Thalassemia, also called Mediterranean anemia, was first described by Cooley and Lee in 1925 (1). Thalassemia is a highly clinically heterogeneous group of autosomal recessive hereditary blood disorders, characterized by the reduced or absent synthesis of globin chains, leading to anemia and microcytosis (2). α- and β-Thalassemia are the most common forms of thalassemia, resulting from mutations in the α- and β-globin gene clusters (HBA1, HBA2, and HBB) on chromosome 16 and chromosome 11, respectively (2). Clinically, thalassemia can be divided into mild (thalassemia gene carriers), intermediate and severe thalassemia according to anemia status (2). Patients with mild thalassemia may show mild hypochromic microcytic anemia, those with intermediate thalassemia show obvious anemia symptoms and may require irregular blood transfusions, and those with severe thalassemia may develop edematous fetuses (α-thalassemia) or developmental jaundice, prognathism, and hepatosplenomegaly. Children with β-thalassemia major tend to die before the age of 5 without treatment. Patients with severe thalassemia depend heavily on blood transfusions to support life, which imposes substantial economic burdens on affected families and society.

It has been recently estimated that there are approximately 350 million carriers of hemoglobinopathies and thalassemias worldwide, of which 270 million are carriers of α-thalassemia and 80–90 million are carriers of β-thalassemia (3). A high prevalence of thalassemia has been mainly reported in tropical and subtropical regions, such as the Mediterranean region, the Middle East, North Africa, India, and Southeast Asia (4). In China, the overall prevalence of α-thalassemia, β-thalassemia, and both α- and β-thalassemia patients was 7.9% (95% confidence interval (CI): 5.54–10.23), 2.2% (95% CI: 1.93–2.48), and 0.5% (95% CI: 0.18–0.79), respectively (5). There were approximately 300,000 patients with severe thalassemia or intermediate thalassemia in China (6). Previous reports indicated a high incidence of thalassemia in southern China, mainly south of the Yangtze River, especially in Guangxi, Guangdong, Hainan, Chongqing, Sichuan, Guizhou, Yunnan, and Fujian Provinces. The incidence of thalassemia is relatively high in Hainan. The carrier rate of thalassemia in the global population is approximately 4.8% (7), while that of thalassemia in Hainan Province is approximately 15%, and could be as high as 55% in the ethnic minority population. As populations in different ethnic and geographical regions have different globin gene mutation spectra, a comprehensive evaluation of the molecular epidemiological characteristics of thalassemia is important for developing appropriate prevention strategies in regions with a high prevalence of thalassemia.

To date, there is no effective treatment for thalassemia, except gene therapy and bone marrow transplantation. To date, several countries have carried out comprehensive national prevention programs, including public education and awareness, carrier screening, genetic counseling, prenatal diagnosis, and preimplantation diagnosis, and have achieved great success in preventing the birth of children with thalassemia major, leading to a consistent decline in the birth rate of children with thalassemia major in some countries such as Greece, Italy, Iran, Pakistan, Singapore, and Thailand (8). In recent years, large-scale surveys of thalassemia have been conducted in different parts of China, including Hainan Province. However, the prevalence of thalassemia is still high. Furthermore, with economic improvement and population migration, thalassemia is spreading to all parts of China, including all parts of Hainan. Only a few studies have reported the prevalence rates and mutations associated with α- and β-thalassemia in this region (9–11), and the current epidemiological characteristics (molecular spectrum, ethnic, and regional) of thalassemia are not completely understood.

In this study, we analyzed α- and β-thalassemia genotypes in patients who underwent thalassemia screening (routine blood tests, hemoglobin electrophoresis) and genetic detection in the southern area of Hainan Province. The results of this study will reveal the detailed molecular spectrum, ethnic and geographical distribution of thalassemia, and provide theoretical bases for genetic counseling and preventive measures to reduce the number of cases of severe thalassemia.

Hainan Province has a high incidence of thalassemia, second only to Guangxi and Guangdong Province in China. In this study, we reported the molecular characteristics, ethnic distribution, and geographical distribution of thalassemias in the southern area of Hainan Province. A total of 6,924 thalassemia-positive subjects (70.6%) were identified among 9,813 suspected carriers of thalassemia in our hospital. Of the 6,924 thalassemia-positive subjects, 83.9% were α-thalassemia mutation carriers, 5.3% were β-thalassemia mutation carriers, and 10.7% were both α- and β-thalassemia mutation carriers. The results revealed that the carrier frequency was higher than those in previous reports in other provinces of China, such as Guangdong (11.6%) (12), Sichuan (3.4%) (13), Dongguan (Guangdong) (47.4%) (14), Chongqing (7.8%) (15), Wuhan (57.3%) (16), Jiangxi (50.3%) (17), Guangxi (50.1%) (18), Guangxi (67.0%) (19), Yunnan (31.9%) (20), and Hubei (34.9%) (21), but lower than those described in Yunan (84.2%) (22), Baise (Guangxi) (92.5%) (23), and Yulin (Guangxi) (87.1%) (24). This difference was probably due to the screening strategy. In the present study, the subjects were mainly selected as suspected thalassemia carriers, and most of them had microcytosis, while in some studies (12, 13), healthy subjects undergoing routine genetic screening were also included.

