Hematologic and biosynthetic studies in homozygous hemoglobin Constant Spring

Coinherited Hemoglobin H/Constant Spring Disease and Heterozygous Hemoglobin Tak Causing Severe Hemolytic Anemia in a Thai Boy

Hemoglobin (Hb) H/Constant Spring disease is a common nondeletional Hb H disease, typically causing a more severe phenotype than the deletional Hb H disease counterpart. Hb Tak, resulting from a dinucleotide insertion (+AC) at codon 146 of beta-globin gene, has an increased oxygen affinity and usually presents with polycythemia. We studied a case of a 4-year-old Thai boy with a severe, early-onset anemia. To our knowledge, he is the first reported patient with Hb H/Constant Spring disease and heterozygous Hb Tak. Trio-whole-exome sequencing does not identify other genetic variants that may contribute to the severity of anemia. The observation suggests that coinherited Hb H/Constant Spring and heterozygous Hb Tak lead to severe hemolytic anemia.

Fetal Anemia and Hydrops Fetalis Associated with Homozygous Hb Constant Spring (HBA2: c.427T > C)

Hb Constant Spring (Hb CS, HBA2: c.427T > C) is a common nondeletional α-thalassemia (α-thal) that results from a nucleotide substitution at the termination codon of the α2-globin gene. Homozygosity for Hb CS (α(CS)α/α(CS)α) is relatively rare, and generally characterized with mild hemolytic anemia, jaundice, and splenomegaly. In this report we present a fetus with cardiomegaly, pericardial effusion, enlarged placenta and increased middle cerebral artery-peak systolic velocity (MCA-PSV) at 24 weeks' gestation. Fetal blood sampling revealed the severe anemia [hemoglobin (Hb) level being 4.8 g/dL] and Hb H (β4) disease-like hematological findings with Hb Bart's (γ4) level of 17.9%. DNA sequencing of the α-globin genes found that both partners were Hb CS carriers and the fetus was an Hb CS homozygote. Therefore, this was a rare case of homozygous Hb CS which demonstrated an unusual and serious anemia and hydrops fetalis in utero.

Acute haemolytic crisis in a Thai patient with homozygous haemoglobin Constant Spring (Hb CS/CS): a case report

Acute haemolysis associated with mild upper respiratory tract infection was observed in a Thai boy who presented with a rapid decline in haemoglobin (Hb) levels, haemoglobinuria and evidence of intravascular haemolysis. Several possible causes giving rise to such a condition were excluded including G6PD deficiency, which is extremely common in Thailand. Subsequent haematological and molecular analyses demonstrated that the patient was homozygous for Hb Constant Spring (Hb CS/CS), an a globin haemoglobinopathy. It has been shown previously that patients with homozygous Hb CS had mild haemolytic anaemia secondary to an accumulation of alpha(CS) chains, which are toxic to red blood cell membrane cytoskeletons. Increased body temperature might induce more precipitation of this a globin variant. This report highlights the importance of Hb CS/CS as a potential predisposing cause of acute haemolysis in children that might be aggravated by acute bacterial or viral infections. This is particularly relevant for patients of Southeast Asian descent where this abnormal haemoglobin is highly prevalent.

Hemoglobin variants: biochemical properties and clinical correlates

Diseases affecting hemoglobin synthesis and function are extremely common worldwide. More than 1000 naturally occurring human hemoglobin variants with single amino acid substitutions throughout the molecule have been discovered, mainly through their clinical and/or laboratory manifestations. These variants alter hemoglobin structure and biochemical properties with physiological effects ranging from insignificant to severe. Studies of these mutations in patients and in the laboratory have produced a wealth of information on hemoglobin biochemistry and biology with significant implications for hematology practice. More generally, landmark studies of hemoglobin performed over the past 60 years have established important paradigms for the disciplines of structural biology, genetics, biochemistry, and medicine. Here we review the major classes of hemoglobin variants, emphasizing general concepts and illustrative examples.

