Hemoglobinopathies, including sickle cell anemia and β-thalassemia, are highly prevalent monogenic gene disorders in Saudi Arabia (1–10). β-thalassemia is caused by point sequence variations or large sequence deletions, that are heritable and either prevent the synthesis of the β-globin chain completely (β⁰ variants) or alters the function of β-globin chain (β⁺ variants) (1–10). The phenotype of β-thalassemia in the Saudi population is highly varied, ranging from asymptomatic to severe transfusion-dependent anemia (1,4,9,11). Furthermore, it has been previously demonstrated that the α-globin gene deletions, gene conversion [hemoglobin-α 12 (HBA12)] and point mutations, are also highly prevalent in the Saudi population, particularly in the densely populated regions of Qatif and Al-Ahssa in the Eastern Province (13.41% of sickle cell disease carriers; 5.9% β-thalassemia carriers) of Saudi Arabia (1–6,9,10,12,13). Previous studies have investigated the prevalence of the α-globin gene deletion in Arab populations, however, very little has been determined with regards to the prevalence of the α-globin gene deletion in Saudi patients with thalassemia (14–20). It has previously been revealed that the coinheritance of α-thalassemia in patients with heterozygous β-thalassemia results in an increase of the mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) blood concentration, however, carriers of β-thalassemia mutations have an increased blood concentration of hemoglobin A₂ (HbA₂) (21). However, not all carriers of β-thalassemia mutations exhibit this phenotype, and some may in fact exhibit normal HbA₂ blood concentrations (22), which has previously been attributed to a number of factors, including α-globin gene deletions (23). A pre-marriage screening for β-thalassemia mutations in these regions depends on the determination of HbA₂ blood concentrations (24,25). The most common α globin gene deletion is the 3.7 kb rightward deletion (−α^3.7), which is caused by the breakage of DNA molecules in the α globin genes (HBA2 and HBA1) region and rejoining of the broken ends by leaving α globin genes region with single functional gene. The present study aimed to determine the effect of α-globin deletion on fetal hemoglobin (HbF) and HbA₂ blood concentrations in Saudi populations.

The results of the present study demonstrated that the −α^3.7 gene deletion is the most prevalent (43.5%) among the population of the Eastern province of Saudi Arabia. The second most prevalent gene deletion was the HBA₂: c.95+2_95+6het_delTGAGG (α₂^HphI) deletion, with a prevalence score of 24.3%. The −α^3.7 gene deletion is characterized by the deletion of 3,804 base pairs (Fig. 2). Further α-globin gene mutations revealed to be present in the tested population were: α₁–^4.2 (1.78%), α^1polyA-1α₂ 1.78% and double gene deletions were at a prevalence rate of 1.39% for −α₂^(20.5) and <1% for ^−-FIL and ^−-MED. The prevalence of the recently identified α12 (HBA12) allele was demonstrated to be 3.78% in the investigated population. A number of genotypes, namely −^3.7α₂/α₁α₂, −^3.7α₂/α₁α₁₂, −^3.7α₂/−^3.7α₂, −^3.7α₂^HphI/α₁α₂^HphI, −^3.7α₂/ α₁- ^4.2, −^3.7α₂/α₁^polyA-1α₂, −^3.7α₁₂/α₁α₁₂, −−^FIL/−^3.7α₂ and −^3.7α₂/−^3.7α₂^{Hb Villiers le Bel} were observed in the population. In addition, >10% of the total population carried other types of α-gene deletion, namely −α₂^(20.5)/α₁α₂, −−^MED/α₁α₂^HphI, α₁α₂^init/α₁^polyA-1α₂, α₁–^4.2/α₁α₁, −−^MED/α₁α₂, −−^FIL/α₁α_2, −−^FIL/α₁^polyA-1α₂, α₁α₂/α₁α₂^HphI, α₁α₂^HphI/α₁α₂^HphI, α₁α₂/α₁α₂^{Hb Handsworth} and α₁^polyA-1α₂/α₁α₂.

