Clonal Evolution of Multiple TET2 mutations In the Transformation of Chronic Myelomonocytic Leukemia to Acute Myeloid Leukemia and Its Subsequent Relapse

Abstract 2699

Transformation from chronic myelomonocytic leukemia (CMML) to acute myeloid leukemia (AML) is associated with a poor prognosis. Clinical features have been used to score those at high risk, e.g. increased marrow blast percentage. Recently, several gene mutations have been identified including TET2, KRAS, NRAS, CBL and RUNX1, of which TET2 is the most frequent (Kohlmann et al 2010, Kosmider et al 2009). Initial descriptions of TET2 mutations in myeloid malignancy suggested that they were early mutations (Delhommeau et al 2009), however subsequent studies have shown in some patients with myeloproliferative neoplasia (MPN) they can be preceded by mutations in JAK2 (Schaub et al 2010). We report a male patient who was diagnosed with CMML aged 43 years. His marrow blast count was 6% and within 2 months he had transformed to AML (FAB M4) with marrow blasts of 50%. He was treated with daunorubicin and AraC (Barts 12 schedule) and obtained a complete morphological remission (CR). However he relapsed 88 days after CR in his skin, central nervous system and marrow. He died with conservative treatment after 6 months. Using this case as a model for transforming CMML, we aimed to identify clonal changes characteristic of the three stages of his disease, chronic leukaemia, transformation to acute leukemia and subsequent relapse. We used cytogenetic analysis, single nucleotide polymorphism (SNP) arrays (Affymetrix SNP 6.0 and 10K; 10K data previously reported by Raghavan et al 2008) to identify the clonal changes. Gene mutation analysis of genomic and cDNA was performed using PCR, cloning and Sanger and amplicon deep-sequencing for the mutational burden of TET2, CBL and KRAS (454 Life Sciences, Branford, CT). At all stages of his disease, he had a normal karyotype, a NPM1 mutation, but no mutations of FLT3, JAK2, KRAS or CBL (Table). Uniparental disomy (UPD) was demonstrated on chromosome 4q at relapse but not present at diagnosis, suggesting an associated TET2 mutation that would be heterozygous at diagnosis and homozygous at relapse. The heterozygous mutation was indentified in the CMML and at transformation, in a region adjacent to the splice site of the end of exon 7. The transcript resulted in a mis-splicing event leading to a stop codon. A second truncated mutant due to a deletion in exon 6 was found expressed at low level. Deep-sequencing of genomic DNA confirmed a deletion in exon 6 consistent with this product at 2.4% (918-fold coverage) in the CMML, and increased to 5.7% (865-fold coverage) burden at transformation. At relapse, the genomic mutation was homozygous. Expression of only the mis-spliced product was observed, with no wild type TET2, and no evidence of the second truncated product by cloning or deep-sequencing (832 reads). Homozygous TET2 mutations are associated with 4qUPD in MPN and myelodysplastic syndromes (Mohamedali et al 2009, Janowska et al 2010), but this is the first description of clonal evolution of a hetero- to homozygous TET2 mutation at disease progression. The case is consistent with the hypothesis that TET2 mutations evolve early in the pathogenesis of myeloid malignancies. However, it also shows there is considerable instability of TET2 mutations, with multiple mutations at various levels, and with mitotic recombination being a late event leading to relapse.