Detection of M-Protein in Acetonitrile Precipitates of Serum Using MALDI-TOF Mass Spectrometry: A Novel Methodology

Background:

Multiple myeloma (MM) and plasmacytoma(s) belong to a group of clonal plasma cell dyscrasias. In 97-98% of all cases, they are characterised by the detection of a monoclonal protein (M-protein) in the blood, and sometimes in the urine. MALDI-TOF-mass spectrometry (MS) has demonstrated excellent analytical sensitivity for the screening and detection of M-protein. We present the results of a novel methodology for M-protein analysis by MALDI-TOF MS.

Patients and Methods:

Blood samples from patients and controls were collected after obtaining Institutional Ethics Committee approval. The work was carried out in accordance with the Declaration of Helsinki after obtaining written informed consent. Patients with confirmed multiple myeloma or plasmacytoma and M-protein detected by serum protein electrophoresis (SPEP), serum immunofixation electrophoresis (IFE), and serum free light chains (FLC) were included for MALDI-TOF MS analysis. IFE and FLC analysis were sent to independent laboratories for external validation of the MALDI-TOF MS results.

Reagent-based extraction

The serum fraction was separated from whole blood by centrifugation at 5000 rpm for 15 minutes and stored at -80^oC until further analysis. Twenty-five μL of the serum sample was mixed with 50% acetonitrile (ACN) to form a precipitate. After precipitation and incubation, the mixture was centrifuged. The protein precipitate was washed with 20% ACN. After centrifugation, the supernatant was discarded, and the precipitate was reconstituted in a buffer comprising 10% formic acid (FA) and 50 mmol/L tris(2-carboxyethyl)phosphine hydrochloride (TCEP).

The MALDI-TOF MS results were validated using immunoenrichment by anti-kappa (κ) and anti-lambda (λ) biotin-labelled antibodies immobilised on streptavidin magnetic beads. MALDI-TOF MS measurements were obtained for intact proteins using alpha-cyano-4-hydroxycinnamic acid as a matrix. The images obtained were overlaid on apparently healthy serum samples to confirm the presence of M-protein. The samples were then analysed using Ultraflex^TM LT, Bruker MALDI/TOF-TOF mass spectrometer. The mass spectra for each sample was exported to FlexAnalysis 3.3 (Bruker Daltonics) and background subtracted. A sample was considered positive for M-protein if there was a sharp or broad peak within the κ or λ mass/charge (m/z) range- κ m/z- [M+2H]²⁺: 11550-12300 Da; [M+H]⁺: 23100-24600 Da), and λ m/z- [M+2H]²⁺: 11100-11500 Da; [M+H]⁺: 22200-23100 Da. All the images were acquired at a m/z range of 10000-29000 Da. Mass measurement was analysed with a summation of 500-5000 shots depending on the intensity of the M-peak.

Results:

Twenty-seven patient samples: 24- multiple myeloma, and 3- plasmacytoma with an M-protein identified by other biochemical tests, were chosen for ACN precipitation and analysed by MALDI-TOF MS. The median age was 62 years (range:44-72); males-12 (44%). A mass spectrometrist, S.J was blinded to the IFE and FLC results- blinded analyst. N.M was the unblinded analyst.

Neat sample (without dilution) was spotted on the MALDI plate for all the control and patient samples. The Gaussian distribution of κ and λ light chains were obtained by analysing 20 serum samples of apparently healthy blood donors. All the 27 samples (100%) with M-protein confirmed by the other biochemical techniques, demonstrated a peak suggestive of M-protein with mass/charge (m/z) falling within the κ or λ range on MALDI-TOF MS: 24 patients with κ peak, and 3 with λ peak. (Fig. 1a and 1b) Immunoenrichment was performed on two samples- 1 with κ peak, and the other with λ peak, analysed by MALDI-TOF-MS by ACN precipitation. The mass spectra by immunoenrichment and ACN precipitation were found to be identical with the light chain m/z falling within their respective range. (Fig. 2 and 3) Three samples were labelled as confounders due to low peak intensity. However, their peaks matched their corresponding IFE and FLC reports. Concordance between MALDI-TOF MS and IFE was observed in 21/23 patients (91%); concordance between MALDI-TOF MS and FLC was observed in 23/24 patients (96%).

Conclusions:

We report the results of a low-cost, reagent-based extraction process using ACN precipitation to enrich for κ and λ light chains, which can be used for screening and for qualitative analysis of M-protein. Further studies are required to identify the immunoglobulin isotype, and to quantify the M-protein by this methodology.