Hemoglobin Titusville, a low oxygen affinity variant hemoglobin, in a family of Northern European background

In Silico Analysis of the Effects of Point Mutations on α-Globin: Implications for α-Thalassemia

Hemoglobinopathies are inherited diseases that impair the structure and function of the oxygen-carrying pigment hemoglobin (Hb). Adult Hb consists of two α and two β subunits. α-Thalassemia (α-thal) affects the genes that code for the α-globin chains, HBA1 and HBA2. Mutations can result in asymptomatic, mild or severe outcomes depending on several factors, such as mutation type, number of mutations and the location at which they occur. PredictSNP was used to estimate whether every possible single nucleotide polymorphism (SNP) would have a neutral or deleterious effect on the protein. These results were then used to create a plot of predicted tolerance to change for each residue in the protein. Tolerance to change was negatively correlated with the residue's sequence conservation score. The PredictSNP data were compared to clinical reports of 110 selected variants in the literature. There were 29 disagreements between the two data types. Some of these could be resolved by considering the role of the affected residue in binding other molecules. The three-dimensional structures of some of these variant proteins were modeled. These models helped explain variants which affect heme binding. We predict that where a point mutation alters a residue that is intolerant to change, is well conserved and or involved in interactions, it is likely to be associated with disease. Overall, the data from this study could be used alongside biochemical and clinical data to assess novel α-globin variants.

First family case of haemoglobinopathy Titusville in France and literature overview

Normal haemoglobin is a tetramer molecule, consisting of two α and β haemoglobin chains. Haemoglobinopathies occur when abnormalities in these proteins are present. More than 1000 naturally occurring human haemoglobin variants with single amino acid substitution throughout the molecule have been identified and can be discovered through their clinical and biological manifestations. Here, we report the case of a 60-year-old woman for whom no oximetry results were obtained during blood gas analysis (BGA) and the values of oxygen saturation obtained from pulse oximetry (73%) and co-oximetry (90%) differed. Haemoglobin analysis demonstrated the presence of a variant in the alpha chain. Clinical history of the patient and her family revealed they carry a haemoglobin variant (Titusville type), thus representing the first French family case reported. Those results raised the question whether the presence of this variant could be the cause of the errors encountered during BGA.

Characterization of deletional and non-deletional alpha globin variants in a large cohort from Spain between 2009 and 2014

The hemoglobinopathies are a group of disorders passed down through families (inherited) in which there is abnormal production or structure of the hemoglobin molecule. They are among the most common inherited diseases around the world. Those that produce abnormal hemoglobin are called structural hemoglobinopathies while thalassemia is another type of disorder that is caused by a defect in the gene production of the globin chains. In a study ambispective comprising 1623 patients, 153 subjects showed an abnormal hemoglobin and 1470 with hypochromic and microcytic anemia, and of these 1470, 23 patients were studied for simultaneously α-thalassemias and structural hemoglobinopathies. Among the α-thalassaemia cases, 1282 cases (87.2%) were deletional α-thalassemia, 172 cases (11.7%) were non-deletional α-thalassemia, and 16 cases (1.1%) were deletional and non-deletional α-thalassamias simultaneously. Thus, approximately 12% of the cases were non-deletional α-thalassaemia. Clinical diagnosis, only 19 severe cases (1 hydrops fetalis and 18 instances of Hb H disease), 1200 thalassamias traits, and 160 thalassaemia silent carriers were recorded within the α-thalassaemia. Regarding structural hemoglobinopathies, there were only 2 cases of hemoglobinopathies with low oxygen affinity and 1 case of hemoglobin M; the remaining 150 were silent hemoglobinopathies. Non-deletional α-thalassaemia represented 12% of all α-thalassemias in our region; the most common deletion in our area was the 3.7-kb deletions, followed by Asian --(SEA) and --(FIL). The alterations responsible for non-deletional α-thalassaemia are most represented by the Hph and Hb Groene Hart and, in the case of structural hemoglobinopathies, Hb Le Lamentin and Hb J-Paris.

