Hemoglobin Abraham Lincoln, beta32 (B14) leucine leads to proline. An unstable variant producing severe hemolytic disease

Diagnostic Dilemma of Hb Perth [β32(B14)Leu→Pro; HBB: c.98T > C] in Mainland China

Unstable hemoglobin (Hb) variants represent a rare etiology of congenital hemolytic anemia. Correct diagnosis can be a challenge due to the relative rarity or lack of awareness of this disorder. We report an 18-month-old girl, who presented with a long-standing hemolytic anemia. Her diagnosis of unstable Hb Perth [β32(B14)Leu→Pro, HBB: c.98T > C] had not been made until gene sequencing of the β-globin gene was performed.

Two missense mutations in the beta-globin gene can cause severe beta thalassemia. Hemoglobin Medicine Lake (beta 32[B14]leucine-->glutamine; 98 [FG5] valine-->methionine)

We studied the molecular basis of transfusion-dependent hemolytic anemia in an infant who rapidly developed the phenotype of beta thalassemia major. DNA sequence of one beta-globin gene of the proband revealed two mutations, one for the moderately unstable hemoglobin (Hb) Köln and another for a novel codon 32 cytosine-thymidine-guanine-->cytosine-adenine-guanine transversion encoding a leucine-->glutamine mutation. A hydrophilic glutamine residue at beta 32 has an uncharged polar side chain that could potentially distort the B helix and provoke further molecular instability. This new hemoglobin was called Hb Medicine Lake. Biosynthesis studies showed a deficit of beta-globin synthesis with early loss of beta-globin chains. An abnormal unstable hemoglobin, globin chain, or tryptic globin peptide was not present, demonstrating the extreme lability of this novel globin. Hb Medicine Lake mRNA was present, but an aberrantly spliced message was not. Absence of an abnormal beta-globin gene in the mother makes it likely that a de novo mutation occurred in the proband. The molecular pathogenesis of Hb Medicine Lake illustrates a mechanism whereby the phenotype of a genetic disorder, like the mild hemolytic anemia associated with a hemoglobinopathy, can be modulated by a coincident mutation in the same gene.

Rapid molecular characterization of mutations leading to unstable hemoglobin beta-chain variants

Characterization of unstable hemoglobins by protein analysis is often difficult. However, it is facilitated by DNA analysis, especially in the case of hyperunstable beta-chain variants, which produce a beta-thalassemia phenotype. We have applied an efficient strategy to the detection of such variants at the DNA level, based on computer-designed denaturing gradient gel electrophoresis (DGGE) of amplified DNA fragments. This approach makes it possible to detect any anomaly in the beta-globin gene. We describe the use of the DGGE method for rapid characterization of beta-chain variants and report a new missense mutation in the beta-globin gene third exon, beta 127 CAG-CGG/Gln-Arg, which is responsible for the synthesis of a highly unstable hemoglobin.

A new variant, HB Muscat [alpha 2 beta (2)32(B14)Leu----Val] observed in association with HB S in an Arabian family

The silent Hb Muscat with a Leu----Val replacement at position beta 32 was discovered by reversed phase high performance liquid chromatography in two members of an Arabian family from Oman; in one person Hb Muscat occurred with Hb S and in the other with Hb A. Hb Muscat is slightly unstable but its presence has no apparent adverse effect on the health of its carriers. Additional hemoglobin abnormalities observed in this family were a common alpha-thalassemia-2 (-3.7 kb) and Hb S. The beta S haplotypes in the heterozygous carriers and the two sickle cell anemia patients were #19 (Benin) and #20 (Bantu); the latter likely originated from an East African population.

Thalassemic hemoglobinopathies

Hemoglobinopathies are due to changes in the normal amino acid sequence of globin. Thalassemias result from imbalance in the normal coordinated synthesis of the globin subunits that make up the hemoglobin tetramer. It is now apparent that a single globin gene can have coding region mutations which simultaneously produce a structural defect (hemoglobinopathy) and a biosynthetic defect (thalassemia). It is likely that two distinct mutations within the same gene can occur and produce a hemoglobinopathy with features of thalassemia. In this review the authors discuss such disorders and include the Hb Lepore and Constant Spring variants, hyper-unstable globins, mutations which create alternative sites for mRNA splicing, and amino acid substitutions likely to be associated with an additional thalassemia lesion within the same gene.

Unstable haemoglobin haemolytic crises: contributions of pyrexia and neutrophil oxidants

Pyrexia and the production of oxidants by phagocytic cells have been examined as two possible causes of haemolytic crises associated with infections in carriers of unstable haemoglobins. Three unstable haemoglobins were examined, both in red cells and after purification. Incubation at 40 degrees C rather than 37 degrees C resulted in only a slight increase in autoxidation rate, but a considerable increase in the rate of precipitation of the haemoglobins as Heinz bodies. Oxidants produced by activated neutrophils were capable of oxidizing haemoglobin both in solution and red cells, though there was no preferential effect on the unstable as opposed to the normal haemoglobin. It is concluded that haemolytic crises associated with infections in carriers of unstable haemoglobins can be explained by increased intracellular precipitation of the haemoglobin as Heinz bodies caused by the accompanying pyrexia.

