Variants in genetic modifiers of β-thalassemia can help to predict the major or intermedia type of the disease

The prevention of thalassemia

The thalassemias are among the most common inherited diseases worldwide, affecting individuals originating from the Mediterranean area, Middle East, Transcaucasia, Central Asia, Indian subcontinent, and Southeast Asia. As the diseases require long-term care, prevention of the homozygous state constitutes a major armament in the management. This article discusses the major prevention programs that are set up in many countries in Europe, Asia, and Australia, often drawing from the experience in Sardinia. These comprehensive programs involve carrier detections, molecular diagnostics, genetic counseling, and prenatal diagnosis. Variability of clinical severity can be attributable to interactions with α-thalassemia and mutations that increase fetal productions. Special methods that are currently quite expensive and not widely applicable are preimplantation and preconception diagnosis. The recent successful studies of fetal DNA in maternal plasma may allow future prenatal diagnosis that is noninvasive for the fetus.

Hydroxyurea treatment in β-thalassemia patients: to respond or not to respond?

Hydroxyurea (HU) is a drug that induces fetal hemoglobin production. As a result, HU is widely used to treat β-thalassemia (β-thal) patients. However, the response of these patients to HU varies. Some β-thal patients respond favorably to treatment while others do not respond at all. HU has a number of side-effects and therefore its targeted prescription is beneficial. Hence, identifying the genetic determinants which lead to the differential HU response is important. This review summarizes recent findings which have shed light on this topic. Special emphasis is given to the mechanisms and genetic loci which may govern these differences. These findings have helped identify several single nucleotide polymorphisms which associate with the response to HU in both β-thal and sickle cell disease patients.

Clinical experience with fetal hemoglobin induction therapy in patients with β-thalassemia

Recent molecular studies of fetal hemoglobin (HbF) regulation have reinvigorated the field and shown promise for the development of clinical HbF inducers to be used in patients with β-thalassemia and sickle cell disease. However, while numerous promising inducers of HbF have been studied in the past in β-thalassemia patient populations, with limited success in some cases, no universally effective agents have been found. Here we examine the clinical studies of such inducers in an attempt to systematically review the field. We examine trials of agents, including 5-azacytidine, hydroxyurea, and short-chain fatty acids. This review highlights the heterogeneity of clinical studies done on these agents, including both the patient populations examined and the study end points. By examining the published studies of these agents, we hope to provide a resource that will be valuable for the design of future studies of HbF inducers in β-thalassemia patient populations.

Genetic association studies in β-hemoglobinopathies

Characterization of the molecular basis of the β-thalassemias and sickle cell disease (SCD) clearly showed that individuals with the same β-globin genotypes can have extremely diverse clinical severity. Two key modifiers, an innate ability to produce fetal hemoglobin and coinheritance of α-thalassemia, both derived from family and population studies, affect the pathophysiology of both disorders at the primary level. In the past 2 decades, scientific research had applied genetic approaches to identify additional genetic modifiers. The review summarizes recent genetic studies and key genetic modifiers identified and traces the story of fetal hemoglobin genetics, which has led to an emerging network of globin gene regulation. The discoveries have provided insights on new targets for therapeutic intervention and raise possibilities of developing fetal hemoglobin predictive diagnostics for predicting disease severity in the newborn and for integration into prenatal diagnosis to better inform genetic counseling.

Recent advances in the molecular understanding of non-transfusion-dependent thalassemia

Thalassemias are a group of inherited autosomal recessive hematologic disorders that occur because of defects in the alpha (α)- and beta (β)-globin genes of adult hemoglobin (Hb). An imbalance in the synthesis of one or more of the globin chains can result in a wide spectrum of phenotypes depending on the type and amount of globin synthesized and additional genetic modifiers. In patients with thalassemia intermedia, a condition known as non-transfusion-dependent thalassemia (NTDT), transfusion requirements are absent or episodic. Non-transfusion-dependent thalassemia includes β-thalassemia intermedia, HbE β-thalassemia, and α-thalassemia intermedia, also known as Hb H disease. This article focuses on the molecular features and genetic mutations specific to NTDT.

Genetic modifiers of sickle cell disease

Sickle cell anemia is associated with unusual clinical heterogeneity for a Mendelian disorder. Fetal hemoglobin concentration and coincident α thalassemia, both which directly affect the sickle erythrocyte, are the major modulators of the phenotype of disease. Understanding the genetics underlying the heritable subphenotypes of sickle cell anemia would be prognostically useful, could inform personalized therapeutics, and might help the discovery of new "druggable" pathophysiologic targets. Genotype-phenotype association studies have been used to identify novel genetic modifiers. In the future, whole genome sequencing with its promise of discovering hitherto unsuspected variants could add to our understanding of the genetic modifiers of this disease.

