Background: One of the most important phenotypic modifying factors for thalassemia is the presence of Xmn1 polymorphism. This retrospective study was performed to investigate the overall prevalence of Xmn1 polymorphism among Iranian β-thalassemia patients with homozygote IVSII-1mutation and to assess the relationship between Xmn1 polymorphism with patients’ hemoglobin levels and the response to hydroxyurea (Hu) therapy.
Materials and Methods: The present cross sectional study included 112 β-thalassemia patients with homozygote IVSII-1 mutation. Laboratory investigations included complete blood count and routine hematological indices were measured by Sysmex K1000 (Japan) blood auto analyzer.  To find the state of Xmn1 polymorphism, blood samples were collected from patients using EDTA containers for genomic DNA analysis. DNA extraction and amplification-refractory mutation to determine the Xmn-1 polymorphism were performed.
Results: In total, 206 thalassemia patients including 112 patients diagnosed as thalassemia major and 94 patients diagnosed as thalassemia intermediate entered the study. The mean age at the start of transfusion was 5 ± 6.4 years old, and all of the patients received hydroxyurea. Twenty eight patients (14%) did not show any Xmn 1 polymorphisms (- / -), and 178 patients (86%) showed polymorphism either in one loci (- / +, 44 patients, 21.3%) or both loci   (+ / +, 134 patients, 65%).  Patients with Xmn1 polymorphism showed significantly higher age at diagnosis (p=0.002), higher age at start of transfusion (p=0.001), higher hemoglobin levels after treatment with hydroxyurea (p=0.005), and lower transfusion dependency (P=0.044).
Conclusion: The presence of Xmn1 polymorphism led to a delay in onset of blood transfusions, higher hemoglobin levels, better response to hydroxyurea treatment and milder phenotypic presentation among thalassemia patients with IVSII-1 mutation.