The thalassemias and related disorders

The thalassemias, sickle cell disease, and other hemoglobinopathies represent a major group of inherited disorders of hemoglobin synthesis. The abnormal hemoglobins were reviewed in the July 2006 issue of Baylor University Medical Center Proceedings. Because of immigration patterns and population flow, these disorders are becoming increasingly more prevalent in the USA. In this article, the clinical aspects of the more common thalassemia syndromes are reviewed. For most symptomatic patients with thalassemia, there is no definite cure; only supportive management of the anemia is possible. A very limited number of patients with thalassemia may be cured by bone marrow transplantation from HLA-identical donors. Other tentative approaches to management include stimulation of fetal hemoglobin synthesis and attempts at somatic cell gene therapy. Prevention of disease transmission by carrier screening programs along with prenatal diagnosis remain of paramount importance in the reduction of these diseases worldwide.

Anemia and hydrops in a fetus with homozygous hemoglobin constant spring

Hemoglobin Constant Spring (Hb CS) is an unstable hemoglobin (Hb) variant that results from a nucleotide substitution at the termination codon of the alpha2-globin gene. The compound heterozygosity of alpha-thalassemia and Hb CS (--/alphaCSalpha) results in a Hb H/CS disease which is clinically more severe than deletional Hb H disease. Homozygosity of Hb CS (alphaCSalpha/alphaCSalpha) is generally characterized with mild hemolytic anemia, jaundice, and splenomegaly. The authors report 1 case with Hb CS homozygosity who presented with fetal anemia and hydrops. Intrauterine transfusions were given which rendered a favorable outcome. This report demonstrates an unusual and serious in utero complication in a fetus with Hb CS/CS.

The usefulness of glycosylated hemoglobin (HbA1C) in discriminating between iron deficiency and thalassemia

This study assessed whether glycosylated hemoglobin could be used as an index to distinguish between iron-deficiency anemia and thalassemia minor. Glycosylated hemoglobin was measured by high-pressure liquid chromatography in 40 ss -thalassemia minor patients, 20 iron-deficiency anemia patients, and 38 healthy controls, all nondiabetic. Median glycosylated hemoglobin was lower in ss -thalassemia minor than in the iron-deficiency and control groups (p = .000). There was no difference between iron-deficiency patients and healthy controls (p = .095). Glycosylated hemoglobin was not different in iron-replete and iron-deficient traits (p > .05). A cutoff value of 5% has provided a sensitivity of 95% and specificity of 75.7% for distinguishing between these two entities. Positive and negative predictive value were 96.6 and 67.9%. These values were superior to the traditional discriminants' values calculated on the same individuals. Glycosylated hemoglobin could be useful in discriminating between iron-deficiency anemia and thalassemia minor. Further studies are needed, but the point that it can also be used when both conditions exist simultaneously seems to be clinically important.

Pathophysiology of thalassaemia

Most of the major clinical manifestations of the beta-thalassaemias can be related to the deleterious effects of imbalanced globin chain synthesis on erythroid maturation and red cell survival. The destruction of red cell progenitors and their progeny results from an extremely complex series of mechanisms all related to the presence of excess alpha-globin chain production. These include mechanical damage, interference with cell division and oxidative destruction of both organelles and components of the red cell membrane. The unequal distribution of gamma-globin chains between different precursors, and the intense selection of those with relatively higher levels of gamma chain production, lead to an extremely heterogeneous cell population in the peripheral blood. Iron overload, due to increased gastrointestinal absorption and blood transfusion is the major cause of tissue damage, morbidity and death.

The alpha / beta and alpha 2 / alpha 1-globin mRNA ratios in different forms of alpha-thalassemia

The present study provides information about the alpha / beta and alpha 2 / alpha 1-mRNA ratios in reticulocytes of normal adults and individuals with different alpha-globin gene deficiencies; it found its origin in analytical data of blood samples from a Laotian couple and their newborn baby. The father carried the 4.2 kb deletion on one chromosome and a TAA --> CAA mutation at the terminating codon of the alpha 2 gene (Hb Constant Spring or CS) on the other chromosome. The mother had the 3.7 kb deletion on one chromosome and a TA A --> TAT mutation at the terminating codon of the alpha 2-globin gene (Hb Paksé) of the second chromosome. The baby was a compound heterozygote for the two termination codon mutations. The mRNA data for this family were compared to those for persons with several well-defined alpha-globin gene deficiencies. The results confirm the importance of the alpha 2 alpha 1-mRNA for the synthesis of alpha chains in alpha-thalassemia-2 homozygotes (-alpha/-alpha) and in patients with Hb H disease due to the deletion of three alpha-globin genes (-alpha/--). Furthermore, the MRNA production of the alpha 1-globin gene on the chromosome with the alpha CS mutation (alpha CS alpha) is only one-half of that by the alpha 2 alpha 1-globin gene of a chromosome with a 3.7 or 4.2 kb deletion, explaining the greater severity of, and higher Hb H level in Hb H patients with the alpha CS alpha condition (alpha CS alpha/--) as compared to those with the three gene deletion (-alpha/--). The methodology could be useful as a preliminary screening for the presence of point mutations leading to the functional loss of a single alpha-globin gene, provided common deletional alleles have been excluded.

