This is a document in a five-part series|
on blood cells and anemia:
|1. Blood cells and the CBC|
2. Anemia: Pathophysiologic
Classification, and Clinical Investigation
3. Nutritional Anemias
Anemia of Chronic Disease
|4. Hemolytic Anemias|
|5. Hemoglobinopathies and Thalassemias|
These conditions comprise a very large number of genetic biochemical/ physiological entities, most of which are academic curiosities whose major effect on medicine is to add to the surfeit of useless scientific information. However, several of these conditions (e.g., sickle cell anemia, hemoglobin SC disease, and some thalassemias) are common major life-threatening diseases, and some others (e.g., most thalassemias, hemoglobin E disease, and hemoglobin O disease) are conditions that produce clinically noticeable -- if not serious -- effects and can cause the unaware physician a lot of frustration and the hapless patient a lot of expense and inconvenience. We will study a few hemoglobinopathies and thalassemias of special importance. It should be kept in mind, though, that there are literally hundreds of diseases in these categories.
Hemoglobinopathy: A genetic defect that results in abnormal structure of one of the globin chains of the hemoglobin molecule. Although the suffix "-pathy" would conjure an image of "disease," most of the hemoglobinopathies are not clinically apparent. Others produce asymptomatic abnormal hematologic laboratory findings. A very few produce serious disease. The genetic defect may be due to substitution of one amino acid for another (as with the very common Hb S and Hb C and the great majority of the other abnormal hemoglobins), deletion of a portion of the amino acid sequence (Hb Gun Hill), abnormal hybridization between two chains (Hb Lepore), or abnormal elongation of the globin chain (Hb Constant Spring). The abnormal chain that results may be the chain (Hb GPhiladelphia), chain (Hb S, Hb C), chain (Hb FTexas), or chain (Hb A2Flatbush). These abnormal hemoglobins can have a variety of physiologically significant effects, discussed below in greater depth, but the most severe hemoglobinopathies (Hb S and Hb C diseases) are characterized by hemolysis.
Thalassemia: A genetic defect that results in production of an abnormally low quantity of a given hemoglobin chain or chains. The defect may affect the , , , or chain, or may affect some combination of the , , and chain in the same patient (but never the and chain together). The result is an imbalance in production of globin chains and the production of an inadequate number of red cells. The cells which are produced are hypochromic/microcytic and contain a surfeit of the unaffected chains which cannot stoichiometrically "mate" with the inadequate supply of thalassemic chains. These "bachelor" chains can produce adverse effects on the red cell and lead to destruction of the red cell in the marrow (ineffective erythropoiesis) and in the circulation (hemolysis). Note that these two definitions are not mutually exclusive -- some hemoglobinopathies may also be thalassemias, in that a structurally abnormal hemoglobin (hemoglobinopathy) may also be underproduced (thalassemia). Some, but not all, hemoglobinopathies and thalassemias are hemolytic anemias. These nosologic concepts are summarized by the Venn diagram below.
Messing around with the amino acid sequence of a globin chain has something of a red kryptonite effect. While some positions on the protein chain can tolerate a lot of substitutions without compromising the physiologic integrity of hemoglobin, other positions are very sensitive to amino acid substitutions. For instance, substitution of valine or lysine for glutamate at position 6 of the chain produces hemoglobins S and C, respectively, which form intraerythrocytic tactoids (see below) and crystals (again respectively) that cause premature destruction of the rbc (hemolysis). On the other hand, substitution of glutamate, asparagine, and threonine for lysine at position 59 of the chain produces, respectively, hemoglobins IHigh Wycombe, JLome, and JKaoshiung, all of which are physiologically indistinguishable from normal Hb A. Without venturing too deeply into tedious stereochemistry, we can say that abnormal globin structure can functionally manifest itself in one or more of the following ways:
These hemoglobins tend to result when mutations affect the portions of the amino acid sequence that compose 1) the regions of contact between and chains, 2) the C-terminal regions, and 3) the regions that form the pocket which binds 2,3-DPG. The hemoglobin eagerly scarfs up the O2 from the alveoli but then only stingily gives it up to the peripheral tissues. The kidney, always compulsively vigilant for hypoxia, cranks out the erythropoietin thinking that a few extra red cells might help out matters. Erythropoiesis then is stimulated, even though there is no anemia, and erythrocytosis (increased total body rbc mass, increased blood hemoglobin concentration, increased hematocrit) is the result.
