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

Comparison of glycation degrees of HbG-Coushatta and HbG-Taipei with HbA using liquid chromatography with tandem mass spectrometry

BACKGROUND

Hemoglobin A_1c(HbA_1c) is widely used to measure glycemic status and control in diabetes testing and treatment and is an important risk factor forcomplications of diabetes. Hemoglobin variants can interfere with the HbA_1c testing method by reducing the life span of erythrocytes or due to differences in glycation degrees. In this study, glycation levels of the HbA, HbG-Coushatta, and HbG-Taipei β-chains (βA, βG-Coushatta, and βG-Taipei, respectively) were examined.

METHODS

Blood samples from heterozygotic patients (HbG-Coushatta/HbA, HbG-Taipei/HbA) were analyzed. Glycation rateswere determined using high-performance liquid chromatography with tandem mass spectrometry. Ratios of glycated βG-Coushatta to glycated βA and glycated βG-Taipei to glycated βA were calculated by comparingareas under the curves from extracted ion chromatograms.

RESULTS

βG-Coushatta and βG-Taipei were 6.08 ± 1.38% and 5.95 ± 0.93% glycated (respectively), which were significantly higher than βA chains(4.55 ± 1.30% and 4.51 ± 0.91%, respectively; p = 0.000). The total glycation degree (α + β) in HbG-Coushatta and HbG-Taipei heterozygotes were estimated to be 9% and 8% higher than those of HbA homozygotes (P＜0.001), respectively.

CONCLUSION

βG-Coushatta and βG-Taipei glycation degrees were significantly higher than βA, while the differences in total hemoglobin (α + β) were small and unlikely to impact the clinical interpretation of HbA_1c results.

Capillary Zone Electrophoresis-Top-Down Tandem Mass Spectrometry for In-Depth Characterization of Hemoglobin Proteoforms in Clinical and Veterinary Samples

Hemoglobin (Hb) constitutes an important protein in clinical diagnostics-both in humans and animals. Among the high number of sequence variants, some can cause severe diseases. Moreover, chemical modifications such as glycation and carbamylation serve as important biomarkers for conditions such as diabetes and kidney diseases. In clinical routine analysis of glycated Hb, sequence variants or other Hb proteoforms can cause interference, resulting in wrong quantification results. We present a versatile and flexible capillary zone electrophoresis-mass spectrometry screening method for Hb proteoforms including sequence variants and modified species extracted from dried blood spot (DBS) samples with virtually no sample preparation. High separation power was achieved by application of a 5-layers successive multiple ionic polymer layers-coated capillary, enabling separation of positional isomers of glycated α- and β-chains on the intact level. Quantification of glycated Hb was in good correlation with the results obtained in a clinical routine method. Identification and characterization of known and unknown proteoforms was performed by fragmentation of intact precursor ions. N-Terminal and lysine glycation could be identified on the α- and β-chain, respectively. The versatility of the method was demonstrated by application to dog and cat DBS samples. We discovered a putative new sequence variant of the β-chain in dog (T38 → A). The presented method enables separation, characterization, and quantification of intact proteoforms, including positional isomers of glycated species in a single run. Combined with the simple sample preparation, our method represents a valuable tool to be used for deeper characterization of clinical and veterinary samples.

The Prevalence and Role of Hemoglobin Variants in Biometric Screening of a Multiethnic Population: One Large Health System's Experience

OBJECTIVES

To characterize and quantitate hemoglobin (Hb) variants discovered during biometric hemoglobin A1c (HbA1c) analyses in a large multiethnic population with a focus on the effect of variants on testing method and results.

METHODS

In total, 13,913 individuals had their HbA1c measured via ion-exchange high-performance liquid chromatography. Samples that had a variant Hb detected or HbF fraction more than 25% underwent variant Hb characterization and confirmation by gel electrophoresis. RBC indices were also evaluated for possible concomitant thalassemia.

RESULTS

Of the 13,913 individuals evaluated, 524 (3.77%) had an Hb variant. The prevalence of each variant was as follows: HbS trait (n = 396, 2.85%), HbSS disease (n = 4, 0.03%), HbC trait (n = 85, 0.61%), HbCC disease (n = 2, 0.01%), HbSC disease (n = 5, 0.04%), HbE trait (n = 18, 0.13%), HbD or G trait (n = 9, 0.06%), HbS β-thalassemia + disease (n = 1, 0.01%), hereditary persistence of HbF (n = 2, 0.01%), and HbMontgomery trait (n = 1, 0.01%). Concomitant α-thalassemia was detected in 20 (3.82%) of the 524 individuals with an Hb variant.

