Langerhans cell histiocytosis (LCH) is a rare, clonal, haematological disease of myeloid origin involving infiltration of neoplastic cells resembling Langerhans cells. The classification of histiocytoses has recently been revised. LCH belonging to the “L” group harbor BRAF mutations or other mutations involved in the MAPK pathway. We describe a case of a 73-year-old male presenting with LCH affecting the skin and concomitant chronic myelomonocytic leukaemia (CMML). NGS revealed TET2 and DNMT3A mutations in the bone marrow, which are well characterized mutations in CMML. In LCH skin lesions, TET2, DNMT3A, and a BRAF mutation, were found. The findings indicate potential coevolution of CMML and LCH from a common haematopoietic progenitor cell. The patient’s skin lesion were refractory to conventional skin treatment. Because skin manifestations in CMML are associated with aggressive disease progression the patient initiated azacitidine (AZA). This led to marked improvement of the debilitating skin affliction suggesting that AZA may be a novel effective treatment modality for LCH with refractory skin involvement.
Histiocytoses, LCH, CMML, Azacitidine