It is important to know that these rare increased O2 affinity hemoglobins exist to prevent diagnostic errors from occurring in working up patients presenting with erythrocytosis (which is much more commonly caused by other conditions, including polycythemia vera [a neoplasm], cigarette smoking, psychosocial stress, chronic residence at high altitudes, and chronic lung disease). Examples of these include Hb Chesapeake and Hb JCapetown.
This is the other side of the coin. These hemoglobins are reluctant to pick up O2 from the lung. The result is a decreased proportion of hemoglobin that is oxygenated at a given PO2. The remainder of the hemoglobin is, of course, deoxygenated and is blue. If the level of blue hemoglobin exceeds 5 g/dL in capillary blood, the clinical result is cyanosis, a bluish discoloration of skin and mucous membranes.
Again, it is important to know about these hemoglobins and keep them in the back of your mind when working up cases of cyanosis, a condition much more commonly caused by pulmonary dysfunction or right-to-left cardiovascular shunts. Examples of low O2 affinity hemoglobins include Hb Seattle, Hb Vancouver, and Hb Mobile.
These hemoglobins are a special class of low O2 affinity hemoglobin variants that are characterized by the presence of heme that contains iron in the ferric (Fe+++) oxidation state, rather than the normal ferrous (Fe++) state. These methemoglobins are all designated "Hb M" and further divided by the geographic site of their discovery, e.g., Hb MSaskatoon and Hb MKankakee. The affected patients have cyanosis, since the methemoglobin is useless in O2 binding.
Methemoglobinemia due to hemoglobinopathy should be distinguished from methemoglobinemia due to other causes, such as NADH-diaphorase deficiency. This enzyme is needed for the reduction (to heme) of metheme that accumulates as a result of normal metabolic processes. Congenital absence of NADH-diaphorase causes an accumulation of metheme, despite the fact that the structure of the globin chain is normal. Toxic methemoglobinemia occurs in normal individuals exposed to certain oxidizing drugs and other compounds in the environment, even though these individuals have normal hemoglobin structure and a normal complement of NADH-diaphorase. In such victims, the oxidizing power of the toxin overwhelms the normal antioxidant defenses.
Since methemoglobin is a brown pigment, patients with clinically severe methemoglobinemia have obviously brown blood. This observation allows one to make a clever and memorable diagnosis at the bedside during the patient's first venipuncture.
Certain abnormalities in the globin chain sequence produce a hemoglobin that is intrinsically unstable. When the hemoglobin destabilizes, it forms up into erythrocyte inclusions called Heinz bodies. It is important to know that Heinz bodies are not visible in cells stained with the routine Wright stain. It is necessary for the cells to be stained with a supravital dye (such as brilliant cresyl blue, which can also be used to demonstrate reticulocytes) to be visible. These inclusions attach to the internal aspect of the rbc membrane and reduce the deformability of the cell and basically turn it into spleenfodder. The result is hemolytic anemia. All of these hemoglobins are rare; inheritance is autosomal dominant. Homozygotes have not been described. Examples of unstable hemoglobins are Hb Gun Hill, Hb Leiden, and Hb Köln.
These phenomena occur respectively in Hb S and Hb C, the most important of the abnormal hemoglobins. We will deal with these in greater depth next.
