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Sickle Cell Anemia: a case study in evolution

Actually, we're not going to be talking about sickle cell anemia here for quite a while, because we've got a lot to cover first. Sickle Cell Anemia is far more complicated than most people realize. For instance, most people don't realize that it's an advantage to the vast majority of people who carry the gene. But explaining why takes a lot of background. So let's get started.

For a moment, I want you to stop thinking of yourself as a businessman or poet or mother or football star or potential model or angel in training or whatever and start thinking of yourself as a whole bunch of meat. Because to a disease that how you look: you're lunch.

Diseases, including parasites, have existed for hundreds of millions of years, and the reason we have an immune system is to protect ourselves against all the very tiny creatures who want to eat us.

By analogy our defenses are in two levels, a passive one and an active one. Think of a medieval castle filled with soldiers armed with spears. (Sorry, no crossbows.) The first defense is the castle wall, in this case our skin. And it is extremely effective, which is why burn patients who lose large parts of their skin are in such danger.

However, the castle wall has several entrances which are not well guarded. By far the best from the point of view of diseases is our respiratory systems, where the lining of the nose and the lung are thin and easily invaded. The second best one is our digestive tract, but that one's harder because the disease has to run a fairly stiff gauntlet in the stomach before it can reach the small intestine and finally reach a thin part of the wall.

Once inside, however, there are all those soldiers with spears; white T-cells and B-cells and antibodies which do not like invaders and are ready to fight to the death to destroy them. It does however take a couple of days before the immune system recognizes a new disease and gears up to fight it. What happens is that one kind of white blood cell recognizes the invader, learns how to make appropriate antibodies, and the reproduces like mad so that there are millions of them pouring out antibodies. The antibodies attach themselves to the invaders, and that becomes a signal for other white blood cells to attack, engulf and destroy the invader. And with most diseases what then happens is a pitched battle between the disease and the immune system, which the immune system ultimately wins over the course of a few days, and you get over the disease (but usually not until you've given it to a couple of friends).

But there are other ways in. Mosquitoes have the ability to burrow through our skin to reach the blood which they suck. But if they immediately started to suck, the blood would clot in their proboscis, clogging it, which would do them no good at all. So after they've finally penetrated the skin, the first thing they do is inject some saliva. The saliva contains a very powerful anticoagulant. Then as the mosquito sucks out blood, the blood it gets contains some of this saliva, and thus does not clot and clog the proboscis.

But if the mosquito were to suck all the saliva out then it would be sucking blood without the anticoagulant and it would clot. So the mosquito stops before then, and as a result leaves some of the saliva behind. And that's why a mosquito bite is annoying; because it leaves behind some things which cause a local immune reaction; so you get a lump and it itches for a couple of days.

Some diseases have gotten a commensal relationship with the mosquito: when the mosquito sucks blood from a victim who is infected with that disease, the disease travels through the mosquito's blood system from its stomach to its saliva glands, and there it waits. And when the mosquito bites someone else, the disease pathogen hitches a ride on the saliva which gets injected. A lot of it gets sucked back out, but as mentioned some is left behind and thus some of the disease pathogens remain, right there in the blood, on the inside side of the wall, in the yummy juicy part of you.

But there are still all those soldiers. The second trick is to figure out how to fool them, and various diseases and parasites have found interesting ways of doing that. There's a rather large parasite called a fluke; one version lives in your liver, another in your lungs, another in your heart, and they can be there for years and the immune system ignores them completely. That's because they coat themselves in a layer of your own cells. So when the immune system takes a look at one of these monsters, it sees "self" not "invader". It's as if the fluke is wearing a stolen a uniform from one of your own soldiers.

Another trick is to hide where the immune system cannot go; the favorite being to cross the "blood-brain barrier" which despite the name actually applies to the entire nervous system everywhere. Herpes does this. When you get herpes, it reproduces in your blood, and your immune system fights back and wins. But before it's done so, some herpes viruses have crossed the blood-brain barrier and gone dormant in nerve cells and the immune system cannot follow them there, because it does not cross the blood-brain barrier. Every once in a while, for no reason anyone has ever determined, the herpes viruses decide to come out for a rematch and again they battle the immune system, and again the immune system wins. But in the mean time the supply of viruses in the nerves has been replenished, and this keeps happening all your life, which is why Herpes has no permanent cure, and why a person infected with it will suffer many attacks. Herpes is not the only virus which does this; there are many. The one which causes warts does exactly the same thing.

