Genome-wide association studies: is there a genotype for cognitive decline in older persons with type 2 diabetes?

There is a dramatic increase in the number of elderly persons on a worldwide scale with an increase in chronic comorbidities, especially type 2 diabetes (T2DM) and dementia. Although cognitive faculties commonly deteriorate in non-diabetic persons as they age, several studies have concluded that diabetes is uniquely associated with cognitive decline and is associated with a two-fold risk of Alzheimer's Disease (AD). Studies have also suggested that good glycemic has shown to improve cognitive status, however whether the use of specific anti-diabetic oral agents may play an additional role in controlling against cognitive deterioration is unknown. In addition, excitotoxicity from the overstimulation of glutamate receptors is considered a major cause of neuron death in AD and statins may be promising agents for protecting against memory loss. Possible pathophysiologic mechanisms common to both T2DM and AD are glucose toxicity and a direct effect of insulin on amyloid metabolism. In fact, AD and T2DM have comparable pathological features in the islet and brain (amyloid derived from amyloid β protein (β-amyloid) in the brain in AD and islet amyloid derived from islet amyloid polypeptide in the pancreas in T2DM). Evidence is growing linking precursors of amyloid deposition in the brain and pancreas to the pathogenesis of AD and T2DM, respectively. Indeed, the need to identify agents capable of correcting such pathological features may in turn significantly protect against the accumulation of β-amyloid in the brain, known to interfere with correct cognitive function. Cholesterol may also be directly involved in β-amyloid aggregation: abnormal oxidative metabolites such as cholesterol-derived aldehydes can modify β-amyloid, firstly promoting Schiff base formation, then accelerating the early stages of amyloidogenesis. At the moment, genome-wide association studies (GWAS) have begun to elucidate the genetic architecture of chronic diseases including, T2DM and AD. Thus, one of the challenges for a successful GWAS in the future will be to identify a genotype in older persons with T2DM for good drug response, which in turn may protect against cognitive decline and AD. The literature has suggested that the use of insulin sensitizers and statins is correlated with a lower rate of cognitive decline in older persons. In this paper, we will explore recent findings regarding diverse single nucleotide polymorphisms from GWAS on T2DM, AD and both. We will also shed light on future pathways, as the basis of improving drug and diagnostics development for a better integration of genetic studies for precise drug-development focusing on the role of genetic variation in maintaining metabolic control and cognitive performance.