Alzheimer’s Disease: The Role of Apolipoprotein E

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Alzheimer’s Disease: The Role of Apolipoprotein E

21 September 2019

Alzheimer’s Disease: The Role of Apolipoprotein E

Since 2012, September has been devoted to raising awareness of Alzheimer’s disease (AD) with Alzheimer’s Day on 21st September each year. Dementia is the medical name attributed to a set of symptoms affecting the brain, including: difficulties with problem solving, thinking, language and memory loss. AD is the most common form of dementia accounting for 60 – 80% of cases and it is believed that half of patients with Alzheimer’s dementia (dementia due to AD) have Alzheimer’s disease 1, 2.

About Alzheimer’s Disease (AD)

AD is one of the most devastating and complex diseases characterised by:

. Neurodegeneration resulting in memory loss 2

. Neurofibrillary tangles composed of tau amyloid fibrils which associates with synapse loss 2

. Accumulation of β-amyloid (Aβ) plaques 2

. Other cognitive functions 2

It is believed that AD is expected to begin 20 years prior to symptom onset, as the small changes in the functioning of the brain are unnoticeable to the person affected. Overtime, the symptoms progress and begin to interfere with the patient’s ability to perform everyday tasks. The final stages of AD leaves the patient bed-bound, requiring 24/7 care. Ultimately, AD is fatal. Age has been identified as a risk factor for AD with 10% of people over the age of 65 affected. Moreover, AD has been recognised as a leading cause of morbidity and the sixth leading cause of mortality, but the fifth leading cause of death in over 65’s in the US 3.

Figure 1: Alzheimer’s Disease Demographic, 2019 3

Physiological Significance of Apolipoprotein E

Apolipoprotein E (Apo E) is a lipoprotein composed of 299 amino acids with a molecular weight of 34kDa.  Apo E is responsible for the regulation of homeostasis through the mediation of lipid transport from and to bodily cells and tissues. Apo E comprises of three common isoforms: apo E2, apo E3 and apo E4. The apo E isoforms differ due to differences in either the 112 and 158 amino acids, whether either arginine (ARG) or cysteine (CYS) is present 4.

Apo E3 is the parent form of apo E and is responsible for the clearance of triglyceride-rich lipoproteins. Apo E3 is associated with normal lipid plasma concentrations. Apo E2 is the rarest of the apo E isoforms and differs slightly compared to the apo E3 isoform through the substitution of a single amino acid, ARG158Cys, located near the low-density lipoprotein receptor (LDLR) recognition site. Apo E2 displays impaired binding to the receptor, prohibiting the clearance of triglyceride-rich lipoprotein remnant particles. Apo E2 is strongly associated with type-III hyperlipoproteinemia. Apo E3 also differs from apo E4, again through the substitution of a single amino acid, Cys112Arg. The main difference between apo E3 and apo E4 is that apo E4 is unaffected by the binding of the isoform to LDLR. However, apo E4 is strongly associated with dyslipidemia 5. Fig. 2 provides a visual representation of the variations in the Apo E isoforms.

Figure 2: Variations in the Apo E Isoforms 4

Apo E is expressed in numerous bodily organs with the liver presenting with the highest expression followed by the brain. Astrocytes and, to a lesser extent, microglia are the major cells responsible for the expression of apo E in the brain. In the brain, apo E, apo J and apo A-1 are predominantly expressed on distinct high-density-like lipoprotein particles. Whilst apo A-1 is the major apolipoprotein of high-density lipoproteins (HDL), in the central nervous system (CNS), apo E is the predominant apolipoprotein of HDL-like lipoproteins. HDL-like lipoproteins are the only lipoproteins present in the CNS. It is believed that the cholesterol released from apo E supports synaptogenesis 6.

