A Novel β+-Thalassemia Mutation (Codon 10 GCC → GCA) and a Rare Transcriptional Mutation (−28A → G) in Indians

Molecular basis of β-thalassemia in the western province of Saudi Arabia: identification of rare β-thalassemia mutations

This study aimed at the identification of the spectrum of mutations in patients with β-thalassemia (β-thal) in the western province of Saudi Arabia. Screening for the mutations was done using the polymerase chain reaction-amplification refractory mutation system (PCR-ARMS) technique to test for 12 mutations, and direct automated DNA sequencing for the unknown samples. The study included 172 patients; of these 15 patients had sickle cell anemia and one Hb S [β6(A3)Glu→Val, GAG>GTG]/β-thal. A total of 23 mutations were identified to cause the disease in the western area. Seven common mutations were responsible for the β-thal alleles in 78% of patients and could be detected by the ARMS technique: IVS-II-1 (G>A), IVS-I-110 (G>A), IVS-I-5 (G>C), codon 39 (C>T), codon 26 (G>A) [Hb E or β26(B8)Glu→Lys, GAG>AAG], frameshift codons (FSC) 8/9 (+G), and IVS-I-1 (G>A). DNA sequencing of uncharacterized alleles detected eight less common mutations: FSC 41/42 (-TCTT), IVS-I 25 bp deletion, codon 37 (G>A), FSC 44 (-C), Cap site +1 (A>C), IVS-I-6 (T>C), FSC 5 (-CT) and IVS-I-1 (G>T), and eight rare mutations: -87 (C>G), initiation codon -1 (T>G), codon 15 (G>A), FSC 16 (-C), FSC 20/21 (+G), codon 27 (G>A), IVS-I-130 (G>C) and IVS-II-837 (A>C). Four alleles were normal by DNA sequencing. Genetic heterogeneity was observed in this study, 10 mutations were of Asian or Asian/Indian origin, two were Kurdish, one Chinese, one Turkish, one Saudi, and the remainder were of Mediterranean origin. The presence of a large population of immigrants in the western province is responsible for the great heterogeneity at the molecular level, and for the difference observed in the frequencies of mutations from those reported in the eastern province of Saudi Arabia. Screening for β-thal mutations using PCR-ARMS for the seven most frequent mutations in the Saudi population followed by DNA sequencing of the unknown alleles could be useful for the implementation of a strategy for carrier detection and preimplantation genetic diagnosis in high risk families.

Impact of β globin gene mutations on the clinical phenotype of β thalassemia in India

The beta thalassemias are one of the commonest group of autosomal recessive disorders in India. Although majority of patients are severe and transfusion-dependent, about 10-15% of cases have a milder phenotype. We evaluated the role of beta gene mutations in modulating the clinical presentation of 342 beta thalassemia patients which included 278 severe thalassemia major (TM) and 64 thalassemia intermedia (TI) cases (severe TI: 27; mild TI: 37) from this region. Thirteen beta thalassemia mutations were characterized by reverse dot blot hybridization or amplification refractory mutation system (ARMS); denaturing gradient gel electrophoresis (DGGE) analysis and DNA sequencing helped to characterize the remaining nine mutations. Majority of the patients in the thalassemia major and thalassemia intermedia groups had severe beta+ or beta0 mutations. IVS 1-5 (G-->C) was the commonest mutation in the three groups. The six severe and common Indian mutations [(IVS 1-5 (G-->C), 619 bp deletion, IVS 1-1 (G-->T), codons 8/9 (+G), codon 15 (G-->A), codons 41/42 (-CTTT)] accounted for 92.0% of molecular lesions in the thalassemia major group, 86.8% in the severe TI group, and 72.9% in the mild TI group. IVS 1-1 (G-->T) and codon 30 (G-->C) were significantly more common in thalassemia intermedia cases. The mild capsite +1 (A-->C) mutation was present in both severe and mild cases. Three other mild beta+ mutations, poly A (T-->C), -28 (A-->G), and -88 (C-->T), were seen only in the thalassemia intermedia cases. These four mild mutations in combination with other severe beta+ or beta0 mutations resulted in a very variable clinical presentation. This study reveals that, in majority of Indian patients, the beta genotype cannot predict the phenotype.

