Depression (MDD) is prodromal to, and a component of, Alzheimer’s disease (AD): it may also be a trigger for incipient AD.
MDD is not a unitary disorder, so there may be particular subtypes of early life MDD that pose independent high risks for
later AD, though the identification of these subtypes is problematical. There may either be a common pathological event underlying
both MDD and AD, or MDD may sensitize the brain to a second event (‘hit’) that precipitates AD. MDD may also accelerate brain
ageing, including altered DNA methylation, increased cortisol but decreasing DHEA and thus the risk for AD. So far, genes
predicting AD (e.g. APOEε4) are not risk factors for MDD, and those implicated in MDD (e.g. SLC6A4) are not risks for AD,
so a common genetic predisposition looks unlikely. There is as yet no strong indication that an epigenetic event occurs during
some forms of MDD that predisposes to later AD, though the evidence is limited.
Glucocorticoids (GCs) are disturbed
in some cases of MDD and in AD. GCs have marked degenerative actions on the hippocampus, a site of early β-amyloid deposition,
and rare genetic variants of GC-regulating enzymes (e.g. 11β-HSD) predispose to AD. GCs also inhibit hippocampal neurogenesis
and plasticity, and thus episodic memory, a core symptom of AD. Disordered GCs in MDD may inhibit neurogenesis, but the contribution
of diminished neurogenesis to the onset or progression of AD is still debated. GCs and cytokines also reduce BDNF, implicated
in both MDD and AD and hippocampal neurogenesis, reinforcing the notion that those cases of MDD with disordered GCs may be
a risk for AD. Cytokines, including IL1β, IL6 and TNFα, are increased in the blood in some cases of MDD. They also reduce
hippocampal neurogenesis, and increased cytokines are a known risk for later AD.
Inflammatory changes occur in both
MDD and AD (e.g. raised CRP, TNFα). Both cytokines and GCs can have pro-inflammatory actions in the brain. Inflammation (e.g.
microglial activation) may be a common link, but this has not been systematically investigated. We lack substantial, rigorous
and comprehensive follow-up studies to better identify possible subtypes of MDD that may represent a major predictor for later
AD. This would enable specific interventions during critical episodes of these subtypes of MDD that should reduce this substantial