Hb Knossos (HBB: c.82G > T), β-globin CD 5 (−CT) (HBB: c.17_18delCT) and δ-globin CD 59 (−a) (HBD: c.179delA) mutations in a Syrian patient with β-thalassemia intermedia

Thalassemia (thal) is one of the most common inherited blood disorder in the world. This disease caused by reduced or/and absent synthesis of the globin chains of hemoglobin (Hb), which leading to imbalance of the globin chains [1, 2]. Beta-thalassemia (β-thal) is one of the major types of thalassemia and it results from decrease in lack of beta-globin (β-globin) chain production [3]. Thalassemia intermedia (TI) or non-transfusion-dependent thalassemia (NTDT) is a moderate clinical form of the β-thal disease.

It has a broad clinical spectrum, spanning in severity from asymptomatic thalassemia minor to transfusion-dependent thalassemia major (TM) phenotype [4]. Thalassemia intermedia can result from the inheritance of one or two β-thal alleles [5‐8]. On the other hand, no clinical significance have been observed for delta-globin (δ-globin) gene mutations, but this gene has important relevance for the screening of β-thal carriers [9]. Hemoglobin A2 (Hb A2), is a minor adult hemoglobin, its levels ranged between (2–3.2%) of the total circulating haemoglobin in healthy adults [10]. The increase in Hb A2 level more than borderline levels is the most important parameter for the identification of thalassemia carriers [2]. The presence of δ-thal mutation, however, interferes with this typical β-thal phenotype, affecting population screening programs for β-thal carriers. Delta/beta-thalassemia (δβ-thal) is as a consequences of a deletion in both the delta and beta genes on chromosome 11. This deletion leads to increase of production of gamma globin (y-globin) gene, which increases the amount of fetal hemoglobin (Hb F). The δβ-thal heterozygotes clinically display characteristics of thalassemia minor. However, homozygous δβ-thal state could appear a clinical description of thalassemia intermedia (TI) with a mild anemia [11]. Some genome-wide association studies have reported that there are at least three major loci that play a major role in increasing Hb F levels [12]. One of them, is the − 158 C > T (HBG2: c.-211C > T) in the promoter gene Gamma 2 (locus XmnI). This locus simultaneously has an influence on up to 20–50% of Hb F variation in patients with β-thal or in healthy adults [13].

Here, we report a β-thal affected proband with low level of Hb A2 who had point mutation in codon 5 [−CT] combined with Hb Knossos codon 27 (G > T) on the β-globin gene associated with codon 59 [−A] mutation on the δ-globin gene. To the best of our knowledge, this is the first report which described the Hb Knossos /codon 5 [−CT] genotype associated with δ-thal mutations in Syrian patient.

Hb Knossos is a rare Hb variant in the world, it was first described in a Greek family [18]. The β⁺ Hb Knossos, codon 27 (G > T) (HBB: c.82G > T, p.Ala28Ser) activates a cryptic splice site in the β-globin gene which competes with the normal splice site thereby resulting in reduced production of Hb Knossos mRNA [19]. It is described to produce the classical phenotype of β-thal intermedia in association with β⁰-thal trait. Also, it has been reported in combination with different β-thal mutations like IVS-I-1 (G > A) [20] and IVS-I-110 (G > A) [21] causing a moderate phenotype, whereas its association with the IVS-II-745 (C > G) resulted in a major phenotype [22]. The β⁰ Codon 5 [−CT] mutation was well-known to complete inhibition of β-chain synthesis through the formation of a premature termination signal at codon 21. In our case, the combination of β⁺ Hb Knossos with β⁰ codon 5 [−CT] mutation is reported for the first time, and it leads to β-IT phenotype.

The δ-globin gene mutations have no clinical implication. However, the co-inheritance of δ-globin gene variant with β-thal may camouflage the β-thal carrier status by decreasing the Hb A2 levels [11, 23, 24]. The codon 59 [−A] mutation is one of the rare δ⁰-globin gene mutation, it deletes a single A in codon 59 leading to premature termination in codon 60. Hb Knossos mutation was reported to be linked to δ⁰-globin gene codon 59 [−A] mutation in the majority of North African and Mediterranean countries [25‐27]. This combination correlated to normal or borderline red blood cell indices and also with low Hb A2 levels [25, 28]. In this study, the δ⁰ codon 59 [−A] mutation was observed for the first time in a Syrian family, and it was associated with the β⁺ Hb Knossos mutation for the proband and mother. In addition, a coinheritance of β⁰ codon 5 [−CT] mutation with δ⁰ codon 59 [−A] mutation was observed for the proband. Two mutations shown to be inherited in trans, the proband inherited the β⁰ codon 5 [−CT] mutation from his father and the δ⁰ codon 59 [−A] mutation from his mother. On the other hand, the level of Hb F was (62.2%) for the proband, and the genotype of Xmn-I polymorphism was homozygous [TT]. This factor may be contributed to produce a high level of Hb F as previously reported [29, 30].

In conclusion, we present here a case of rare β⁺ Hb Knossos codon 27 (G > T) variant associated with β⁰ codon 5 [−CT] mutation in β-globin gene and δ⁰ codon 59 [−A] mutation in δ-globin gene which were found in Syrian male proband for the first time in a Syrian family. However, As Syria is one of the countries where β-thal is prevalent, δ-thal mutations should be investigated at the β-thal carriers when we have low level of Hb A2, due to interactions between these haemoglobinopathies which can failing to diagnose the β-thalassemia carriers.