An inherited gene linked to Alzheimer's disease can affect the brains and thinking ability of young children in the first years of life, research suggests.
A part of the brain vital to memory was found to be smaller and less well developed in children younger than eight who had the gene variant.
Some of the youngest affected children also performed less well in tests of working memory, planning and organisational skills, and attention.
However, scientists found that even children with the Alzheimer's mutation achieved normal scores in thinking tests after the age of eight.
Previous research has shown that people with a particular variant of the apolipoprotein-E gene called epsilon 4 are more likely to develop Alzheimer's than those with two other versions of the gene.
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For the new study, researchers analysed DNA samples from 1187 individuals aged three to 20 to identify those with the Alzheimer's variant.
They also conducted brain scans and tests of memory and thinking skills. None of the participants had brain disorders or other problems that could have affected their brain development, such as exposure to drugs in the womb.
A key discovery was that the hippocampus - a brain region essential to memory that is especially vulnerable to Alzheimer's damage - was smaller and less well assembled in young children with the epsilon 4 variant.
Lead researcher Dr Linda Chang, from the University of Hawaii in Honolulu, said: "These findings mirror the smaller volumes and steeper decline of the hippocampus volume in the elderly who have the epsilon 4 gene."
Two other versions of the apolipoprotein-E gene exist besides the epsilon 4 variant, epsilon 2 and epsilon 3.
The brain scans showed that some children with two copies of epsilon 4, or one copy of epsilon 4 paired with one copy of epsilon 2, had lower scores in tests of memory and thinking.
The youngest of them with an epsilon 4/epsilon 4 combination performed especially poorly in tests of executive function - which governs planning and organisational ability - and working memory.
Some of the youngest children with an epsilon 2/epsilon 4 pairing were similarly disadvantaged in tests of attention.
But the differences were no longer seen in children older than eight, the scientists found.
The findings are published in the latest online issue of the journal Neurology.
Dr Chang said: "Studying these genes in young children may ultimately give us early indications of who may be at risk for dementia in the future and possibly even help us develop ways to prevent the disease from occurring or to delay the start of the disease."
