Are genetic diseases evolutionarily beneficial? It sounds like a crazy question, but the answer is “yes, some are.” In my blog post, “Why are Black People Black and White People White,” we discussed how sickle cell trait (primarily in people of African descent) and thalassemia (a group of diseases that is mostly found in Southern Europeans and in Asians) are both protective against malaria. Inherit one copy of the sickle cell gene and you gain substantial and lifelong protection against malaria, historically one of the most deadly diseases to afflict mankind, the downside being that the rare person who inherits two copies of the sickle cell gene ends up dying from a horrible genetic disease. The genetics of thalassemia follow this concept, although the actual inheritance patterns are more complicated and not something that I’m going to bore you with. The take home message is that on a population level, at least historically, having the sickle cell gene and the thalassemia genes floating around in the population did a lot more good than harm (on a net basis) and that this is the reason why these genes are still with us today!
The impetus for my writing “Why are Black People Black and White People White” was the excellent book The World Until Yesterday, by Jared Diamond (he’s also the author of Guns, Germs, and Steel — another exceptionally great read), which reminded me of some things that I’d learned years ago in medical school, found fascinating, and just never gotten around to blogging about. Following on this theme, I thought that it would be interesting to briefly explore some additional genetic diseases that are present today because they were evolutionarily beneficial, at least on the population level, to people in the past. Most of these diseases work in the same basic way — lots of people inherit one copy of the defective gene and gain protection against something in the environment with little downside. Unfortunately, a few people inherit two copies of the defective gene and end up suffering from a full blown genetic disease.
Cystic Fibrosis: The mutation that causes cystic fibrosis causes your body to retain salt from the gut. Inheriting one copy of the mutated gene provides protection against cholera, a bacteria that is contracted by drinking contaminated water and that causes your gastrointestinal system to waste salt which results in profuse diarrhea. Cholera killed millions upon millions of people in Europe during the 19th century and this terrible disease recently reared its ugly head in the Western Hemisphere when Nepalese U.N. Peacekeepers inadvertently carried cholera with them to Haiti during their earthquake relief mission, unfortunately causing a deadly epidemic in the worst possible place at the worst possible time. While having one copy of the cystic fibrosis gene protects against “King Cholera” at little cost to the carrier of the defective gene by making the carrier resistant to loss of salt through the gastrointestinal tract, inheriting two copies of this defective gene causes cystic fibrosis, a terrible disease. Patients with cystic fibrosis cannot excrete adequate amounts of salt into their bowels or their airways and, since water follows salt, this results in the accumulation of thick and dry mucus that clogs up the airways and the gut and that causes all sorts of really awful problems, eventually resulting in early death in most cases (although this outlook is slowly improving with advances in medical science).
This is a cholera clinic in India with patients lined on “cholera cots (cots coated with easily cleanable plastic).” Notice the white buckets below each cot — those are for diarrhea. Cholera is caused by a bacterium that causes profuse diarrhea. Cholera kills its victims by dehydration. This deadly pathogen has been largely eradicated in the developed world due to effective water sanitation efforts.
A diagram of the effects of cystic fibrosis. Mucus plugs in the lungs provide an excellent habitat for difficult to eradicate bacterial infections, especially by the pathogen Pseudomonas aeruginosa, a bacterium that is notoriously resistant to antibiotics. Patient’s with cystic fibrosis also suffer from mucus plugging in the gastrointestinal tract and this causes a range of complications including potentially fatal pancreatitis.
HIV Resistance: CCR5 is the name of a gene on a group of your white blood cells called CD4+ T cells, and these are the cells that the HIV virus loves to work its mischief inside. Approximately 10% of people of European descent have inherited one mutated copy of the CCR5 gene and these lucky people are resistant to the HIV virus, and thus resistant to getting AIDS even if they are exposed to HIV. People with two copies of the mutated CCR5 gene — about 1% of Europeans — are immune to HIV (as far as we know) and apparently can’t get AIDS even if they are exposed to HIV. The CCR5 mutation is not found in non-European peoples and it is thought that this mutation was selected for hundreds of years ago because it provided some protection against either bubonic plague (“The Black Death”) or smallpox, both of which were major killers in medieval Europe. Since people with the CCR5 mutation were less likely to die from the above said diseases, the gene eventually became widespread in Europe because people dying from smallpox or from bubonic plague (people without the mutation) are a heck of a lot less likely to have children than healthy people for obvious reasons! The downside of having the CCR5 mutation is that it probably weakens immunity to some other infectious diseases, including the West Nile Virus, which is an avian (bird) virus that causes meningoencephalitis (an infection of the brain and its supporting tissues, the meninges) in humans.
