Tuesday, January 13, 2015

Happy Accidents: Beneficial Mutations in Humans

A while back, I was talking with a group of young students about evolution and natural selection, and I was teaching them about the role of mutations when I was asked an interesting question that I failed to answer.

The concept of mutations can be tough to grasp, because mutation sounds like a really bad thing, and indeed it often is. A mutation is essentially an accident in your DNA, and while most of them are harmless, most of the rest have unfortunate side effects. And yet mutation is one of the driving forces of evolution: natural selection weeds out the bad accidents and encourages the good ones, and life changes through the generations.

Which brings me to this question I was asked: Are there examples of good mutations in humans?

Great question. At the time, I had about 20 seconds to answer, and nothing came to me. I knew there were examples, but nothing sprung to mind, and then my time was up, and so I failed to answer the question. That educational let-down has bothered me ever since. Every time I learn of a cool human mutation, I remember that kid with that question. Well these days I know a bunch of great examples! So, inquisitive kid, I hope you’re reading: here are some of my favorite beneficial human mutations.

The incredible human eye. From Wikimedia Commons.
Maureen Seaberg says “you really don’t want to go clothes shopping with me.” She has a hard time finding clothes with colors that match. You might be surprised when you bring her a great matching outfit only to have her shake her head and insist the colors are wrong. It’s not your fault, you just can’t see like she can. Maureen is a mutant, with super-color-vision.

We humans perceive color using special cells in our eyes called cones. We are trichromats, meaning we have three types of cones, each attuned to a different wavelength of light: short wavelengths (blue), medium wavelengths (green), and long wavelengths (red). With all three, we can see about a million shades of those colors, and all the colors between them. As you’d imagine, a mutation affecting your cones is typically bad. A mutation in your red-cone genes might make you see slightly different shades of red than most people, fewer shades of red, or even no red at all (which would make you a dichromat, like cats and dogs). When any of these mutations is so bad that you notice it, you have color-blindness.

But the genes for red and green cones are on the X chromosome. A man with color-blindness has a defective gene on his X-chromosome. A woman with color-blindness would have to have a defective gene on both her X chromosomes (a much rarer condition). But Maureen is different. Maureen has normal cone genes on one of her X chromosomes, and slightly “tweaked” genes on the other, which means she has blue, green, and red cones, plus an extra type of cone that picks up slightly different shades. She is a tetrachromat (a condition she shares with some birds, fish, and reptiles): she has four types of cones.

There has been a fair amount of research on tetrachromacy in humans, particularly in the amazing case of Concetta Antico, an artist whose genes and artistic training might give her the best color-vision of any human on record. Researchers estimate that this four-coned “super-vision” might be present in a significant percentage of women, and possibly even in some men. These women don’t see different colors than the rest of us, but they have a visual acuity that can distinguish far more shades of color than we can, and they might also have enhanced low-light vision. As a young flirtatious Charles Xavier might say, it’s a very groovy mutation.

Natural Immunity
You’ve seen this movie, right? Some kind of awful virus is tearing through human cities, killing millions of people (or turning them into zombies, depending on the movie), but wait! The protagonist is immune to the deadly virus! If the doctors can get hold of her blood, they can synthesize a cure and save humanity! But why is she immune? Where did her immunity come from?

First: viruses. A virus gets into your body, infects your cells, kills them, and spreads to other cells. The Human Immunodeficiency Virus (HIV), for example, attacks the cells of your immune system, causing AIDS. In your immune system, there is a group of cells that have a protein called CCR5 on their surface. This protein is used to receive signals from other cells in the immune system, allowing cells to communicate and work together to fight off disease. But many strains of HIV have evolved to use this protein to latch on and infect the cells, turning your own cell structures against you.

But about 1% of Caucasians have a mutation that changes the shape of the CCR5 protein, meaning those strains of HIV can’t grab hold. These people are immune to most forms of HIV, and with little to no adverse side effects. And just like in the movies, scientists have been studying these people to see if they can use their bodies as an example to develop ways to fight or cure the deadly virus.

