Abstract

Clonal expansion of CD5-expressing B cells, commonly designated as monoclonal B lymphocytosis (MBL), is a precursor condition for chronic lymphocytic leukemia (CLL). The mechanisms driving subclinical MBL B-cell expansion and progression to CLL, occurring in app. 1% of affected individuals, are unknown. An autonomously signaling B-cell receptor (BCR) is essential for CLL pathogenesis. The objective of this study was functional characterization of the BCR of MBL in siblings of CLL patients and comparison of genetic variants in MBL-CLL siblings pairs. Screening of peripheral blood by flow cytometry detected 0.2-480 clonal CLL-phenotype cells per µL (median: 37/µL) in 34 of 191 (17.8%) siblings of CLL patients. Clonal BCR isolated from highly purified CLL-phenotype cells induced robust calcium mobilization in BCR-deficient murine pre-B cells in the absence of external antigen and without experimental cross-linking. This autonomous BCR signal was less intense than the signal originating from CLL BCR of their CLL sibling. According to genotyping by single nucleotide polymorphism array, whole exome, and targeted panel sequencing, CLL risk alleles were found with high and similar prevalence in CLL patients and MBL siblings. Likewise, the prevalence of recurrent CLL-associated genetic variants was similar between CLL and matched MBL samples. However, copy number variations and small variants were frequently subclonal in MBL cells, suggesting their acquisition during subclinical clonal expansion. These findings support a stepwise CLL pathogenetic model wherein autonomous BCR signaling leads to a non-malignant (oligo)clonal expansion of CD5+ B cells, followed by malignant progression to CLL after acquisition of pathogenic genetic variants.