The Analysis of Human Haemoglobin Variants using Mass Spectrometry

Significance of the Topic

Human haemoglobin variants affect oxygen transport and can lead to serious clinical conditions such as thalassaemias and sickle cell disease. Accurate identification of these variants is crucial for appropriate counselling, treatment, and genetic screening. Mass spectrometry (MS) has emerged as a powerful tool for detecting both known and electrophoretically silent haemoglobin variants, complementing traditional techniques and providing high precision in mass measurement.

Aims and Overview of the Study

This work describes a streamlined, five-step analytical workflow for detecting and characterising human haemoglobin variants using electrospray ionisation mass spectrometry (ESI-MS), supported by charge-sensitive chromatography: 1) screening by cation exchange-HPLC (ce-HPLC) or isoelectric focusing (IEF), 2) ESI-MS of intact globin chains, 3) identification of hybrid haemoglobins, 4) tryptic peptide mapping by ESI-MS, 5) tandem MS sequencing of variant peptides. This approach reduces sample preparation and analysis times to under two hours per sample and detects over 98 % of variants encountered in routine screening.

Methodology and Instrumentation

• Sample Preparation: Whole blood is diluted 500-fold in 50 % aqueous acetonitrile/0.2 % formic acid, desalted on cation-exchange resin beads and directly infused into the MS. For peptide mapping, blood is diluted 50-fold, denatured, and digested with trypsin in 30 min, then diluted 10-fold and analysed.
• Instrumentation: Triple quadrupole ESI-MS (e.g. Micromass Quattro Ultima) is operated in positive ion mode, scanning m/z 930–1210 for intact chains and 200–1650 for digests. Tandem MS (MS-MS) uses collision-induced dissociation with argon, typically 2.5 × 10⁻³ mbar and 18–26 V collision energy.
• Data Processing: Raw spectra are background subtracted, smoothed, calibrated internally using the major α-chain peaks, and deconvoluted with maximum entropy (MaxEnt) to yield true mass spectra. Quantitation of minor components (δ-chain, haem adducts, glycated chains, carbonic anhydrase) is obtained from mass profile peak intensities.

Main Results and Discussion

• Precision: Internal calibration with the α-chain achieves ±0.05 Da SD for the β-chain, enabling detection of ±1 Da variants at >10 % abundance.
• Charge-State Correlation: ce-HPLC elution shifts correlate with predicted charge changes from amino acid substitutions, guiding the assignment of small mass shifts (<±6 Da) to α- or β-chains.
• Minor Component Detection: δ-chain levels (Hb A₂ equivalent) are quantified, distinguishing homozygous from trait states. Glycated α- and β-chains (Hb A₁c) are measured, with calibration to clinical Hb A₁c values. Carbonic anhydrase 1 and hybrid haemoglobin chains are also detected.
• Variant Identification: Over 300 variants were identified over 30 years, including 15 novel before 2001 and further novel variants through 2012. Some variants (e.g. Hb Constant Spring, Lepore-Baltimore, Kenya, P-Nilotic) are recognised directly from intact chain spectra; others require tryptic digest MS and MS-MS peptide sequencing for confirmation.

Benefits and Practical Applications

• Rapid Turnaround: From screening to final identification in under two hours, supporting clinical and diagnostic workflows.
• High Sensitivity: Detection of low-abundance variants and electrophoretically silent mutations.
• Comprehensive Coverage: Identification of point mutations, insertions, deletions, C-terminal extensions, and hybrid genes without extensive chromatographic separations or derivatization.
• Quantitative Information: Simultaneous measurement of variant abundance, δ-chain levels for thalassaemia screening, and glycation levels for diabetes monitoring.

Future Trends and Potential Applications

• Automation and High-Throughput: Integration of nano-ESI, microfluidics, and advanced software for real-time variant screening.
• Structural Characterisation: Top-down proteomics and high-resolution orbitrap or time-of-flight MS to resolve complex variant patterns.
• Expanded Panels: Inclusion of post-translational modifications, chain cross-talk, and rare hybrid haemoglobins in newborn screening programs.
• Digital Pathology Integration: Linking mass spectrometry data with clinical databases and AI-driven diagnostic platforms.

Conclusion

Electrospray ionisation mass spectrometry, combined with simple desalting and tryptic digestion, provides a rapid, sensitive, and comprehensive approach for identifying human haemoglobin variants. This methodology complements traditional electrophoretic and chromatographic techniques, enhances diagnostic accuracy, and supports effective genetic counselling. Continued technological advances promise further improvements in throughput, sensitivity, and integration into clinical laboratories.

Reference

1. Roepstorff P. and Fohlman J. (1984) Proposal for a common nomenclature for sequence ions in mass spectra of peptides. Biomed. Mass Spectrom. 11:601–601.