Human gene mutations affecting RNA processing and translation

Gene mutations affecting mRNA processing and translation are not common causes of human genetic disease. Their analysis has nevertheless provided important insights into the basic biochemical mechanisms underlying mRNA transcription and translation.

A new alpha chain variant Hb Tonosho [alpha 110(G17)Ala----Thr]: subunit dissociation during cation exchange chromatography for Hb A1c assay

A new alpha chain variant, alpha 110(G17)Ala----Thr, was detected because of subunit dissociation during the determination of the Hb A1c by automated cation exchange high performance liquid chromatography. The abnormal hemoglobin overlapped the cathodic edge of the band of Hb A in isoelectrofocusing. It was slightly unstable in the isopropanol test and had a slightly increased oxygen affinity. The abnormal alpha chain eluted slightly faster than the normal alpha chain in reversed phase high performance liquid chromatography. The amino acid substitution was determined by purification of S-alkylated alpha T-12,13 tryptic peptide, chymotryptic digestion, and sequencing of an octapeptide alpha 110-117. The abnormal alpha chain comprised about 14% of the total alpha chain. A biosynthetic study did not suggest selective loss of the abnormal chain in reticulocytes.

Frameshift mutation near the 3' end of the COL1A1 gene of type I collagen predicts an elongated Pro alpha 1(I) chain and results in osteogenesis imperfecta type I

Osteogenesis imperfecta (OI) is a heterogeneous disorder of type I collagen of which OI type I, an autosomal dominant condition, is the mildest and most common form. Affected individuals have blue sclerae, normal stature, bone fragility without significant deformity and osteopenia. Fibroblasts from most affected individuals produce about half the expected amount of structurally normal type I collagen as a result of decreased synthesis of one of its constituent chains, pro alpha 1(I), but the nature of the mutations which result in OI type I are unknown. We describe a three generation family with OI type I in which all affected members have one normal COL1A1 allele and another from which the intragenic Eco RI restriction site near the 3' end of the gene is missing. Amplification by polymerase chain reaction and sequence determination of the normal allele and of the mutant allele in the domain that normally contains the Eco RI site demonstrated a 5-bp deletion from the mutant allele. The deletion changes the translational reading-frame beginning at the Eco RI site and predicts the synthesis of a pro alpha 1(I) chain that extends 84 amino acids beyond the normal termination. Although the mutant pro alpha 1(I) chain is synthesized in an in vitro translation system, we are unable to detect its presence in intact cells, suggesting that it is unstable and rapidly destroyed in one of the cell's degradative pathways. Our analysis of individuals with OI type I from 20 families indicates that this is a unique mutation and suggests that the phenotype can result from multiple mechanisms that decrease the synthesis of normal type I procollagen molecules, including those that alter protein stability.

Molecular screening for haemoglobin constant spring

Haemoglobin H/Constant Spring is an important cause of severe haemoglobin H disease, but the Constant Spring protein is difficult to detect by electrophoresis. A technique for allele specific polymerase chain amplification of the 3'-end of the alpha 2 globin gene improved detection of the alpha cs alpha haemoglobin variant in DNA samples by slot-blot hybridisation. The alpha cs alpha mutation was confirmed in subjects that had been previously diagnosed by haemoglobin electrophoresis, and it was also detected in patients who were negative by protein electrophoresis. 10 of 103 unrelated Laotians with HbE were alpha cs alpha heterozygotes. Of these, 3 were negative to the normal probe because they had -alpha 3.7/alpha cs alpha with a single alpha globin deletion. 5 samples did not amplify or hybridise to either probe because they had deletions of both alpha 2 globin regions. The gene frequency for alpha cs alpha is about 0.05 for Laotians. This technique, which is highly specific and sensitive for rapid detection of the alpha cs alpha mutation, is suitable for clinical diagnoses and population studies. The true incidence of alpha cs alpha may prove to be greater than previously suspected from protein electrophoresis.

