miRNA Analysis Identifies a Unique Expression in Waldenström Macroglobulinemia B Cells and Plasma Cells

MicroRNAs (miRNAs) are small noncoding RNAs that are approximately 20–22 nucleotides with critical functions in cell growth, survival, and differentiation. These conserved sequences can regulate expression of multiple genes and are often tissue specific and dysregulated in malignancies. Thus, miRNA profiling has been used to create signatures for many solid tumors. These profiles have been used to classify tumors and to help predict survival and outcome. In the present study, we utilized the DiscovArray miRNA profiling service (Asuragen Services, Austin, TX) which utilizes a custom-manufactured Affymetrix GeneChip^® from Ambion that covers miRNAs derived from the Sanger miRBase (http://microrna.sanger.ac.uk/sequences/index.shtml) and over 11,000 predicted miRNAs derived from published reports. The signal processing implemented was a multi-step process involving probe-specific signal detection calls, background estimation and correction, constant variance stabilization and global normalization. For each probe, an estimated background value was subtracted derived from the median signal of a set of G-C-matched anti-genomic controls. Arrays within a specific experiment were normalized together according to variance stabilization method. Detection calls were based on a Wilcoxon rank-sum test of the miRNA probe signal compared to the distribution of signals from GC-content matched anti-genomic probes. For statistical hypothesis testing, a two-sample t-test, with assumption of equal variance, was applied. One-way ANOVA was used for multiple group comparison. Probes were considered to be differentially expressed based on two criteria: a p-value of < 0.001 and glog₂ difference > 1. miRNA expression was analyzed in all malignant B lineage cells (CD19+ CD138+) (n=8), malignant B cells alone (CD19+) (n=6) and plasma cells alone (CD138+) (n=3) from Waldenström macroglobulinemia (WM) patients. The expression was compared to malignant CD19+ B cells from chronic lymphocytic leukemia (CLL) patients (n=5), malignant plasma cells (CD138+) from multiple myeloma (MM) patients (n=5) and to B lineage cells (CD19+ CD138+) (n=4), CD19+ B-lymphocytes (n=3) and CD138+ plasma cells (n=6) from healthy donors. Data analysis based on a total of approximately 11,000 miRNAs analyzed shows that CD19+ CD138+ cells (double sorting) from WM patients did not cluster as a unique group. Some samples had a pattern similar to CLL, some similar to MM and others similar to CD19+ CD138+ cells from healthy controls. This lack of clear signature was observed by others in gene expression profiling and CGH arrays. We therefore hypothesized this lack of clustering was due to the lymphoplasmacytic nature of WM cells and therefore we analyzed B cells (CD19+) and plasma cells (CD138+) separately. miRNA expression in B cells (CD19+) identifies a signature in normal B cells that is absent in both WM B cells (CD19+) and CLL cells. There is also a set of miRNAs that are absent in normal B cells that are expressed in WM B cells and CLL. In addition, WM B cells had a unique miRNA signature that is unique compared to CLL and normal B cells. An additional set of miRNAs were expressed and clustered only in CLL patients. Similar to B cells, plasma cell (CD138+) analysis in WM, MM and healthy donors shows a clustering pattern that identifies normal plasma cells from MM plasma cells. WM plasma cells had a miRNA signature that is unique only to WM patients, however, a subset of miRNAs shared an expression pattern with MM plasma cells. While miRNAs can target multiple genes, some of the genes that are targets of the miRNAs identified in this analysis include XBP-1, Blimp-1, IRF-4, Bcl-6 and TACI. These target genes are known to be important in B cell and plasma cell development. In summary, we have analyzed miRNA expression in malignant B cells (CD19+) and malignant plasma cells (CD138+) from WM patients and compared their expression pattern to their normal counterpart as well as malignant counterpart in CLL B cells and MM plasma cells. Our analysis shows that WM B cells have a miRNA signature unique to WM only and one that is shared by CLL cells. Similarly, WM plasma cells have a unique miRNA signature but also has some miRNAs that are shared by malignant plasma cells in MM. These miRNAs target genes involved in B cell differentiation. Analysis of the functional roles of these miRNAs will and their regulation will further our understanding of the regulation of B cells development in normal and malignant conditions.