Perioperative Research Into Memory GENomics in the Intensive Therapy Unit: Alzheimer's (PRIMoGenITA): The Genomics of Cognitive Impairment After Admission to a Burns Intensive Care Unit

Long-lasting post-operative cognitive dysfunction (POCD) is now a well-recognised complication of surgery, anaesthesia and critical care. It has been shown that cognitive impairment in critical care survivors can be worse than cohorts diagnosed with Mild Cognitive Impairment and approaching those suffered moderate Traumatic Brain Injury. POCD most significantly impacts upon the elderly, comorbid and those with pre-existing cognitive impairment, and has a socially and psychologically significant impact on quality of life.

The current central dogma suggests an underlying neuroinflammatory dysregulation affecting neuronal function. However, the underlying pathological processes have not yet been fully elucidated. Although some risk factors have been recognised we cannot accurately predict outcome, suggesting other unidentified determinants. The discovery of further risk factors would allow new insights into the underlying mechanism, and open up future avenues of investigation and potential interventions.

Thus far, little work has been performed looking into the impact of genomics on cognitive outcome after critical care. There are an estimated 20,000 protein coding genes in the human genome. Such large numbers of whole exome sequence variables necessitate using either an extremely large population or testing a small number of candidate genes. Critical Care research is hampered by small population sizes, ruling out whole genome analysis. The ideal candidate gene would be a known modulator of neuroinflammation, synthesised in the central nervous system (or able to traverse the blood-brain barrier) and have a known association with cognitive impairment. In terms of feasibility it should have relatively high incidence allele proportions, and be modifiable for in order to test any association for causality.

The most common gene associated with cognitive impairment is APOE, which codes for apolipoprotein E (ApoE). There are three common alleles of the APOE gene: APOE-∈2, APOE-∈3 and APOE-∈4. Possession of an APOE-∈4 allele increases the risk of developing late-onset Alzheimer's Disease. ApoE demonstrates immunomodulatory properties both systemically and in the central nervous system. It is secreted by astrocytes and oligodendrocytes after brain injury and is implicated in the cholesterol distribution necessary for membrane growth after axonal injury. ApoE-deficient murine glial cultures express more nitric oxide (NO) and Tissue Necrosis Factor α (TNFα) after simulated injury (LPS / closed head injury) than wild-type cells. Incubating the deficient glial cultures in recombinant apoE suppresses the TNF-α expression. This suppression occurred to a much greater extent with recombinant apoE-∈3 and apoE-∈2 than with apoE-∈4. Finally, microglia treated with secreted amyloid precursor protein α (sAPP-α, a secreted derivative of β amyloid precursor protein (β-APP)) will produce higher levels of activation markers and enhance the production of neurotoxins. This effect can be blocked by prior incubation apoE3, but not with apoE4. Administration of peripheral ApoE mimetic peptide (a truncated protein able to traverse the blood brain barrier) suppresses systemic and brain Il-6 and TNFα levels. Lastly, ApoE isoforms also have different affects on cholinergic neuron destruction, through increased synthesis and reduced elimination of amyloid beta.

Critical care patients typically experience systemic inflammation secondary to sepsis, surgery or trauma, or a combination of the three. Central nervous system (CNS) ApoE mRNA expression is upregulated in wild-type mice exposed to repeated restraint stress. As such, it is possible that APOE genotype may influence cognitive outcome through mediation of the neuroinflammatory response. The APOE-∈4 allele is known to increase the risk of POD after surgery and to increase the duration of delirium in the intensive care unit (ITU). The presence and duration of post-operative delirium (POD) are recognised predictors of long-term cognitive impairment (LTCI). Neither the incidence of delirium nor LTCI according to APOE genotype have been investigated in critical care patients.