LAUSR

The steps in the progression from subclinical clonal haemopoiesis to acute myeloid leukaemia (AML) have not been defined, nor has its likelihood or latency. NPM1 mutations were not found to drive clonal haemopoiesis in haematologically normal individuals(Genovese et al, 2014; Jaiswal et al, 2014; Xie et al, 2014; McKerrell et al, 2015), highlighting their strong leukaemogenic pedigree, but making them difficult to capture at a stage prior to frank AML. As occasional cases of NPM1-mutated myelodysplastic syndrome/myeloproliferative neoplasm have been described(Caudill et al, 2006; Peng et al, 2016), we hypothesised that detailed analysis of such cases could offer insights into leukaemic evolution after the acquisition of NPM1 mutations. Here, we describe the genetic events driving rapid progression of a case of NPM1 mutant chronic myelomonocytic leukaemia (CMML) to full-blown AML. A 50-year-old woman with an eight-week history of tiredness was found to be anaemic (Hb 75 g/l) and thrombocytopenic (platelet count 116 9 10/l). Her white cell (WCC) (8 7 9 10/l) and neutrophil (5 48 9 10/l) counts were normal but she had monocytosis (1 74 9 10/l). The bone marrow aspirate was haemodilute, but trephine biopsy showed marked myeloid hyperplasia and <5% blasts (Figure S1). The karyotype was normal and a diagnosis of CMML was made. After 83 d she developed fever, abdominal discomfort and a WCC of 209 9 10/l with 82% myeloblasts. A diagnosis of AML was confirmed on bone marrow aspiration (Figure S2) and molecular testing identified NPM1 and FLT3 internal tandem duplication (ITD) mutations. Analysis of CMML DNA identified the NPM1 but not the FLT3-ITD mutation. The subsequent clinical course, summarized in Fig 1, unfortunately ended when our patient succumbed to multiplyrelapsed AML 11 months later. To understand the molecular events following acquisition of the NPM1 mutation, we performed exome sequencing [SureSelect Human Exon 50 Mb Kit baits (Agilent, Santa Cara, CA), HiSeq2000 (Illumina, San Diego, CA)] of bone marrow DNA at CMML diagnosis, AML diagnosis and first complete remission. Caveman analysis identified 43 single nucleotide variants (SNV) in AML and 62 in CMML DNA, of which 23 were shared (Tables SI and SII). Pindel analysis identified ten indels in AML and eight in CMML, of which