Prenatal origin of KRAS mutation in a child with an acute myelomonocytic leukaemia bearing the KMT2A/MLL‐AFDN/MLLT4/AF6 fusion transcript

Abstract

Acute myeloid leukaemia (AML) accounts for 15–20% of paediatric leukaemia and almost 60% of children are cured (Pession et al, 2013). AML results from a multistep process that requires two cooperative types of genetic events because one aberration is not sufficient to induce leukaemia (Balgobind et al, 2011). Type I aberrations occur as mutations in specific genes involved in signalling transduction pathways, such as KRAS and PTPN11, which leading to uncontrolled proliferation and/or survival of leukaemic cells (Balgobind et al, 2011). Type II aberrations are often chromosomal rearrangements of transcription factors [e.g. rearrangements of 11q23/KMT2A (previously termed MLL)] that lead to impaired differentiation of cells (Balgobind et al, 2011). Somatic mutations in the RAS pathway occur in 80% of juvenile myelomonocytic leukaemia (JMML) cases (Stieglitz et al, 2015). KRAS mutations are detected at diagnosis in 8% and 10% of children with AML (Meshinchi et al, 2003) and acute lymphoblastic leukaemia (ALL) (Liang et al, 2018), respectively. 11q23/KMT2A rearrangements are usually present in 25–30% of children with AML, particularly in French-American-British (FAB) M4 and M5 subtypes (Balgobind et al, 2009). The KMT2A-AFDN (previously termed AF6/MLLT4) fusion transcript of t(6;11)(q27;q23) is highly frequent in acute myelomonocytic leukaemia (AMML), conferring a very poor prognosis (Balgobind et al, 2009). The only recurrent mutations in t(6;11)(q27;q23) positive AML were RAS mutations, which occurred in a higher proportion of the cases (45%) than all other AML subtypes (Coenen et al, 2014). According to the ‘two-hits theory’ in leukemogenesis, KMT2A rearrangements, as type II mutations, initiate the leukaemic process. On the other hand, RAS mutations (type I) are suggested to be secondary hits, providing a proliferative advantage (Balgobind et al, 2011). Given that childhood cancer shows relatively short latencies, its embryonic origin derives from a silent pre-leukaemic clone, detectable on a neonatal blood specimen (Guthrie cards – GC) at birth (Wiemels et al, 2010). The existence of RAS mutations at birth that precedes KMT2A rearrangement in infant ALL has been recently demonstrated (Emerenciano et al, 2015). We now report, for the first time to date, a child who showed a pre-leukaemic clone with KRAS mutation at birth, and who further developed AMML with a KMT2A-AFDN rearrangement, demonstrating an inverse sequence of the occurrence of type II and type I mutations. The methods used are reported in details in Supplementary Materials. Our case was a male child diagnosed with AMML at 18 months of age, who presented with hyperleucocytosis, a mild thrombocytopenia and hepato-splenomegaly (Table I). Based on characteristics at presentation, we firstly hypothesized a typical onset of JMML. However, immune-phenotypic and cytogenetic analyses revealed an AML. We detected a complex karyotype, with hyperdiploidy (Table II) and molecular biology analyses revealed the presence of a KMT2A-AFDN rearrangement and a mono-allelic exon-2 point mutation (G38A) of the KRAS gene (Table I; Fig 1A). The child was treated according to the Associazione Italiana di Ematologia e Oncologia Pediatrica (AIEOP)-AML 2002/01 protocol (Pession et al, 2013), achieving a complete remission after the second induction course. Unfortunately, immediately after consolidation, he suffered an early relapse and died of progressive disease. We analysed leukaemic cells from relapse, which showed a lack of KRAS diagnostic mutation. However, we detected a mono-allelic exon-3 point mutation (C12T) in