Sattelite virus, viruses types, viral particals, prions, viroids, low molecular weight viruses, diseases causes by viroids and prions

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2. SUBVIRALPARTICLES
Contents
 Sub viral particle types (3 types)
• Their Discovery
• Their Structure
• Their Replication and
• Diseases caused by them
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3. SUBVIRALPARTICLES
TYPES 3:
 Satellite virus (has only Nucleic acid)
Hepatitis Delta virus
 Viroids (has only RNA)
Infect Plants
 Prions (has only proteins)
Cause degenerative diseases
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4. DISCOVERY
• Satellite viruses are also known as virusoids.
• In 1977, an Italian doctor named Mario Rizzetto
discovered a new nuclear antigen in the liver cells
of patients infected with Hepatitis B Virus (HBV).
• The antigen was thought to be a new protein
encoded by HBV, and it was labeled as the delta
antigen. Subsequent research on chimpanzees,,
indicated that this antigen was derived from a new
virus, named the Hepatitis Delta Virus (HDV).
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5. 5

6. SATELLITE VIRUS
STRUCTURE:
• HDV consists of a single stranded, negative sense,
circular RNA genome, 1,700 fold, with an envelope
made up of HBAg. Smallest known virus.
• Virions are 35-43 nm and are roughly spherical, with
no distinct nucleocapsid structure.
• The nucleocapsid is made up of 60 large and small
delta antigens. These are the only proteins encoded
by HDV.
• HDV relies on host cell machinery for replication,
and the viral genome (and antigenome) serves as
ribozymes for self-ligation and cleavage.
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7. • Viral replication occurs in the nucleus of primary
hepatocytes using a double-rolling circle mechanism.
• New virions can be assembled only in the presence of
hepatitis B virus.
• The stages of the viral life cycle, including replication,
assembly, and transport, depend on the ratio of small to
large delta antigen.
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8. REPLICATION:
• The Hepatitis delta virus genome consists of a circular,
unbranched RNA.
• It does not encode an RNA dependent RNA polymerase,
and so must rely on cellular enzymes or components to
replicate its genome. Cellular polymerases (Pols)
replicate the viral genome.
• HDV can replicate continuously in culture for up at least
one year if provided with a source of HDAg-S mRNA.
• Chang et al examined the nature of the changes to the
nucleotide sequence of HDV after a year of replication in
culture.
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9. • HDV sequences after one year were just as effective in
replication as the original sequences.
• The findings suggest that HDV in chronic hepatitis D can
continue to replicate and may be just as virulent a year
or longer into the infection as in the initial stages of
infection.
• Wang and Chao demonstrate that RNA recombination of
HDV is both possible in culture and occurs naturally in
individuals infected with multiple HDV genotypes.
• One proposed mechanism is template switching by the
cellular polymerase during replication at certain
secondary structures in the RNA.
• The results suggest a powerful mechanism for HDV
heterogeneity that could easily lead to new HDV strains
with new properties.
9

10. •Viral particles consist of
HDV RNA (black lines)
•HDV antigen (blue)
•Encapsidated by HBsAg
(pink)
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11. Envelope:
• HDV envelope consists of gene products of HBV s &
pre-s genes.
•HBV pre-s/s genes contains 3 start signals & 1
termination signal.
•Three potential gene products are synthesized, which
are glycosylated to form 6 gene products/ glycoproteins
(of mol. Wt. 23, 25, 33, 36, 39 & 42 kilo daltons)
6 GENE
PRODUCTS
Small proteins
Intermediate
proteins
Large proteins
• Designated as P23, GP25, GP33, GP36, P39 & SP42
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12. • Two Small molecules are more abundant in both virion
and surface antigen
• Two Intermediate size molecule are abundant in
virions than in HBsAg called as Intermediate/middle
proteins.
• Two Large molecules are abundant in virions
• Middle proteins contains site receptor for Human
Albumin
• Large protein contains site Hepatocytes & involve in
entry into the cell.
• Envelope of HDV is completely derived from HBS
surface
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13. DISEASE:
• Hepatitis D virus (HDV) causes hepatitis D (hepatitis
delta)
Transmission & Epidemiology HDV:
• Transmitted by the same means as is HBV sexually, by
blood perinatally.
• In the United States, most HDV infections occur in
intravenous drug users who share needles.
• HDV infections occur worldwide with a similar
distribution to that of HBV infections
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15. HDV Pathogenesis & Immunity:
• The pathogenesis of hepatitis caused by HDV and HBV is
the same the virus-infected hepatocytes are damaged by
cytotoxic T cells.
• There is some evidence that delta antigen is cytopathic
for hepatocytes.
• IgG antibody against delta antigen is not detected for
long periods after infection uncertain whether long term
immunity to HDV exists.
• Because HDV can replicate only in cells also infected
with HBV, hepatitis delta can occur only in a person
infected with HBV.
• A person can either be infected with both HDV and HBV
at the same time, ie, be “coinfected,” or be previously
infected with HBV and then be infected with HDV
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16. HDV Laboratory Diagnosis:
• Detecting either delta antigen or IgM antibody to delta
antigen in the patient’s serum.
Treatment:
• Alpha interferon can mitigate some of the effects of the
chronic hepatitis caused by HDV but does not eradicate
the chronic carrier state.
Prevention of HBV infection:
• Vaccine hyperimmune
globulin
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vaccination

