Hb Niigata [beta 1 (NA1) Val-->Leu]: the fifth variant with retention of the initiator methionine and partial acetylation

MetAP2 inhibition modifies hemoglobin S to delay polymerization and improves blood flow in sickle cell disease

Sickle cell disease (SCD) is associated with hemolysis, vascular inflammation, and organ damage. Affected patients experience chronic painful vaso-occlusive events requiring hospitalization. Hypoxia-induced polymerization of sickle hemoglobin S (HbS) contributes to sickling of red blood cells (RBCs) and disease pathophysiology. Dilution of HbS with nonsickling hemoglobin or hemoglobin with increased oxygen affinity, such as fetal hemoglobin or HbS bound to aromatic aldehydes, is clinically beneficial in decreasing polymerization. We investigated a novel alternate approach to modify HbS and decrease polymerization by inhibiting methionine aminopeptidase 2 (MetAP2), which cleaves the initiator methionine (iMet) from Val1 of α-globin and βS-globin. Kinetic studies with MetAP2 show that βS-globin is a fivefold better substrate than α-globin. Knockdown of MetAP2 in human umbilical cord blood-derived erythroid progenitor 2 cells shows more extensive modification of α-globin than β-globin, consistent with kinetic data. Treatment of human erythroid cells in vitro or Townes SCD mice in vivo with selective MetAP2 inhibitors extensively modifies both globins with N-terminal iMet and acetylated iMet. HbS modification by MetAP2 inhibition increases oxygen affinity, as measured by decreased oxygen tension at which hemoglobin is 50% saturated. Acetyl-iMet modification on βS-globin delays HbS polymerization under hypoxia. MetAP2 inhibitor-treated Townes mice reach 50% total HbS modification, significantly increasing the affinity of RBCs for oxygen, increasing whole blood single-cell RBC oxygen saturation, and decreasing fractional flow velocity losses in blood rheology under decreased oxygen pressures. Crystal structures of modified HbS variants show stabilization of the nonpolymerizing high O2-affinity R2 state, explaining modified HbS antisickling activity. Further study of MetAP2 inhibition as a potential therapeutic target for SCD is warranted.

3 cases of variant hemoglobin Hb A2-Niigata detected by falsely high HbA1c values

BACKGROUND

HbA1c measurements in blood are used to monitor diabetes status.

METHODS

We detected 3 cases (1 diabetic case, 1 borderline diabetic case, and 1 case with normal glucose tolerance) of variant hemoglobin Hb A2-Niigata based on falsely high HbA1c values measured by high-performance liquid chromatography (HPLC).

RESULTS

In all 3 cases, HbA1c values measured by the HPLC method were higher than the reference range whereas the HbA1c values measured by immunoassay, plasma glucose and glycated albumin were within the reference range. The results of the genetic test revealed heterozygous mutation GTG (Val) → GCG (Ala) in codon 1 of the δ-globin gene in all 3 cases, based on which Hb A2-Niigata was found. Although until now Hb A2-Niigata has been reported in 3 cases, this is the first report on Hb A2-Niigata with falsely high HbA1c values.

CONCLUSIONS

In this report, 3 cases of Hb A2-Niigata were found in a single institution in a short period; therefore, Hb A2-Niigata would exist frequently in a certain area. As a pathology causing falsely high HbA1c values, Hb A2-Niigata should be kept in mind.

Two cases with Hb Andrew-Minneapolis showing high or low-normal HbA1c levels depending on the measurement method

We experienced two cases of Hb Andrew-Minneapolis with high or low-normal HbA1c levels depending on the measurement method. Case 1 was a 25-year-old male, and case 2 was a 32-year-old pregnant woman. Both cases showed normal glucose tolerance levels and glycated albumin within the reference range. In both cases, the high-performance liquid chromatography (HPLC) method (standard mode) showed high HbA1c levels of 6.8% and 6.5%, respectively, while the HbA1c levels measured by immunoassay were low normal at 4.6% in both cases. Globin gene analysis detected heterozygous β-chain mutations (β144Lys → Asn) in both cases, which resulted in the diagnosis of Hb Andrew-Minneapolis. In case 1, a high-resolution HPLC chromatogram showed multiple abnormal peaks; two unknown peaks in addition to variant hemoglobin (HbX0) and glycation products of variant hemoglobin (HbX1c) were observed after in vitro glycation reaction. Although the details of unknown peaks were not identified, those might be modified hemoglobin associated with variant hemoglobin. The presence of unknown peaks could cause high HbA1c levels measured by HPLC (standard mode). Furthermore, the HbA1c level measured by immunoassay was increased to 4.9% within the reference range after adjustment for modified hemoglobin in case 1. Consequently, the high HbA1c levels measured by HPLC (standard mode) and the low-normal HbA1c level measured by immunoassay might be due to modified hemoglobin associated with variant hemoglobin.

