Hemoglobins with Altered Oxygen Affinity, Unstable Hemoglobins, M-Hemoglobins, and Dyshemoglobinemias



Hemoglobins with Altered Oxygen Affinity, Unstable Hemoglobins, M-Hemoglobins, and Dyshemoglobinemias


Martin H. Steinberg



More than 1,100 mutations affecting the globin subunits of hemoglobin have been described (http://globin.cse.psu.edu/), and among these, sickle cell disease along with hemoglobinopathies associated with HbE, HbC, and the β– and α-thalassemias are mankind’s most common single-gene disorders (see Chapters 33 and 34). Much less common are hemoglobin mutations discussed in this chapter that affect the ability of the molecule to bind and release oxygen, that reduce its stability, and that allow its heme iron to be oxidized. Exogenous agents can also oxidize hemoglobin, interfering with oxygen transport.


HEMOGLOBINS WITH ALTERED OXYGEN AFFINITY

The affinity of hemoglobin for oxygen is characterized by the amount of oxygen bound at any given oxygen tension. Oxygen affinity is usually designated by the P50, which is the partial pressure of oxygen at which hemoglobin is 50% saturated. Oxygen affinity can be modified by pH, temperature, and organic phosphates. The normal co-operativity of hemoglobin, or heme-heme interactions in the hemoglobin tetramer, determines the sigmoidal shape of the hemoglobin-oxygen dissociation curve. This is a result of the fact that the deoxygenated T (tense) form of hemoglobin has a lower affinity for oxygen than the oxygenated R (relaxed) form (see Chapter 6). Consequently, globin gene mutations that alter areas of the molecule involved with T-R interactions can lead to alterations in oxygen affinity. Both high- and low-oxygen-affinity hemoglobin variants are encountered and all globin genes have been affected.


High-oxygen-Affinity Hemoglobins

Globin gene mutations can increase the affinity of the hemoglobin molecule for oxygen and cause erythrocytosis. More than 90 high-oxygen-affinity variants have been reported in http://globin.cse.psu.edu/ and Wajcman and Galacteros.1 Familial erythrocytosis is a valuable clue to the presence of a high-oxygen-affinity hemoglobin variant; isolated cases are caused by new mutations. These α– or β-globin chain mutations are dominant disorders, expressed clinically in the heterozygote. They may be lethal in homozygotes if they affect the α-globin chain and are expressed in utero. One γ-globin variant (HbF-Monserrato-Sassari, HBG2 cys93arg) in a normal newborn and a possible δ-globin variant (Hb Noah Mehmet Oesteurk, HBD his143tyr) have been described.

Several homozygotes for β-globin high-oxygen-affinity variants have been described and these individuals can have more severe disease.2, 3, 4, 5 Compound heterozygotes with a high-oxygen-affinity hemoglobin and β0-thalassemia have also been described, mimicking the homozygous state as normal adult hemoglobin A (HbA) is not present.




α-Globin Chain Variants

Because there are four α-globin genes, most stable α-globin variants form 25% or less of the total hemoglobin, compared with 40% to 50% for β-globin variants. As a result, the clinical effects of α-globin variants are less striking than those of β-globin variants. However, the coincident inheritance of a β-thalassemia gene can modulate the concentration of high-affinity α-variant hemoglobins and homozygosity for a high-oxygen-affinity variant affecting α-globin chains has been described.

Hb Chesapeake (HBA arg92leu) was the first reported highaffinity hemoglobin variant.11 It was discovered in an 81-year-old patient with erythrocytosis, an abnormal hemoglobin detected by hemoglobin electrophoresis, and erythrocytes with increased oxygen affinity. Fifteen members of the proband’s family were similarly affected. Hb Chesapeake represented ˜20% of the total hemoglobin. With a P50 of 19 mm Hg (normal ˜26 mm Hg), Hb Chesapeake produced moderate erythrocytosis. The mutation affected an invariant residue that stabilizes the R state at the α1β2 area of contact, making the T conformer less favored.

Hb Nunobiki (HBA1 arg141cys), is one of four mutations of this invariant residue, all of which exhibit high oxygen affinity and moderate to mild erythrocytosis.12 This group of mutations represents an interesting cluster of variants that illustrates the effects of different mutations at the same amino acid residue. As a mutant of the 3′-HBA1 gene that is expressed to a lesser extent than the 5′-HBA2 gene, Hb Nunobiki makes up ˜13% of the hemolysate and is accompanied by only mild erythrocytosis. High oxygen affinity is a result of the breaking of the C terminal-to-C terminal salt bridge that is indispensable for the stabilization of the T state, favoring the R state.