17. Geographic Distribution of HDV
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19. Viroids
• A VIROID = VIR (virus) OID (like) particle
• Very small, covalently closed, circular RNA molecules
capable of autonomous replication and induction of
disease, range in size from approximately 20 nm.
• No coding capacity - do not program their own
polymerase, use host-encoded polymerase for
replication.
• More than 40 viroid species and many variants have
been characterized.
• Viroids differ from viruses in that viruses, at their
most basic level, consist of genetic material (DNA or
RNA) contained within a protective protein shell.
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20. • Infects only plants
• Mechanically transmitted; often seed transmitted.
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21. 1974:
Confirmation that
viroids are non-
coding
1973: Electron
micrograph shows
viroid’s hairpin
structure
Discovery
Early 1960s:
Raymer and
O’Brien develop a
bioassay for the
agent causing
potato spindle
tuber disease
1965: Raymer
teams up with
Diener; they
show that agent
is not a typical
virion
1971: Diener
demonstrates
that the agent is a
free non-coding
RNA, coins the
term viroid
1968:
Characterization of
chrysanthemum
stunt and citrus
exocortis as non-
typical viruses
1978: PSTVd is
sequenced
1976: EM
shows that
viroids form
closed circular
RNAs
Theodore O. Diener

22. Structure:
• The smallest viroid identified so far is a 220
nucleobase scRNA (small cytoplasmic RNA)
associated with the rice yellow mottle sobemovirus
(RYMV) (Collins et al. 1998).
• In comparison, the genome of the smallest known
viruses capable of causing an infection by
themselves are around two kilobases in size.
• Many viroids consist of only 300 to 400 nucleotides.
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23. •Two main groups of viroids: self-cleaving and non-self-
cleaving
•Non-self cleaving viroids replicate in nucleus and fold
into “dog bone” or rod-like structure
•All are covalently closed circular RNAs fold to tightly
base-paired structures
•Five domains identifiable in non-self-cleaving
Terminal left (TL)
Terminal right (TR)
Pathogenicity (P)
Central (C)
Variable (V)
•The closed single-stranded RNA circle has extensive
intrastrand base pairing and interspersed unpaired
loops.

24. • Viroids have five domains. Most changes in viroid
pathogenicity seem to arise from variations in the
P and TL domains.
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25. Schematic models of viroid structures
a) Rod-like secondary structure proposed for PSTVd, the type
member of family Pospiviroidae.
b) Quasi rod-like secondary structure proposed for ASBVd, the
type member of family Avsunviroidae.
c) Complex branched conformation proposed for PLMVd.

26. Viroid movement
Pospiviroidae:
• Intra cellular movement
• Cell-to-cell through plasmodesmata
• Long distance through phloem
Host factors responsible for viroid movement
• Cellular proteins
• Phloem proteins- Phloem lectin PP2(CsPP2)
• Viroid binding protiens-Virp1
• Specific sequence