CONCLUSIONS

This study represents one of the largest epidemiologic investigations into the prevalence of Hb variants in a North American metropolitan, multiethnic workforce and their dependents and reinforces the importance of method selection in populations with Hb variants.

Degree of discrepancy between HbA1c and glycemia in variant hemoglobin is smaller when HbA1c is measured by new-type Arkray HPLC compared with old-type HPLC

OBJECTIVES

Although variant hemoglobin mainly demonstrates inappropriate HbA1c values measured by high-performance liquid chromatography (HPLC), these values differ depending on the HPLC model. In the 1990s, old-type HPLC models were replaced with new-type HPLC models which could separate stable HbA1c from labile HbA1c and modified hemoglobin. This study compared HbA1c values in subjects with variant hemoglobin measured using old-type Arkray HPLC (HA-8150) and new-type Arkray HPLC (HA-8160 or HA-8180).

DESIGN AND METHODS

This study included non-diabetic subjects with apparently low HbA1c values who had variant hemoglobins due to a β-chain heterozygous mutation. HbA1c was measured by old-type HPLC in 28 subjects with 12 variant hemoglobins (group 1) and new-type HPLC in six subjects with four variant hemoglobins (group 2). When HbA1c was measured by HPLC (HPLC-HbA1c), HbA1c measured by immunoassay (IA-HbA1c) and glycated albumin (GA) were also measured.

RESULTS

IA-HbA1c and GA did not significantly differ between both groups. However, HPLC-HbA1c in group 2 was significantly higher than that in group 1 (group 1: 2.9 ± 0.7% vs. group 2: 3.7 ± 0.2%, P = 0.006).

CONCLUSIONS

When HbA1c in subjects with variant hemoglobin is measured by new-type Arkray HPLC, the degree of discrepancy between HbA1c and glycemia is smaller compared with that measured by old-type HPLC.

Classification of variant forms of haemoglobin according to the ratio of glycated haemoglobin to glycated albumin

Background

Asymptomatic variant haemoglobin is increasingly being found in the measurement of glycated haemoglobin (HbA1c) for the management of diabetes mellitus. We compared the HbA1c concentrations measured by high-performance liquid chromatography (HPLC) and immunoassay and glycated albumin (GA) concentrations and calculated the respective ratios in order to classify the variant haemoglobin.

Methods

Twenty different haemoglobin variants from 43 subjects were identified by mass spectrometry and DNA analysis. Since GA accurately reflects glycaemic control in patients with variant haemoglobin, we calculated respective ratios of HbA1c and GA. Haemoglobin variants causing a low ratio of HbA1c measured by HPLC (HPLC-HbA1c) to GA with a normal ratio of HbA1c measured by immunoassay (IA-HbA1c) to GA were classified as C1. A further classification of α and β was used with abnormalities of the α chain or β chain in the haemoglobin gene. Other haemoglobin variants were classified as non-C1. Eight diabetic patients with stable glycaemic control were used as controls.

Results

Twenty forms of variant haemoglobins were classified as C1 α (2 variants; I-Interlaken and Hb J-Meerut), C1 β (15 variants) and non-C1 (3 variants; Hb Himeji, Hb Woolwich, Hb Peterborough). Positive correlations between GA and HPLC-HbA1c or IA-HbA1c were seen in the C1 β patients with diabetes mellitus. The regression line between GA and HPLC-HbA1c, but not that between GA and IA-HbA1c, showed a downward shift in comparison with the data obtained from the diabetic controls.

Conclusions

Variant haemoglobin could be classified by calculating the ratios of HPLC-HbA1c, IA-HbA1c and GA.

