Medicine and evolution, Part 3: A trypanosome shows the way

Earlier this week, I wrote about how the principles of population evolution can be applied to premalignant lesions in order to predict which lesions would progress to cancer. This time around, I'd like to discuss how using evolutionary principles can provide insights to human disease that would not be as obvious or that would take much longer to discover without considering evolution. One of the beautiful things about evolution and applying it to medicine is that one can find connections in unexpected places that may actually shed light on the pathogenesis of human diseases and even suggest new ways to treat disease. One such example was featured in Nature a couple of weeks ago in an article out of Lancaster University entitled, Flagellar motility is required for the viability of the bloodstream trypanosome.

This article is remarkable for a number of reasons. First, it shows a new and unexpected function of flagellar proteins in a human parasite. Next, it reveals one of those unexpected connections between organisms that never cease to amaze. Finally, it provides more evidence (as if any more were needed) that the concept of the "irreducible complexity" of flagella that "intelligent design" (ID) creationists like to point to as "evidence" against evolution by natural selection and for an "intelligent designer" is dubious at best.

The trypanosome studied was Trypanosoma brucei. This nasty little bug is a parasite carried by the tsetse fly in Africa that can cause a devastating disease known as sleeping sickness in humans and animals when transmitted by fly bite. Migration of this parasite between the gut and the salivary glands of the tsetse fly vector is absolutely critical to its life cycle (see image below), but the role of motility in its extracellular bloodstream form was unclear. In both forms, a single flagellum emerges from a posterior flagellar pocket and is made up of a membrane-bound axoneme and a paraflagellar rod. Not surprisingly, from an evolutionary perspective, the overall structure of flagella in various microorganisms is highly conserved. In diverse organisms, such flagella consist of nine microtubules surrounding two or zero core microtubules. It had been assumed that this conservation at the level of a larger structure like a flagellum implied conservation of structure in the core proteins making up the structure.

Broadhead et al did a detailed proteonomic analysis of the proteins making up the T. brucei axoneme. First, they isolated the axonemal proteins using detergents and salts and then subjected them to one- and two-dimensional gel electrophoresis. They then cut the protein bands from the gels, ultimately identifying 338 proteins making up the proteome of the the axoneme. They then digested the isolated proteins with trypsin, and subjected them to mass spectroscopy. The results of this study revealed surprising diversity among organisms of the proteins that make up the axoneme. Some 208 of the 338 T. bruceii proteins seemed to be Trypanosomid-specific, most likely reflecting Trypanosome-specific functions.

However, when the investigators did a search against various databases looking for proteins in other organisms that resemble the proteins they found in the trypanosome flagellum, it turned out that several of these flagellar proteins have homologues in the human genome. Moreover, these human proteins resembling the trypanosomal proteins are not just any proteins; they are proteins that have been linked to human diseases. And it's not just any diseases that these genes were associated with, but diseases involving cilia, finger-like projections from specialized cells that are involved in cellular motility, absorption, and transport. For example, the cilia in your trachea beat constantly back and forth in order to help move the mucus produced in your lungs upward, so that it doesn't settle. When it was all done, the investigators identified 34 genes that ultimately mapped to 25 loci. These had been associated with human diseases and roughly mapped by genetic techniques, but the precise genes responsible for the diseases in question have not yet been identified. The genes identified were all associated with diseases involving the cilia, including primary ciliary dyskinesia, polycystic kidney disease, macular and cone-rod dystrophies, retinitis pigmentosa, and other diseases.

It's this sort of finding that makes me marvel at the elegance of evolutionary theory, of how genes encoding for proteins in primitive unicellular organisms like T. brucei can be found in very similar form in a much different organism (humans) serving functions related to their original function in the ancient unicellular organism. On a philosophical level, it all provides evidence of the oneness of life and how humans are part of web of life that stretches back hundreds of millions of years. And the finding makes sense, because cilia appear to have evolved from flagella, and this study shows that, for all the diversity in the structure of flagella from different organisms, certain core proteins have been conserved right up to humans, and that those core proteins can, when not functioning properly, cause disease related to ciliary function.

This study would have been fascinating enough if the investigators had stopped there, but they took it one step further. They blocked the function of individual trypanosome proteins associated with human disease using RNA interference, starting with the gene responsible in humans for cryptic hydrocephalus, and observed severe defects in motility, strongly suggesting that interference with ciliary function is the mechanism by which impaired function of the human orthologues of these genes cause disease in humans.

Finally, the investigators looked at five more proteins, but this time forms assciated with the bloodstream form of the parasite. To their astonishment, knocking down the levels of these proteins resulted in monstrous cells that could not complete cell division, as shown in the pictures I borrowed from the article, leading them to conclude that flagellar motility is essential for correct cell division in T. brucei. On the left is a scanning electron micrograph of a normal trypansome; on the right is what happens when the expression of one of these flagellar proteins is blocked using RNAi.

This study shows quite clearly how the use of evolutionary concepts can directly contribute to our understanding of human disease. Not only did the study of a "primitive" organism like T. brucei lead directly to the identification of twenty-five candidate genes responsible for several human diseases resulting from compromised ciliary function, but the discovery that some of the flagellar proteins in T. bruceii are essential for proliferation of this parasite, suggesting that designing drugs that target the five flagellar proteins that are also essential for trypanosome proliferation could represent a promising strategy for developing new treatments for a devastating human disease. This study highlights the rationale and advantages of studying ancient "primitive" organisms.

It's likely that the genes responsible these human diseases would eventually have been identified without invoking evolution and using genetic techniques to identify human proteins that resemble ancient trypanosomal proteins. Likely they would have been identified one at a time here and there over many years. The power of the evolutionary approach was such that twenty five of them were identified in a fraction of the time.

What other discoveries related to human disease can be made using evolution, and will the U.S. be the nation to make them if we abandon the teaching of evolution in high school and continue to deemphasize it in medical school? it can be argued that, even in the absence of a strong grounding in evolution among physicians, biologists specializing in evolution could still use such techniques to study human disease even if physicians don't. Unfortunately,evolutionary biologists often do not have a good understanding of human disease. Certainly they do not have the in-depth understanding that a clinician who treats such human diseases day in and day out does for the simple reasons that they do not treat disease and they do not study disease in the same way that physicans do. Similarly, physicians who do not have a firm grasp of at least basic-to-intermediate principles of evolution won't even think to apply evolution to the study human disease as Broadstreet et al did because they don't have the understanding necessary to do so. It is for these reasons that evolutionary biology should be considered just as much of a basic medical science that medical students must learn in their first two years as physiology, biochemistry, pharmacology, anatomy, histology, and pathology.