Glycated albumin; clinical usefulness

The main purpose of treating diabetes is to prevent the onset and progression of diabetic chronic complications. Since the mechanism of onset of chronic complications is still not well understood, the main strategy to achieve this purpose is to bring plasma glucose levels as close as possible to those in healthy subjects and maintain good glycemic control over the long term. Since glycation among various proteins is increased in diabetic patients compared with non-diabetic subjects, glycated protein can be used as a glycemic control indicator. Currently, among these glycated proteins, HbA1c is used as the gold standard of glycemic control indicators. However, HbA1c does not accurately reflect the actual status of glycemic control in some conditions with rapid changes in glycemic control and in patients with anemia (hemolytic anemia, iron deficiency anemia, etc.) and variant hemoglobin. In comparison, glycated albumin (GA) more accurately reflects changes in plasma glucose during the short term and postprandial plasma glucose. GA also reflects glycemic control in patients with hematologic disorders whereas GA does not reflect glycemic control in patients with disorder of albumin metabolism. GA is a glycemic control indicator which overcomes most of the disadvantages of HbA1c, and could be therefore expected to replace HbA1c as the standard glycemic control indicator in the near future. However, it is necessary to accumulate more evidences from large research studies on the effective directions for measuring GA.

Hb St. Jozef, A Val→Leu N-Terminal Mutation Leading to Retention of the Methionine, and Partial Acetylation Found in the Globin Genein Ciswith a −α3.7Thalassemia Deletion

We report a new hemoglobin (Hb) variant found in a 6-year-old girl of Moroccan origin, living in the Dutch city of Gouda. The child was referred because of microcytic and hypochromic parameters. A normal zinc protoporphyirin (ZPP) value excluded iron deficiency and gap-polymerase chain reaction (gap-PCR) revealed a heterozygosity for the common -alpha(3.7) thalassemia deletion, partially justifying the hematological picture. The Hb pattern on alkaline electrophoresis and capillary electrophoresis was normal, while a fraction of 9% preceding the Hb A peak, remained visible on different high performance liquid chromatography (HPLC) devices. This fraction, located in front of the Hb A peak, is usually considered as a Hb A derivate that becomes more expressed in older samples. However, the sample was freshly collected and the peak unusually evident. Therefore, direct sequencing of the alpha-globin genes was performed revealing a GTG-->CTG transversion at codon 1 of the alpha1-globin gene or of the hybrid gene. This point mutation induces a single amino acid substitution from valine to leucine. Electrospray-mass spectrometry (ES-MS) analysis revealed, in addition to this substitution, that the N-terminal methionine was retained and that about 20% of the variant was acetylated. As expected for an association with a -alpha(3.7)-thalassemia (thal) deletion, the non acetylated and acetylated abnormal alpha chain amounted to 32% of the total alpha chains. Family studies revealed that the mutated codon was located in cis of the deletion.

Detection and characterization of variant and modified structures of proteins in blood and tissues by mass spectrometry

Some variant proteins cause diseases, and some diseases result in increases of proteins with abnormally modified structures. The detection, characterization, and estimation of the relative amounts of protein variants and abnormally modified proteins are important for clinical diagnosis and for elucidation of the mechanisms of the pathogenesis of diseases. Analysis of the covalent structures of proteins using matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF-MS) and liquid chromatography-electrospray ionization MS (LC-ESI-MS), which had been developed by the early 1990s, have largely replaced analyses by conventional protein chemistry. Here, we review the detection and characterization of hemoglobin variants, HbA1c measurement, detection of carbohydrate-deficient transferrin, and identification of variants of transthyretin (TTR) and Cu/Zn-superoxide dismutase (SOD-1) using soft ionization MS. We also propose the diagnostic application of the signals of modified forms of TTR, that is, S-sulfonated TTR and S-homocysteinyl TTR. The relative peak height ratio of the abnormal/normal components gives valuable information about the instability of variants and enables the detection of unstable Hb subunits or thalassemia heterozygotes. We found unique modified structures of TTR that suggested changes in amyloid fibrils.

Development of an isotope dilution mass spectrometry assay for HbA1c based on enzyme-cleaved peptide analysis

HbA1c is an index of control in diabetes patients. We report a highly reproducible measurement method for HbA1c based on analysis of the enzyme-cleaved peptide by electrospray ionization mass spectrometry using deuterium-labeled synthetic peptides as internal standards. Intra- and inter-assay coefficients of variation for the novel method ranged from 1.23 to 1.99% for samples with high and low HbA1c. Using this method, we clarified the extent of discrepancies among the indices of diabetes measured by conventional methods and the ESI method for clinical samples including those from patients with Hb variants. High-performance liquid chromatography (HPLC) methods for most samples with variants underestimate the true HbA1c value, although a few variants give a positive error for HbA1c. Immunoassays may also underestimate the values, if the reactivity of the antibody is low against the glycated N-terminal of the variant beta-chains by conformational change. The method proposed here is an important step to establish a candidate definitive method, and is also useful in assessing specific HbA1c test systems using samples containing Hb variants.

