Single-cell analysis of MYD88L265P and MYD88WT Waldenström macroglobulinemia patients

Waldenström macroglobulinemia (WM) is characterized by the expansion of clonal lymphoplasmacytic cells; the MYD88L265P somatic mutation is found in >90% of patients, but malignant B cells may still display intra-clonal heterogeneity. To assess clonal heterogeneity in WM, we generated and performed single-cell RNA sequencing of CD19+ sorted cells from five patients with MYD88 L265P and two patients with MYD88 WT genotype as well as two healthy donors. We identified distinct transcriptional patterns in the clonal subpopulations not only between the two genetically distinct WM subgroups but also among MYD88 L265P patients, which affected the B cell composition in the different subgroups. Comparison of clonal and normal/polyclonal B cells within each patient sample enabled the identification of patient-specific transcriptional changes. We identified gene signatures active in a subset of MYD88L265P patients, while other signatures were active in MYD88 WT patients. Finally, gene expression analysis showed common transcriptional features between patients compared to the healthy control but also differentially expressed genes between MYD88 L265P and MYD88 WT patients involved in distinct pathways, including NFκΒ, BCL2, and BTK. Overall, our data highlight the intra-tumor clonal heterogeneity in WM with potential prognostic and therapeutic implications.

Who Knew? Dopamine Transporter Activity Is Critical in Innate and Adaptive Immune Responses

The dopamine transporter (DAT) regulates the dimension and duration of dopamine transmission. DAT expression, its trafficking, protein-protein interactions, and its activity are conventionally studied in the CNS and within the context of neurological diseases such as Parkinson's Diseases and neuropsychiatric diseases such as drug addiction, attention deficit hyperactivity and autism. However, DAT is also expressed at the plasma membrane of peripheral immune cells such as monocytes, macrophages, T-cells, and B-cells. DAT activity via an autocrine/paracrine signaling loop regulates macrophage responses to immune stimulation. In a recent study, we identified an immunosuppressive function for DAT, where blockade of DAT activity enhanced LPS-mediated production of IL-6, TNF-α, and mitochondrial superoxide levels, demonstrating that DAT activity regulates macrophage immune responses. In the current study, we tested the hypothesis that in the DAT knockout mice, innate and adaptive immunity are perturbed. We found that genetic deletion of DAT (DAT^-/-) results in an exaggerated baseline inflammatory phenotype in peripheral circulating myeloid cells. In peritoneal macrophages obtained from DAT^-/- mice, we identified increased MHC-II expression and exaggerated phagocytic response to LPS-induced immune stimulation, suppressed T-cell populations at baseline and following systemic endotoxemia and exaggerated memory B cell expansion. In DAT^-/- mice, norepinephrine and dopamine levels are increased in spleen and thymus, but not in circulating serum. These findings in conjunction with spleen hypoplasia, increased splenic myeloid cells, and elevated MHC-II expression, in DAT^-/- mice further support a critical role for DAT activity in peripheral immunity. While the current study is only focused on identifying the role of DAT in peripheral immunity, our data point to a much broader implication of DAT activity than previously thought. This study is dedicated to the memory of Dr. Marc Caron who has left an indelible mark in the dopamine transporter field.

IgM-MM is predominantly a pre-germinal center disorder and has a distinct genomic and transcriptomic signature from WM

Immunoglobulin M (IgM) multiple myeloma (MM) is a rare disease subgroup. Its differentiation from other IgM-producing gammopathies such as Waldenström macroglobulinemia (WM) has not been well characterized but is essential for proper risk assessment and treatment. In this study, we investigated genomic and transcriptomic characteristics of IgM-MM samples using whole-genome and transcriptome sequencing to identify differentiating characteristics from non-IgM-MM and WM. Our results suggest that IgM-MM shares most of its defining structural variants and gene-expression profiling with MM, but has some key characteristics, including t(11;14) translocation, chromosome 6 and 13 deletion as well as distinct molecular and transcription-factor signatures. Furthermore, IgM-MM translocations were predominantly characterized by VHDHJH recombination-induced breakpoints, as opposed to the usual class-switching region breakpoints; coupled with its lack of class switching, these data favor a pre-germinal center origin. Finally, we found elevated expression of clinically relevant targets, including CD20 and Bruton tyrosine kinase, as well as high BCL2/BCL2L1 ratio in IgM-MM, providing potential for targeted therapeutics.

