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Epigenetic changes during B-cell differentiation generate distinct DNA methylation signatures specific for B-cell subsets, including memory B cells (MBCs) and plasma cells (PCs). Waldenström macroglobulinemia (WM) is a B-cell malignancy uniquely comprising a mixture of lymphocytic and plasmacytic phenotypes. Here, we integrated genome-wide DNA methylation, transcriptome, mutation, and phenotypic features of tumor cells from 35 MYD88-mutated WM patients in relation to normal plasma and B-cell subsets. Patients naturally segregate into 2 groups according to DNA methylation patterns, related to normal MBC and PC profiles, and reminiscent of other memory and PC-derived malignancies. Concurrent analysis of DNA methylation changes in normal and WM development captured tumor-specific events, highlighting a selective reprogramming of enhancer regions in MBC-like WM and repressed and heterochromatic regions in PC-like WM. MBC-like WM hypomethylation was enriched in motifs belonging to PU.1, TCF3, and OCT2 transcription factors and involved elevated MYD88/TLR pathway activity. PC-like WM displayed marked global hypomethylation and selective overexpression of histone genes. Finally, WM subtypes exhibited differential genetic, phenotypic, and clinical features. MBC-like WM harbored significantly more clonal CXCR4 mutations (P = .015), deletion 13q (P = .006), splenomegaly (P = .02), and thrombocytopenia (P = .004), whereas PC-like WM harbored more deletion 6q (P = .012), gain 6p (P = .033), had increased frequencies of IGHV3 genes (P = .002), CD38 expression (P = 4.1e-5), and plasmacytic differentiation features (P = .008). Together, our findings illustrate a novel approach to subclassify WM patients using DNA methylation and reveal divergent molecular signatures among WM patients., (© 2020 by The American Society of Hematology.)

The ETS-domain transcription factors divide into subfamilies based on protein similarities, DNA-binding sequences, and interaction with cofactors. They are regulated by extracellular clues and contribute to cellular processes, including proliferation and transformation. ETS genes are targeted through genomic rearrangements in oncogenesis. The PU.1/SPI1 gene is inactivated by point mutations in human myeloid malignancies. We identified a recurrent somatic mutation (Q226E) in PU.1/SPI1 in Waldenström macroglobulinemia, a B-cell lymphoproliferative disorder. It affects the DNA-binding affinity of the protein and allows the mutant protein to more frequently bind and activate promoter regions with respect to wild-type protein. Mutant SPI1 binding at promoters activates gene sets typically promoted by other ETS factors, resulting in enhanced proliferation and decreased terminal B-cell differentiation in model cell lines and primary samples. In summary, we describe oncogenic subversion of transcription factor function through subtle alteration of DNA binding leading to cellular proliferation and differentiation arrest. SIGNIFICANCE: The demonstration that a somatic point mutation tips the balance of genome-binding pattern provides a mechanistic paradigm for how missense mutations in transcription factor genes may be oncogenic in human tumors. This article is highlighted in the In This Issue feature, p. 681 ., (©2019 American Association for Cancer Research.)