RETRO VIRUSES ARE REQUIRED FOR PLACENTAL DEVELOPMENT
Image care of Gore 2006
A team of scientistsfrom Texas A&M University have shown (using gene knock-out technology) that endogenous retroviruses are required for sheep placental formation.
The retroviruses studied were related to Jaagsiekte sheep retrovirus (enJSRVs) and are present in the genome of all mammals, including humans.
Retroviruses use the genetic code of their hosts to power their replication and include the human immunodeficiency virus (HIV).
Dr. Thomas Spencer, Texas A&M’s chief reproductive biologist, published his findings on September 11 in the Proceedings of the National Academy of Sciences.
Understanding retrovirus function in sheep should provide researchers with a better understanding of their function in their mammalian counterparts including humans.
WHAT IS A RETROVIRUS?
Retroviruses are viruses that use the genetic code of their hosts to power their reproduction. Retro Viruses are called Retro viruses because they insert their DNA code into the DNA of their host that allows the virus to use the hosts’ DNA transcription molecules to replicate itself.
Many retroviruses are damaging to their host e.g. the most widely known retrovirus is the human immunodeficiency virus (HIV). However, some retroviruses have been evolved together with their hosts and have provided (by chance) and evolutionary advantage to the host. This is what has happened in the mammal genome as has been prooven by Dr Thomas Spencer and his team at Texas A&M.
The figure below shows how a virus (the red line D) has inserted itself into the host’s DNA so that when the host replicates its DNA or produces mRNA (E), the virus is also replicated (shown as red line C) and uses the cellular machinary to then transcribe its own replications (B&A) that can then infected other cells and their gene sequences.
Figure 1, a cartoon of how a retrovirus replicates (care of: http://www.baclesse.fr)
RETRO VIRUSES & EVOLUTION
Dr Spencer’s results have shown that the enJSRV retro virus must be present and functional within the sheep’s DNA for full placental development and therefore successful reproduction.For 30 years scientists have suggested that endogenous retroviruses provided a key step in the evolution of the mammalian placenta (dailyscience). What Dr.Spencer’s results show is that enJSRV must have provided some evolutionary benefit and have been heritable in the early mammals it infected, for it to be present and highly conserved today in sheep and all placental mammal. This follows the basic dogma if evolution that if a mutation provides a better survival advantage and is heritable it is likely to become dominant within a population. This theory was first developed by Charles Darwin.
Figure 1, a photo of the father of evolution, Charles Darwin.
PROOVING THAT A RETRO VIRUS IS REQUIRED FOR SHEEP REPRODUCTION
Dr Spencer’s team prooved that enJSRV was neccessary for successful placental formation and therefore successful reproduction by knocking-out the gene that normally codes for the enJSRVs retrovirus.
Genes can be knocked out by inserting a backwards copy of the gene into the host’s genome. When the forward and backward genes are read and created by the genetic machinary they form opposite messengers (mRNA). These opposite messengers are then like polar magnets and are attracted to one another. When two messengers are attached to one another then they can no longer create the message (protein) they have been created for. A video summary of this technique referred to as RNAi can be found in the links section of this blog.
Therefore by knocking-out the gene for the enJSRV retro virus Sheep that had the retrovirus gene knocked-out showed inhibited growth of the placenta cell type called giant binucleate cells, which led to miscarriage.
RESEARCH KEY FINDINGS & SUMMARY
Dr Spencer showed that:
- EnJSRVs are necessary for mammalian placental development by knocking out the gene that codes for enJSRVs.
- The enJSRVs Retro virus has co-existed as part of the highly conserved genetic sequence within placental mammals since the days of the early mammal
- Mammalian knowledge gained through sheep research could be utilised to better understand human reproductary systems, minimise the risk of miscarriage and further human reproductive science.
REFERENCE LIST
Baclesse, retrovirus image, URL: http://www.baclesse.fr/cours/fondamentale/7-carcino-virale/Images/Retrovirus.gif, accessed 12/10/06
Dailyscience, Researchers discover that sheep need retroviruses for reproduction, URL: http://www.sciencedaily.com/releases/2006/09/060911233630.htm, accessed 12/10/06
Gore, URL: http://www.goreplace.org/SHEEP-PIX/sheep-lamb-high-res.jpg, accessed 12/10/06
Wiki, Charles Darwin photo, URL, www.wikipedia.com, accessed 12/10/06
RETRO VIRUSES ARE REQUIRED FOR SHEEP PLACENTAL DEVELOPMENT 
