Explication simple
The liver makes a protein called transthyretin that normally carries hormones around the body. In ATTR amyloidosis that protein misfolds, clumps together, and deposits in tissues — most damagingly in the heart muscle and the nerves. There is no way to clear the deposits, so treatment aims to stop more forming. This is the disease where doctors first delivered CRISPR into a person's bloodstream and edited a gene inside their liver, rather than editing cells outside the body.
Aller plus loin
ATTR amyloidosis results from destabilisation of the transthyretin tetramer, allowing monomer misfolding and amyloid fibril deposition. Hereditary forms follow autosomal dominant TTR variants; wild-type ATTR is an age-related disease affecting predominantly older men. Cardiomyopathy (ATTR-CM) and polyneuropathy (ATTR-PN) are the dominant phenotypes. Because circulating TTR is almost entirely hepatic, knocking out TTR in hepatocytes is a rational one-time intervention, and lipid nanoparticles reach hepatocytes efficiently — which is why this became the first in vivo CRISPR indication.
Why this disease came first for in vivo editing
Three things aligned. The target tissue is the liver, the one organ lipid nanoparticles reach reliably. The therapeutic goal is to reduce a protein, which a knockout does well. And people with congenitally low transthyretin appear healthy, so removing it is unusually well-supported by human genetics. Very few diseases line up that neatly.
Where the programmes stand
Intellia's nexiguran ziclumeran (nex-z, formerly NTLA-2001) knocks out TTR in the liver after a single infusion, with Phase 1 data showing deep and durable reductions in serum transthyretin. It advanced into the Phase 3 MAGNITUDE trial in ATTR cardiomyopathy and MAGNITUDE-2 in polyneuropathy.
In late 2025 a participant in MAGNITUDE — a man in his early eighties — developed grade 4 liver transaminase elevation and raised bilirubin after dosing and subsequently died of liver dysfunction. Intellia paused dosing and screening voluntarily, and the FDA placed both studies on clinical hold. The hold on MAGNITUDE was subsequently lifted. This site records that sequence rather than the headline alone, because it is the clearest available illustration of what a serious adverse event in a one-time therapy actually looks like: the change cannot be withdrawn.
What else is available
This is a disease with real competition from non-editing medicines: TTR stabilisers such as tafamidis and acoramidis, and RNA-based silencers including patisiran, vutrisiran and eplontersen, all of which reduce or stabilise transthyretin and have approvals in various indications. A one-time edit would be a genuine convenience advantage; established medicines have years of safety data and can be stopped. That comparison, not the novelty of editing, is what will determine where this goes.
Common questions
What was the first in vivo CRISPR treatment?
NTLA-2001, now nexiguran ziclumeran, given to patients with hereditary ATTR amyloidosis with polyneuropathy from 2021. It was the first time a CRISPR editor was infused into a person's bloodstream to edit a gene inside their body, rather than editing cells outside it and infusing them back.
Sources
- New England Journal of Medicine (Gillmore et al.) · 2021
CRISPR-Cas9 in vivo gene editing for transthyretin amyloidosis ↗ - Intellia Therapeutics · 2025
Intellia provides update on MAGNITUDE clinical trials of nexiguran ziclumeran ↗ - Intellia Therapeutics · 2025
Intellia announces FDA lift of clinical hold on MAGNITUDE Phase 3 clinical trial in ATTR-CM ↗