Juvenile myelomonocytic leukaemia (JMML) is characterized by gene variants that deregulate the RAS signalling pathway. Children with neurofibromatosis type 1 (NF-1) carry a defective NF1 allele in the germline and are predisposed to JMML, which presumably requires somatic inactivation of the NF1 wild-type allele. Here we examined the two-hit concept in leukaemic cells of 25 patients with JMML and NF-1. Ten patients with JMML/NF-1 exhibited a NF1 loss-of-function variant in combination with uniparental disomy of the 17q arm. Five had NF1 microdeletions combined with a pathogenic NF1 variant and nine carried two compound-heterozygous NF1 variants. We also examined 16 patients without clinical signs of NF-1 and no variation in the JMML-associated driver genes PTPN11, KRAS, NRAS or CBL (JMML-5neg) and identified eight patients with NF1 variants. Three patients had microdeletions combined with hemizygous NF1 variants, three had compound-heterozygous NF1 variants and two had heterozygous NF1 variants. In addition, we found a high incidence of secondary ASXL1 and/or SETBP1 variants in both groups. We conclude that the clinical diagnosis of JMML/NF-1 reliably indicates a NF1-driven JMML subtype, and that careful NF1 analysis should be included in the genetic workup of JMML even in the absence of clinical evidence of NF-1.

Department of Hematology and Oncology Hospital Sant Joan de Déu Barcelona Spain

Department of Pediatric Hematology and Oncology Bambino Gesù Children's Hospital Catholic University of the Sacred Heart Rome Italy

Department of Pediatric Hematology and Oncology Dr v Hauner Children's Hospital University Hospital LMU Munich Germany

Department of Pediatric Hematology and Oncology Oslo University Hospital Oslo Norway

Department of Pediatric Hematology Oncology Charles University and Univ Hospital Motol Prague Czech Republic

Department of Pediatric Oncology Hematology and Immunology University of Dusseldorf Dusseldorf Germany

Department of Pediatric Oncology Hematology Skåne University Hospital Lund Sweden

Department of Pediatrics Aarhus University Hospital Skejby Aarhus Denmark

Department of Pediatrics and Adolescent Medicine St Anna Children's Hospital Medical University of Vienna Vienna Austria

Diagnostic Laboratory DCOG Laboratory Princess Maxima Center for Pediatric Oncology Utrecht Netherlands

Division of Hematology Oncology and Stem Cell Transplantation Children's Hospital Helsinki University Hospital Helsinki Finland

Division of Pediatric Hematology and Oncology Department of Pediatrics and Adolescent Medicine Medical Center Faculty of Medicine University of Freiburg Freiburg Germany

Faculty of Biology University of Freiburg Freiburg Germany

German Cancer Consortium partner site Freiburg Freiburg Germany

Human Genetics University of Magdeburg Magdeburg Germany

Institute of Medical Bioinformatics and Systems Medicine Medical Center Faculty of Medicine University of Freiburg Freiburg Germany

Pediatric Oncology and Hematology Unit Lalla Seràgnoli IRCCS Azienda Ospedaliero Universitaria di Bologna Bologna Italy

Princess Maxima Center for Pediatric Oncology Utrecht Netherlands

St Anna Children's Cancer Research Institute Vienna Austria

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