Unique Gain-of-Function of Mutated c-CBL Tumor Suppresor in Myeloid Neoplasms.

Abstract 2970

Poster Board II-946

Acquired uniparental disomy (aUPD) is a common feature of myeloid neoplasms, especially myelodysplastic syndromes (MDS) / myeloploriferative neoplasms (MPN). aUPDs preferentially affected several chromosomal arms in distinct subsets of patients, and frequently associated with mutated oncogenes and tumour suppressor genes. Among these, the most common aUPDs are those involving 11q, which defined a unique subset of myeloid neoplasms that were clinically characterized by frequent diagnosis of chronic myelomonocytic leukaemia (CMML) with normal karyotypes.

Recently, we and other groups reported that 11qUPD are genetically defined by the presence of homozygous mutations of C-CBL. C-CBL proto-oncogene is the cellular homolog of the v-Cbl transforming gene of the Cas NS-1 murine leukemia virus. C-CBL is thought to be involved in the negative modulation of tyrosine kinase signalling, primarily through their E3 ubiquitin ligase activity that is responsible for the down-regulation of activated tyrosine kinases. As expected from the latter function, we demonstrated that wild-type C-CBL has tumour suppressor functions; c-Cbl null mice showed expanded hematopoietic progenitor pools, promoted blastic crisis induced by a bcr/abl transgene, and spontaneous development of late-onset invasive cancers in complete penetrance. On the other hand, mutated C-CBL showed clear oncogenic potential; all tested mutants strongly transformed NIH3T3 fibroblasts, and prolonged replating capacity of hematopoietic progenitors.

All reported C-CBL mutations involved the linker-RING finger domains that are central to the E3 ubiquitin ligase activity. We demonstrated that mutated C-CBL not only lost their E3 ubiquitin ligase activity, but also inhibited that of wild-type C-CBL, leading to prolonged activation of a broad spectrum of tyrosine kinases after ligand stimulations in fibroblasts and hematopoietic cells. In accordance with this, c-Cbl^−/− hematopoietic stem/progenitor cells (HSPCs) showed enhanced sensitivity to a variety of cytokines, but unexpectedly, transduction of C-CBL mutants into c-Cbl^−/− HSPCs further augmented the sensitivity to a broader spectrum of cytokines, indicating the presence of gain-of-function in mutated C-CBL that is not simply mediated by inhibition of wild-type C-CBL functions. The gain-of-function effects of C-CBL mutants on cytokine sensitivity of HSPCs largely disappeared in the c-Cbl^+/+ background or by co-transduction of wild-type C-CBL, which may suggest the pathogenic importance of loss of wild-type c-Cbl alleles found in most cases of C-CBL-mutated myeloid neoplasms. Our findings provide a novel insight into a role of gain-of-function mutations of a tumour suppressor associated with aUPD in the pathogenesis of some of myeloid cancer subsets. Currently, further functional studies regarding the molecular mechanism of the gain-of-function are ongoing.