Oxygenation during general anesthesia and thoracic surgery in a patient with Titusville low-oxygen affinity hemoglobin

The purpose of this case study is to describe the physiological characteristics of a patient with the low-oxygen affinity Titusville hemoglobin variant. A 46-yr-old man with exertional dyspnea was diagnosed with a mediastinal lymphadenopathy of unknown origin and, to obtain definitive diagnosis by biopsy, underwent endobronchial ultrasound-guided transbronchial needle aspirate under sedation and video-assisted thoracoscopy under general anesthesia. High inspired fraction of oxygen ( FIO2 ) was used to guarantee adequate oxygenation even during the one-lung ventilation needed for thoracoscopy. Following radial and pulmonary arterial catheterization, continuous mixed-venous oxygen saturation ( SVO2 ), cardiac output, oxygen delivery (DO₂), oxygen consumption (V̇o₂), and oxygen extraction ratio (ERO₂) were measured. Serial blood gas analyses were obtained at different FIO2 . Anesthesia and surgery were carried out safely. Data obtained during the clinical case were utilized to 1) construct an in vivo Titusville hemoglobin dissociation curve and 2) describe oxygen delivery and consumption of a human with Titusville mutation. Titusville hemoglobin showed relatively high P50 (i.e., 30 vs. normal of 27) and very low cooperativity (Hill coefficient of 1.45 vs. normal 2.27), which was compensated in our patient by increases in cardiac output, rather than by augmenting oxygen extraction.

Low SpO2 With Normal SaO2 During General Anesthesia: A Case Report

A 25-year-old Caucasian man with a history of spherocytosis, splenectomy, recurrent blood transfusion, and no cardiopulmonary disease presented for an emergent laparoscopic cholecystectomy with a baseline pulse oximetric saturation (SpO2) of 88% while breathing room air. The SpO2 increased to only 89% during preoxygenation with an FIO2 1.0. Multiple arterial blood samples revealed SaO2 as high as 100% with PaO2 averaging 390 mm Hg. He was subsequently diagnosed with a dyshemoglobin, hemoglobin Köln. The simultaneous presentation of a stable patient from a cardiopulmonary perspective with normal arterial oxygen tension and saturation in the blood gas analyses despite a low SpO2 measurement outlines the importance of integrating the history of present illness and both the importance and the limitation of the pulse oximetry.

Direct comparison of oligochaete erythrocruorins as potential blood substitutes

While many blood substitutes are based on mammalian hemoglobins (e.g., human hemoglobin, HbA), the naturally extracellular hemoglobins of invertebrates (a.k.a. erythrocruorins, Ecs) are intriguing alternative oxygen carriers. Specifically, the erythrocruorin of Lumbricus terrestris has been shown to effectively deliver oxygen in mice and rats without the negative side effects observed with HbA. In this study, the properties of six oligochaete Ecs (Lumbricus terrestris, Eisenia hortensis, Eisenia fetida, Eisenia veneta, Eudrilus eugeniae, and Amynthas gracilis) were compared in vitro to identify the most promising blood substitute candidate(s). Several metrics were used to compare the Ecs, including their oxidation rates, dissociation at physiological pH, thermal stability, and oxygen transport characteristics. Overall, the Ecs of Lumbricus terrestris (LtEc) and Eisenia fetida (EfEc) were identified as promising candidates, since they demonstrated high thermal and oligomeric stability, while also exhibiting relatively low oxidation rates. Interestingly, the O₂ affinity of LtEc (P₅₀ = 26.25 mmHg at 37 °C) was also observed to be uniquely lower than EfEc and all of the other Ecs (P₅₀ = 9.29–13.62 mmHg). Subsequent alignment of the primary sequences of LtEc and EfEc revealed several significant amino acid substitutions within the D subunit interfaces that may be responsible for this significant change in O₂ affinity. Nonetheless, these results show that LtEc and EfEc are promising potential blood substitutes that are resistant to oxidation and denaturation, but additional experiments will need to be conducted to determine their safety, efficacy, and the effects of their disparate oxygen affinities in vivo.