The concentrations of HbF and HbA₂ in the blood are presented in Table I and Fig. 1. A gradual increase in the levels of both HbF and HbA₂ in the β thalassemia patient groups was demonstrated as the number of α-gene deletions increased (Fig. 1 and Table II; P<0.05). However, in healthy patients, the concentration of HbA₂ was revealed to decrease as the number of gene deletions increased. The post-hoc Tukey, Bonferroni and Holm multiple comparison tests revealed significant differences with regards to the blood concentration of HbF (f-ratio=3.42806; P=0.020334 for ANOVA of all data groups) and HbA₂ (f-ratio=9.72308; P=0.000012 for ANOVA of all data groups; Table III), in different patient groups.

The high prevalence of these disorders has previously been attributed to the high endemicity of malaria in affected areas (29). β-thalassemia disorders represent a group of heterogeneous hemoglobin disorders, characterized by either the absence or reduced synthesis of the β-globin chain. Such disorders can be classified into three groups according to the severity of their associated clinical representation: β-thalassemia carrier (low severity), thalassemia intermedia (moderate severity) and thalassemia major (high severity). An excess of α-globin chain production, which aggregates in red blood cell precursors forming inclusion bodies, characterizes thalassemia major. This results in the destruction of red blood cells in the bone marrow and ineffective erythropoiesis, which results in the development of anemia as well as intense proliferation and expansion of the bone marrow (30).

The phenotypic variation exhibited by patients with β-thalassemia is due to the heterogeneity in genetic mutations associated with the disease. However, the phenotype can also be modified by other genetic factors. In addition to the influence of HbF concentration on the β-thalassemia phenotype, deletions of α-globin genes have also been demonstrated as having a significant effect on patient phenotype (20,21).

Furthermore, it has been revealed that the coinheritance of α-thalassemia in individuals heterozygous for β-thalassemia results in elevated levels of MCV and MCH (31). This most frequently occurs when two α-globin genes are silenced due to gene deletion or another type of mutation (32). These individuals also exhibit an increased concentration of HbA₂: A characteristic that is exploited for the identification of β-thalassemia carrier state.

Previously, we have demonstrated that the α-globin gene deletion is highly prevalent in the populations of the Al-Qatif and Al-Ahssa regions (4,9,10). Furthermore, it has previously been demonstrated that the association of the α-gene deletion with β-thalassemia and sickle cell anemia leads to amelioration of disease severity (33).

The results of the present study suggest that the frequency of α-gene deletions is increased in both normal and β-thalassemia populations in the Eastern province compared with other provinces, which is in agreement with the results of previous studies, including reports on African populations (34). Furthermore, previous studies have demonstrated that the prevalence of the α-gene deletion in Arab populations is varied, ranging from 28% in Kuwait and the United Arab Emirates to as high as 75% in Lebanon (14–20,35). This difference in the reported prevalence frequencies of α-globin gene deletions may be attributed to variation in the sample size as well as the inclusion criteria of different studies. Genome-wide association studies have revealed that the α-globin gene deletions have a significant effect on the blood concentrations of HbA and HbA₂, whereas it has no significant effect on the concentration of HbF (36). The results of the present study suggest that there is an association between the −α^3.7 deletion and elevated concentrations of HbA₂, and is therefore in agreement with the aforementioned study. In addition, the β-thalassemia mutations were demonstrated as being associated with an increased concentration of HbF (36–38). Therefore, it can be concluded that the elevated HbF concentration in the present study is predominantly associated with β-thalassemia mutations as opposed to α-gene deletions. Furthermore, the results of the present study also revealed new α-gene deletion genotypes prevalent in the studied populations, namely: α₁α₂/α₁α₂^HphI, α₁α₂^HphI/α₁α₂^HphI, α₁α₂/α₁α₂^{Hb Handsworth}, −^3.7α₂^HphI/α₁α₂^HphI, −^3.7α₂/−^3.7α₂^{Hb Villiers le Bel} and −−^MED/α₁α₂^HphI, which, to the best of our knowledge, have not previously been reported.