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Diabetic Phenotypes and Late-Life Dementia Risk: A Mechanism-specific Mendelian Randomization Study

Stefan Walter1, Jessica R Marden, Laura D Kubzansky

  • 1*Department of Epidemiology & Biostatistics, University of California, San Francisco, San Francisco ∥Department of Medicine, Division of General Medical Disciplines, Stanford University, Stanford, CA Departments of †Social and Behavioral Sciences §Nutrition, Harvard School of Public Health, Boston, MA ‡Department of Medicine, Division of General Internal Medicine, University of Washington, Seattle, WA.

Alzheimer Disease and Associated Disorders
|December 10, 2015
PubMed
Abstract

Insights

Type 2 diabetes (T2D) may not directly cause Alzheimer's disease (AD). However, impaired insulin sensitivity, a factor in T2D, appears to increase AD risk, suggesting a specific mechanism is involved.

Area of Science:

  • Genetics
  • Neuroscience
  • Metabolic Disorders

Background:

  • Previous Mendelian Randomization (MR) studies found no association between type 2 diabetes (T2D) and Alzheimer's disease (AD).
  • This study investigates potential underlying mechanisms linking T2D and AD using a refined MR approach.

Purpose of the Study:

  • To explore the surprising lack of association between T2D and AD reported in prior MR studies.
  • To investigate if specific T2D-related mechanisms, such as insulin sensitivity, impact AD risk.

Main Methods:

  • Employed inverse-variance weighted MR analysis using genetic data from large consortia (DIAGRAM, IGAP).
  • Examined associations between T2D and AD using 39 single nucleotide polymorphisms (SNPs).
  • Evaluated mechanism-specific genetic subscores (beta-cell function, insulin sensitivity, adiposity) and validated findings in the Health and Retirement Study.

Main Results:

  • The overall T2D polygenic score did not predict AD risk in the International Genomics of Alzheimer's Project (IGAP).
  • However, an insulin sensitivity polygenic score significantly predicted higher AD risk (OR=1.17, 95% CI: 1.02-1.34).
  • Replication in the Health and Retirement Study showed polygenic scores associated with T2D risk, but insulin sensitivity's link to cognitive phenotypes was not significant.

Conclusions:

  • Findings suggest insulin sensitivity, rather than T2D overall, may be a key factor influencing Alzheimer's disease risk.
  • This highlights the importance of investigating specific pathophysiological pathways in complex disease associations.

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