The total tau level in blood plasma appears to predict both onset and progression of dementia and could be used to help refine research cohorts.
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Blood samples from two large dementia research cohorts confirmed the finding: Each standard deviation in plasma tau above the median is associated with a 29% greater risk of incident all-cause dementia and a 35%increase in the risk of incident Alzheimer’s disease, Matthew P. Pase, PhD and colleagues wrote in JAMA Neurology. It also correlated positively with some neuropathological aspects of dementia: smaller hippocampus and a higher burden of neurofibrillary tangles in the medial temporal lobe, said Dr. Pase of The Florey Institute for Neuroscience and Mental Health. Victoria, Australia.
Plasma tau isn’t the highly sought Holy Grail of a simple Alzheimer’s blood test. But the finding could benefit the research world. As a study entry criteria, it could substantially decrease the number of subjects needed to validate an outcome of either all-cause dementia or Alzheimer’s disease. Abnormal tau is also a required finding for an Alzheimer’s diagnosis in revised NIA-AA Research Framework. And, the authors noted, although plasma tau wasn’t quite as accurate a predictor as CSF tau, a needle in the arm would be much more acceptable to many more patients than a lumbar puncture.
“Whereas we do not expect plasma t-tau cutoffs to enhance diagnostic certainty for any single patient, our results suggest that plasma t-tau could be associated with improved risk stratification at a population level, targeting persons for inclusion in prevention trials, thus improving the power and precision of clinical trials and potentially accelerating therapeutic pipelines and drug discovery,” the team wrote.
The study drew on stored plasma samples from subjects enrolled in the Framingham Heart Study (1,453) and the Memento study, a multicenter cohort of persons with mild cognitive impairment or subjective cognitive complaints recruited from memory clinics across France (367).
The Framingham cohort was followed for up to 10 years between baseline examination to incident event (median 6 years).
Over that time, 134 (9.2%) cases of dementia developed; most of these (105) were due possible, probable, or definite Alzheimer’s.
Plasma tau levels rose linearly as the cohort aged. Higher plasma t-tau levels were associated with proven AD risk factors, including female sex, lower education, and higher vascular risk factors. They did not differ by apolipoprotein epsilon 4 status (APOEe4).
After adjusting for age and sex, each stand deviation unit increase in the log of tau was associated with a 29% greater risk of incident all-cause dementia, and a 35% increase in the risk of incident Alzheimer’s dementia. Subjects with tau levels above the median had a 62% increased risk of all-cause dementia and a 76% greater risk of AD. Adding APOEe4 status and vascular risk factors to the analysis didn’t alter the associations.
“Plasma t-tau level improved risk discrimination for all dementia and AD dementia beyond age and sex,” the investigators wrote. “[It] was associated with improved risk discrimination … in both APOEe4 carriers and noncarriers.”
In a hypothetical 5-year clinical trial, enrolling subjects with total plasma tau greater than the median could reduce the estimated necessary sample size by 38% for an outcome of all-cause dementia and by 50% for one of Alzheimer’s. Selecting those with both elevated plasma tau and APOEe4 carriage could reduce the required sample by 69% for all-cause dementia outcomes and by 80% for Alzheimer’s outcomes.
In the neuropathologic study, each standard deviation unit increase was associated with more neurofibrillary tangles in the medial temporal lobe, more microinfarcts, and smaller hippocampal volume. There was no association with amyloid plaque in any brain region.
Subjects in the Memento study had a meant of 4 years of follow-up. Over that time, there were 76 cases of incident dementia, 55 of which were probable Alzheimer’s.
Each standard deviation unit increase was associated with a nonsignificant 14% greater risk of all-cause dementia and a significant 54% increase in the risk of incident Alzheimer’s.
CSF was drawn on the same day as plasma in 140 of these subjects. The addition of CSF boosted the predictive value; each standard deviation increase more than doubled the risk of both Alzheimer’s (HR 2.33). Each standard deviation unit increase in CSF t-tau increased the risk by 2.14.
“Plasma t-tau was weakly correlated with CSF t-tau in our study. This finding is consistent
with previous studies showing that the associations of plasma t-tau with CSF t-tau have been weak or nonexistent,” the authors wrote. But, “Despite a weak correlation between plasma and CSF t-tau, plasma t-tau was at least as strongly associated with the development of incident AD dementia.”
“Use of plasma t-tau in this manner could be likened to the measurement of the APOEe4 allele, which is not a biomarker of AD pathology providing diagnostic certainty for AD dementia but is still routinely used to power clinical trials by selecting at-risk individuals,” they concluded.
Dr. Pase had no financial disclosures.
SOURCE: Pase, M et al JAMA Neurol 2019 doi:10.1001/jamaneurol.2018.4666