Transcription
Paroxysmal nocturnal hemoglobinuria. Paroxysmal means episodic or in the form of attacks. Nocturnal means at night. Hemoglobinuria: hemoglobin in the urine. The problem is a PIGA gene mutation. This is medical perfection. We continue our series on hematology. Okay. PNH is an acquired chronic hemolytic anemia. Contrast that with thalassemia or sickle cell anemia; those were inherited in an autosomal recessive fashion. PNH is acquired; it's not congenital, it's not inherited; it's acquired after you're born. You acquire this disease due to the fact that in the myeloid stem cells—myeloid stem cells will give rise to red blood cells, neutrophils, and platelets and others—the mutation is in the PIGA gene on the short arm of the X chromosome, position 22.2. The mutation occurs after birth; hence, acquired. By the way, PIGA stands for phosphatidylinositol glycan anchor biosynthesis class A. Holy guacamole! The PIGA gene encodes an enzyme that synthesizes something called membrane-associated proteins. These regulate the complement and they include GPI, which stands for glycosylphosphatidylinositol. Who knew? If there is no GPI, there are no complement regulatory proteins, which include cluster differentiation 55, also known as decay-accelerating factor, and cluster differentiation 59, also known as membrane inhibitor of reactive lysis, also known as homologous restriction factor. Did you understand that thing? Of course not, but that's why Medicosis is here. Let's dissect everything so that you can understand all of these crazy words.
Let's review some immunology: complement system. Why the name is complement? Because it complements the ability of antibodies to do their job. Three different pathways, depending on who pulls the trigger, who will start the cascade. If it's antigen-antibody complex, this is the classical pathway. If it's the bacterial endotoxin, this is the alternative pathway. If mannose-binding lectin, this is the lectin pathway. In PNH, the problem is usually in the alternative pathway. So let's elaborate now: alternative pathway of the complement. What triggers the cascade is bacterial endotoxin, which is a lipopolysaccharide in the outer membrane of gram-negative bacteria. That was nice microbiology. There we start with factor C3 hydrolysis in the presence of water into factor C3b. It will bind with factor B, and then factor D will convert factor B into Ba and Bb. Factor Bb will combine with C3b to form C3 convertase. This is a big deal. C3 convertase will convert C3 into C3a and C3b. C3b will bind with C3 convertase to form C5 convertase. C5 convertase will do what? Converts C5 into C5a and C5b. C5b, together with C6, 7, 8, and 9, will form something called the MAC. This is not anti-Microsoft, and this is not McDonald's either. This is the membrane attack complex. It will form a pore in the cell membrane of the bacteria, which will disrupt the ionic and osmotic environment of the cell, and now the bacteria will die. Go to hell. Shakespeare time: To attack or not to attack, this is the question. How does the complement differentiate between self and non-self? Non-self: bacteria are foreigners, and we should destroy them. But self are our own cells, such as red blood cells; we should never touch them. How does the complement know? Thanks to complement regulatory proteins. Think of regulatory as inhibitory; they inhibit the complement from attacking. These include CD55 and CD59. So this is the cell membrane of your red blood cell, or neutrophil, or platelets. These are the myeloid lineage; there are no lymphocytes here because these are the lymphoid stem cell origin. Now, the PIGA gene will encode an enzyme that will give rise to GPI. GPI is an anchor protein; it will anchor CD55 and CD59 to the cell membrane. CD55 will prevent the formation of C3 convertase, so the complement cannot attack. CD59 will block the binding of C9, so there is no MAC. When there is no MAC, there is no attack, and everybody is happy, and the complement can recognize the cell and not attack it. This is normal.
Let's see now. You will understand every single word in this page. PNH is an acquired chronic hemolytic anemia due to a defect in the myeloid stem cells. Yes. PIGA gene mutation; correct. PIGA gene codes for GPI. If there is no GPI, there is no CD55 or CD59, so the complement will destroy the normal blood cells—the self. The complement attacks the red blood cell in an intravascular hemolysis. Most of the time, PNH is an intrinsic, intravascular, acquired hemolytic anemia. You may have some hemolysis throughout the day, but when you go to bed, you increase your urine concentration. This is true because the first morning urine sample is the highest concentration, so you will see your urine darker, and it has hemoglobin in it. So that patient is shouting, "Dr., help me! There is blood in my urine!"
Complications of PNH include iron deficiency anemia—why? Because you are losing hemoglobin in the urine—as well as acute myeloid (not lymphoid) myeloid leukemia (most common cause of death), thrombosis (especially venous). What's the cause of thrombosis? Nobody knows. A theory suggests that of the myeloid lineage—our platelets—when platelets are destroyed by the complement, they produce some factors which are procoagulant and lead to thrombosis. Okay. Venous thrombosis in the hepatic veins, known as Budd-Chiari syndrome, portal or cerebral. And the second most common cause of death is infection.
Now, clinically, a nice triad: hemolytic anemia, pancytopenia, and venous thrombosis. Hemolytic anemia will have its symptoms as splenomegaly, as we have discussed in our famous video: intravascular versus extravascular hemolysis. Pancytopenia: no neutrophils (infections), no platelets (bleeding), no red blood cells (fatigue and anemia). Okay. Aplastic anemia can coexist with the pancytopenia of PNH. Venous thrombosis, depending on the location: in hepatic veins, this is Budd-Chiari syndrome, in the absence of any liver disease. So the patient's liver is completely fine; no cirrhosis, no hepatitis, no cancer, but the veins are having thrombosis. This is PNH. Abdominal veins: splenic necrosis and severe abdominal pains. Renal vein thrombosis: stroke. Veins of the skin: skin nodules. Esophageal spasms may coincide with hemoglobinuria. This is high-yield. PNH can cause fever of unknown origin.
Now let's go to the lab: CBC. How about hemoglobin and hematocrit? Both are decreased because this is the definition of anemia. How about reticulocytes? Are increased because this is a response from the bone marrow to hemolysis, except if there is aplastic anemia; in that case, reticulocytes will be low, but most of the times they are high. MCV is usually normal, which means 80 to 100, but there are some tiny exceptions. One of the complications of PNH is iron deficiency anemia, which will lead to low MCV. Another exception: if reticulocytosis exists, MCV will be high because, as you know, reticulocytes are immature red blood cells, and they are larger than red blood cells, raising the MCV. In the serum: LDH is high, unconjugated bilirubin is high, haptoglobin is low. Go back to my video on hemolysis. Direct Coombs test is negative because there is no antigen-antibody reaction; there is no classical complement pathway. Contrast that with autoimmune hemolytic anemia, where the Coombs test is positive because then there is antigen-antibody reaction. Sucrose hemolysis test is very unreliable and it's a screening test. Acidified serum test, or HAM, is very reliable, used for confirmation. Flow cytometry is the gold standard; it can detect the absence of CD55 and CD59, which is PNH.
How to treat PNH? PNH is a hemolytic anemia; give iron and folate. If severe: RBC transfusion. Glucocorticoids can be used in hemolytic anemias, but specifically in PNH they are contraindicated. Eculizumab is a new miraculous drug for PNH. Eculizumab means monoclonal antibody: antibody against one antigen and one antigen only. This one antigen is C5. Eculizumab will bind to C5 and prevents its cleavage into C5a and C5b by the enzyme C5 convertase. There is no C5a and C5b, so there is no MAC and no attack. It reduces mortality, so the patient can live longer. If you are giving eculizumab, vaccinate against Neisseria meningitidis. Definitive treatment, of course, is stem cell transplant. If PNH coexists with aplastic anemia, do a bone marrow transplant. See you in the next video: a mnemonic on PNH.