The modifying effect of Xmn1-HBG2 on thalassemic phenotype is associated with its linked elements in the beta globin locus control region, including the palindromic site at 5′HS4

The core sequence of 5′HS4-beta globin locus control region and Xmn1-HBG2 site were analyzed and compared among 86 thalassemia patients with homozygous or compound heterozygous beta globin gene mutations and 101 normal individuals. Frequency of the G allele in the polymorphic palindromic sequence of 5′HS4 (TGGGG A/G CCCCA) and positive Xmn1-HBG2 profile was significantly higher in thalassemia patients compared to the normal population. Linkage disequilibrium was observed between the G allele and positive Xmn1-HBG2 profile in patient population. Furthermore, dominance of IVSII-1 in the mutation spectrum of the patients enabled us to identify linkage disequilibrium relationships between IVSII-1, positive Xmn1-HBG2 and the G allele at 5′HS4. The frequency of milder clinical phenotype was significantly higher in patients with GG/++ than cases with AA/−− genotypic pattern in 5′HS4/Xmn1-HBG2 loci. These data together with biochemical evidence suggesting a role for the A/G polymorphism at 5′HS4 palindromic site on modifying chromatin structure and in the absence of any evidence from functional studies relating the Xmn1-HBG2 site to the increased gamma chain expression, suggest that the phenotype modifying role long time assigned to Xmn1-HBG2 is possibly played by more functionally potent elements linked to it in LCR.