Thalassemia intermedia: heterozygous beta-thalassemia and co-inheritance of an a gene triplication

Hematological and genetic profiles of persons with co-inherited heterozygous β-thalassemia and supernumerary α-globin genes

INTRODUCTION

Thalassemias are common monogenic autosomal recessive hemoglobin disorders. The usually asymptomatic heterozygotes (β-thalassemia traits, βTT) may rarely develop non-transfusion-dependent-thalassemia (NTDT) due to co-inheritance of supernumerary α-globin genes. Literature on phenotypic/genotypic features of these rare combinations is limited.

MATERIALS AND METHODS

We studied the demographic, clinical, and laboratory data from 47 persons with co-inherited βTT + supernumerary α-globin genes. HBB mutations were tested for by ARMS-PCR and/or Sanger sequencing, ααα^(anti3.7) /ααα^(anti4.2) and deletional α-thalassemia testing by multiplex gap-PCRs, and Xmn1^G γ genotyping by PCR-RFLP.

RESULTS

The 47 cases comprised 0.08% of 61 010 hemoglobinopathy screenings during the study period. Mean age was 31.9 ± 14.7 years (range 5.5-83 years), with 57.4% males. Thirty (63.8%) had NTDT-phenotype, 16 (34%) were asymptomatic/minimally symptomatic, and 1 became transfusion-dependent at the age of 20 years. Anemia/pallor and jaundice were the commonest complaints (76% each); 40% had required blood transfusions. Twenty-one had splenomegaly, 14 had hepatomegaly. Mean hemoglobin was 9.0 ± 1.9 g/dl (range 4.0-13.0). HbA2 was 5.1 ± 0.7% (3.4%-6.3%) and HbF% 4.2 ± 3.2% (0.5%-18.4%). Forty-four (93.6%) had ααα^anti3.7 , while 3 (6.4%) had ααα^anti4.2 triplications. HBB:c.92+5G>C (47%), HBB:c.27_28insG (14.9%), and HBB:c.47G>A (8.5%) were the commonest β-globin mutations. One case showed HBB:c.-138C>T (β⁺⁺ ), while the rest had β⁰ or severe-β⁺ mutations. Symptomatic cases had significantly lower hemoglobins and higher HbF% than asymptomatic ones.

CONCLUSION

This largest Indian and globally second-largest study reports the βTT + ααα^4.2 state for the first time in such genotypically-complex Indian cases. Supernumerary α-genes should be suspected in all βTT with disproportionate clinical symptoms, mild-to-moderately elevated HbF, and unexplained anisopoikilocytosis.

Alpha-globin gene triplication and its effect in beta-thalassemia carrier, sickle cell trait, and healthy individual

The genotype and phenotype correlation between coinheritance of heterozygous beta-thalassemia with the alpha-globin triplication is unclear. In this study we have investigated and reviewed alpha triplication frequency in beta-thalassemia carriers, sickle cell trait, and healthy individuals and its effect on hematological and phenotypical changes. In this study, 4005 beta-thalassemia carriers, 455 sickle cell trait, and 2000 healthy individuals were included. Molecular characterization of beta and alpha-thalassemia was performed. The frequencies of alpha-globin triplication in beta-thalassemia carriers, sickle cell trait, and healthy individuals were 67 (1.67%), 4 (0.88%), and 18 (0.9%), respectively. In total, the frequency of alpha-triplications is approximately 89 (1.39%) in Khuzestan province, South of Iran population. We have compared the average hematological parameters of beta-thalassemia carriers, sickle cell trait, and healthy individuals with and without alpha gene triplication. This mutation did not show any significant effect on the change of blood indices, neither in healthy individuals nor in sickle cell trait and beta-thalassemia carriers. Therefore, there is no need to take more notice of anti 3.7 mutation in beta-thalassemia carriers is opposed with some studies reported that the presence of excess alpha-globin genes in beta-thalassemia carriers can lead to the phenotype of beta-thalassemia intermedia. Therefore, not every individual with triplicated alpha globin coinherited with beta-thalassemia trait will have a significantly lower Hb than normal, and it is highly likely that none of them will need transfusion.

