OBJECTIVE: To develop an understanding of the basic concepts of virology by examining the nature, structure and classification of viruses.

I. ACELLULAR INFECTIOUS AGENTS

A. Prions -

"Small proteinaceous infectious particles which resist inactivation by procedures that modify nucleic acids" (Stanley B. Prusiner)

Diseases - degenerative disease of CNS; Kuru (human, infectious), Creutzfeldt-Jakob disease (human, sporadic and familial), Gerstmann- Straüssler Syndrome (human, genetic and familial), Fatal Familial Insomnia (human), Scrapie (animal model, sheep and goats), TME (transmissible mink encephalopathy; mink), CWD (chronic wasting disease, mule deer, elk), BSE (bovine spongiform encephalopathy, “MAD COW DISEASE, cows).

HUMAN

CJD, GSS and FFI are diseases that are both inherited and infectious; new animals models are speeding studies - trisomy 16 (mouse Down syndrome) 17 days from 153 days.

Kuru (shivering or trembling) - identified 1957; Fore tribe of cannibals in New Guinea; ceremonial ritual for children and young women; degeneration of cerebellum manifested by ataxia, disturbed balance, clumsy gait, death within year of onset; transmittable to chimps, monkeys, minks and ferrets. The tribe called it the "laughing death."

CJD - fatal presenile dementia of midadult (>50) life NOT geographically restricted; 1 per 106 in US and Europe; 100 times more frequent in Libyan Jew; epidemiology ?; has been transmitted by corneal transplant; nonrecombinant hGH; electroencephalographic electrodes sterilized only with 70% ethanol and formaldehyde vapor; original transmission - poorly cooked sheep brains or eyeballs. CJD typically occurs a decade later than GSS and has cerebral involvement so dementia is more common and patient seldom survives a year.

GSS - occurs at about 2% of the rate of CJD. GSS is distinct from CJD, it occurs typically in the 4th-5th decade, characterized by cerebellar ataxia and concomitant motor problems, dementia less common and disease course lasts several years to death.

Emerging Human Diseases

FFI - pathology is characterized by severe selective atrophy of the thalamus.

Alpers syndrome - is the name given to prion diseases in infants.

SIMILAR BUT UNRELATED HUMAN DISEASES

Alzheimer's Disease - similarities in neuropathologies to CJD including amyloid plaques, however amyloid material is different; no transmission yet; a recent study indicates that there is a similar glial association with plague formation in CJD and Alz. but amyloid centers are different

ANIMAL MODELS

Scrapie - Animals lose coordination becoming so incapacitated that they cannot stand. They also become irritable and, in some cases, develop an intense itch that leads them to scrape off their wool or hair (hence the name “scrapie"); recessive trait in sheep; experimental transmission - sheep 0 to 80%, goats 100%, mice and hamsters reduced incubation time; high titer recoverable for brain, spinal cord, and eye (same locations as pathology)

MAD COW DISEASE - Clinical Signs

Cattle affected by BSE experience a progressive degeneration of the nervous system. Affected animals may display changes in temperament, such as nervousness or aggression; abnormal posture; incoordination and difficulty in rising; decreased milk production; or loss of body weight despite continued appetite. Affected cattle die.

The incubation period (the time from when an animal becomes infected until it first shows disease signs) is from 2 to 8 years. Following the onset of clinical signs, the animal's condition deteriorates until it either dies or is destroyed. This usually takes from 2 weeks to 6 months. Most cases in England have occurred in dairy cows between 3 and 5 years of age.

Epidemiology There are different scientific hypotheses concerning the origins of BSE. The epidemiologic data suggests that BSE in England may have been caused by feeding cattle rendered protein produced from the carcasses of scrapie-infected sheep. An alternate theory is that BSE had existed in undetectable levels in the British cattle population prior to 1986. The practice of using products such as meat and bone meal in cattle rations as a source of protein has been common for several decades. Scrapie has a long incubation period--up to 60 months--and has been endemic in Great Britain for centuries. Changes in rendering operations in the early 1980's-- particularly the removal of a solvent-extraction process that included a steam-heat treatment --may have played a part in the appearance of the disease and the large number of cases that developed.

