Lecture 22 (1).pdf

 Medical Virology Classification of viruses Prof. Dr. Hazem Aqel Basic Medical Sciences Dept. Faculty of Medicine Al-Balqa’ Applied University Classification of viruses  Nucleic acid  Capsid  Presence of envelope  Replication strategy  CLASSIFICATION NUCLEIC ACID  RNA or...

 Medical Virology Classification of viruses Prof. Dr. Hazem Aqel Basic Medical Sciences Dept. Faculty of Medicine Al-Balqa’ Applied University Classification of viruses  Nucleic acid  Capsid  Presence of envelope  Replication strategy  CLASSIFICATION NUCLEIC ACID  RNA or DNA  segmented or non-segmented  linear or circular  single-stranded or double-stranded  if single-stranded RNA  is genome mRNA (+) sense or complementary to mRNA (-) sense  ENVELOPE  Obtained by budding through a cellular membrane (except poxviruses)  Possibility of exiting cell without killing it.  Contains at least one virally coded protein.  Attachment protein.  Loss of envelope results in loss of infectivity.  Properties of naked viruses  Stable in hostile environment  Not damaged by drying, acid, detergent, and heat  Released by lysis of host cells  Can sustain in dry environment  Can infect the GI tract and survive the acid and bile  Can spread easily via hands, dust, fomites, etc  Can stay dry and still retain infectivity  Neutralizing mucosal and systemic antibodies are needed to control the establishment of infection Naked viruses (Non - Enveloped)  Adeno-associated Virus (AAV) Adenovirus B19 Coxsackievirus - A Coxsackievirus - B Echovirus Hepatitis A Virus (HAV) Hepatitis E Virus (HEV) Norwalk Virus The Baltimore classification system Based on genetic contents and replication strategies of viruses. According to the Baltimore classification, viruses are divided into the following seven classes: 1. dsDNA viruses 2. ssDNA viruses 3. dsRNA viruses 4. (+) sense ssRNA viruses (codes directly for protein) 5. (-) sense ssRNA viruses 6. RNA reverse transcribing viruses 7. DNA reverse transcribing viruses where "ds" represents "double strand" and "ss" denotes "single strand". Virus Classification - the Baltimore classification  All viruses must produce mRNA, or (+) sense RNA  A complementary strand of nucleic acid is (–) sense  The Baltimore classification has + RNA as its central point  Its principles are fundamental to an understanding of virus classification and genome replication, but it is rarely used as a classification system in its own right Viral genome strategies  dsDNA (herpes, papova, adeno, pox)  ssDNA (parvo)  dsRNA (reo, rota)  ssRNA (+) (picorna, toga, flavi, corona)  ssRNA (-) (rhabdo, paramyxo, orthomyxo, bunya, filo)  ssRNA (+/-) (arena, bunya)  ssRNA (+RTase) (retro, lenti)  Sub-viral agents  Satellites  Contain nucleic acid  Depend on co-infection with a helper virus  May be encapsidated (satellite virus)  Mostly in plants, can be human e.g. hepatitis delta virus  If nucleic acid only = virusoid  Viroids  Unencapsidated, small circular ssRNA molecules that replicate autonomously  Only in plants, e.g. potato spindle tuber viroid  Depend on host cell polII for replication, no protein or mRNA  Prions  No nucleic acid  Infectious protein e.g. BSE Viroids & Prions  Viroids  ss RNA genome and the smallest known pathogens.  Affects plants  Prions  Infectious particles that are entirely protein.  No nucleic acid  Highly heat resistant  Animal disease that affects nervous tissue  Affects nervous tissue and results in  Bovine spongiform encepahltits (BSE) “mad cow disease”,  scrapie in sheep  kuru & Creutzfeld-Jakob Disease (CJD) in humans  Viroids  Viroids are small (200-400nt), circular RNA molecules with a rod-like secondary structure which possess no capsid or envelope which are associated with certain plant diseases. Their replication strategy like that of viruses - they are obligate intracellular parasites.  Viroids do not encode any proteins and unlike satellites they are not dependent on the presence of another virus  Viroid replication  Viroids utilize cellular RNA polymerases for their replication  Replication is performed by “rolling circle mechanism”  The resulting long RNA molecule is cut in pieces and ligated either autocatalytically or by cellular factors (depending on a viroid)  So in a sense, at least some viroids are ribozymes... Examples of plants, infected with various viroids  Hepatitis d virus – a chimeric molecule, half viroid, half satellite  Viroid like properties  Satellite like properties - Rod-like RNA molecule - Encodes a protein, which is necessary both for - Rolling circle replication encapsidation and - Self-cleaving activty replication - Dependent on presence another virus – HBV - Genome larger than for viroids (1640 nt)  Prions  Prions are rather ill-defined infectious agents believed to consist of a single type of protein molecule with no nucleic acid component.  Confusion arises from the fact that the prion protein & the gene which encodes it are also found in normal 'uninfected' cells.  These agents are associated with diseases such as Creutzfeldt-Jakob disease in humans, Scrapie in sheep & Bovine Spongiform Encephalopathy (BSE) in cattle.  