But the disease I'm most interested in for this discussion is malaria. You get it by being bitten by a mosquito who had previously bitten someone else who was infected; malaria does that trick of moving from the mosquito stomach to the mosquito saliva gland. (However, it's very specific; it has to be the Anopheles mosquito. No other will do. The Anopheles mosquito is a friendly beast and loves to share; it also spreads yellow fever.) For a much more technical and specific description of the disease, consult this.

The malarial parasite is extremely small, and inside you its life cycle consists primarily of invading red blood cells (which are huge by comparison) and eating out the insides and using that material to make more malarial parasites. Eventually you have a loose bag filled with parasites, and it bursts.

While the parasites are out in the blood plasma, the immune system is capable of recognizing and attacking them. But they don't have to search far to find a new red blood cell to invade, and while they're inside the red blood cell they've vanished as far as the immune system is concerned.

Malaria is a vicious disease and while it often doesn't directly kill its victim it so seriously weakens him that he's liable to die from many other things which ordinarily would not be fatal.

Now the malarial parasite has a normal metabolism just like any other living thing; it utilizes oxygen and generates carbon dioxide as a waste. This is critical.

I'm afraid I need to define some technical terms because we'll be using them extensively. The first two words we need are homozygous and heterozygous. Everyone knows that humans have 23 pairs of chromosomes and except for the sex pair, each pair is essentially identical in gross. But they are not necessarily identical in detail. A given chromosome may contain the formula for hair color at a certain location, and if so, its compatriot will also contain the formula for hair color at that same location. But they may contain the same formula or different ones.

If the two chromosomes contain exactly the same formula at that point, then the individual is referred to as being homozygous for that gene, which means that the zygote (the person) only has one ("homo") formula. If the two chromosomes are different at that location then the individual is referred to as being heterozygous which means the zygote (the person) has multiple ("hetero") kinds of gene. (Just to clarify, the words have nothing whatever to do with sexual preference.)

Now genes like that are sometimes dominant or recessive or co-dominant.

The eye-color gene is the classic example of dominance and recessiveness. If a person is homozygous with the blue-eyed gene, then they will have blue eyes. If the person is homozygous with the brown eyed gene they will have brown eyes. But if the person is heterozygous, with one of each, he will still have brown eyes because the brown eye gene is dominant and the blue-eyed gene is recessive.

Yet two more odd words: genotype and phenotype. The genotype is the genes the person carries. The phenotype is what the resulting individual looks like.

A person whose genotype for eye color is homozygous brown and the person who is heterozygous for eye color both have the same phenotype: brown eyes. There is no external way to tell them apart.

Finally, one last word to define and perhaps the most important of all: co-dominant means that if the person is heterozygous, he will express in his phenotype both genes, and thus will be different than a homozygous individual with either of the genes. The reason this is important is that the sickle gene is codominant with the normal gene at that location.

Sorry about all the jargon, but we need it to talk precisely about what happens with malaria.

Hemoglobin is the critical compound in our red blood cells, which carries iron atoms and gives the red cells the ability to carry oxygen from the lungs to the tissues. This is necessary because oxygen does not dissolve very well in water, and the plasma alone is not capable of carrying enough oxygen to the tissues to keep you alive. Hemoglobin is horrendously complicated, consisting of two each of two different large proteins which wind together so as to hold a few iron atoms into certain very precise positions. There are hundreds of amino acids in Hemoglobin.

Another function of the blood is to carry back the CO₂ to the lungs so it can be exhaled. But unlike oxygen, CO₂ is readily soluble in water. It forms carbonic acid (H₂C0₃) by combining with a water molecule. At the lungs it just as readily returns to its normal gaseous form and can be exhaled. So unlike oxygen, no special mechanism is needed to carry CO₂ to the lungs. But while dissolved, it is an acid and actually a fairly potent one.

The sickle gene turns out to be a single change, the alteration of a single amino acid, in the formula for one of the two protein chains which makes up hemoglobin. It has no effect at all on the ability of the modified hemoglobin to carry oxygen.

