Molecular Evidence of Ongoing Antigen Driven Somatic Hypermutation in the Natural History of Classic and Variant Hairy Cell Leukemia

Classic hairy cell leukemia (HCL) is a clonal chronic B-cell malignancy representing 2% of all leukemias, with neoplastic B lymphocytes in the blood, bone marrow and organs of the classic reticuloendothelial system. HCL variant (HCLv) is less common, typically has inferior prognosis, and usually lacks several HCL-related features including BRAF V600E, CD25, and Annexin A1.  The malignant cells in HCL and HCLv patients have one or in unusual cases two different functional immunoglobulin heavy chain rearrangements. Significant variability can be observed in the 3rd complementarity determining region (CDR3), comprising the variable heavy (IGHV), diversity (IGHD), and joining (IGHJ) domains. A role for continued antigen selection in the pathogenesis and evolution of HCL and HCLv has been suggested but limited evidence has been found.

We examined HCL and HCLv-derived IGHV genes by massive parallel deep sequencing in 15 HCL and 7 HCLv cases. In all studied samples, a productive monoclonal tumor-derived IGH sequence was detected. Analysis of all reads sharing clonal CDR3 motifs revealed the existence of dominant subclone expansion and multiple subclonal variants. Analysis of phylogenetic trees indicated multiple branching and therefore exposure to multiple rounds of somatic hypermutation (SHM) in the evolution of clonally-related malignant cells. To investigate a possible cause of the high SHM activity and the large number of subclones in HCL and HCLv, we studied the expression of activation induced cytidine deaminase (AID), an enzyme essential for somatic hypermutations, expressed by germinal center (GC) B-cells where SHM occurs.  In 5 HCL and 3 HCLv cases we detected a high expression level of AID mRNA, including wild-type and 2 splice variants; in 10 HCL, 4 HCLv, and 3 normal peripheral blood samples, AID expression was not detected using standard end-point PCR conditions. However, more sensitive real-time quantitative PCR detected AID transcripts in virtually all tested HCL and HCLv cases, although the range of transcript levels was large between different cases and varied with individual cases over time.

The data obtained for both HCL and HCLv fit a model of tumorigenesis in which the BRAF V600E mutation (or another event in HCLv) initiates neoplastic transformation in a GC B-cell committed to terminal differentiation, but still thereafter still targeted by ongoing SHM