Decreased sensitivity of artesunate and chloroquine of Plasmodium falciparum infecting hemoglobin H and/or hemoglobin constant spring erythrocytes

Plasmodium falciparum infecting hemoglobin (Hb) H and/or Hb Constant Spring erythrocytes in vitro was relatively more resistant than that infecting normal erythrocytes to artesunate and chloroquine, while the sensitivity to pyrimethamine was unchanged. The 50% inhibitory concentrations (IC50) for artesunate in HbH (alpha-thal 1/alpha-thal 2), HbH (alpha-thal 1/Hb Constant Spring), and homozygous Hb Constant Spring erythrocytes were 4.5 +/- 2.8, 8.5 +/- 3.2, and 2.6 +/- 1.6 nM compared with 0.82 +/- 0.35 nM in normal erythrocytes (P less than 0.002 for all three cases). The IC50 for chloroquine were 97 +/- 46, 162 +/- 67, and 93 +/- 36 nM, respectively, in the variant erythrocytes, compared with 48 +/- 13 nM in normal erythrocytes (P less than 0.002, 0.002, and 0.02, respectively). The differences in sensitivity to artesunate and chloroquine of the parasite infecting HbH erythrocytes are probably related to their oxidative mode of action and relatively high amounts of antioxidant enzymes in the host erythrocytes. This novel example of dependence on the host of the malarial parasite drug sensitivity may have implications for chemotherapy of malaria in patients with genetically variant erythrocytes.

The molecular basis of AE-Bart's disease

AE-Bart's disease is a thalassemia intermedia resulting from the interaction between alpha-thalassemia and heterozygous Hb E. In this study we analyzed the alpha-globin genes of 25 patients designated as AE-Bart's disease by starch gel electrophoresis. Twenty-one cases had Hb Constant Spring in addition to Hbs E + A + Bart's, and the remaining four cases had only Hbs E + A + Bart's. DNA mapping revealed the alpha-globin genotype of alpha-thalassemia-1/alpha-thalassemia-2 in four patients who had Hbs E + A + Bart's, whereas the alpha genotype of the remainder is alpha-thalassemia-1/nondeletion alpha-thalassemia. The nondeletion alpha-thalassemia is Hb Constant Spring as indicated by starch gel electrophoresis. Hematologic data and hemoglobin analysis showed that Constant Spring-AE-Bart's disease is a more severe clinical syndrome than AE-Bart's disease.

Alpha-thalassemia in Thailand

The alpha-thalassemia syndromes are remarkable both for their phenotypic diversity and for their different clinical severity. They are associated with variable degrees of alpha-chain deficits; the clinical manifestations range from asymptomatic cases with normal hematologic findings to the totally lethal Hb Bart's hydrops fetalis syndrome. Recent molecular biology studies have clarified the defects in these alpha-thalassemia syndromes around the world. This paper describes the alpha-thalassemias in Thailand, and covers the types, molecular defects, incidence of each genotype, and their phenotypic expression.

Different hematologic phenotypes are associated with the leftward (-alpha 4.2) and rightward (-alpha 3.7) alpha+-thalassemia deletions

We have compared the phenotypes of the two common deletion forms of alpha+-thalassemia by analysis of umbilical cord blood samples from Melanesia. Homozygotes for the leftward, 4.2-kilobase, deletion (-alpha 4.2) had significantly higher levels of Hb Bart's at birth than homozygotes for the rightward, 3.7-kilobase, deletion (-alpha 3.7). Compound heterozygotes for each deletion had intermediate values. Although deletion forms of alpha 0 thalassemia were not found in this survey, nondeletion alpha-thalassemia was present at low frequency. Since the predominant rightward deletion in this population, -alpha 3.7III, entirely removes the alpha 1-gene and the 4.2-kilobase deletion deletes the alpha 2-gene, these data indicate that the alpha 2-globin gene has a higher output than the alpha 1-gene, on single alpha-gene chromosomes.

