Hemoglobin Mississippi (beta 44ser----cys). Studies of the thalassemic phenotype in a mixed heterozygote with beta +-thalassemia

Hb Narges Lab, a Novel Hemoglobin Variant of the β-Globin Gene

In this study, we describe a new missense variant on the β-globin gene in a heterozygous form in a female individual. Standard methods were used to determine red blood cell indices and perform hemoglobin analyses. Molecular studies were performed on the genomic DNA isolated from peripheral blood cells. Beta-globin genes were amplified and sequenced. We report a novel mutation on the β-globin gene (HBB), c.134 C>T; p.S44F variant, in the heterozygote state which was detected in a female of Persian ethnic origin in the Khuzestan province, southern Iran, that we named Hb Narges Lab (HbNL) variant. This mutation was predicted to be disease-causing in all except one in silico prediction tools. This variant was reported for the first time worldwide, had no shown hematological abnormalities but should be considered when inherited in the compound heterozygous form with β- thalassemia (β0-thal) carrier, which might result in the phenotype of thalassemia intermedia.

Hemoglobin Hakkari: an autosomal dominant form of beta thalassemia with inclusion bodies arising from de novo mutation in exon 2 of beta globin gene

Certain beta globin gene mutations produce a thalassemia major phenotype in the heterozygous state. While most such patients have thalassemia intermedia, we describe a young Guatemalan child with a de novo mutation in the beta globin gene, codon 31 T --> G (Hemoglobin Hakkari), who developed severe anemia at the age of 10 months and remains transfusion-dependent. The substitution of B13 leucine with arginine in the beta globin results in alteration of a critical heme contact point resulting in an extremely unstable variant hemoglobin and a clinical picture that is characterized by ineffective erythropoiesis and numerous intracytoplasmic inclusions within the erythrocyte precursors of the bone marrow. .

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.

Isolation and characterization of the translation product of a beta-globin gene nonsense mutation (beta 121 GAA----TAA)

The beta o-thalassaemia gene of an individual who was a mixed heterozygote for this allele (GAA to TAA in codon 121) and beta(+)-thalassaemia (IVS-1 position 110 G to A) was examined to determine if the beta o-thalassaemia allele directed the synthesis of any detectable protein product. This beta o-thalassaemia allele was of particular interest, because it is the only example of a premature chain termination codon in the third exon of the beta-globin gene that produces thalassaemia. A very small amount of an abnormal protein was found in the red blood cells of the proband and was purified by preparative column chromatography. This abnormal protein was digested with trypsin, the peptides were separated by reverse phase high performance liquid chromatography (HPLC), and the amino acid content of each peptide was determined. All of the soluble beta-globin peptides were found except for T-13, T-14 and T-15 (residues 121-146), indicating the presence of a truncated protein that corresponded to the translation product of the beta 121 Glu----Term mRNA. This truncated globin was estimated to comprise between 0.05% and 0.1% of the total non-alpha-globin protein. These results may explain the phenotype of inclusion body beta-thalassaemia in heterozygotes, which is atypical of heterozygous beta o-thalassaemia.

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.