1. PLANT VIRUS1. PLANT VIRUS2. INFECTIOUS AGENTS 2. INFECTIOUS AGENTS

(Virus-like agents)(Virus-like agents)VIROIDS, VIRUSOIDS VIROIDS, VIRUSOIDS AND PRIONSAND PRIONS

LESSON OUTCOME

• Definition – viroids, virusoids and prions

• Explain these three

• Similarities and differentiation between these virus-like agents

• Compare between virus and viroids/virusoids.

• Characteristics to compare prions, viroids and viruses

• Relationship between virus and cancer

Plant virusesPlant viruses• Plant viruses face special problems initiating an infection.

- The outer surfaces of plants are composed of protective layers of waxes and pectin, but more significantly, each cell is surrounded by a thick wall of cellulose overlying the cytoplasmic membrane.

- no plant virus is known to use a specific cellular receptor of the type that animal and bacterial viruses use to attach to cells.

- Plant viruses rely on a mechanical breach of the integrity of a cell wall to directly introduce a virus particle into a cell. - - This is achieved either by the vector associated with transmission of the virus or simply by mechanical damage to cells.

Transmission Plant virusesTransmission Plant viruses

• Through sap - entry

• Seeds: due to external contamination of the seed with virus particles, or due to infection of the living tissues of the embryo.

• Vectors: Many different groups of living organisms can act as vectors and spread viruses from one plant to another:

– Bacteria (e.g. Agrobacterium tumefaciens - the Ti plasmid of this organism has been used experimentally to transmit virus genomes between plants)

– Fungi

– Nematodes

– Arthropods: Insects - aphids, leafhoppers, planthoppers, beetles, thrips, etc.

– Arachnids - mites

• Mechanical: Mechanical transmission of viruses is the most widely used method for experimental infection of plants and is usually achieved by rubbing virus-containing preparations into the leaves, which in most plant species are particularly susceptible to infection. However, this is also an important natural method of transmission. Virus particles may contaminate soil for long periods and may be transmitted to the leaves of new host plants as wind-blown dust or as rain-splashed mud.

Family Potyviridae

(Genus Potyvirus) - potato virus Y - papaya ringspot virus (PRSV)

Flexiviridae - potato virus X

Comoviridae- cowpea mosaic virus

Genus Tobamovirus- tobacco mosaic virus

RNA Viruses - Class IV Class IV (+ sense) ssRNA

Family: Sequiviridae - Plant virus

Genus: Waikavirus – rice tungro spherical virus

Plant viruses: example

INFECTOUS AGENTSINFECTOUS AGENTS• Virion = a complete virus particle, including envelope (if any)

• Viroid = an infectious RNA particle, smaller than a virus, lacking a capsid, that causes various plant diseases

• Virusoid (satellite nucleic acids) = same as viroid; small, ssRNA molecule, usually 500 to 2000 nucleotides in length, lacking a capsid, lack genes required for the replication virusoid require a helper (satellite) virus to replicate, causes various plant diseases.

OTHER THAN VIRUS : IN PLANTOTHER THAN VIRUS : IN PLANT

- depend on virus to replicate, its nucleotide only encode structural proteins

SIMILARITIESSIMILARITIES

Viroid Nucleic acid – ssRNAVirusoid Lack capsid, cause plant diseases

DIFFERENCESViroids Virusoids

1. Does not require helper virus 1. Require helper virus for replication

2. Does not encode protein 2. Encode proteins

3. Replication takes place in host cell nucleus

3. Replication takes place in host’s cytoplasm

Animal virusoid = hepatitis delta virusoid – the helper virus is Hepatitis B virus.

ExampleViroid

- avocado sunblotch viroid, peach latent mosaic, potato spindle tuber, coconut cadang-cadang, tomato plant macho viroid, citrus bent leaf viroid, pear blister canker viroid

-Virusoid - barley yellow dwarf satellite RNA , tobacco ringspot virus satellite RNA

Viroid - cause lethal plant diseases; potato spindle tuber disease, chrysanthemum stunt disease, cucumber pale fruit disease, coconut cadang-cadang disease, chrysanthemum stunt disease, tomato apical stunt disease.

