Molecular analysis of the beta-thalassemia phenotype associated with inheritance of hemoglobin E (alpha 2 beta2(26)Glu leads to Lys)

The Characteristics of Compound Heterozygosity for Hemoglobin G-Makassar with Hb E in Malaysia

Background

Human hemoglobin of G-Makassar and hemoglobin E (Hb E) are hemoglobin variants that affect Beta (β) globin. Hb G-Makassar is a very rare variant while Hb E is estimated to affect at least one million people worldwide. Both Hb G-Makassar and Hb E can be inherited in the heterozygous, homozygous or compound heterozygous state. This case series describes the characteristics of four individuals with compound heterozygosity for Hb G-Makassar/Hb E cases in Malaysia. To the best of our knowledge, these are the only four individuals with this genotype reported in the literature.

Case Series

We present four cases of compound heterozygosity for Hb G-Makassar/Hb E identified from October 2014 to January 2021. All the cases were incidental findings whereby the screening Hb analysis showed the presence of peaks in both Hb S and Hb E zones on capillary electrophoresis (CE) and cation-exchange high-performance liquid chromatography (HPLC). Molecular analysis confirmed the findings of compound heterozygous Hb G-Makassar/Hb E. Two cases had a history of anemia secondary to unrelated conditions that resolved with treatment of the underlying cause. The other two cases were asymptomatic individuals who were detected through Malaysia's National Thalassemia Screening program. On the last follow-up, all the individuals were well, non-transfusion dependent, and had no reported history of chronic anemia, bleeding, hemolysis or thromboembolism complications.

Conclusion

The cases reported here highlight the possibilities for rare compound heterozygous states in multi-ethnicity populations such as Malaysia. Compound heterozygous Hb G-Makassar/Hb E individuals are clinically silent with laboratory values suggesting microcytic and hypochromic red blood cells. Further local epidemiology or population studies with genotyping tests are required for a better understanding of the diversity of its clinical phenotype.

Clinical and haematological characteristics of 38 individuals with Hb G-Makassar in Malaysia

Haemoglobin (Hb) G-Makassar is a rare Hb variant. It presents a diagnostic challenge as it imitates sickle Hb (Hb S) in standard electrophoresis and high-performance liquid chromatography assays requiring DNA analysis to confirm diagnosis. Both have point mutations in codon 6, exon 1 in the β-globin (HBB) gene with different pathogenicities. This study describes the clinical phenotype, haematology and genotype of Hb G-Makassar. Clinical and laboratory data of 38 cases of Hb G-Makassar over 8 years were analysed. Hb G-Makassar was confirmed by a direct sequencing of HBB gene and co-inheritance of α-thalassaemia determined through multiplex gap-PCR and multiplex Amplification Refractory Mutation System polymerase chain reaction. All cases were Malays, predominantly from Terengganu (n = 20, 52.6%). There were 14 (36.8%) males and 24 (63.2%) females with median age of 25 years. Majority (n = 33, 86.8%) had features of thalassaemia trait with mean ± SD for Hb, mean cell volume (MCV) and mean cell haemoglobin (MCH) as 13.21 g/dL ± 1.69, 73.06 ± 4.48 fL and 24.71 ± 1.82 pg, respectively. None had evidence of haemolysis or thromboembolic complications. Six genotypes were identified; ßG-Makassar/ß,αα/αα (n = 19, 50.0%), ßG-Makassar/ßE,αα/αα (n = 4, 10.5%), ßG-Makassar/ßNewYork,αα/αα (n = 1, 2.6%), ßG-Makassar/ß,αα/-α (n = 11, 28.9%), ßG-Makassar/ß,αα/αAdanaα (n = 2, 5.3%) and ßG-Makassar/ß,αα/-SEA (n = 1, 2.6%). The ßG-Makassar/ß,αα/αα showed that features of thalassaemia trait with mean ± SD for Hb, MCV and MCH were 13.74 g/dL ± 2.40, 76.18 ± 6.02 fL and 25.79 ± 2.41 pg, respectively. This is the largest study reporting a significant number of Hb G-Makassar in Malaysia. Although the mutation is similar to Hb S, the phenotype is benign.

