Gene Expression Profiling of IgM Monoclonal Gammopathy of Undetermined Significance (IgM-MGUS)

The 2^nd IWWM tried to define reproducible criteria for the diagnosis of IgM-MGUS and Waldenstrom's Macroglobulinemia. IgM-MGUS was defined as asymptomatic condition characterized by serum IgM monoclonal protein (MC) without morphologic evidence of bone marrow (BM) lymphoplasmacytic infiltration. The proposal of the guidelines was to classify as MGUS also patients with equivocal evidence of BM infiltration, such as those presenting clonal B-cells by multiparameter flow cytometry (MFC) in the absence of morphologic evidence of BM infiltration, as well as those with equivocal BM infiltrates not confirmed by immunophenotypic studies.

Patients

The diagnosis of IgM-MGUS was made in 11 patients (6 males, 5 females) according to the consensus panel criteria. The median age at diagnosis was 73 (range, 60-77). Ten patients had K light chains. The median erythrocyte sedimentation rate was 11. The MC level at diagnosis ranged from 0.1 to 1.2 g/dL (median 0.4). Only one patient had MC value > 1.0 g/dL. The median IgM value was 697 mg/dL (range 116-1790). Five of 11 IgM-MGUS patients showed a small clonal B-cell population (light-chain-isotype-positive B-cells) detected by MFC without histologic evidence of BM infiltration. Therefore, patients were divided in 2 groups: group 1 (n=5) showing a clonal B-cell population, and group 2 (n=6) with polyclonal B-cells at MFC.

Methods and results

We isolated BM CD19+ cells in the 11 IgM-MGUS patients using Miltenyi Microbeads and performed microarray with Affymetrix-HG-U133 Plus 2.0 array. Gene set enrichment analysis (GSEA) was performed and different sets of genes were defined based on REACTOME pathways, KEGG pathways and GO Biological Process Terms. Interestingly, 17 top-ranking gene sets including differently expressed genes, reached a nominal p-value lower than 0.001; 2 gene sets were upregulated (while 15 gene sets were downregulated in monoclonal vs. polyclonal IgM-MGUS (table 1).

No genes resulted significantly differentially expressed between group 1 and group 2 using a classic SAM test for microarrays and correcting for multiple testing with a false discovery rate (FDR) threshold of 5%. Similarly, IgM and MC were not differentially expressed between the two groups, although IgM showed a nominal p-value of 0.09 (t-test).

However, when using linear regression to explain each gene expression data as a function of both IgM and MC, UBTF, TRIM5, FLJ35816, RDH10 genes were selected based on a FDR equal to 5%, applied to the F-statistic p-value. In particular, the model fitting UBTF had a p-value of 9.461e-07 and an adjusted R-squared of 0.9786; table 2 displays the coefficients of the model and the related p-values, showing a positive co-regulation of UBTF with MC.

Conclusions

In conclusion, microarray of IgM-MGUS gives insights into gene expression differences in IgM-MGUS. Notably, UBTF is a transcription factor which plays a crucial role in the transcription of rRNA in ERK-pathway, suggesting a possible role of ERK-pathway in IgM-MGUS. Additional gene expression measurements are ongoing in a larger cohort of IgM-MGUS patients.

Downregulated gene sets in monoclonal vs. polyclonal IgM-MGUS