|This is a document in a five-part series
on blood cells and anemia:
|1. Blood cells and the CBC|
|2. Anemia: Pathophysiologic Consequences,
Classification, and Clinical Investigation
|3. Nutritional Anemias and
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.
III. Pathophysiology of hemoglobinopathies
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:
- Increased O2 affinityThese 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.
- Decreased O2 affinityThis 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.
- MethemoglobinemiaThese 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.
- Unstable hemoglobin (Heinz body anemia)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.
- Sickling and crystallizationThese 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.
IV. Specific hemoglobinopathies
A. Hemoglobin S and sickle cell disease
1. Epidemiology and genetics
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.
- Hereditary persistence of fetal hemoglobin (HPFH) has established itself in the black population and allows Hb F to so dilute the Hb S that sickling does not occur or is less prominent. In these people the Hb F gene does not “turn off” in infancy but persists indefinitely.
- G-6-PD deficiency has been suggested as an ameliorative condition for sickle cell disease. This is controversial; the pathophysiologic basis of any such effect must be pretty obscure.
2. Clinical findings
Sickle cell anemia is a particularly bad disease in that not only is it a hemolytic anemia, but also a vaso-occlusive condition. The clinical findings can then be divided into one of these two groups:
- a. Effects of chronic hemolysis
- Anemia. Pretty much self-explanatory
- Jaundice, due to rapid heme turnover and subsequent generation of bilirubin
- Cholelithiasis. It has been classically taught that sickle cell patients are prone to the formation of calcium bilirubinate gallstones due to excess bilirubin secretion into the hepatobiliary tree.
- Aplastic crisis. Many of us have brief episodes of marrow aplasia as a result of common viral infections. With a normal erythrocyte life span of 120 days, no anemia results from an unnoticed marrow shut-down of a few days. However, the sickle cell patients, with their markedly abbreviated rbc life span, can have a precipitous fall in hematocrit (and retic count) under such conditions. This may be life-threatening.
- Hemolytic crisis. Most sickle cell patients establish a stable, tonic level of hemolysis. Rarely, for obscure reasons, they experience a catastrophic fall in hematocrit, increasing intensity of jaundice, and increasing reticulocyte count. This is called a “hemolytic crisis.”
- b. Effects of vaso-occlusion
- Dactylitis. Resulting presumably from infarction or ischemia of the bones of the hands and feet, this is often the presenting manifestation of sickle cell disease in a six-months-old infant. The hands and feet are swollen and painful.
- Autosplenectomy. In childhood, the spleen is enlarged due to excess activity in destruction of the sickled erythrocytes. Gradually, the spleen infarcts itself down to a fibrous nubbin.
- Priapism. This refers to a painful and sustained penile erection, apparently due to sludging of sickled cells in the corpora cavernosa. Sometimes the penis has to be surgically decompressed. Repeated episodes of priapism cause the spongy erectile tissues to be replaced by fibrous tissue, with impotence being the end result.
- Renal papillary necrosis. The physiologic function of the loops of Henle make the renal medulla an eldritch, unbodylike area of high hematocrit, high osmolarity, low pH, hemodynamic stasis, and low PO2. All of these conditions predispose to sickling and infarctive loss of the papillae of the pyramids. The result is inability to concentrate and dilute urine. Even sickle cell trait individuals may experience episodes of hematuria, presumably due to this mechanism.
- Infarctive (painful) crisis. Increased sickling activity may be brought about by any general stress on the body, especially infection. Almost any organ may suffer acute infarction (includinmg the heart), and pain is the chief symptom.
- Sequestration crisis. This occurs mostly in infants and young children and is characterized by sudden pooling of sickled erythrocytes in the RES and vascular compartment. This produces a sudden fall in hematocrit. Sequestration crisis may be the most common cause of death in sickle cell patients in the youngest age group.
- Leg ulcers. After all of the disasters mentioned above, this seems trivial. However, the deep, nonhealing ulcers of skin and tela subcutanea (classically around the medial malleolus) may be the only clinical manifestation of sickle cell disease in an otherwise well-compensated patient. These may be the only bugaboo standing between the patient and a productive, financially solvent life.
B. Hemoglobin C
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.
C. Hemoglobin E
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:
B. Biochemistry and pathophysiology
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.|
|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).|
C. Beta thalassemia
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:
- 0 thalassemiaThis 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.
- + thalassemiaThis 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:
- Individuals heterozygous for any of the thalassemia genes are either silent carriers or have minimal clinical effects, usually manifested as a borderline anemia (Hct ~ 35 cL/L) with disproportionate microcytosis (MCV ~ 60 fL) and a reciprocally high rbc count (~ 6 x 106/µL). The Hb A2 is increased. This clinical presentation is called thalassemia minor. It makes for interesting wine-tasting party conversation if you have this condition, and all that your friends can muster is chronic fatigue syndrome. Your kids should have no problems if you just marry a Teuton, Slav, Balt, or Lapp.
