Genetic Modifiers of Homozygous Beta Zero Thalassemia.

Introduction: Extensive studies over the last 50 years have derived two major modifiers of the clinical expression of beta-thalassemia: innate ability to produce fetal hemoglobin (Hb F) and co-inheritance of alpha-thalassemia. Recently genetic variants at the BCL11A locus and HBS1L-MYB intergenic region, that can modulate Hb F levels have been identified ( Thein et al 2007, Menzel et al 2007). Moreover, BCL11A variant has been shown to moderate the phenotype of homozygous beta-thalassemia (Uda et al 2008). In this study we evaluated the contribution of these Hb F modulating genetic markers and of alpha-thalassemia in ameliorating the severity of homozygous beta-thalassemia.

Methods: We studied 50 patients with the mild non-tranfusion dependent thalassemia intermedia phenotype and 75 with the severe tranfusion dependent thalassemia major. All patients were homozygotes for the beta 39 non-sense C → T mutation and were negative for the Xmn I -158 Gγ polymorphism. Alpha thalassemia was detected by GAP-PCR technique (deletion defects) and restriction enzyme digestion (nondeletion defects).Genotyping of HbF modulating genetic variants was performed using TaqMan® SNP genotyping assay (Applied Biosystems, Warrington, UK) according to the manufacturer’s protocol. Evaluation of the association, OR calculation and test for interaction were performed fitting a logistic regression.

Results: We found that the frequency of the BCL11A and HBS1L-MYB Hb F modulating markers and of co-inherited alpha- thalassemia is significantly increased (p= 2.05 x 10⁻⁵, p= 8.1 x 10⁻⁴, and p= 9.9 x 10⁻⁵ respectively) in the group of patients with thalassemia intermedia. Interestingly, the contribution of BCL 11A gene appears to be greater than HBS 1L-MYB locus ( OR= 5.15 and 4.61 respectively) and both are striking larger than the effect attributable to the co-inheritance of alpha-thalassemia (OR= 3.32). Furthermore, to visualize the combined effects of all loci, we considered a score variable defined as the number of positive alleles (i.e. those associated with the amelioration of the phenotype) carried from each patient. When considered all together the three loci explain 79% of the variation, evaluated as the correlation between fitted and observed values in the logistic regression. All three analyzed loci act in an additive fashion, where each copy of the minor allele contributes to the amelioration of the phenotype expression.

Conclusions: Molecular genetic analysis and particularly genome-wide association scans are producing rapid advances in defining the genetic variants for complex diseases and quantitative traits. The information reported are relevant in understanding the correlation between genotype and phenotype in thalassemia syndromes.