Molecular Basis and Genetic Modifiers of Thalassemia

Beta thalassemia syndromes: New insights

Beta thalassemia (β-thalassemia) syndromes are a heterogeneous group of inherited hemoglobinopathies caused by molecular defects in the beta-globin gene that lead to the impaired synthesis of beta-globin chains of the hemoglobin. The hallmarks of the disease include ineffective erythropoiesis, chronic hemolytic anemia, and iron overload. Clinical presentation ranges from asymptomatic carriers to severe anemia requiring lifelong blood transfusions with subsequent devastating complications. The management of patients with severe β-thalassemia represents a global health problem, particularly in low-income countries. Until recently, management strategies were limited to regular transfusions and iron chelation therapy, with allogeneic hematopoietic stem cell transplantation available only for a subset of patients. Better understanding of the underlying pathophysiological mechanisms of β-thalassemia syndromes and associated clinical phenotypes has paved the way for novel therapeutic options, including pharmacologic enhancers of effective erythropoiesis and gene therapy.

A Variable Clinical Presentation of Hemoglobin City of Hope

Hemoglobin City of Hope (Hb-COH), NC_000011.9(NM_000518.5):c.208G > A; NP_000509.1:p.(Gly70Ser), has rarely been described. The presentation ranges from asymptomatic heterozygosity to significant anemia in patients carrying an additional pathogenic variant in β-globin. To elucidate the clinical spectrum of Hb-COH, we analyzed 31 individuals carrying the variant, including, for the first time, homozygous individuals. Seven patients who were compound heterozygous for Hb-COH and an additional variant in β-globin, presented with mild-to-severe microcytic anemia and elevated hemoglobin-A2. Three (43%) of these also had elevated fetal hemoglobin, but none required blood transfusions. Seven patients coinherited Hb-COH with an -α3.7-deletion (NG_000006.1:g.34247_38050del), their presentation ranged from mild microcytic anemia to normal blood counts. Three homozygous and 14 heterozygous individuals for Hb-COH had normal blood counts. Most Hb-COH alleles whose origin was traceable were from Ashkenazi Jews (70.4%). To conclude, while isolated Hb-COH appears asymptomatic even in the homozygous state, it may cause significant anemia when coinherited with an additional pathogenic variant in β-globin. Understanding the full impact of Hb-COH is crucial for optimal patient management and for genetic counseling.

Genetic Modifiers of Hemoglobin Expression from a Clinical Perspective in Hemoglobinopathy Patients with Beta Thalassemia and Sickle Cell Disease

Hemoglobinopathies, namely β-thalassemia and sickle cell disease (SCD), are hereditary diseases, characterized by molecular genetic aberrations in the beta chains of hemoglobin. These defects affect the normal production of hemoglobin with severe anemia due to less or no amount of beta globins in patients with β-thalassemia (quantitative disorder), while SCD is a serious disease in which a mutated form of hemoglobin distorts the red blood cells into a crescent shape at low oxygen levels (qualitative disorder). Despite the revolutionary progress in recent years with the approval of gene therapy and gene editing for specific patients, there is an unmet need for highlighting the mechanisms influencing hemoglobin production and for the development of novel drugs and targeted therapies. The identification of the transcription factors and other genetic modifiers of hemoglobin expression is of utmost importance for discovering novel therapeutic approaches for patients with hemoglobinopathies. The aim of this review is to describe these complex molecular mechanisms and pathways affecting hemoglobin expression and to highlight the relevant investigational approaches or pharmaceutical interventions focusing on restoring the hemoglobin normal function by linking the molecular background of the disease with the clinical perspective. All the associated drugs increasing the hemoglobin expression in patients with hemoglobinopathies, along with gene therapy and gene editing, are also discussed.

A novel case of Hb Bart's hydrops fetalis following prenatal diagnosis: Case report from Huizhou, China

Objective

Presentation of a novel case of a patient with Hb Bart's hydrops fetalis, which was accurately identified by SMRT sequencing leading to expand the mutation spectrum of α-thalassemia.