Estimation of the difference in HbF expression due to loss of the 5' δ-globin BCL11A binding region

BCL11A was the focus of recent studies on its inhibiting effect when bound onto the β-globin cluster in the mechanism of hemoglobin switching and HbF downregulation. We examined a cohort of 10 patients displaying different HbF levels and short deletions within the γβ-δ intergenic region to find a possible correlation with the BCL11A binding site located 5' to the δ-globin gene. Precise characterization of deletions was achieved using a custom DNA-array chip and breakpoint sequencing. The α-globin cluster and major SNP associated with HbF expression were genotyped. Our results show that the loss of the BCL11A binding domain located 5' to the δ-globin gene is correlated with a strong HbF difference (mean+2.7 g/dL, ratio 2.81). This result provides evidence for the use of BCL11A level down-regulation or this domain blockage for new therapies in sickle cell disease and β-thalassemia major patients.

The XmnI and BCL11A single nucleotide polymorphisms may help predict hydroxyurea response in Iranian β-thalassemia patients

Hydroxyurea (HU), a drug which can reactivate fetal hemoglobin (Hb F) production, is frequently prescribed to β-thalassemia (β-thal) patients. However, transfusion requirements of only a subset of patients are reduced upon HU treatment. Because of its potential side-effects, targeted prescription of HU is imperative. To identify genetic markers that correlate with drug response, we have carried out a retrospective association study of single nucleotide polymorphisms (SNPs) in three Hb F quantitative trait loci (QTLs): the XmnI polymorphism, BCL11A, and the HBS1L-MYB intergenic region, with the response to HU in a cohort of 81 transfusion-dependent Iranian β-thal patients. An increase in blood transfusion intervals post-therapy was used to measure drug response. Our results suggest that presence of the XmnI T/T genotype or the BCL11A rs766432 C allele correlates strongly with response to HU (p <0.001). Accordingly, these markers may be used to accurately predict the HU response of Iranian β-thal patients.

Genetic modifiers of β-thalassemia and clinical severity as assessed by age at first transfusion

BACKGROUND

The clinical and hematologic features of β-thalassemia are modulated by different factors, resulting in a wide range of clinical severity. The main factors are the type of disease-causing mutation and the ability to produce α-globin and γ-globin chains. In the present study we investigated the respective contributions of known modifiers to the prediction of the clinical severity of β-thalassemia as assessed by the patients' age at first transfusion.

DESIGN AND METHODS

We studied the effect of seven loci in a cohort of 316 Sardinian patients with β(0)-thalassemia. In addition to characterizing the β-globin gene mutations, α-globin gene defects and HBG2:g.-158C>T polymorphism, we genotyped two different markers in the BCL11A gene and three in the HBS1L-MYB intergenic region using single nucleotide polymorphism microarrays, imputation and direct genotyping. We performed Cox proportional hazard analysis of the time to first transfusion.

RESULTS

According to the resulting model, we were able to explain phenotypic severity to a large extent (Harrell's concordance index=0.72; Cox & Snell R(2)=0.394) and demonstrated that most of the model's discriminatory ability is attributable to the genetic variants affecting fetal hemoglobin production (HBG2:g.-158C>T, BCL11A and HBS1L-MYB loci: C-index=0.68, R(2)=0.272), while the remaining is due to α-globin gene defects and gender. Consequently, significantly distinct survival curves can be described in our population.

CONCLUSIONS

This detailed analysis clarifies the impact of genetic modifiers on the clinical severity of the disease, measured by time to first transfusion, by determining their relative contributions in a homogeneous cohort of β(0)-thalassemia patients. It may also support clinical decisions regarding the beginning of transfusion therapy in patients with β-thalassemia.

Fetal hemoglobin levels and morbidity in untransfused patients with β-thalassemia intermedia

To evaluate the association between fetal hemoglobin (HbF) levels and morbidity in β-thalassemia intermedia (TI), we analyzed data from 63 untransfused patients who had also never received HbF induction therapy. Patient records were reviewed for any history of 10 predefined morbidities. Laboratory measurements for markers of ineffective erythropoiesis were also obtained. The mean age of patients was 32.1 years, 47.6% were males, and the median HbF level was 37.2%. HbF levels correlated positively with total hemoglobin, yet negatively with growth differentiation factor-15 and non-transferrin-bound iron levels. Median HbF levels were significantly lower in patients with the majority of evaluated morbidities than in those without. There was a strong negative adjusted linear correlation between the HbF level and the total number of morbidities (R(2) = 0.825, P < .001). The HbF threshold of 63.7% had 95.5% sensitivity and 100% specificity for ensuring absence of morbidity. There exists a strong association between HbF levels and morbidity in the subset of untransfused patients with TI.