Genetic Modulators: Prevalence and Their Effect on the Phenotype of HbE/β Thalassemia Patients.

HbE, a β-globin chain variant occurs due to GAG to AAG i.e. Glu-Lys substitution at codon 26.In heterozygous and homozygous state it usually remain asymptomatic but the compound heterozygous state with β thalassemia (HbE/β thalassemia) results in a variable and often severe anemia, with the phenotype ranging from transfusion dependence to a complete lack of symptoms. The basis of the interaction and the cause of the variability remains unexplained. Some of the possible explanations for the observed variable clinical severity are coinheritance of mild β mutations, Effect of alpha gene number and increased production of HbF. Aims of the study are to determine the frequency of primary (β mutations) and secondary modulators (α deletion and α triplication, XmnI polymorphism, HPFH-3 and Asian-Indian Inv/Del^Gγ(^Aγδbeta;)⁰ deletion) in HbE/β thalassemia patients and to study their effect on the phenotype of the Patients. Subjects were 180 HbE/β thalassemia patients diagnosed by HPLC and confirmed by PCR-RFLP. Patients were divided into three subgroups according to a scoring system based on seven clinical criteria as mild (score 0–3.5)[Gp1], moderate (score 4–7)[Gp2] and severe (score 7.5–10) [Gp3]. β-mutations were studied by ARMS PCR, XmnI polymorphism was studied by PCR-RFLP while α deletions, HPFH-3 and Asian-Indian Inv/Del^Gγ (^Aγ δβ)⁰ deletions were studied by GAPPCR and alpha triplication by a modified single tube PCR. The frequency of β mutation was almost similar in all the groups (IVS 1–5 G-C (58.8%) was the predominant one). α deletion was found in 19 (10.5%) out of which 13 (12 αα/−α^3.7 & 1 αα /--SA) were from Gp1 and 6 (αα/-α^3.7) were from Gp2. α triplication was found in 11(6.1%) out of these 11 patients 8 were (αα/ααα^anti3.7) from Gp3 and 3 were (αα/ααα^anti3.7) from Gp2. XmnI was found in 112(62.2%), out of which 98 were heterozygous (+/−) and 14 were homozygous (+/+).XmnI +/+ was present in 10 Gp1 & 4 Gp2 patients, while XmnI +/− was present in 36 Gp1 ,43 Gp2 & 33 Gp3 patients.HPFH-3 deletion was present in 8 (4.4%) out of which 5 were from Gp1 and 3 were from Gp2. Asian-Indian Inv/Del^Gγ (^Aγδβ)⁰ deletion was present in 2(1.1%) patients one from Gp2 and another from GP3. Primary modulator (β mutation) is not the actual factor responsible for the disease heterogeneity. Patients with coexisting α deletion required lesser transfusions and had less severe phenotype while patients with α triplication were on frequent transfusions and had severe phenotype. XmnI polymorphism in homozygous state was observed to alleviate the severity while in heterozygous state it had no effect on the disease severity except the delay of the onset of disease up to some extent. Patients having HPFH3 deletions and Asian-Indian Inv/DelG γ (Aγδβ)⁰ deletion showed mild to moderate disease severity. Thus it can be concluded that secondary modulators play an important role in the disease heterogeneity of HbE/βthalassemia.