Hb Hope [β136(H14)Gly→Asp (GGT→GAT)]: Interactions with Hb S [β6(A3)Glu→Val (GAG→GTG)], Other Variant Hemoglobins and Thalassemia

Hb Hope [beta136(H14)Gly-->Asp (GGT-->GAT)] was first described in an African-American family in 1965. Since then, it has been found in combination with several different globin gene mutations in many other families of divergent ethnic backgrounds. The basis for its relatively frequent occurrences remains unexplained. This variant hemoglobin (Hb) is mildly unstable and has reduced oxygen affinity, but is generally innocuous clinically. This variant Hb can present as a confounding factor in arriving at a correct diagnosis by either electrophoresis or high performance liquid chromatography (HPLC), particularly during the neonatal period. DNA-based diagnostics can help solve this potential problem.

The range of hemoglobin A(2) in hemoglobin E heterozygotes as determined by capillary electrophoresis

Capillary electrophoresis (CE) is capable of distinguishing hemoglobin E (HbE) from hemoglobin A(2) (HbA(2)), thus permitting quantification of HbA(2) in patients with HbE. In this study, routine samples submitted for evaluation of hemoglobinopathy that demonstrated HbE were studied by high-pressure liquid chromatography and CE. The data for 52 samples from adult HbE heterozygotes were compared with those for a control group consisting of 209 patients. The mean HbA(2) of patients with HbE trait was 3.4% (SD, 0.4%), which was significantly higher (P < .001) than the 2.6% (SD, 0.4%) for the control group. Seven samples from adults homozygous for HbE were also evaluated. The mean HbA(2) of HbE homozygotes was 4.4%, which was significantly greater (P < .001) than the HbA(2) values for the HbE heterozygotes. Data from these cases provide an estimate of the range of HbA(2) in patients with HbE when evaluated by CE.

Hemoglobin E disease in North Indian population: a report of 11 cases

Hemoglobin E (beta26Glu --> Lys) is the most common hemoglobin (Hb) variant in Southeast Asia and the second most prevalent worldwide. However in India, it is prevalent in Bengal and the north-eastern region, but relatively rare in the rest of the country. Identification of this Hb variant is important, because the doubly heterozygous state for HbE and beta-thalassemia is characterized clinically by thalassemia major, a situation different from other compound heterozygous states for structural beta-chain variants and beta-thalassemia. Thus, the affected individual may be symptomatic and transfusion dependent at an early age. This paper reports four cases with Hb E trait, three cases with hemoglobin E disease and another four cases with Ebeta-thalassemia. Laboratory investigations are based on RBC indices and high performance liquid chromatography (HPLC). A negative correlation has been found to exist between levels of HbA(2) and RBC indices including the MCV and MCH. A similar correlation has been seen between levels of HbF with Hb, RBC count, and MCV. The main aim is to increase the awareness of this relatively rare disorder, so that it can be included in the differential diagnosis of patients presenting clinically like thalassemia intermedia or thalassemia major. This awareness may also help in prenatal diagnosis, genetic counseling and clinical management. The clinical, hematological and laboratory features of this disorder are also discussed.

Hemoglobin SE disease: a concise review

An infant with Hb SE disease is reported. He was clinically well. Review of the literature shows that patients aged 18 and younger are usually well. On the other hand, more than half of those aged 20 and older developed sickling-related complications, including potentially life-threatening acute chest syndrome. These patients have 60-65% Hb S, similar to the percent Hb S in patients with Hb S/beta(+)-thalassemia. Their hematological features and clinical course appear to parallel those of Hb S/beta(+)-thalassemia. Patients have variable levels of anemia, and some develop clinical complications. With population migrations and increasing racial intermarriages, Hb SE disease is expected to be encountered more often around the globe. Patients with Hb SE disease should be followed and managed in a similar fashion as those with Hb S/beta(+)-thalassemia, and treated appropriately when they develop sickling-related symptoms and complications.

