"The loss of smell - or olfactory dysfunction - has been known for more than a decade as an early sign of several neurodegenerative diseases, but we have never been able to link it to a pathological entity that is measurable over time," said Richard Doty, PhD, Professor and Director of Penn's Smell and Taste Center, who is also the team leader of the study. "By tying decrements in the ability to smell to the presence of key disease proteins, such as tau, we may well be able to assess the degree of progression of selected elements of Alzheimer's disease and related disorders by scores on quantitative smell tests."
A total of ten mice were evaluated in this experiment - five mice that were genetically engineered to be a model for human Alzheimer's and Parkinson's disease, and five normal control mice that do not overexpress tau proteins. Olfactory dysfunction was evaluated by measuring the amount of time the mice spent investigating unfamiliar odors, such as peppermint or vanillin. Unlike normal mice, those with smell deficits do not spend much time investigating such odors, and do not show a preference for "novel" odors over "familiar" odors.
The results of the Penn study showed that only the control mice, with no excess of tau proteins, expressed an interest in new odors, indicating a normal sense of smell. The mice that had excess in tau protein showed little or no interest in such odors, implying olfactory dysfunction. Analysis of brain tissue from the diseased mice confirmed a link between the olfactory loss and the presence of excess tau proteins in brain structures important for smelling. Additionally, the genetically engineered mice exhibited a significant amount of neurofibrillary tangles, structures also linked to Alzheimer's disease.
No test currently exists for the detection of Alzheimer's disease. A definitive diagnosis is only confirmed upon death when the brain tissue becomes available for testing of the presence of such proteins and other physiological markers of the disease.
The process for diagnosis before death is less certain and involves several kinds of tests - for memory, problem solving, attention and counting - plus review of a patient's complete medical history from a primary care physician, and possibly brain scans and consultations from other specialists. With several diagnostic tools and criteria, physicians can make a diagnosis of moderate to severe stages of Alzheimer's disease with 90 percent accuracy. However, diagnosis is less certain for the early stages of the illness.
No explanation exists for what causes the alterations in smell perception associated with neurodegenerative disorders. What is known is that key physiological markers for these illnesses - deposits of neurofibrillary tangles or amyloid plaques, Lewy bodies or tau and alpha-synuclein proteins - are commonly found in brain regions associated with the perception of odors in the bodies of people who suffer from these illnesses.
"The mice who overexpress tau, a protein associated with a family of neurological disorders that includes Alzheimer's disease and Parkinson's disease, have decreased ability to smell," said Doty. "We next plan to study the progression of the disease entities in the brain as it relates to smell dysfunction. These experiments are part and parcel to better understanding the physical causes of such neurodegenerative diseases."
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