The Hb S gene is found primarily in populations of native tropical African origin (which include most African-Americans). The incidence of the gene in some African populations is as high as 40%; in African-Americans the incidence is 8%. The gene is also found with less frequency in non-Indo-European aboriginal peoples of India and in the Middle East. Rare cases have been reported in Caucasians of Mediterranean descent. The gene established itself in the tropical African population presumably because its expression in heterozygotes (sickle cell trait) affords some protection against the clinical consequences of Plasmodium falciparum infestation. Unfortunately, homozygous expression produces sickle cell disease, which is a chronic hemolytic anemia and vaso-occlusive condition that usually takes the life of the patient.
Hemoglobin S has the peculiar characteristic of expressing its biochemical instability by precipitating out of solution and forming up into long microtubular arrays called tactoids. The erythrocytes which contain the Hb S stretch around the tactoids to form the characteristic long, pointed, slightly curved cells called (with somewhat liberal imagination) "sickle cells." Only the deoxygenated form of Hb S (deoxy-Hb S) makes tactoids. The greater the proportion of Hb S in the cell, the greater is the propensity to sickle. Therefore, persons with 100% Hb S (being homozygotes) sickle under everyday conditions, while typical heterozygotes (who usually have about 30-40% Hb S) do not sickle except possibly under extraordinary physiologic conditions. Since Hb S is a chain mutation, the disease does not manifest itself until six months of age; prior to that time the Hb S is sufficiently "watered down" by Hb F (22), which of course has no chain.
In post-infancy individuals homozygous for the Hb S gene, 97+% of the hemoglobin is Hb S, the remainder being the normal minor hemoglobin, Hb A2 (22). Several coexisting genetic "abnormalities" (actually godsends) prevalent in African populaitons may ameliorate the course of the disease:
-thalassemia carriers (which comprise 20% of African-Americans!) have a lower MCHC than normal individuals. It has been suggested that a low MCHC is beneficial in decreasing the vaso-occlusive properties of sickled cells. These sickle cell patients live longer and have a milder disease than do non-thalassemic patients. Thalassemia is discussed in greater detail below.
The gene for Hb C is also prevalent in the African-American population but with less frequency (2-3%) than that of the sickle cell gene. Hb C does not form tactoids, but intracellular blunt ended crystalloids. The result is decreased rbc survival time; however, hemolysis is not as severe as in sickle cell disease, and the vaso-occlusive phenomena, so devastating in sickle cell disease, are not generally noted. Like sickle cell trait, the Hb C trait is asymptomatic. Homozygotes (and some heterozygotes) for Hb C often have many target cells (codocytes) in the peripheral smear, but the crystals, although pathognomonic, are only occasionally seen. The prognosis of homozygous Hb C disease is excellent.
An individual may inherit a Hb S gene from one parent and a Hb C gene from the other. The result of this double whammy is Hb SC disease. The clinical severity of this condition is intermediate between that of sickle cell disease and Hb C disease, except that visual damage due to retinal vascular lesions is characteristically worse in SC disease than in sickle cell anemia. The intracellular bodies that occur upon hemoglobin destabilization in SC disease are curious hybrids of the blunt-ended crystalloids of Hb C and the sharp-pointed tactoids of Hb S, in that they often have one pointed end and one blunt end, thus vaguely resembling arrowheads.
This is a very common chain mutation among Southeast Asians. The Thai and Khmer groups have the highest frequency, followed by Burmese and Malays, then Vietnamese and Bengalis. The gene does not occur in ethnic Han Chinese or Japanese. The heterozygous state is asymptomatic but causes microcytosis without anemia, thus resembling some cases of thalassemia minor (see below). The homozygous state has more severe microcytosis and hypochromia, but little, if any, anemia (this is also reminiscent of thalassemia minor). Hemoglobin E should always be considered working up an unexplained microcytosis in a member of one of the affected ethnic groups.
