2.4-kb Deletion of Α1-globin Gene (hba1) Identified in Two Chinese Patients with Hemoglobin H Disease

Two Chinese patients with mild and moderate hemoglobin (Hb) H disease were investigated for rare mutations in the α-globin genes (HBA1, HBA2) in addition to --(SEA) deletion. One patient was a 41-year old man with mild anemia (Hb 11.3 g/dl). Multiplex ligation-dependent probe amplification (MLPA) revealed a rare 2392-base deletion involving the entire HBA1. Mapping by gap-polymerase chain reaction (PCR) defined the exact breakpoints of this deletion (HBA1:g36859_39252del2392) and confirmed its identity with a recently reported HBA1 deletion found in Southern Chinese. The other patient was a 53-year old man with moderate anemia (Hb 9.5 g/dl). Automated direct nucleotide sequencing identified a novel single nucleotide deletion at codon 40 of HBA2 (HBA2:c123delG). This leads to a frameshift which modifies the C-terminal sequence to (40)Lys-Pro-Thr-Ser-Arg-Thr-Ser-Thr(47)COOH and the introduction of a stop codon TGA 23 nucleotides downstream. These two cases demonstrate the power of MLPA and direct nucleotide sequencing to detect and characterize rare and novel mutations. They also highlight the differential effect of HBA1 and HBA2 gene mutation on α thalassemia phenotype due to their different transcriptional activity. Hemoglobin H disease is typically caused by deletion of both HBA1 and HBA2 on one chromosome 16 and either a complete deletion or a point mutation in HBA1 or HBA2 on the other homologue. This results in the deletional and non-deletional form of Hb H disease, respectively. In the southern Chinese population, the overwhelming majority of the two-gene deletion in cis is --(SEA). Single-gene deletions -alpha(4.2) or -alpha(3.7) are found in deletional Hb H disease, while Hb Constant Spring (CS), Hb Quong Sze (QS) and codon 30 (ΔGAG) deletion predominate in the non-deletional form. More than 90% of Hb H disease can therefore be genotyped by a simple multiplex gap-PCR and allele-specific PCR priming approach targeted at these common mutations (1). In rare instances, these routine techniques fail to reveal the complete α-globin genotype and more empirical genotyping methods are indicated. We describe here the detection of two rare mutations in two patients with Hb H disease. Patient A was a 53-year old Chinese man with Hb 9.5 g/dL, mean corpuscular volume (MCV) 74.7 fL, mean corpuscular Hb (MCH) 19.3 pg and red cell distribution width (RDW) 21.6%. Supravital staining with brilliant cresyl blue showed 98% of red cells bearing Hb H inclusion bodies. Patient B was a 41-year old Chinese man with Hb 11.3 g/dL, MCV 73.2 fL, MCH 20.4 pg and RDW 22.4%. Hemoglobin H inclusion bodies were seen in 90% of red cells. Routine α-globin genotyping only detected heterozygous --(SEA) in both patients. Rare or novel mutations were suspected and automated direct nucleotide sequencing of HBA1 and HBA2 was performed as previously described (1). In Patient A, a hemizygous novel single nucleotide deletion was found in codon 40 of HBA2 (AAG to AA-; HBA2:c123delG), which causes a frameshift and creates a new TGA stop codon 23 nucleotide downstream (Figure 1). This predicts the production of a truncated polypeptide 47-amino acid in length with a carboxyl end Lys-Pro-Thr-Ser-Arg-Thr-Ser-Thr. However, the mutated mRNA is more likely to undergo nonsense-mediated decay without significant translation. No functional globin chain production is therefore expected from this novel mutation. HBB sequencing results were normal. No sequence variation was detected in Patient B by Sanger sequencing of HBA1, HBA2 and HBB. An unusual large deletion of HBA1 or HBA2 was suspected. Alpha MLPA was performed to screen for deletion in the α-globin gene cluster. Probes and reaction mixture for ligation and PCR were purchased