Germline Mutations in CBL Cause a Predisposition to Juvenile Myelomonocytic Leukemia.

Juvenile myelomonocytic leukemia (JMML) is an aggressive myeloproliferative neoplasm (MPN) of early childhood. Up to 60% of patients harbor activating mutations in either NRAS, KRAS, or PTPN11, while another 15% of children have neurofibromatosis type 1 (NF-1) and demonstrate loss of the wildtype NF1 allele in their hematopoietic cells at diagnosis. We recently described that an additional 10–15% of children with JMML harbor missense homozygous mutations in exons 8 and 9 of CBL (Loh, Blood, 2009). Cbl is a complex protein that functions primarily as an E3 ubiquitin ligase but also serves numerous important adaptor functions. Mutations in CBL have recently been reported in adults with MPNs and the available evidence in adults indicates that these lesions are somatically acquired. We noted that a number of children with JMML and CBL mutations had neurological conditions including developmental delay and dysmorphic stigmata, although these features were not 100% penetrant. Based on these observations, we performed mutational studies in fibroblasts and buccal epithelial cells, which were available from 13 JMML patients with homozygous CBL mutations in their bone marrow at diagnosis. We now show that in all 13 patients the initial CBL mutation occurred as a heterozygous germline event (Table 1). Interestingly, a child with the 1222 T>C later developed a brain tumor with a homozygous CBL lesion. Mutational studies on parental DNA were informative in 11 cases and indicated autosomal inheritance in 6 families. Furthermore, two of these children had extensive family histories in which several young family members died of JMML. There were no known features of NF-1 in either pedigree. Subsequent analysis of these two pedigrees revealed a pattern of autosomal dominant inheritance that spanned 4 generations in 1 family and 3 generations in the other. To rule out normal genetic variation, a cohort of 240 healthy individuals were screened without detection of a CBL abnormality.

One hallmark feature of JMML myeloid progenitor cells is their sensitivity to granulocyte-macrophage colony stimulating factor (GM-CSF) in colony-forming assays. Retroviral transduction of the 371 Tyr>His and 384 Cys>Arg mutations into wildtype murine fetal liver cells failed to induce a hypersensitive phenotype. However, recently published data suggest that a full oncogenic phenotype is conferred when the wildtype allele is deleted (Sanada, Nature, 2009), as occurs in the human diseases. Transduction of the most common human JMML mutations into BaF3-EpoR cells that have had murine Cbl knocked down is ongoing, as is further biochemical analysis. In summary, germline mutations in CBL cause a clinical syndrome with a predisposition to JMML, and importantly, can be inherited in an autosomal dominant fashion, thus establishing CBL as a new familial cancer predisposition gene.

No relevant conflicts of interest to declare.