Understanding the thalassemias can be facilitated by reviewing the genesis of the normal post-embryonal hemoglobins:
Chromosome 16 contains the genes for the all-important chain. The genes for all of the other important globin chains are on chromosome 11, where they are closely linked. The linkage means (if you will briefly abuse yourself by recalling basic genetics) that the genes tend to be inherited as a group, as opposed to non-linked (or distantly linked) genes which assort independently due to crossing over during gametogenesis. Because of the linkage, a mutation that affects the rate of production of the chain not uncommonly affects rate of production of the adjacent chain. An individual carrying such a mutation would then have a gene for " thalassemia." He or she could pass on the thalassemia gene to offspring but would essentially never, say, pass a thalassemia gene to one child and a thalassemia gene to another. Conversely, since the genome for the chain is on a completely different chromosome than the genes for all the other chains, one would expect no mutation in a chromosome 11 chain gene (,,) to affect chain production. Moreover, if some poor shlimazel happened to inherit an thalassemia gene from one parent and a thalassemia gene from another, he would not tend to pass both abnormal genes on as a unit to his or her offspring. One kid (out of a representative Mendelian sibship of four) would get the abnormal gene, one would get the abnormal , one would get neither, and one would get both.
But enough of Mendel! We're in med school to learn about hemoglobin, right? Whatever the genetics, the clinical problem in the thalassemias is the inability to maintain a balance between the synthesis rate of one type of globin chain vis-à-vis that of its mate. Even though thalassemias have been described for all four of the above chains, we will consider only those that involve the chain (the thalassemias and thalassemias) and the chain ( thalassemias). It will be useful to review what kind of hemoglobins you can build by mixing and matching globin chains:
|A||22||The only physiologically important adult hemoglobin in normal individuals. Includes the post-translational glycosylated hemoglobins A1a, A1b, and A1c, the last being important in monitoring diabetics.|
|F||22||The major physiologic hemoglobin in post embryonal fetuses. Adapted best for lowered intrauterine O2 tension because of its left-shifted Hb-O2 dissociation curve (allowing O2 to be more readily picked up from maternal circulation). Production normally turns off in early infancy. Proportion of circulating Hb F fades to insignificance at about 6 months of age.|
|A2||22||Medical philosopher's proof of the existence of God (and God's love of physicians). Apparently put here solely as a marker for doctors trying to figure out whether a patient has iron deficiency anemia or thalassemia. Normally less than 3% of circulating hemoglobin (thus physiologically insignificant), Hb A2is slightly elevated in most thalassemias, but normal or decreased in iron deficiency, thus making it a nifty marker for evaluating microcytic, hypochromic anemias.||Gower 1
|Very early normal embryonal hemoglobins that disappear after 8 weeks of gestation. The only one of clinical importance is Hb Portland, which may be seen at birth in cases of the severest form of thalassemia.|
|H||4||Abnormal hemoglobin produced in cases of thalassemia, when excess chains decide to get it on with each other, there not being enough chains to go around. Intrinsically unstable, Hb H produces Heinz bodies in the erythrocytes and subsequent hemolysis.|
|Bart's||4||Delinquent youth gang analogue of Hb H. This abnormal hemoglobin is found in infants with thalassemia. Detecting presence of Hb Bart's in cord blood may be the only practical way to screen for the very large number of individuals who are silent carriers of one type of thalassemia (see below).|
Although this is the classic form of thalassemia it is not the most common. The first description was written by Dr. Thomas Cooley in 1925. The term "Cooley's anemia" has been used synonymously with clinically severe forms of thalassemia, although the remainder of Cooley's career was so undistinguished as to cause some to suggest that his name is not worthy of eponymous immortality. Cooley's anemia was a fatal microcytic anemia of children of Mediterranean descent. The name "thalassemia" was coined to reflect the original geographic home of the target population ( "thalassa" is the classical Greek name for the Mediterranean Sea). Over the years, it became clear that many other groups (Africans, African-Americans, Arabs, Indians, and Southeast Asians) are affected. In fact, thalassemias in general tend to affect races of people that hail from a tropical belt that girdles the Mediterranean and extends all the way through the Indian subcontinent to Southeast Asia.
