A novel sickle hemoglobin: hemoglobin S-south end

Biophysical chemistry behind sickle cell anemia and the mechanism of voxelotor action

Sickle cell anemia disease has been a great challenge to the world in the present situation. It occurs only due to the polymerization of sickle hemoglobin (HbS) having Pro-Val-Glu typed mutation, while the polymerization does not occur in normal hemoglobin (HbA) having Pro-Glu-Glu peptides. It is also well confirmed that the oxygenated HbS (OHbS) does not participate in the polymerization, while the deoxygenated HbS (dHbS) does, which causes the shape of red blood cells sickled. After polymerization, the blood has a low oxygen affinity. Keeping this fact into consideration, only those drugs are being synthesized that stabilize the OHbS structure so that the polymerization of HbS can be stopped. The literature data showed no systematic description of the changes occurring during the OHbS conversion to dHbS before polymerization. Hence, an innovative reasonable study between HbA and HbS, when they convert into their deoxygenated forms, was done computationally. In this evaluation, physiochemical parameters in HbA/HbS before and after deoxygenation were studied and compared deeply. The computationally collected data was used to understand the abnormal behaviour of dHbS arising due to the replacement of Glu6 with Val6. Consequently, during the presented computational study, the changes occurring in HbS were found opposite/abnormal as compared to HbA after the deoxygenation of both. The mechanism of Voxelotor (GBT-440) action to stop the HbS polymerization was also explained with the help of computationally collected data. Besides, a comparative study between GBT-440 and another suggested drug was also done to know their antisickling strength. Additionally, the effect of pH, CO, CO2, and 2,3-diphosphoglycerate (2,3-DPG) on HbS structure was also studied computationally.

Non-S Sickling Hemoglobin Variants: Historical, Genetic, Diagnostic, and Clinical Perspectives

Apart from hemoglobin-S (HbS), there are other Hb variants (non-S sickling Hb variants) that cause sickle cell disease. However, the profiles of these non-S sickling Hb variants have neither been collated nor harmonized. A literature search revealed 14 non-S sickling Hb variants (HbC-Harlem, HbC-Ziguinchor, HbS-Travis, HbS-Antilles, HbS-Providence, HbS-Oman, HbS-Cameroon, HbS-South End, Hb Jamaica Plain, HbC-Ndjamena, HbS-Clichy, HbS-San Martin, HbS-Wake, and HbS-São Paulo). Generally, the non-S sickling Hb variants are double mutants with the HbS mutation (GAG>GTG: βGlu6Val) and additional β-chain mutations. Consequently, non-S sickling Hb variants give positive solubility and sickling tests, but they differ from HbS with respect to stability, oxygen affinity, and electro-chromatographic characteristics. Similarities and discrepancies between HbS and non-S sickling Hb variants create diagnostic pitfalls that can only be resolved by elaborate electro-chromatographic and/or genetic tests. It is therefore imperative that tropical hematologists should have a thorough understanding of these atypical sickling Hb variants. Collated and harmonized appraisal of the non-S sickling Hb variants have not been previously undertaken. Hence, this paper aims to provide a comprehensive but concise historical, genetic, comparative, diagnostic, and clinical overview of non-S sickling Hb variants. The elaborate techniques often required for precise diagnosis of non-S sickling Hb variants are regrettably not readily available in low resource tropical countries, which paradoxically carry the heaviest burden of sickling disorders. We strongly recommend that tropical countries should upgrade their diagnostic laboratory facilities to avoid misdiagnosis of these atypical Hb mutants.

Pitfalls in the genetic diagnosis of Hb S

Patients homozygous for Hb S need to be properly identified to start as early as possible a treatment that should avoid complications. For prevention and genetic counseling, carriers of Hb S have to be screened. Hb S is easily detected by several analytical systems, but other variants, usually harmless, may behave as Hb S, leading to false positive diagnosis. Some interactions may also cause difficulties in the qualitative or quantitative interpretation of a chromatography or electrophoresis profile. These problems may result from several causes among which the simultaneous presence of an α chain variant leading to the formation of tetramers having both an α and a β chain modified, the presence of a second mutation within the Hb S allele, the existence of a compound heterozygous state leading to some "Hb S trait with dominantly transmitted sickle cell disease (SCD)", and the presence of thalassemic allele affecting the intracellular proportion of Hb S. In case of any "dominant Hb S trait" a thorough Hb study is always required. This work reports some of the difficulties observed by us, or reported in the literature, and propose how to avoid them and reach a correct diagnosis.

