Asymptomatic Multiple Myeloma - Background of Progression, Evolution, and Prognosis

BACKGROUND. Asymptomatic multiple myeloma (AMM) evolves from monoclonal gammopathy of unknown significance (MGUS) and progresses to symptomatic myeloma characterized by end-organ damage. Aim of our study was to address the determinants of evolution and progression of AMM, their molecular background, and whether they are present upfront or evolve de novoin a multistep process on the background of an ongoing genetic instability.

METHODS. CD138-purified plasma cell samples of 2369 consecutive patients with MGUS, asymptomatic, and symptomatic myeloma were investigated by fluorescence-in-situ-hybridization (n=304/432/1633), 951 (n=62/259/630) by gene expression profiling. Sixty-five paired samples at AMM and disease progression were assessed by iFISH, 28 of these were further assessed by array-comparative-genomic-hybridization, as well as whole exome- (WES), and RNA-sequencing. Serum/urine samples (n=8398) allowed modelling of plasma cell accumulation in AMM and MGUS, respectively (n=322/196).

RESULTS.Up-front tumor mass, plasma cell accumulation rate and molecular characteristics, including alterations in gene expression and presence of progression-associated chromosomal aberrations, i.e. t(4;14), deletions of 13q14, 17p13, 8p21, gains of 1q21, as well as hyperdiploidy, drive and predict evolution and progression of AMM. But for hyperdiploidy, the same factors drive progression from symptomatic to relapsed myeloma and also in AMM rather their number than the specific single aberration impact on time to progression. This means that the mechanisms driving progression to symptomatic myeloma are (at least in part) the same driving progression under treatment. Molecularly, all chromosomal aberrations, most transcriptomic changes, and most frequent mutations detected in symptomatic myeloma including NRAS, KRAS, DIS3, HIST1H1E are already present in MGUS or AMM. In paired AMM/MM samples, 22/27 (81%) show a stable clonal pattern, 5/27 (19%) the de novo appearance of expressed clones, including KRAS or FAM46C. No significant transcriptomic differences are found by RNA-sequencing. (Sub-)Clonal complexity with 4-5 discernable clusters of 103-363 single nucleotide variants with an allele frequency of ≥10% remains fairly constant during disease progression with most being detectable in both AMM and MM, incompatible with clonal outgrowth to any reason in these patients.

In CONCLUSION, evolution and progression of AMM are driven and can be well predicted by factors being present upfront, i.e. tumor mass, plasma cell accumulation rate, and the set of molecular alterations. Progression is, contrary to current thinking, in the vast majority of patients not driven by de novo acquired expressed clonal alterations. This is proven in our set of paired samples on the level of chromosomal numeric or structural alterations (as per iFISH and aCGH), expressed clonal single nucleotide variants (as per whole exome- and RNA-sequencing), and remaining subclonal complexity. This in turn disproves other de novo alterations (e.g. methylation), as the subclone harboring these would then need to become clonal.