MSACL 2023 Abstract

Introduction:
Monoclonal gammopathy of undetermined significance (MGUS) is the most common plasma cell disorder found in approximately 3% of the population over 50 years old. Patients with MGUS are usually asymptomatic and have persistent risk of progression to multiple myeloma or other plasma cell disorders of 1% per year. Since the rate of risk does not decrease over time, lifelong follow-up is required. It is important to identify risk factors to predict groups of MGUS patients with high risk of progression, which can be essential for defining frequency of monitoring, and early diagnosis of multiple myeloma or related disorders. N-glycosylation of monoclonal light chains has been identified as an important risk factor for progression to primary amyloidosis. A MALDI-TOF MS based-assay with use of isotype-specific nanobody enrichment (MASS-FIX) has been developed and validated to detect and type monoclonal light chains in plasma cell disorders. MASS-FIX is more analytically sensitive, specific, cost-effective, and efficient compared with immunofixation with gel electrophoresis, and it enables easy identification of glycosylated monoclonal immunoglobulins.

Objectives:
In this study we aimed to assess the prevalence of light chain glycosylation at the time of recognition of MGUS.

Methods:
Our study cohort consisted of 849 serum samples from unique individuals who lived in the 11 counties of southeastern Minnesota. They had samples collected within 30 days of a previously established MGUS diagnosis at Mayo Clinic, and samples were kept frozen at -80oC until testing. Samples were tested for serum protein electrophoresis using agarose gels (Helena Laboratories), immunofixation (Sebia Inc.) and free light chains (FreeLite, Binding Site) were tested by nephelometry on a Siemens BNII. In addition, MASS-FIX was used to detect glycosylation of the immunoglobin light chains. Serum samples were incubated with agarose beads coupled with antibodies targeting κ or λ light chain constant domains respectively for immuno-enrichment. Beads then washed, reduced, spotted, and analyzed separately on MALDI-TOF MS (Bruker Corporation). The spectra were analyzed using FlexAnalysis software (Bruker Corporation). All analyses were conducted using R (version 4.2.1).

Results:
Of the total 849 MGUS patients tested, median age was 72 years old (range 24 to 96), and 44.9% were female. Median serum median M-spike concentration was 0.80 g/dL (range 0.20 to 4.5) although 140 out of 849 patients did not have an M-spike available. The most common isotype identified was IgG kappa (352, 41.8%), followed by IgG lambda, (219, 26.0%) and IgM kappa (92, 10.9%). Free light chain testing results for the kappa/lambda ratio were within reference intervals (0.26 to 1.65) in 66.8% of the cohort, with the remainder being abnormal. 45 patients (5.3%) were found to have glycosylated light chain. This is aligned with prevalence of glycosylation found in previous studies. Patients with and without glycosylated light chains have similar characteristics including age, gender, hemoglobin, serum creatinine, M-spike, and prevalence of abnormal free light chain ratio results (all P≥.10). 71.1% were kappa glycosylated light chains, and 28.9% were lambda glycosylated light chains, free light chain ratio was normal in 55.6% of cases. The lowest M-spike value associated with glycosylated light chains was 0.40 g/dL.

Conclusion:
MALDI-TOF mass spectrometry-based assay (MASS-FIX) can easily detect and identify glycosylation of monoclonal immunoglobulins. The prevalence of light chain glycosylation at the initial diagnosis of MGUS is about 5%. Glycosylation further characterization can provide valuable information on disease patterns, and follow-up information from patients over time may add value in stratifying patients into groups that would be at higher risk of progression to a malignancy at early stages of MGUS.