β-Amyloid-Dependent and -Independent Genetic Pathways Regulating CSF Tau Biomarkers in Alzheimer Disease

Towards cascading genetic risk in Alzheimer's disease

Alzheimer's disease typically progresses in stages, which have been defined by the presence of disease-specific biomarkers: amyloid (A), tau (T) and neurodegeneration (N). This progression of biomarkers has been condensed into the ATN framework, in which each of the biomarkers can be either positive (+) or negative (-). Over the past decades, genome-wide association studies have implicated ∼90 different loci involved with the development of late-onset Alzheimer's disease. Here, we investigate whether genetic risk for Alzheimer's disease contributes equally to the progression in different disease stages or whether it exhibits a stage-dependent effect. Amyloid (A) and tau (T) status was defined using a combination of available PET and CSF biomarkers in the Alzheimer's Disease Neuroimaging Initiative cohort. In 312 participants with biomarker-confirmed A-T- status, we used Cox proportional hazards models to estimate the contribution of APOE and polygenic risk scores (beyond APOE) to convert to A+T- status (65 conversions). Furthermore, we repeated the analysis in 290 participants with A+T- status and investigated the genetic contribution to conversion to A+T+ (45 conversions). Both survival analyses were adjusted for age, sex and years of education. For progression from A-T- to A+T-, APOE-e4 burden showed a significant effect [hazard ratio (HR) = 2.88; 95% confidence interval (CI): 1.70-4.89; P < 0.001], whereas polygenic risk did not (HR = 1.09; 95% CI: 0.84-1.42; P = 0.53). Conversely, for the transition from A+T- to A+T+, the contribution of APOE-e4 burden was reduced (HR = 1.62; 95% CI: 1.05-2.51; P = 0.031), whereas the polygenic risk showed an increased contribution (HR = 1.73; 95% CI: 1.27-2.36; P < 0.001). The marginal APOE effect was driven by e4 homozygotes (HR = 2.58; 95% CI: 1.05-6.35; P = 0.039) as opposed to e4 heterozygotes (HR = 1.74; 95% CI: 0.87-3.49; P = 0.12). The genetic risk for late-onset Alzheimer's disease unfolds in a disease stage-dependent fashion. A better understanding of the interplay between disease stage and genetic risk can lead to a more mechanistic understanding of the transition between ATN stages and a better understanding of the molecular processes leading to Alzheimer's disease, in addition to opening therapeutic windows for targeted interventions.

Disentangling genetic risks for development and progression of Alzheimer's disease

This scientific commentary refers to ‘Towards cascading genetic risk in Alzheimer’s disease’ by Altmann et al. (https://doi.org/10.1093/brain/awae176).

Alzheimer's disease genetic pathways impact cerebrospinal fluid biomarkers and imaging endophenotypes in non-demented individuals

INTRODUCTION

Unraveling how Alzheimer's disease (AD) genetic risk is related to neuropathological heterogeneity, and whether this occurs through specific biological pathways, is a key step toward precision medicine.

METHODS

We computed pathway-specific genetic risk scores (GRSs) in non-demented individuals and investigated how AD risk variants predict cerebrospinal fluid (CSF) and imaging biomarkers reflecting AD pathology, cardiovascular, white matter integrity, and brain connectivity.

RESULTS

CSF amyloidbeta and phosphorylated tau were related to most GRSs. Inflammatory pathways were associated with cerebrovascular disease, whereas quantitative measures of white matter lesion and microstructure integrity were predicted by clearance and migration pathways. Functional connectivity alterations were related to genetic variants involved in signal transduction and synaptic communication.

DISCUSSION

This study reveals distinct genetic risk profiles in association with specific pathophysiological aspects in predementia stages of AD, unraveling the biological substrates of the heterogeneity of AD-associated endophenotypes and promoting a step forward in disease understanding and development of personalized therapies.

HIGHLIGHTS

Polygenic risk for Alzheimer's disease encompasses six biological pathways that can be quantified with pathway-specific genetic risk scores, and differentially relate to cerebrospinal fluid and imaging biomarkers. Inflammatory pathways are mostly related to cerebrovascular burden. White matter health is associated with pathways of clearance and membrane integrity, whereas functional connectivity measures are related to signal transduction and synaptic communication pathways.

Polygenic risk for Alzheimer's disease is associated with neuroaxonal damage before onset of clinical symptoms

INTRODUCTION

Establishing valid diagnostic strategies is a precondition for successful therapeutic intervention in Alzheimer's disease (AD).

METHODS

One hundred forty-four healthy 75-year-old participants from the Vienna-Transdanube-Aging longitudinal cohort study were tested for neuroaxonal damage by single molecular array (Simoa) plasma neurofilament light chain (NfL) levels at baseline, 30, 60, and 90 months, and onset of AD dementia. Individual risk for sporadic AD was estimated by continuous shrinkage polygenic risk score (PRS-CS, genome-wide association study).