Virusoid - cause tobacco necrosis

Disease

The potato spindle tuber viroid and tomato plant macho viroid, members of the family Pospiviroidae,

Avsunviroidae, two known members are the avocado sunblotch viroid and the peach latent mosaic viroid.

PRIONS• A unique infectious agent, protein infectious particle

• Prion = a small infectious particle consisting of protein and lack nucleic acid.

• Prion – features:-Resistant to inactivation by heating to 90o C, which inactivate virus-The infection is not sensitive to radiation; radiation damages virus genomes-Prions are not destroyed by enzymes that digest nucleic acids-Sensitive to protein denaturing agents; urea, phenol-Prions have direct pairing of amino acids

• Diseases –1.Creutzfeldt-Jakob – mental degeneration, loss of motor function and death (human)

2.Scrapie and bovine spongiform encephalopathy (BSE) – loss of neuronal function that leads to death in sheep and dairy cattle. 3.Kuru – a neurological disorder in human Holes brain –

“spongiform”

Cerebral cortex of a normal human brain (right), patient with CJD (left).

Once present in the brain, prions cause normal proteins to refold into abnormal shapes. As these abnormal proteins multiply, they destroy neurons and eventually cause brain tissue to become riddled with holes. Prions can only be destroyed through incineration.

Mad – cow disease

PRIONS

Viruses, Viroids and Prions

Characteristic between virus, viroid and prion

Nucleic acid type, NA strandedness, host range and structural features

Virus and CancerVirus and Cancer

VIRUS AND CANCER - INTRODUCTION

• Cancers are the result of a disruption of the normal restraints on cellular proliferation. It is apparent that the number of ways in which such disruption can occur is strictly limited and there may be as few as forty cellular genes in which mutation or some other disruption of their expression leads to unrestrained cell growth.

• There are two classes of these genes in which altered expression can lead to loss of growth control: 

(a) Those genes that are stimulatory for growth and which cause cancer when hyperactive. Mutations in these genes will be dominant. These genes are called oncogenes.

(b) Those genes that inhibit cell growth and which cause cancer when they are turned off. Mutations in these genes will be recessive. These are the anti-oncogenes or tumor-suppressor genes.• Viruses are involved in cancers because they can either carry a copy of one of these genes or can alter expression of the cell's copy of one of these genes. – tumor viruses

REMEMBER: Animal virus- Not all virus infection will cause immediate lysis to host cell

- some maintain a carrier relationship, these so called persistent infections can last from a few weeks to the remainder of the host’s life.

- Example: measles virus, herpes simplex viruses (cold sores and genital herpes) and herpes zoster virus (chicken pox and shingles).

Latency/latent state

Virulent virus

Viruses and CancerViruses and Cancer Some animal viruses enter their host cell and permanently alter its genetic material, leading to cancer. These viruses are termed oncogenic, Their effect on the cell is called transformation. - A startling feature of these viruses is that their nucleic acid is consolidated into the host DNA. - Transformed cells have an increased rate of growth; alterations in chromosomes; changes in the cell’s surface molecules; and the capacity to divide for an indefinite period, unlike normal animal cells.

Mammalian viruses capable of initiating tumors are called oncoviruses.

DNA viruses such as papillomavirus (genital warts are associated with cervical cancer), herpesviruses (Epstein-Barr virus causes Burkitt’s lymphoma), and hepatitis B virus.

A process when virus catalyze the conversion of a normal cell to a tumor cell.

Latent state

Related with tumor formation – eventually producing cancerous cell

TUMOR VIRUSES

CLASSES OF TUMOR VIRUSES• There are two classes of tumor viruses: 1.DNA tumor viruses 2.RNA tumor viruses, the latter also being referred to as RETROVIRUSES.

