Beta-thalassemia alleles and unstable hemoglobin types among Russian pediatric patients

Double Heterozygosity for Hb Durham-N.C. (HBB: c.344T>C) [β114(G16)Leu→Pro] and the IVS-I-110 (HBB: c.93-21G>A) Causing a Severe β-Thalassemia Phenotype

The evaluation of a 10-month-old girl of Sicilian origin with a clinical phenotype of severe thalassemia led to the identification of two β-globin gene defects, a β-thalassemia (β-thal), mutation at IVS-I-110 (HBB: c.93-21G>A) and a variant hemoglobin (Hb) mutation at codon 114 (HBB: c.344T>C) on the other allele, reported as Hb Durham-N.C. (also known as Hb Brescia) [β114(G16)Leu→Pro] in the HbVar database. A very low Hb level (Hb 3.5 g/dL), microcytosis [mean corpuscular volume (MCV) 63.2 fL] and hypocromia [mean corpuscular Hb (MCH) 19.6 pg], increased red blood cell (RBC) distribution width (RDW) (36.0%), higher reticulocytes (6.2%), anisocytosis, poikilocytosis, hypocromia, basophilic stippling and inclusion body formation, were present in the affected subject. Analysis of other family components showed the presence of HBB: c.93-21G>A defect in the mother and in her brother, while Hb Durham-N.C. was absent in all other relatives, thus, this mutation has arisen as a de novo defect. This is the first case described as a severe thalassemic phenotype in a compound heterozygote carrier of this unstable Hb and a common β-thalassemic allele. The important information gained from this case is that a rare dominant or recessive mutation may arise in every individual, even if this is a very rare event.

The molecular basis of β-thalassemia

The β-thalassemias are characterized by a quantitative deficiency of β-globin chains underlaid by a striking heterogeneity of molecular defects. Although most of the molecular lesions involve the structural β gene directly, some down-regulate the gene through distal cis effects, and rare trans-acting mutations have also been identified. Most β-thalassemias are inherited in a Mendelian recessive fashion but there is a subgroup of β-thalassemia alleles that behave as dominant negatives. Unraveling the molecular basis of β-thalassemia has provided a paradigm for understanding of much of human genetics.

Two French Caucasian Families with Dominant Thalassemia-Like Phenotypes Due to Hyper Unstable Hemoglobin Variants: Hb Sainte Seve [Codon 118 (− T)] and Codon 127 [CAG→TAG (Gln→Stop])

We report two French Caucasian families suffering from dominant thalassemia-like phenotypes due to hyper unstable hemoglobin (Hb) variants. In both cases, molecular analysis revealed a defect localized in the third exon of the beta-globin gene, resulting in dramatic changes of the Hb structure. The first one is a new variant, Hb Sainte Seve, that is associated with a frameshift mutation at codon 118 (-T). In the second family, the disease resulted from a truncated protein due to a stop mutation at codon 127 [CAG-->TAG (Gln-->Stop)]. These two observations are additional evidences of the important role played by helix H in Hb stability: its partial absence, or a large structural modification, seems to be the major reason for the hyper instability of such molecules.

Hb Florida: a novel elongated C-terminal beta-globin variant causing dominant beta-thalassemia phenotype

We report here a new frameshift mutation in exon 3 of the beta-globin gene, a single nucleotide deletion (-C) in between codons 140/141 (GCC/CTG-->GCC/TG), found in an 8-year-old Argentinean girl with clinical picture of thalassemia intermedia. It leads to a beta-chain that is elongated to 156 amino acids [(141)Trp-Pro-Thr-Ser-Ile-Thr-Lys-Leu-Ala-Phe-Leu-Leu-Ser-Asn-Phe-(156)Tyr-COOH]. The resulting hemoglobin, which we named Hb Florida, was not detected in peripheral blood; however, erythroid hyperplasia and dyserythropoiesis with large inclusion bodies on methyl violet staining were observed in bone marrow, suggesting that this is a hyperunstable variant producing a dominant beta-thalassemia phenotype, since the other beta-allele was completely normal.

A Korean family with a dominantly inherited beta-thalassemia due to Hb Durham-N.C./Brescia

We describe the molecular and the hematological characteristics of a Korean family with a dominantly inherited beta-thalassemia. Carriers were characterized by moderate anemia, hypochromia, microcytosis, elevated Hb A2 and Hb F levels, and splenomegaly. DNA analysis revealed a CTG (Leu) to CCG (Pro) substitution at codon 114 of the beta-globin gene, that leads to a highly unstable hemoglobin variant, Hb Durham-N.C./Brescia, and this was linked to the beta haplotype V, [+----+-], and framework 2. RNA analysis showed that the proband had comparable levels of mutant and normal beta-mRNA. Translation of the mutant mRNA would give rise to non-functional hyperunstable beta-globin chains, and their degradation would, by placing an additional burden on the proteolytic process of the red blood cell precursors, result in a more severe phenotype.