Sometimes life gives you one of lime and the other of sand. Leaving aside the eternal question about which is good and which is bad, it should be noted that this is a very valid statement in terms of genetics it means. And it is that the same genetic variant can protect us from a illness, even saving our lives, while condemning us to a greater propensity to suffer another. There are multiple examples and the big problem is that, precisely because of this initial protectionsaid variant has a greater probability of lasting over time.
It has recently been discovered that this could occur with the Black Death. That great epidemic that struck down almost half of the European population in just four years, was able to shape the immune system of the survivors. They would be protected from disease. But, curiously, now it has been seen that those who inherited their superpower are much more prone to autoimmune diseases such as Crohn’s disease.
And it is not the first couple of diseases of this type that is known. Similar relationships have also been found with sickle cell anemia and malaria or with cystic fibrosis and cholera. In short, these are genetic diseases that somehow protect against infections that cause large epidemics.
The case of the black plague and autoimmune diseases
The relationship between the black plague and autoimmune diseases recently shown in a study published in Nature. In it, he analyzes Ancient DNA from remains of individuals from England and Denmark who died in the Middle Ages, before, during, or after the great epidemic of the Black Death. It is logical to think that those who survived could have some genetic variant that somehow protected them from the disease. For this reason, when they focused on those related to the immune system, there were four major ones in that group that caught their attention, all associated with two genes: ERAP2 and TICAM2.
The first is activated in the macrophageswhich are cells that quickly respond to an infection, attacking the pathogen, digesting it and presenting a part of what is left of it to the rest of the immune cells, so that they know where to attack
As for the second gene, it carries the instructions for the synthesis of a protein that is involved in the detection of gram negative bacteriain whose group is Yersinia pestis, the cause of the black plague. It seemed obvious that there was a relationship. But, to verify it with more certainty, they took blood from people with these variants today and exposed their macrophages to that bacterium that devastated Europe in the Middle Ages. As might be expected, they were much more efficient eliminating the bacteria than those who did not have this genetic variant.
All this led to the selection of those people genetically protected against Y. pestis. However, the authors of this study have seen that there is also a negative face in the history. And it is that today people with these variants seem to be more predisposed to suffer from certain autoimmune diseases.
These are very common diseases today, while the plague, although it has not been eradicated, it is not so widespread anymore. In addition, the existence of antibiotics makes it more complicated for large epidemics to occur. For this reason, what in its day was a blessing, today becomes a condemnation.
sickle cell anemia and malaria
The sickle cell anemia is a genetic disease characterized by the presence of Red blood cells with an abnormal shape, like half Moon. The result of this conformation is that these cells they break easilythus decreasing iron reserves and causing severe anemia.
In addition, this shape of the red blood cells causes them to clog small blood vessels, such as capillaries, producing ischemic vasculo-occlusive accidents. All of this can be very serious in people who inherit the affected gene from both parents. However, in those who only inherit it from one, the symptoms are very slight or even imperceptible. This is because they have red blood cells that are crescent-shaped, but also the usual round and flat ones.
On the other hand, the parasite that causes malaria, Plasmodium falciparum, infects precisely the red blood cells. Nevertheless, finds it more difficult to do in those that are crescent-shaped, since they break in the process, making it more difficult for the disease to occur in these people. This has also led to episodes of malaria epidemicsespecially in certain regions, individuals carrying sickle cell anemia have been genetically selected. Unfortunately, as it is precisely something that occurs above all in very specific areas, it is easier for these individuals to end up mating and, finally, their offspring carry the two mutated copies of the gene and, therefore, seriously develop sickle cell anemia.
More diseases: cystic fibrosis and cholera
Cystic fibrosis is a genetic disease characterized by mutations in a gene that contains the instructions for the synthesis of a chlorine channel called CFTR. Chlorine channels are proteins that are embedded in cell membranes and act as a gate for the passage of chlorine ions, opening or closing, as appropriate. These ions and others, such as sodium or potassium, are involved in a large number of functions. Some examples are nervous arousal, blood pressure control, or cell proliferation.
The presence of this mutation causes, above all, a excessive buildup of mucus in those systems that normally contain it, such as the digestive or respiratory systems. It becomes very serious, especially since patients sometimes choke on their own mucus or can develop more infections. However, as with sickle cell anemia, severe symptoms only occur in those they inherit both mutated copies.
As for cholera, it is an infectious disease caused by the bacterium Vibrio cholerae. When they reach the intestine, the toxins from this bacterium excessively activate the CFTR chloride channels, causing a excess fluid production to compensate for this increase in ion concentration. For this reason, the stools become very watery, causing diarrhea so intense that it can quickly lead to death from dehydration.
The curious relationship between the two
People who carry the mutated gene that causes cystic fibrosis have been found to be more resistant to effects of cholera. And it is not clear why. However, it is suspected that it may be because they have part of their calcium channels atrophied. This would not generate worrying symptoms, but it would hinder the effects of the toxins of the bacteria that cause cholera.
Cholera is especially common in places with poor water sanitation. Today it is reduced to areas with few economic resources or places where there have been natural disasters. However, in the past it was much more common. Therefore, the genes of cystic fibrosis carriers had more advantages to survive. This occurred to the point that it is estimated that 1 in 25 people are carriers of the disease, so it is not unlikely that two will end up having children who inherit both copies and develop the disease. That would be lime. Or the sand. Whatever the bad of them.
