Genetic epidemiology of β‐thalassemia in sicily: Do sequences 5′ to the G γ gene and 5′ to the β gene interact to enhance HbF expression in β‐thalassemia?

The present epidemiological study of the molecular characteristics of β‐thalassemia in Sicily was prompted by the disparate phenotypic expression (in clinical status and absolute HbF level) observed in two β‐thalassemic homozygotes who were also homozygous for the β‐like globin gene cluster haplotype III. We suspected that polymorphisms within haplotype III could be the cause for the discrepancy. Based on the association of particular conformations of the (AT) x T y motif (‐540 5′ to the β gene) with milder forms of thalassemia and sickle cell anemia, 38 homozygous β‐thalassemia patients were studied to define their haplotypes, the −158 site 5′ to the G γ gene (linked to haplotype III) and the structure of the (AT) x T y motif. We found that the patient who was phenotypically mild and homozygous for β‐thalassemia, haplotype III, and the −158 C→T mutation was homozygous for the rare (AT) 9 T 5 motif. In contrast, the patient homozygous for β‐thalassemia, haplotype III, and the —158 mutation, but exhibiting a severe clinical course, was homozygous for the (AT) 7 T 7 configuration. Others have suggested that (AT) 9 T 5 is a negative regulatory protein binding sequence, and it is a silent carrier state for β‐thalassemia. The usual configuration (AT) 7 T 7 , has considerably less affinity for regulatory protein binding, and it is the most common configuration in Sicilian β‐thalassemics (67 of the 78 chromosomes studied). Within the 38 patients studied, seven were informative because they had various combinations of the (AT) 9 T 5 and (AT) 7 T 7 motif, and the −158 C→T mutation. The results in these patients suggest that only the co ‐presence of the (AT) 9 T 5 configuration and a C→T change at −158.5′ to the G γ gene is associated with high HbF expression and a mild clinical phenotype. We postulate that these two regions of the β‐like globin gene cluster interact, when endowed with the proper sequences, to enhance the expression of HbF secondary to anemia. © 1992 Wiley‐Liss, Inc.