A new study from MIT neuroscientists reveals how rare variants of a gene called ABCA7 may contribute to the development of Alzheimer's in some of the people who carry it.
Dysfunctional versions of the ABCA7 gene, which are found in a very small proportion of the population, contribute strongly to Alzheimer's risk. In the new study, the researchers discovered that these mutations can disrupt the metabolism of lipids that play an important role in cell membranes.
This disruption makes neurons hyperexcitable and leads them into a stressed state that can damage DNA and other cellular components. These effects, the researchers found, could be reversed by treating neurons with choline, an important building block precursor needed to make cell membranes.
"We found pretty strikingly that when we treated these cells with choline, a lot of the transcriptional defects were reversed. We also found that the hyperexcitability phenotype and elevated amyloid beta peptides that we observed in neurons that lost ABCA7 was reduced after treatment," says Djuna von Maydell, an MIT graduate student and the lead author of the study.
Li-Huei Tsai, director of MIT's Picower Institute for Learning and Memory and the Picower Professor in the MIT Department of Brain and Cognitive Sciences, is the senior author of the paper , which appears today in Nature.
Membrane dysfunction
Genomic studies of Alzheimer's patients have found that people who carry variants of ABCA7 that generate reduced levels of functional ABCA7 protein have about double the odds of developing Alzheimer's as people who don't have those variants.
ABCA7 encodes a protein that transports lipids across cell membranes. Lipid metabolism is also the primary target of a more common Alzheimer's risk factor known as APOE4. In previous work , Tsai's lab has shown that APOE4, which is found in about half of all Alzheimer's patients, disrupts brain cells' ability to metabolize lipids and respond to stress.
To explore how ABCA7 variants might contribute to Alzheimer's risk, the researchers obtained tissue samples from the Religious Orders Study/Memory and Aging Project (ROSMAP), a longitudinal study that has tracked memory, motor, and other age-related changes in older people since 1994. Of about 1,200 samples in the dataset that had genetic information available, the researchers obtained 12 from people who carried a rare variant of ABCA7.
The researchers performed single-cell RNA sequencing of neurons from these ABCA7 carriers, allowing them to determine which other genes are affected when ABCA7 is missing. They found that the most significantly affected genes fell into three clusters related to lipid metabolism, DNA damage, and oxidative phosphorylation (the metabolic process that cells use to capture energy as ATP).
To investigate how those alterations could affect neuron function, the researchers introduced ABCA7 variants into neurons derived from induced pluripotent stem cells.
These cells showed many of the same gene expression changes as the cells from the patient samples, especially among genes linked to oxidative phosphorylation. Further experiments showed that the "safety valve" that normally lets mitochondria limit excess build-up of electrical charge was less active. This can lead to oxidative stress, a state that occurs when too many cell-damaging free radicals build up in tissues.
Using these engineered cells, the researchers also analyzed the effects of ABCA7 variants on lipid metabolism. Cells with the variants altered metabolism of a molecule called phosphatidylcholine, which could lead to membrane stiffness and may explain why the mitochondrial membranes of the cells were unable to function normally.
A boost in choline
Those findings raised the possibility that intervening in phosphatidylcholine metabolism might reverse some of the cellular effects of ABCA7 loss. To test that idea, the researchers treated neurons with ABCA7 mutations with a molecule called CDP-choline, a precursor of phosphatidylcholine.
As these cells began producing new phosphatidylcholine (both saturated and unsaturated forms), their mitochondrial membrane potentials also returned to normal, and their oxidative stress levels went down.
The researchers then used induced pluripotent stem cells to generate 3D tissue organoids made of neurons with the ABCA7 variant. These organoids developed higher levels of amyloid beta proteins, which form the plaques seen in the brains of Alzheimer's patients. However, those levels returned to normal when the organoids were treated with CDP-choline. The treatment also reduced neurons' hyperexcitability.
In a 2021 paper, Tsai's lab found that CDP-choline treatment could also reverse many of the effects of another Alzheimer's-linked gene variant, APOE4, in mice. She is now working with researchers at the University of Texas and MD Anderson Cancer Center on a clinical trial exploring how choline supplements affect people who carry the APOE4 gene.
Choline is naturally found in foods such as eggs, meat, fish, and some beans and nuts. Boosting choline intake with supplements may offer a way for many people to reduce their risk of Alzheimer's disease, Tsai says.
"From APOE4 to ABCA7 loss of function, my lab demonstrates that disruption of lipid homeostasis leads to the development of Alzheimer's-related pathology, and that restoring lipid homeostasis, such as through choline supplementation, can ameliorate these pathological phenotypes," she says.
In addition to the rare variants of ABCA7 that the researchers studied in this paper, there is also a more common variant that is found at a frequency of about 18 percent in the population. This variant was thought to be harmless, but the MIT team showed that cells with this variant exhibited many of the same gene alterations in lipid metabolism that they found in cells with the rare ABCA7 variants.
"There's more work to be done in this direction, but this suggests that ABCA7 dysfunction might play an important role in a much larger part of the population than just people who carry the rare variants," von Maydell says.
The research was funded, in part, by the Cure Alzheimer's Fund, the Freedom Together Foundation, the Carol and Gene Ludwig Family Foundation, James D. Cook, and the National Institutes of Health.
