Learning Outcomes
After completing this lesson, students will be able to:
i. Define prions and viroids and distinguish them from traditional viruses.
ii. Explain the unique structure of prions, consisting solely of misfolded proteins.
iii. Describe the characteristics of viroids, the smallest known infectious agents composed of circular RNA.
iv. Discuss the diseases caused by prions, including Creutzfeldt-Jakob disease (CJD) and bovine spongiform encephalopathy (BSE).
v. Explain the transmission and pathogenesis of prion diseases.
vi. Understand the limited treatment options available for prion diseases.
vii. Describe the impact of viroid infections on plants, including potato tuber spindle dwarf disease and citrus exocortis disease.
viii. Explain the transmission and replication of viroids.
ix. Discuss the control measures implemented to manage viroid infections in agriculture.
i. Introduction: Beyond Viruses – Prions and Viroids
The world of infectious agents extends beyond traditional viruses, encompassing a diverse array of entities with unique structures and modes of action. Prions and viroids are two such unconventional pathogens that have challenged our understanding of infectious diseases.
ii. Prions: Infectious Proteins with a Misfolded Structure
Prions, first identified in the 1970s, are infectious agents composed solely of misfolded proteins. Unlike viruses, which contain genetic material (DNA or RNA), prions lack nucleic acid and rely solely on their abnormal protein structure to transmit their infectious potential. This unique characteristic sets prions apart from all other known pathogens.
iii. Diseases Caused by Prions: A Puzzle of Misfolding
Prion diseases, also known as transmissible spongiform encephalopathies (TSEs), are a group of neurodegenerative disorders characterized by the accumulation of abnormally folded proteins in the brain. These misfolded proteins, known as prions, lead to neuronal damage, progressive loss of brain function, and ultimately death.
Creutzfeldt-Jakob disease (CJD) is the most common human prion disease, typically affecting individuals in their 60s. Symptoms include progressive dementia, muscle coordination problems, and involuntary movements.
Bovine spongiform encephalopathy (BSE), also known as mad cow disease, is a prion disease that affects cattle. The consumption of BSE-infected meat was linked to the variant form of Creutzfeldt-Jakob disease (vCJD) in humans, highlighting the concern of prion transmission through food.
iv. Transmission and Pathogenesis of Prion Diseases: An Enigma Unfolding
The exact mechanisms of prion transmission and pathogenesis remain incompletely understood. Prions are believed to transmit by direct contact with infected tissues or fluids. Once introduced into the body, prions can interact with normal proteins and induce them to misfold, leading to a chain reaction of abnormal protein accumulation and subsequent neurodegeneration.
v. Treatment Options for Prion Diseases: A Grim Outlook
Currently, there are no effective treatments or cures for prion diseases. The misfolded proteins that characterize these diseases are highly resistant to degradation, making it challenging to target them with therapeutic agents. The focus of prion research lies in understanding the mechanisms of prion transmission and pathogenesis, with the hope of developing preventive measures and potential therapies in the future.
vi. Viroids: The Smallest Infectious Agents – A Circular RNA Enigma
Viroids, the smallest known infectious agents, are composed of circular single-stranded RNA molecules. Unlike viruses, viroids lack a protein coat and replicate independently within host cells. Their small size and unique structure have posed challenges in understanding their biology and mode of infection.
vii. Impact of Viroid Infections on Plants: A Threat to Agriculture
Viroid infections can cause significant damage to various plant species, leading to reduced crop yields and economic losses. Some notable examples include:
Potato tuber spindle dwarf viroid: This viroid infects potatoes, causing stunted growth, spindle-shaped tubers, and reduced yield.
Citrus exocortis viroid: This viroid infects citrus trees, causing stunted growth, gummosis (excessive gum production), and reduced fruit quality.
viii. Transmission and Replication of Viroids: A Hitchhiking Strategy
Viroids are typically transmitted through mechanical means, such as contaminated tools or grafting, or through pollen and seeds. Once inside a host cell, viroids hijack the cell's machinery to replicate and produce new viroid RNA molecules.
ix. Control Measures for Viroid Infections: Protecting Crops
Managing viroid infections in agriculture requires a combination of preventive measures, including:
- Using certified virus-free planting material
- Implementing strict sanitation practices to prevent mechanical transmission
- Developing viroid-resistant plant varieties
