RARE CASE OF ALZHEIMER’S TRANSMISSION: WHAT YOU NEED TO KNOW

Medically Reviewed by Dr. Sony Sherpa (MBBS) - February 09, 2024

Alzheimer's disease, a progressive neurodegenerative disorder, profoundly impacts individuals and society at large. The condition, characterized by memory loss, cognitive decline, and behavioral changes, significantly burdens the families of affected individuals as well as healthcare systems. Alzheimer's is becoming more common as the world's population ages, which makes it a serious public health concern.

Traditionally viewed as a non-transmissible condition, recent research suggests rare instances where transmission may occur, particularly through medical accidents. A recent study published in the journal Nature Medicine suggests a potential connection between early-onset dementia and a discontinued pituitary human growth hormone treatment used decades ago. The study, believed to be the first of its kind, investigates individuals who received the treatment as children who went on to exhibit symptoms of Alzheimer's disease at an unusually young age. Researchers suspect the link could be traced to the contaminated human growth hormone and eventual transmission of amyloid beta protein, a hallmark factor associated with Alzheimer's. However, despite this rare finding, it is important to note that the scientific community does not view Alzheimer’s as an infectious disease.

That said, these insights may prove valuable in preventing future cases and enhancing healthcare practices. It opens avenues for improved patient care and more informed medical procedures and, ultimately, contributes to the broader goal of tackling this debilitating disease.

Insights from Instances of Alzheimer's Disease Transmission

Recent studies have expanded our understanding of Alzheimer's disease (AD) transmission, emphasizing its potential in certain medical contexts.

Alzheimer’s Disease and Prion Diseases: A Pathological Connection

Alzheimer’s Disease, a primary form of dementia, is characterized by the accumulation of misfolded amyloid-β (Aβ) and tau proteins in the brain. This accumulation is central to the disease's pathology, including the formation of senile plaques and cerebral amyloid angiopathy (CAA), which are consistently present in Alzheimer’s patients.

On the other hand, prions are infectious agents composed wholly of a protein material that can fold in multiple, structurally distinct variations, at least one of which can be transmitted to other prion proteins, leading to a chain reaction of misfolding.

Prions cause a variety of neurodegenerative diseases, including CJD in humans, bovine spongiform encephalopathy (BSE, also called “mad cow disease”) in cattle, and scrapie in goats and sheep. The pathogenicity of prions is associated with their ability to induce abnormal folding of normal cellular prion proteins in the brain, resulting in brain damage and the distinctive signs and symptoms of prion diseases. However, despite significant advances in elucidating the prion-like behavior of these proteins, effective treatments for prion-induced NDs remain elusive. The quest for therapeutics is ongoing, highlighting the urgent need for further research to translate our growing understanding of prion biology into viable treatments for these debilitating diseases.

The similarities between AD and prion diseases like Creutzfeldt-Jakob disease (CJD), Gerstmann–Sträussler–Scheinker syndrome, and kuru are noteworthy, particularly regarding the propensity of Aβ to misfold, aggregate, and spread akin to prions. Research has demonstrated that misfolded forms of Aβ and tau proteins in Alzheimer's, as well as α-synuclein in conditions like PD and MSA, can induce pathology in a prion-like fashion, suggesting a common underlying mechanism across these diseases. In Alzheimer’s, amyloid beta proteins misfold (similarly to prion proteins in prion diseases) and aggregate in a way that causes them to become infectious, propagating the misfolding to other normally folded proteins, leading to progressive neurodegeneration. This mechanism underscores the potential for Alzheimer's to exhibit prion-like characteristics, especially in the context of disease transmission via medical procedures. This insight into the transmissible nature of protein aggregates within the brain has profound implications for understanding disease progression and developing therapeutic strategies.

How Tau Disrupts Brain Cells

Normally, tau helps stabilize microtubules in the axons of neurons, which are essential for normal cell function and nutrient transport. However, in Alzheimer's disease, tau proteins undergo abnormal chemical changes, causing them to detach from microtubules and stick to other tau molecules.

These tau proteins form tangles inside neurons, disrupting cell function and contributing to cell death. The process of tau protein accumulation and tangle formation interferes with the neuron's transport system, leading to the malfunction and eventual death of brain cells.

How Tau Tangles Grow

Tau tangles begin to form when tau proteins become hyperphosphorylated and accumulate inside neurons. Initially, these tangles form in areas of the brain responsible for memory, such as the entorhinal cortex and hippocampus.

Over time, as Alzheimer's disease progresses, the tangles spread to other regions of the brain, correlating with the spread of symptoms from memory loss to more widespread cognitive decline. The growth and spread of tau tangles are associated with the severity of dementia symptoms.

Role of Beta-Amyloid Protein and Amyloid Plaques in Alzheimer's

Beta-amyloid is another protein implicated in Alzheimer's disease. It is derived from the larger amyloid precursor protein (APP) through the action of enzymes. In Alzheimer's disease, beta-amyloid fragments abnormally accumulate to form plaques around neurons in the brain.

These amyloid plaques are a hallmark component of Alzheimer's and contribute to the disease process by disrupting cell function, triggering inflammation, and leading to neuronal death. The presence of amyloid plaques is often considered an early event in the disease process, which precedes and possibly facilitates the formation of tau tangles and the progression of Alzheimer's disease.