The genotype distribution in the current study was different from those in previous reports, as the proportion of the α-thalassemia mutation was nearly sixteen times that of the β-thalassemia mutation. The frequencies of α- and β-thalassemia were 83.9% and 5.3% in the total subjects in our study, respectively. Previous studies have shown that α-thalassemia is mainly caused by three types of gene deletions, including -α^3.7, -α^4.2, and --^SEA. In the current study, eight α-thalassemia deletion genotypes were identified, including -α^3.7/αα, -α^4.2/αα, -α^3.7/-α^4.2, -α^3.7/-α^3.7, --^SEA/αα, -α^4.2/-α^4.2, -α^3.7/--^SEA, and -α^4.2/--^SEA, which accounted for 69.2% of all α-thalassemia mutations in carriers. -α^3.7 and -α^4.2 deletions were the most common deletions in the southern part of Hainan. However, the --^SEA deletion was the most common α-thalassemia genotype in the southern region of China, such as Guangdong (14, 25), Guangxi (18, 19, 23, 24, 26), Yunnan (20, 22), Sichuan (13), Jiangxi (17), Hubei (16, 21) and Hunan Province (27). Wang et al. (11) reported that --^SEA/αα was identified in 39.1% of α-thalassemia mutation carriers in Hainan. However, --^SEA/αα was identified only in 6.0% of all α-thalassemia mutation carriers in the southern area of Hainan. Ethnic differences may also play a significant role in genetic differences, so we also analyzed thalassemia genotypes in the Han and the minority ethnic groups. In the present study, 74.1% of --^SEA/αα carriers were the Han people, and 19.7% were the Li people, which is consistent with previous studies (11, 28), indicating that the --^SEA/αα mutation is more common in the Han than the Li people.

β-Thalassemias are mainly caused by point mutations, while a few of them are caused by gene deletions. At present, 945 mutation types of β-thalassemia have been identified worldwide, and over 145 different β- thalassemia gene mutations have been detected in China. The most common mutations are β^CD41−42, β^CD17, β^{IVS−II−654}, β^−28M, β^CD71−72, β^−26M, β^IVS−I−1, and β^−29M, and these mutations account for more than 95.0% of all β mutations found in the Chinese population (29). In the current study, 13 genotypes were detected, and β^CD41−42/β^N (69.6%) was the most common β-thalassemia genotype in the southern part of Hainan, which was similar to that in an earlier study (28) and different from that in another study (11). The proportion of β^CD41−42 (88.0%) among β-thalassemia in the southern part of Hainan was the highest and was higher than those in other provinces of China (12, 16, 17, 30). In the Li people, nearly 97.0% of β-thalassemia mutations were β^CD41−42, whereas in the Han population, 71.3% of β-thalassemia mutations were β^CD41−42. The β^{IVS−2−654} mutation was identified in 2.4% of β-thalassemia patients in the southern part of Hainan, which was similar to that in Guangxi Province (2.7%) (23). However, it was the most common mutation in Wuhan (41.2%) (16), Fujian (41.2%) (30), and Hunan (33.7%) (27). β^CD17 was detected in 1.5% of β-thalassemia cases in the southern part of Hainan, which was lower than that in other provinces of southern China, such as Guangdong (12), Guangxi (18, 19, 23), Yunnan (20, 22), and Sichuan (13). It is the most common mutation in Yunan (20), Guizhou (31), and Baise (Guangxi Province) (23). This result may be explained by the distinctive customs and identities of different ethnicities. Due to special geographical locations, customs, and cultures, the proportion of intermarriages between some regions of Hainan Province and other regions is relatively small, which consequently leads to an overlapping distribution of thalassemia genes and results in a high number of thalassemia carriers. Compared with other countries, the most frequent genotype of β-thalassemia was similar to that in Thailand (23), while it was different from Pakistan (24), Syria (25), Iran (20), Vietnam (26) Malaysia (27), and India (28).

Among α-thalassemia mutations, -α^4.2 was the most common mutation in Ledong, Lingshui, Baoting, and Wuzhishan, -α^3.7 was the most common mutation in Sanya, which was different from those in previous research in the corresponding city of Hainan Province (11). Among the β-thalassemia mutations, β^CD41−42 accounted for the largest proportion among the five regions, which is consistent with a previous study that reported it to be the most frequent genotype in Sanya, Ledong, Lingshui, Baoting, and Wuzhishan of Hainan Province (11). These findings may be explained by the migration of the population and the special location of Hainan Province, which is located at the southeastern end of China.

The genotypes of α- and β-thalassemia in Hainan Province in China are characterized by a wide distribution, a high carrier rate, genetic heterogeneity, significant geographical differences and ethnic specificity. This study enriches the knowledge of the thalassemia mutation spectrum in the Chinese people and provides valuable information for genetic counseling, prenatal diagnosis, and prevention of thalassemia in Hainan Province. We also recommend further studies on contributing factors that can affect the accessibility of thalassemia interventions to provide a scientific basis for government decision-making.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by the Ethics Committee of Sanya Women and Children's Hospital. The patients/participants provided their written informed consent to participate in this study.

GY, ZL, and HW conceived and designed the project. YY, CJL, YG, CYL, DL, JW, and HW collected the samples and clinical information. YY and GY analyzed and interpreted the data. GY wrote the manuscript. All authors read and approved the final manuscript.

The project was supported by grant from the Health Commission of Hainan Province, China (20A200237), grant from Hainan Province Clinical Medical Center, and the Golden Coconut Seed Funding (JYZZ-ZD-202102).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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