Cloning and functional characterization of a novel c-mpl variant expressed in human CD34 cells and platelets

The thrombopoietin receptor, c-mpl, is a crucial element not only in thrombopoietin (TPO)-initiated signaling pathways but also in the regulation of the circulating amount of TPO. We have identified a new c-mpl isoform, called c-mpl-del, that lacks 72 bp (24 amino acids) in the extracellular region of c-mpl and arises as a consequence of alternative RNA splicing between exons 8 and 9. c-mpl-del is expressed along with c-mpl-wt in blood mononuclear cells, CD34(+)cells, megakaryocytes, and platelets prepared from either normal donors or ET patients, although its relative expression appears to increase with megakaryocyte differentiation. The c-mpl-del-transfected cells expressed greater amounts of c-mpl-del RNA and protein than the comparable c-mpl-wt-transfected cells, however flow cytometry analysis could not detect any c-mpl receptor on the surface of the c-mpl-del-transfected cells. Further evidence for the absence of surface c-mpl-del was that in contrast to cells transfected with c-mpl-wt, those transfected with c-mpl-del did not grow in response to TPO, failed to undergo tyrosine phosphorylation of TPO-specific signal molecules, and did not bind(125)I-rHuTPO. Taken together, these results demonstrate that c-mpl-del, a naturally occurring variant of c-mpl, fails to be incorporated into the cell membrane but might serve as a mechanism to decrease the overall expression of functional c-mpl late in megakaryocyte differentiation.

Relative quantitation of mRNA in beta-thalassemia/Hb E using real-time polymerase chain reaction

beta-Thalassemia and Hb E patients, with seemingly identical genotypes, have a remarkable variability in severity. Reduction in red cell survival in beta-thalassemia is correlated with the amount of intracellular unmatched alpha-globin chains. However, it was only recently realized that mRNA, whose translation is prematurely terminated, is also unstable. No systematic attempts have been made to investigate mRNA stability in beta-thalassemia arising from nonsense mutations located upstream from the normal termination codon. In this study, one-step real-time polymerase chain reaction has been employed to compare the levels of alpha- and beta-globin mRNA in reticulocytes from beta-thalassemia/Hb E subjects. The results showed the highest alpha/beta-globin mRNA ratio (median = 5.70, n = 13) in frameshift codons 41/42 (-TTCT)/Hb E individuals compared to normal subjects (median = 1.02, n = 6), or those with Hb E trait (median = 2.15, n = 8). In addition, there was a concomitant increase in the alpha/beta-globin mRNA ratio with decrease in hemoglobin level, i.e., increase in severity. The difference in the ratio among beta-thalassemia/Hb E patients with the same genotype may be attributed to individual variations of efficiency in betaE-globin mRNA splicing and in the destruction of prematurely terminated mRNA.

MRNA analysis in reticulocytes of subjects with Hb D, Hb Porto Alegre, Hb E, and different types of unstable hemoglobin variants

Using a reverse transcription-polymerase chain reaction (RT-PCR) technique we determined the alpha 2/alpha 1, alpha/beta, and gamma/beta mRNA ratios in reticulocytes of 11 patients with seven different unstable beta chain variants, of 4 patients with two unstable alpha chain variants, in hemoglobin (Hb) D, Hb Porto Alegre, and Hb E heterozygotes, and in 8 patients with Hb X-beta 0-thalassemia (thal) (three D-beta 0-thal, one Porto Alegre = beta 0-thal, one Lulu Island-beta 0-thal, and three E-beta 0-thal). In addition, we determined the beta X/beta A mRNA ratios (X = unstable) in some Hb D heterozygotes and in 6 subjects with an unstable beta chain variant. Normal alpha/beta and beta X/beta A mRNA ratios were found in all heterozygotes tested, indicating that the respective mutations did not alter the stability of the mRNAs. The alpha/beta mRNA ratio in four Hb E heterozygotes averaged 4.21 (normal, 4.47), and that in 2 patients with Hb E-beta 0-thal and four alpha-globin genes (alpha alpha/alpha alpha) averaged a high 22.4. The gamma mRNA level in the Hb E heterozygotes was < 1% but varied greatly in patients with Hb E-beta 0-thal; the alpha/(gamma + beta) mRNA ratios in the 2 patients were 15.5 and 16.7, respectively. The large differences in alpha/beta and alpha/(gamma + beta) mRNA ratios in reticulocytes of subjects with AE and with E-beta 0-thal may be due to differences in the levels of normally-spliced beta E and abnormally-spliced beta E mRNAs. Only the latter is unstable and is preferentially produced in bone marrow and reticulocytes of Hb E-beta 0-thal patients, where it is rapidly degraded.

