Alzheimer’s disease is virtually guaranteed for people who carry two copies of the APOE4 gene variant, a new study suggests.

Scientists have known for the last 30 years that people with the copies — making up between 2-3% of the general population or 15% of people with Alzheimer’s disease — have significantly higher risk of the neurodegenerative disorder compared with people with the most common version, known as APOE3.

But the study published on Monday in Nature Medicine could result in APOE4 gene homozygosity being considered a new genetic form of the disease.

Dr Juan Fortea of the University of Barcelona in Spain and colleagues studied more than 3000 donated brains from the US National Alzheimer’s Coordinating Center, as well as biological and clinical data on more than 10,000 individuals from five datasets in three countries.