A multi-center study in order to further define the molecular basis of beta-thalassemia in Thailand, Pakistan, Sri Lanka, Mauritius, Syria, and India, and to develop a simple molecular diagnostic strategy by amplification refractory mutation system-polymerase chain reaction

The spectrum of the beta-thalassemia mutations of Thailand, Pakistan, India, Sri Lanka, Mauritius and Syria has been further characterized by a multi-center study of 1,235 transfusion-dependent patients, and the mutations discovered used to assess the fidelity of a simple diagnostic strategy. A total of 44 beta-thalassemia mutations were identified either by allele-specific oligonucleotide hybridization, amplification with allele-specific primers, or DNA sequencing of amplified product. The results confirm and extend earlier findings for Thailand, Pakistan, India, Mauritius and Syria. This is the first detailed report of the spectrum of mutations for Sri Lanka. Two novel mutations were identified, codon 55 (-A) and IVS-I-129 (A-->C), both found in Sri Lankan patients. Two beta-thalassemia mutations were found to coexist in one beta-globin gene: Sri Lankan patients homozygous for the beta0 codon 16 (-C) frameshift were also homozygous for the beta+ codon 10 (C-->A) mutation. Studies of Sri Lankan, Pakistani, and Indian carriers suggest the codon 10 (C-->A) mutation is just a rare polymorphism on an ancestral allele, on which the beta0 codon 16 (-C) mutation has arisen. Each country was found to have only a few common mutations accounting for 70% or more of the beta-thalassemia alleles. A panel of primers to diagnose the majority of the mutations by the amplification refractory mutation system was developed, enabling a simple molecular diagnostic strategy to be introduced for each country participating in the multi-center study.

Distribution of beta-thalassemia mutations in the Indian population referred to a diagnostic center

Hemoglobinopathies are the most commonly inherited genetic disorders in India. Population screening has identified certain communities in India with high risk of beta-thalassemia, the prevalence of carrier status in some being as high as 17%. Over a period of 6 years we have conducted DNA analysis on 1,233 carriers of 23 beta-thalassemia mutations and Hb E, using the amplification refractory mutation system. The studies included analyses of five common mutations for Asian Indians, namely IVS-I-5 (G-->C), 619 bp deletion, IVS-I-1 (G-->T), and the frameshifts at codons 8/9 (+G) and 41/42 (-TTCT). The occurrence of these was seen in 1,066 (86.45%) of the carriers referred to us, the percentage of mutations varying between 5.03-42.58%. We found codon 15 (TGG-->TAG) in 47 (3.81%) samples which was also considered a common mutation in the Indian population. Rare beta-thalassemia mutations were found in 87 (7.06%) individuals. We have designed five new primers and modified four primers for nine rare mutations. These were seen in nine (0.73%) samples. The beta-thalassemia anomaly in 33 (2.68%) carriers remained uncharacterized. State-wide and community-wide distribution patterns of mutations indicated that IVS-I-5 (G-->C) is the most common beta-thalassemia allele in the Indian population. Sindhis settled in Gujrat, and Maharashtra and Lohanas from Gujrat showed a prevalence of the 619 bp deletion mutation in 49.2 and 45.5% carriers, respectively. High frequency of the IVS-I-1 (G-->T) mutation was also found in Sindhis (25.5%), Punjabi Hindus (34.7%), and Lohanas (31.2%). These studies of mutation patterns in different communities have helped us in the quick identification of beta-thalassemia mutations for prenatal diagnosis.

Potential of denaturing gradient gel electrophoresis for scanning of beta-thalassemia mutations in India

Over the last few years, substantial progress has been made in developing strategies for the detection and characterization of various mutations causing beta-thalassemia. The Indian population comprises of numerous endogamous caste groups and beta-thalassemia is seen in almost all of them. Knowledge of the spectrum of beta-thalassemia mutations in the population is a prerequisite for successful implementation of a prevention programme. Among the different approaches available today, Denaturing Gradient Gel Electrophoresis (DGGE) offers a valid technical approach which is applicable for screening of known mutants and polymorphisms as well as in locating regions of DNA bearing unknown mutations. We analysed 356 unrelated beta-thalassemia heterozygotes by DGGE and detected 30 anomalous DGGE patterns. Fifteen mutations were characterized after sequencing 25 anomalous patterns. Of these, codon 10(GCC --> GCA) is a recently reported novel beta-thalassemia mutation while -28(A --> G) and codon 121(G --> T) are being reported for the first time in the Indian population. HbS and HbE also showed two anomalous DGGE patterns each. Framework (FW) linkage studies showed that four mutations were associated with different beta-globin gene frameworks. Linkage of IVSI-5(G --> C) and cap site +1(A --> C) to FW2 and 619-bp deletion to FW1 is being observed for the first time. Multiple DGGE patterns corresponding to the same mutation is one of the major drawbacks of this technique. In spite of this, if sufficient preliminary work has been carried out to compile a comprehensive catalogue of DGGE patterns; this is a powerful approach to characterize the mutation or to localize a small region of DNA in the case of rarer mutations.

Beta-thalassemias: expression, molecular mechanisms and mutations in Indians

The beta-thalassemias are a heterogenous group of inherited disorders of hemoglobin (Hb) synthesis characterized by a reduction (beta+) or absence (beta zero) of synthesis of the beta globin chains of Hb, resulting in an imbalanced chain synthesis. To understand their expression and molecular basis in Indians, it is essential to review briefly the genetic control of normal Hb production and the structure, organization and regulation of different globin genes. The Indian beta-thalassemia mutations and strategies for prevention are described.