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.

Laboratory diagnosis of hemoglobinopathies

Diagnostic tests for most common hemoglobinopathies and recent advances in structural analysis of variant hemoglobins are reviewed. Routine and newly introduced methods that apply to the diagnosis of sickle cell anemia, thalassemia and the hemoglobin E disorders are presented. A brief description of the clinical course for each of these disorders is given, and potential pitfalls in diagnosis are discussed. Application of high-performance liquid chromatography and various mass spectrometric techniques (electrospray ionization mass spectrometry, liquid secondary ion mass spectrometry, and tandem mass spectrometry) for evaluation of hemoglobinopathy is presented.

Thalassemia: molecular pathology and management

Recent advances in molecular biology have allowed us to develop an almost complete picture of the molecular pathology of the thalassemia syndromes. The different classes of mutations that are responsible for the thalassemia syndromes will be discussed along with the special insights they have provided into the controls of eukaryotic gene expression. While management of these disorders has not kept pace with our understanding of their cause, there have been notable advances in treatment. Perhaps even more exciting is what the future holds, as the continued march of molecular biology is melded with novel approaches to the definitive treatment of thalassemias.

Further characterization of Hb Henri Mondor or alpha 2 beta 2(26)(B8)Glu----Val

A second case of Hb Henri Mondor is reported. The subject, homozygous for Hb Henri Mondor, is of Algerian origin. The electrophoretical behavior and structural characterization are given and discussed. Hb Henri Mondor, which is characterized by the replacement of the lysine residue in position beta 26, as is the case for Hb E, has normal functional properties and is normally expressed.

Effects of hemin on a lymphoblastoid cell line that expresses the human epsilon- and gamma-globin genes

Northern blot analysis using probes specific for each of the human embryonic (epsilon), fetal (gamma), and adult (beta) globin genes indicates that the human lymphoblastoid F-265 cells express the embryonic and fetal globin genes. Unlike the erythroid cell line K562, in which globin RNA levels increase during treatment with hemin in culture, globin RNA levels decrease in F-265 cells in the presence of hemin. This effect is reversible after passage of F-265 cells in fresh medium without hemin. Both the rates of globin RNA synthesis and the presence of DNase I-hypersensitive sites in hemin treated and untreated F-265 cells were investigated to identify the levels at which globin gene expression is controlled.

(A gamma delta beta) thalassaemia: similarity of phenotype in four different molecular defects, including one newly described

Globin gene mapping of DNA from families with (A gamma delta beta) thalassaemia has revealed a previously unreported gene deletion responsible for this condition. The deletion removes the A gamma, delta and beta genes and while its 5' end is in a similar position to that described in a previous deletion of this type, the 3' ends of the two deletions are quite different. In addition we have observed further examples of two other previously described deletions which result in this disorder. Phenotypic comparisons of families with (A gamma delta beta) thalassaemia, in which the molecular basis has been defined, show a remarkable similarity among the four different deletion defects, with important implications with regard to the mechanism by which deletions allow the continued expression of gamma genes.