And the sickle gene is co-dominant. So a person who is heterozygous with the sickle gene will have half their hemoglobin in the normal form and half with the modification. (Actually, what happens is that one quarter of the hemoglobin complexes will be made with two normal copies of the protein, one quarter will be made with two altered copies, and one half will have one normal and one altered copy.)

The modification has one critical effect: it make the hemoglobin sensitive to high levels of acid. In the presence of high quantities of acid, the hemoglobin complexes which include the altered protein will stack up like poker chips and form long chains, and as a result the shape of the red blood cell changes fom the normal platelet shape to something more like a banana. This is called sickling.

And when this happens, the immune system labels the cell as aberrant, and a white blood cell will engulf and destroy it.

For a person who is heterozygous with the sickle gene, almost the only thing which can make this happen is the presence of malarial parasites inside the red blood cell. They are excreting CO₂ and it is dissolving in the water inside the red blood cell and being converted to carbonic acid, and when the concentration reaches a certain point, the hemoglobin complexes which contain one or two altered copies of that protein will cause the cell to sickle, and make the immune system engulf and destroy the red blood cell.

Before the parasites get out. This is a Good Thing.

Not every single time, but most of the time, and the result is that the disease never get out of control. It's a chronic infection which never really affects the health of the heterozygous individual.

That's the point: the sickle gene protects its heterozygous carriers against malaria. They're not immune to the disease, but it can't kill them because it doesn't reach sufficient quantity in their blood to do so.

But it's an imperfect solution because it protects the heterozygous individuals at the expense of everyone else. They're doing fine, but everyone else suffers because they exist.

First there is the disease we call "sickle cell anemia". That happens when both parents of a child are heterozygous, and the child gets the sickle gene from both of them, approximately one chance in four, thus becoming homozygous with the modified gene. The effect of this is to dramatically lower the amount of acid which is needed to set off a sickling attack.

In a person who is homozygous with the sickle gene, their own normal CO₂ (in the form of carbonic acid in their blood plasma) can be sufficient to cause their red blood cells to sickle, and when it happens it happens to many if not most of the red cells all at once. At which point the immune system goes hog wild and starts destroying nearly every red cell it can find (which has sickled). And that's why it is called "anemia", because in fairly short order there aren't many red blood cells left. Modern medical treatment for such an attack is to put the person into a nearly pure oxygen atmosphere to permit them to utilize such oxygen carrying capacity as they have left, and once things have calmed down a bit, to treat with transfusion of normal red blood cells.

But in an impoverished community in Somalia in some obscure village out in the bush, or for any human anywhere before transfusion became safe and common, that would be out of the question. So all you can do is watch the child and feel bad because of the pain and hope it doesn't die. Absent modern medical treatment, few who are homozygous with the sickle gene survive past age 12.

Because heterozygous individuals make up a minority of the population, and even when they do marry other heterozygous individuals, only a quarter on average of their children are homozygous with the sickle gene, that means it doesn't actually kill a very large proportion of the population. On the other hand, all those heterozygous individuals are saved from malaria by the gene, so it saves more lives than it costs in areas where malaria is endemic. If 10% of the population is heterozygous, then 1% of children have both parents heterozygous, and on average 0.25% of children born are homozygous with the sickle gene and are killed by it, but 10% of the population is resistant to malaria. For every child killed by the gene, 40 are protected from malaria. (If 20% of the population is heterozygous then 4% of children have both parents heterozygous and 1% of children are homozygous with the sickle gene and die, one for every 20 people protected by the gene.)

What's less obvious is that the heterozygous community also represent a serious health threat to the people who are homozygous with the normal gene for hemoglobin.

The heterozygous individuals are not immune to malaria. They get it. They don't get over it. It just doesn't severely affect them. So they represent a pool of infected individuals that the mosquitoes can bite which causes the mosquitoes themselves to become infected, which raises the chance that the mosquito which bites a homozygous normals will carry malaria and give it to him. So the presence in the population of a significant number of heterozygous individuals will raise the chance of the homozygous normals getting and dying from the disease.

Now it is mathematically impossible for every individual in the population to be heterozygous. Even if they began that way, the next generation would not be.