Transmission of Gut Microbiota Dysbiosis and Its Impact on Alzheimer's Disease Pathogenesis

Another study has highlighted the transmission of gut microbiota dysbiosis from Alzheimer's disease (AD) transgenic (Tg) mice to wild-type (WT) mice cohabiting with them, termed ADWT mice, leading to AD-associated pathogenesis and cognitive impairment in the latter. Butyric acid's impact on the acetylation-regulated phosphorylation of glycogen synthase kinase 3 beta (GSK3β) is proposed as a mechanism behind the cognitive impairment seen in ADWT mice, pointing to the alteration of GSK3β activity and subsequent Tau protein phosphorylation as a result of gut microbiota dysbiosis.

The findings parallel clinical observations where partners of AD patients also exhibited similar dysbiosis. This suggests a potential for the transmission of gut microbiota and related AD pathogenesis and cognitive impairment from AD individuals to those not affected by the disease.

The study underscores the significant role of gut microbiota (GMB) transmission in the development of AD pathogenesis, including the phosphorylation of Tau protein and accumulation of amyloid-beta (Aβ), facilitated by the decrease in butyric acid levels in both AD Tg and ADWT mice. The research also suggests the importance of gut-brain axis metabolites in modulating neurological health and disease progression.

Evidence of Iatrogenic (Medically-Acquired) Transmission

Intriguingly, a connection has been observed in patients who developed iatrogenic CJD (iCJD) following childhood treatment with human cadaveric pituitary-derived growth hormone (c-hGH) contaminated with both prions and Aβ.

This contamination is linked to the development of early-onset Alzheimer's in some patients. These findings suggest that Aβ seeds might propagate from person to person under specific circumstances. However, the study, led by Prof. John Collinge, notes that this form of transmission is highly unusual and does not occur through regular interactions or care.

In the 1980s, it was discovered that growth hormone could transmit Creutzfeldt-Jakob Disease (CJD), a rare and fatal degenerative brain disorder caused by prions. This realization led to the discontinuation of hormone therapies derived from cadaveric human sources and spurred the development and adoption of recombinant growth hormone, which is laboratory-produced and does not carry the risk of CJD.

Further research has demonstrated that injection of c-hGH containing Aβ seeds can accelerate Aβ plaques and CAA in transgenic mouse models of AD. Biochemical analysis of c-hGH revealed the presence of Aβ and tau proteins, with subsequent experiments confirming the seeding activity of these proteins in inducing AD-like pathology in mice. This evidence indicates the enduring nature of Aβ seeding activity and its potential for person-to-person transmission.

However, many aspects remain unclear, including the exact mechanisms of Aβ's prion-like activity and the long incubation period post-infection. Also, while tau proteins were present in c-hGH, their role in disease transmission is still being studied.

Animal model studies have further implicated the propagation and spread of Aβ proteins in AD's pathogenesis. These studies suggest that Aβ aggregates might behave similarly to prions, raising questions about potential transmission routes in humans.

Potential Transmission Through Medical Procedures

Investigating the possibility of AD transmission through other medical procedures, such as blood transfusions or neurosurgery, is an ongoing area of research. While some studies have not found an increased risk of AD from blood transfusions, the potential of surgical equipment to contribute to AD transmission cannot be overlooked.

The recognition of potential iatrogenic forms of AD underscores the importance of stringent protocols, decontamination measures, and thorough screening in medical practices to prevent disease transmission. As Aβ assemblies may exhibit structural diversity, understanding these mechanisms is critical for developing effective therapeutic strategies and ensuring medical safety.

Implications and Preventive Measures

The recent findings on Alzheimer's disease transmission have significant implications for healthcare practices. Firstly, they underscore the importance of effective decontamination of surgical instruments and the adherence to stringent protocols when handling human tissues. This is crucial in preventing the inadvertent transmission of diseases, including those potentially linked to misfolded proteins.

Moreover, there is a pressing need for ongoing research to deepen our understanding of the risks and mechanisms underlying Alzheimer's disease transmission. Such research is vital in developing more effective preventative strategies and treatments.

These results emphasize how important it is for the public and healthcare professionals to be more aware of the possible risks of transmission. Educating these groups about the findings and their implications can lead to more informed healthcare practices and a better understanding of Alzheimer's disease.

Conclusion

Rare instances of Alzheimer's disease transmission through medical accidents have been explored, highlighting the role of misfolded proteins such as amyloid-beta and their similarities to prion diseases. The importance of stringent healthcare protocols and decontamination measures to minimize transmission risks is emphasized, alongside the necessity for ongoing research into Alzheimer's transmission mechanisms. The discussion also stresses the significance of raising awareness about these potential risks among healthcare professionals and the general public. However, researchers concur that there is no cause for alarm regarding Alzheimer’s transmissibility since the pathway discovered by Prof. John Collinge et al. is extremely rare. The transmission of gut microbiota dysbiosis, a factor linked to AD, from those with Alzheimer’s to their partners is another aspect that has been studied. In a nutshell, the possibility of Alzheimer’s spreading from person to person represents an intriguing topic for further medical studies.

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