B10 - Endogenous Retroviruses & Cellular Transformation

What are v-onc?

Viral oncogenes

What do Rapid Transforming Viruses encode alongside the core set of retroviral genes?

Also encode viral oncogenes (v-onc)

• There are related to a cellular gene (c-onc) but have been pirated by the virus to control cell proliferation

What are the five classes of oncogenes?

• Growth hormones

• Receptors for extracellular growth signals

• G proteins

• Protein kinases

• Transcription factors

What are viral oncogenes usually missing when compared to their cellular counterparts?

Usually are missing key regulatory regions.

This means viral oncogenes are always ‘on’

What happens to most viruses with encoded oncogenes?

These viruses are usually replication defective.

• May have variable deletions of one or more core retroviral genes

• Many retroviral oncogenes can also become expressed as a gag-onc fusion

• These viruses can only replicate if the virus is also infected with a non-defective virus

How can cells become transformed without viral oncogenes?

Insertion of a provirus into the cellular genome can influence expression of proto-oncogenes

• Can produce a novel transforming factor

How can proviral LTRs affect cellular gene expression?

Viral LTRs can act as promoters for cellular oncogenes

• Enhancer sequences in the U3 region of the LTR can activate c-oncs

Provirus integration may result in a fusion transcript

• This creates a novel protein

Provirus integration may stabilise c-onc mRNA

What protein does Human-T-cell leukaemia virus type 1 (HTLV-1) cause production of?

Tax protein

What does HTLV-1 Tax protein cause?

• Transcriptional activation of viral genome

• Transcriptional activation of cellular genes

• Stimulates cell cycle progression

• Causes chromosomal instability

What are Endogenous Retroviruses?

Retroviral sequences which are fixed in our genomes

• Due to multiple insertional events

What is the most recent ERV event?

HERV-K subtype

How much of the human genome is estimated to be of viral origin?

8%

ERVs are sites for what?

Genetic recombination

• Duplications

• Deletions

• Chromosomal reorganisation

What is Exaptation?

An adaptation which fulfils a new function distinct from its originally selected function

What is an example of a HERV which has been modified for our benefit?

Syncytin-1/2 which are used for cell to cell fusion during placental formation

• Originally were HERV env proteins and are near identical to retroviral envelope protein genes

How are some genes controlled by HERV insertions?

Some genes have acquired HERV LTRs and these LTRs have become the functional promoters for that gene

• This allows for host control over gene expression

Why can most ‘old’ HERVs not make new virus particles?

They have accumulated too many mutations over time making them functionally unable to produce virus particles

Why could HERV-K subtypes potentially be able to produce virus particles?

These HERVs have been seen to be competent and have all the genes necessary to produce virus particles.

• Still unclear if these virus particles are infectious or harmful

Why are ERVs potentially important during transplantation?

Porcine ERVs can produce particles which can infect human cells

• Therefore PERVs pose a risk during xenotransplantation

However PERV transmission has yet to be observed

What are Transposons?

‘Jumping genes’

Genes which can move around the genome

What are LTR Retrotransposons?

Long Terminal Repeats (LTRs) which flank a coding region

Using a RNA intermediate, these LTRs can allow for the coding region to move inside the genome

How are LTR Retrotransposons capable of moving genes?

LTR Retrotransposons contain all the genes needed to replicate (gag, pro and pol) but lack the env gene

This means they lack an extracellular phase (cannot produce a virion)

What is a Retroviral DNA vector?

Contains the retroviral LTRs, the RNA packaging signal (Ψ) with the desired gene cloned in-between

What is a Packaging Cell Line?

Cell line which expresses retroviral proteins:

• Gag

• Pol

• Env

This allows for packaging of retroviral vector RNA into a virion

How does Retroviral Gene Therapy work?

Retroviral Vector DNA is transfected into a Packaging Cell Line

The desired gene of interest is then packaged into a virion and released from the cell

This virion can then infect the target cell and integrate the gene of interest into the target cell’s genome

What is done to produce Retroviral Pseudotypes?

The env protein ORF is replaced

• This means no replication in target cells as gag-pol ORF is not present in the final Virus-Like particles

What are the advantages of Lentiviral/Retroviral vectors?

• Small genome which is easy to work with

• Will infect many cell types

• Stable gene expression over time

• Gene of interest is integrated into the genome of the target cell

What are the disadvantages of Lentiviral/Retroviral Vectors?

• Limited to ORFs <10kb

• Transgene can insert near a proto-oncogene and result in cancer