Monoclonal gammopathy of indeterminate potential (MGIP) as a stable premalignant entity preceding hematologic malignancy

Abstract INTRODUCTION Recent advances in mass spectrometry (MS) have enabled the detection of low-level monoclonal immunoglobulins previously undetectable by conventional electrophoresis, termed monoclonal gammopathy of indeterminate potential (MGIP). MGIP typically presents below the 0.2 g/L detection threshold of electrophoresis, and its biological and clinical significance remains under investigation. In this study, we sought to assess whether MGIP is an early indicator of progression to multiple myeloma (MM) or other hematologic malignancies, and to define its clonal properties using multi-platform genomic and transcriptomic approaches. METHODS A total of 2,194 serum samples from 491 individuals in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial (PLCO) cohort was used for MS screen, including 122 who developed MM (progressors) and 369 with stable monoclonal gammopathy of undetermined significance (MGUS) but did not progress to MM (non-progressors). To validate the persistence of MGIP, we analyzed an independent set of 932 samples from 461 participants in the Predicting Progression of Developing Myeloma in a High-Risk Screened Population (PROMISE) study, which is a cohort of individuals at high risk for MM. Additional cases were obtained from the Dana-Farber Cancer Institute observational Precursor Crowd (PCROWD) cohort and the Personalised progression prediction in patients with monoclonal gammopathy of undetermined significance or smouldering multiple myeloma (PANGEA) study, both focused on monoclonal gammopathies and MM progression. We leveraged these cases and conducted single-cell RNA sequencing (scRNA-seq), single-cell B cell receptor sequencing (scBCR-seq), and whole-genome sequencing (WGS) to investigate clonal relationships and molecular features of MGIP-associated cells. RESULTS MS analysis of longitudinal samples from 491 individuals in the PLCO cohort revealed that MGIP frequently precedes MGUS and MM. Among 122 progressors, 16% exhibited persistent MGIP and 10% transitioned from MGIP to MGUS (≥0.2g/L) prior to MM diagnosis. In the PROMISE cohort (n=461), 60 to 72% of MGIP cases remained MS-positive at follow-up, with 13% of MGIP-high individuals progressing to MS-MGUS. Serial MS profiling revealed multiple dynamic patterns of M-protein behavior. Among 105 progressors with ≥2 MS-positive time points, 31% showed monoclonal persistence, 51% exhibited stable single dominance, and 17% displayed shifting dominance of M-protein peaks. A change in the isotype class of the dominant M-protein was observed in 18% of progressors initially presenting with IgM dominance. Similar dynamics were seen among non-progressors. M-proteins that expanded from MGIP to MGUS levels were more likely to be IgG or IgA in progressors and IgM in non-progressors (p<0.01). Growth modeling showed that M-proteins expanding to MGUS-level in progressors had significantly higher maximum M-protein concentration than non-progressors. Single-cell analysis of a t(11;14) MM patient with multiple MS-detected peaks revealed three distinct tumor clones, including one producing MGIP-level M-protein. Comparison of V(D)J sequences between these clones and the germline sequence, along with CNV profiling, suggested early divergence of the MGIP clone from a common ancestor, followed by emergence of more advanced subclones harboring additional genomic alterations. Finally, in individuals with persistent IgM MGIP from the PROMISE and PCROWD cohorts, we identified clonally expanded B-cell populations responsible for MGIP, and these clones showed CLL-like transcriptional profiles and harbored somatic driver mutations and copy number alterations associated with lymphoid malignancies. CONCLUSION Our findings establish MGIP as a stable and biologically relevant state that precedes clinically recognized MGUS and MM. The integration of MS-based detection with single-cell and genomic profiling reveals clonal complexity and evolutionary trajectories, underscoring the need for early monitoring strategies in individuals harboring low-level M-proteins.