- Individuals homozygous for all of the thalassemia genes [except the + (Negro) gene] have severe anemia and some or all of the pathophysiological consequences given in the diagram below. This is classic Cooley’s anemia and is termedthalassemia major. This is bad news.
- Individuals homozygous for the + (Negro) gene and several other miscellaneous types of mildly behaving genes have a relatively mild clinical anemia called thalassemia intermedia. These patients may require transfusion, but only later in life than is the case in the very sick children with thalassemia major.
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.
- Since there is a decrease in the synthesis of chains, there is a net decreased synthesis of Hb A. With less Hb A available to fill the red cells, the result is microcytic anemia. Whereas in iron deficiency microcytosis occurs because there is not enough heme, in thalassemia the same thing occurs because there is not enough globin.
- Since the body cannot make enough chains, it makes a feeble attempt to compensate by trying to churn out chains. The result is increased Hb A2, which can be measured easily and inexpensively by column chromatography. This is a pretty specific test for the diagnosis of thalassemia. Pitfall: both and chains are decreased in thalassemia, which is not rare and presents like thalassemia, except that the Hb A2 is not elevated. You would expect this since Hb A2contains chains).
- In some cases of thalassemia, there is attempt at compensation by maintaining some production of Hb F. This has some pathophysiologic consequences (as shown above) and also provides a laboratory marker to assist in diagnosis. Retention of Hb F production is not as common as increased rbc Hb A2 content.
- In severe forms of thalassemia, the anemia due to failure to make adequate amounts of Hb A is compounded by the hemolysis, ineffective erythropoiesis, and extramedullary hematopoiesis brought on by precipitation of 4 tetramers (which are unstable). In classic Cooley’s anemia, the ineffective erythropoiesis dominates the clinical picture by producing tremendous expansion of the marrow space, manifested by the so-called “tower skull” with an x-ray showing innumerable vertical bony striae between the inner and outer tables of the calvarium. This radiographic feature is fancifully called the “hair-on-end appearance” by radiologists, and the “guy-who-accidentally-sat-on-a-Van-de-Graaff-generator appearance” by those wacky electrical engineers. Extramedullary hematopoiesis and hemolysis causes splenomegaly, which produces hypersplenism, and more hemolysis.
- The high turnover state caused by the tremendous erythroproliferative activity causes wastage of folate and may produce a complicating megaloblastic anemia. Another effect of the high turnover state is hyperuricemia (due to catabolism of the purine content of cellular DNA).
- Classically in thalassemia major, the treatment is the cause of death. The children are maintained by transfusions until about age ten years, at which time they start to show symptoms of excess iron loading. This happens because the transfusion bypasses the body’s normal gastrointestinal mechanism of iron intake and excretion. The iron is poured into the bloodstream directly; the body cannot excrete it fast enough. First iron (as hemosiderin) fills the cytoplasm of the RES phagocytes and then starts to be deposited in the parenchymal cells of just about every organ of the body. The pancreas, liver, myocardium, adrenals, and gonads are among the organs most sensitive to iron toxicity. The clincial result is diabetes mellitus, hepatic cirrhosis, congestive heart failure, adrenal insufficiency, and failure to undergo puberty. Death used to occur in the second or third decade of life, the most common immediate cause being complications of heart failure. Nowadays, thal major patients live longer because of advances in chelation therapy. To achieve such longevity, they must submit to daily subcutaneous injections of the parenteral chelating agent, deferoxamine. These injections are given by pump, usually overnight, and last 9 to 12 hours each. An oral chelating drug in presently under development in Europe.
D. Alpha 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:
- Unlike thalassemia, thalassemia is present even before birth, since the chain is integral to all hemoglobins past the very primordial Hb Gower 1 and Hb Portland. Thus, individuals, born and unborn, who carry one of these genes underproduce Hb Gower 2, Hb F, Hb A, and Hb A2. Obviously then, elevated Hb A2 cannot be used as a diagnostic marker for thalassemia.
- As noted in an illustration above, each individual has four genes for the globin chain, which can be denoted as /. Each haplotype () is inherited from a parent as an indestructible, non-crossoverable pair. In thalassemias, anywhere from one to all four of these genes of the diplotype (/) can be deleted (or transformed into some “bad guy” gene that doesn’t do anything), producing a dose-related depression of chain synthesis. Thus, the (-/) diplotype produces the mildest condition, while (–/–) produces the fatal intrauterine disease. The (-) gene is called the “ thalassemia-2″ gene while the (–) gene is termed the “ thalassemia-1″ gene. One can be homozygous or heterozygous for each of these, to give four possible diplotypes (-/, -/-, –/, –/–). An additional diplotype is the double heterozygote for thalassemia-1 and thalassemia-2 (-/–). Since the thalassemia-1 and thalassemia-2 genes tend to occur in different races (Asians and Africans, respectively, between which there is a low rate of intermarriage), this combination is not common. Let us look at the various nefarious gene combinations individually:
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.