Molecular basis of beta-thalassemia in Japan: heterogeneity and origins of mutations

Dominant beta-thalassemia with hemoglobin Hradec Kralove: enhanced hemolysis in the spleen

We describe a 6-year-old girl and her mother with dominant beta-thalassemia due to hemoglobin Hradec Kralove (Hb HK). Both patients presented microcytic anemia, jaundice, splenomegaly, cholelithiasis, and recurrent hemolytic bouts. Osmotic resistance tests using saline and coiled planet centrifugation revealed the increased fragility of the red cell membrane. On the other hand, the glycerol lysing time was prolonged, and results of the isopropanol test were weakly positive. Despite mimicking the features of hereditary spherocytosis, the results of the genetic analyses verified the second reported family with Hb HK (codon 115, GCC [Ala] --> GAC [Asp]). Splenectomy was effective for the amelioration of hemolysis. Of 7 reported patients with Hb variants at beta-globin codon 115 (Hb Madrid and Hb HK), 5 underwent splenectomy. Because of the variable augmentation of extramedullary hemolysis in dominant beta-thalassemias, genotyping is necessary for determining the clinical indication of splenectomy.

Beta-thalassemia in the Korean population

Beta-thalassemia is uncommon in the Korean population, however it must be considered in the differential diagnosis of hypochromic anemia. The molecular characterization of beta-thalassemia is absolutely necessary for molecular diagnosis as well as any genetic epidemiological study in this region. We analyzed the molecular basis of beta-thalassemia in 47 Korean families. Using direct sequencing of genomic DNA amplified through PCR and haplotype analysis, 44 beta-thalassemia genes were characterized, all of which were heterozygous. Fourteen different mutations were identified. The common mutations noted included the initiation codon (CD) ATG-->AGG (23.4%), CD 17 A-->T (21.2%), and IVS-II-1 G-->A (12.7%). Interestingly, mutations causing dominantly inherited beta-thalassemia were common (17.0%). All cases of IVS-II-1 G-->A mutations were linked to the silent mutation of CD 91 C-->T of the -globin gene. The initiation CD ATG-->AGG and IVS-II-1 G-->A with CD 91 C-->T were found in the Far East only, and may be inherited from a common origin for each mutation, at least in Koreans. CD17 A-->T and CDs 41/42-TTCT were suggested to be introduced by gene-flow from southern China. Otherwise, Hb Korea, CDs 89/90 -GT and a novel beta-thalassemia mutation, CD 131 CAG-->TAG, were only identified in Koreans. This mutation spectrum is characteristic of the low prevalent area of beta-thalassemia, however it is quite different even from the adjacent countries, Japan or China.

Beta-thalassemia in the Korean population

Beta-Thalassemia is uncommon in the Korean population, however, it must be considered in the differential diagnosis of hypochromic anemia. The molecular characterization of beta-thalassemia is absolutely necessary for molecular diagnosis as well as any genetic epidemiological study in this region. We analyzed the molecular basis of beta-thalassemia in 38 Korean families. Using direct sequencing of genomic DNA amplified through polymerase chain reaction and haplotype analysis, 35 beta-thalassemic genes were characterized, all of which were heterozygous. Twelve different mutations were identified. The most common mutations noted included the initiation codon ATG-->AGG (beta0) (28.9%), codon 17 (beta0) (A-->T) (18.4%), and IVS-II-1 (beta0) (G-->A) (10.5%). Interestingly, mutations causing dominantly inherited beta-thalassemia were also common (15.7%). All four cases with the IVS-II-1 (G-->A) mutation were linked to the silent mutation of codon 91 (C-->T) of the beta-globin gene. The initiation codon A7G-->AGG and IVS-II-1 (G-->A) with codon 91 (C-->T) mutations were found in the Far East only, and may be inherited from a common origin for each mutation, at least in Koreans. The codon 17 (A-->T) and codons 41/42 (beta0) (-TTCT) were suggested to be introduced by gene-flow from southern China. Otherwise, only Hb Korea [codons 33/34 (beta0) (-GTG)] and a novel beta-thalassemic mutation, codons 89/90 (beta0) (-GT), were identified in Koreans. This mutation spectrum is characteristic of the low prevalence area of beta-thalassemia in Korea, it is, however, quite different from the adjacent countries, Japan and China.

A novel amber mutation in a beta zero-thalassaemia gene (beta 37TGG-->TAG), with direct detection by mapping the restriction fragments in amplified genomic DNA

A novel amber mutation, a G to A substitution at the second position of codon 37 in the beta-globin gene that changes the tryptophan coding triplet (TGG) to a termination codon (TAG), was found in a Chinese beta-thalassaemia carrier. The mutant gene creates an additional Dde I recognition site and eliminates the Ava II site, so this point mutation can be directly identified by restriction enzyme analysis.