Assessment of the effect of hemoglobin variants on routine HbA1c measurements by electrospray ionization mass spectrometry

We applied electrospray ionization mass spectrometry (ESI-MS) to identify hemoglobin (Hb) variants, and to assess the effect of the variants on routine measurements of a glycated Hb, HbA1c. Over the past 8 years, we have diagnosed 83 cases, including 42 kinds of variant Hb, using MS as the main technology. Of these variants, 3 were new, and 9 were the first cases in Japan. Some abnormal Hbs cause diseases, and most cause erroneous values of HbA1c measured by various methods. ESI-MS was also successfully used for the accurate determination of glycated Hb. We and other groups proposed methods to examine glycated Hb by ESI-MS using enzyme-digested peptides, or intact globin without enzyme digestion. Using the peptide method, we clarified the extent of discrepancies among the HbA1c values measured by conventional methods and accurate values for samples containing various Hb variants identified by the MS method. We applied the globin method to measure the ratio of the glycated component of a variant chain and that of a normal chain obtained from the same erythrocytes. Although the glycation degree on most variant chains was similar to that on normal chains obtained from the same erythrocytes, the content of the glycated component of a particular variant beta-chain was approximately three times as much as that of the normal beta-chain. Abnormal Hbs cause erroneous values for HbA1c to various extents with commercial measurement methods, and MS may offer an unrivaled strategy to correct these errors.

Detection and identification of protein variants and adducts in blood and tissues: an application of soft ionization mass spectrometry to clinical diagnosis

The detection and identification of protein variants and abnormally increased modified proteins are important for clinical diagnosis. We applied soft ionization mass spectrometry (MS) to analyze proteins in blood and tissues from various patients. Over the past 8 years, we diagnosed 132 cases (55 kinds) of variant proteins including hemoglobin (Hb), transthyretin (TTR), and Cu/Zn-superoxide dismutase (SOD-1), using MS as the leading technology. Of these variants, eight were new, and nine were the first cases in Japan. Some abnormal Hb cause diseases, and most of them cause erroneous levels of glycated Hb, HbA1c, i.e., a popular index of diabetes. Most of the variant TTR causes amyloidotic polyneuropathy. Variant SOD-1 causes amyotrophic lateral sclerosis. We first showed that immunoprecipitation by a specific antiserum is a reliable and simple method to prepare protein from sera and tissues for analysis by matrix-assisted laser desorption time-of-flight MS, and liquid chromatography-electrospray ionization MS (LC-ESI-MS). The use of this technology has become widespread. Using an immunoprecipitated target protein and LC-ESI-MS, we showed that the ratios of tetra-, di- and a-sialo-transferrin from two cases of congenital glycoprotein deficient syndrome were clearly distinguishable from those of control samples. We first reported a unique modified form of TTR, that is, S-sulfonated TTR, which increased markedly and specifically in three cases with molibdenum cofactor deficiency. We proposed that S-sulfonated TTR is a useful marker for screening this disease. ESI-MS was successfully used for the accurate determination of HbA1c, and we clarified the extent of discrepancies between the HbA1c value measured by conventional methods and the accurate values for samples containing various Hb variants determined by the MS method.

Determination of ionization efficiency of glycated and non-glycated peptides from the N-terminal of hemoglobin beta-chain by electrospray ionization mass spectrometry

We compared the ionization efficiency of glycated and non-glycated peptides for the HbA1c measurement method developed by Kobold et al. [Clin. Chem., 43 (1997) 1944] based on LC-ESI-MS analysis of the N-terminal peptides of the beta-chains released by cleavage of the hemoglobin with endoproteinase Glu-C. Taking half the peak area of the doubly charged ion and adding it to the area of the singly charged ion, we determined that the slope of the resulting calibration curve was nearly equal to 1, and the reproducibility of the added values was better than the values calculated by the doubly or the singly charged ion alone.

Establishment of a combined strategy of genetic and mass spectrometric analyses for characterizing hemoglobin mutations. An example of Hb Hoshida (beta43Glu-->Gln)

Structural analysis of mutant hemoglobins has been efficiently accomplished by a consecutive mass spectrometric strategy: molecular mass measurement of the protein to detect mutation and to determine the molecular mass change, and fragmentation analysis with collision-induced dissociation to determine the site and type of mutation. A flaw of this method is an inherent inability to detect a mutation associated with no or little change of the molecular mass. In the present study, the strategy was improved by incorporating genetic analysis prior to mass spectrometry, which confirms the resulting amino acid change and searches for possible post-translational modifications. The method was applied to an unknown mutant, and elucidated a substitution of glutamine for glutamic acid at position 43 of beta-globin subunit, the mutation of Hb Hoshida.