Viruses & neoplasia Flashcards

1
Q

What are examples of retroviruses (RNA viruses) that cause cancer?

A

Avian leukosis virus – lymphoid, myeloid tumors, sarcomas

Feline retroviruses – lymphoid tumors

Jaagsiekte sheep retrovirus (JSRV) – lung adenocarcinoma

Bovine leukosis virus (BLV) – lymphoma

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2
Q

What are examples of DNA viruses that cause tumours?

A

Marek’s disease virus (herpesvirus) – lymphoid tumours

Papillomaviruses – cause skin & mucosal warts & papillomas in cattle, horses & dogs

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3
Q

What is a proto-oncogene?

A

Normal gene that can become oncogene due to mutation or increased (uncontrolled) expression

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4
Q

What are examples of proto-oncogene functions?

A

Receptor kinases
Adaptor proteins
Small binding proteins
Kinases
Transcription factors

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5
Q

How do proto-oncogenes become oncogenes?

A

Most cellular proto-oncogenes are normal components of growth factor signalling pathways
But…increased activity leads to increased cell growth:
- Mutation of proto-oncogene (cellular oncogene or c-onc)
- Viral transduction of an oncogene (v-onc)
- Viral insertion affecting production of cellular oncogene

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6
Q

What are the main mechanisms by which retroviruses cause cancer?

A

Transduction of an oncogene
Insertional activation of a cellular oncogene
Other mechanisms via specific viral proteins

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7
Q

How does retroviral transduction lead to cancer?

A

Retrovirus carries cellular oncogene & inserts it into host genome

This oncogene is overexpressed, leading to uncontrolled cell growth

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8
Q

How does a retrovirus integrate into the host genome?

A
  1. Virus enters host cell via receptor
  2. RNA genome is reverse transcribed into cDNA
  3. cDNA enters nucleus & is integrated into host genome using viral enzyme integrase
  4. Once integrated, viral genome remains in host for life, unless cell dies
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9
Q

What happens when a retrovirus acquires a cellular oncogene, and why are most defective oncogenic retroviruses not transmitted?

A

They often lose essential genes, making them “defective” & unable to replicate independently. As a result, they arise de novo in each infection & are not passed to new hosts

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10
Q

How is Rous Sarcoma Virus (RSV) different from other oncogenic retroviruses?

A

RSV can still replicate & transmit despite carrying oncogene, unlike most defective oncogenic retroviruses

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11
Q

What oncogene does RSV contain, and how does it cause cancer?

A

RSV carries v-src (viral homolog of c-src with C-terminal deletion) making it constitutively active & leading to uncontrolled cell growth

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12
Q

How does Rous Sarcoma Virus (RSV) cause rapid oncogenic transformation?

A

RSV carries v-src oncogene, which promotes uncontrolled cell division, leading to rapid transformation of normal cells into cancerous ones

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13
Q

What are key cellular changes after RSV infection?

A

Loss of contact inhibition (cells no longer stop growing when they touch each other)

Increased cell density (cells pile up abnormally)

Increased growth rate

Anchorage-independent growth (cells grow without needing a solid surface)

Tumorigenicity (cells can form tumours in appropriate hosts)

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14
Q

What type of retrovirus is RSV classified as?

A

Acute transforming retrovirus, meaning it induces cancer quickly due to direct oncogene activation

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15
Q

What is insertional activation in retroviral oncogenesis?

A

Retrovirus integrates near proto-oncogene (c-onc), leading to its abnormal activation & potential cancer formation

Process of oncogenesis is slower compared to acute transforming retroviruses

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16
Q

How do retroviruses cause cancer through insertional activation?

A

Unlike transducing retroviruses, these retroviruses don’t carry oncogene

Instead, their Long Terminal Repeat (LTR) regions act as promoters & enhancers, increasing transcription of nearby proto-oncogenes

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17
Q

What are the two main ways retroviral insertion can activate a proto-oncogene?

A

Promoter activation – viral LTR promoter enhances transcription of proto-oncogene

Enhancer activation – viral LTR enhancer elements increase expression even if insertion is some distance away

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18
Q

What is an example of an insertional activation retrovirus?

A

Feline leukemia virus (FeLV) & Avian Leukosis virus

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19
Q

How is avian leukosis virus transmitted?

A

From the hen to the egg

Chicks hatch with persistent infection & become immunotolerant to viral antigens & develop tumours

Incubation period for tumour development is > 4 months

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20
Q

What type of tumors does Avian Leukosis Virus (ALV) cause?

A

ALV primarily causes B-cell tumours, including:
- Tumours in bursal follicles (due to viral integration activating proto-oncogenes)
- Diffuse liver tumors (resulting from viral recombination & gene capture)

21
Q

How is ALV controlled?

A

Control by eradication in breeder flocks

  1. Select virus-free hens by screening eggs before hatching
  2. Check eggs over 14-day period for ALV antigen in albumen by ELISA
  3. Hatch chicks and rear in isolation
  4. Test for ALV antigen in blood
  5. Maintain virus-free breeders

Virus is susceptible to disinfectants but can be transmitted by mating

22
Q

What is Bovine Leukemia Virus (BLV), and how does it cause disease?

A

BLV is delta retrovirus that infects B lymphocytes, leading to enzootic bovine leukosis

Virus becomes latent in host genome, meaning there is no free virus in blood, but infected cattle produce antiviral antibodies

23
Q

What viral protein is responsible for BLV-induced oncogenesis, and how does it work?

A

Tax protein transactivates cellular genes, promoting uncontrolled cell growth

Products of these transactivated genes may be oncogenic, contributing to lymphoma formation

24
Q

How does the Tax protein contribute to cancer development?

