Not Exactly Rocket Science

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Millions of people in Latin America have been invade by a parasite – a trypanosome called Trypanosome cruzi. They are passed on through the bite of the blood-sucking assassin bug and they cause Chagas disease, a potentially fatal illness that affects the heart and digestive system. The infections are long-lasting; it can take decades for symptoms to show and a third of infected people eventually die from the disease. But T.cruzi does much more than invade our flesh and blood; it also infiltrates our genomes.

T.cruzi is unusual in that a massive proportion of its DNA, around 15-30%, lies outside of its main genome. These accessory sequences are stored in the form of thousands of interlinked DNA rings. In the parasite, these sequences are found in the mitochrondria – small structures that provide it with energy – but they have found a way to spread much further.

According to new research from Mariana Hecht and a team of Brazilian scientists, T.cruzi has the ability to inveigle its DNA rings into the genomes of those it infects. Once inside, the parasite genes can hop around, hitchhiking from one chromosome to another and leaving genetic chaos in their wake. They can even be passed on from one generation to the next. Hitching a ride aboard sperm and eggs, they can add themselves to the genomes of children, who’ve never been in direct contact with trypanosomes.

Hecht’s discovery suggests that T.cruzi is an unexpected source of genetic diversity in our species. It’s certainly not the only parasite to do this. Viruses have been infiltrating our genes since time immemorial and a massive part of our genome has a viral origin. These events, where viruses joined our family tree, provided raw material for natural selection. Some viral genes wreaked havoc by disrupting important genes, while others were eventually domesticated to act as helpful, even necessary, parts of our genome.

But T.cruzi is a different story. Despite its microscopic, single-celled nature, it’s a vastly complex creature compared to a simple virus. And it continues to breach our DNA today. Now that we’re getting technically better at detecting such “horizontal gene transfers“, we may find that many other parasites are also smuggling their genes into ours. In Hecht’s words, the “human population may be a patchwork of all the organisms to which it has ever been exposed.”

i-f666df8112bd5188ad40fc034c8f6b1c-Assassin_bug.jpgHecht’s discovery began with a simple observation – some rabbits and human patients would still die from chronic Chagas disease even after antibiotics had exterminated the last of the trypanosomes in their bodies. The hearts of these individuals were under attack from their own immune system – a common Chagas symptom. Hecht suggested that genetic fragments left behind by the parasites could be sustaining these chronic attacks, so she set out to find them.

Hecht analysed the genes of 87 people from five Brazilian families, whose oldest members all suffered from Chagas disease. These family heads, along with some of their descendants, still had antibodies that targeted T.cruzi, meaning that they still had active infections. Twenty-five of these people had trypanosome DNA in their genomes.

The trypanosome mostly inserts its DNA into parts of the human genome that are already mobile. They’re especially fond of jumping into small sequences called LINEs that can copy and paste themselves into new parts of the genome. LINEs make up around a fifth of our genome so they’re plentiful targets for the trypanosomes. During an infection, Hecht envisages the parasite flooding the nucleus of host cells with thousands of DNA rings that form hundreds of junctions with LINE sequences.

The parasite can also infect the “germ-line cells” that produce sperm and eggs, giving it a chance to transfer its genes into not just the current host, but all the host’s children too. Indeed, among the three families that Hecht worked with, 29 sons and daughters had trypanosome DNA in their genes, despite never having been infected by the parasite themselves. Trypanosome genes can become unplanned heirlooms, passed down the generations and jumping into new places every time.

And the close association between these genetic parasites allows the trypanosome sequences to hop into new sites from their initial staging grounds. This invading DNA can have dire consequences depending on where it’s inserted. In many cases, Hecht found that they fused disparate parts of different chromosomes, and one in eight insertions knocked out important genes, involved in everything from cell communication to the immune system to our sense of smell.

i-2f34ac765eded7895ace252dbf68b717-Chagas_heart.jpgHecht thinks that in some cases, the fusions of human and parasite genes could produce actual proteins. They may not do anything, but their presence could draw the attention of the immune system, leading to many of the autoimmune symptoms of Chagas disease. For now, this is just speculation. To truly understand how these jumping genes contribute to Chagas, we’ll need to sequence the complete genomes of many patients and compare symptoms with sequences.

T.cruzi has the potential to be an important driver of genetic diversity. It infects millions of people from Latin America, about a million of whom are migrating to other continents. It also affects over 1,250 species of wild mammals.

This might just be the tip of the iceberg. Other complex parasites set up shop in the human body for long, persistent infections, including other species of trypanosome. We don’t know whether these body-snatchers also donate their DNA to us but finding out should occupy Hecht and other scientists for years to come. What other genetic duets await discovery?

