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Causes of Alzheimer's and "Mad-Cow" Diseases

Alzheimer's and "mad-cow" diseases are unique in that their infectious agents are not viruses or germs, but rather proteins. The brains of patients who suffered from Alzheimer's or cows that died of "mad-cow" disease show deposits of abnormal tissue called amyloid plaques. The primary component of these plaques is a protein called prion protein or PrP. Chemical and biochemical analysis showed that there was no difference in composition or primary structure between the normal, cellular form of PrP (PrP^C, shown at right) and the disease form of PrP (PrP^Sc). Further analysis showed that PrP^C can change into PrP^Sc when two of the α helices (shown in green) change into ß sheets. This ß sheet can then induce a similar change in another molecule of PrP^C and hydrogen bond to it. The PrP^Scs then polymerize and come out of solution, forming the plaques found in Alzheimer's patients and mad cows. How the plaques cause the symptoms of the diseases is still not clear, but the prion protein holds the unique distinction of causing a disease solely through a small alteration in secondary structure.