Hemoglobin Milledgeville (alpha 44 (CD2) Pro leads to Leu): a new variant with increased oxygen affinity

Hemoglobin Milledgeville, a new hemoglobin structural variant, was identified in three members of a black American family. The oxygen affinity of blood and hemoglobin samples from the affected individuals was markedly increased (p50 O2 of whole blood 11-15 mmHg at 37 degrees C, pH 7.4), and the abnormality was associated with mild erythrocytosis. The variant hemoglobin did not separate from Hb A by electrophoresis or by chromatography or isoelectric focusing, and efforts to isolate an abnormal globin chain were also unsuccessful. The Hb A2 fraction as well as Hb A from erythrocytes of affected individuals exhibited increased oxygen affinity, indicating that the altered oxygen equilibrium was the result of a hemoglobin alpha chain abnormality. Fractionation of trypsin and chymotrypsin digests of isolated alpha chains demonstrated a single abnormal peptide representing a Pro leads to Leu substitution at alpha 44 (CD2). Properties of Hb Milledgeville include low cooperativity (n = 1.1-1.4), a normal alkaline Bohr effect (delta logp50/delta pH = -0.62), and normal interaction with 2,3-diphosphoglycerate. The alpha CD2 proline residue normally participates in the formation of the alpha 1 beta 2 subunit interface in the deoxy quaternary conformation, but not in oxyhemoglobin; the leucine substitution may produce destabilization of the deoxy conformation with a resulting shift in equilibrium toward the oxy conformation.

Unstable hemoglobins, hemoglobins with altered oxygen affinity, and m-hemoglobins

Most patients with chronic Heinz body anemia do not require treatment. Dietary folic acid supplementation is recommended when hemolysis is chronic and severe. During infection, patients should be observed carefully because of the possibility of aplastic or hemolytic crises. Individuals with hemoglobins with altered oxygen affinity or M-hemoglobins do not require treatment, and should be counseled about the benign nature of their condition. Unnecessary procedures to exclude cardiac or pulmonary disease should be avoided.

Thalassemia intermedia caused by heterozygosity for both beta-thalassemia and hemoglobin Saki [beta 14 (A11) Leu replaced by Pro]

The syndrome thalassemia intermedia can be the clinical expression of heterozygosity for different tyes of thalassemia, beta-thalassemia and hereditary persistence of fetal hemoglobin, beta-thalassemia and Hb-Lepore, and in blacks it may even represent a true beta-thalassemia homozygote. This report describes thalassemia intermedia in a white male due to beta-thalassemia and an unstable hemoglobin. Chain-synthesis studies showed an excess of alpha-chain production over beta-chain production in the propositus and his mother but balanced chain synthesis in the clinically normal father, who is heterozygous for the unstable hemoglobin. The unstable hemoglobin was found to be beta14 (A11) Leu leads to Pro, which has previously been described in a clinically normal African woman, and named Hb-Saki. This hemoglobin is not distinguishable from Hb-A on routine electrophoresis at alkaline or acid pH and tests for unstable hemoglobins are necessary for its detection. The increasing list of such hemoglobin variants and previous cases of heterozygosity for beta-thalassemia and unstable hemoglobins are reviewed.

Studies of hemoglobin denaturation and Heinz body formation in the unstable hemoglobins

The sequential changes that occur during the precipitation on mild heating of the unstable hemoglobins, Hb Christchurch, Hb Sydney, Hb Köln, and Hb A, were examined with particular attention to the possibility of an accompanying oxidative process. Hb Christchurch, Hb Sydney, and Hb A precipitated with equal amounts of alpha- and beta-chains and full heme complement. Hb Köln, however, was one-half hemedepleted and showed a slight excess of precipitated beta-chains. In all cases the spectrum of the precipitated material was typical of a hemichrome. There was no evidence that sulfhydryl oxidation contributed to the precipitation process. Reduced glutathione was unable to protect the hemoglobin against precipitation, and mixed disulfide formation between the precipitating hemoglobin and glutathione was insignificant, even in the presence of excess glutathione. No blockade of beta93 cysteines could be demonstrated in the unstable hemoglobins. Precipitation of oxyhemoglobin and carboxyhemoglobin in all cases gave nonspecific oxidation of approximately two of the six hemoglobin sulfhydryl groups to give intra- and intermolecular disulfide bonds. Single alpha- and beta-chains, plus polymers of up to five or six chains linked by disulfide bridges, were demonstrated by polyacrylamide gel electrophoresis. This disulfide oxidation was not observed with deoxy- or methemoglobin and did not appear to influence the rate of precipitation. These findings fit the theoretical prediction that autoxidation of oxy- and carboxyhemoglobin is accompanied by formation of a free radical, with the reactions of this free radical being confined intramolecularly.Together, these results are in keeping with predictions based on the known structural abnormalities of the unstable hemoglobins, all of which result in greater molecular flexibility. Our findings support the conclusion that the usual precipitating event is altered bonding at the heme to give the formation of hemichromes. There is no evidence of an accompanying oxidative process that could pose a threat to the integrity of the red cell.