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  • Review Article
  • Published:

Genetic variants in Alzheimer disease — molecular and brain network approaches

Key Points

  • Genetic findings in late-onset Alzheimer disease (AD) have not yet resulted in strategies to prevent or treat AD

  • Examining the position of genes carrying disease-associated variants in large-scale molecular networks can aid identification of coherent disease mechanisms

  • Not all network approaches are equal: recent approaches involve networks that are directed (with causal links), and specific to the tissue and disease state

  • AD neuropathology and AD-associated genetic variants decrease efficiency of information transfer in the brain connectome, which can be quantified by measuring structural and functional patterns in brain networks

  • Construction of multiscale models of the effects of AD-associated genetic variants with large neuronal simulations is now feasible, and will be useful to understand the effects of such variants and to screen therapeutics in silico

Abstract

Genetic studies in late-onset Alzheimer disease (LOAD) are aimed at identifying core disease mechanisms and providing potential biomarkers and drug candidates to improve clinical care of AD. However, owing to the complexity of LOAD, including pathological heterogeneity and disease polygenicity, extraction of actionable guidance from LOAD genetics has been challenging. Past attempts to summarize the effects of LOAD-associated genetic variants have used pathway analysis and collections of small-scale experiments to hypothesize functional convergence across several variants. In this Review, we discuss how the study of molecular, cellular and brain networks provides additional information on the effects of LOAD-associated genetic variants. We then discuss emerging combinations of these omic data sets into multiscale models, which provide a more comprehensive representation of the effects of LOAD-associated genetic variants at multiple biophysical scales. Furthermore, we highlight the clinical potential of mechanistically coupling genetic variants and disease phenotypes with multiscale brain models.

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Figure 1: Effects of Alzheimer disease genetic variants on molecular networks and global brain topology.
Figure 2: Components of a multiscale model of the effects of genetic variants.

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Acknowledgements

This work was supported by NIH grants U01AG46152, R01AG36042, P30AG10161, RF1AG15819, R01AG17917, R01AG36836.

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Glossary

Genetic variants

Insertion, deletion or alternative coding of DNA

LOAD

Late-onset Alzheimer disease (LOAD) is the most common form of the neurodegenerative disease, typically diagnosed clinically after the age of 65 years and definitively diagnosed postmortem. LOAD is associated with functionally diverse, weak genetic variants

Multiscale models

Mathematical or conceptual models that couple processes that occur on a varying range of physical or temporal scales, which are typically studied in isolation from each other

Amyloid hypothesis

Proposal according to which the root cause of Alzheimer disease is the accumulation of amyloid-β (Aβ), with nuances around sufficiency and form of amyloid-β responsible for pathogenesis

Amyloid precursor protein

Amyloid precursor protein (APP) is cleaved to form amyloid-β peptides. Presenilin (PSEN1 and PSEN2) mutations promote the cleavage of APP into plaque-forming peptides

Apolipoprotein E

Apolipoprotein E is a protein that physically interacts with amyloid-β (Aβ) and tau; its interaction with Aβ influences plaque aggregation. Dosage of the ε4 allele of the APOE gene is the strongest genetic risk factor for late-onset Alzheimer disease

PPAR-γ

Peroxisome proliferator- activated receptor γ (PPAR-γ) is a component of a nuclear receptor complex that includes the retinoid X receptor. This complex is activated endogenously by fatty acids and leads to transcription of the apolipoprotein E gene, among other genes

Epistasis

When a combination of two or more genetic variants have a greater effect on a phenotype than their linear combination would predict

Pleiotropic effects

The contribution of a single gene, or variants of that gene, to two or more 'non-related' phenotypes

Directed networks

Networks where elements are connected by links with a specific direction that represents an asymmetric temporal or transfer function

Cognitive reserve

Tolerance and adaptation to neuropathology, in part attributed to genetic and lifestyle-associated factors such as education and social activity, and their neural correlates

Small-world organization

Network structure in which most nodes are connected by a small number of hops, yet form relatively isolated (modular) clusters

Clustering coefficient

Measure of modularity around a network node. The coefficient represents the number of connections among neighbours of a node divided by the maximum possible number of connections among those neighbours

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Gaiteri, C., Mostafavi, S., Honey, C. et al. Genetic variants in Alzheimer disease — molecular and brain network approaches. Nat Rev Neurol 12, 413–427 (2016). https://doi.org/10.1038/nrneurol.2016.84

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