The following points highlight the three main viral agents that cause various diseases. The viral agents are: 1. Viroids 2. Virusoids 3. Prions.
Viral Agent # 1. Viroids:
Viroids differ from viruses in having no coat enclosing the nucleic acid which in these agents is a small single stranded circular RNA. The naked RNA can infect many plants causing at least 20 diseases. Some of these diseases result in heavy economic loss.
Viroids are among the smallest pathogenic agents which can replicate within infected hosts with the help of host enzymes, because the viroid RNA does not code for any proteins. Replication takes place mainly in the nucleus of infected cells.
The naked RNA of viroids consists of 246 to 375 nucleotides and has a molecular weight of 1.1 to 1.3 x 106 Daltons. In comparison, the smallest viruses have a genome size of about 4,000 nucleotides. Although the viroid RNA is single-stranded and generally circular, it possess considerable amount secondary structure due to intra-strand base-pairing with intermittent loops produced by unpaired bases. This gives a more or less rod-like shape to the RNA resembling a double-stranded molecule with closed ends (Fig. 6.31).
In many viroids, the folded RNA molecule contains five domains having specific functions. The left and right terminal domains (Fig. 6.31) are involved in replication. The changes in the pathogenicity of the viroid are controlled by the pathogenicity domain located near the left terminal domain.
The central portion contains the conserved sequence, while the base sequences are variable in the variable domain. This correlation of structure and function is observed in most viroids. A change of few nucleotides in the pathogenicity domain may change a viroid from a mildly pathogenic to a highly virulent one.
It appears that the changes in the base sequences result in a structural change of the viroid and consequently in its shape. The pathogenicity of a viroid is in all probability related to its molecular shape. Under electron microscope, the viroid RNA measures about 40 nm in length and it has the thickness of double-stranded RNA.
The viroid RNA does not code for any protein. The RNA cannot function as messenger-RNA as shown in vitro protein synthesis system. The mechanism of their replication is not clearly understood. Evidences available suggest that the viroid is replicated by direct copying with the help of a host RNA dependent RNA polymerase.
The components building up new viroids are also obtained from the host cells. Replication of the viroid (+) RNA appears to take place by the rolling circle model in which the (+) strand acts as the template to produce concatemers of (-) strand. The linear (-) RNA then acts as template for synthesis of a concatemer of (+) RNA strand. The (+) strand so formed is cleaved into segments of the size of the viroid genome. These are circularized as individual copies of the viroid.
The suggested mechanism of viroid replication is schematically represented in Fig. 6.32.
An interesting speculation regarding the origin of viroids is that they evolved from introns which are segments of ribonucleic acid spliced out during formation of messenger RNA in eukaryotic systems. Such speculation is based on the similarity in base sequences of viroids and introns. There is also similarity in size and the circularity of the RNA molecules in both. The absence of any genes specifying polypeptides in both viroids and introns has also been suggested to support the speculation.
The mechanism of pathogenicity of viroids is also very little understood. In some host plants, viroid infection may remain latent without showing any damaging symptoms, although the viroid replicates in such infected hosts. The same viroid may cause severe disease in another host species.
It has been suggested that viroids might interfere with the gene regulation of the susceptible hosts which leads to the expression of the disease symptoms. It is obvious, however, that the metabolism of viroid- infected susceptible hosts is profoundly affected, though it is not clear how.
Evidence has been advanced to show that viroid RNA under in vitro system activates a protein kinase. The degree of activation depends on the virulence of the viroid strain. It has been suggested that the activated kinase may trigger metabolic changes leading to the development of disease symptoms.
Among the important diseases caused by viroids, are the potato spindle tuber, citrus exocortis, chrysanthemum stunt, chrysanthemum chlorotic mottle, cucumber pale fruit and cadang-cadang of coconut. The characteristics of the viroids, symptoms of the diseases, transmission and measures of control are briefly described below.
Potato spindle tuber disease is highly destructive and its viroid was the first to be recognized. The tuber yield is reduced by at least 25%. The viroid (PSTV) consists of an RNA strand with 359 nucleotides. The viroid remains effective after heating for 10 min at 75-80°C.
The viroid spreads through the entire body of infected plants. The plants are dwarfed and tubers become spindle-shaped with pointed ends bearing numerous eyes. The disease is transmitted mechanically through tools like knives and possibly also through pollen and seeds.
