A Novel beta-delta Globin Gene Fusion Leads to Over-Expression of Delta Globin Chain and a Mild Thalassemia Intermedia Phenotype When Co-Inherited with β0-Thalassemia.

Abstract

Anti-Lepore hemoglobins are rare βδ fusion variants that arise from nonhomologous crossover during meiosis, resulting in a δ-βδ-β configuration in the β globin cluster on the short arm of chromosome 11. No such variant has previously been reported in Chinese. We studied 2 unrelated Chinese families with anti-Lepore due to a novel βδ fusion. A moderate increase of Hb A2 to between 16.8 and 19.2% of total Hb was seen in 4 clinically and hematologically normal carriers, while in 1 patient with concurrent β thalassemia allele and mild thalassemia intermedia phenotype, the Hb A2 was markedly raised to 42.2%. Potential βδ fusion gene was amplified using PCR with primer 5′ to β globin gene Cap site and 3′ to δ globin gene poly A region in these 5 subjects. Direct sequencing of the PCR products revealed a crossover between the β and δ globin gene within a 54 bp region spanning the junction of CAP and exon 1 in all 5 subjects. Active transcription of this novel βδ fusion gene was studied in 2 subjects (1 normal carrier and the patient with thalassemia intermedia). Using primer pair 5′ to β globin gene Cap site and at the 3′ end of δ globin gene exon 3, a 655 bp PCR product was obtained from cDNA in both subjects. Direct sequencing confirmed the presence of a βδ fusion mRNA. The predicted protein product of this fusion gene was identical to δ globin, since only the 5′ untranslated region was β specific. Determination of α/β mRNA ratios by quantitative real-time PCR was performed in 2 normal carriers (ratio 3.28 and 3.95) and the patient with thalassemia intermedia (ratio 25.59). These were compared with ratios ± standard deviation from 20 normal controls (1.92±0.55), 5 unrelated patients with β thalassaemia minor (17.23±5.38) and 3 unrelated patients with β thalassemia major (139.21±39.50). Mild down-regulation of the β gene in cis due to this interposed βδ fusion gene was demonstrated. Although heterozygotes of this novel anti-Lepore Hb had normal red cell indices and were clinically silent, compound heterozygotes with β0 mutation in trans produced a mild thalassemia intermedia phenotype with a markedly raised Hb A2 level that might mimic clinically mild Hb E-β+ thalassemia. Awareness of this new anti-Lepore Hb will help to resolve diagnostic problems in regions with significant prevalence of globin disorders.