Conundrum of elevated HbA1C and hypoglycemia-a rare cause

A white diabetic patient on insulin therapy presented with recurrent hypoglycemia despite very high glycosylated hemoglobin (HbA1c) values. Hemoglobin (Hb) variants, chemically modified Hb, and abnormalities of red cell turnover cause errors in HbA1c measurement. Widely prevalent Hb variants affecting HbA1c estimation include HbS and HbC in African Americans, HbE in southeast Asians, and carbamyl-Hb in uremic patients. In addition, there are at least 893 other Hb variants as of 2005, many of which affect HbA1c estimation. HbA1c values are also affected by methodology of estimation. Our patient had HbJ, which is rare amongst whites. The relationship between HbA1c values and mean plasma glucose allows estimation of expected HbA1c. Significant discrepancy between expected and measured HbA1c should be evaluated. Considering Hb variants, evaluating for the same and estimating HbA1c with the appropriate method under such circumstances are described. Numerous new or rare Hb variants will be diagnosed if suspicion is appropriately entertained.

Effects of 7 Hemoglobin Variants on the Measurement of Glycohemoglobin by 14 Analytical Methods

Background: Hemoglobin variants (HbVAR) are not uncommon in the Korean population, with Hb G-Coushatta and Hb Queens being the 2 most common HbVAR. Hb G-Coushatta is also the most common HbVAR in Chinese people from the Silk Road region, as well as in some North American Indian tribes. However, data are scarce on the effect of these HbVAR on the different methods used for analyzing HbA1c.

Methods: Specimens from 24 individuals with 7 HbVAR (Hb G-Coushatta, Hb Queens, Hb G-Hsi-Tsou, Hb Ube-4, Hb G-Waimanalo, Hb Inglewood, and Hb Bologna-St.Orsola) were collected and tested using the International Federation of Clinical Chemistry primary reference method as well as 14 routine HbA1c assay methods.

Results: Hb G-Coushatta showed a clinically significant effect on the measured HbA1c, particularly when analysis was performed with ion-exchange HPLC methods with short elution times. This interference could be resolved by measuring the HbA1c using other methods such as HPLC with a long elution time, immunoassay, boronate affinity chromatography, and enzymatic assay. Hb Queens showed a clinically significant difference, defined as a >10% deviation from regression lines, in results from the 2 HPLC methods but not in the other methods. The remaining 5 rare HbVAR showed different HbA1c results in the different assays.

Conclusion: Hb G-Coushatta, Hb Queens, and other rare HbVAR can interfere with glycohemoglobin assays, including ion-exchange HPLC methods with short elution times, but the interference can be resolved using other unaffected methods. It is important to identify these HbVAR through a careful inspection of the chromatograms and apply other noninterfering methods for accurate measurements of the HbA1c.

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.

Conditions associated with very low values of glycohaemoglobin measured by an HPLC method

AIMS

To identify the causes of very low glycohaemoglobin (GHb) values in a sample of patients with diabetes in southern Brazil using high performance liquid chromatography.

METHODS

Between August 1996 and December 2001 all samples from patients with diabetes at a university hospital with GHb values below the reference range (4.7-6.0% HbA(1c)) were submitted to cellulose acetate electrophoresis. Medical records were reviewed to identify conditions that might be associated with these low values.

RESULTS

Among 29 657 samples analysed, 130 patients had GHb < 4.7%. Seventy three patients (56%) were heterozygous for HbS, HbC, or HbD (19 black, two mulatto, and 52 white patients). The other 57 patients (44%) without Hb variants had low haematocrit and haemoglobin values (42 patients) or other conditions such as pregnancy, lipaemia, malignancy, cirrhosis, acetylsalicylic acid use, and absence of diabetes (15 patients).

CONCLUSIONS

The presence of an Hb variant may falsely lower GHb measurements. However, anaemia is also a source of negative interference. The haematological status should be considered for the correct interpretation of GHb results.

First case of a single heterozygote of an abnormal hemoglobin, Hb Stanmore, [beta111(G13)Val-->Ala]

We describe a hemoglobin beta-chain mutant detected incidentally in an unusual profile of glycated hemoglobin (HbA1c) measured by ion-exchange HPLC. Analysis of intact globin by electrospray ionization mass spectrometry (ESI-MS) and peptide analysis by on-line HPLC-ESI-MS-MS revealed the substitution, [beta111(G13)Val-->Ala], which was confirmed by DNA analysis. This was the second case of Hb Stanmore. As the first case combined beta(0)-thalassemia, and the family study in that case showed no case of Hb Stanmore without combined thalassemia, the case presented here is the first case of single heterozygote, and the first Japanese case. Hb Stanmore is isoelectrophoretically silent with only mild clinical symptoms, although stability by isopropanol test was positive.