Case report

A 26-year-old pregnant woman and her husband underwent molecular analysis of thalassemia due to abnormal hematological results. The molecular analysis showed that the pregnant woman carried -α3.7/--SEA, while her husband exhibited a negative result. Accordingly, the pregnant woman continued the pregnancy until the 19-week gestational age. She was subsequently referred to our department for genetic counseling due to abnormal ultrasound findings in the fetus. A novel deletional α-thal mutation was detected for the husband by MLPA, and the precise location of the mutation was determined through SMRT sequencing, which revealed a 45.2 kb deletion. Later, an interventional umbilical cord blood puncture was offered for the pregnant woman. The cord blood was subjected to capillary electrophoresis, which revealed apparent Hb Bart's and Hb Portland peaks associated with Hb Bart's hydrops fetalis syndrome.

Conclusion

It is imperative that Hb Bart's hydrops fetalis syndrome be diagnosed with the utmost expediency. If results of molecular analysis are not consistent with the clinical hematological findings, the presence of a novel thalassemia could be suspected. To identify the novel genotype, the SMRT sequencing represents an effective method for achieving an accurate diagnosis.

Associations between BCL11A and HBS1L-MYB polymorphisms and thalassemia risk

Objectives

This study investigated the associations of the rs4671393, rs1427407, and rs11886868 genetic variants of the BCL11A gene and the rs9399137 variant of the HBS1L-MYB gene with thalassemia in patients from the population of Punjab, Pakistan.

Methods

A cohort of 600 participants, comprising 300 patients with thalassemia and 300 age- and sex-matched healthy controls, was recruited from various hospitals in Punjab, Pakistan. DNA was extracted from whole blood samples from all participants. Specific DNA regions containing four genetic variants of interest were amplified with polymerase chain reaction.

Results

The genotypic frequencies of the rs4671393 SNP of BCL11A indicated that the heterozygous (AG) genotype of this SNP was significantly associated with a nearly two-fold increased thalassemia risk, with respect to the control group (OR = 1.77; 95% CI = 1.77-2.80; p = 0.01). Combining all genotypes into a joint model further confirmed their significant association with thalassemia risk. Similarly to the findings for rs4671393, the heterozygous (CT) genotype of rs11886868 exhibited a significant association with thalassemia risk (approximately two-fold increased risk. Analysis of both genotypes together revealed a marginally significant association (one-fold increased risk) with thalassemia in individuals carrying any variant allele of rs11886868. The allelic distribution of the rs1427407 SNP of BCL11A indicated that the heterozygous (GT) genotype of this SNP was significantly associated with thalassemia (approximately two-fold increased risk, with respect to the control group). Combining all genotypes into a joint model confirmed a significant association between the presence of any variant allele of rs1427407 and thalassemia (two-fold increased risk). The genotypic distribution frequencies of the heterozygous (CT) genotype for the rs9399137 SNP of HBS1L-MYB was similar to that of rs1427407, and exhibited a highly significant association with thalassemia risk (nearly two-fold increased risk). Analysis of both genotypes together revealed a significant association with thalassemia risk (one-fold increase) for individuals carrying any variant allele of rs9399137.

Conclusion

BCL11A and HBS1L-MYB polymorphisms were significantly associated with increased thalassemia risk.

α0-Thalassemia Caused by a Novel α-Globin Gene Cluster Deletion (-LB) Found in a Chinese Family

We report a novel large α-globin gene cluster deletion in a Chinese family from the Guangxi Zhuang Autonomous Regionfor the first time. The proband was a 20-year-old male who presented with microcytic hypochromatosis. Routine genetic analysis showed none of the common mutations in theα-globin and β-globin genes. Multiplex ligation-dependent probe amplification (MLPA) of the α-globin chain revealed there was a large deletion, which removed the entire HBA2 and HBA1 genes, HBQ gene, HBZ gene, and major regulatory element HS-40, eliminating more than 134 kb from the α-globin chain. Subsequently, pedigree analysis revealed that the proband inherited the novel deletion from his father. By consultation of literature and databases, it was confirmed as a hitherto undescribed chain deletion and named Laibin deletion (-LB) for the origin of the proband.

