HbH disease is considered the most symptomatic form of α-thalassaemias that is compatible with life. The disease, however, has a heterogeneous clinical and haematological phenotype ranging from very mild phenotypes to more severe forms requiring regular or irregular blood transfusion. The severity of the disease is assumed to be related to the degree of α:β globin chain imbalance, which is principally determined by the nature of the underlying α-globin mutations [1, 10]. However, variations in the clinical phenotypes have been observed even in the presence of similar genotype, which is probably due to genetic and environmental modifiers .
Erbil province in northern Iraq covers an area of 15,075 km2 with a population of about two million people. Majority of the population are Kurds; the minorities of Turkmen, Assyrians, and Arabs comprise about 5% of its total population. The province is bordered by Turkey from the north-east and Iran from the east. We know much about β-thalassaemia in northern Iraq. Many studies have described the spectrum of beta-globin gene mutations, the carrier rate, and the genotypic mechanisms in both thalassaemia major and thalassaemia intermedia [14–19]. Scrutinizing α-thalassaemia is not as easy as for β-thalassaemia because of the lower carrier rate in our region, and the difficulty in confirming diagnosis α-thalassaemia carriers via the routine Hb-electrophoresis. The carrier rate of α-thalassaemia in Iraq is estimated at < 1%, but no study has yet rectified the exact carrier rate in this region. So far, a couple of small studies by Al-Allawi et al. have described the spectrum of α-globin gene mutations among carriers in the neighboring provinces of Dohuk and Sulaimaniyah [20, 21]. The current study is the first to tackle a relatively large cohort of patients with HbH disease, characterized their genotypes, and correlated genotypes to patients’ clinical phenotypes.
The clinical and haematological features of the enrolled patients showed obvious heterogeneity. Substantial variation was noticed in the patients’ age, Hb level, and transfusion requirement. Many clinical parameters strongly related to patients’ age, primarily the ferritin level and transfusion requirement. It is widely reported that the clinical severity of thalassaemias and other hemoglobinopathies is extremely age dependent .
In the current study, we have identified eight different α-globin gene mutations, four of which are deletion mutations (--MED, -α3.7, -α4.2, and -(α)20.5) and the remaining four are non-deletion mutations (α2 poly-A1, Hb Adana, α2 IVS1 (-5nt), and α2 poly-A2). Comparable to the results of earlier reports by Al-Allawi et al [20, 21], we found the spectrum of α-globin gene defects restricted to a relatively limited number of mutations. However, this finding is unlike to what have been observed in Iran, Turkey, Sardinia, and Cyprus, where the mutations are more heterogeneous [23, 24]. Within our HbH disease patients, the --MED double-gene deletion was the most commonly encountered mutation (33 alleles) followed by the worldwide prevalent -α3.7 single gene deletion (31 alleles). Similar findings have been reported by the vast majority of studies done in the Mediterranean and Middle East regions . The α2 poly-A1 non-deletion mutation, which is known to be prevalent in the Arabian Peninsula, was encountered in 14 alleles and was the most frequently encountered non-deletion mutation. Homozygosity for the latter mutation is the main genotype causing HbH disease in Saudi Arabia, UAE, Kuwait, and Bahrain [26–29]. None of the south-east Asian mutations were observed in this cohort. A previous study has reported the --FIL double-gene deletion in one β-thalassaemia intermedia case who was homozygous for IVS-II-1 (G > A). The same deletion was also observed in Hatay Province of Turkey [18, 25].
We, in the current cohort, observed nine different genotypes causing HbH disease, most of which were deletion genotypes (70.45%, 31 patients). The --MED/-α3.7 arrangement was the most frequently encountered genotype as in the neighboring areas of Turkey and Iran. High prevalence of deletion type HbH disease have been reported in the Mediterranean area, Turkey, Iran, Greece, and Sardinia, (80%, 50.5%, 56.1%, and 86% respectively) [10, 26, 30, 31]. We detected 19 alleles carrying non-deletion mutations of which 17 involved the α2 gene. Mutations of the α2-globin gene are associated with more severe phenotype because more than two-third of the α-globin chain synthesis is controlled by the dominant α2 globin gene transcription . Hb Adana was the only detected α1 globin gene mutation in our patients; it was observed in compound heterozygosity with --MED deletion in one patient and with -α3.7 deletion in another. We could not find any previous study reporting the latter combination (-α3.7/ααAdana) to have a HbH equivalent phenotype; though, we surprisingly found it in a 1.5 year-old boy with Hb of 7.1 g/dL. Coinheritance of -α3.7 deletion, with many other point mutations in the α2 globin gene, are reported in many studies . In our cohort, coinheritance of two point mutations was detected in six patients (13.6%) only, who all were homozygous for α2 poly-A1 mutation. Non-deletion genotypes are reported to be the predominant HbH disease genotypes in the Arabia area, Jordan, and northern Iran [23, 27, 34, 35].
It has been previously documented that patients with deletion/deletion HbH disease genotype are of least severe phenotype, followed by deletion/non-deletion arrangement, and the non-deletion/non-deletion genotype which is supposed to be the most severe [6, 30]. In our cohort, we did not find any significant difference in the clinical and haematological parameters, which we studied in both deletion and non-deletion HbH disease subgroups. The median Hb and red cell count were notably lower in the non-deletion HbH subgroup, however not to a statistically significant level. The remaining red cell parameters, the ferritin level, the HbH fraction, splenectomy, and blood transfusion status did not show any difference in the two subgroups.
In summary, our HbH disease patients had variable clinical and haematological phenotypes ranging from very mild cases not requiring transfusion to more severe others needed irregular transfusions. The spectrum of α-globin gene mutations and then the underlying genotypic arrangements were not many. The majority of our HbH disease patients were found to have deletion genotypes; however, their clinical phenotypes did not differ much from those having non-deletion genotypes. Knowing all these will definitely have a positive impact on thalassaemia diagnosis and management guidelines in this region.