Abstract IA39: Investigating malignant transformation in Waldenstrom's macroglobulinemia
Authors: Hunter Zachary R. , Guerrera Maria Luisa , Yang Guang , Tsakmaklis Nicholas , Lu Xia , Treon Steven P. 
Published: December 2020

Waldenström's macroglobulinemia (WM) is an uncommon B-cell lymphoma that corresponds to the histopathologic classification of IgM-secreting lymphomplasmacytic lymphoma. It is also a disease characterized by a series of highly recurrent mutations. Whole-genome sequencing of CD19+ bone marrow cells from patients with WM led to the discovery of a somatic heterozygous c.978T&gt;C mutation in Toll-like receptor adaptor protein MYD88, resulting in a leucine-to-proline substitution p.Leu265Pro (L265P) in over 90% of WM patients. Somatic activating mutations in MYD88 induce constitutive homodimerization and downstream signaling through the nuclear factor kappa-light-chain-enhancer of activated B cells (NFKB) pathway independent of receptor activation. Other somatic events characteristic of WM include deletions in chromosome 6q21-23 and activating nonsense and/or frameshift mutations in the carboxyl-terminal tail of CXCR4, found in 40-60% and over 30% of WM patients, respectively. Both of these events are found predominantly in MYD88 mutant WM and are frequently restricted to a subclonal population.

Similar to related lymphomas, WM is thought to evolve from IgM-secreting monoclonal gammopathy of undermined significance (MGUS). The MYD88 mutations are easily detectible IgM MGUS stage, suggesting that it is a very early event in clonal development but may predate the malignant transformation into WM. This theory is support by several transgenic mouse models of mutant MYD88 that have demonstrated that p.Leu265Pro MYD88 mutation in B cells alone is not sufficient for lymphomagenesis (Knittel et al., Blood 2016; Sewastianik et al., Blood Adv 2019). Mutations in CXCR4 have been detected in IgM MGUS, though at lower rates than are typical of WM. Given that this and the transcriptional profile of CXCR4 activating mutations in WM are consistent with a pattern of suppression of tumor suppressors upregulated by mutant MYD88, it has been proposed as a possible transformation event from IgM MGUS to asymptomatic WM. Likewise, a number of careful genetic studies of IgM MGUS have suggested the emergence of somatic copy number alterations, particularly the deletions in 6q, to be related to the malignant transformation of WM (Schop et al., Cancer Genet Cytogenet 2006; Paiva et al., Blood 2015).

We therefore reanalyzed our whole-genome sequencing data of WM patients and performed allele-specific PCR and 6q copy number assays to study the rates and relationship of CXCR4 mutations and chromosome 6q deletions in untreated MYD88 mutated WM. Both our 30-patient whole-genome and our 25-patient PCR cohort found no relationship between symptomatic and asymptomatic WM and the somatic events, indicating that they both were occurring at an earlier stage of WM development. Both studies also found that the large clonal deletions in chromosome 6q were not observed in the presence of CXCR4 mutations (p&lt;0.01 for both). Using our previous RNASeq data of 55 WM patients, we compared the transcriptional signatures for CXCR4-mutated and 6q-deleted WM relative to WM patients with the MYD88 mutation alone, revealing an overlapping signature of 19 genes. Notably, the magnitude and relative direction of the changes were similar for CXCR4-mutated and 6q-deleted WM. Impacted genes included biologically relevant targets such as FAM110C, WNK2, CDK14, FOXO3, IGF2R, and HRK. Preliminary validation studies using qPCR and immunohistochemistry have confirmed these findings. We therefore propose that CXCR4 mutations and 6q deletions are not only appearing during the transformation from MGUS to WM but target a related set of genes that may play a critical role in the pathogenesis of MYD88 mutant WM. Functional studies to further characterize these genes and evaluate their potential as therapeutic targets for WM are ongoing.

Citation Format: Zachary R. Hunter, Maria Luisa Guerrera, Guang Yang, Nicholas Tsakmaklis, Xia Lu, Steven P. Treon. Investigating malignant transformation in Waldenstrom's macroglobulinemia [abstract]. In: Proceedings of the AACR Virtual Meeting: Advances in Malignant Lymphoma; 2020 Aug 17-19. Philadelphia (PA): AACR; Blood Cancer Discov 2020;1(3_Suppl):Abstract nr IA39.