DISEASE AGENT

size of prions - <5nm

composition - ONLY protein (glycoprotein with Asn-linked oligosaccharides); SAF - scrapie-associated fibrils; MW ~27-30; PrPSc (scrapie form); PrPC (cellular form); gene mapped to short arm of chr 20, two exons + 1 intron, with single start site; with GSS - several different types of mutations have been identified; with familial CJD - a 144-bp insertion in the open reading frame has been identified; .

Theories at one time included - 1) prion is a "reverse translatase" that goes from protein to a RNA molecule; or 2) prion is a regulatory protein that is not normally produced, and when present it activates a gene in the host that encodes for the prion; or conventional virus with extremely small genome; or small RNA (noncoding) that binds PrPC with high affinity.

Current Theory is that this protein is found predominantly on the surface of neurones attached by a glycoinositol phospholipid anchor, and is protease sensitive. Thought to be involved in synaptic function. The mutations in the gene allow the protein to occasionally “miss fold”. When this happens it causes other normal proteins to unfold and then refold the wrong way.

B. Viroids - low-molecular weight nucleic acids (250-400 nucleotides) that are devoid of proteins. They have the properties of infectious agents. They are resistant to inactivation by UV and ionizing radiation. Replication is difficult to imagine (cells do not have an RNA replicase and viroids are too small to encode one.

size - <5nm

COMPOSITION - ssRNA only ("double-stranded"? extensive looping back with many unpaired bubbles)

HOSTS - plants only; single-stranded RNA of the hepatitis delta virus shows a strong degree of nucleotide homology with the potato spindle tuber viroids.

C. Viruses - are infectious agents; composed of both protein and nucleic acids; obligate intracellular parasites that utilize the host cell's synthetic and energy-yielding apparatus; 1898 Foot-Mouth disease. size - 20-200 nm

composition - nucleic acid and protein

hosts - bacteria to humans

diseases - benign to acute

II. VIRAL STRUCTURE

A. Nomenclature

virion - is the complete particle; the term denotes both intactness of structure and the property of infectivity

protomer - individual proteins making up capsomer

capsomer - are morphologic subunits of viral proteins that make up the viral capsid. They are composed of only a few types of proteins

capsid - is the protein molecules surrounding the viral genome in the form of a layer or shell

nucleocapsid - is the structure consisting of the shell plus the nucleic acid.

envelopes - are membranes of cellular origin that surround some viruses.

B. Basic structure: nucleic acid protected by protein layer or shell

genome - may be either DNA or RNA, double stranded (ds) or single stranded (ss), single stranded may be either positive or negative stranded, linear or circular, single segment (monopartite) or multi segment (multipartite), size may be between 3,000 and 300000 bases. Some viral genomes are infectious (picornavirus, papovavirus, caliciviruses, togaviruses, adenoviruses, herpesviruses). The criteria for infectivity - either the genome can act as mRNA (+ stranded) directly or it can be transcribed by host-encoded polymerases. If they require virus-encoded polymerases they are not infectious. GENERALLY VIRUSES ARE HAPLOID (EXCEPTION - RETROVIRUSES). SOME PLANT VIRUSES CONTAIN ONLY A PORTION OF THE GENOME AND SEVERAL VIRIONS ARE NEEDED TO PROVIDE THE WHOLE GENOME (I.E. ALL OF THE PARTS OF THE GENOME MUST BE IN THE SAME CELL TO CAUSE AND INFECTION.)

capsid - The capsid is composed of numerous repeating subunits (identical or belonging to only a few different species) that are arranged in a defined pattern. The simplest subunit is a single protein, sometimes referred to as a protomer. The capsomer is a more complex subunit composed of several identical or a few different protein molecules. The building blocks of the capsid are believed to composed of a limited number of proteins because it limits the amount of genetic information required and because this leads to efficient assembly (capsid proteins have a strong tendency to bind together). The capsid functions as protection and in some cases to introduce the viral genomes into host cells by adsorbing to the cell surface.

nucleocapsid - consists of nucleic acid and capsid proteins. There are two basic patterns that have been well studied.

