-G change in the PCSK9 gene. That site was picked because it’s not similar to anything else in the genome (so the guide RNA should take the machinery only to the place where it’s being targeted), and once that single-base change is made, the expression of the PCSK9 gene will be altered at the first exon/intron boundry. That will lead to “read-through” for some of intron 1, adding a few completely unnecessary and totally disruptive amino acids to the exon 1 piece of the actual PCSK9 protein, which will leave the affected liver cells with no real PCSK9 function at all. This editing will remain for the rest of these cells’ lifetimes and be passed on after mitosis: in theory, this will be a one-and-done therapy, but we’ll see what happens in practice. The patients will not, though, be passing this on to their children, since this won’t be editing the germ line – rather, they should just end up with permanently altered livers.
The Verve team first demonstrated this in primate studies, and I wrote about those in detail here. That was the first example of such base-editing technology in such a species, as opposed to “classic CRISPR”, which goes through double-strand DNA breaks and can lead to undesired effects. And here it is in humans: the first patient has been dosed. The therapy, Verve-101, is a lipid nanoparticle formulation of mRNA to code for the needed CRISPR enzymes and a guide RNA to send them to the right spot on the genome. PCSK9’s expression is heavily biased towards the liver to start with, so that makes it a perfect candidate for this sort of thing, because if you infuse LNP formulations that’s where all the particles are going to pile up anyway. The same goes for the siRNA therapy, for that matter, and the earlier attempts at antisense PCSK9 treatment as well. Getting these things to work somewhere other than in the liver, now that’s more of a challenge, so what we’re seeing now is the low-hanging fruit. Which is fine – it’s a great test bed for such ideas in general. I should note that there are other CRISPR therapies being tried in humans right now, but these are generally using earlier CRISPR techniques and/or doing the gene modification outside the patients’ bodies, with later injection or transplants of modified cells (as in the sickle cell anemia trials going on now).
The trial is supposed to enroll 40 patients, with a first round of ascending dose to look for the best effects, followed by a round of treatment at the selected dose, and perhaps another second dose for patients who got the lower doses the first time around or did not respond well. We’ll see how that goes – how efficient the CRISPR base editing really is in humans, how beneficial it is if it does work, and what other things may happen along the way. The future is unwritten, but that’s about to change.
Editor’s Note: The summary bullets for this article were chosen by Seeking Alpha editors.