Development of pre-implantation genetic testing protocol for monogenic disorders (PGT-M) of Hb H disease

Hb H disease is the most severe form of α-thalassemia compatible with post-natal life. Compound heterozygous α0-thalassemia- SEA deletion/α+-thalassemia- 3.7kb deletion is the commonest cause of Hb H disease in Thailand. Preimplantation genetics testing for monogenic disorders (PGT-M) is an alternative for couples at risk of the disorder to begin a pregnancy with a healthy baby. This study aims to develop a novel PCR protocol for PGT-M of Hb H disease- SEA/-3.7kb using multiplex fluorescent PCR. A novel set of primers for α+-thalassemia- 3.7kb deletion was developed and tested. The PCR protocol for α0-thalassemia- SEA deletion was combined for Hb H disease- SEA/-3.7kb genotyping. The PCR protocols were applied to genomic DNA extracted from subjects with different thalassemia genotypes and on whole genome amplification (WGA) products from clinical PGT-M cycles of the families at risk of Hb Bart's. The results were compared and discussed. The results showed three PCR products from α+-thalassemia- 3.7kb primer set, and three from α0thalassemiaSEA primer set. The results were consistent with the known thalassemia genotypes. The novel -α3.7 primers protocol was also tested on 37 WGA products from clinical PGT-M cycles giving accurate genotyping results and a satisfying amplification efficiency with the ADO rates of 2.7%, 0%, and 0% for HBA2, HBA1, and internal control fragments, respectively. This novel PCR protocol can precisely distinguish Hb H disease- SEA/-3.7kb from other genotypes. Additionally, this is the first PCR protocol for Hb H disease- SEA/-3.7kb which is optimal for PGT-M.

The Relevance of β-Thalassemia Heterozygosity in Pediatric Clinical Practice: Croatian Experience

(1) Background: Thalassemia syndromes are common monogenic disorders that represent a significant global health issue. No systematic epidemiological or molecular investigations on thalassemias in the Croatian population have been reported to date. (2) Methods: This prospective study included 70 children with a presumptive diagnosis of thalassemia and their 42 first-degree relatives. Molecular characterization was performed using direct sequencing and gap-PCR methods. (3) Results: We identified 46 (30 children and 16 first-degree relatives) β-thalassemia heterozygous carriers from 24 unrelated families, carrying eight different mutations and one hemoglobin variant. Five variants account for approximately 85% of all affected β-globin alleles: Hb Lepore-Boston-Washington (32.6%), HBB:c.93-21G>A (19.6%), HBB:c.315+1G>A (13.1%), HBB:c.92+1G>A (10.9%), and HBB:c.92+6T>C (8.7%) variants. (4) Conclusions: β-thalassemia carriers need more detailed genetic profiling since genetic modifiers can significantly impact their phenotype. Our study provides important new insights into the relevance of β-thalassemia heterozygosity in pediatric clinical practice.

Thalassemia screening by third-generation sequencing: Pilot study in a Thai population

Background

Conventional thalassemia screening takes a stepwise approach and has limitations in comprehensively identifying all spectrums of mutations. This study aimed to investigate the performance of third-generation sequencing (TGS) compared to conventional molecular testing.

Methods

TGS was applied to validate all known variants detected by conventional testing and to detect missing variants in undiagnosed cases. The study was conducted at Maharaj Nakorn Chiang Mai Hospital between December 2021 and April 2022.

Results

In total, 19 cases were included in this study, among which 52.6% (10/19) had known thalassemia variants, while 47.7% (9/19) cases were undiagnosed by conventional methods. All 16 variants previously detected were validated by TGS, and TGS additionally detected 43.8% (7/16) thalassemia variants for 36.8% (7/19) cases.

Conclusion

TGS could provide additional genetic diagnoses compared with conventional methods. Further cost-effectiveness studies with a larger sample size are needed to explore the role of TGS in clinical practices.

