Immunophenotypic, cytogenetic, and mutational features of chronic lymphocytic leukemia/small lymphocytic lymphoma with atypical immunophenotype

Flow cytometric analysis plays an important role in the diagnosis of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL). Most CLL cases show a typical immunophenotype characterized by the expression of CD5, CD23, CD43, ROR1, and CD200, along with dim expression of B-cell markers. However, some show an atypical immunophenotype. The immunophenotypic, cytogenetic, and mutational profiles of atypical CLL are not well characterized. In this study, we aim to address these gaps by analyzing a cohort of 270 CLL cases with a focus on those with atypical immunophenotypes. Their detailed immunophenotype as assessed by flow cytometry is presented along with cytogenetics and mutational data. Fifty-three (19.6%) cases exhibited an atypical immunophenotype. The common atypical immunophenotypic features detected included increased CD20 in 17 (32.1%), negative CD43 in 17 (32.1%), negative ROR1 in 16 (30.1%), and increased surface light chain in 11 (20.8%) cases. Trisomy 12 was more frequently detected in atypical versus typical CLL cases (58.5% vs. 20.7%, p < 0.01), including those with decreased to negative expression of CD5, CD23, CD43, and ROR1, and increased expression of CD20 and CD22. Cases with increased CD20 expression more often had mutated IGHV. BIRC3 is the most frequent mutation in the atypical CLL group, and alterations in BIRC3 (p = 0.02), KRAS (p = 0.03), NRAS (p < 0.01), KMT2D (p < 0.01), and SMARCA4 (p = 0.02) were more frequently detected in atypical CLL when compared to typical CLL. In summary, approximately 20% of CLL cases show an atypical immunophenotype, and these cases have cytogenetic abnormalities and mutation profiles that differ in frequency from typical CLL cases. Recognition of the immunophenotype of atypical CLL can be helpful in the diagnosis and differential diagnosis in low-grade B-cell neoplasms.