Genome's tale of 'conquer and enslave'

Toronto scientists uncovered how viral remnants helped shape control of our genes.

If genes were lights on a string of DNA, the genome would appear as an endless flicker, as thousands of genes come on and off at any given time. Tim Hughes, a Professor at the University of Toronto's Donnelly Centre, is set on figuring out the rules behind this tightly orchestrated light-show, because when it fails, disease can occur.

Genes are switched on or off by proteins called transcription factors. These proteins bind to precise sites on the DNA that serve as guideposts, telling transcription factors that their target genes are nearby.

In their latest paper, published in Nature Biotechnology, Hughes and his team did the first systematic study of the largest group of human transcription factors, called C2H2-ZF.

Despite their important roles in development and disease, these proteins have been largely unexplored because they posed a formidable challenge for researchers.

C2H2-ZF transcription factors count over 700 proteins -- around three per cent of all human genes! To make matters more complicated, most human C2H2-ZF proteins are very different from those in other organisms, like those in mice. This means that scientists could not apply insights gained from animal studies to human C2H2-ZFs.

Hughes' team found something remarkable: the reason C2H2-ZFs are so abundant and diverse -- which makes them difficult to study -- is that many of them evolved to defend our ancestral genome from damage caused by the notorious "selfish DNA."

Selfish DNA are bits of parasitic DNA whose only purpose is to multiply, a kind of virus for our genome. They seize a cell's resources to make copies of themselves, which they insert randomly across the genome -- causing harmful mutations along the way.

Almost half the human genome is made of selfish DNA, which probably came from ancient retro-viruses which, similar to modern counterparts, inserted their DNA into the host's genome. When this happens in an egg or sperm, the viral DNA gets passed on to the next generation, and the selfish DNA is then known as endogenous retro-elements (EREs).

Evolutionary biologists believe that selfish DNA was instrumental in making genomes bigger, giving natural selection additional DNA material to tinker with.

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Genome's tale of 'conquer and enslave'