Transcription
We have spent the last four videos talking about sickle cell anemia. Now let's turn our attention to sickle cell trait. Sickle cell trait again—Africa, sub-Saharan Africa to be precise. And now this African kid, I'm asymptomatic for the most part, and I'm protecting against malaria, especially Plasmodium falciparum malaria. That's why there is a sickle cell trait. Hemoglobin genotype is hemoglobin AS. Contrast that with sickle cell anemia, where the hemoglobin was genotype SS.
In sickle cell trait, the cells are normal. The red blood cells do not sickle for the most part; they can circle. But under extreme conditions—this is the huge difference between sickle cell trait and sickle cell anemia—and sickle cell anemia, yes, the cells are sickled. And sickle cell trait, they are normal for the most part; they can cycle only under extreme conditions. Now, normal hemoglobin is hemoglobin A, and the genotype is AA. In sickle cell disease, it's called hemoglobin S, and the genotype is SS. In sickle cell trait, you have a mixture: hemoglobin A and hemoglobin S, so the genotype is AS. This is a co-dominance phenomenon in genetics where you have normal hemoglobin as well as abnormal hemoglobin in the same body.
So sickle cell trait is heterozygous AS, while sickle cell disease is homozygous SS. Of course, you should remember this: sickle-cell disease is an autosomal recessive. So you have 25% of the offspring will be normal, 50% will be carriers—we call this hemoglobin AS, or the carrier state, or the sickle-cell trait—and 25% will be abnormal. This is the sickle-cell anemia, and this is hemoglobin SS. Hope this is clear.
In sickle cell trait, sickling follows incomplete dominance. This is a genetic phenomenon where sickling occurs only under certain conditions. Incomplete dominance: if it were complete dominance, sickling will be present all of the time. But now it's incomplete dominance; sickling occurs only at low oxygen concentration, such as the renal medulla. There's hypoxia and hypertonicity in the renal medulla, which will force these nice cells to sickle only under certain conditions. This is sickle-cell trait, not sickle-cell anemia.
And now in sickle cell trait, you have 55% of the hemoglobin is hemoglobin A, 43% is hemoglobin S, and the rest 2% is hemoglobin A2. So there are two main types of hemoglobin, and we call this phenomenon co-dominance. Okay, remember our story of to sickle or not to sickle—this is the question. What are the causes of sickling? You have to have a hemoglobin S concentration of more than 60%. In sickle cell trait, it's only about 43%, which is definitely less than 60%. So for the most part, there is no sickling. However, there is sickling only under extreme conditions, such as hypoxia and hypertonicity, which occur in the renal medulla. The renal medulla is notorious for hypoxia and hypertonicity. That's why the only clinical symptoms that you can have are microscopic hematuria—which is blood in the urine, but it's microscopic, so the patient cannot see blood in the urine; it's only under the microscope—and something called hypostenuria. We have talked about hypostenuria; it's kind of the same. Hypostenuria is even a better term, which means less strength or less concentration—inability to concentrate urine because the renal medulla is now suffering because the cells are sickling under these extreme conditions. Some sources also add cramping as a symptom in sickle cell trait.
Diagnosis: hemoglobin electrophoresis is the best. It tells you the type of hemoglobin and the amount of hemoglobin, which is great. Sodium metabisulfite is also great when it detects hemoglobin S; it is detectable in the test tube. Pay the greatest attention here: there are no sickle cells in sickle cell trait; there are no target cells in sickle cell trait, only in sickle cell anemia, in the hemoglobin SS. But in the hemoglobin AS, or the sickle cell trait, there are no sickle cells or target cells. Management: nothing. It's a very benign condition, and it needs no treatment. That's it for sickle cell trait; pretty straightforward. And I'll see you in the next video, a nice comparison between sickle cell trait and sickle cell anemia. See you then.