A gene that is found to be mutated in a type of blood cancer exhibits properties of both a growth-suppressing tumour suppressor and a growth-promoting oncogene.
Myeloproliferative neoplasms (MPNs) are a group of cancers characterized by excessive production of one or more types of differentiated blood cell. Mutations in genes that encode components of cellular signalling networks are a hallmark of MPNs, and cells from affected patients are often abnormally sensitive to growth factors called cytokines1. On page 904 of this issue, Sanada et al.2 report that mutations in the CBL gene, which are found in a subset of patients with MPNs, contribute to the development of cancer in unexpected and complex ways, suggesting that the normal protein encoded by this gene both promotes and inhibits the growth of blood cells.
Sanada and colleagues2 performed genome-wide studies of bone-marrow samples from patients with MPNs to uncover acquired genetic changes that might contribute to abnormal cellular growth. The authors were particularly interested in detecting a type of genetic alteration called acquired uniparental disomy (aUPD) that occurs in somatic cells (non-gametes). In this alteration, a region of DNA on one chromosome is lost in tumour cells, but is replaced by a copy of the corresponding segment from the partner chromosome that harbours a mutant gene. This mechanism can contribute to leukaemia through loss of the normal copy (allele) of a tumour-suppressor gene3 or by duplicating genes that promote cancer (oncogenes)4,5. Because this alteration results in replacement of the deleted DNA, the region of aUPD eludes discovery by conventional techniques. To surmount these difficulties, the authors2 and other groups6,7,8 have analysed samples using single nucleotide polymorphism (SNP) arrays — a technique that has become a valuable tool in cancer biology. One of the many advantages of SNP analysis is that it can identify aUPD by detecting a chromosomal region in which the DNA is derived from a single parental chromosome (so-called loss of heterozygosity).
Using this technique, researchers have detected aUPD spanning a region of chromosome 11 that includes the CBL gene in MPN specimens2,6,7,8, and have gone on to discover mutations in this gene. The Cbl protein is expressed in various cell types and regulates cellular signalling networks by acting as a multifunctional adaptor molecule and as an E3 ubiquitin ligase9, an enzyme that attaches a ubiquitin molecule to growth-factor receptors and other cellular proteins. Ubiquitination of growth-factor receptors triggers their internalization and degradation, thereby reducing the signalling cascades that promote cellular proliferation. The missense CBL mutations found in MPNs introduce amino-acid substitutions that disable ubiquitin-ligase activity.
Consistent with a previous report10, Sanada et al.2 show that mice lacking the Cbl gene produce increased numbers of immature blood cells. They also find that Cbl inactivation promotes the development of leukaemia in mice engineered to express the pro-leukaemic BCR–ABL gene. Mutant Cbl proteins inhibit ubiquitination of growth-factor receptors in blood-cell lines, even in cells that retain a normal copy of the CBL gene2,7, and the authors demonstrate that this inhibition is associated with prolonged receptor activation and an enhanced proliferative response to cytokine growth factors. Although these studies provide evidence that CBL is a tumour-suppressor gene, Sanada and colleagues' data2 also suggest a more complex role for CBL in leukaemogenesis. For example, if mutation of a single CBL allele is sufficient to disrupt normal ubiquitin-ligase activity, it is unclear why MPN cells also inactivate the normal copy of the gene through aUPD. The authors' findings that the effects of the mutant Cbl proteins are more pronounced in cells that lack a normal CBL gene suggest that other biochemical properties contribute to tumour growth. This idea is consistent with the observation that mice lacking Cbl do not spontaneously develop MPNs, as would be expected if the gene acted purely to suppress tumour formation. Finally, when the authors over-expressed mutant CBL in fibroblasts, these cells showed cancerous properties2.
So what are we to make of CBL? Is it a tumour suppressor or an oncogene, or, as Shakespeare might have put it, “more than kin and less than kind”? On the one hand, there is strong selective pressure to delete the normal CBL allele in tumour cells, resulting in loss of ligase activity that restrains the output of activated growth-factor receptors. These are impeccable credentials for a tumour-suppressor protein. However, the blood cells of patients with MPNs invariably retain at least one gene that encodes a functional, albeit mutant, Cbl protein. Furthermore, mutated Cbl proteins seem to acquire unexpected growth-promoting functions2 — this gain-of-function characteristic is not seen in blood stem cells that lack CBL. Such features implicate CBL as a bona fide oncogene.
Tumour suppressor or oncogene? Perhaps a solution to this conundrum is that this multi-domain protein fine-tunes the growth of blood stem cells and progenitor cells by simultaneously promoting and restraining growth through distinct protein–protein interactions. Recent studies offer intriguing clues about the potential growth-promoting biochemical properties of mutant Cbl proteins. First, Sanada et al.2 find that expression of the mutant Cbl proteins is associated with aberrant phosphorylation of STAT5, an activator of gene transcription. Aberrant phosphorylation is a biochemical feature of some types of MPN11, and although in this report2 it may be due to loss of Cbl-mediated ubiquitination of cytokine receptors that activate STAT5, other mechanisms are possible. Second, CBL mutations and mutations in the oncogene NRAS were mutually exclusive in the adult MPN patients studied by the authors. CBL mutations were also found only in specimens from children with juvenile myelomonocytic leukaemia (a type of MPN) without mutations in NRAS or KRAS8. As this childhood leukaemia is an MPN in which a hyperactive form of the cell-signalling molecule Ras has a central role, there is likely to be a connection between mutant Cbl proteins and Ras signalling. Data suggesting that Cbl regulates Ras trafficking in cells are intriguing in this respect12. The elegant functional studies of Sanada et al.2 thus raise fascinating questions about the nature of oncogenic Cbl-mediated interactions in MPNs and how such interactions might be targeted to treat these disorders.
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International Journal of Hematology (2010)