New name for tau-only brain pathology – a matter of semantics?

A group of scientists have proposed a new name for  an apparently common disorder in older people in which neurofibrillary tangles composed of abnormal tau protein accumulate unaccompanied by the amyloid-beta plaques found in Alzheimer’s disease.   Both neurofibrillary tangles and amyloid-beta plaques are necessary for a diagnosis of Alzheimer’s disease.  A disorder which includes only neurofibrillary tangles is called a tauopathy.  The new term that has been proposed for this pathology  is PART , acronym for  primary age-related tauopathy.

Alzheimer's disease

Diagram of the brain of a person with Alzheimer’s Disease. Credit: Wikipedia/public domain.

Figure retrieved from MedicalXpress.

The  article proposing the new term can be found here, and a discussion of it here.  But even more interesting is this article at ALZForum which critiques the new definition.

Scientists Propose a New Definition for Tau-Only Pathology

Tau tangles in the brain are an inevitable consequence of aging, and now a consortium of neuropathologists have coined a name for the condition: primary age-related tauopathy, or PART. They define PART mainly by what is missing; brains contain little if any Aβ. The consortium, led by first author John Crary of the Columbia University Medical Center in New York and senior author Peter Nelson of the University of Kentucky in Lexington, offer up the new term in a consensus paper in the October 28 Acta Neuropathologica online. “We see this pathology every day, but we have never had a name for it,” said Nelson. By christening the condition PART, he and his co-authors hope to instigate a wave of research on tau-only pathology.

PARTial Tauopathy. A section (left) from the medial temporal lobe of someone with PART exhibits extensive phospho-tau staining (brown). Right, silver stain picks up tau tangles (dark spots). [Image courtesy of Jean-Paul Vonsattel, Columbia University Medical Center, New York.]

PART is a pathological phenomenon, diagnosed only at autopsy, consisting of tau tangles that permeate the medial temporal lobe. The person might have appeared neurologically healthy, had mild cognitive impairment, or even had dementia. Previously, neuropathologists labeled PART with a variety of terms, such as “tangle-only dementia.” Those were inappropriate, the authors argue, because most people with PART do not experience dementia or senility. “For people with tauopathy only, we had no diagnosis to give them,” Nelson said. “All we could say is they did not have Alzheimer’s disease.”

Reaction to the new definition was mixed. “What they are now renaming PART is something that has been commonly accepted for decades,” said John Morris of the Washington University School of Medicine in St. Louis. Morris was not involved in the study. “However, if this construct, PART, brings more interest and inquiry to this abnormality, then I am all for it,” he said. Study co-author David Knopman of the Mayo Clinic in Rochester, Minnesota, admitted that the new term may be a matter of semantics. “But it is really important semantics,” he said.

Some pathologists questioned the necessity of the term, saying it simply represents a stepping-stone to AD. “PART is a subset of full-blown Alzheimer’s disease,” said Charles Duyckaerts of the Hôpital de La Salpetriere in Paris. In a commentary accompanying the paper, Heiko Braak and Kelly Del Tredici of the University of Ulm in Germany argued that with time, so-called PART cases will pick up amyloid pathology and become simply AD.

Knopman disagreed.  He told Alzforum that tauopathy could exist apart from Alzheimer’s. “When you mix them together, that is the toxic brew,” he said. “I do not believe that there is any evidence that medial temporal tauopathy induces amyloidosis.” Morris agreed that not all tauopathy would culminate in AD.

Morris was concerned that the precise borders of PART seem fuzzy. Indeed, scientists are still debating where to draw the lines between no pathology, PART, and AD, Nelson said. How much amyloid must be present to stop calling a case PART and label it Alzheimer’s? On the other end, even 20-somethings can show evidence of tauopathy in the brain (Elobeid et al., 2012), so when does that reach the level a pathologist should designate PART?

“PART, in some sense, is an inevitable consequence of aging,” Knopman said. Nelson said everyone who lives long enough will get PART, but the tauopathy appears to halt in the medial temporal lobe so long as Aβ does not get involved and spark Alzheimer’s. But even though it’s part and parcel of normal aging, that does not mean PART is harmless, Nelson said. He compared it to osteoarthritis, an unavoidable and uncomfortable consequence of aging. For some elderly individuals, osteoarthritis is no big deal. For others, it is crippling, so a cure would be a good thing, Nelson points out. Similarly, even if PART interfered with thinking in only some people, he said, it would still be worth identifying and fixing.

Read more.

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