See Hansson and Mormino (doi:10.1093/brain/awy065) for a scientific commentary on this article.
Where should measurements be taken to best capture tau accumulation in ageing and Alzheimer's disease? ...Jack et al. report that in clinically symptomatic stages of Alzheimer's disease, tau accumulation occurs throughout the brain, rather than only in specific areas. Rate measurements from simple meta-regions of interest may be sufficient to capture progressive within-person tau accumulation.
Abstract
See Hansson and Mormino (doi:10.1093/brain/awy065) for a scientific commentary on this article.
Our objective was to compare different whole-brain and region-specific measurements of within-person change on serial tau PET and evaluate its utility for clinical trials. We studied 126 individuals: 59 cognitively unimpaired with normal amyloid, 37 cognitively unimpaired with abnormal amyloid, and 30 cognitively impaired with an amnestic phenotype and abnormal amyloid. All had baseline amyloid PET and two tau PET, MRI, and clinical assessments. We compared the topography across all cortical regions of interest of tau PET accumulation rates and the rates of four different whole-brain or region-specific meta-regions of interest among the three clinical groups. We computed sample size estimates for change in tau PET, cortical volume, and memory/mental status indices for use as outcome measures in clinical trials. The cognitively unimpaired normal amyloid group had no observable tau accumulation throughout the brain. Tau accumulation rates in cognitively unimpaired abnormal amyloid were low 0.006 standardized uptake value ratio (SUVR), 0.5%, per year but greater than rates in the cognitively unimpaired normal amyloid group in the basal and mid-temporal, retrosplenial, posterior cingulate, and entorhinal regions of interest. Thus, the earliest elevation in accumulation rates was widespread and not confined to the entorhinal cortex. Tau accumulation rates in the cognitively impaired abnormal amyloid group were 0.053 SUVR (3%) per year and greater than rates in cognitively unimpaired abnormal amyloid in all cortical areas except medial temporal. Rates of accumulation in the four meta-regions of interest differed but only slightly from one another. Among all tau PET meta-regions of interest, sample size estimates were smallest for a temporal lobe composite within cognitively unimpaired abnormal amyloid and for the late Alzheimer's disease meta-region of interest within cognitively impaired abnormal amyloid. The ordering of the sample size estimates by outcome measure was MRI < tau PET < cognitive measures. At a group-wise level, observable rates of short-term serial tau accumulation were only seen in the presence of abnormal amyloid. As disease progressed to clinically symptomatic stages (cognitively impaired abnormal amyloid), observable rates of tau accumulation were seen uniformly throughout the brain providing evidence that tau does not accumulate in one area at a time or in start-stop, stepwise sequence. The information captured by rate measures in different meta-regions of interest, even those with little topographic overlap, was similar. The implication is that rate measurements from simple meta-regions of interest, without the need for Braak-like staging, may be sufficient to capture progressive within-person accumulation of pathologic tau. Tau PET SUVR measures should be an efficient outcome measure in disease-modifying clinical trials.
Whether novel risk variants of Alzheimer’s disease (AD) identified through genome-wide association studies (GWAS) also influence MRI-based intermediate phenotypes of AD in the general population is ...unclear. We studied association of 24 AD risk loci with intracranial volume (ICV), total brain volume (TBV), hippocampal volume (HV), white matter hyperintensity (WMH) burden, and brain infarcts in a meta-analysis of genetic association studies from large population-based samples (N=8,175–11,550). In single-SNP based tests, AD risk allele of
APOE
(rs2075650) was associated with smaller HV (p=0.0054) and
CD33
(rs3865444) with smaller ICV (p=0.0058) In gene-based tests, there was associations of
HLA-DRB1
with TBV (p=0.0006) and
BIN1
with HV (p=0.00089). A weighted AD genetic risk score was associated with smaller HV (beta±SE=−0.047±0.013, p=0.00041), even after excluding the
APOE
locus (p=0.029). However, only association of AD genetic risk score with HV, including
APOE
, was significant after multiple testing correction (including number of independent phenotypes tested). These results suggest that novel AD genetic risk variants may contribute to structural brain aging in non-demented older community persons.
Abstract
Background
This study aimed to examine if the association of cerebral perfusion with gait speed differs across systolic blood pressure (SBP) and age.
Methods
Cerebral perfusion was measured ...via arterial spin labeled (ASL)-MRI among community-dwelling adults aged 31–94 years in the population-based Mayo Clinic Study of Aging. Usual gait speed was assessed over 5.6 meters on an electronic mat. Sex- and body mass index (BMI)-adjusted linear regression models estimated cross-sectional gait speed associations with ASL and modifying effects of age and SBP using 3-way and 2-way interaction terms between continuous age, SBP, and ASL. Results report estimated differences in gait speed per standard deviation (SD) lower ASL for exemplar SBPs and ages.
Results
Among 479 participants (mean age 67.6 years; 44% women; mean gait speed 1.17 m/s), ASL relations to gait speed varied by age (ASL-x–age interaction: p = .001) and SBP (ASL-x–SBP interaction: p = .009). At an SBP of 120 mmHg, each SD lower ASL was associated with a 0.04 m/s (95% confidence interval CI: 0.01, 0.07) slower gait speed at 65 years, 0.07 m/s (0.04, 0.10) at 75 years, and 0.09 m/s (0.05, 0.13) at 85 years. At an SBP of 140 mmHg, ASL associations with gait speed were attenuated to 0.01 (−0.01, 0.04), 0.04 (0.02, 0.06), and 0.06 (0.04, 0.09) m/s slower gait speed at ages 65, 75, and 85, respectively.
Conclusion
Poorer cerebral perfusion is associated with clinically meaningful slower gait speeds, particularly with older age, while higher perfusion markedly attenuates age differences in gait speed.
Null mutations in the progranulin gene (PGRN) were recently reported to cause tau-negative frontotemporal dementia linked to chromosome 17. We assessed the genetic contribution of PGRN mutations in ...an extended population of patients with frontotemporal lobar degeneration (FTLD) (N=378). Mutations were identified in 10% of the total FTLD population and 23% of patients with a positive family history. This mutation frequency dropped to 5% when analysis was restricted to an unbiased FTLD subpopulation (N=167) derived from patients referred to Alzheimer's Disease Research Centers (ADRC). Among the ADRC patients, PGRN mutations were equally frequent as mutations in the tau gene (MAPT). We identified 23 different pathogenic PGRN mutations, including a total of 21 nonsense, frameshift and splice-site mutations that cause premature termination of the coding sequence and degradation of the mutant RNA by nonsense-mediated decay. We also observed an unusual splice-site mutation in the exon 1 5' splice site, which leads to loss of the Kozac sequence, and a missense mutation in the hydrophobic core of the PGRN signal peptide. Both mutations revealed novel mechanisms that result in loss of functional PGRN. One mutation, c.1477C>T (p.Arg493X), was detected in eight independently ascertained familial FTLD patients who were shown to share a common extended haplotype over the PGRN genomic region. Clinical examination of patients with PGRN mutations revealed highly variable onset ages with language dysfunction as a common presenting symptom. Neuropathological examination showed FTLD with ubiquitin-positive cytoplasmic and intranuclear inclusions in all PGRN mutation carriers.
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