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Competing Interests: Kazimierz Groen: BMS and Beigene: speakers bureau (no personal funding). Febe Smits: No conflicts of interest. Kazem Nasserinejad: No conflicts of interest. Mark‐David Levin: Support for attending meetings and/or travel: Janssen, Takeda. Josien C. Regelink: No conflicts of interest. Gert‐Jan Timmers: Participation on an Advisory Board: Novartis; Travel, Accommodations, Expenses; Novartis, Janssen. Esther G. M. de Waal: No conflicts of interest. Matthijs Westerman: No conflicts of interest. Gerjo A. Velders: No conflicts of interest. Koen de Heer: No conflicts of interest. Rineke B. L. Leys: No conflicts of interest. Roel J. W. van Kampen: No conflicts of interest. Claudia A. M. Stege: Speaker's Bureau: Sanofi, Celgene/Bristol Myers Squibb, Takeda; Consulting or Advisory Role: Sanofi, Janssen. Maarten R. Seefat: No conflicts of interest. Inger S. Nijhof: Payment or honoraria for lectures, presentations, or educational events: Janssen, Celgene/Bristol Myers Squibb, Sanofi. Ellen van der Spek: Payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events: Janssen. Saskia K. Klein: No conflicts of interest. Niels W. C. J. van de Donk: Consulting or Advisory Role: Janssen, Celgene, Bristol Myers Squibb, Novartis, Amgen, Servier, Takeda, Bayer, Roche, Pfizer, Abbvie, Adaptive (no personal funding); Research Funding: Janssen, Celgene, Amgen, Novartis, Bristol Myers Squibb, Cellectis. Paula F. Ypma: Payment or honoraria for presentations: Janssen; Support for attending meetings and/or travel: Janssen. Sonja Zweegman: Consulting or Advisory Role: Janssen‐Cilag, Takeda, Celgene/Bristol Myers Squibb, Sanofi, Oncopeptides (no personal funding); Research Funding: Janssen, Takeda.

Patients with lymphoplasmacytic lymphoma/Waldenström macroglobulinemia (LPL/WM) occasionally develop diffuse large B-cell lymphoma (DLBCL). This mostly results from LPL/WM transformation, although clonally unrelated DLBCL can also arise. LPL/WM is characterized by activating MYD88 L265P (>95%) and CXCR4 mutations (~30%), but the genetic drivers of transformation remain to be identified. Here, in thirteen LPL/WM patients who developed DLBCL, the clonal relationship of LPL and DLBCL together with mutations contributing to transformation were investigated. In 2 LPL/WM patients (15%), high-throughput sequencing of immunoglobulin gene rearrangements showed evidence of >1 clonal B-cell population in LPL tissue biopsies. In the majority of LPL/WM patients, DLBCL presentations were clonally related to the dominant clone in LPL, providing evidence of transformation. However, in 3 patients (23%), DLBCL was clonally unrelated to the major malignant B-cell clone in LPL, of which 2 patients developed de novo DLBCL. In this study cohort, LPL displayed MYD88 L265P mutation in 8 out of eleven patients analyzed (73%), while CXCR4 mutations were observed in 6 cases (55%). MYD88 WT LPL biopsies present in 3 patients (27%) were characterized by CD79B and TNFAIP3 mutations. Upon transformation, DLBCL acquired novel mutations targeting BTG1, BTG2, CD79B, CARD11, TP53, and PIM1 . Together, we demonstrate variable clonal B-cell dynamics in LPL/WM patients developing DLBCL, and the occurrence of clonally unrelated DLBCL in about one-quarter of LPL/WM patients. Moreover, we identified commonly mutated genes upon DLBCL transformation, which together with preserved mutations already present in LPL characterize the mutational landscape of DLBCL occurrences in LPL/WM patients., Competing Interests: The authors have no conflicts of interest to disclose., (Copyright © 2023 the Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the European Hematology Association.)

Competing Interests: FHS: Grants or contracts from Celgene, Janssen, Oncopeptides, Sanofi, GSK Targovax. Payments or honoraria for lectures/presentations/speakers bureus/article writing or educational events: Amgen, BMS, Takeda, Abbvie, Janssen, Novartis, SkyliteDX, Oncopeptides, Sanofi, Pfizer, Daiki-Sankyo, GSK. Participation on a Data Safety Monitoring Board or Advisory Board: Abbvie, GSK, Celgene, Takeda, Janssen, Oncopeptides, Sanofi, BMS. MH: Royalties of payments for lectures: Janssen, BMS, Pfizer. EvdS: Royalties for lectures Janssen. PFY: Royalties of payments for lectures/article preparations/etc: Janssen and Amgen: participation in a company sponsored speakers’ bureau. UHM: Employment/Consultation: Advisory boards: Amgen, Sanofi, Pfizer. Royalties of payments for lectures/article preparations/etc: Lecture honoraria: Amgen, Janssen, BMS, Sanofi, GSK. NWCJvdD: Research support from Janssen Pharmaceuticals, AMGEN, Celgene, Novartis, Cellectis and BMS, and serves in advisory boards for Janssen Pharmaceuticals, AMGEN, Celgene, BMS, Takeda, Roche, Novartis, Bayer, Adaptive, and Servier. PS: Consulting or Advisory Role: Celgene, Janssen, Amgen, Karyopharm Therapeutics, CARsgen Therapeutics, Pfizer. Research Funding: Janssen, Amgen, BMS/Celgene. NA: Research grant, honorarium and advisory board, Takeda. SZ: Consulting or Advisory Role: Janssen-Cilag, Takeda, Celgene/Bristol Myers Squibb, Sanofi, Oncopeptides (no personal funding). Research Funding: Janssen, Takeda. All the other authors have no conflicts of interest to disclose.