from MRC-Holland (SALSA MLPA kit P140-B3 HBA, MRC-Holland, Amsterdam, the Netherlands). Testing and data analysis procedures were done as previously described (2). Apart from the heterozygous loss of probe products contributed by --(SEA), which spans from the probe binding site between pseudogenes HBA2P and HBA1P to the probe binding site 3.7 kb downstream of HBA1, there was further loss of products of all probes targeting from exon 1 of HBA1 to a site 0.5 kb downstream of HBA1. This suggests a deletion of the entire HBA1 in addition to --(SEA) (Figure 2). The exact breakpoints were mapped by a gap-PCR using the MLPA probes flanking the putative HBA1 deleted region (alpha MLPA 256 nucleotide probe as forward primer: 5' TTCTCTGCCCAAGGCAGCTTACCC 3'; alpha MLPA 310 nucleotide probe as reverse primer: 5' AGGTGTTTCTTCAGGGCAGTGAAC 3'). A 2.2-kb PCR product was obtained (Figure 3). Direct nucleotide sequencing of this PCR product identified the 5' breakpoint at nucleotide 36859 of NG_000006.1 GenBank reference sequence, which is 684 bp upstream of the start point of HBA1 whereas the 3' breakpoint lies at nucleotide 39252, which is 867 bp downstream of the end of HBA1. Due to sequence micro-homology flanking the breakpoints, alternative breakpoints at nucleotide 36853 and nucleotide 39245 can also generate the same deletion (Figure 4). Results confirmed the presence of a 2392-base deletion of HBA1. A search of the literature revealed three reports of this HBA1 deletion in Chinese (3-5). Simple heterozygotes are reported to have a very mild phenotype, which is consistent with a lesser contribution to globin production by HBA1 compared with HBA2. The mild deletional Hb H disease phenotype observed in our patient is also consistent with this. Notably, all three previously reported Hb H disease patients of this same genotype (compound heterozygous for –SEA and 2.4 kb HBA1 deletion) had moderate disease severity with Hb ranging from 8.3 to 9 g/dL. One of them was a teenage female with a very low MCV of 58 fL; the other was a pregnant woman, and the age and clinical details of the remaining patient were not stated. Other contributing factors of Hb level such as age, sex, iron status, physiological hemodilution and concurrent diseases might have affected the phenotype. These two cases demonstrate the power of direct nucleotide sequencing and MLPA in the detection and characterization of novel and rare thalassemic mutations. Moreover, they illustrate clearly the phenotypic difference between HBA1 and HBA2 mutations in the setting of Hb H disease. Finally, the study confirms independently the occurrence of the rare and mild 2.4-kb HBA1 deletion in Southern Chinese. Declaration of Interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article. Acknowledgements: The study was supported in part by a small project funding fromThe University of Hong Kong. REFERENCES1. Chan AY, So CC, Ma ES, Chan LC. A laboratory strategy for genotypinghaemoglobin H disease in the Chinese. J Clin Pathol 2007;60(8):931-4.2. So CC, So AC, Chan AY, Tsang ST, Ma ES, Chan LC. Detection andcharacterisation of beta-globin gene cluster deletions in Chinese using multiplexligation-dependent probe amplification. J Clin Pathol 2009;62(12):1107-11.3. Lin M, Wu JR, Huang Y, Lin F, Zhan XF, Lin CP, Tong X, Luo ZY, Yang HT,Yang LY, Wang Q, Zheng L, Zhong TY. Clinical and molecular characterization of arare 2.4 kb deletion causing alpha(+) thalassemia in a Chinese family. Blood CellsMol Dis 2012;49(2):83-4.4. Zhao JB, Zhao L, Fei YJ, Liu JC, Huisman TH. A novel alpha-thalassemia-2(-2.7-kb) observed in a Chinese patient with Hb H disease. Am J Hematol1991;38(3):248-9.5. Eng B, Walsh R, Walker L, Patterson M, Waye JS. Characterization of a raresingle alpha-globin gene deletion in a Chinese woman with Hb H disease.Hemoglobin 2005;29(4):297-9.