Rous Sarcoma Virus

 

 

DEFINITION OF RSV

 

     As we saw before, the Rous Sarcoma Virus is a retrovirus.  Historical evidence suggests that this was one of the first retroviruses discovered.

 

Rous experiments involved injecting cell-free extract of chicken tumor into the cells of a healthy chicken.  This resulted in oncogenesis whereby the chickens developed sarcomas.  According to Temin and Rubin (2004), the Rous sarcoma virus “changes a chick fibroblast into a "morphologically new and stable cell type”.  

 

 

 

 

STRUCTURE OF RSV GENOME

 

     Scientifically, the genes of retroviruses are encoded in RNA, not DNA. The virus contains a reverse transcriptase. This is a DNA polymerase that uses RNA as its template.

 

The RSV genome has terminal repeats.  This allows the RNA to integrate into the host genome.  It also causes an over-expression of the RSV genes.

 

The RSV is constructed with a simple genetic structure. It comprises of four (4) genes. The nature and function of each gene is outlined:

 

DIAGRAM SHOWING THE GENOME OF RSV.  (Image adapted from Cooper, 2000)

 

• gag  codes for the capsid protein
• pol codes the reverse transcriptase
• env codes the envelope protein
• src codes a tyrosine kinase which is an enzyme that attaches phosphate groups to Tyr residues on a variety of host cell proteins (Rous Sarcoma Virus (RSV), 2005)

 

The (gag, pol and env) code for proteins which are needed to replicate the viral genome and to encapsulate it for further transmission. The fourth gene, the v-src, codes for a protein that on its own, can induce itself in cultured cells.  This is the morphological and behavioral changes that scientists characterize as changes in tumor cells.  Hence it increases the virus' virulence, but it not essential for RSV proliferation.

 

The v-src gene is responsible for the tumor causing potential of the virus and was the first of many tumor-causing genes, or oncogenes, to be isolated from viruses. An oncogenic virus is a virus that is capable of causing cancer.  

 

The oncogenic properties of the v-src protein arise from disruptions in an internal control mechanism of the host DNA that normally prevents the activation of the protein in the absence of external signals.

 

 

 

 

Diagram illustrating the genomes of the murine leukemia virus and the Rous sarcoma virus.  Note that RSV has the extra fourth gene: src. (NCBI, 1971)

 

 

 

 

PROPERTIES OF RSV

 

 

1.  Temperature: A controlling factor

 

     Rous Sarcoma Virus possesses one particular property: temperature sensitivity whereby it is sensitive to temperature changes. Whenever a cell transformed by the retrovirus is being placed under a climate of 36 Celsius degrees, the virus becomes active. When it is under a climate of 41.5 Celsius degrees, however, it is inactive. This phenomenon is reversible, thus, only requiring a change in the temperature to change its activity (Martin et al., 1971).

   

 

Diagram illustrating the activity at different tempertures. (Retrieved March 31, 2010, from The Rous Sarcoma Virus (RSV), 2005)

 

  

 

2.  RSV has a Proto-oncogene

 

DEFINITION OF PROTO-ONCOGENE

     

     A proto-oncogene is the normal cell genes from which the retroviral oncogenes are derived. In many cases, these important cell regulatory genes encode proteins which function in the signal transduction pathways controlling normal cell proliferation (e.g., src, ras, and raf). However, the oncogenes are abnormally expressed or mutated forms of the corresponding proto-oncogenes. As a result of such alterations, the oncogenes induce tumor development and abnormal cell proliferation. 

 

     An oncogene incorporated into a retroviral genome differs in several respects from the corresponding proto-oncogene. Firstly, rather than being controlled by the normal transcriptional regulatory sequences of the proto-oncogene the viral oncogene is transcribed under the control of viral promoter and enhancer sequences. As a consequence, oncogenes are usually expressed at much higher levels than the proto-oncogenes and are sometimes transcribed in inappropriate cell types. Thus, in several cases, such abnormalities of gene expression are sufficient to convert a normally functioning proto-oncogene into an oncogene that drives cell transformation.

 

 

DIFFERENCE BETWEEN VIRAL-ONCOGENES AND CELLULAR-ONCOGENES (or PROTO-ONCOGENES)

 

      Viral oncogenes are quite different from cellular oncogenes.  The cellular-oncogene is a  proto- oncogene and it is designated c-src. Viral oncogenes are referred to as v-src.   However, c-oncogenes are not identical to their corresponding v-oncogenes. As a result, it seems that the virus has picked up a cellular growth controlling or differentiating gene and after the gene was acquired by the virus it was subjected to mutation. Cellular-oncogene differs from viral-oncogene in two main ways:

 

1. c-src is a split gene, and it is similar to most eukaryotic genes. It contains many exons which are separated by introns.
2. v-src genes always contain mutations, and as a result they are distinguished from c-src of chickens. (The Rous Sarcoma Virus, 2005)

 

 

 

 

Diagram illustrating position of the src gene within the RSV genome (taken from The Rous Sarcoma Virus, 2005)

 

 

 

V-onc causes havoc in the cell and c-onc does not because of a difference in genes. Mutations occur in the genes once it was picked up by the virus.  These changes include:

 

1. Amino acid substitutions or deletions which result in altered translation products

 

2. Many v-onc proteins are fusion proteins translated from a v-onc that is a hybrid gene of a c-onc and a viral gene

 

3. V-oncs are inserted into the host genome along with LTR's which contain promoters/enhancers. This is likely to result in  over expression of a gene that we know is probably involved in DNA transcription and replication.

 

 

 

CHARACTERISTICS OF CELLULAR-ONCOGENES

 

           These are classic cellular genes with typical control sequences and as with most eukaryotic genes, most have introns (while retroviral oncogenes - v-oncs - do not).

     

      They show normal Mendelian inheritance because they are normal genes, essential to the functions of the cell. However, they are always at same place in genome as with all genes in the eukaryotic genome and it contains  no Long Terminal Repeats sequences (v-oncs always are in an LTR context).

 

     Viral oncogenes are similar the c-onc of the animal from which the virus is thought to have acquired the gene. Thus, v-src of RSV is more like chicken src than human src.

 

     Therefore it is important to note that v-onc was long ago acquired accidentally by the virus from the genome of a previous host cell. Cellular oncogenes are expressed by the cell at some period in the life of the cell, often when the cell is growing, replicating and differentiating normally. They are usually proteins that are involved in growth control and cellular oncogenes are highly conserved

 

 

 

Proto-Oncogene Video (http://www.youtube.com/watch?v=2wIVwZksIt4)

 

 

 

For more information, go to: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC389513/pdf/pnas00086-0122.pdf (temperature-sensitive)

 

 

 

 

 

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