Down syndrome is usually due to each cell in the human body having three copies, rather than two copies, of chromosome 21. This heartbreaking illness leads not just to a spate of physical abnormalities and cognitive dysfunction, but often to early-onset Alzheimer’s disease.
A pivotal brain region affected by Down syndrome is the hippocampus. Now scientists at Stanford University School of Medicine have made two important findings about what occurs in that region in a mouse model of Down syndrome, which purportedly translates to people with the syndrome.
One is a loss of two types of neurotransmitter-producing neurons in the hippocampus—acetylcholine-producing neurons and norepinephrine-producing neurons. The other is that this loss is linked with the overexpression of the amyloid precursor protein gene in the hippo-campus. Mutations in this gene, which is located on chromosome 21, are known to lead to early-onset Alzheimer’s, and it may be that early onset of Alzheimer’s pathology in people with Down syndrome is due in part to overexpression of the amyloid precursor protein gene.
These findings have provocative therapeutic implications for people with Down syndrome, the scientists also pointed out online September 27 in Biological Psychiatry. For instance, although the amyloid precursor protein gene should be a primary therapeutic target in Down syndrome, there are no safe and effective medications on the market to reduce the gene’s expression. In contrast, since a paucity of norepinephrine-producing neurons in the hippo-campus also seems to contribute to Down syndrome, medications that enhance norepinephrine levels in the brain and are already on the market to treat attention-deficit/hyperactivity disorder might be of therapeutic benefit to individuals with Down syndrome.
Moreover, such medications might also subdue the action of the amyloid precursor protein gene in such individuals, they speculated.
“We are indeed working on this group of drugs—drugs that are able to increase norepinephrine levels and that have already been approved by the Food and Drug Administration—in our mouse models,” Ahmad Salehi, M.D., Ph.D., a clinical associate professor of psychiatry at Stanford and the study’s senior investigator, told Psychiatric News. “This strategy could speed up the development of a treatment for cognitive function in Down syndrome enormously.”
The scientists’ discovery of a paucity of acetylcholine-producing neurons in the hippocampus of an animal model of Down syndrome holds therapeutic promise, the researchers noted. In fact, other researchers reported eight years ago that the Alzheimer’s drug donepezil, which slows the break down of acetylcholine in the brain, might improve cognitive scores and expressive language in children and adults with Down syndrome.
Yet if donepezil improves cognitive function and language in these individuals, is there reason to believe that medications that increase norepinephrine would be superior to donepezil in that regard? Asalehi believes there is. “I think using norepinephrine-ergic drugs would be far superior to cholinergic ones for the following reasons: The norepinephrine system has some regulatory effects on the cholinergic one. It has been shown that lesions in the former lead to increased severity of cholinergic deficits; most adults with Down syndrome will show Alzheimer’s-related pathology, particularly amyloid plaques. There are new studies showing that increasing norepinephrine levels in mouse models of Alzheimer’s significantly reduce amyloid accumulation. These findings suggest that using norepinephrine-ergic drugs might not only restore cognition in kids with Down syndrome, but also reduce Alzheimer’s-related pathology in adults with the syndrome.”
“Studies such as this one help to further our overall understanding of central nervous system function and in particular differences seen in individuals with Down syndrome,” Melanie Manning, M.D., director of the Center for Down Syndrome at Stanford’s Lucile Packard Children’s Hospital, told Psychiatric News.
“Many of the families of individuals with Down syndrome follow results from research studies such as this one with great interest. They express a desire to learn more about potential clinical applications that lie ahead.”
The research was funded by the Mental Illness Research, Education, and Clinical Center Department of Veterans Affairs; Down Syndrome Research and Treatment Foundation; Thrasher Foundation; and Alzheimer’s Association.
Ahmad Salehi, M.D., Ph.D.:
An abstract of “Neurobiological Elements of Cognitive Dysfunction in Down Syndrome: Exploring the Role of APP” is posted at <www.biologicalpsychiatryjournal.com/article/S0006-3223(11)00822-5/abstract>.