Wednesday, February 17, 2010

Shhhhh - quieting the troublesome genes

Welcome to the inaugural post of Why You Will Love Science.   Today we discuss a paper published in the Proceedings of the National Academy of Sciences discussing some really cool emerging technology.  The title, "Lipid-like materials for low-dose, in vivo gene silencing" is a great summary.   This seemed like an excellent paper to start with because it delves into some relatively recently understood fundamental basics of biology, the authors work diligently to show it works across different species, including nonhuman primates, and the work shows some exciting progress towards new types of medicines - specifically in the hot arena of gene therapy. So let's begin, shall we?

So you're not left hanging, here's the punchline.  The authors have developed a way to get small nucleic acid particles targeting specific genes to turn down the expression of those genes into liver cells of animals.  Not only did they turn down the expression - or silence - one gene, but up to five genes simultaneously in the same animal.   And they were able to do this at very low levels of "drug" with no obvious side effects.  Pretty cool? Definitely.

First a bit of background.
siRNA are little pieces of genetic material that takes the product from a gene, the messenger RNA, and diverts it from making a protein, and generally targets it for destruction.  It's a way your cells have of regulating genes, amazingly only discovered about 20 years ago.  This paper looks at using siRNA to do gene therapy - modifying a gene to treat a disease.  This is a tricky thing to do because not only is it really hard to get genetic material into a cell - there's a little something called an immune system in the way - and once genetic material gets to the target cell, it's difficult to generate a sustained response in the cell.

So what did they do?

They first created a library of fat-like particles called lipidoids of varying sizes in a way that was fast (3 days) and could be completely automated.  This is important for future applications.  Next, they tested how effective each little fat bubble was at getting the siRNA into a cell.  Most were not effective but a few looked really promising.  They chose a few really potent ones and tested in mice.

The first test in mice was to target a liver-specific gene, Factor VII.  They chose one particle type that was effective at a very low dose, and added a few modifications to make it more invisible to the immune system and less sticky with other blood proteins, giving the particles an extra chance to get into cells. The most potent one was several hundred fold more potent than the current method.  They dubbed the particle C12-200.  With a very low dose they observed lowered levels of Factor VII for up to 25 days, with little side effects in the mice.

They next wanted to see if they could target multiple genes at the same time.  They added in siRNAs that targeted four other liver-specific genes  At low doses that were well tolerated by mice they could  successfully knock down all five of the genes simultaneously.  Very cool.

Next they looked to see if the best lipidoid would work in non-human primates.  Again they chose a liver-specific gene.  And it worked.  At very low doses.  With no apparent side-effects.

So why are these little fat bubbles so interesting?  Some diseases, such as hepatitis C, have no effective treatments but has unique genes that would be great siRNA targets. We might be able to use these little fat bubbles to target the genes in HepC, shutting them off, and providing a cure.  Or, if you had a disease with multiple genes involved, such as cancer or metabolic syndrome, you could target those genes, altering their expression and stopping the disease in their tracks.

What is still missing?  Well, they only looked at genes active in one organ, the liver.  This made for clean experiments but for other applications, it will be important to know how wide-spread the siRNAs have their effect.  For example, in a cancer cell, turning off a gene might be a good thing, but in a normal cell it might have dire consequences.  This still needs to be tested.  Also, in mice the length of time of gene silencing was quite a long time, but it would be nice to know if they can give a second dose and quiet those genes again.


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