The Evolution and Medicine Review

…bridging the gap.

An ecological approach to understanding differential cancer susceptibility in organs

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Cell Press

Why is there a large difference in susceptibility to cancer among the different body organs? Why, for instance, is it rare to find cancer in the small bowel and heart, but common in the colon and breast? And why, although it often produces benign fibroid tumors, is cancer in the uterus a fairly rare event? A number of extrinsic and intrinsic factors have been proposed to explain these anomalies, such as the fact that some organs have higher rates of cellular division than others and so by the laws of chance should accumulate more mutations. Or some organs, like the lungs and the liver, are more susceptible to environmental burdens like tobacco smoke and alcohol. But in a recent paper in Trends in Cancer, “Evolutionary Ecology of Organs: A Missing Link in Cancer Development?” Frederic Thomas, Randy Nesse, Bob Gatenby, together with several other authors including Beata Ujvari, ask us to consider organs as distinct but connected ecosystems whose different vulnerabilities to malignant transformation may be partially explained by how essential each organ is for survival through the age of reproduction.

For instance, they argue, selection for mechanisms to suppress cancer will be greatest in organs that are relatively small and can, therefore, be compromised by even small tumors or are critical for survival and reproduction, such as the heart, brain, and uterus (i.e., keystone organs). While organs that are large or paired, and thus can maintain function even when a relatively large tumor is present, will be less subject to anticancer selection forces. Variation in cancer risk across human organs, they say, may be analogous to ‘Peto’s paradox’: an observed lack of correlation between cancer risk and body mass (and life span) across the animal kingdom. In the same way that long-lived massive animals, like elephants, do not suffer inordinately from cancer thanks to multiple copies of the p53 “guardian of the genome” gene, tissues with high levels of turnover, like the small intestine, which rejuvenates its lining every 5 days, may have evolved especially powerful anticancer mechanisms.

Can a systems ecology approach that applies systems theory to the correlation between the ecosystems of organs and oncogenesis, be a powerful heuristic that could allow us to understand differential oncogenesis in organs more successfully?

The threat (or not) of insecticide resistance for malaria control

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           Andrew Read

Evmed correspondent Andrew Read has this interesting, counter-intuitive, piece about malaria resistance, written with colleague Mark Thomas. It is a commentary on a recent paper in PNAS by Viana, Hughes, Matthiopoulos, Ranson and Ferguson. “Delayed mortality effects cut the malaria transmission potential of insecticide-resistant mosquitoes.”

Here is the opening paragraph of their commentary: “Malaria burdens have fallen dramatically this century, in large part because around a billion long-lasting insecticide-treated bed nets (LLINs) have been in- troduced into Africa. Hanging over this success is a key question: What if the insecticides stop working? Pyrethroids are the only chemical class currently approved for bed net use and, predictably, pyrethroid resistance is emerging in Anopheles populations. How much does this matter? Incredibly, we have little idea. The paper by Viana et al. in PNAS explores one piece of the puzzle.

In agriculture, the impact of insecticide resistance is relatively straightforward to anticipate: insects eat crops and insects that survive otherwise effective insecticide exposure continue to eat crops and reduce yield. But in public health, the situation is more nuanced. What matters is not simply whether mosquitoes survive insecticide exposure but, rather, whether insecticide resistance enhances the ability of mosquitoes to acquire and transmit pathogens (vectorial capacity). For example, most mosquitoes do not survive the 10–14 days it typically takes malaria parasites to become infectious. Insecticides work by reducing the number of survivors still further. Whether insecticide resistance negates this reduction depends critically on the lifespan of resistant mosquitoes. If resistance is incomplete—so the mosquitoes nonetheless die younger after repeated insecticide exposure—or if resistance itself is a life-shortening trait because it is metabolically costly, the impact of resistance on disease transmission might be negligible. Could it be that LLINs will continue to function in the face of increasing resistance?”