Thalassemia-like abnormalities of the red cell membrane in hemoglobin E trait and disease

In recent studies, we observed a decrease of KMapp, an abnormally biphasic kinetics of the red cell membrane neutral phosphatase and an increased binding of hemoglobin to the membrane in various forms of beta-thalassemia. Since the gene encoding the beta chain (beta E chain) of hemoglobin E (HbE) is endowed with some thalassemic characteristics, we studied the erythrocyte membrane in 25 individuals with Hb E trait or disease. The apparent Michaelis-Menten constant for p-nitrophenylphosphate (the artificial substrate used) was significantly decreased, as in beta-thalassemia. However, the kinetics was monophasic in all the heterozygotes and in four of the homozygotes. It was biphasic only in the three other homozygotes. Vmax was also significantly reduced, a fact that is masked, when not reversed in beta-thalassemia, owing to the rejuvenation of the red cell population. In 5 mM phosphate buffer (pH 8.00), the binding of Hb E to the erythrocyte ghosts was increased in the homozygotes. In the heterozygotes, Hb A binding was also increased, as is the case in beta-thalassemia. This latter fact suggests that the membrane binding site(s) of hemoglobin is (are) altered. We found a highly significant increase of Hb F in EE subjects. The present study extends to the red cell membrane the beta-thalassemic phenotype associated with the beta E gene.

Hemoglobin E in Europeans: further evidence for multiple origins of the beta E-globin gene

We have determined haplotypes for the known restriction site polymorphisms in the beta-globin gene cluster in two families of European ancestry containing individuals who are heterozygous for hemoglobin E. In both families, the beta E mutation is associated with a haplotype not previously found among the haplotypes of beta E chromosomes in Southeast Asia. Moreover, in one family, the mutation is present in a beta-gene framework not found in beta E chromosomes of Southeast Asia. These data provide further evidence of multiple independent origins of the beta E mutation in human populations.

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.

Abnormal RNA processing due to the exon mutation of beta E-globin gene

As is typical of all beta-thalassaemias, the erythroid cells of individuals with the variant haemoglobin E (alpha 2 beta 2(26Glu leads to Lys)) exhibit a quantitative deficiency in their content of beta-globin (in this case beta E-globin) and its messenger RNA2,3. To determine the molecular basis of this phenotype, we have investigated the structure and expression of cloned beta E-globin genes. We report here that the complete nucleotide sequence of a beta E-gene revealed the expected GAG leads to AAG change in codon 26 but no other mutations. Expression of beta E-globin genes introduced into HeLa cells revealed two abnormalities of RNA processing: slow excision of intervening sequence-1 (IVS-1) and alternative splicing into exon-1 at a cryptic donor sequence within which the codon 26 nucleotide substitution resides. These results demonstrate a disturbance in the expression of the beta E-gene attributable solely to the exon mutation-a novel mechanism for gene dysfunction.

Hemoglobin Queens: alpha 34 (B15) Leu-Arg structural and functional properties and its association with Hb E

Hemoglobin Queens: alpha 34 (B15) Leu-Arg was found in association with Hb E in a Vietnamese boy. The observed microcytosis and hypochromia were due to Hb E, which also occurred in other members of this family. The functional properties of Hb Queens in the absence and presence of allosteric effectors were normal. The abnormal hemoglobins appear to be unrelated to the hyperbilirubinemia that was observed at birth.

Evidence for multiple origins of the beta E-globin gene in Southeast Asia

To investigate whether recurrent mutation has contributed to the high frequency of the beta E-globin gene in Southeast Asia, we used the haplotypes at three polymorphic restriction sites within and to the 3' side of the beta-globin gene to predict the framework of 23 beta E-globin genes. These haplotypes suggested that beta E-globin genes are present in two different beta-globin gene frameworks. DNA sequence determination of one gene representing each framework demonstrated that the same mutation (GAG leads to AAG at codon 26) was present in both frameworks. Moreover, the frameworks differed at three nucleotide positions known to be polymorphic in Mediterraneans. These polymorphic sites are located 70 nucleotides to the 5' side of the beta E mutation and 382 and 1032 nucleotides to the 3' side of it. The existence of the beta E mutation in these two beta-globin gene frameworks can be explained by (i) recurrent mutation giving rise to beta E-globin, (ii) a double crossing-over event, or (iii) two single crossing-over events. Mathematical analysis suggests that the first alternative, recurrent mutation of G leads to A at the first nucleotide of codon 26, is most likely.