Interaction of an α-Globin Gene Triplication with β-Globin Gene Mutations in Iranian Patients with β-Thalassemia Intermedia

The 3.7 kb triplicated α-globin gene (ααα(anti 3.7)) mutation has been found in most populations. It results from an unequal crossover between misaligned homologous segments in the α-globin gene cluster during meiosis. The pathophysiology and clinical severity of β-thalassemia (β-thal) are associated with the degree of α chain imbalance. The excess of α-globin chains plays an important role in the pathophysiology of β-thal. When heterozygous/homozygous β-thal coexists with an α gene numerical alteration, the clinical and hematological phenotype of thalassemia could change to mild anemia in case of an α deletion (-α/αα) or severe anemia in the case of an α triplication (αα/ααα). The coexistence of an ααα(anti 3.7) triplication is considered an important factor in the severity of β-thal, exacerbating the phenotypic severity of β-thal by causing more globin chain imbalance. This study shows that the ααα(anti 3.7) triplication is an important factor in the causation of β-thal intermedia (β-TI) in heterozygous β-thal. This type of phenotype modification has rarely been observed and reported in the Iranian population. Here we report the coinheritance of a triplicated α-globin gene arrangement and heterozygous/homozygous β-thal in 23 cases, presenting with a β-TI or β-thal major (β-TM) phenotype. Some of these patients were considered to have a mild β-TI phenotype as they needed no blood transfusions; some occasionally received blood transfusions in their lifetime (for example on delivery) but some are dependent on regular blood transfusions (every 20 to 40 days). Our study was focused on the importance of detecting the α-globin gene triplication in genotype/phenotype prediction in Iranian thalassemia patients.

Sickle cell anemia--molecular diagnosis and prenatal counseling: SGPGI experience

OBJECTIVE

To study the issues and dilemmas in prenatal diagnosis of Sickle cell anemia (SCA) and to evaluate the role of genetic modifiers in counseling the families.

METHODS

The authors studied the genotype in 47 individuals with increased HbS and three representative families were taken as an example for describing various issues which need to be sorted out for appropriate counseling.

RESULTS

Of 47 individuals 24 were S beta thalassemia, 14 were homozygous sickle cell anemia (SS) and 9 were HbS trait. In the S beta thalassemia and homozygous SS cases, anemia was presenting manifestation in all. The transfusion requirement in these varied from 0-12 transfusions/ year. Hepatosplenomegaly was seen in 27 cases (71%) and only splenomegaly in 9 cases (23.7%). Jaundice was observed in 34 cases (84.2%). All the 47 subjects (including HbS trait) were studied by Hb Variant system and underwent DNA analysis for beta globin gene mutations, alpha globin gene number and XmnI polymorphism. One or two alpha gene deletion of 3.7 kb (-α3.7/αα or -α3.7/-α3.7) was found in 11 out of 47 cases whereas alpha triplication was found in 2 cases. 28 cases were heterozygous (+/-) for XmnI polymorphism, 9 were homozygous negative (-/-) and 10 were homozygous positive (+/+). Patients with SCA co-inherited with α-thalassemia have less hemolysis as revealed by lower reticulocyte counts than with normal alpha genotype. The authors further discuss the issues and dilemmas faced during prenatal counseling of three families during this study.

CONCLUSIONS

The knowledge of the relationship between genotype and phenotype, effect of the modifier genes has an important role in genetic counseling and for planning individualized treatment for sickle cell anemia.

Genotyping of alpha-thalassemia in microcytic hypochromic anemia patients from North India

Microcytic hypochromic anemia is a common condition in clinical practice and alpha-thalassemia has to be considered as a differential diagnosis. Molecular diagnosis of alpha-thalassemia is possible by polymerase chain reaction. The aim of this study was to evaluate the frequency of alpha-gene numbers in subjects with microcytosis. In total, 276 subjects with microcytic hypochromic anemia [MCV<80fl; MCH<27pg] were studied. These include 125 with thalassemia trait, 48 with thalassemia major, 26 with sickle-cell thalassemia, 15 with E beta-thalassemia, 40 with iron-deficiency anemia, 8 with another hemolytic anemia, and 14 patients with no definite diagnosis. Genotyping for -alpha3.7 deletion, -alpha4.2 deletion, Hb Constant Spring, and a-triplications was done with polymerase chain reaction. The overall frequency of -alpha3.7 deletion in 276 individuals is 12.7%. The calculated allele frequency for a-thalassemia is 0.09. The subgroup analysis showed that co-inheritance of a-deletion is more frequent with the sickle-cell mutation than in other groups. We were able to diagnose 1/3 of unexplained cases of microcytosis as a-thalassemia carriers. The a-gene mutation is quite common in the Indian subcontinent. Molecular genotyping of a-thalassemia helps to diagnose unexplained microcytosis, and thus prevents unnecessary iron supplementation.

Modifier Genes and Oligogenic Disease

It is now increasingly apparent that modifier genes have a considerable role to play in phenotypic variations of single-gene disorders. Intrafamilial variations, altered penetrance, and altered severity are now common features of single gene disorders because of the involvement of several genes in the expression of the disease phenotype. Oligogenic disorders occur because of a second gene modifying the action of a dominant gene. It is now certain that cancer occurs due to the action of the environment acting in combination with several genes. Although modifier genes make it impossible to predict phenotype from the genotype and cause considerable difficulties in genetic counseling, they have their uses. In the future, it is hoped that modifier genes will allow us to understand cell and protein interactions and thus allow us to understand the pathogenesis of disease.