Helical symmetry - the nucleic acid occurs as an extended filament with the protein surrounding it in a helical fashion. Disk forms first with 2 units, then associates with NA. Finally helix forms.

Icosahedral symmetry - the nucleic acid is condensed and the capsid forms a shell around the nucleic acid. The capsomers are arranged in a very precise structure such that when the capsid is turned you can seen the following axises: 5:3:2. Empty capsids are possible.

envelope - are composed of a lipid bilayer that is derived form the cell's plasma membrane or nuclear membrane. The envelope contains only virus specified proteins and glycoproteins (herpesviruses and retroviruses still contain some host-coded proteins). The shape of the virus appears to be roughly spherical, but the shape is not rigid and therefore is referred to as pleomorphic. With enveloped helical viruses the nucleocapsid is coiled within the envelope.

Complex virions - the poxvirus does not conform to one of the above shapes instead it has several coats around the nucleic acid. Some Bacteriophages are also complex having additional structures.

III. CLASSIFICATION

A. International Committee on Nomenclature of Viruses (1966)

Originally proposed by David Baltimore it is frequently referred to the Baltimore Classification System

ds DNA, enveloped

Poxviridae - brick shaped complex particles that contain a monopartite linear genome surrounded by an envelope. Ex. Variola major causes small pox

Iridoviridae - icosahedral capsid, not an important human pathogen (at this time). The sole mammalian virus is enveloped.

Herpesviridae - linear genome. Several human pathogens including Varicella zoster and Herpes simplex

ds DNA, nonenveloped

Adenoviridae - linear DNA, monopartite. Classified by subgroups based on serology. Some are benign, others cause mild respiratory tract infections, some can induce tumors in hamsters.

Papovaviridae - circular genome. The term papo a was derived as an acronym for the three original viruses (Papillomavirus, Polyomaviruses, Vacuolating viruses ). There are now two genera - papillomaviruses and polyomaviruses (the vacuolating viruses are now classified as polyomaviruses). Human papillomaviruses are cause warts and some have been associated with cervical carcinomas. Polyomavirus is a mouse virus that produces a variety of tumors. Human polyomaviruses include BK virus and JC virus.

ss DNA, nonenveloped

Parvoviridae - the genome is linear and the nucleic acid that is encapsulated may be either + or - sensed. The autonomously infected parvoviruses normally package only the one strand (the minus sense), while other viruses like the Adeno-associated virus encapsulates either sense with equal efficiency.

ds RNA, nonenveloped

Reoviridae (Respiratory, Enteric, Orphan) - linear genome, multipartite (10 - 11 strands), have complex capsids. Orbivirus causes Colorado tick fever (encephalitis)

ss RNA, enveloped

NO DNA in replication

positive sense

Togaviridae - linear, monopartite, enveloped. Contains viruses previously classified as arboviruses. Eastern equine encephalitis virus, Yellow fever.

Coronaviruses - helical nucleocapsid, enveloped, monopartite. Infectious Bronchitis Virus cause an acute upper respiratory virus.

minus sense

nonsegmented

Paramyxoviruses - helical nucleocapsid surrounding monopartite genome. The virus is enveloped. Mumps and parainfluenza

Rhabdoviridae - enveloped virus having a helical nucleocapsid. The genome is monopartite. The shape is unusual (bullet like appearance). Group includes Rabies virus.

segmented

Bunyaviridae - enveloped with icosahedral capsid. Has multipartite genome (3 segments). Ex. California encephalitis virus.

Orthomyxoviridae - enveloped, multipartite with 6 - 8 segments, helical capsid. Includes influenza virus.

Arenaviridae - linear multipartite genome (2 segments). Group contains Lassa Fever virus.

DNA in replication

Retroviridae - Diploid genome. Positive sensed RNA surrounded by icosahedral capsid and envelope. Ex. HIV-1.

ss RNA, nonenveloped, positive

Picornaviridae - linear, monopartite. Poliovirus, ECHO viruses, Rhinoviruses,

Caliciviridae - linear, monopartite. Norwalk virus may be in this group. Caliciviruses have been reported to cause gastroenteritis.