Viroids and Prions

A viroid is the smallest identified infectious agent composed simply of circular, single-stranded RNA without a protein coat. Like viruses, viroids are noncellular and require a host to replicate. A viroid's RNA is considered to be "naked." Although viroid RNA is single-stranded, extensive base pairing within the circular viroid can give it a double-stranded appearance. Viroids are known to infect only plants.

Viroids initially enter plant cells through wounds caused by physical damage or insects. Once inside, viroids can move between plant cells through plasmodesmata, cell structures that connect plant cells by penetrating cell walls. Viroid replication takes place in the host cell nucleus or chloroplast. A viroid cannot drive its replication within a plant cell in the same way a virus does because its RNA does not code for protein formation. Instead, the viroid relies on replication by host cell RNA polymerase enzymes. This process produces a string of viroid units forming an RNA molecule. The viroid molecule is capable of ribozyme activity, RNA enzyme function similar to proteins, that cuts or cleaves the RNA molecule into individual viroids.

Scientists are unclear on exactly how viroids cause plant diseases, but most of their effects are seen in altered development and hampered growth. One well-studied viroid that induces plant disease is the potato spindle tuber viroid. Another, the coconut cadang-cadang viroid, killed over 10 million coconut palms in the Philippines, causing tremendous economic harm.

A prion is a misfolded protein that causes the proteins around it to become misfolded as well. Like viruses and viroids, prions are noncellular infectious agents, but they are structurally different from both. Prions lack nucleic acids, the source of genetic replication of most pathogens, but consist solely of protein. The name prion arose as a shortened version of proteinaceous infectious particle.

Prions are known to cause neurological diseases in animals, including humans. A native form of the prion protein is found in healthy animal neurons, primarily in the brain. The pathogenic form of the prion protein can enter a host animal's brain, either by infection or through mutation of a protein-encoding gene, causing its original amino acid chain to fold incorrectly. Benign proteins consist of flexible coils or helices, whereas prions are stretched and flattened. Infectious prions induce the healthy prion proteins to refold into the pathogenic form. Other properties are also altered, as prions—unlike the native proteins—are insoluble and resistant to breakdown by proteases. These transformations create a chain reaction in which misfolded prions accumulate in neural cells. As the prions propagate, the aggregates can break apart, "seeding" additional transformation of benign proteins. The accumulations of prions trigger neurological symptoms as they destroy brain tissue and related nervous system tissue. Eventually, holes created in neurological tissues give it a spongelike pattern.

Prion diseases present through three mechanisms. Infectious prion disease is transmitted from one animal (including a human) to another. Sporadic prion disease results from the random misfolding of an individual's native protein. An inherited prion disease results from a genetic mutation. While prions have similarities to viruses, they are very different entities. Prions are the random misfolding of the host's normal proteins.

Examples of the several known prion-induced diseases are bovine spongiform encephalopathy (BSE or mad cow disease) in cattle, scrapie in sheep, and kuru and Creutzfeldt-Jakob disease in humans. All these diseases cause tremendous neurological degeneration and are eventually fatal. Disease monitoring in England in the mid-1990s revealed that BSE can be transmitted to humans through consumption of infected beef. These findings prompted an international health scare and a major blow to the European cattle industry.