Genetic and phenotypic analysis of a rare asymptomatic case of a homozygous Chinese Gγ+(Aγδβ)0-thalassemia deletion in a Chinese family
Introduction
Thalassemia, one of the most common monogenetic disorders, is widely distributed throughout the world. Thalassemia is mainly classified into α-, β-, δβ-thalassemia and hereditary persistence of fetal hemoglobin (HPFH) according to the type of defective globin gene involved. δβ-Thalassemia is caused by large deletions in both the δ- and β-globin genes, which leads to decreased or absent synthesis of δ- and β-globin chains. As a compensatory mechanism, enhanced expression of fetal γ-chain synthesis increases fetal hemoglobin (Hb F) levels in adult life [1]. With high levels of Hb F, the phenotype of δβ-thalassemia heterozygotes is similar to that of those with the β-thalassemia trait, whereas homozygotes for δβ-thalassemia tend to have variable severities ranging from intermedia anemia to thalassemia major [2], [3], [4], [5], [6], [7]. Chinese Gγ+(Aγδβ)0-thalassemia (HGVS: NC_000011.9:g.5191125_5270051delinsGTGATAGTTTGCTGAGGATGATAGTTTCCAGCTTCA), which involves an approximately 80-kb deletion, is the most common large deletion of the β-globin cluster in southern China [8], [9], [10]. Nonetheless, individuals who are homozygous for Chinese Gγ+(Aγδβ)0-thalassemia have rarely been detected. On the basis of previously reported cases, the clinical phenotype of the homozygous state manifests intermedia or severe anemia [11], [12]. Surprisingly, we identified a patient with Chinese Gγ+(Aγδβ)0-thalassemia in the homozygous state with no clinical manifestation.
β-Thalassemia patients with the same genotype may exhibit remarkable phenotypic diversity, which is an area of intense research. Previous studies have clearly suggested that genetic defects of the β-globin gene are not the only determinant of clinical outcomes and that other conditions that reduce α- versus non-α-globin chain imbalance or the presence of genetic variants that upregulate γ-globin expression can also alter phenotypes. Similarly, the disease severity of δβ-thalassemia is influenced by these modifications. In recent years, several genetic polymorphisms and genome-wide association studies have identified quantitative trait loci associated with increased Hb F levels, including single-nucleotide polymorphisms (SNPs) in the HBG1 or HBG2 gene in the β-globin cluster, the HBS1L-MYB intergenic region on chromosome 6q23, and the gene encoding B-cell lymphoma/leukemia 11A protein (BCL11A) on chromosome 2p16, and mutations in Kruppel-like factor (KLF1) on chromosome 19p13 [13], [14], [15].
Here, to explore the reasons why the present asymptomatic case differs from cases previously reported, the genotype of six members of the proband’s family was examined with regard to the thalassemia definition and evaluated for selected modifier genes.
Section snippets
Materials and methods
The proband (Fig. 1; Ⅱ3) was a 25-year-old woman from Dongyuan City, Guangdong Province. She attended the antenatal clinic for her first pregnancy and was found by the routine thalassemia screening program to express 100% Hb F. She had no history of anemia, and a physical examination revealed normal results. Her husband was negative for thalassemia based on testing at a local hospital. All members of her family agreed to participate in our study and signed informed consent. Peripheral blood
Results
The hematological indices and genotypes of the family members are summarized in Table 2. Among them, four members, including the proband, showed a decreased mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) level. Quantitative hemoglobin assessment of the proband revealed the complete absence of Hb A and Hb A2, with Hb F being the only detectable Hb (Fig. 2). The suspension-array system determined that the four members who had hypochromic microcytic red cells all carried
Discussion
There are at least 80 different large deletions in the β-globin gene cluster described in the human hemoglobin variant (HbVar) database, and approximately 10 types of δβ-thalassemia have been reported worldwide [19]. Several types of δβ-thalassemia have been distinguished in the Chinese population. Previous epidemiological studies have reported a higher carrier rate of Chinese Gγ+(Aγδβ)0-thalassemia than other types [8], [9]. Chinese Gγ+(Aγδβ)0-thalassemia (HGVS:
Funding
National Key Research and Development Program of China, 2016YFC1000700, 2016YFC1000703.
Guangdong Medical Science and Technology Research Project, 2016118171659322.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Li Du and Danqing Qin contributed equally to this work.