Hemoglobin Titusville: a rare low oxygen affinity hemoglobinopathy

The relevance of this case lies in the extensive diagnostic workup that can be avoided with proper laboratory evaluation of relatively unsophisticated tests.

Diagnosis and clinical presentation of hemoglobin Kirksey

More than 1000 hemoglobin (Hb) variants have been identified. Hb Kirksey, a rare Hb variant involving mutation at codon 94 of α2-globin, is associated with low oxygen affinity. To our knowledge, there is no case report on Hb Kirksey in the literature, and, accordingly, the clinical features of patients with Hb Kirksey are currently unknown. We report here the case of a neonate who presented with asymptomatic low oxygen saturation on pulse oximetry (SpO2 ) just after birth, together with clinical analysis of the sister and cousin. Based on the presentations, Hb Kirksey does not seem to be associated with clinical abnormalities. Discrepancy between SpO2 and partial pressure of arterial oxygen (PaO2 ) detected on arterial blood gas analysis provided a clue to the diagnosis. Hb variants should be suspected and arterial blood gases should be measured in pediatric cases of unexplained low SpO2 .

Characterization of Homozygous Hb Setif (HBA2: c.283G>T) in the Iranian Population

Hemoglobin (Hb) variants are abnormalities resulting from point mutations in either of the two α-globin genes (HBA2 or HBA1) or the β-globin gene (HBB). Various reports of Hb variants have been described in Iran and other countries around the world. Hb Setif (or HBA2: c.283G>T) is one of these variants with a mutation at codon 94 of of the α2-globin gene that is characterized in clinically normal heterozygous individuals. We here report clinical and hematological findings in two homozygous cases of Iranian origin for this unstable Hb variant.

Reaction trajectory revealed by a joint analysis of protein data bank

Structural motions along a reaction pathway hold the secret about how a biological macromolecule functions. If each static structure were considered as a snapshot of the protein molecule in action, a large collection of structures would constitute a multidimensional conformational space of an enormous size. Here I present a joint analysis of hundreds of known structures of human hemoglobin in the Protein Data Bank. By applying singular value decomposition to distance matrices of these structures, I demonstrate that this large collection of structural snapshots, derived under a wide range of experimental conditions, arrange orderly along a reaction pathway. The structural motions along this extensive trajectory, including several helical transformations, arrive at a reverse engineered mechanism of the cooperative machinery (Ren, companion article), and shed light on pathological properties of the abnormal homotetrameric hemoglobins from α-thalassemia. This method of meta-analysis provides a general approach to structural dynamics based on static protein structures in this post genomics era.

Case-control studies of novel hemoglobin anomalies as differential diagnosis in sleep apnea syndrome

PURPOSE

Pulse oximetry plays an essential role in the diagnosis of sleep apnea syndrome. We discovered two novel hemoglobin anomalies, Hb Bonn and Hb Venusberg, which initially resulted in avoidable sleep disorder examinations and therapeutic consequences due to their low oxygen saturation levels as measured by pulse oximetry.

METHODS

Hematological as well as clinical chemical diagnosis was carried out. Hemoglobin anomalies were detected through electrophoresis, chromatography, spectrophotometry, and pulse oximetry as well as hemoglobin gene sequencing.

RESULTS

Hb Bonn is a novel hemoglobin mutation of the proximal α1 globin with an additional absorption maximum of the oxyhemoglobin at 668 nm. This results in pulse oximetry measurements of false low oxygen saturation due to incorrect calculations at the pulse oximetry measuring point 660 m. Hb Venusberg is a novel oxygen-affine hemoglobin mutation of the β-globin which is electrophoretically silent. Clinical symptoms include intermittent low oxygen saturation levels, cyanosis of lips and nail beds, and limited physical resistance to stress.

