In Utero Cltc-ALK Fusion In Hematopoietic Progenitor Cells As a First Step Of Leukemogenesis In Blastic Plasmacytoid Dendritic Cell Neoplasm

Introduction Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a subtype of myeloid leukemia mainly affecting the elderly and often accompanied by cutaneous legions. It is a rare disease, and neither the genetic nor clonal origin of the disease is known. We report the first case of BPDCN with clathrin heavy chain (CLTC)-anaplastic lymphoma kinase (ALK) fusion gene. We performed a detailed analysis to understand the origin of the tumor cells and the leukemic process involved. Samples and Results Samples were collected from a female infant who was admitted under the diagnosis of hemophagocytic lymphohistiocytosis (HLH) at 1 month of age. One month later, leukemic blasts appeared in the peripheral blood showing karyotypic abnormality 46,XX,t(2;17;8)(p23;q23;p23). Fluorescence in situ hybridization with break apart probes covering the ALK gene revealed translocation of the 3’-ALK signal to der(17) and loss of the 5’ ALK signal on der(2). CLTC-ALK fusion was identified by direct sequencing of the RT-PCR product obtained from the peripheral blood specimen. Although HLH symptoms improved after one course of chemotherapy, blast cells re-appeared in the peripheral blood and bone marrow after 3 courses of chemotherapy, with a karyotype of 45, XX, t(2;17;8)(p23;q23;p23), -7. Multicolor flow cytometry showed the blast cells were weakly positive for CD4 and negative for CD3, and expression of CLTC-ALKwas confirmed in these cells. Some of the blasts were highly positive for CD123 and CD303, indicating the plasmacytoid dendritic cell phenotype and leading to the diagnosis of BPDCN. The rest of the blasts were positive for CD56 and weakly positive for CD123. Nearly half of this CD4+CD56+ population was also positive for monocytic marker, CD14. The possibility of in utero origin of the leukemic cells was tested by analyzing the presence of CLTC-ALK fusion in the Guthrie card. The genomic breakpoint of the CLTC-ALKfusion was determined by inverse PCR, and then 24 pieces of the Guthrie card containing the neonatal blood were tested for the existence of the cells carrying the same fusion breakpoint. The testing revealed the prenatal origin of the fusion gene. To explore the origin of leukemic transformation in the patient, the presence of the CLTC-ALK fusion gene was assessed in genomic DNA extracted from subpopulations sorted from the patient’s peripheral blood. As well as leukemic CD4+CD3- cells, most of the monocytes possessed the CLTC-ALK fusion gene, and a small portion of T cells, B cells and neutrophils were also positive for genomic CLTC-ALK fusion. Immature cells with high CD34 expression but without lineage markers separated from the peripheral blood were also positive for CLTC-ALKfusion. Conclusions The CLTC-ALK fusion gene was identified for the first time as the leukemia-promoting abnormality in an infant case of myeloid neoplasm BPDCN, indicating the tumorigenic potential of CLTC-ALK in myeloid progenitor cells. In addition, activated monocytes with the CLTC-ALK fusion might be responsible for the occurrence of HLH in the patient. Formation of the CLTC-ALK fusion was suggested to have occurred in a hematopoietic progenitor cells in utero, and the subsequent acquisition of monosomy 7, one of the myeloid lineage-oriented abnormalities, might have determined the cell fate to a myeloid neoplasm in this patient. Disclosures: No relevant conflicts of interest to declare.