Role of polymorphic sequences 5′ to theGγ gene and 5′ to the β gene on the homozygous β thalassemic phenotype

Sixty‐seven homozygous male and female thalassemic patients with different phenotypes, aged between 8 and 33 years, were divided into three groups, according to the severity of their β‐thalassemia (thal) mutations. We investigated whether some co‐inherited genetic factors could influence the phenotype. Patients with milder β‐thal defects, homozygotes or compound heterozygotes for the IVS‐I‐6 (T→C) or −87 (C→G) mutations had a milder disease. In addition, determination of the co‐inheritance of the −158 (C→T) Gγ polymorphism and the (AT)9T5 repeat motif in the region −540 to −525, 5′ to the β‐globin gene, showed that in some patients with severe or mild/severe β‐thal mutations, linked to haplotype III, there was higher Hb F expression. We conclude that in homozygous β‐thal patients, the severity of the mutations is the most important factor influencing the phenotype, but some polymorphisms such as the −158 (C→T) Gγ and (AT)9T5 repeat motif, increasing the Hb F expression and ameliorate the clinical course o...