Hemoglobinopathies and Thalassemias

2. Clinical findings

• a. Effects of chronic hemolysis
  1. Anemia. Pretty much self-explanatory
  2. Jaundice, due to rapid heme turnover and subsequent generation of bilirubin
  3. 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.
  4. 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.
  5. 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
  1. 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.
  2. 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.
  3. 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.
  4. 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 P_O2. 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.
  5. 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.
  6. 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.
  7. 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.

V. Thalassemia

A. Genetics

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.

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:

Hemo- globin	Globin chain composition	Notes
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 Gower 2 Portland	22 22 22	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 thalassemia

  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.

• ⁺ thalassemia

  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:

1. 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.

2. 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 termed thalassemia major. This is bad news.

3. 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.

While studying the illustration, consider the following observations concerning thalassemia:

1. 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.

2. 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 A₂ contains chains).

3. 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.

4. 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.

5. 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).

6. 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:

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.