There are a multiplicity of different thalassemia genes that give rise to a clinically heterogeneous spectrum ranging from asymptomatic expression to classical, deadly Cooley's anemia. It is convenient to group the various thalassemias into two groups, based on the amount of globin chain production:
This abnormal gene allows no production of chains. Individuals homozygous for this gene produce only Hb A2, Hb F (and very little of that after six months of age), and unstable 4 tetramers that trash the red cells while they are still in the marrow. As you might imagine, these people are in pretty dire straits unless some guardian angel has given them another, independent gene for hereditary persistence of fetal hemoglobin (HPFH). This prevents the Hb F spigot from turning down to a trickle at six months. Such persons can live to ripe old age and still be young at heart.
This abnormal gene allows some, but still subnormal, production of chains. People homozygous for this gene will make a subnormal amount of Hb A but will still have trouble with the destructive effects of 4 tetramers on the erythrocytes and erythrocyte precursors in the marrow. The + genes can be further subdivided into the classic + (severe) form, seen in Mediterranean Caucasians, and the mild + (Negro) form seen in blacks. Nowadays this gene has its highest population concentration in Liberia.
Although these genes are remarkably varied in their effect on chain synthesis rate, one can make up some useful rules of thumb:
The pathophysiology of thalassemia major is best understood by going and getting yourself a beer (or politically correct beverage), watching a little TV, doing one or two other chores to postpone the inevitable, and then sitting down to study the next diagram.
While studying the illustration, consider the following observations concerning thalassemia:
The thalassemias include the most common of all hemoglobinopathies and thalassemias. One form of thalassemia is very common in African-Americans. Fortunately this form is so mild that its very detection is almost impossible in adult heterozygotes, and even homozygotes are asymptomatic with mild laboratory abnormalities. Yet another form of thalassemia, fortunately uncommon, produces the most severe disease of all the hemoglobinopathies and thalassemias and usually takes the life of its victim even before birth. Surely thalassemia is a disease of extremes!
It is helpful to consider two concepts concerning thalassemia:
In the above diagram, the normal diplotype is compared with the heterozygous state for thalassemia-2. The latter occurs in 20% of all African-Americans. It produces no symptoms and no abnormalities on routine laboratory tests. The only practical way to detect it is to screen all black infants at birth for Hb Bart's (4), and even this technique will miss some individuals. It is thought that the reason for the high prevalence of this gene in the black population is that it may be an "anti-sickle cell" gene. It turns out that Hb S homozygotes have a milder course of sickle cell anemia if they also are silent carriers for thalassemia.
The the two diplotypes given above, two of the four genes are trashed by "bad guy" mutations. Such individuals are usually not anemic but may have microcytosis. They become victims of the medical system when they are subjected to expensive and time consuming testing in a quixotic search for some serious hematologic condition that does not exist. About the only way to make a diagnosis of one of these conditions is to eliminate other causes of microcytosis and/or check rbc indices on all available blood (no pun intended) relatives. Amidst all this shenanigans lurks a somber note -- if an thalassemia-1 heterozygote finds his or her true love in another thalassemia-1 heterozygote, one-fourth of the issue of such a union will suffer the always lethal homozygous state.
The double heterozygote on the left has "hemoglobin H disease," so named because of the presence of a significant proportion of the hemoglobin composed of four chains. Affected infants will, of course, show some Hb Bart's as well. These people have a hemolytic anemia which varies from very mild to that which clinically resembles thalassemia major.
The thalassemia-1 homozygote, on the right, is allowed no production of chains. The only hemoglobins present are Hb Bart's, Hb H, and Hb Portland. Most of the affected die in utero or within hours after birth. Autopsy shows massive extramedullary hematopoiesis in virtually every parenchymatous organ of the body. The severe anemia causes congestive heart failure and subsequent massive total body edema, termed "hydrops fetalis." Parenthetically, it should be noted that hydrops (from which springs the term "dropsy") is not limited to thalassemia but is seen in any conditon that causes severe heart failure in utero, such as in the anemia due to alloimmune hemolytic disease of the newborn.