As the body ages, cells naturally accumulate dozens of genetic mutations each year. New research reported by researchers at Boston Children’s Hospital suggests that the brain’s resident immune cells, microglia, amass mutations in specific cancer-driving genes, yet they don’t manifest as cancer. Instead, these mutations may help drive Alzheimer’s disease.

The research team, led by Christopher Walsh, MD, PhD, chief of the Division of Genetics and Genomics at Boston Children’s and an investigator of the Howard Hughes Medical Institute, and collaborators Alice Eunjung Lee, PhD, and August Yue Huang, PhD, also in the Division of Genetics and Genomics—who are all professors at Harvard Medical School and associate members of the Broad Institute of MIT and Harvard—say their study findings may provide insights into new Alzheimer’s disease diagnostics and treatments.

“We find that to some extent, Alzheimer’s disease is a little like cancer—driven by the same mutations that drive blood cancers like lymphoma and leukemia,” said Walsh. “This is helpful because we have a lot of drugs to fight cancer and some of them might be useful therapeutically for Alzheimer’s disease.”

The researchers reported on their work in Cell, in a paper titled “Somatic cancer variants enriched in Alzheimer’s disease microglia-like cells drive inflammatory and proliferative states.”

Microglia function as the brain’s resident immune cells, acting as garbage collectors, eating debris and infected or dying cells. “The importance of microglia in Alzheimer’s disease (AD) pathogenesis has been demonstrated by large-scale genetic association studies, which have identified AD risk variants in a growing list of microglia-related genes,” the authors wrote. “Once abnormally reactive in AD, microglia can promote synaptic and neuronal loss while exacerbating tau proteinopathy.”

Unlike the rest of the immune system cells that circulate in the blood throughout the body, microglia don’t cross the blood brain barrier—or so experts thought. For their newly reported study the research team sequenced 149 cancer-driving genes from tissue samples in 190 brains donated from people with Alzheimer’s disease compared to 121 healthy brains. The Alzheimer’s samples had more single DNA letter changes than the healthy tissue with the most changes found repeatedly in the same five cancer driver genes, meaning the microglia were amassing mutations in specific genes. “Deep (>1,000×) panel sequencing of 311 brain samples revealed enrichment of somatic single-nucleotide variants (sSNVs) in cancer driver genes in AD brains, especially in genes associated with clonal hematopoiesis (CH),” the team stated.

The cancer gene mutations the researchers discovered in the microglia are commonly found in blood cancers. Because of this, the team tested blood samples from people with Alzheimer’s disease for these same mutations. The team didn’t expect the blood to have these mutations. However, Walsh’s team found the blood cells of the same Alzheimer’s patients carried the same cancer mutations too.

“These sSNVs were associated with clonal expansion and carried by both microglia-like brain macrophages (MLBMs) in multiple brain regions as well as paired blood, suggesting a likely hematopoietic origin,” the investigators stated. “It was actually a really unexpected finding that suggests a totally new mechanism for Alzheimer’s disease pathogenesis,” said Huang. “The findings mean that the blood’s immune cells with cancer mutations are likely getting into the brain and contributing to disease.”

The researchers theorize that the blood-brain barrier weakens, either by age or injury, allowing the blood’s immune cells to cross into the brain. These new arrivals then convert into microglia-like cells. Separately, clumps of proteins accumulate in the brain, triggering microglia to proliferate and respond. The cells most likely to dominate are those with a selective advantage, such as the microglia-like cells with the cancer mutations. However, these mutant microglia also make the environment more inflammatory and hostile than that of the healthy microglia, causing innocent bystander neurons to die off, which leads to Alzheimer’s disease. “These findings suggest that clonal somatic driver variants in MLBMs are enriched in AD, potentially promoting neuroinflammation and neurodegeneration,” the researchers noted. “Potential roles of somatic cancer driver variants in AD pathogenesis open up a whole new range of therapeutic avenues in AD, complementary to approaches emphasizing amyloid and tau.”

Lee added, “Because it’s hard to access brain tissue in a living patient, genetic screens using blood samples could be developed to test whether a person carries these mutations, and has an increased risk of developing Alzheimer’s disease.” Lee and Huang performed a follow-up study, now posted as a preprint on bioRxiv. Here, they demonstrated that cancer driver mutations observed in patient blood samples increased risk of Alzheimer’s disease independently of a well-established genetic risk factor, APOE4.

The post Alzheimer’s Linked to Cancer Mutations in Brain Immune Cells appeared first on GEN – Genetic Engineering and Biotechnology News.

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