Defining Risk of MGUS and AMM Progression to Myeloma By Ig Heavy-Chain FISH

Interphase FISH analysis using probes for the immunoglobulin heavy chain constant region (IGHC) and variable regions (IGHV) revealed that plasma cells from healthy individuals and from patients with plasma cell (PC) dyscrasias had different patterns of signal-combinations of the two probes. We hypothesized that these patterns indicate diagnostic and prognostic significance.

Patients and Methods: Bone marrow specimens from 39 healthy donors, 111 patients with MGUS, 89 patients with AMM, and 96 patients with MM were examined using interphase FISH with probes for IGHC and IGHV simultaneously (Tian et al. Gene Chr Can 2014) and immune staining for cytoplasmic Ig isotypes. For each specimen, the combination patterns of IGHC and IGHV probes were scored separately for Kappa and Lambda expressing PCs. In addition, gene expression profiles (GEP) of CD138⁺-purified PCs were analyzed. FISH were also performed with the probes to chromosome 1q21 (CKS1B), 1p13 (AHCYL1), 17p13 (TP53), and 17q12 (ERBB2) loci.

Results:

1. IGHC and IGHV pattern diversity of PCs was present in all specimens, including healthy individuals. A bivariate distribution of polyclonal diversities among healthy individuals was calculated to create an ellipse defining the normal population pattern. Applying the ellipsoid definition to the patient populations, two subgroups could be distinguished in MGUS/AMM cases: one within the ellipse, representing patients with polyclonal PCs (without a dominant clone), while the other, who had a dominant clone with identical IGHC/IGHV FISH pattern, fell outside of the ellipse and was termed asymptomatic monoclonal gammopathy (Figure). With 99% confidence, 54 of 111 (48.7%) of MGUS and 66 of 89 (74.2%) of AMM were classified as monoclonal. In MM, 80 of 96 (86.0%) were stratified outside of the ellipse.

2. The FISH-defined monoclonal subgroup (outside the ellipse) among AMM patients was at high risk of progressing to symptomatic myeloma; one year progression to MM requiring therapy was 26%, 2-year progression was 37%, and the 5-year progression rate was estimated at 64%. The subgroup with polyclonal PCs in AMM had significantly lower risk of disease progression (<10% in 2 years, p<0.0001). In univariate Cox regression analysis, besides FISH-defined monoclonality (HR 2.72, p<0.004), serum albumin (< 3.5 g/dL), GEP70 risk score (> -0.26), t(4;14), excess lambda light chain, and gain of chromosome1q21 region (>2) were significant progression factors of FISH-defined monoclonal patients (Hazard Ratios >2.2; p<0.005). In multivariate analysis, FISH-defined monoclonality (HR 2.15), age (>65 yr, HR 2.05), serum albumin (< 3.5 g/dL, HR 3.21) and having AMM (HR 3.02) remained significant predictors of early disease progression with an R² of 0.434.

Conclusion:

We established a FISH predictive model on the basis of healthy bone marrow. Based on normal distribution of PC clonal diversities, patient subgroups with polyclonal PC and with monoclonal PC were detected in MGUS and AMM patients. The presence of a dominant clone in the bone marrow at the time of diagnosis identifies, among patients with MGUS and AMM, a subgroup at high-risk of early disease progression to myeloma requiring therapy. We are currently developing a model to identify with high accuracy patients will progress within 1, and 2 years.

Figure: A 100(1-α)% prediction ellipse based on clonal distribution of healthy donors (left) defines the polyclonal PC region. MGUS, AMM, and MM patients with monoclonal PCs are outside this region.