Prions Prions are proteinaceous transmissible pathogens responsible for a series of fatal neurodegenerative diseases (in humans, Creutzfeld-Jakob disease and kuru, in animals, bovine spongioform encephalopathy) A prion (proteinaceous infectious particle, analogy for virion) is a type of infectious agent that does not carry the genetic information in nucleid acid Prions are proteins with the pathological conformation that are believed to infect and propagate the conformational changes of the native proteins into the the abnormally srtructured form Disease name Natural host Prion name PrP isoform Scrapie Sheep, goat Scrapie prion OvPrPSc Transmissible mink Mink TME prion MkPrPSc encephalopathy (TME) Chronic wasting disease Elk, mule deer CWD prion MDePrPSc (CWD) Bovine spongioform Cattle BSE prion BovPrPSc encephalopathy (BSE) Feline spongioform Cat FSE prion FePrPSc encephalopathy (FSE) Exotic unguale Greater kudu, EUE prion NyaPrPSc encephalopathy (EUE) nyala Kuru Human Kuru prion HuPrPSc Creutzfeldt-Jakob disease Human CJD prion HuPrPSc (CJD) Gerstmann-Straussler- Human GSS prion HuPrPSc Scheinker syndrome (GSS) Fatal familial insomnia Human FFI prion HuPrPSc (FFI)  Prion diseases: rare neurodegenerative disorders (one person per million) 1. Sporadic (85 %) In the sixth or seventh decade, rapidly progressive (death in less than a year) Creutzfeldt-Jakob disease (CJD) 2. Familial (inherited-15%) Mutations in the PrP gene that favour the transition from the cellular form to the pathological form of PrP Gerstmann-Straussler-Scheinker disease (GSS), Fatal familial insomnia (FFI) 3. Transmissible (rare; a source of great concern) Propagation of kuru disease in New Guinea natives (ritualistic cannibalism) Recently, it has been discovered that BSE had been transmitted to humans in Europe after consumption of infected beef, producing a variant of the CJD called vCJD  Transmissible spongioform encephalopathy (TSE)=prion disease  A group of progressive conditions that affect the brain and nervous system of humans and animals and are transmitted by prions  The pathology: vacuolar degeneration, neuronal loss, astrocytosis and amyloid plaque formation  The clinical signs: loss of motor functions (lack of coordination, ataxia, involuntary jerking movements), personality changes, depression, insomnia, confusion, memory problems, dementia, progressive tonic paralysis, death  Definitive diagnostic test: biopsy of brain tissue (histopathological examination and immunostaining for PrPSc)  There is no cure α-helix β-sheet Conformational change Normal protein Disease-associated protein (folded structure) (misfolded structure) Aggregation Gain of toxic Loss of biological activity function  PrPC PrPSc The normal protein The abnormal, disease-producing protein is called PrPC (for cellular) is called PrPSc (for scrapie) is a transmembrane glycoprotein has the same amino acid sequence (neurons, lymphocytes); its function is (primary structure) unknown; it binds Cu2+ (regulation its homeostasis) has dominant secundary structure has dominant secundary structure β-sheets α-helix is insoluble is easily soluble is multimeric and resistant to digestion by is monomeric and easily digested by proteases proteases When PrPSc comes in contact with PrPC, is encoded by a gene designated PRNP it converts the PrPC into more of itself located on the chromosome 20 These molecules bind to each other forming aggregates  Molecular models of the structure of: PrPC PrPSc Predominantly α-helix (3) β-sheets (40%), α-helix (30%)  Replication cycle The presence of an initial PrPSc: exogenous (infectious forms) or endogenous (inherited or sporadic forms) This first prion will initiate PrPSc accumulation by sequentially converting PrPC molecules into PrPSc in replication cycle PrPSc molecules aggregate  Summary The prions are proteins that carry information for self-reproduction (contradict the central dogma of modern biology) The prions are expressed in cells of healthy humans and animals; their abnormal conformations (PrPSc) are insoluble, resistent to digestion and aggregate The PrPSc attacks the native prion PrPC, changes its conformation into an abnormal form and causes an exponential production of insoluble proteins; they aggregate and form the fibrillar structure Prion disease are rare fatal degenerative disorders; a portion of them can be transmitted; this mechanism is not clear (e.g. transmision of BSE to human) One part of the prion protein can cause apoptosis, or programmed cell death Prions induce no immune reactions within the human  DNA VIRUSES DOUBLE STRANDED SINGLE STRANDED COMPLEX NON-ENVELOPED ENVELOPED ENVELOPED NON-ENVELOPED PARVOVIRIDAE POXVIRIDAE HERPESVIRIDAE HEPADNAVIRIDAE CIRCULAR LINEAR PAPILLOMAVIRIDAE ADENOVIRIDAE All families shown are POLYOMAVIRIDAE icosahedral except for (formerly grouped together as the poxviruses PAPOVAVIRIDAE) DNA viruses From Principles of Virology Flint et al ASM Press  RNA VIRUSES SINGLE STRANDED SINGLE STRANDED DOUBLE STRANDED positive sense negative sense ENVELOPED NONENVELOPED ENVELOPED NONENVELOPED ICOSAHEDRAL HELICAL ICOSAHEDRAL HELICAL ICOSAHEDRAL FLAVIVIRIDAE CORONAVIRIDAE PICORNAVIRIDAE ORTHOMYXOVIRIDAE REOVIRIDAE TOGAVIRIDAE CALICIVIRIDAE PARAMYXOVIRIDAE RETROVIRIDAE ASTROVIRIDAE RHABDOVIRIDAE FILOVIRIDAE BUNYAVIRIDAE ARENAVIRIDAE RNA viruses From Principles of Virology Flint et al ASM Press Dr.T.V.Rao MD BASIC STEPS IN VIRAL LIFE CYCLE  ADSORPTION  PENETRATION  UNCOATING AND ECLIPSE  SYNTHESIS OF VIRAL NUCLEIC ACID AND PROTEIN  ASSEMBLY  RELEASE