A

Activates cytokine genes (e.g. IL-2, GM-CSF), promoting immune cell proliferation

Upregulates IL-2 receptors, creating positive feedback loop for continuous growth signals

Disrupts cell cycle regulation & causes chromosomal instability, increasing risk of transformation

25
Q

How is BLV transmitted?

A

Via infected cells (e.g. milk, blood)
Can spread vertically (mother to calf) or horizontally

26
Q

What disease does Jaagsiekte Sheep Retrovirus (JSRV) cause?

A

Ovine Pulmonary Adenocarcinoma (OPA) (“panting sickness”)

27
Q

How does Jaagsiekte Sheep Retrovirus (JSRV) cause cancer?

A

Viral Env protein activates cellular signaling pathways, leading to uncontrolled cell division in type II pneumocytes & Clara cells

28
Q

What are key respiratory signs of Jaagsiekte Sheep Retrovirus (JSRV) in infected sheep?

A

Loss of condition
Dyspnea (panting, difficulty breathing)
Clear or frothy lung fluid discharge
Slow progression, sudden death possible

29
Q

How is JSRV transmitted and controlled?

A

Spread through respiratory secretions & requires close contact

More common in housed sheep

Controlled through isolation and culling

Diagnosis via histopathology and RT-PCR

30
Q

What is a common DNA virus causing tumours in cattle?

A

Bovine papillomatosis caused by bovine papillomavirus (BPV)

Virus is tropic for epithelial and mucous tissues

31
Q

How does bovine papillomavirus (BPV) persist and contribute to cancer development?

A

BPV establishes persistent infection & remains latent in host cells

Over time, expression of early viral genes activates host signaling pathways, leading to abnormal cell growth

32
Q

What factors increase the risk of BPV-associated cancer?

A

Bracken fern exposure induces immunosuppression & genetic changes, weakening immune response & promoting neoplastic transformation

33
Q

Why is BPV-associated cancer relatively rare?

A

Neoplastic progression is slow, multistep process, requiring additional factors like immune suppression & long-term viral persistence for tumour formation

34
Q

What viruses are associated with alimentary and bladder cancers in cattle?

A

BPV-2: Causes bladder carcinomas & enzootic haematuria

BPV-4: Leads to alimentary carcinomas in oesophagus, rumen & reticulum

35
Q

What disease does Marek’s Disease Virus (MDV)/Gallid herpesvirus 2 cause?

A

T-cell lymphomas in poultry

36
Q

How is Marek’s Disease Virus (MDV) transmitted?

A

MDV is shed from feather follicle cells & spreads through dander & dust, which is inhaled by other birds, infecting respiratory tract

37
Q

After inhalation, where does MDV disseminate in the body?

A

Virus spreads via blood to immune organs:
- Bursa of Fabricius
- Thymus
- Spleen

38
Q

What are the clinical signs of Marek’s disease?

A

Neurological signs (paralysis)
Tumors in muscles and organs
Ocular changes
Immunosuppression
Cutaneous nodules

39
Q

What are the effects of MDV on different immune cells?

A

B cells & macrophages → Undergo cell death, leading to immunosuppression

T cells → Initially undergo activation & proliferation, then transform into tumour cells, causing lymphomas

40
Q

What are the three types of virus interactions with host cells in Marek’s disease?

A

Differ depending on cell type

Cytopathic (lytic) & cell-associated – Virus kills B-cells & macrophages

Non-productive (latent) & cell-associated – Virus remains in tumour cells in T lymphocytes

Cell-free & productive – Virus spreads from feather follicle cells, key source of infection

41
Q

What factors make Marek’s Disease epidemiology complex?

A

Carrier status (latency)

Environmental survival for months

Viral virulence (mild to highly virulent strains)

Host factors, including MHC-dependent immune response & stress levels

42
Q

How is Marek’s Disease diagnosed?

A

Clinical signs & pathology

Virus isolation, PCR & antibody detection (supportive but not confirmatory in absence of characteristic clinical signs)

Differential diagnosis is avian leukosis (doesn’t cause neurological signs)

43
Q

What are the main strategies for controlling Marek’s Disease?

A

Disinfection, biosecurity & all-in-all-out management

Vaccination using:
- Turkey herpesvirus (HVT)
- Gallid herpesvirus-3 (SB-1 strain)
- CVI988/Rispens (attenuated Gallid herpesvirus-2)

44
Q

How is Marek’s Disease vaccination administered, and how long does immunity take to develop?

A

Given in-ovo (before hatching) or to day-old chicks

Immunity takes 7 days to develop

45
Q

What is the function of the gag gene in a retrovirus?

A

Gag gene encodes structural proteins, including:
- Matrix protein
- Capsid protein
- Nucleoprotein

These proteins form virus structure & protect viral RNA

46
Q

What is the function of the pol gene in a retrovirus?

A

Pol gene encodes key enzymes needed for viral replication, including:
- Reverse transcriptase (converts viral RNA into DNA)
- Integrase (helps insert viral DNA into the host genome)
- Protease (processes viral proteins for assembly)

47
Q

What is the function of the env gene in a retrovirus?

A

Env gene encodes viral envelope proteins, which help virus attach to & enter host cells

48
Q

What are Long Terminal Repeats (LTRs) and how do they contribute to viral oncogenesis?

A

LTRs are repeating sequences at both ends of viral genome that contain promoter & enhancer regions, which regulate gene expression

LTRs can:
- Increase viral gene expression.
- Disrupt normal cellular gene regulation when virus integrates into host genome
- Activate oncogenes if they insert near proto-oncogene, leading to uncontrolled cell growth

49
Q

What are endogenous retroviruses?

A

Proviruses integrated into host genome. If they enter germ cells (sperm or eggs), they are inherited by next generation; if in somatic cells, they are lost when animal dies