Hb Cibeles [α2 CD25(B6) (Gly → Asp)]: a novel alpha chain variant causing alpha-thalassemia.

Thalassemias are the most frequent monogenic disorders around the world and are a serious health problem in areas with a high incidence. Thalassemias are particularly frequent in Mediterranean countries, the Middle East, Africa, the Indian subcontinent, and in the Southeast Asia. Of these, α-thalassemia is inherited as an autosomal recessive disorder. α-thalassemias are due to a deficiency or absence of hemoglobin (Hb) α-chain synthesis and are characterized by microcytic and hypochromic cells anemia and a clinical phenotype varying from nearly asymptomatic to a lethal hemolytic anemia. Compound heterozygotes and some homozygotes have a moderate to severe form of α-thalassemia called HbH disease. Hb Bart's hydrops fetalis is a lethal form in which no α-globin chain is synthesized. In this study we show a new structural variant of α-chain, Hb Cibeles [alpha 25(B6) Gly → Asp], in heterozygous state, which was undetectable by electrophoretic or chromatographic methods. Hb Cibeles is thus a hyper-unstable hemoglobinopathy. In this new globin chain variant, an apolar amino acid is replaced by a negatively charged amino acid. This change may be responsible for the molecular hyper-instability similar to the mutation in the adjacent residues.

Association in cis of the mutations +20 (C>T) in the 5' untranslated region and IVS-II-745 (C>G) on the ß-globin gene.

In the 5' untranslated region (5'UTR), which is transcribed but not translated and is involved in posttranscriptional regulation of the gene promoting and stabilizing the mRNA translation, several mutations associated with mild ß-thalassemia (ß-thal) have been described. One of these, the +20 (C>T) mutation, is described in the HbVar database as a mutation responsible for ß(+)-thal. In heterozygote cases, it gives rise to a phenotype of ß-thal minor and ß-thal intermediate (ß-TI) when the mutation is associated with ß(+) IVS-II-745 (C>G). To clarify if this mutation is responsible for ß(+)-thal, we studied nine cases where we found an association of the +20 and IVS-II-745 mutations. All patients were carriers of four α genes. Three patients carried ß-thal major (ß-TM), two were compound heterozygotes for IVS-II-745 and codon 8 (-AA) or codon 39 (C>T), and the third was homozygous for IVS-II-745; all had the +20 mutation in the 5'UTR. The remaining patients showed the mutation IVS-II-745 associated with a replacement of C>T at nucleotide (nt) +20 of the 5'UTR. Contrary to reports in the HbVar database, the +20 mutation should be considered as an innocuous single nt polymorphism associated with the IVS-II-745 mutation in cis.

Study of three families with Hb Agrinio [α29(B10)Leu→Pro, CTG>CCG (α2)] in the Spanish population: three homozygous cases.

Most α-thalassemia (α-thal) mechanisms are deletions of one or both α-globin genes and less than 5.0-10.0% are point mutations. Hb Agrinio [α29(B10)Leu→Pro, CTG>CCG (α2)] is a hyperunstable α chain structural variant in which the thalassemic phenotype is determined by a post translational precipitation of the structurally anomalous chain in erythroid precursors. This study involved 14 cases with Hb Agrinio from three families. Selective sequencing of the α2 gene showed a CTG(Leu)>CCG(Pro) mutation at codon 29. The mutation was found in a heterozygous state in 11 cases and in a homozygous state in three cases. These are the first cases with Hb Agrinio described in Spain. In all cases where a leucine is exchanged for a proline, an unstable hemoglobin (Hb) will occur both in the α and the ß chain. Some of these are as unstable as Hb Agrinio and their presence is difficult to detect except by DNA sequencing.

Hb Extremadura [ß64(E8)Gly→Ser;ß133(H11)Val→Leu]: a new molecular analysis.

Unstable hemoglobin (Hb) variants account for 9.5% of structural hemoglobinopathies. The majority of these unstable variants are the result of gene point mutations resulting in the substitution of a single amino acid by another. The presence of two mutations in the same allele is infrequent: of the 781 variants of the ß-globin cluster described, only 32 are due to two point mutations (4.1%). Hb Extremadura is a structural variant that is included within the so-called unstable Hb anomalies. It was first described in 1989, employing the most up-to-date techniques available at that time, reversed phase high performance liquid chromatography (HPLC) to separate the abnormal chain (ß(X)) digesting it with trypsin and analysis of the fragments with an automatic analyzer.