Early Serum Protein Clonal Dynamics Preceding Monoclonal Gammopathy of Undetermined Significance and Multiple Myeloma Years before Diagnosis

Background

We previously demonstrated that monoclonal gammopathies can be detected in a population at risk for myeloma using mass spectrometry (MS), identifying two entities: monoclonal gammopathy of indeterminate potential (MGIP) and MS detected MGUS (MS-MGUS). However, it remains unclear whether these early clones progress to clinically detected MGUS or multiple myeloma (MM).

To investigate this, we utilized archived blood samples from individuals in the screening arm of the National Cancer Institute Prostate, Lung, Colorectal, and Ovarian Cancer Screening (PLCO) Trial, collected up to 22 years before their clinical diagnosis with MGUS or MM.

Methods

Samples were obtained from healthy adults aged 55-74 enrolled between Nov 1993 and July 2001. We focused on participants who developed either MM or MGUS based on clinical records of serum protein electrophoresis (SPEP and IFX) and serum free light chain (sFLC) tests conducted by Landgren et al.

We used matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) MS coupled with turbidimetry (EXENT and Optilite, Binding Site part of Thermo Fisher Scientific) to detect and quantify monoclonal peaks in the serum. Monoclonal peaks were categorized as previously described: MGIP (0.015-0.2 g/L) or MS-MGUS (>0.2 g/L). Participants were classified into: MS-MGUS, MGIP or Negative.

Results

We analyzed 2,219 serial samples from 509 participants (median 5 samples, range 1-6) who later developed MM (n=134; median 13 yrs [IQR: 10-17]) or MGUS (n=375: median 17 yrs [IQR: 13-19]).

Among the 128 participants who progressed to MM (excluding 6 with light chain only disease), 123 (96%) had at least one sample positive for MGIP or MS-MGUS before clinical diagnosis. Of these, 90 cases (73%) showed an MS-MGUS peak, while 33 cases (27%) an MGIP peak at the earliest sample. MGIP (n=23) or MS-MGUS (n=31) was detected in 54 out of 56 patients (96%) who had available samples before the clinical MGUS diagnosis (median 3 yrs, range 1-6 yrs). A log-rank test showed no significant difference in time of progression to MM between individuals whose first sample had a monoclonal protein detected at concentrations of MGIP (median: 13, IQR 10-17 yrs) vs. MS-MGUS (median: 14, IQR 11-17 yrs) (p=0.92). We captured progression of the MGIP to MS-MGUS in 6 and 26 participants, before MM or MGUS clinical diagnosis, respectively.

We leveraged the sensitivity of MS to determine the number of monoclonal peaks present in each sample, their isotype, and mass-to-change ratio and then followed those clones across time in the serial samples. The purpose of this was to gain insight into early clonal dynamics among expanded B cell/plasma cell clones and elucidate how these monoclonal populations compete over time. Several distinct patterns were observed in 422 individuals who had at least 2 positive samples serially tested. In 324/422 participants (77%), there was a dominant clone present in all samples and had minimal dynamic alterations over time. In 127 of the 324 (39%) participants with a persistent dominant clone, MS detected only one clone in the first sample, which remained dominant with follow up until clinical detection. For the remaining cases, 197/324 (61%) participants, MS demonstrated the presence of multiple monoclonal clones (median of 2 clones, range: 2-13) in which one clone remained dominant. In the remaining 98/422 (23%) participants, clonal dominance varied significantly over time with different clones becoming dominant at various timepoints. When clones were competing for dominance, IgM isotype clones were the first o be detected at the MGIP stages and were replaced by IgG clones later becoming the dominant clone in the majority (43%) of the observed cases during follow up over time. This phenomenon of transitioning clones was observed at similar rates among participants regardless of their progression to MGUS or MM. The clonal transition was similar in all cases regardless of the clone’s concentration at first available sample (MGIP or MS-MGUS) or isotype (IgG, IgA, or IgM).

Conclusion

Our data demonstrates that monoclonal gammopathies detected by MS precede the clinical diagnosis of MGUS or MM by up to 22 years. In most cases, a dominant clone persists over time, while in 23%, there is notable clonal transition. Further investigation is needed to determine if these IgM to IgG changes constitute isotype class switch within the same B cell clone during plasma cell differentiation.