Origin of Waldenstrom's macroglobulinaemia

Waldenstrom's macroglobulinaemia (WM) is an MYD88^L265P-mutated lymphoplasmacytic lymphoma that invades bone marrow and secretes monoclonal immunoglobulin M (IgM). WM cells are usually unable to undergo class switch recombination, and have mutated IGHV, with a typical immunophenotype CD19⁺/CD22^low+/CD23^-/CD25⁺/CD27⁺/CD45⁺/CD38^low+/SmIgM⁺ (negative for CD5, CD10, CD11c, CD103). This immunophenotype matches memory B cells (smIgM^-/+/CD10^-/CD19⁺/CD20⁺/CD27⁺/CD38^low+/CD45⁺), representing 30% of B cells in the blood. Fifty percent of them have not undergone class switch recombination and are IgM⁺. These cells have suffered somatic hypermutation as WM cells. Genetic abnormalities do not abrogate the capacity to progress to plasma cells that usually belong to the clonal WM compartment, with a normal immunophenotype and functional characteristics. However, some WM cells are CD27^-, MYD88^WT, without somatic hypermutation, or with class switch recombination capable of reactivation. Thus, most data support a B-memory-cell origin for WM, but a small fraction of cases may have a different origin.

HCK is a survival determinant transactivated by mutated MYD88, and a direct target of ibrutinib

HCK transcription and activation is triggered by mutated MYD88, and is an important determinant of pro-survival signaling. HCK is also a target of ibrutinib, and inhibition of its kinase activity triggers apoptosis in mutated MYD88 cells.

Transcriptome sequencing reveals a profile that corresponds to genomic variants in Waldenström macroglobulinemia

Whole-genome sequencing has identified highly prevalent somatic mutations including MYD88, CXCR4, and ARID1A in Waldenström macroglobulinemia (WM). The impact of these and other somatic mutations on transcriptional regulation in WM remains to be clarified. We performed next-generation transcriptional profiling in 57 WM patients and compared findings to healthy donor B cells. Compared with healthy donors, WM patient samples showed greatly enhanced expression of the VDJ recombination genes DNTT, RAG1, and RAG2, but not AICDA Genes related to CXCR4 signaling were also upregulated and included CXCR4, CXCL12, and VCAM1 regardless of CXCR4 mutation status, indicating a potential role for CXCR4 signaling in all WM patients. The WM transcriptional profile was equally dissimilar to healthy memory B cells and circulating B cells likely due increased differentiation rather than cellular origin. The profile for CXCR4 mutations corresponded to diminished B-cell differentiation and suppression of tumor suppressors upregulated by MYD88 mutations in a manner associated with the suppression of TLR4 signaling relative to those mutated for MYD88 alone. Promoter methylation studies of top findings failed to explain this suppressive effect but identified aberrant methylation patterns in MYD88 wild-type patients. CXCR4 and MYD88 transcription were negatively correlated, demonstrated allele-specific transcription bias, and, along with CXCL13, were associated with bone marrow disease involvement. Distinct gene expression profiles for patients with wild-type MYD88, mutated ARID1A, familial predisposition to WM, chr6q deletions, chr3q amplifications, and trisomy 4 are also described. The findings provide novel insights into the molecular pathogenesis and opportunities for targeted therapeutic strategies for WM.

Clonotypic analysis of immunoglobulin heavy chain sequences in patients with Waldenström's macroglobulinemia: correlation with MYD88 L265P somatic mutation status, clinical features, and outcome

We performed IGH clonotypic sequence analysis in WM in order to determine whether a preferential IGH gene rearrangement was observed and to assess IGHV mutational status in blood and/or bone marrow samples from 36 WM patients. In addition we investigated the presence of MYD88 L265P somatic mutation. After IGH VDJ locus amplification, monoclonal VDJ rearranged fragments were sequenced and analyzed. MYD88 L265P mutation was detected by AS-PCR. The most frequent family usage was IGHV3 (74%); IGHV3-23 and IGHV3-74 segments were used in 26% and 17%, respectively. Somatic hypermutation was seen in 91% of cases. MYD88 L265P mutation was found in 65,5% of patients and absent in the 3 unmutated. These findings did not correlate with clinical findings and outcome. Conclusion. IGH genes' repertoire differed in WM from those observed in other B-cell disorders with a recurrent IGHV3-23 and IGHV3-74 usage; monoclonal IGHV was mutated in most cases, and a high but not omnipresent prevalence of MYD88 L265P mutation was observed. In addition, the identification of 3 patients with unmutated IGHV gene segments, negative for the MYD88 L265P mutation, could support the hypothesis that an extra-germinal B-cell may represent the originating malignant cell in this minority of WM patients.