Chronic lymphocytic leukemia with IGH/BCL2 fusion and clonal heterogeneity: phenotypic and molecular profiling analysis of a rare case.

Clonal heterogeneity within chronic lymphocytic leukemia (CLL) has been increasingly recognized as a factor that may influence disease progression, treatment response, and prognosis. Here, we report a rare case of CLL with two distinct B-cell populations displaying distinct immunophenotypic characteristics, genetic mutations, and genetic evolution. The two aberrant mature B cell populations were identified in peripheral blood (PB) and bone marrow (BM) through multiparametric flow cytometry (MFC) with identical light chain restriction but distinct immunophenotypes. The first population (CD5 + CD23+CD200+) was consistent with typical CLL, while the second (CD5-CD23-CD200-) displayed atypical features. Further characterization through flow sorting, fluorescence in situ hybridization (FISH), and next-generation sequencing (NGS) revealed that both populations shared the same BCL2-IgH translocation and an identical IGHV gene usage, but exhibited different IGHV mutation rates. NGS revealed that a common progenitor cell with BCL2-IgH translocation and mutations in TP53, KMT2A, KMT2C, and KMT2D gave rise to two distinct subclones: one (CD5 + CD23+CD200+) CLL driven by mutations in TNFAIP3 and BCORL1, and the other (CD5-CD23-CD200-) driven by mutations in EP300, NOTCH1, and BCL2. This case highlights the significance of clonal heterogeneity in CLL and underscores the crucial role of MFC, flow sorting, and molecular genetics in diagnosing and understanding the complex evolution of this disease.