The only effective means of control is to use viroid- free seed-tubers for planting. Several strains of PSTV have been detected which vary greatly in pathogenicity. PSTV can also infect tomato without causing much harm.
Citrus exocortis viroid (CEV) can infect various species of Citrus, like oranges, mandarin, lemons, limes etc. Its RNA consists of 370-375 nucleotides (mostly 371). Infected plants are characterized by vertical strips of cracks on the stem; the bark becomes loose giving a scaly appearance. The yield is lowered by about 40%. The disease is spread through grafting equipment. It is also known to be spread through dodder. The viroid is highly resistant to heat and sterilizing chemicals except hypochlorite solution.
Chrysanthemum stunt viroid has 354 nucleotides and causes dwarfing of the plants with much reduced flower yield. Cadang-cadang disease of coconut plants is a very serious disease occurring in the Philippines killing more than a million palms every year. The disease develops slowly over a period of 8 to 15 years and the trees are infected after they reach the flowering stage.
The cadang-cadang viroid has 246 nucleotides. In later stages of infection, viroids which are longer in size having 296-297 nucleotides are present. These longer forms are produced by duplication of parts of the RNA. The viroid may spread mechanically through cutting instrument and possibly also through pollen. No measure of control has been developed.
Viral Agent # 2. Virusoids:
Like viroids, virusoids are also small circular single-stranded RNA. But they differ from viroids in their inability to replicate by themselves and in their requirement of a helper virus. Thus, whereas viroids occur independently in the infected cells, virusoids are always associated with a helper virus. Till now, only a few of such agents have been reported.
They are all associated with some or other plant diseases. They may occur in infected host cells either as free circular molecules or within the capsid of the helper virus. The helper viruses associated with viroids include velvet tobacco mottle virus, Solarium nodiflorum virus, Lucerne transient streak virus etc. The length of the virusoids RNA is more or less of the size of viroid RNA containing some 324 to 388 nucleotides.
Virusoids resemble the satellite viruses which are defective viruses unable to grow by themselves and require a helper virus for replication. One such satellite virus is associated with cucumber mosaic virus (Cucumovirus). Hepatitis D virus is another example which can replicate only in presence of hepatitis B virus causing a co-infection.
Although the RNA genome of hepatitis D virus contains the information for its nucleoprotein core, the outer capsid of this satellite virus is composed of hepatitis B surface antigen. During replication in the nucleus of the infected cell, circular genome is formed and is associated with hepatitis B surface antigen before release.
Viral Agent # 3. Prions:
Prions were discovered in 1982 by Stanley Prusiner as the causative agent of an infectious disease of sheep, known as scrapie. Prusiner was awarded the Nobel Prize in 1997 for his pioneering work on this new class of infectious agent. The most significant feature of prions is that they have been claimed to consist of only protein.
The name, prion was coined to describe these proteinacious infectious particles. Prions survive heat, radiation and chemical treatments that can normally inactivate viruses. They are susceptible to some protein hydrolyzing enzymes, but are resistant to nucleases which show that they do not contain any nucleic acid, either DNA or RNA.
This raises an important question: how can an infectious agent consisting of proteins direct its own replication, because a universal characteristic of all cellular organisms as well as viruses or even viroids is to store genetic information in nucleic acids, DNA or RNA. Prions are exceptions and their ability to replicate in infected hosts goes against the Central Dogma.
Research during the last decade has shown that the Prion protein (PrP) is a harmless normal constituent present on the surface on the neurons and glial cells of the brain in vertebrates and invertebrates. This protein is produced initially as a precursor protein containing 253 amino acid residues. Its role has been shown to bind copper ions and thereby helping the brain cells to resist the toxic effects of highly reactive oxygen radicals.
The enzyme, superoxide dismutase, which protects cell from oxygen radicals, requires copper for its activity. In absence of PrP, the cells cannot bind copper and the enzyme remains inactive. This leads to death of the cells due to oxygen toxicity. The normal PrP is encoded by host cell chromosomal genes. The normal protein was designated by Prusiner as PrPc and the abnormal scrapie-inducing proteins as PrPsc.
The abnormal protein has been shown to possess the same amino acid sequence as the normal one, but they differ in conformation. As a result of such conformational change, the abnormal protein shows difference in properties.
