Evidence that chronic lymphocytic leukemia arises from a multiclonal selection process

Abstract Although chronic lymphocytic leukemia (CLL) is diagnosed by identifying in a patient’s blood a B-cell clone that exceeds 5x10⁶/µL, additional clones have on occasion been identified. To characterize such clones in patients with CLL, we analyzed by NGS the IGHV-IGHD-IGHJ gene rearrangements in circulating CD5⁺ B cells from 57 untreated patients. In every patient, we identified additional distinct clonotypes (ADCs) from the clinically relevant clone (CRC). Even when defining clonal expansion using a rigorous threshold, 68% of patients had at least one expanded ADC. The predominant ADC (pADC) reached numbers qualifying as a CLL clone in two patients and qualifying as low/high-count monoclonal B-cell lymphocytosis in each other case. Over time, pADCs enlarged or shrank, suggesting an ongoing selection process. Stereotyped IGHV-D-J rearrangements were enriched in ADCs and IGHV usage and mutations revealed similarities specifically between pADCs and CRCs, while minor ADCs more closely resembled normal CD5⁺ B-cells. Thus, finding multiple expanded clones within the CD5⁺ B-cell population of patients with CLL is more the rule rather than the exception. Thus, leukemogenesis involves a competitive selection for dominance of more than one B-cell clone, with the process being influenced to some degree by the structure of the B-cell receptor.