β-Globin Chain Variants

All possible single-base mutations of the β99 site disturbing the α1β2 area of contact have been described and include Hb Kempsey (HBB asp99asn), Hb Yakima (asp99his), Hb Radcliffe (asp99ala), Hb Ypsilanti (asp99tyr), Hb Hotel-Dieu (asp99gly), Hb Chemilly (asp99val), and Hb Coimbra (asp99glu). As expected for stable β-globin chain variants, all are present at 40% to 50% of the hemolysate, exhibit moderately high oxygen affinity, and are characterized clinically by erythrocytosis. Hbs Kempsey, Radcliffe, and Hotel Dieu have a decreased response to 2,3-DPG. Hbs Ypsilanti and Radcliffe form stable hybrid tetramers in the hemolysates in which the abnormal β chains coexist with normal β chains.

Six of the possible seven mutations of the C-terminal CAC (tyr) codon have also been described. One of them, Hb Cochin-Port Royal (tyr146arg), has nearly normal oxygen affinity but decreased 2,3-DPG interaction and Bohr effect.13

Three mutations of β82 lys have been described: Hb Rahere (lys82thr), Hb Helsinki (lys82met), and Hb Providence (lys82asn). All have moderately high oxygen affinity and moderate erythrocytosis.

These mutants have drastically reduced 2,3-DPG binding as a result of the elimination of one of the normal binding sites for this allosteric effector. Hb Porto Alegre (HBB ser9cys) has high oxygen affinity and a tendency to aggregate, but erythrocytosis is not present.14 Polymerization of Hb Porto Alegre is based on the formation of disulfide bonds in oxygenated samples and is different from HbS polymerization. Polymerization of this mutant diminishes heme-heme interaction and increases the oxygen affinity.

Hb Tak (HBB 147(+AC), modified C-terminal sequence: 147thr-lys-leu-ala-phe-leu-leu-ser-asn-phe-157tyr-COOH), is elongated by 11 amino acid residues.15, 16 It forms 40% of the hemolysate, has a very high oxygen affinity with no co-operativity, and no allosteric interaction with pH or 2,3-DPG. The C terminus of the β-globin chain is actively involved in the conformational changes of the hemoglobin molecule by stabilizing the T state. By having these stabilizing interactions disrupted, Hb Tak is totally frozen in the R state. Hb Tak is also slightly unstable. In spite of these severe functional abnormalities, a heterozygous patient did not have erythrocytosis. The extreme biphasic nature of the hemoglobin-oxygen affinity curve observed in mixtures of Hb Tak and HbA suggests that hybrid tetramer (α2βAβTak) formation is absent. The top portion of the oxygen equilibrium curve is normal, and it begins to be abnormal only at <40% saturation. Because physiologic oxygen exchange occurs most commonly above that level of saturation, the tissues may not be hypoxic, removing the stimulus for increased erythropoiesis.


Clinical Features

Patients with high-affinity hemoglobins and erythrocytosis have a benign clinical course and rarely have complications, apart from a ruddy complexion. Splenomegaly is typically absent. Hemoglobin concentration and hematocrit are increased variably, and usually only moderately, suggesting that modulation by variations in other genes might affect the physiologic response to hypoxia. Some patients with Hb Malmo (HBB his97gln) have been reported to be symptomatic and to benefit from phlebotomy and the transfusion of normal blood, but this clinical course is an exception.17

Many cases of high-oxygen-affinity hemoglobins are diagnosed during a routine hematologic examination or when the family of a proband known to have erythrocytosis is examined. In very limited studies, exercise capacity in the laboratory and the indices of working capacity and cardiac tolerance were similar in patients with high-oxygen-affinity hemoglobins and in controls.18 It has been suspected that carriers of these variants may have enhanced athletic performance under some circumstances, and this has led to the unfortunate and sometimes fatal use of erythropoietin or transfusion to enhance performance in competitive athletics.