27. Viroid Replication
• Viroid RNA does not code for any known protein;
some even lack the AUG initiation codon.
• Nonetheless, they replicate autonomously in host
cells.
• The replication mechanism involves interaction
with RNA polymerase II, an enzyme normally
associated with synthesis of messenger RNA, and
"rolling circle" synthesis of new RNA.
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Asymmetric rolling circle replication
• Viroids replicate autonomously by using host-encoded
RNA polymerase
• Member of Pospiviroidae replicate via an asymmetric
• Incoming (+)-circular RNA initially is transcribed into
concatemeric linear (-)-strand RNA
• Which then serves as the replication intermediate for
the synthesis of concatemeric, linear (+)- strand RNA
• This (+)- strand RNA subsequently is cleaved into unit
length monomers that are ligated into circles
Enzymes involved in replication (3)
• RNA polymerase, RNA cleavage, RNA ligase

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Symmetric rolling circle replication
• Member of Avsunviroidae replicate via an symmetric
• The circular (+)-RNA is transcribed into linear, concatemeric (-)-
strand RNA
• Instead of serving as the direct template for the synthesis of
linear concatemeric (+)- strand RNA
• The concatemeric (-)- strand RNA is cleaved into unit length
molecules followed by circularization
• The circular (-)- RNA then serves as the template for the
synthesis of linear, concatemeric (+)- strand RNA
• When subsequently is cleaved into unit-length monomers and
circularized

32. Genus Pospiviroids:
PSTVd (potato spindle tuber)
Genus Hostuviroids:
HSVd (hop stunt)
Genus Cocadviroids:
CCCVd (coconut cadang-cadang)
Genus Apscaviroids:
ASSVd (apple scar skin)
Genus Coleviroids:
CbVd 1
(coleus blumei 1)
Genus Avsunviroids:
ASBVd (avocado
sunblotch)
Genus Pelamoviroids:
PLMVd (peach latent mosaic)
Disease:

33. Potato spindle tuber viroid
(PSTVd)
 May be limiting to potato growers
 First viroid characterized
 Many variants described
 Control with detection in mother
stock, clean seed
PSTVd in tomato
PSTVd in potato

34. Citrus exocortis viroid (CEVd)
• Causes stunting of plants, shelling of
bark
• May result in little yield loss
• May be useful to promote dwarfing
for agronomic advantage
• Transmitted through stock, graft
• Control by removal of infected
plants, detection, clean stock

35. Viroid disease control
• Disinfection of cutting tool
• Cold treatment
E.g storage at 4 °C for 6months or more, followed by
apical shoot-tip-culture, grafting, can be used to
eliminate CSVd and HSVd.
• Pre-inoculation with protective mild strains of viroid
has proved effective to control PSTVd
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1982 Dr. Stanley Prusiner coins the term "prion“, Highly purified PrP-
res is shown to be infectious.

38. • Prions (PREE-ons; Shortened term for: Proteinaceous
Infections Particle) are proteins that are unique in their
ability to reproduce on their own and become
infectious. They can occur in two forms called PrP-sen
and PrP-res.
38
I don’t
disturb/ trigger your
immune system

39. Basic Structure:
• Normal prions contain about 200-250 amino acids
twisted into three telephone chord-like coils known as
helices, with tails of more amino acids.
• The mutated, and infectious, form is built from the same
amino acids but take a different shape.
• 100 times smaller than the smallest known virus.
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NORMAL MUTATED

40. • Both PrP-sen and PrP-res are made up of the exact
same string of amino acids, the building blocks that
make up proteins. However, the two forms have
different shapes.
• PrP-sen is produced by normal healthy cells. The sen
stands for “sensitive” because this version of the
protein is sensitive to being broken down.
• PrP-sen is present mainly in neurons in the brain, but
is also found in other cell types.

41. • Scientists don’t know the exact function of PrP-sen, but
there is evidence that it may be involved in
communication between neurons, cell death, and
controlling sleep patterns. Interestingly, mice that are
genetically engineered to produce no PrP-sen seem to
be healthy.
• The second type of prion protein, known as PrP-res, is
the disease-causing form. Organisms with it develop
spongiform disease. “res” stands for “resistant”
because this version of PrP is resistant to being broken
down.

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43. • Unlike other infectious agents, prions do not contain
genetic material. However, once they infect an
individual, prions can replicate. How is this possible?
• Scientists are still working out the details, but evidence
supports the idea that when PrP-sen comes into
contact with PrP-res it is converted to PrP-res. The
result is a chain reaction that multiplies copy after
copy of the infectious prion.
• Because of their abnormal shape, PrP-res proteins tend
to stick to each other. Over time, the PrP-res molecules
stack up to form long chains called “amyloid fibers”.