Strategy for identification by mass spectrometry of a new human hemoglobin variant with two mutations in Cis in the beta-globin chain: Hb S-Clichy [beta6(A3)Glu-->Val; beta8(A5)Lys-->Thr]

Hemoglobinopathies are the most frequent genetic diseases in the world. Among them, the Hb S variant [beta6(A3)Glu-->Val], which, in the homozygous state, produces a severe disease known as sickle cell anemia with polymerization of Hb S inside red blood cells under hypoxic conditions. Additional mutations, in cis or in trans of the beta(S)-globin chain, may inhibit or enhance the polymerization process. We describe here a new hemoglobin (Hb) variant (Hb S-Clichy) which carries the beta(S)-globin chain and an additional mutation beta8(A5)Lys-->Thr. The variant was detected by routine electrophoretic techniques and cation exchange liquid chromatography (CE-LC). Globin chain separation by reversed phase LC (RP-LC) showed normal and abnormal beta chains, confirming that the additional abnormality was located in cis to Hb S. Electrospray ionization mass spectrometry (ESI-MS) gave a 57 Da mass decrease for the abnormal globin chain. The abnormal chain was isolated and submitted to trypsin digestion. Normal peptides betaT-1 and betaT-2 were not observed on the matrix-assisted laser desorption-time of flight (MALDI-TOF) mass spectrum but a new peptide betaT-1,2 was detected. Nano LC-ESI-MS/MS of the new peptide showed that the glutamic acid at codon 6 was replaced by a valine residue, and the lysine at codon 8 was replaced by a threonine residue, as confirmed by DNA sequencing. This example demonstrates that in a population where Hb S is present, every unidentified Hb needs to be clearly characterized to prevent major sickle cell syndromes. In addition, the identification of these variants must be considered in newborn screening for sickle cell disease, using either classical biochemical methods or MS techniques.

Sickle cell disease caused by Hb S/Québec-CHORI: treatment with hydroxyurea and response

Sickle hemoglobin (Hb S;betaGlu 6 Val) is due to an A>T transversion in codon 6 of the beta-globin gene. Other variant hemoglobins mimic Hb A, S, or C on newborn screening and clinical laboratory diagnostic tools, thus making their correct identification potentially difficult. Sickling disorders can result in individuals who are compound heterozygous for beta-globin mutations (e.g., Hb SC, HbSO(Arab)). The authors report a second case of HbS/Québec-CHORI, a severe compound heterozygous sickling disorder and their experience managing this patient with hydroxyurea.

Hemoglobinopathies mimicking Hb S/beta-thalassemia: Hb S/S with alpha-thalassemia and Hb S/Volga

There are approximately 1200 known natural mutations of the human globin genes. In most clinical laboratories, the diagnosis of hemoglobin disorders is based on blood counts, hemoglobin electrophoresis, or column chromatography, which can identify common variant hemoglobins such as Hb S, C, and E, but are unable to definitively diagnose most other hemoglobin variants and thalassemia mutations. We report two unrelated cases, both thought initially to have Hb S/beta-thalassemia. Subsequent mutational analyses revealed that one is Hb S/S with compound heterozygosity for nondeletional alpha-thalassemia mutations. The other is the first reported case of compound heterozygosity for Hb S and an unstable hemoglobin, Hb Volga. Correct diagnoses of these hereditary disorders are needed for prognosis and proper management and also for genetic counseling. These studies underscore the importance to correlate clinical course with laboratory diagnosis and to make DNA-based diagnostics more widely available for patients with unusual or complicated hemoglobin disorders.

Promoting children's health through understanding of genetics and genomics

PURPOSE

To describe the effects of genetics and genomics on children's health care.

ORGANIZING CONSTRUCT

The breakthroughs in the Human Genome Project have great potential for disease prediction, treatment, and prevention in the health care of children with chronic health conditions. Most childhood conditions based on a single gene are influenced by a complex interaction of genetic and environmental factors.

METHODS

A review of the literature was conducted to determine the most common childhood diseases linked to genetic causes.

FINDINGS

Two examples were selected to depict how a health professional would use genetic knowledge to provide holistic health promotion and disease prevention.

CONCLUSIONS

Knowledge of the interaction of the genetic profile coupled with a person's lifestyle, work environment, and family context provide a more holistic picture of a person's health profile. The clinical implications are that this knowledge will provide opportunities for health professionals to advise families on individualized treatment options or to tailor health promotion to future disease states based on genes and their interaction with the environment.

Dominantly inherited beta thalassaemia intermedia caused by a new single nucleotide deletion in exon 2 of the beta globin gene: Hb morgantown (beta91 CTG>CG)

Family members in multiple generations of an Irish-American family were investigated for moderate to severe microcytic anaemia, inherited in an autosomal dominant fashion. A novel frameshift mutation of the beta globin gene was discovered. This study highlights the importance of considering dominantly inherited beta thalassemia in the investigation of anaemia, even in patients with ethnic backgrounds not usually associated with beta thalassaemia.