Waldenström’s Macroglobulinemia (WM) Is Preceded By Clonal Lymphopoiesis Including MYD88 L265P in Progenitor B Cells

Background: The transformation from a normal to a cancer cell is driven by the multistep acquisition of genetic alterations. Recently, shared mutations between clonal B cells in MBL/CLL and CD34+ hematopoietic progenitor cells (HPC) have been identified. Similarly, a HPC origin of BRAFV600E mutations in hairy cell leukemia (HCL) has been uncovered, strengthening the notion that at least a fraction of somatic mutations may occur in CD34+ HPC before the malignant transformation of some B cell neoplasms. Since almost all WM patients have mutated MYD88L265P, it is worthy to investigate if this disease follows a similar pathogenic process than that of MBL/CLL or HCL. Aim: Define the cellular origin of WM by comparing the genetic landscape of WM cells to that of CD34+ HPC, B cell precursors and residual normal B cells. Methods: We used FACSorting to isolate 57 cell subsets from bone marrow (BM) aspirates of 10 WM patients: CD34+ HPC, B cell precursors, residual normal B cells (if detectable), WM B cells, plasma cells (PCs) and T cells (germline control). Whole-exome sequencing (WES, mean depth 79x) was performed with 10XGenomics Exome Solution for low DNA-input due to limited numbers of some cell types. Single-cell RNA and B-cell receptor sequencing (scRNA/BCRseq) was performed in total BM B cells and PCs (n=32,720) from 3 IgM MGUS and 2 WM patients. Accordingly, the clonotypic BCR detected in WM cells was unbiasedly investigated in all B cell maturation stages defined according to their molecular phenotype. In parallel, MYD88p.L252P (orthologous position of the human L265P mutation) transgenic mice were crossed with conditional Sca1Cre, Mb1Cre, and Cγ1Cre mice to selectively induce in vivo expression of MYD88 mutation in CD34+ HPC, B cell precursors and germinal center B cells, respectively. Upon immunization, mice from each cohort were necropsied at 5, 10 and 15 months. Results: All 10 WM patients showed MYD88L265P and 3 had mutated CXCR4. Notably, we found MYD88L265P in B cell precursors from 1/10 cases and in residual normal B cells from 4/10 patients, which were confirmed by ASO-PCR and ddPCR. Indeed, these more sensitive methods detected MYD88L265P in B cell precursors from 6/10 cases and in residual normal B cells from 6/10 patients. CXCR4 was simultaneously mutated in B cell precursors and WM B cells from one patient. Overall, CD34+ HPC, B-cell precursors and residual normal B cells shared a median of 2 (range, 0-45; mean VAF, 0.13), 3 (range, 1-44; mean