Other examples:
-A similar mutation may provide immunity to Ebola.
-Of course, there’s the famous case of the mutation which protects against malaria if you have one copy of the mutation, but causes sickle-cell anemia if you have two copies.
-Some recent research has uncovered a genetic mutation that guards against Alzheimer’s

The examples above are rare mutations, present only in a small percentage of people. Let’s look at some good mutations that have taken hold, and become more widespread:

The Tibetan Plateau is the highest plateau in the world, with average
elevation over 15,000 feet.  Image from Wikimedia Commons.
Extreme Living
In 1993, Karl Gordon Henize died on Mount Everest. During his life, he had been an astronomer, an astronaut, and a space scientist. At the age of 66, he joined a NASA research expedition up the slopes of the mountain, and developed a case of high-altitude sickness, which led to his death at 21,000 feet. Dr. Henize’s name is one of a substantial list of people who have met their demise thousands of feet above sea level.

Humans don’t do well at high altitudes. Our bodies just aren’t built for that environment. Once you get up to 8,000 or 10,000 feet, the low levels of oxygen in the air can cause altitude sickness, which can include dizziness, headaches, and even (as was the case with Dr. Henize) deadly edema. And if you get past the short-term effects, people who spend many years living at such altitudes often develop respiratory or circulatory problems. We really are just better off sticking to the lowlands. And yet, millions of people in the Himalayas and the Andes spend their entire lives at 10,000 to 15,000 feet, with very few problems, and they’ve been at it for thousands of years. How do they do it?

Research on these high-altitude peoples has found a whole collection of genetic mutations that appear to help them survive hypoxic (low-oxygen) conditions:
-In both the Andes and the Himalayas, native people commonly have mutations in the genes that control the body’s response to hypoxia. These people do not suffer the same long- or short-term health problems that most people would at high altitude.
-A 2010 study found that certain mutations common in Tibetans help their bodies to utilize oxygen more efficiently in low concentrations.
-Another study looked at Tibetan women who had mutations that helped their blood store more oxygen, and found that the women with these mutations also had lower rates of infant mortality than women without those mutations.

These high-altitude adaptations are a beautiful example of natural selection in humans. Thousands of years ago, these mutations were rare in those people, just as they are in low-land people today. But as they continued to help people survive at high altitudes, the mutations became more widespread, and now these “errors” are the norm for these incredible super-survivors. 

Lactose Tolerance

Diarrhea; bloating; cramps; gas; nausea; vomiting. It can be rough to have lactose intolerance. If you don’t want to suffer those symptoms, you have to stay away from milk, ice cream, sometimes also yogurt or certain types of candy. What an unfortunate disorder to have.
But wait a minute. Is lactose intolerance a disorder? Consider this: about 65% of human adults are lactose intolerant. Oh, and get this: pretty much all mammals are lactose intolerant. If you can enjoy a bowl of ice cream without dietary problems, you’re the one with the disorder.

So what is lactose intolerance? Well, the primary sugar in milk is called lactose, and it can be broken down by an enzyme in your body called lactase. Most human babies (mammal babies, in general, actually) produce lactase, which is great since one of the hallmarks of being a mammal is drinking mother’s milk. But most mammals stop producing lactase when they grow up. Without lactase, lactose doesn’t break down in the body; it just sits there causing nasty intestinal problems: the body can’t tolerate it.

But if you’re human, and have European ancestors, there’s a very good chance you have a mutation in a gene called MCM6. MCM6, among other things, regulates another gene called LCT, which directs the production of lactase. The mutation stops MCM6 from turning off LCT, so the body goes on producing lactase into adulthood. This tiny mutation is what allows me to eat ice cream, and I am extremely grateful. 

The mutation first appeared in European farming culture about 7,500 years ago, and these mutants suddenly had a new food source: milk from those cattle they’d been farming. So natural selection took hold and today most people of European descent have this lactose tolerance mutation.
Intriguingly, lactose tolerance has also appeared in parts of Africa, Asia, and the Middle East, as a result of different, independent mutations of the MCM6 gene. Who knows, maybe someday all of humanity will be able to share a glass of milk.

These are just a few of my favorite examples of beneficial mutations in humans; there are plenty more. If I missed your favorite, let me know in the comments! 

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