The world of genetics is filled with stories that are as gripping as the plot of any thriller. Take the IRGM gene – its saga, played out over millions of years, has all the makings of a classic drama. Act One: setting the scene. By duplicating and diverging, this gene thrived in the cells of most mammals as a trinity of related versions that played vital roles in the immune system.
Act Two: tragedy strikes. About 50 million years ago, in the ancestors of today’s apes and monkeys, the entire IRGM cluster was practically deleted, leaving behind a sole survivor. Things took a turn for the worse – a parasitic chunk of DNA called Alu hopped into the middle of the remaining gene, rendering it useless. IRGM was, for all intents and purposes, dead and it remained that way for over 25 million years of evolution.
Act Three: the uplifting ending. The future looked bleak, but IRGM’s fortunes were revived in the common ancestor of humans and great apes. Out of the blue, a virus inserted itself into this ancient genome in just the right place to resurrect the long-defunct gene. A fall from grace, a tragic demise and an last-minute resurrection – what more could you ask for from a story?
This twisting tale lies hidden in the genomes of the world’s mammals and it was discovered and narrated by Cemalettin Bekpen from the University of Seattle. To reconstruct the evolutionary story of the IRGM gene, Bekpen searched for it in a variety of different species.
(Oh come on – you try to find an image to illustrate this story!)
Most mammals, such as the dog and the mouse, have three copies located one after the other. This trio of genes are “paralogs” – slightly different duplicates of a single ancestor. Two species of lemur – the ring-tailed and mouse lemurs – also share the same three genes, but all monkeys have just the one. This suggests that the IRGM cluster shrank from a trinity into a single gene in the last common ancestor of all apes and monkeys, which lived about 40 million years ago.
If that decline wasn’t bad enough, Bekpen found that the remaining IRGM gene of monkeys is actually a dead remnant. It’s much shorter than those of other mammals, and it has been so heavily abridged that it has lost the ability to encode a protein. In technical terms, it’s a “pseudogene” and to paraphrase the Python boys, it’s an ex-gene.
The villain that struck the killing blow to the IRGM family will be familiar to geneticists – it’s a mobile piece of DNA called Alu. It could well be described as a genetic parasite, for it has the ability to cut itself out of its resident genome and insert itself into new locations. And it’s a common feature of primate genomes – as much as 11% of our own DNA is made up of Alu sequences.
In the ancestor of monkeys and apes, an Alu sequence jumped into the middle of the last remaining IRGM, disrupting its sequence. Every three “letters” of a gene’s sequence corresponds to one amino acid. To make a working protein, the gene needs to be “read” from the right position in order to manufacture the right chain of amino acids. With the insertion of Alu, that process starts from the wrong place leading to a wholly different chain of amino acids, and one that ends prematurely. It’s called a “frameshift mutation” and it renders the gene useless.
Across 15 species of monkey from around the world, from marmosets to macaques to baboons, Bekpen found the same frameshift mutation. This means that Alu must have infiltrated the primate genome before the various monkeys diverged from one another, about 40 million years ago.
For 25 million years, the IRGM gene was effectively dead. But then, in the common ancestor of humans and the great apes, something unexpected happened. The gene somehow regained its ability to produce a protein, albeit a shortened one. The gene had been resurrected, and ironically enough, its saviour was another genetic hitchhiker that inserted itself in just the right place.
The new intruder was an “endogenous retrovirus” – a group of viruses that can integrate their DNA into the genomes of their hosts. The virus jumped into a position at the very start of the gene and its own sequence then drove how the gene was read and activated. Its presence negated the harmful effects of Alu, effectively getting rid of the frameshift mutation.
We humans have this resurrected IRGM gene, and Bekpen found that it’s active in a variety of different tissues including the heart, brain, kidneys and particularly the testicles. The results don’t provide direct evidence that the gene actually produces a working protein, but there’s good reason to believe that it does.
Mutations in the IRGM gene are associated with Crohn’s disease – an inherited disease, where the immune system attacks the digestive tract, leading to chronic inflammation. That strongly suggests that the IRGM gene has regained some sort of role in the immune system, and it was one of the reasons why Bekpen started investigating the gene in the first place.
But as he says on his homepage, “Who cares about Crohn’s disease? We just discovered the Jesus gene!”
