Junius G. Adams

Alpha thalassaemia in adults with sickle-cell trait.

Summary. Mild forms of α thalassaemia are difficult to detect in adults. Since α thalassaemia existing with structural defects of the β chain of haemoglobin may lead to decreased levels of the abnormal haemoglobin, we examined individuals having sickle‐cell trait for the possible coexistence of α thalassaemia. Patients with sickle‐cell trait having haemoglobin‐S (Hb‐S) levels less than commonly expected were compared to two control groups—one with sickle‐cell trait and the usual levels of Hb S and one with normal haemoglobin. Twenty‐one patients with sickle‐cell trait having Hb‐S concentrations below 35% had 65.8% Hb A, 31.8% Hb S, and a mean corpuscular volume of 81.6 fl. Studies of the relative rates of α‐, βs‐ and βA‐chain synthesis in 14 of these individuals showed a mean α:β ratio of 0.76. In patients with normal haemoglobin as well as in sickle‐cell trait with Hb‐S levels above 35%, this ratio was unity. These findings are consistent with the presence of α thalassaemia in patients with sickle‐cell trait who have lower than usual levels of Hb S and micro‐cytosis.

The effects of alpha-thalassaemia in HbSC disease

Summary. In HbSC disease, as in sickle cell anaemia, there is a spectrum of clinical severity. A reduced mean corpuscular volume and haemoglobin concentration, traits typical of thalassaemia, might retard sickling. We therefore ascertained the prevalence of α‐thalassaemia in 53 adults with HbSC disease and related α‐globin gene deletion to the haematologic and clinical findings. Alpha‐globin genotype was identified by restriction endonuclease gene mapping. Indirect ophthalmoscopy and fluorescein angiography were used to document the presence of proliferative retinopathy. Bone necrosis and infarction were determined roentgenographically or by radionuclide scanning. Either heterozygous or homozygous alpha‐thalassaemia‐2 was present in 26% of patients. There was no relationship between α‐globin genotype and haematocrit, pain crises, bone lesions, proliferation retinopathy or clinical severity score.

HB Mississippi [β44(CD3)SeråG]: A New Variant with Anomalous Properties

Hb Mississippi was discovered in a 6-year-old Chinese girl with chronic anemia and thalassemia intermedia. Family studies revealed that she had inherited the Hb Mississippi from her father as well as inheriting a gene for β+-thalassemia from her mother. Electrophoretic analyses of the hemolysate of the father of the father and the proband on polyacrylamide gels at pH 8.6 showed that the abnormal hemoglobin had three distinct mobilities. A similar pattern was also observed by isoelectric focusing. In addition, multiple abnormal peaks were observed by high performance liquid chromatographic hemoglobin separations as well as high performance liquid chromatographic globin chain separation. Structural analysis of the abnormal hemoglobin demonstrated a single abnormality; the substitution of serine to cysteine at position 44 (CD3) of the β-globin chain. Since CD3 is on the surface of the β-globin chain, it was thought that polymerization of the abnormal hemoglobin by disulfide linkages might have been responsib...

Interaction Between HBS-β°-Tha lassemia and α-Thalassemia

We have defined the clinical and laboratory characteristics of a group of patients with HbS-βo+α-thalassemia plus α-thalassemia, by analysis of erythrocyte indices, hemoglobin A2 and F levels, globin biosynthesis studies and α-globin gene mapping. Patients with HbS-βo + α-thalassemia closely resembled individuals with HbS-βo-thalassemia except for balanced globin synthesis ratios and a lower HbF level. The frequency of painful crises, leg ulceration, aseptic necrosis of bone and acute chest syndrome was similar in HbS-βo + α-thalassemia patients and controls with sickle cell anemia (HbSS), HbSS-α-thalassemia and HbS-βo-thalassemia. These findings are consistent with previous work which failed to show a reduction in the vaso-occlusive severity of sickle cell disease by the coexistence of α-thalassemia.

β-Thalassemia Intermedia with Exceptionally High Hemoglobin A2: Relationship to Mutations in the β-Gene Promoter

Small deletions of the 5′ portion of the β-globin gene that remove the promoters but stop 3′ to the δ-globin gene are recognized as the sole cause of β-thalassemia with exceptionally high hemoglobin A2 (HbA2) levels. Two patients with β-thalassemia intermedia and exceptionally high levels of HbA2 (10.4 and 12.0%) were examined. One patient was a combined heterozygote for the − 88 C→T and a novel − 87 C→A mutation, while the other was homozygous for the − 29 A→G β+-thalassemia mutation. The remainder of the β genes were normal. There was no evidence for deletions involving the 5′ portion of the β gene or the region between the β and δ genes. Gene mapping studies excluded the possibility of a βδ-anti-Lepore hemoglobin gene with β promoters and δ coding sequences. There were no mutations in the promoters of the Gγ or Aγ- globin genes that have been associated with the hereditary persistence of HbF phenotype. The δ-globin gene promoters were normal from codon 17 to position − 145 relative to the mRNA capping site. There appears to be considerable heterogeneity of HbA2 and HbF levels in patients who are homozygous or mixed heterozygotes for mutations in the TATA box and other promoter elements of the β-globin gene. The capacity for proteolysis within the erythrocyte may vary among individuals. The authors hypothesize that in the exceptionally high HbA2 β-thalassemia intermedia phenotype, proteolysis of superfluous α-globin chains is less efficient than in patients with customary levels of HbA2. The relative surfeit of α-chains may combine with δ-globin and account for the unusually high HbA2 levels.