Allergy Enhances Neurogenesis and Modulates Microglial Activation in the Hippocampus

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Barbara Klein.  Paracelsus Medical University

There is a great deal of scientific work on the relationship between states of inflammation in the body – routinely caused by bacterial or viral infection – and the brain. Specifically, the idea that inflammation in the periphery can communicate itself to the brain and cause a complementary state of neuro-inflammation, involving primed and hostile brain immune cells called microglia, that can easily get out of hand and cause states of depression and social withdrawal in the short-term, and the type of neuro-degeneration we see in Alzheimer’s and other degenerative cognitive diseases, in the long-term. In fact, in Body by Darwin I cite the work by Clive Holmes and Hugh Perry, from the University of Southampton, which started with the observation that those dementia patients that suffered from any one of a number of chronic inflammatory conditions like cancer, heart disease, diabetes or arthritis, together with the spike of a recent infection, were cognitively declining at a much faster rate than Alzheimer’s patients who were free of this disease background.

So you might imagine that an allergic reaction in the body, which causes an aberrant immune system response involving a whole slew of inflammatory chemical messenger molecules called cytokines, would have the same detrimental effect on cognition and brain health. But according to this recent paper in Frontiers in Cellular Neuroscience, written by Barbara Klein et al from Paracelsus Medical University in Austria, quite the opposite is true. Allergy appears to cause neurogenesis in the hippocampus (the part of the brain heavily involved in learning and memory) and a strong regulation of microglial activity.

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Controlled fire use in early humans might have triggered the evolutionary emergence of tuberculosis

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Mycobacterium tuberculosis cdc.gov

When did tuberculosis arrive as a major lung pathogen of humans? Rebecca Chisholm, James Trauer, Darren Curnoe and Mark Tanaka, all from the University of New South Wales in Sydney, have presented a very nice story purporting to demonstrate how human cultural innovation can lead to social innovation and, in turn, to diseased respiratory systems that might have provided the perfect environment for a benign soil mycobacterium to turn nasty and become transmissible.

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More shattering news about cancer chromosomes

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    Glioblastoma. drug discovery.com

Back in 2011, in the journal Cell, Philip Stephens and a host of colleagues drawn mainly from the University of Cambridge and the Sanger Institute, published a dramatic account of extreme cancer evolution in a paper they titled “Massive Genomic Rearrangement Acquired in a Single Catastrophic Event during Cancer Development”. The massive event they documented had come from cell samples collected from a 62 year-old woman with chronic lymphocytic leukaemia. They counted a massive 42 genomic rearrangements that had occurred, in one fell crisis, in the long arm of chromosome 4. Detailed sequencing of the chromosome revealed that these break points had been healed by DNA repair enzymes but in a random way where bits of chromosome were re-attached together such that the new conjoined fragments of DNA had originally been separated by megabases of DNA. They concluded that chromosome 4 had literally blown itself apart, some time before the patient had been diagnosed, and had been repaired, extraordinarily, to form a new “hodge-podge” chromosome which bore little structural resemblance to the original chromosome. They coined a name for the phenomenon – chromothripsis – which literally means “chromosome shattering into pieces”. The patient had subsequently declined rapidly – her cancer relapsed because it had acquired resistance to alemtuzumab – the drug being used to treat her.

So, had they discovered an extremely rare extreme form of cancer evolution – a one in a trillion random event that had, against all odds, proved fortuitous – for the cancer – or is chromothripsis more commonplace? Ever since 2011, cases of chromothripsis have been appearing regularly in the scientific literature. As a mechanism for cancer evolution, and occasionally as a method to rectify a disease process, it is now becoming treated as a mainstream, if extreme, form of evolution. So much so that scientists from the College of Life Sciences at Sichuan University in China have created an online database of chromothripsis examples called ChromothripsisDB. Now Robert Rennert and Clark Chen, from UC San Diego, have added another dramatic case of chromothripsis to this growing pile – this time involving a particularly nasty type of brain cancer called glioblastoma.

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Symposium on Evolution and Medicine October 14, 2016 at UCSD

From uniquely human ailments, to hunter-gatherer diseases, to the evolution of sleep disorders and chronic inflammation, the next CARTA symposium, “Implications of Anthropogeny for Medicine and Health,” will offer insights relevant to all humans and our origins.

When:   Friday, October 14, 2016   1:00-5:30 pm
Where:  Mandeville Center, UC San Diego
Co-sponsored by: CARTA and The ASU Center for Evolution & Medicine
Event Chairs: Ajit Varki and Randolph Nesse
Special Guest: Baba Brinkman with his Rap Guide to Medicine.