Impact of genotype on multi-organ iron and complications in patients with non-transfusion-dependent β-thalassemia intermedia

We evaluated the impact of the genotype on clinical and hematochemical features, hepatic and cardiac iron levels, and endocrine, hepatic, and cardiovascular complications in non-transfusion-dependent (NTD) β-thalassemia intermedia (TI) patients. Sixty patients (39.09 ± 11.11 years, 29 females) consecutively enrolled in the Myocardial Iron Overload in Thalassemia project underwent Magnetic Resonance Imaging to quantify iron overload, biventricular function parameters, and atrial areas and to detect replacement myocardial fibrosis. Three groups of patients were identified: homozygous β+ (N = 18), heterozygous β0β+ (N = 22), and homozygous β0 (N = 20). The groups were homogeneous for sex, age, splenectomy, hematochemical parameters, chelation therapy, and iron levels. The homozygous β° genotype was associated with significantly higher biventricular end-diastolic and end-systolic volume indexes and bi-atrial area indexes. No difference was detected in biventricular ejection fractions or myocardial fibrosis. Extramedullary hematopoiesis and leg ulcers were significantly more frequent in the homozygous β° group compared to the homozygous β+ group. No association was detected between genotype and liver cirrhosis, hypogonadism, hypothyroidism, osteoporosis, heart failure, arrhythmias, and pulmonary hypertension. Heart remodelling related to a high cardiac output state cardiomyopathy, extramedullary hematopoiesis, and leg ulcers were more pronounced in patients with the homozygous β° genotype compared to the other genotypes analyzed. The knowledge of the genotype can assist in the clinical management of NTD β-TI patients.

Αlpha-thalassemia: A practical overview

α-Thalassemia is an inherited blood disorder characterized by decreased synthesis of α-globin chains that results in an imbalance of α and β globin and thus varying degrees of ineffective erythropoiesis, decreased red blood cell (RBC) survival, chronic hemolytic anemia, and subsequent comorbidities. Clinical presentation varies depending on the genotype, ranging from a silent or mild carrier state to severe, transfusion-dependent or lethal disease. Management of patients with α-thalassemia is primarily supportive, addressing either symptoms (eg, RBC transfusions for anemia), complications of the disease, or its transfusion-dependence (eg, chelation therapy for iron overload). Several novel therapies are also in development, including curative gene manipulation techniques and disease modifying agents that target ineffective erythropoiesis and chronic hemolytic anemia. This review of α-thalassemia and its various manifestations provides practical information for clinicians who practice beyond those regions where it is found with high frequency.

Identification of double heterozygous -α4.2Ⅰ/-α4.2Ⅱ using third-generation sequencing

OBJECTIVE

The 4.2 kb deletion (-α4.2/) is a common a+-thalassemia with a carrier rate, followed by the South-East Asian deletion (-SEA) and the 3.7 kb deletion (-α3.7/). There are few reports about 4.2 kb deletion sub-types. Herein, we present a patient with double heterozygous -α4.2Ⅰ/-α4.2Ⅱwho was identified using third-generation sequencing (TGS).

METHODS

Hematology and hemoglobin fraction analysis were carried out by complete blood count (CBC) and capillary electrophoresis (CE). Gap-PCR was used to detect the common deletional α-thalassemia, and multiple ligation-dependent probe amplification (MLPA) was performed to screen the large deletion. Sanger sequencing identified the variant. The different deletions were confirmed by TGS.

RESULTS

CBC showed the patient with microcytic hypochromic anemia, and CE indicated the presence of a Hb variant. Gap-PCR and MLPA detected 4.2 kb deletion homozygotes (-α4.2/-α4.2). The Hb variant was confirmed as Hb Q-Thailand by Sanger sequencing. The patient was identified as compound heterozygous of 4.2 kb deletion and Hb Q-Thailand (-α4.2/-α4.2-Q-Thailand, -α4.2Ⅰ/-α4.2Ⅱ) using TGS.

CONCLUSIONS

Hb Q-Thailand (-α4.2-Q-Thailand/) complex 4.2 kb deletion heterozygote (-α4.2/) is easily misdiagnosed as 4.2 kb homozygous using Gap-PCR and MLPA. The TGS enables the identification of the two different 4.2 kb deletion sub-types.

Measurement of Erythrocyte Lifespan Using a CO Breath Test in Patients with Thalassemia and the Impact of Treatment

Background: Thalassemia comprises a diverse group of genetic disorders that affects the synthesis of globin chains, with a global distribution. The use of erythrocyte lifespan (ELS) to assess differences among patients with different types of thalassemia and the efficacy of splenectomy has not been implemented.