Chronic lymphocytic leukemia often arises by a multiclonal selection process

Although chronic lymphocytic leukemia (CLL) is diagnosed by identifying a circulating B-cell clone that exceeds 5x103/μL, additional distinct clones (ADC) have been identified in various studies. Notably, the numbers of ADC documented in these studies has increased as the various technologies evolved. To better define the frequency and the characteristics of ADC in CLL, we used a next-generation sequencing platform that affords high sequencing depth along with steps that limit overcounting to analyze IGHV-IGHD-IGHJ gene rearrangements in circulating CD5+ B cells from 57 patients. Notably, all patients had at least one ADC, in addition to the clinically relevant clone. In 46 patients for whom lymphocyte count data were available, 44 had at least one ADC above the threshold of 1 B cell/μL and, remarkably, the average number of ADC was 12 per patient. Notably, in two patients, the predominant ADC qualified clinically as a separate CLL clone and in the remaining cases as low/high-count monoclonal B-cell lymphocytosis clones. Moreover, in 11 patients studied longitudinally, predominant ADC were persistent and often increased in number. ADC in patients with CLL exhibited 4-fold more stereotyped IGHV-IGHD-IGH rearrangements than found in CD5+ B cells from healthy individuals, and IGHV use, somatic mutations, and Ig isotype distribution were similar between predominant ADC and clinically relevant clones. Thus, finding multiple expanded clones within the CD5+ B cells is the rule in patients with CLL, indicating that leukemogenesis is a multiclonal process that likely involves competition among B cells with special B-cell receptor features.