Globin Biosynthesis in Erythroid Bursts of Heterozygous α or β Thalassaemia

Summary. We examined globin chain synthesis in erythroid bursts (BFU‐E) of patients with heterozygous α or β thalassaemia. BFU‐E were cloned from circulating mononuclear cells, labelled with [H]leucine and globin chains purified by gel filtration and column chromatography. In six patients heterozygous for β thalassaemia, globin synthesis in BFU‐E was nearly balanced, with an α/non α ratio of 1.05 0α12. These BFU‐E produced 33.8 12.7%γ globin chain, an amount similar (P >1 0.05) to that found in 10 controls with sickle cell anaemia (25.6 6.7) but greater (P <0.05) than that of five normal controls (17.2 2.2). The balanced globin synthesis appeared due to the large amounts of γ chain made by BFU‐E. In two α thalassaemia carriers, who also had sickle cell trait, the BFU‐E α/non‐α ratio was 0.67 and 0.79. These BFU‐E produced 15% and 20%γ chain and 39% and 45%βS globin. The synthesis of βS globin in BFU‐E exceeded the erythrocyte levels of 20% and 29% HbS and indicated nearly equal expression of βA and βB globin genes in these proliferating erythroid precursors. This provides further evidence that the low levels of HbS in sickle cell carriers with α thalassaemia are due to post‐translational events resulting from the differing affinity of βS and βA globin for α chain and the destruction of excessive βS chain.

Effect of Lead and Ethanol upon γ-Globin Synthesis in Sickle Reticulocytes

There is evidence from both in vivo and in vitro studies that the synthesis of hemoglobin can be modified by post-translational alterations in the assembly of the tetrameric molecule. Globin biosynthesis in reticulocytes of patients with sickle cell disease was studied to ascertain the effects of lead and ethanol on γ-globin chain synthesis and hemoglobin assembly. In incubations containing lead (400 μg/dl) or ethanol (1.0 M) there were 86.7 ± 139.7% and 542.7 ± 397.0% increases in the relative synthesis of the γ-globin chain. This was associated with a relative reduction in α-chain synthesis, as estimated by changes in the α/γ + βs synthesis ratio, as well as a marked reduction in total globin synthesis.

Laboratory Identification, Screening, Education, and Counseling for Abnormal Hemoglobins and Thalassemias

With presently available laboratory methodology, virtually all of the clinically significant hemoglobin disorders as well as their heterozygous carrier states can be identified accurately, rapidly, and by relatively inexpensive means. In addition, most of these conditions can now be reliably detected in the newborn and in the fetus in utero. By the application of these diagnostic techniques, affected carriers can be identified and provided accurate information about their genetic risks, and if desired antenatal diagnosis may also be carried out to determine the hemoglobin genotype of a potentially affected fetus. The development of centers that offer these types of genetic services has progressed rapidly since the early 1970’s, and these programs are continuing to assume increasing importance in many areas of the world. As an introduction to a discussion of these issues, the following section briefly reviews the most commonly employed laboratory methods for the identification of these disorders and their carrier states, as well as the rationale for their application.

The Globin Gene Mutations

The known human globin gene mutations now total more than 400, representing a considerably larger number than for any other human protein system. By far the most numerous of these mutations are those resulting from replacements of single nucleotide bases, and most of these mutant genes are expressed by the synthesis of structurally abnormal globin chains having substitutions of one of their amino acid residues. The substantial number of hemoglobin variants that were well characterized as long as 20 years aga permitted the specific base changes of many of the globin gene mutations to be inferred even before the genetic code was fully known (Smith, 1962). After the complete triplet code was determined it could be shown that virtually all of the amino acid substitutions in these mutant globins could be explained by single base changes in affected codons. Although only few of the genes for the abnormal globins have thus far been sequenced, those that have been analyzed have fully corroborated the mutation codon assignments that were predicted from the genetic code. In many of the other forms of globin structural abnormalities, which include deletions and insertions of amino acid residues, the formation of abnormally shortened or elongated globin chains, and frameshift mutations, the underlying mutations could also be predicted reliably from known codon relationships in the genetic code. It was therefore possible to develop a comprehensive classification of the mutations underlying most of the structural globin variants well before the time that methods for the analysis of globin gene structure became available.

The Geographic Distribution of Globin Gene Variation

With relatively few exeeptions, the globin gene mutations that are tabulated in the Appendix occur only rarely, with a majority of these mutations having been found in only one or a very limited number of individuals or families. Some of the other globin gene mutations have been shown to occur with relatively high frequency, but only in individuals within highly isolated populations. On the other hand, the group that includes Hb S, Hb C, Hb E, and the thalassemias includes enormous numbers of people and extends over a very wide geographical area. (For a more detailed account see Bowman, 1983, and Winter, 1985).