Reference: Hecht, M., Nitz, N., Araujo, P., Sousa, A., Rosa, A., Gomes, D., Leonardecz, E., & Teixeira, A. (2010). Inheritance of DNA Transferred from American Trypanosomes to Human Hosts PLoS ONE, 5 (2) DOI: 10.1371/journal.pone.0009181

More on horizontal gene transfer:

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Comments

  1. #1 Brian Schmidt
    February 14, 2010

    Very interesting, as usual. Is the gene transfer an accidental by-product of infestation, or is it part of a life cycle? It’s hard to imagine how T.cruzi would evolve this capability unless it’s purely accidental, or unless there’s some way for the genes that hop into humans to hop back into T.cruzi and direct it to keep doing what it’s doing. Or maybe the genes don’t hop back, but they alter human biology somehow in a way that benefits the parasite.

  2. #2 Comrade PhysioProf
    February 14, 2010

    I like how you used the word “inveigle”!

  3. #3 David Kroll
    February 14, 2010

    Ed, what a lovely discussion of an absolutely fascinating article.

    In my work on DNA topoisomerase II, I came to use the kinetoplast DNA networks of another trypanosome, Crithidia fasciculata as a substrate for the unlinking of the networks via the double-strand DNA cleavage, strand passage, and religation activity of the enzyme. When run by agarose gel electrophoresis, the network remain in the wells while the decatenated “minicircles” run into the gel in proportion to topoisomerase II activity.

    The authors note that topoisomerase II could be involved in the recombination event at LINE sequences, but they do not mention whether it may be due to the enzyme in the trypanosome of the cellular host. Human topoisomerase II inhibiting drugs are used primarily for cancer chemotherapy (etoposide, doxorubicin and, most recently, pixantrone) but we commonly use as antibiotics bacterial topoisomerase II (DNA gyrase) poisons such as the fluoro-4-quinolone antibiotics because they are more selective for bacteria vs. human topo II.

    Therefore, I’ll be interested to learn of follow up studies to this paper on the mechanism of recombination. Should the event be catalyzed by the trypanosomal topoisomerase II, there may be a therapeutic opportunity to selectively inhibit this activity responsible for the long-term sequelae of Chagas disease.

    Fascinating. Thank you.

  4. #4 Michael Meadon
    February 15, 2010

    Interesting in the extreme. Thanks, Ed.

    I’m wondering… There is a longstanding controversy about what disease afflicted Charles Darwin. Peter Medawar argued it was Chagas. (There was also a recent article in the Christmas edition of BMJ that suggested it was cyclical vomiting syndrome or something). Could it be possible to test the Chagas hypothesis by looking at the DNA of his descendants?

  5. #5 Ed Yong
    February 15, 2010

    Well that gets my vote for Comment of the Year.

  6. #6 Poodle Stomper
    February 15, 2010

    Fascinating! I haven’t had the time to read the whole paper yet but I’m wondering if the study showed whether the transferred sequences are from T.cruzi itself or from parasitic transposon-like elements that previously parasitized the T.cruzi genome? Does it mention which genes were transferred?

  7. #7 zackoz
    February 15, 2010

    IANAS (I am not a scientist), but as I understand it if a parasite somehow inveigles itself into a host’s germ line, as you mention sometimes happens here, it eventually reaches a less hostile relationship with the host. The host’s survival becomes a positive factor for the parasite instead of a matter of indifference to it, because host reproduction helps carry on the parasite’s line too. Charles Darwin just needed to wait a further 100,000 years or so. (No mockery intended of the immortal Charles).

  8. #8 Stephen Wells
    February 16, 2010

    Suddenly, a thousand B-movies become plausible; being bitten by a bloodsucker really can alter your genes.

  9. #9 Daniel J. Andrews
    February 16, 2010

    I imagine it still might be possible to get some of Darwin’s DNA directly. (?)

  10. #10 Michael Meadon
    February 17, 2010

    Getting Darwin’s DNA directly? I don’t know. I suppose it might be, but it all depends on what’s preserved of him and how well. (Is there hair?). I doubt we could exhume his body…

  11. #11 Gabriel Fazito
    February 20, 2010

    Truly fascinating stuff and also quite logical. The intracellular amastigote cycle of the parasite makes it likely to occur DNA swapping if the cell reacts to the infection or if the cell dies from the infection. It probably works both ways but it might be a little harder to identify human genes in the right T. cruzi unless you do a populational study. Hm, that’s actually more interesting than my old line of research on Chagas disease.