Clinical Significance of Apolipoprotein E in Alzheimer’s Disease

Whilst apo E3 is the most abundant of the three isoforms, apo E4 has been known for decades to be the most significant genetic risk factor for late-onset AD. Inheriting the one copy of the apo E4 gene increases the risk of AD 2-3-fold, whilst inheriting two copies increases the risk of AD up to 12-fold 7. Whilst the underlying mechanism of apo E’s contribution to AD risk is still unclear and debatable, apo E has been identified as promoting amyloid β (Aβ) deposition and clearance as well as neurofibrillary tangles in the brain. Interestingly, Aβ-independent pathways exist for apo E in AD, which led to the unearthing of the new roles of apo E including the most recent, iron metabolism and mitochondria dysfunction 8, 9. Captivatingly, sex-related hormones may play a role in AD in apo E4 carriers as AD has been recognised to be more pronounced in women 10. Apo E4 has also been identified as impairing lipid transport, microglial responsiveness, glucose metabolism, synaptic plasticity and integrity, and cerebrovascular function and integrity. Some of these pathogeneses are independent of Aβ pathways. Furthermore, therapeutic strategies are aiming to modulate the quantity, lipidation, structural properties, Aβ interaction and receptor expression of Apo E 11.

Key Features of the Randox Apolipoprotein E Assay

Randox are one of the only manufacturers to offer the apo E assay in an automated clinical chemistry format. Utilising the immunoturbidimetric method, the Randox apo E assay is available in a liquid ready-to-use format. Not only does the Randox apo E suffer from limited interferences from bilirubin, haemoglobin, intralipid® and triglycerides for truly accurate results, it has an excellent measuring range of 1.04 – 12.3mg/dl for the comfortable detection of clinically important results. Moreover, apolipoprotein calibrator and controls are available for a complete testing package. Applications are available detailing instrument-specific settings for the convenient use of the Randox apo E assay on a wide range of clinical chemistry analysers.

Biochip Technology – Alzheimer’s Array

Utilising the Biochip Technology, Randox have developed an array to identify the risk of Alzheimer’s disease in just 3 hours with one effective test. In addition to a rapid and accurate diagnosis, this also introduces both cost and time-saving benefits. The apo E4 array is a research use only product developed for the Evidence Investigator, a semi-automated benchtop immunoassay analyser which can process up to 2376 test per hour as well as up to 44 analytes screened per biochip. The apo E4 array measures both total apo E protein levels and apo E4 protein levels directly from plasma samples as well as using a ratio, it can classify patients as negative or positive for apo E4. In turn, we can then assess their risk for the development of Alzheimer’s disease.

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Rare Disease Day: 28th February 2019

28th February 2019

Rare Disease Day: 28th February 2019

Rare Disease Day raises awareness of rare diseases and how patients’ lives are affected. Many rare diseases remain incurable and many go undiagnosed. 1 in 20 people will live with a rare disease at some point in their life and this is why it is so important to raise awareness.1

What is a rare disease?

There is no single definition for a rare disease, as many countries identify them differently. In the United States, the Rare Diseases Act of 2002 defines a rare disease by its prevalence: “any disease or condition that affects fewer than 200,000 people in the United States”. However, the EU defines a rare disease as a condition that affects less than 5 in 10,000 of the population. There are approximately 7000 rare diseases and disorders and 50% of people affected by rare diseases are children.2,3

Hyperlipoproteinemia type III

This rare disease day, Randox will be raising awareness of hyperlipoproteinemia type III.  Hyperlipoproteinemia type III, also known as dysbetalipoproteinemia or broad beta disease, is a rare genetic disorder characterised by improper breakdown of lipids, specifically cholesterol and triglycerides.  The condition is caused by mutations in the Apo-E gene, however the inheritance of this condition is complicated due to the development of symptoms having to be triggered by a secondary factor to raise lipid levels. These factors include diabetes, obesity or hypothyroidism.

It is unknown exactly what the prevalence of the condition is, but it is estimated to affect approximately 1 in 5,000 – 10,000 of the general population and it has been found that it affects males more often than females, with women rarely being affected until after menopause.4,5

Figure A. Example of cholesterol and lipid build-up [6] 

Symptoms

Symptoms for hyperlipoproteinemia type III will vary for each individual and some people may even be asymptomatic. The most common symptom is the development of xanthomas which are deposits of fatty material, the lipids, in the skin and underlying tissue. Xanthomas may appear on the palms of the hands, eyelids, soles of the feet or on the tendons of the knees and elbows.

