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MYD88 L265P is the most common mutation in lymphoplasmacytic lymphoma/Waldenström macroglobulinemia (LPL/WM) and one of the most frequent in poor-prognosis subtypes of diffuse large B-cell lymphoma (DLBCL). Although inhibition of the mutated MYD88 pathway has an adverse impact on LPL/WM and DLBCL cell survival, its role in lymphoma initiation remains to be clarified. We show that in mice, human MYD88L265P promotes development of a non-clonal, low-grade B-cell lymphoproliferative disorder with several clinicopathologic features that resemble human LPL/WM, including expansion of lymphoplasmacytoid cells, increased serum immunoglobulin M (IgM) concentration, rouleaux formation, increased number of mast cells in the bone marrow, and proinflammatory signaling that progresses sporadically to clonal, high-grade DLBCL. Murine findings regarding differences in the pattern of MYD88 staining and immune infiltrates in the bone marrows of MYD88 wild-type (MYD88WT) and MYD88L265P mice are recapitulated in the human setting, which provides insight into LPL/WM pathogenesis. Furthermore, histologic transformation to DLBCL is associated with acquisition of secondary genetic lesions frequently seen in de novo human DLBCL as well as LPL/WM-transformed cases. These findings indicate that, although the MYD88L265P mutation might be indispensable for the LPL/WM phenotype, it is insufficient by itself to drive malignant transformation in B cells and relies on other, potentially targetable cooperating genetic events for full development of lymphoma., (© 2019 by The American Society of Hematology.)

Despite recent advances in treatment, human precursor B-cell acute lymphoblastic leukemia (B-ALL) remains a challenging clinical entity. Recent genome-wide studies have uncovered frequent genetic alterations involving RAS pathway mutations and loss of the INK4A /ARF locus, suggesting their important role in the pathogenesis, relapse, and chemotherapy resistance of B-ALL. To better understand the oncogenic mechanisms by which these alterations might promote B-ALL and to develop an in vivo preclinical model of relapsed B-ALL, we engineered mouse strains with induced somatic Kras G12D pathway activation and/or loss of Ink4a/Arf during early stages of B-cell development. Although constitutive activation of Kras G12D in B cells induced prominent transcriptional changes that resulted in enhanced proliferation, it was not sufficient by itself to induce development of a high-grade leukemia/lymphoma. Instead, in 40% of mice, these engineered mutations promoted development of a clonal low-grade lymphoproliferative disorder resembling human extranodal marginal-zone lymphoma of mucosa-associated lymphoid tissue or lymphoplasmacytic lymphoma. Interestingly, loss of the Ink4a/Arf locus, apart from reducing the number of apoptotic B cells broadly attenuated Kras G12D -induced transcriptional signatures. However, combined Kras activation and Ink4a/Arf inactivation cooperated functionally to induce a fully penetrant, highly aggressive B-ALL phenotype resembling high-risk subtypes of human B-ALL such as BCR-ABL and CRFL2 -rearranged. Ninety percent of examined murine B-ALL tumors showed loss of the wild-type Ink4a/Arf locus without acquisition of highly recurrent cooperating events, underscoring the role of Ink4a/Arf in restraining Kras-driven oncogenesis in the lymphoid compartment. These data highlight the importance of functional cooperation between mutated Kras and Ink4a/Arf loss on B-ALL., Competing Interests: Conflict-of-interest disclosure: The authors declare no competing financial interests.

Diagnostics and supportive care for patients with non-Hodgkin lymphoma (NHL) in lower- and middle-income countries (LMICs) are lacking. We hypothesized that high-throughput transcription-based diagnostics could classify NHL specimens from Malawi amenable to targeted therapeutics. We established tissue microarrays and classified 328 cases diagnosed by hematoxylin and eosin as NHL at University of Malawi College of Medicine using immunohistochemistry (IHC) for conventional markers and therapeutic targets. A subset was analyzed using NanoString-based expression profiling with parsimonious transcriptional classifiers. Overall, 72% of lymphomas were high-grade B-cell tumors, subsets of which were enriched for expression of MYC, BCL2, and/or PD-L1. A 21-gene transcriptional classifier, previously validated in Western cohorts, divided 96% of diffuse large B-cell lymphomas (DLBCLs) with 100% of B-cell lymphomas, unclassifiable, into 1 cluster and 88% of Burkitt lymphomas into a separate cluster. Cell-of-origin categorization of 36 DLBCLs by NanoString lymphoma subtyping test (LST) revealed 69% concordance with IHC. All discordant cases were classified as germinal center B cell-like (GCB) by LST but non-GCB by IHC. In summary, utilization of advanced diagnostics facilitates objective assessment and segregation of biologically defined subsets of NHL from an LMIC without expert review, thereby establishing a basis for the implementation of effective and less toxic targeted agents., Competing Interests: Conflict-of-interest disclosure: D.G., C.D.C, and S.J.R. have a pending patent application (WO/2015/157291) for MYCKEY NanoString-based molecular signature. The remaining authors declare no competing financial interests.