In a population-based study of erythrocytosis, high-oxygen-affinity variants accounted for 3% of all cases.19 By early diagnosis of high-affinity hemoglobins, unnecessary invasive diagnostic procedures and inappropriate therapeutic interventions, such as cardiac catheterization, can be avoided. Patients have received 32P treatment based on a mistaken diagnosis of polycythemia vera.

Increased morbidity or mortality in mothers with high-oxygen-affinity hemoglobins or their offspring has not been observed, suggesting that the affinity of the mother’s hemoglobin is irrelevant with respect to oxygen delivery to the fetus.18 Low ambient pO2, as in unpressurized airplanes and ascent to altitude, do not represent a risk, because high-affinity hemoglobins are avid for oxygen.

Hypothetically, carriers should be less prone to “the bends” during deep sea diving, because of slower oxygen release during ascension.




Low-oxygen-Affinity Hemoglobins

Hemoglobin variants with reduced affinity for oxygen are in many respects the converse of high-oxygen-affinity variants. About half as many low-oxygen-affinity variants have been described compared to high-oxygen-affinity variants. They are expressed in the heterozygote, with homozygosity likely to be embryonic lethal. A major clinical feature is anemia that, at times, is accompanied by cyanosis.




Clinical Features

Three low-oxygen-affinity variants have been described at β102. Hb Kansas, the best-studied variant, has a whole-blood P50 of ˜70 mm Hg, decreased co-operativity, and a normal Bohr effect.21 The β102 asn residue is invariant among β-globin chains and participates in the only hydrogen bond between asn 102 and asp 94 across the α1β2 interface in oxyhemoglobin. This bond is broken when the molecule assumes the T state. The new thr residue is incapable of forming this bond, and low oxygen affinity results from destabilization of the R conformer. The changes induced by this substitution at the α1β2 interface allow Hb Kansas to dissociate into αβ dimers, the near opposite of the high-oxygen-affinity Hb Chesapeake.

Hb Beth Israel (HBB asn102ser) was found in a patient with cyanosis of the fingers, lips, and nail beds.23 The P50 was 88 mm Hg, and arterial blood was only 63% saturated despite a normal pO2. The hemolysate also had a low oxygen affinity and a normal Bohr effect. Erythrocyte 2,3-DPG was mildly elevated. The molecular mechanism of reduced oxygen affinity is the same as for Hb Kansas, although the defect may be more disruptive locally, because the serine side chain is shorter than that of threonine.

Hb Bologna (HBB lys61met) is informative because it was present as a compound heterozygote with β0-thalassemia and comprised 90% of the hemolysate.24 Adults were neither cyanotic nor anemic despite having a P50 of 37.6 mm Hg. During gestation, the high concentration of HbF makes it doubtful that this mutation would have an effect on fetal development.

Hb Bruxelles (HBB phe42del) is a deletion of the most conserved amino acid residue of hemoglobin.25, 26 Phenylalanine residues at β41 and β42 are conserved in all normal mammalian non-α-globin chains and are indispensable for the structural integrity and oxygen-binding functions of the molecule. From age 4 years, the index case of Hb Bruxelles had severe hemolytic anemia and cyanosis, requiring blood transfusion once. Later in life, her hemoglobin concentration stabilized at 10 g/dL. Reasons for this “switch” of phenotype are unknown. Other mutations of β41 and β42, which are predominately unstable hemoglobins, are discussed later.




UNSTABLE HEMOGLOBINS

The unstable hemoglobins result from globin chain mutations that cause hemoglobin tetramer instability and intracellular precipitation of its globin subunits. These intraerythrocytic precipitates are detectable by supravital staining and appear as globular aggregates called Heinz bodies. These inclusions reduce the life of the erythrocyte by binding to the membrane, decreasing cell deformability, and increasing membrane permeability. The resultant hemolytic disorder is sometimes called congenital Heinz body hemolytic anemia. Heinz bodies and hemolysis also occur with certain hereditary erythrocyte enzyme deficiencies (see Chapter 28). More than 140 unstable variants of both the β– and α-globin chains with widely varying clinical severity have been reported, almost always as heterozygotes for the mutation, although some homozygous cases have been reported. The major clinical features are anemia, reticulocytosis, pigmenturia, and splenomegaly.

Oct 21, 2016 | Posted by in HEMATOLOGY | Comments Off on Hemoglobins with Altered Oxygen Affinity, Unstable Hemoglobins, M-Hemoglobins, and Dyshemoglobinemias

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