PCSK9

In one line: PCSK9 is a gene that destroys the receptors which clear cholesterol from your blood — so the people born with a broken copy have low cholesterol, protected hearts, and became the blueprint for a whole new class of medicine.

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The gene

PCSK9 — proprotein convertase subtilisin/kexin type 9 — sits on chromosome 1p32.3. Your liver makes its protein for a single, counter-intuitive job: it hunts down the LDL receptors on the surface of liver cells — the ports that pull LDL ("bad") cholesterol back out of the blood — and marks them for destruction. More PCSK9 → fewer receptors → more cholesterol left circulating.

The hunt

The gene was found twice, from opposite directions, and the two teams should not be merged:

• 2003, France. Marianne Abifadel, Catherine Boileau and colleagues were studying families with dangerously high cholesterol (autosomal-dominant familial hypercholesterolemia). They traced it to gain-of-function mutations in PCSK9 — a gene working too hard. It was the third such cholesterol gene found, after LDLR and APOB.
• 2006, Dallas. Jonathan Cohen, Helen Hobbs and colleagues, mining the Dallas Heart Study, found the mirror image: people carrying a loss-of-function copy. Carriers of the nonsense variants Y142X / C679X had ~28% lower LDL cholesterol and ~88% lower risk of coronary heart disease. (A milder missense variant, R46L, gave ~15% lower LDL and ~47% lower risk — kept off-screen so the two number-pairs can't be confused.)

The mechanism

Liver cells fly LDL receptors to their surface to catch LDL and pull it out of the blood, then recycle those receptors and do it again. PCSK9 sabotages the loop: it grabs a receptor and escorts it to the lysosome to be degraded instead of recycled. Fewer receptors means more cholesterol stays in the blood. Remove or block PCSK9, and the receptors survive — and keep clearing the blood. That is the entire therapeutic idea, and nature had already run the experiment.

The stakes, and the frontier

Heart disease is the world's leading cause of death, and LDL cholesterol is one of its biggest modifiable drivers. So when Hobbs and Cohen described a healthy woman with essentially no working PCSK9 (a compound heterozygote), her LDL near 14 mg/dL — a fraction of normal — and otherwise well, she wasn't a patient. She was a living proof-of-concept.

If nature could switch the gene off safely, so could medicine — at three different levels:

1. Antibodies that mop up the PCSK9 protein in the blood: evolocumab (Repatha) and alirocumab (Praluent), both FDA-approved in 2015.
2. siRNA that silences the message inside the cell: inclisiran (Leqvio), dosed about twice a year (EU 2020 / US 2021).
3. In-vivo base editing that rewrites the gene itself in liver DNA — a potential one-time treatment, currently investigational / in clinical trials, not approved.

The 2026 frontier

The field is moving fast, and two developments define this year:

• The oral pill. Merck's enlicitide decanoate (MK-0616) is an investigational once-daily oral PCSK9 inhibitor — the first to deliver antibody-grade LDL lowering (~56% vs placebo at 24 weeks in the Phase 3 CORALreef program) from a pill rather than an injection. It received an FDA National Priority Voucher (Dec 2025) for expedited review. As of June 2026 it is Phase 3-positive but not yet approved — if cleared, it would move PCSK9 inhibition from a specialty injectable into everyday primary care.
• One-and-done editing. VERVE-102 (Verve Therapeutics, now Eli Lilly) reported 2026 Phase 1b data: a single infusion that base-edits the PCSK9 gene in the liver lowered PCSK9 by up to ~88% and LDL by up to ~60%, with durable effect — the clearest sign yet that a one-time genetic treatment for high cholesterol is becoming real. Still investigational.

(The freshest actual approval in the class is lerodalcibep / LEROCHOL, a once-monthly anti-PCSK9 binding protein, FDA-approved Dec 2025 — kept off-screen to keep the cure beat to three clean levels.)

Sources

Full claim-by-claim evidence is in references.md. Primary anchors:

• Gene/locus & function: NCBI Gene 255738 · MedlinePlus: PCSK9
• Gain-of-function → disease (2003): Abifadel et al., Nat Genet 34:154-156 — PMID 12730697
• Loss-of-function → protection (2006): Cohen et al., NEJM 354:1264-72 — PMID 16554528
• Natural knockout, LDL ≈ 14: Zhao et al., Am J Hum Genet 79:514-523 — PMID 16630924
• Therapies: Repatha FDA approval, 2015 · Leqvio/inclisiran (siRNA) · In-vivo base editing, Nature 2021
• 2026 frontier: Merck enlicitide (oral PCSK9), CORALreef Phase 3 · JACC 2026 (enlicitide vs oral non-statins) · FDA National Priority Voucher, Dec 2025 · Lilly/Verve VERVE-102 base editing, 2026 · Lerodalcibep FDA approval, Dec 2025

Accuracy note: the episode is careful with three things people routinely get backwards — (1) gain-of-function raises LDL (disease); loss-of-function lowers it (protection) — breaking the gene is the good outcome; (2) the −88% / −28% figures belong specifically to the Y142X/C679X loss-of-function carriers, not to the milder R46L variant; and (3) the three therapies act at three different levels — antibody (protein), siRNA (mRNA), base editor (DNA) — and only the base editor is gene editing, and it is labeled in trials.