Gain-of-function c-CBL mutations associated with uniparental disomy of 11q in myeloid neoplasms

Abstract

c-CBL (CBL) encodes a multifunctional protein engaged in the regulation of intracellular signaling pathways. It was first identified as a cellular counterpart of the viral oncogene, v-CBL, that causes murine lymphoma. Although no genetic evidence existed suggesting its role in human carcinogenesis, the recent discovery of c-CBL mutations in myeloid cancers has unveiled a unique oncogenic mechanism mediated by gain-of-function of a mutated tumor suppressor, closely associated with allelic conversion of 11q arms. In this review, we summarize our current knowledge about c-CBL mutations and discuss the molecular mechanisms of their gain-of-function.

Figure 1

In cancer cells, LOH is frequently associated with a mutated tumor suppressor locus, in which a normal copy of the tumor suppressor is lost by simple allelic deletion (A), or replaced by the mutated copy through allelic conversion that leads to copy number neutral LOH or aUPD (B). In either case, the common consequence is biallelic loss-of-function of the tumor suppressor. In addition, LOH caused by aUPD is also implicated in the common mechanism of homozygous mutations of proto-oncogenes. A number of gain-of-function oncogenic mutations found in aUPD regions have been shown to exist in a homozygous state, including mutations of JAK2 (9pUPD), MPL (1pUPD), NRAS (1pUPD), KRAS (12pUPD), BRAF and FLT3 (13qUPD). The clonal outgrowth of aUPD-positive clones indicates that two copies of mutations confer a growth advantage to aUPD positive cells through their gain-of-function.

Figure 2

(A) Structure of CBL family proteins. CBL family proteins in mammals have highly conserved domains, where an N-terminal TKB domain, consisting of a four-helix bundle (4H), a Ca²⁺-binding EF (EF), and a src-homology (SH2) domains, is connected to a RING finger domain via a linker. c-CBL and CBL-b, but not CBL-c, have a proline-rich and other C-terminal components that end with a ubiquitin-associated and leucine zipper (UBA/LZ) domain. Their viral form, v-CBL, is truncated just after its SH2 domain. (B) CBL family proteins interact with a number of signal transducing molecules. Through their TKB domain, CBL family proteins target phosphrylated tyrosine kinases, including growth factor receptors and cytokine receptors, as well as, non-receptor tyrosine kinases. Ubiquitin conjugating enzymes have contact with CBL proteins via the linker/RING finger domain, which is central to the E3 ubiquitin ligase activity. The proline-rich domain provides a binding site for SH3 domains of Grab2, CAP and Src-family kinases. The C-terminal portion contains three tyrosine residues, Y700, Y731 and Y774, which are the major phosphorylated tyrosines, and which bind to the p85 subunit of PI3 kinase (Y731), Vav (Y700) and Crk proteins (Y700 and Y774).

Figure 3

Possible mechanisms of gain-of-function of mutated TP53 and c-CBL. The gain-of-function of TP53 mutants is associated with their potential to induce carcinoma in mice as well as in human, which is considered to be mediated by inhibition of TP63 and TP73. TP53-deficient mice frequently develop sarcomas and lymphomas but only rarely carcinomas, which are thought to be suppressed by TP53 homologues, TP63 and TP73, in epithelial tissues, in the face of loss of TP53. Mutant TP53 inhibits tumor suppressor functions of TP63 and TP73, and compromises TP53-like activity. Similarly, the gain-of-function of CBL mutants found in MDS/MPN may be explained by the inhibition of CBL-b (red arrow), which would result in more profound defects in negative regulation of tyrosine kinase signaling compared to simple loss of c-CBL. On the other hand, c-CBL is thought to have positive regulatory functions that are not directly related to the E3 ubiquitin ligase activity and could be the source of the gain-of-function of c-CBL mutants (blue arrow).

Figure 4

Positive regulation of signal transduction by c-CBL. (A) Having E3 ubiquitin ligase activity for negative regulation of signaling, c-CBL also works as an adaptor protein for multiple signal transduction molecules. When bound to phosphorylated tyrosine kinases, c-CBL is rapidly phosphorylated at multiple tyrosine residues, which in turn provide binding sites for a number of signal transduction molecules. Several lines of evidence suggest that binding to these molecules plays important roles in positive regulation of signal transduction (red arrows). Normally, phosphorylated c-CBL undergoes degradation, which is mediated by its E3 ubiquitin ligase activity. Thus, degradation of mutated c-CBL could be retarded, leading to prolonged transmission of positive signals (B).