44. • Amyloid fibers are toxic to cells, and ultimately kill
them.
• Cells called astrocytes crawl through the brain
digesting the dead neurons, leaving holes where
neurons used to be. The amyloid fibers remain.

45. Replication:
• When a prion enters a healthy organism, it induces existing,
properly folded normal proteins to convert into the disease-
associated, prion form.
• The normal form of the protein is called PrPC, while the
infectious (prion) form is called PrPSc
• These newly formed prions can then go on to convert more
proteins themselves; this triggers a chain reaction that
produces large amounts of the prion form.
• Prions cause other similar proteins to also misfold lose
function, cause disease

46. Chain-reaction Conversion of PrPc to PrPsc
PrPc PrPsc Chain-reaction Conversion

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49. Diseases:
• Creutzfeldt-Jakob Disease (CJD)
• Gerstmann-StraussIer-Scheinker (GSS)
• Fatal familial insomnia
• Kuru, human variety (cannibalism)
• Scrapie (sheep and goats)
• Mink transmissible encephalopathy
• Chronic wasting disease (deer and elk)
• Bovine Spongy Encephalopathy/ Mad Cow
1730s
Earliest written record of Scrapie in English sheep;
already prevalent in central Europe. 1950s
High levels of kuru appear among the Fore people of
New Guinea.
I’m not a
mad cow

50. 1960s
Scientists experimentally transmit Kuru and CJD to
chimpanzees, demonstrating the transmissible nature of
these diseasorm Encephalopathy (BSE)
1980s
60 people die from CJD after being infected by contaminated
surgical instruments. 85 people die after receiving prion-
infected growth hormone injections
1985
Scientists identify the PrP gene and discover that uninfected
people produce a normal form of the PrP protein.
1986
By the year 2000, nearly 180,000 cattle will become infected.
To stop the spread, thousands of cattle are killed.
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52. Common Symptoms
 Behavioural changes
 Communication problem
 Seizers
 Memory loss
 Blurry vision
 Difficulty in swallowing
 Loss of muscle control
52
Prion Infected
Brain
Healthy
Brain
I’m PRION, I’m
very smart b’coz I
eat your brain

53. Classic CJD or Creutzfeldt-Jakob disease (human)
•The most prevalent of the spongiform diseases
•Occurs spontaneously in 1 out of a million
people
•10% of cases are inherited mutations in the
PRPN gene
•Usually strikes people age 50 to 75
Fatal Familial Insomnia (human)
•All cases are inherited mutations in the
PrP gene
•Usually strikes people age 36 to 61
•Disruption of sleep/wake cycle leads to
coma, then death
Scrapie (goats, sheep)
•Occurs as infection in genetically
susceptible sheep
•There is no evidence of spread to
humans

54. Microscopic
observation of
Brain Tissue
54

55. Bovine Spongiform Encephalopathy
(BSE) or MAD COW (cattle)
•Infected animals act strangely and can
be aggressive
•Spread rapidly through Britain by
rendering
Kuru (human)
•Members of the Fore tribe in the 1960s
•Muscle weakness, loss of coordination,
tremors, inappropriate episodes of laughter
or crying
•Transmitted by ritual cannibalism as part
of funeral ceremonies

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Chronic Wasting Disease (deer)
Infectious disease in wild deer and elk primarily in
the western United States
Drooling, difficulty swallowing, weight loss

57. Controversy
 DNA and RNA are the only
substances now known to replicate
in body tissues, so how do prions
make copies of themselves without
any nucleic acids?
 Some believe TSEs are caused by an
unidentified slow-acting virus.
Others believe a small virus
accompanies a prion and they work
together to cause disease.

58. • VIROID = VIRus; OID: Like
• Small naked RNA molecule
• Discovered & coined by
T.O. Diener (1971)
• Cause disease in Plants
• Single stranded circular RNA
• Smaller than viruses
• Inactivated by Ribonuclease
digestion but resistant to
proteinase K & Trypsin
digestion
• PRION = INfectious Protein
• Protein particles
• Discover ed & coined by
Stanley B. Prusiner
• Cause disease in Animals
• No DNA or RNA
• Smaller than viruses & viroids
• Not inactivated by UV &
ionising radiation due to lack of
DNA/RNA (nucleic acid)
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