Whereas, the normal protein is soluble in some detergents and is destroyed by proteases, the abnormal protein is not. Moreover, the abnormal protein has mostly a pleated-sheet structure which is absent in normal protein. As a result, the abnormal protein tends to clump together forming aggregates within the infected cells, possibly blocking molecular traffic in the infected cells ultimately leading to their death.
When abnormal prion protein (PrPsc) from a diseased animal is introduced into a healthy individual containing the normal prion-protein (PrPc), the individual becomes diseased, though the incubation period is long. This means that PrPsc is infective. It has been suggested that the abnormal prion-protein binds to the normal protein forming a dimmer (PrPc – PrPsc) and alters the conformation of PrPc to PrPsc.
The newly formed PrPsc molecules can in turn interact with PrPc bringing about similar conformational change. These abnormal molecules accumulate in the brain cells, because they are protease resistant and produce the characteristic symptoms. The brain tissues acquires a sponge-like appearance with gradually increasing empty areas of dead tissues, producing what is known as spongiform encephalopathy.
Prions causing a specific disease, like scrapie, have been shown to possess strains which differ in virulence. Moreover, a particular strain can change in the degree of virulence. In other pathogenic agents, like bacteria and viruses, such changes are known to be due to mutation.
In case of prions too, such changes might be due to changes in the genes coding for the prion protein. Some scientists still believe that prions are really made up of a tiny nucleic acid enclosed in a prion protein molecule and they prefer calling prions as virinos. However, till now no nucleic acid, either DNA or RNA has been detected in prions and the existence of so called virino has not been proved.
Prions were discovered in search of the causative agent of scrapie of sheep. Scrapie is a neurological disorder which is infectious causing a loss of coordination of movement and the affected animals tend to scrape their bodies against walls or rocks. The disease is usually fatal. It was previously called a slow virus disease, but later was found to be caused by prions.
Another animal disease, commonly known as Mad-Cow Disease came into prominence in the Great Britain in 1990s. This was also attributed to an infection by prions. This disease, technically called Bovine Spongiform Encephalopathy (BSE), entered into the cattle by feeding them with protein supplements of sheep containing bones, meat, brains, spinal columns etc.
The sheep-products evidently were contaminated with prions of scrapie. The peak of the Mad-Cow Disease in the British Isles reached during 1992-93 when thousands of cattle heads were afflicted. This heavily affected the meat industry of that country. Not only that, a more serious effect was that the consumption of beef was suspected to cause a fatal human disease, known as Creutzfeldt-Jacob Disease (CJD) which is also a prion-inflicted disease resulting in the development of spongiform encephalopathy.
The disease requires a period of 7 to 8 years for full development of symptoms and is usually fatal. As such, it is a rare disease, but several cases were reported in Great Britain during the last decade and a possible link between the Mad-Cow Disease of cattle and CJD was suspected and officially admitted.
Apart from scrapie and Mad-Cow Disease, prion-infection also causes the mink encephalopathy. Mink, an animal valued for its excellent fur, contacts this prion infection also through feeding of meat of diseased animals. Mink encephalopathy is also a transmissible disease.
Prions are now known to cause several human neurological disorders. Among these are Creutzfeldt-Jacob disease, Gerstmann-Straussler syndrome, Fatal Familial Insomnia and Kuru. Except Kuru, the others are rare heritable diseases.
Creutzfeldt-Jacob disease, might be transmitted through ingestion of meat of diseased cattle, but is generally an inherited disease affecting middle-aged persons. It is thought to be associated with human chromosome 20. The afflicted persons develop symptoms of progressive dementia and generally die within one year.
Gerstmann-Straussler syndrome is also an inherited spongiform encephalopathy affecting middle- aged persons. It is thought to be controlled by an autosomal dominant gene which causes a replacement of proline to leucine in the PrP. The affected persons lose control of coordinated movement (ataxia) and mental disorder.
Fatal Familial Insomnia is a rare autosomal-dominant human disease caused by a mutation of the prion-protein gene. The disease appearing in middle-aged persons is characterized by progressively severe insomnia, ataxia and progressive degeneration of certain tissues of the thalamus of the brain — ultimately leading to death.
Kuru was an once prevalent disease found only in some cannibal tribals of Papua-New Guinea who used to practice the tribal ritual of eating the brains of dead kinsmen. Particularly—women and children used to take part in this ritual. With the abolition of cannibalism the disease is now extinct.