Single‐cell analysis of MYD88^L265P and MYD88^WT 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 MYD88^L265P 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.