And that's why this is such an interesting case study in evolution. For any given gene, all that natural selection is interested in is whether it saves more lives than it costs, and this one does. Far more people are saved from malaria by this gene than are killed by sickle-cell anemia, and since natural selection is simply a statistical process, that's all it takes to select in favor of it. Natural selection doesn't care what effect it has on those who do not carry the gene.

Consider when the original mutation took place. Now since it is a single amino acid change, it is completely plausible; unlikely, but plausible. (Why Africa? Gold is where you find it. It just happened to be there. Rewind the tape and run it forward again, and it might have happened in Malaysia, or in Central America, or Sweden, or it might not have happened at all. But it did happen in Africa. Unlike natural selection, which is stochastic, mutation is random.)

The first individual to get the mutation was heterozygous! She (?) got all the advantages and none of the disadvantages. She marries and has children and on average half of them get the gene and they are all heterozygous (and the rest are homozygous normal). And again, the carriers get all the advantages but none of the disadvantages. So the gene wil spread for a long time; and you won't get a homozygous individual with the sickle gene until two people marry who are sufficiently distantly related so that they don't think they are violating the incest taboo but are both descended from the person who had the original mutation. And even then only one quarter of their children die a horrible death; half are heterozygous, and one quarter are homozygous normal.

And even then, since the majority are homozygous normal, the heterozygous individuals are more likely on average to pick a homozygous normal individual for a mate, and their offspring will be on average half heterozygous and half homozygous normal. A homozygous sickle gene offspring is impossible in such a pairing.

So for a really long time, the sickle gene was an advantage and natural selection would have favored it and caused it to spread. And indeed in many parts of Africa it still is mostly an advantage.

At least, that's the evolutionary explanation. It turns out from an evolutionary standpoint to make perfect sense, because evolution isn't looking for "perfect" answers; just for things which are better than before.

But it represents a really serious problem for creationists, because there's no good explanation for why God would have done this. One explanation was proposed by Mark Twain (though not in this context): "God is a malign thug." But I think most creationists would reject this.

But look at it: it's a half-assed solution to a problem which probably shouldn't have existed in the first place. Why did God make malaria at all? But having done so, He seems to have decided to protect humans from it. Only he did so in such a way that only some are protected (since it is mathematically impossible for it to protect them all with this particular solution), and because of the way He did it, a certain number of innocent children must suffer an agonizingly slow death for no reason other than because they chose their parents wrong. And in the in the mean time, it's causing the normals to die like flies.

And just to make something absolutely crystal clear: you don't get malaria by committing a sin. You get malaria by being bitten by a mosquito, and mosquitos bite saints and sinners alike. The saint/sin status of none of these individuals has anything to do with how this works. They could all be saints, they could all be sinners, and exactly the same thing would occur.

Equally, you don't become homozygous with the sickle gene and thus get sickle cell anemia by committing a sin; you get it solely because both of your parents are heterozygous and you were unlucky enough to hit the 1:4 chance of getting the gene from both of them. There is no sin involved, just bad luck.

Everyone who dies because of this unbelievable kludge is or could be innocent. So any explanation of this as "God punishing sinners" is total nonsense.

From the point of view of God, there are really only three things He could do which make sense: Don't make malaria, make malaria but make ALL humans immune to it, or make malaria and make NO humans immune to it (though it's difficult to understand why He'd do the third one)..

What actually happened make no sense all from a creationist standpoint, because there is no justice whatever in the outcome. As I stated in a different essay elsewhere here, if God designed this mechanism deliberately, then God is an incompetent engineer. But it's also possible that He is insane or He is sadistic, or that Mark Twain was right.

Or... maybe it is the result of evolution (where it make complete sense) and not creation (where it makes no sense at all).

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Since this was written I've learned more about sickle cell anemia. It turns out that each of us has a set of genes for haemoglobin which are active in the womb (g-globin), and a different set (b-globin) which activate shortly after birth (while the former are deactivated again). It is the second set which is changed in people carrying the sickle cell gene, and work is underway to see if there might be a way to reactivate the fetal gene. If this can be done, then people with sickle cell disease could conceivably be cured. This is an extremely exciting prospect.

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