Human Retroviruses

Question 1

Human diseases possibly associated with retroviruses

Answer 1

• Human T-cell lymphotrophic virus 1 (HTLV-1)
  
  • Leukemia/lymphoma
• Human foamy virus (HFV)
  
  • Lymphoblastoid cells
  • Cancer etiology unknown
• Human endogenous retrovirus K (HERV-K)
  
  • CML
  • Breast cancer
  • Prostate cancer
  • Melanoma
  • Autoimmune diseases (MS, RA, psoriasis, T1D)
• HIV
  
  • Kaposi’s sarcoma
  • Non-Hodgkins lymphoma

Question 2

Retrovirus structure

Answer 2

• HIV as “prototypical retrovirus”
• 2 copies of RNA genome packaged inside capsid
• Surrounding capsid - matrix
• Outside matrix - envelope, derived from host cytoplasmic membrane
  
  • 2 membrane proteins: gp120, gp41

Question 3

3 essential genes in viral genome

Answer 3

1. Gag
   
   • Encodes matrix, capsid, nucleic acid-binding proteins
2. Pol
   
   • Encodes protease (PR), reverse transcriptase (RT), integrase (IN)
3. Env
   
   • Proteins determine host range
   • Derived from host cytoplasmic membrane

Question 4

Proteins encoded by pol gene

Answer 4

• Protease (PR)
  
  • Cleaves viral polyproteins
• Reverse transcriptase (TR)
  
  • Copies viral RNA genome –> dsDNA that contains long terminal repeats (LTRs)
    
    • Very inefficient
    • Drug target
• Integrase (IN)
  
  • Integrates dsDNA made by RT into host genome = viral persistence
    
    • Integrated DNA also known as proviral DNA

Question 5

Unique features of retroviruses: reverse transcription

Answer 5

• Initiation of infectious cycle begins 1 of 2 ways
• Integration into host chromosome
  
  • Envelope proteins bind to receptors on host cell and dump contents into cytoplasm OR
  • Endosomes take virus into host cells then dump contents into cell
• Viral uncoating
• In cytoplasm: RT makes dsDNA containing necessary LTRs
• Productive infectious cycle begins when viral DNA enters nucleus, integrates into host DNA
• Viral mRNAs made by host polymerase
• Viral mRNAs processed by host splicing and polyadenylating enzymes
• Retroviruses mature by budding from plasma membrane (just like other enveloped viruses)

Question 6

3 mechanisms by which retroviruses contribute to development of cancer

Answer 6

1. Acute transforming
2. Nonacute transforming
3. Trans-activation

Question 7

Acute transforming mechanism

Answer 7

• Some onco-retroviruses have a viral oncogene, derived from cellular proto-oncogenes

Question 8

Non-acute transforming mechanism

Answer 8

• Addition of virus genome into host’s genome
• 2 types of non-acute transformations:
  
  • Insertion activation: virus sticking strong promoter/enhancer directly in front of proto-oncogene not normally expressed in host’s genome
  • Enhancer activation: genome insertion many thousands of base pairs away
• Can include growth factors, growth factor receptors, signal transduction proteins, and transcription factors

Question 9

Trans-activation

Answer 9

• Very comple - Not covered well in lecture
• Rare
• Involves accessory genes
• Takes a long time

Question 10

Human T-cell lymphotrophic viruses (HTLV): epidemiology and associated diseases

Answer 10

• HTLV-1: δ-retrovirus that infects human T lymphocytes
• Epidemiology:
  
  • Isolated in Japan, Caribbean countries, South America, Africa, parts of Iran
  • Transmission: sexual contact, blood products, needles, mother –> child during breastfeeding
    
    • Effective transmission relies on cell to cell contact
• Associated diseases:
  
  • Adult T-cell leukemia/lymphoma (ATL)
  • HTLV-1 associated myelopathy (HAM) a.k.a. tropical spastic parapesis
  • Overall increased susceptibility to other diseases

Question 11

Adult T-cell leukemia/lymphoma (ATL): types, treatment

Answer 11

• Lymphoproliferative disorder - poor prognosis, difficult to treat
• Tumor derived from single transformed cell infected by virus
• 4 types:
  
  • Acute (50-60%): most patients die within 6 months of dx
  • Chronic type + lymphoma type (together 20%) - 2 year survival
  • Smoldering is slow and least aggressive (5%)
• Treatment: all current chemotherapeutic regimens fail to treat ATL - some therapies are promising:
  
  • Stem cell transplanation
  • Monoclonal Abs against T-cell leukemia cells
  • Anti-virus drugs

Question 12

ATL: clinical characteristics, histologic features

Answer 12

• Enlargement of peripheral lymph nodes
• Skin lesions due to leukemic cell infiltration
• Lytic bone lesions and as a result, hypercalcemia
• If acute, patients also have:
  
  • Increased ATL cells
  • Systemic lymphadenopathy
  • Pulmonary lesions
  • Hepatosplenomegaly
  • Immunodeficiency –> opportunistic infections often present, increase with course of disease
• Histology: flower-like nucleus, easy to distinguish

Question 13

HTLV-1 associated myelopathy (HAM) a.k.a. tropical spastic parapesis

Answer 13

• Autoimmune disease
• Seems to be due to strong immune response to HTLV-1 antigens, including increased chemokine levels
• Resembles MS:
  
  • Progressively paralytic disease of CNS
  • Demyelination of spinal cord nerves –> weakness, stiffness, paralysis of legs, resulting in many patients bedridden within 10 years
  • 2:1 ratio F:M
• Tx: corticosteroids for some symptomatic relief
• Uveitis: swelling/irritation of uvea (middle layer of eye), provides most of the blood supply to the retina

Question 14

AIDS-defining cancers

Answer 14

• Increased incidence with decreased CD4 counts
• Kaposi Sarcoma: HHV-8
• Non-Hodgkins lymphoma: EBV, HHV-8
• Cervical cancer: HPV

Question 15

Non-AIDS defining cancers

Answer 15

• Not affected by CD4 counts
• HAART increasing survival of people with HIV – shift in natural spectrum of HIV related disease –> more non-AIDS defining cancers
• Large range of common cancers

Question 16

“Trends” in HIV-associated cancers

Answer 16

• Cancer developing at young ages
• Become more invasive cancers more rapidly
• Atypical pathology with increased grade
• More aggressive, worse outcomes
• Relapse more common

Question 17

Retroviral gene therapy: mechanism

Answer 17

• Retroviruses used as vector in gene therapy
• Viral genes (gag, pol, env) replaced with therapeutic gene
  
  • Human retroviruses not used
• Areas of backbone homology removed to prevent recombination
• Has been used to successfully treat 20 patients with SCID
  
  • Except for the ones who developed leukemia
• Highly controversial

Question 18

Retroviral gene therapy: benefits

Answer 18

• Wide host range
• Stably integrate into host cell genome
• Long term expression of transgene
• Can easily modify genome for use as vector
• Non-pathogenic in humans

Question 19

Retroviral gene therapy: limitations

Answer 19

• Capacity for therapeutic genes is small (< 8kb)
• Infectivity limited to dividing cells
• Relatively low titers
• Randomly integrates into genome –> insertion mutagenesis