Reference: Cemalettin Bekpen, Tomas Marques-Bonet, Can Alkan, Francesca Antonacci, Maria Bruna Leogrande, Mario Ventura, Jeffrey M. Kidd, Priscillia Siswara, Jonathan C. Howard, Evan E. Eichler (2009). Death and Resurrection of the Human IRGM Gene PLoS Genetics, 5 (3) DOI: 10.1371/journal.pgen.1000403
More on genetics:
March 5th, 2009 at 9:47 pm
The actual paper reference is (Bekpen C, Marques-Bonet T, Alkan C, Antonacci F, Leogrande MB, et al. (2009) Death and Resurrection of the Human IRGM Gene. PLoS Genet 5(3): e1000403. doi:10.1371/journal.pgen.1000403) http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1000403
March 6th, 2009 at 4:35 am
Don’t laugh – if some Christians get hold of this, they’ll crow at scientific proof of resurrection. Luckily, most won’t be able to wrap their heads around it.
March 6th, 2009 at 9:11 am
Framing for the unwashed masses, sacrilicious!
March 6th, 2009 at 10:43 am
Unfortunately, the fact that most of them won’t be able to wrap their heads around it is the problem. They will make it up as they go along and the followers will believe the pseudoscience is real.
March 6th, 2009 at 9:41 pm
Don’t laugh – if some Christians get hold of this, they’ll crow at scientific proof of resurrection. Luckily, most won’t be able to wrap their heads around it.
Unfortunately, the fact that most of them won’t be able to wrap their heads around it is the problem. They will make it up as they go along and the followers will believe the pseudoscience is real.
Posted by: Kay |
Interesting I like many ppl believe in both chritianity and evolution. Your arrogance makes you little different than the ppl you believe yourselves better than.
Ive found the feild of genetics to be very interesting now that were learning so much more.
March 7th, 2009 at 1:37 am
“Your arrogance makes you little different than the ppl you believe yourselves(sic) better than.”
I think the grammatical error reveals how disingenuous you are.
March 7th, 2009 at 4:03 pm
loving the picture.
And I really wouldn’t be too surprised if this article winds up on a blog like Denyse O’Leary’s horrible Post-Darwinist (intentionally not linking heh). Probably with some bizarre title like “evolutionary genetics shows proof of Jesus, scientists bury evidence”
March 7th, 2009 at 4:27 pm
Okay enough folks. Yes I’m sure there’s plenty of potential for people to wildly misinterpret the study. But until they do, how about we stop predicting the ways in which they could and concentrate on the really interesting piece of research here?
It would be a sad day for science if the only reaction to fascinating pieces of research was mass speculation about the potential reactions of the anti-science lobby.
March 8th, 2009 at 8:59 pm
Hi Ed! Just found your blog (thanks to the “Now on Science Blogs” ad above). This post was the first one I read, and, boy oh boy(!), it is a good one! This is fascinating. Consider yourself bookmarked. I’m surprised I haven’t run across this information yet on other science sites I visit.
Incidentally, appreciate your comment above. There’s no need to toss slurs around especially over something that hasn’t happened.
Anyway, off to read some of your older articles.
March 8th, 2009 at 9:14 pm
Thanks Daniel. Glad to have you on board
And as to why this hasn’t been mentioned anywhere else, it may just be that it’s breaking news. The paper was only published last Thursday!
March 10th, 2009 at 1:53 am
Abstruse
What grammatical error? “You” can be either singular or plural in standard English. It seems clear that James was not criticizing both Kay and Steve, both of whom were quoted in his comment. Two people = plural = yourselves.
March 10th, 2009 at 1:54 am
James was not criticizing
D’oh! Strike out the “not”. That’s what happens when you rephrase and forget to proofread …
March 10th, 2009 at 2:25 pm
“Who cares about Crohn’s disease?”
Well, one would suspect those who suffer from it do.
March 10th, 2009 at 9:50 pm
http://www.youtube.com/watch?v=kyAzwREVBZs
I don’t see why you guys turn everything into a Creationism vs. Evolution debate. As I see it, Creationism should at least be everyone’s wish, because only God can save us from what you see in that video overtaking the earth within 50 years. Damn you, evolution. DAMN YOU TO HELL.
March 14th, 2009 at 5:19 pm
What the fuck does Jesus have to do with a story about genetics? How about just reporting the facts in a science blog?
March 14th, 2009 at 6:34 pm
It’s a blog, not a peer reviewed scientific journal (and even they have editorials). Who died and made you the blog police? Or perhaps you’re from the bureau of oversensitivity to mild tongue-in-cheek religious references? Or the department of absurdly narrow interpretations of media classifications?
November 20th, 2010 at 8:58 am
16 comments and NOT ONE is of a scientific nature, and mine makes 17. I’m not knowledgeable enough to say anything scientific on this subject (but I do spell correctly). I read this wonderful Discover website to learn science and usually I learn more science from the comments. But not today.