Gene Expression Profiling of IgM Monoclonal Gammopathy of Undetermined Significance (IgM-MGUS)
Authors: Greco Antonino , Trojani Alessandra , Lodola Milena , Di Camillo Barbara , Tedeschi Alessandra , Morra Enrica 
Published: October 2019

Abstract
The 2nd 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.
Table 1. Upregulated gene sets in monoclonal vs. polyclonal IgM-MGUS GENE SET NAME REACTOME_XENOBIOTICS REGULATION_OF_CHROMOSOME_ORGANIZATION_AND_BIOGENESIS
Downregulated gene sets in monoclonal vs. polyclonal IgM-MGUS
Table 2. Linear regression results of both IgM and MC on UBTF expression GENE SET NAME REACTOME_ROLE_OF_DCC_IN_REGULATING_APOPTOSIS REACTOME_P38MAPK_EVENTS REACTOME_EARLY_PHASE_OF_HIV_LIFE_CYCLE REACTOME_MRNA_DECAY_BY_3_TO_5_EXORIBONUCLEASE KEGG_NICOTINATE_AND_NICOTINAMIDE_METABOLISM CHROMATIN_ASSEMBLY_OR_DISASSEMBLY ESTABLISHMENT_AND_OR_MAINTENANCE_OF_CHROMATIN_ARCHITECTURE PROTEIN_DNA_COMPLEX_ASSEMBLY RESPONSE_TO_DNA_DAMAGE_STIMULUS CHROMATIN_ASSEMBLY CHROMOSOME_ORGANIZATION_AND_BIOGENESIS DOUBLE_STRAND_BREAK_REPAIR CHROMATIN_REMODELING CENTROSOME_ORGANIZATION_AND_BIOGENESIS MICROTUBULE_ORGANIZING_CENTER_ORGANIZATION_AND_BIOGENESIS
Table 3 Coefficient p-value Intercept 7.0738801 6.47e-12 IgM -0.0021593 3.78e-07 MC 1.1925055 0.000501 IgM:MC 0.0010577 0.000152

Disclosures
No relevant conflicts of interest to declare.