new research suggests.
The causal association between disturbed sleep and Alzheimer’s disease that has been observed in previous studies may have resulted from reverse causation, the researchers noted. The current Mendelian randomization analysis also failed to find a causal relationship between Alzheimer’s disease and major depressive disorder. Future studies should examine the genetic heterogeneity of depression syndromes to test for causal relationships between subtypes of depression with distinct causes and Alzheimer’s disease.
Mendelian randomization compares individuals who have different genetic profiles for a given exposure. “Given that genetic variants are inherited at random, these two groups are comparable, and any differences are not likely to be due to other associated factors,” such as confounding bias, said corresponding author Abbas Dehghan, PhD, reader in cardiometabolic disease epidemiology at Imperial College London. “Moreover, given that genetic information is constant over the lifetime, the chances for reverse causation are small.”
The findings were published online August 19 in Neurology.
Causal questions
Many patients with late-life neurodegenerative disorders such as Alzheimer’s disease have comorbid depression, but whether these two disorders have a causal relationship or common risk factors has been unclear, the investigators noted. Abnormal sleep patterns are symptoms of both depression and Alzheimer’s disease. Abnormal sleep is also associated with cognitive decline and anxiety.
The researchers hypothesized that sleep causally affects major depressive disorder and Alzheimer’s disease but that there is no causal relationship between major depressive disorder and Alzheimer’s disease. They conducted a bidirectional, two-sample Mendelian randomization study to test these hypotheses.
The investigators conducted genomewide association studies (GWASs) using data from the prospective, population-based U.K. Biobank. Sleep phenotypes were measured by self-report or accelerometer. Genetic associations were derived from 403,195 patients for chronotype, 237,627 patients for insomnia, 446,118 people for sleep duration, and 85,670 people for accelerometer-derived phenotypes.
Two binary variables from sleep duration were derived: short sleep (duration of less than 7 hours) and long sleep (duration of 9 or more hours). A sleep episode was defined as a period of at least 5 minutes with a change on the dorsal-ventral axis of less than 5 degrees. The durations of all sleep episodes were added to calculate total sleep duration.
Major depressive disorder was diagnosed clinically in accordance with DSM-IV criteria. Genetic associations were derived from 9,240 case patients and 9,519 control participants. Alzheimer’s disease was diagnosed on the basis of physician examination or autopsy results. Genetic associations were obtained from a meta-analysis of GWAS on participants of European ancestry in the International Genomics of Alzheimer’s Project, which included 21,982 case patients and 41,944 control participants.
More risk factor research needed
Results showed no causal relationships between sleep-related phenotypes and major depressive disorder in either direction. Causal relationships between major depressive disorder and Alzheimer’s disease were found in both directions, but neither was statistically significant.
A genetically higher risk for Alzheimer’s disease was associated with being a “morning person,” being at decreased risk for insomnia, having shorter sleep duration on self-report and accelerometer, having decreased likelihood of reporting long sleep, having an earlier timing of the least active 5 hours, and having a smaller number of sleep episodes. However, no analysis supported a causal effect of sleep-related phenotypes on risk for Alzheimer’s disease.
Because APOE4 can influence disease processes that may contribute to Alzheimer’s disease risk, the investigators also conducted a sensitivity analysis that excluded APOE single-nucleotide polymorphisms. In this analysis, the causal associations of Alzheimer’s disease with self-reported and accelerometer-based sleep duration were not significant. The sensitivity analysis did support the other causal associations between Alzheimer’s disease and sleep phenotypes, however.
The causal associations between major depressive disorder and Alzheimer’s disease observed in other studies may have been the result of confounding, and the participants may have had other associated characteristics that put them at risk for the disease, said Dr. Dehghan. Furthermore, the previous studies considered various sleep phenotypes together, whereas in the current study, the investigators examined them separately.
The results suggest that preclinical and clinical Alzheimer’s disease may affect sleep phenotypes differently. Sleep management thus could be an important approach to improving quality of life for patients with Alzheimer’s disease, the researchers wrote.
“Our study indicates that depression and sleep disorders are not likely to be a causal factor for Alzheimer’s disease,” Dr. Dehghan said. “We need to search for other risk factors for the prevention of Alzheimer’s disease.”