A man dying from AIDS. In and of itself HIV does not typically directly kill its victims. Rather, the HIV virus destroys the immune system and leaves its victim vulnerable to environmental pathogens that people with intact immune systems are able to easily ward off.
So that’s a few more examples of predominantly genetic diseases/mutations that are, at least at times, beneficial on the population level, which is why they are still floating around in our gene pool. While a few select genetic diseases fit into this category, most are simply bad diseases and we’ll discuss the “hows and whys” of these afflictions next time (in English!) — hope that you join us!
Dr. Leonardo Noto
Physician and Author of Medical School 101, Intrusive Memory, The Life of a Colonial Fugitive, and The Cannabinoid Hypothesis.
NOTE: The Life of a Colonial Fugitive — my dark historical thriller — is free for your ereader at http://www.smashwords.com/books/view/215272. Thanks for reading!
Author Bio: Dr. Leonardo Noto is the nom de plume of a former airborne battalion surgeon who is now in civilian practice. Dr. Noto is the author of four books and he also writes for a medical education corporation that assists medical students, interns, and residents as they prepare for the medical board examinations. Dr. Noto is the proud father of an extremely spoiled 16-month-old American Bulldog who enjoys slobbering everywhere and tearing up things that he is not supposed to! Dr. Noto is an amateur practitioner of muay Thai and Brazilian jiu jitsu and he recently began learning to play the guitar (but he is currently a quite terrible musician, as his neighbors will readily attest).
Remember to discuss all health concerns with your personal physician (I don’t count!) before making any medical decisions. www.leonardonoto.com is intended to present general medical information for entertainment purposes and not as specific guide to any medical treatment. The author has made every effort to present accurate information; however, due to the ever-changing nature of medicine and the intrinsic caveats that are inherent in any particular case, no medical decisions should ever be made based on information gleaned from the internet (duh!). The internet and self-education are great, but they don’t replace your Doc!
The opinions voiced on this medical blog are solely the author’s own and they do not reflect the opinions or values of Dr. Noto’s employers, past or present. Dr. Noto’s medical blogs should never be used as supporting evidence for legal testimony — this is of course obvious to anyone who isn’t a complete moron, but some people are rather stupid.
1. HIV Resistance Via CCR5 Mutation. Stanford at the Tech: http://genetics.thetech.org/original_news/news13
2. Cholera Clinic. Flickr.com. https://www.google.com/search?as_st=y&tbm=isch&hl=en&as_q=cholera&as_epq=&as_oq=&as_eq=&cr=&as_sitesearch=&safe=images&tbs=sur:fc&biw=1366&bih=622&sei=JlUSUt2DKYro2QX-noHYCw#facrc=_&imgdii=_&imgrc=8PiUOA3RqTPBIM%3A%3BZvgjzhUR9-GmxM%3Bhttp%253A%252F%252Ffarm1.staticflickr.com%252F32%252F66832397_c3d1224de7_o.jpg%3Bhttp%253A%252F%252Fwww.flickr.com%252Fphotos%252Fknobil%252F66832397%252F%3B800%3B600
3. Cystic Fibrosis. Children’s Hospital.org. http://www.google.com/imgres?imgurl=&imgrefurl=http%3A%2F%2Fwww.childrenshospital.org%2Faz%2FSite2934%2FmainpageS2934P0.html&h=0&w=0&sz=1&tbnid=ijQWsIiGnCSJJM&tbnh=225&tbnw=225&zoom=1&docid=PRJDSxdx5tiO8M&hl=en&ei=l1USUsL_E-2yygHEyoFQ&ved=0CAEQsCU
4. AIDS Patient. www.gsdmc.com.