<i>KRAS</i> may facilitate transformation of chronic lymphocytic leukemia to histiocytic sarcoma with indeterminate dendritic cell features

HS is a rare proliferation of malignant tissue histiocytes that is usually primary, although secondary HS exists and often demonstrates mutations in the Ras/Raf/MAPK or PI3K/AKT/mTOR pathways, but HS with indeterminate dendritic cell (IDC) features has not been previously reported. A 77-year-old man with chronic lymphocytic leukemia (CLL) presented with an oropharyngeal mass. Biopsy specimen showed large atypical histiocytic cells with oval-to-irregular indented nuclei. They were positive for CD33, CD4, CD68 (subset, weak), CD163 (subset, weak), BCL6, S100 (subset), CD1a, cyclin D1 (subset), and lysozyme (weak) but negative for Langerin, BRAF V600E, CD21, CD23, CD35, CD123, TCF4, TCL1, MPO, CD20, CD79a, CD10, MUM1, and BCL2. The patient was diagnosed with secondary HS with IDC features as well as chronic myelomonocytic leukemia (CMML) in the bone marrow. Careful genomic dissection of all 3 types of malignant cells showed that SF3B1 p.E622D was present in both CLL and HS but not CMML. In addition, the HS acquired KRAS p.G13D, which we hypothesize drove the transdifferentiation of CLL to HS. Moreover, next-generation sequencing (NGS) clonotypic evaluation of variable-diversity-joining (VDJ) rearrangements in both the HS and CLL established relatedness but not the CMML.Conclusion: This is the first report of secondary HS with IDC features arising from CLL. We establish by both IGH NGS analysis and mutational profiling that the CLL and HS are clonally-related and posit that acquisition of KRAS p.G13D drove transdifferentiation. This has therapeutic implications for targeting the RAS-BRAF-MAPK-ERK pathway.