Simple explanation
Alzheimer's disease damages memory and thinking as abnormal protein deposits build up in the brain and neurons are lost. A gene called APOE strongly influences risk — one version raises it substantially, another lowers it — but risk is not cause: many people with the high-risk version never develop the disease, and many people with the disease do not carry it. Editing a risk factor in a living adult brain is not something anyone can currently do, and it is not obvious it would help if they could.
Go deeper
Late-onset Alzheimer's disease is polygenic with APOE ε4 the strongest common risk allele; rare autosomal dominant forms involve APP, PSEN1 and PSEN2. Editing is used to build isogenic models — for example converting APOE ε4 to ε3 or ε2 in patient-derived neurons — and to study protective variants such as the Christchurch APOE variant and the APP A673T allele. Therapeutic editing faces the same barriers as other brain indications, compounded by the difficulty of intervening before irreversible damage.
Risk is not cause
This distinction matters more here than almost anywhere else on the site. APOE ε4 shifts probability; it does not determine outcome. An intervention against a risk factor would need to be given long before symptoms, to people who might never have developed the disease, with permanent effect — a risk-benefit calculation quite unlike treating an established condition.
Sources
- National Institute on Aging
Alzheimer's disease genetics fact sheet ↗