Registration: Admission is free and open to the public; however, registration is required.  To register, go to: carta.anthropogeny.org/events/implications-anthropogeny-medicine-and-health

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The New Antibiotic Mantra—“Shorter Is Better”

A report by Uranga, et al. of a randomized trial of 312  patients with pneumonia  published recently in JAMA Internal Medicine,  has found that continuing  antibiotics more than 5 days gives no additional benefit if the patient is stable and has been afebrile for 48 hours.  Neither the word “evolution” nor the phrase “natural selection” appear in the article.

An accompanying commentary by Spellberg puts the study in context and makes the connection with evolutionary biology. Among other conclusions it states,”There is no evidence that taking antibiotics beyond the point at which a patient’s symptoms are resolved reduces antibiotic resistance. To the contrary, specifically for pneumonia, studies have shown that longer courses of therapy result in more emergence of antibiotic resistance,6,7 which is consistent with everything we know about natural selection, the driver of antibiotic resistance.” 

The beginning of that commentary is below.
” In ad 321, Roman Emperor Constantine the Great codified that there would be 7 days in a week. Even in the modern era of evidence-based-medicine, this 1695-year-old decree remains a primary reference for duration of antibiotic therapy: it leads physicians to treat infections in intervals of 7 days. Thus, it is gratifying when clinical trials challenge the standard antibiotic duration of 7 to 14 days.”

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Embracing a Fuller History of the Application of Evolution to Medicine

In a recent blog post (http://evmed.asu.edu/blog/evolutionary-medicine-top-ten-questions), Randy Nesse suggests that the presentations and discussions at the second annual conference of the International Society for Evolution, Medicine, and Public Health (ISEMPH) were

“… instigated 25 years ago as George Williams and I discussed and grappled with how evolution could be useful for medicine, and what to call the enterprise.”

In her chapter (Bentley, 2016) introducing the just published book, “Evolutionary Thinking in Medicine: from Research to Policy and Practice,” the author acknowledges activity that can be considered evolutionary medicine in the years prior to 1991 but confines it to before roughly 1940.  Following the end of World War II, Professor Bentley finds little to no evidence of significant work in the field until the 1990s.  Unfortunately, these claims disregard substantial numbers of evolution-related studies that either influenced fundamental understanding of human health and disease or affected medical practice.

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The mother’s curse

This is a fascinating piece in Physorg about the likelihood that mother’s can transmit genes to their children that harm their sons but not their daughters – thanks to an evolutionary arms race between mitochondrial DNA (only transmitted by females in ova) and nuclear DNA. The article points out that, unlike the nuclear genome, which is a product, in offspring, of mother’s and father’s DNA, the mitochondrial genome is only passed on by the mother, making male offspring an evolutionary dead end. As such, it goes on to say, while natural selection actively suppresses mutations in mitochondrial DNA that might weaken females there is no pressure to weed them out of males – this leads to the mother’s curse.

The target paper for this article has been published in eLife by Maulik Patel, from Vanderbilt University, and colleagues, including some from the Fred Hutchinson Cancer Research Center in Seattle, who have discovered an mtDNA mutation in fruit flies that reduces male offspring fertility but does not affect female siblings. They believe this research substantiates the principle of the mother’s curse in animals, including humans.

Humans Never Stopped Evolving

I am acquainted with Steve Jones, professor of zoology at University College, London. He’s a lovely science writer and has talked a lot of sense. But I have never understood why he is so implacably opposed to the idea that Homo sapiens continues to evolve. He sees that there is very little genetic variation across modern human groups – far less than other primate species – and assumes there is therefore little for evolution to bite on. Yet there are 7 billion genomes out there, pole to pole, at the moment, and that number continues to grow. And there are huge differentials in the efficacy of modern medicine, quality of environment, life expectancy, differential reproductive success, exposure to toxins and pathogens, all resulting in differences in frequency of gene variants in human populations. And indeed, the explosion of human culture exerts its own selection pressures. I much prefer the approach of John Hawks, professor of anthropology at the University of Wisconsin-Madison, who believes humans never stopped evolving and has just written this essay in The Scientist, laying out his stall for the view that humans continue to evolve.

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