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Alzheimer's disease

The mechanisms of the disease

Introduction to Alzheimer’s disease


Alzheimer’s disease is a neurodegenerative disease that results in cognitive decline and dementia with increasing age, environmental and genetic factors contributing to its onset. Scientists believe this is the result of protein biomarkers that build-up in the brain and accumulate within neurones. As of 2020, 55 million people suffer with dementia, with Alzheimer’s being a leading cause. Thus, it is crucial we develop efficacious treatments, with final adverse effects. A new drug called Iecanemab, may be the key to a new era of Alzheimer’s treatment…


The disease is most common in people over 65, with 1/14 affected in the UK, thus, there is a huge emphasis on defining the disorder and developing drug treatments.

The condition results in difficulty with memory, planning, decision making and can result in co-morbidities such as depression or personality change. This short article will explain the pathology of the disorder and the genetic predispositions for its onset. It will also explore future avenues for treatment, such as the drug Iecanemab that may provide, “a new era for Alzheimer’s disease”.

 

Pathology and molecular aspects


The neurodegeneration seen in Alzheimer’s has, as far, been associated protein dispositions in the brain, such as the amyloid precursor protein (APP) and Tau tangles. This has been deduced by PET scans and post-mortem study.

 

APP, located on chromosome 21, is responsible for synapse formation and signalling. It is cleaved to b-amyloid peptides by enzymes called secretases, but overexpression of both these factors can be neurotoxic (figure 1). The result is accumulation of protein aggregates called beta-amyloid plaques in neurons, impairing their survival. This deposition starts in the temporo-basal and front-medial areas of the brain and spreads to the neocortex and sensory-motor cortex. Thus, many pathways are affected, resulting in the characteristic cognitive decline.


Tau proteins support nerve cells structurally and can be phosphorylated at various regions, changing the interactions they have with surrounding cellular components. Hyperphosphorylation of these proteins result in the Tau pathology in the form of tau oligomer (short peptides) that is toxic to neurons. These enter the limbic regions and neocortex.


It is not clearly defined which protein aggregate proceeds the other, however, the amyloid cascade hypothesis suggests that b-amyloid plaque pathology comes first. It is speculated that b-amyloid accumulation leads to activation of the brain’s immune response, the microglial cells, which then promotes the hyperphosphorylation of Tau. Sometimes, there is a large release of pro-inflammatory cytokines, known as a cytokine storm, that promotes neuroinflammation. This is common amongst older individuals, due to a “worn-out” immune system, which may in part explain Alzheimer’s disease.

 

Genetic component to Alzheimer’s disease


There is strong evidence obtained through whole genome-sequencing studies (WGS), that suggests there is a genetic element to the disease. One gene is the Apoliprotein E (APOE) gene, responsible for b-amyloid clearance/metabolism. Some alleles of this gene show association with faulty clearance, leading to the characteristic b-amyloid build-up. In the body, proteins are made consistently depending on need, a dysregulation of the recycling process can be catastrophic for the cells involved.


PSEN1 gene that codes for the presenilin 1 protein, part of a secretase enzyme complex. As mentioned, the secretase enzyme is responsible for the cleavage of APP, the precursor for b-amyloid. Variants of this gene have been associated with early onset Alzheimer’s disease, due to APP processing being altered to produce a longer form of the b-amyloid plaque.

 

The genetic aspects to Alzheimer’s disease are not limited to these genes, and in actuality, one gene can have an assortment of mutation that results in a faulty protein. Understanding the genetic aspects, may provide avenue for gene therapy in the future.

 

Treatment


Understanding the point in which the “system goes wrong” is crucial for directing treatment. For example, we may use secretase inhibitors to reduce the rate of plaque formation. An example of this is the g- secretase BACE1 inhibitor. There is a need for this drug-type to be more selective to its target, as has been found to produce unwanted adverse effects.

 

A more selective approach may be to target the patient’s immune system with the use of monoclonal antibodies (mAb). This means designing an antibody that recognises a specific component, such as the b-amyloid plaque, so it may bind and then encourage immune cells to target the plaque (figure 3). An example is Aducanumab mAb, which targets b-amyloid as fibrils and oligomers. The Emerge study demonstrated a decrease in amyloid by the end of the 78-week study. As of June 2021, Aducanumab received approval from the FDA for prescription of this drug, but this is controversial as there are claims it brings no clinical benefit to the patient.


The future of Alzheimer’s disease


Of note, drug development and approval is a slow process, and there must be a funding source in order to carry out plans. Thus, particularly in Alzheimer’s, it is relevant to educate the public and funding bodies to supply the financial support to the process. However, with many hits (potential drug candidates), these often fail at phase III clinical trials.

 

Despite this, another mAb, lecanemab, has recently been approved by the FDA (2023), due to its ability to slow cognitive decline by 27% in early Alzheimer’s disease. The Clarity AD study on Iecanemab, found the drug benefited memory and thinking, but also allowed for better performance of daily tasks. This drug is currently being prescribed on a double-blind basis, meaning a patient may either receive the drug or the placebo. This study shows a hope for those suffering from the disease.

 

Drugs that have targeted the Tau tangles, have as far, not been successful in clinical trials. However, the future of Alzheimer’s treatment may be in the combination therapy directed to both Tau protein and b-amyloid. Washington universities neurology department have launched a trial known as Tau NextGen, in which participants will receive both Iecanemab and tau-reducing antibody.


Conclusion


This article provides a summary to what we know about Alzheimer’s disease and the potential treatments of the future. Overall, the future of Alzheimer’s treatment lies in the combination therapy to target known biomarkers of the disease.


By Holly Kitley

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