• These two classes have very different ways of reproducing themselves but they often have one aspect of their life cycle in common: the ability to integrate their own genome into that of the host cell. Such integration is not, however, a pre-requisite for tumor formation.

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How viruses cause cancerHow viruses cause cancer

- DNA tumor viruses – can exist as provirus

- RNA tumor viruses – use own RTase to transcribe (+) sense RNA into DNA that integrates as provirus

- Cancer is due to unregulated viral proteins in cell causing the host cells to divide uncontrollably. Some viral proteins blocks the tumor-suppressor genes (prevent uncontrolled cell divisions) therefore a tumor develops.

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Viruses and CancerViruses and Cancer• Oncogenes = gene that when mutated and expressed at

high level will form tumor due to uncontrolled host cell divison

- In DNA tumor virus, oncogenes also contain the information for synthesizing viral proteins for viral replication

• Proto-oncogenes = normal gene (from normal host cells), that under the control of a virus can cause uncontrolled cell division; can act as oncogene. The oncogenes are not needed for virus replication.

Oncogene = a gene that can bring about malignant transformation

VIRUS AND CANCER VIRUS AND CANCER Examples of viruses that are associated with human cancers:

1. Epstein-Barr virus (EBV)2. Human papillomavirus (HPV)3. Hepatitis B virus (HBV)

EBV What is the Epstein-Barr virus? Human herpesvirus 4 (HHV-4),• HerpesviridaeHerpesviridae – so dsDNA linear virus, icosahedral capsid icosahedral capsid and has

envelope • EBV is a member of the herpesvirus family and causes more than

90% of cases of mononucleosis. EBV is a double-stranded DNA virus named for the English virologists professor Sir Anthony Epstein and Yvonne M. Barr.

• The designation "mononucleosis" refers to an increase in a special type of white blood cells

(lymphocytes) in the bloodstream

relative to the other blood

components as a result of the EBV

infection.

EBV

• Mono is usually spread by person-to-personcontact with saliva as the primary method of transmission.

• person with mono can also pass on the disease by coughing or sneezing, causing small droplets of infected saliva and/or mucus to be suspended in the air and inhaled by others. Sharing food or beverages from the same container or utensil can also transfer the virus from one person to another, since contact with infected saliva may result.

EBV• The initial symptoms of mono are a general

lack of energy (malaise), loss of appetite, and chills. The more common intense symptoms include a severe sore throat, fever, and swollen glands (lymph nodes) in the neck area. It is typically the severe sore throat that prompts people to contact their doctor.– have recovered will continue to secrete the EBV in their saliva for years due to periodic "reactivations" of the viral infection.

EBV

• This disease rarely fatal in healthy man, Mono tends to be more aggressive in patients with abnormal immune systems, such as people with AIDS or those who are taking medications that suppress immune function. The EBV has been associated with some types of cancers, most commonly lymphomas. As well, some studies have linked EBV to the development of at least one subtype of Hodgkin's disease.

Burkitt’s Lymphoma - caused by Epstein-Barr Virus  2

• In 1958 he reported that the jaw tumours affected children between the ages of two and fourteen years, with a peak incidence occurring at five years. What became known as Burkitt’s lymphoma (BL) was found to occur in a band across tropical Africa, tailing partly down the east coast, and showing a similar distribution to holoendemic malaria. Also cause Nasopharyngeal Carcinoma

Human papillomavirus

• Non-enveloped ds DNA virus• Family Papillomaviridae• HPV has vaccine

• Spread through epithelial – skin contact• Some cause only benign warts.• HPVHPV-cause of cervical cancercervical cancer, genital wartsgenital warts. .

HPVs may play a role in some cancers of the anus, vulva, vagina, and penile cancer (cancer of the penis) – eg. HPV-8 and HPV-16.