RESULTS

Nineteen participants developed AD after a median of 60 months (interquartile range 30). In participants with AD, baseline NfL plasma levels correlated with PRS-CS (r = 0.75, p < 0.001; difference to controls: Fisher's r-to-z: z = 3.89, p < 0.001). PRS-CS combined with baseline plasma NfL predicted onset of AD (p < 0.01).

DISCUSSION

Our data suggest that polygenic risk for AD and plasma NfL closely interact years before onset of clinical symptoms. Peripheral NfL may serve as a diagnostic measure supporting early therapeutic intervention and secondary prevention in AD.

The Biochemistry Behind Cognitive Decline: Biomarkers of Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent type of dementia. Pathologically, the disease is marked by neurofibrillary tangles (NFT), which are aberrant accumulations of the tau protein that develop inside neurons, and extracellular plaque deposits of the amyloid β peptide (Aβ). These pathological lesions are present in the brain before the beginning of clinical manifestations. However, despite advancements in the comprehension of AD pathophysiology, timely and accurate clinical diagnosis remains challenging. Therefore, developing biomarkers capable of detecting AD during the preclinical phase holds enormous promise for precise diagnosis since detecting the disease early is crucial because it enables interventions when treatments may be more effective. This article intends to provide a comprehensive review of AD biomarkers, discussing their significance, classification, and recent developments in the field.

Phosphorylation of MAP 1A regulates hyperphosphorylation of Tau in Alzheimer's disease model

BACKGROUND AND PURPOSE

Hyperphosphorylation of Tau is one of the important pathological features of Alzheimer's disease (AD). Therefore, studying the mechanisms behind Tau hyperphosphorylation is crucial in exploring the pathogenesis of neurological damage in AD.

METHODS

In this study, after the establishment of rat models of AD, quantitative phosphoproteomics and proteomics were performed to identify proteins, showing that phosphorylation of microtubule associated protein 1A (MAP 1A) was lower in the model group. Western blot confirmed the changes of MAP 1A in the SD rats, APP/PS1 transgenic mice and cell AD models. To further study the molecular mechanism of recombinant MAP 1A phosphorylation affecting Tau phosphorylation, interfering siRNA-MAP 1A and protein immunoprecipitation reaction analysis were performed in AD cell models.

RESULTS

Cyclin-dependent kinase 5 (CDK5) showed reduced binding to MAP 1A and increased binding to Tau, resulting in a decrease in phosphorylated MAP 1A (p-MAP 1A) and an increase in phosphorylated Tau (p-Tau), and MAP 1A silencing promoted binding of CDK5-Tau and increased Tau phosphorylation, thereby reducing the cell survival rate.

CONCLUSIONS

In summary, we found that p-MAP 1A downregulation associated with p-Tau upregulation was due to their altered binding forces to CDK5, and MAP 1A could enhance autophosphorylation by competitive binding to CDK5 and antagonise Tau phosphorylation. This leads to neuronal protection and reducing tissue damage levels in AD, which can help better understand the mechanisms of AD pathogenesis.

Experimental evidence for temporal uncoupling of brain Aβ deposition and neurodegenerative sequelae

Brain Aβ deposition is a key early event in the pathogenesis of Alzheimer´s disease (AD), but the long presymptomatic phase and poor correlation between Aβ deposition and clinical symptoms remain puzzling. To elucidate the dependency of downstream pathologies on Aβ, we analyzed the trajectories of cerebral Aβ accumulation, Aβ seeding activity, and neurofilament light chain (NfL) in the CSF (a biomarker of neurodegeneration) in Aβ-precursor protein transgenic mice. We find that Aβ deposition increases linearly until it reaches an apparent plateau at a late age, while Aβ seeding activity increases more rapidly and reaches a plateau earlier, coinciding with the onset of a robust increase of CSF NfL. Short-term inhibition of Aβ generation in amyloid-laden mice reduced Aβ deposition and associated glial changes, but failed to reduce Aβ seeding activity, and CSF NfL continued to increase although at a slower pace. When short-term or long-term inhibition of Aβ generation was started at pre-amyloid stages, CSF NfL did not increase despite some Aβ deposition, microglial activation, and robust brain Aβ seeding activity. A dissociation of Aβ load and CSF NfL trajectories was also found in familial AD, consistent with the view that Aβ aggregation is not kinetically coupled to neurotoxicity. Rather, neurodegeneration starts when Aβ seeding activity is saturated and before Aβ deposition reaches critical (half-maximal) levels, a phenomenon reminiscent of the two pathogenic phases in prion disease.