> Chest pain or other signs of coronary artery disease

> Cramps in the calves when walking

> Sores on toes

> Stroke-like symptoms such as trouble speaking, dropping on one side of the face, weakness in an arm or a leg and a loss of balance6

Complications can arise if the condition is left untreated and these can include: myocardial infarction, ischemic stroke, peripheral vascular disease, intermittent claudication and gangrene of the lower extremities.7

Diagnosis

Although there is no specific diagnostic test for hyperlipoproteinemia type III, diagnosis is based on clinical evaluation and identification of symptoms. Research has indicated that an algorithm comprising a number of dysbetalipoproteinemia indices may be helpful in the diagnosis of the disease.  These include:

> Low apolipoprotein B to total cholesterol ratio

> Elevated levels of triglycerides

> Elevated levels of total cholesterol8

Managing the condition

The condition cannot be cured but treatment is to control conditions such as obesity, hypothyroidism and diabetes. Most patients will go through dietary therapy to control their intake of cholesterol and saturated fat. This prevents xanthomas, high levels of lipids in the blood, exercise will also help to lower lipid levels. However, dietary changes may not be effective for some individuals and this is where drugs may be used to lower lipid levels instead.

How Randox can Help

Randox offer a range of routine and niche assays within the lipid testing panel to monitor lipid levels and to identify associated complications.  Some of these tests include:

Apolipoprotein B

The Randox Apolipoprotein B tests utilises an immunoturbidimetric method, offers a wide measuring range and is available liquid ready-to-use for convenience and ease of use.

Learn more about the Randox Apolipoprotein B Test

Total Cholesterol

The Randox Total Cholesterol test utilises the CHOD-PAP method and offers an extensive measuring range with a wide range of kits available to suit a wide range of laboratory sizes.

Learn more about the Randox Total Cholesterol test

Triglycerides

The Randox Triglycerides test utilises the GPO-PAP method while offering an extensive measuring range with both liquid and lyophilised formats available offering choice and flexibility.

Want to know more?

Contact us or download our Cardiology and Lipid Testing brochure to learn more.




Related Products

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Resource Hub

Lipid Panel Page

  • References

    [1] Rare Disease Day. What is Rare Disease Day? Rare Disease Day. [Online] 2019. [Cited: February 21, 2019.] https://www.rarediseaseday.org/article/what-is-rare-disease-day

    [2] Genetic Alliance UK. What is a Rare Disease? Rare Disease UK. [Online] 2018. [Cited: February 21, 2019.] https://www.raredisease.org.uk/what-is-a-rare-disease/

    [3] NZORD. Rare Disease Facts and Figures. NZORD. [Online] 2019. [Cited: February 21, 2019.] https://www.nzord.org.nz/helpful-information/rare-disease-facts-and-figures.

    [4] NORD. Hyperlipoproteinemia Type III. NORD. [Online] 2019. [Cited: February 21, 2019.] https://rarediseases.org/rare-diseases/hyperlipoproteinemia-type-iii/

    [5] GARD. Hyperlipidemia Type 3. National Centre for Advanciing Translational Sciences. [Online] December 29, 2016. [Cited: February 21, 2019.] https://rarediseases.info.nih.gov/diseases/6703/hyperlipidemia-type-3

    [6] Falck, Suzanne. Everything you need to know about hyperlipidemia. Medical News Today. [Online] December 21, 2017. [Cited: February 21, 2019.] https://www.medicalnewstoday.com/articles/295385.php

    [7] Medline Plus. Familial Dysbetalipoproteinemia. Medline Plus. [Online] May 16, 2018. [Cited: February 21, 2019.] https://medlineplus.gov/ency/article/000402.htm.

    [8] Dysbetalipoproteinemia: Two cases report and a diagnostic algorithm. Kei, Anastazia, et al. 4, s.l. : World Journal of Clinical Cases, 2015, Vol. 3.


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