Structural study of hemoglobin Knossos, beta 27 (B9) Ala leads to Ser. A new abnormal hemoglobin present as a silent beta-thalassemia

A new electrophoretically silent hemoglobin variant is described that produces the classical phenotype of beta thalassemic intermedia in association with beta thalassemia trait. This variant has the expression of a silent beta thalassemia trait. The abnormal hemoglobin was detected by acid-urea-Triton-acrylamide electrophoresis and further demonstrated by isoelectric focusing. The amount of the variant in carrier is approximately 30% of the total hemoglobin. No instability was found. Absence of hemoglobin A in the propositus blood facilitated structural studies. Peptides maps were normal but analysis of individual peptide spots showed an Ala leads to Ser substitution in the beta T3. This variant has been previously called Hb Knossos (beta 27 (B9) Ala leads to Ser).

Hemoglobin E in Indochinese refugees

Hemoglobin E, a beta-chain variant commonly found in Southeast Asian populations, is relatively unfamiliar to American physicians. The hematologic features of 55 patients with hemoglobin E (8 homozygote, 46 heterozygote and 1 genotype E/beta(thal)) are described and compared with previous series. The patients were immigrants from Cambodia, Laos, Thailand and Vietnam. These studies support the association of this beta-globin variant with microcythemia and suggest that it mimics the "thalassemia picture" of microcythemia out of proportion to the degree of anemia. Splenomegaly was not a characteristic. Oxidative hemolysis may occur under conditions that deplete reduced glutathione. The simultaneous presence of the beta-thalassemia trait appears to significantly worsen the prognosis.

Hemoglobin E diseases: hematological, analytical, and biosynthetic studies in homozygotes and double heterozygotes for alpha-thalassemia

Two families with Hb E diseases are described. In the Laotian family S, three homozygous Hb E were found. In the Vietnamese family H, double heterozygous Hb E-alpha-thalassemia-2 and Hb E-Hb H diseases were found. Anemia or hemolysis was absent in Hb E carriers, unless complicated by iron deficiency, the presence of severe alpha-thalassemia gene (Hb H disease), or oxidative drug (paraaminosalicylic acid). Moreover, iron deficiency or concurrent alpha-thalassemia genes resulted in a decreased amount of Hb E in its heterozygous carriers. Mild microcytosis and hypochromia were observed in Hb E heterozygotes, whereas the microcytosis and hypochromia were more pronounced in Hb E homozygotes. Globin chain synthesis studies yielded unbalanced alpha/non-alpha ratios in both heterozygotes and homozygotes (average ratios were 1.13 and 1.56, respectively). The unbalanced biosynthetic ratios with microcytosis and hypochromia in Hb E carriers represented a beta-thalassemia phenotype, which could be a result of reduced synthesis of beta E-globin mRNA, as suggested by recent hybridization studies.

Instability of beta E-messenger RNA during erythroid cell maturation in hemoglobin E homozygotes

Hemoglobin E interacts with beta-thalassemia to produce a disorder of variable severity that is the most common form of symptomatic thalassemia in Southeast Asia. The beta E-globin gene acts as a mild thalassemia gene; there are low levels of beta E-messenger RNA (mRNA) in reticulocytes, and preliminary evidence had suggested that this might be due to instability of the beta E-mRNA. Analysis of beta E-mRNA levels in the nuclei and cytoplasm of bone marrow erythroblasts compared with reticulocytes has shown higher levels of beta E-mRNA in the former, providing direct evidence that this is the case.

Thalassemia: recent insights into molecular mechanisms

Recent advances in defining the molecular basis for the thalassemia syndromes are discussed. We now realize that the causes of the thalassemia phenotype are diverse and include gene deletions, nuclear RNA processing defects, nonsense mutations, fusion genes, termination codon mutants, and unstable globin chains.