CONCLUSIONS

Hemoglobin anomalies, such as Hb Bonn and Hb Venusberg, should be included in differential diagnosis as potential causes of low oxygen saturation especially in case of nonspecific or conflicting findings.

Unexpectedly low pulse oximetry measurements associated with variant hemoglobins: a systematic review

Pulse oximetry estimates arterial blood oxygen saturation based on light absorbance of oxy- and deoxy-hemoglobin at 660 and 940 nm wavelengths. Patients with unexpectedly low SpO₂ often undergo cardio-pulmonary testing to ascertain the cause of their hypoxemia. However, in a subset of patients, a variant hemoglobin is responsible for low SpO₂ measurements. The extent of this problem is unclear. We performed a systematic literature review for reports of low SpO₂ associated with variant hemoglobins. We also reviewed unpublished cases from an academic hemoglobin diagnostic reference laboratory. Twenty-five publications and four unpublished cases were identified, representing 45 patients with low SpO₂ and confirmed variant hemoglobin. Fifty-seven family members of patients had confirmed or suspected variant hemoglobin. Three low oxygen affinity variant hemoglobins had concordantly low SpO₂ and SaO₂. Eleven variant hemoglobins were associated with unexpectedly low SpO₂ measurements but normal SaO₂. Hemoglobin light absorbance testing was reported in three cases, all of which showed abnormal absorption spectra between 600 and 900 nm. Seven other variant hemoglobins had decreased SpO₂, with unreported or uncertain SaO₂. Twenty-one variant hemoglobins were found to be associated with low SpO₂. Most variant hemoglobins were associated with spuriously low SpO₂. Abnormal absorption spectra explain the discrepancy between SpO₂ and SaO(2) for some variants. The differential diagnosis of possible variant hemoglobin ought to be considered in asymptomatic patients found to have unexpectedly low SpO₂. The correct diagnosis will help to spare patients from unnecessary investigations and anxiety.

Mutations in the paralogous human alpha-globin genes yielding identical hemoglobin variants

The human alpha-globin genes are paralogues, sharing a high degree of DNA sequence similarity and producing an identical alpha-globin chain. Over half of the alpha-globin structural variants reported to date are only characterized at the amino acid level. It is likely that a fraction of these variants, with phenotypes differing from one observation to another, may be due to the same mutation but on a different alpha-globin gene. There have been very few previous examples of hemoglobin variants that can be found at both HBA1 and HBA2 genes. Here, we report the results of a systematic multicenter study in a large multiethnic population to identify such variants and to analyze their differences from a functional and evolutionary perspective. We identified 14 different Hb variants resulting from identical mutations on either one of the two human alpha-globin paralogue genes. We also showed that the average percentage of hemoglobin variants due to a HBA2 gene mutation (alpha2) is higher than the percentage of hemoglobin variants due to the same HBA1 gene mutation (alpha1) and that the alpha2/alpha1 ratio varied between variants. These alpha-globin chain variants have most likely occurred via recurrent mutations, gene conversion events, or both. Based on these data, we propose a nomenclature for hemoglobin variants that fall into this category.

Low oxygen saturation by pulse oximetry may be associated with a low oxygen affinity hemoglobin variant, hemoglobin Titusville

The authors describe a 10-year-old Caucasian boy who presented with abnormally low oxygen saturation values on pulse oximetry after a routine tonsillectomy. He was asymptomatic and there was no personal or family history of respiratory, cardiac, or hematologic disorders. Multiple initial investigations were completed without diagnostic yield. Eventually, hemoglobinopathy studies identified the presence of a low oxygen affinity hemoglobin variant, characterized as hemoglobin Titusville. Hemoglobinopathies remain highly prevalent worldwide, with more than 65 low oxygen affinity hemoglobin variants identified to date. Early recognition of abnormal hemoglobin variants in asymptomatic patients may avoid extensive, unnecessary medical investigations.