Interaction of five globin gene abnormalities in a Cambodian family

Members of a Cambodian family with an undiagnosed hypochromic, microcytic anaemia were found by haemoglobin and DNA analysis to have five interacting globin gene abnormalities. One child has Hb E and typical Hb H disease, while his mother has the form of Hb H disease associated with Hb Constant Spring interacting with Hb E. Quantitation of Hbs E and A2 by globin chain separation and triton/urea gel electrophoresis support the concept that Hb H/Constant Spring disease is a more severe form of alpha thalassaemia than Hb H disease. This family illustrates how the remarkably high prevalence of globin gene abnormalities in Southeast Asians can give rise to a series of atypical thalassaemic phenotypes, and how they can be defined by direct globin gene analysis.

Red cell volume distribution curves and intracellular globin chain precipitation in the alpha-thalassaemic mouse, Hbath-J

Red cell volume distribution curves were studied in alpha-thalassaemic mice (Hbath-J/+ mice) and normal mice (+/+ mice) of various ages. Individual Hbath-J/+ mice could not be reliably distinguished from their +/+ littermates on the basis of modal cell volume either at birth or during the first 3 weeks of life. However, between the ages of 4 and 30 weeks Hbath-J/+ mice displayed a degree of microcytosis that enabled them to be readily distinguished from their normal littermates using the criterion of modal red cell volume. Preliminary studies of alpha:beta globin chain synthesis ratios given by blood reticulocytes of Hbath-J/+ and +/+ mice after incubation with 3H-leucine for 5 min and 2 h suggest that there is little or no proteolysis of excess beta-chains in the alpha-thalassaemic mouse. Electron microscope studies revealed that the erythroblasts, marrow reticulocytes and circulating red cells of Hbath-J/+ but not +/+ mice contain stellate and branching intracytoplasmic inclusions, presumed to consist of precipitated beta-chains. These inclusions were ultrastructurally similar to the inclusions which have been previously reported in the erythroblasts and marrow reticulocytes of people with various alpha-thalassaemia syndromes. The proportion of erythropoietic cell profiles with inclusions was higher in Hbath-J/+ mice (in which two of the four alpha-globin genes are deleted) than in Thai patients with HbH disease (in whom there is usually a deletion of three of the four alpha-globin genes); this finding is probably related to a relatively low proteolytic capacity in the more mature mouse erythroid cells when compared with human cells. The presence of inclusion-containing red cells (mainly reticulocytes) in the peripheral blood of unsplenectomized Hbath-J/+ animals contrasts with the absence of such cells in unsplenectomized patients with alpha-thalassaemia I trait and HbH disease; this difference seems to be at least partly due to a poorly-developed pitting function in the mouse spleen.

Compensatory increase in alpha 1-globin gene expression in individuals heterozygous for the alpha-thalassemia-2 deletion

alpha-Globin is encoded by the two adjacent genes, alpha 1 and alpha 2. Although it is clearly established that both alpha-globin genes are expressed, their relative contributions to alpha-globin messenger RNA (mRNA) and protein synthesis are not fully defined. Furthermore, changes that may occur in alpha-globin gene activity secondarily to the loss of function of one or more of these genes (alpha-thalassemia [Thal]) have not been directly investigated. This study further defines the expression of the two human alpha-globin genes by determining the relative levels of alpha 1 and alpha 2 mRNA in the reticulocytes of normal individuals and in individuals heterozygous for the common 3.7-kilobase deletion within the alpha-globin gene cluster that removes the alpha 2-globin gene (the rightward type alpha-Thal-2 deletion). To quantitate accurately the ratio of the two alpha-globin mRNAs, we have modified a previously reported S1 nuclease assay to include the use of 32P end-labeled probes isolated from alpha 1- and alpha 2-globin complementary DNA recombinant plasmids. In individuals with a normal alpha-globin genotype (as determined by Southern blot analysis [alpha alpha/alpha alpha]), alpha 2-globin mRNA is present at an average 2.8-fold excess to alpha 1. In individuals heterozygous for the rightward type alpha-Thal-2 deletion (-alpha/alpha alpha) the alpha 2/alpha 1 mRNA ratio is 1:1. These results suggest that the loss of the alpha 2-globin gene in the alpha-Thal-2 deletion is associated with a 1.8-fold compensatory increase alpha 1-globin gene expression.