More on the viruses inside us: retrotransposons

I've talked about jumping genes, and I've talked about endogenous retroviruses, so now it's time to combine the two and talk about retrotransposons.

Retrotransposons are a subclass of transposons, genetic elements that can make copies of themselves and, once copied, can "jump" to different places in the DNA (hence the name "jumping genes"). Retrotransposons in particular are first transcribed to RNA, and then the RNA is copied back to DNA through reverse transcription, just like retroviruses do.

"Close to half of the human genome is derived from retrotransposons replicating by the copy-and-paste mechanism used by exogenous retroviruses such as HIV. These genetic invaders are both essential motors of evolution and threats whose uncontrolled spread would be fatal to their host [1]."

There are so many intriguing facts to be mentioned about retrotransposons: we seem to have accumulated them through our evolutionary history, as they tend to increase with evolutionary complexity, covering from 3% of the genome in yeast to 44% of the genome in humans. The active ones are responsible for a significant chunk of spontaneous mutations. They are quite abundant in plants, and in fact they were first discovered in maize. And finally, while many retrotransposons derived from endogenous retroviruses that got inserted into the germ line (as I explained last week), there's evidence that also points at the opposite mechanism taking place as well: ancient retrotransposons in the genome that acquired an envelope gene from a viral source and found an escape back to the viral world [2].

Pretty cool, right?

And here's the icing on the cake, which ties it all with yet another earlier post of mine:

"Retroelements are permanently inactivated during embryonic development so as to exhibit a transcriptionally silent state in adult tissues and in the germ line. Their tight regulation is important to prevent insertional mutagenesis and needs to withstand zygotic genome activation, which takes place at the two-cell stage, shortly after fertilization, as well as the ensuing DNA demethylation that is required for reprogramming [1]."

I know, it gets all very complicated, but if you go back to this post, and to the figure in that post in particular, you'll remember how cells "reprogram" themselves during embryogenesis. All epigenetic markers are reset so that cells can start fresh and differentiate into the various tissues needed to make a new being. Well, it turns out, retrotransposons are involved in this process too, and retroelements are silenced through cytosine methylation during embryonic development. The pathways involved are extremely complex and the various elements interact with one another in multiple ways, as "development itself is in fact controlled by temporal and spatial expression and repression of retroelements, and some cellular genes crucial during this period are co-regulated with ERVs and related entities [1]."


[1] Rowe, H., & Trono, D. (2011). Dynamic control of endogenous retroviruses during development Virology, 411 (2), 273-287 DOI: 10.1016/j.virol.2010.12.007

[2] Malik HS, Henikoff S, & Eickbush TH (2000). Poised for contagion: evolutionary origins of the infectious abilities of invertebrate retroviruses. Genome research, 10 (9), 1307-18 PMID: 10984449