"These findings provide a glimpse of the brain shutting down the ability to form new memories gene by gene in Alzheimer's disease, and offer hope that we may be able to counteract this process," said Roderick Corriveau, Ph.D., a program director at NIH's National Institute of Neurological Disorders and Stroke (NINDS), which helped fund the research.
The study was led by Li-Huei Tsai, Ph.D., who is director of The Picower Institute for Learning and Memory at the Massachusetts Institute of Technology and an investigator at the Howard Hughes Medical Institute. It was published online February 29 in Nature.
Dr. Tsai and her team found that a protein called histone deacetylase 2 (HDAC2) accumulates in the brain early in the course of Alzheimer's disease in mouse models and in people with the disease. HDAC2 is known to tighten up spools of DNA, effectively locking down the genes within and reducing their activity, or expression.
In the mice, the increase in HDAC2 appears to produce a blockade of genes involved in learning and memory. Preventing the build-up of HDAC2 protected the mice from memory loss.
Dr. Tsai and her team examined two mouse models of Alzheimer's around the time that the mice begin to show signs of brain cell degeneration. They found that the mice had higher levels of HDAC2, but not other related HDAC proteins, specifically in the parts of the brain involved in learning and memory. This increase in HDAC2 was associated with a decrease in the expression of neuronal genes that HDAC2 is known to regulate.
Use of a gene therapy approach to reduce the levels of HDAC2 prevented the blockade of gene expression. The treatment also prevented learning and memory impairments in the mice. It did not prevent neuronal death, but it did enhance neuroplasticity — the ability of neurons to form new connections.
Dr. Tsai and her team also examined HDAC2 levels in autopsied brain tissue from 19 people with Alzheimer’s at different stages of the disease, and from seven unaffected individuals. Even in its earliest stages, the disease was associated with higher HDAC2 levels in the learning and memory regions of the brain.
"We think that the blockade of gene expression plays a very important role in the cognitive decline associated with Alzheimer's disease," said Dr. Tsai. "The good news is that the blockade is potentially reversible."
Alzheimer's disease is the most common cause of dementia in older adults, and affects as many as 5.1 million Americans. In the most common type of Alzheimer's disease, symptoms usually appear after age 65. A hallmark of the disease is the accumulation of a toxic protein fragment called beta-amyloid in brain cells, which is widely believed to be the initial trigger for neurodegeneration.
Dr. Tsai theorizes that HDAC2 is brought into play by beta-amyloid. Indeed, she and her team found that exposing mouse neurons to beta-amyloid caused them to produce more HDAC2.
"We think beta-amyloid triggers a cascade of damaging reactions. Once of these is to activate HDAC2, which in turn blocks the expression of genes needed for brain plasticity. Once this blockade is in place, it may have a more systemic, chronic effect on the brain," she said.
Vaccines and other therapies aimed at reducing beta-amyloid are in clinical trials. Efforts to reduce HDAC2 may provide a complementary approach to treating Alzheimer's, Dr. Tsai said. She has previously reported that HDAC inhibitor compounds can protect against signs of Alzheimer's disease in mice. A problem with such compounds is that they inhibit not only HDAC2 but related HDAC proteins, leading to broad and potentially toxic effects. The new study supports the possibility of developing drugs more specifically targeted to HDAC2 and the pathology of Alzheimer’s disease, Dr. Tsai said. Her team is working to identify HDAC2-specific inhibitors that could be developed into drugs and moved into trials.
Dr. Tsai's study was supported by NINDS and the National Institute on Aging through the NIH Common Fund Epigenomics Program. Additional support was provided through the NIH Blueprint for Neuroscience Research and its Neuroplasticity initiative.