Overexpression of EGFR is one of the most frequently diagnosed genetic aberrations of glioblastomas, and is often accompanied by deletion mutations. The most frequently detected and best-studied deletion mutant is EGFRvIII (Gan et al., 2009
). This mutated receptor is highly oncogenic and shares both structural and functional similarities with the viral EGFR homolog, v-ErbB (Shu et al., 1991
). The two EGFRvIV mutants, termed EGFRvIVa and EGFRvIVb in this study, have so far evaded biochemical investigation. These mutants lack a significant segment of their noncatalytic carboxyl terminal sequences, yet their extracellular domain is intact. Interestingly, a similar deletion was uncovered in the S3v-ErbB strain of the avian erythroblastosis virus, which is more aggressive than other strains (McManus et al., 1997
Our results indicate that the internal deletions of EGFRvIV enhance basal kinase activity, and confer oncogenic phenotypes. In addition, we show that the signaling pathways induced by these mutations differ from those activated by EGFRvIII. Despite differences, EGFRvIV and EGFRvIII share some similarities. For example, both activate AKT and decrease phosphorylation of a negative regulator, EPHA2 (). This uncoupling of a negative feedback loop potentially enhances the transforming receptors. An additional common property of EGFRvIII and EGFRvIV is their dependence on the chaperoning activity of HSP90 (), which explains the higher abundance of mutated EGFRs relative to WT. Finally, our results suggest that similar to the case of EGFRvIII, basal dimerization may be involved in EGFRvIV activation (). Nevertheless, although relief of extracellular constraints might contribute to dimerization of EGFRvIII, our data favor involvement of the kinase domain in basal dimerization of EGFRvIV (). It is notable, however, that the fraction of pre-dimerized EGFRvIII exceeds the portion of EGFRvIV molecules present within dimers, before EGF binding.
Other receptor tyrosine kinases are regulated by their C-tails (Shewchuk et al., 2000
; Chiara et al., 2004
). However, because of its unique independence of phosphorylation within the activation loop (Gotoh et al., 1992
) and because of the fact that the kinase domain of EGFR does not require large spatial rearrangements to achieve activation (Stamos et al., 2002
), catalytic regulation of this receptor is distinct. On the basis of the structural and computational analyses, Landau et al. (2004)
suggested that a negatively charged segment (residues 979–996) of the carboxyl terminal region of EGFR forms large complementary surfaces with the kinase domain and, thereby, may inhibit its catalytic activity. Furthermore, although they differ in conformation, two crystal structures documented physical interactions between the kinase domain and carboxyl terminal segments of EGFR: the first report entails an inactive kinase stabilized by lapatinib, a tyrosine kinase inhibitor (Wood et al., 2004
; see an illustration in Supplementary Figure S4A
). Accordingly, two α-helical regions, 971–980 and 983–990, bind to the kinase domain, potentially inhibiting its activation. The other structure suggested interactions in trans
, in which the C-tail of one receptor molecule interacts with the kinase of the other receptor, thereby stabilizing an inactive dimer (Jura et al., 2009
) (Supplementary Figure S4B
). This inactive conformation is thought to hinder a kinase–juxtamembrane contact, which is essential for subsequent activation and kinase–kinase interaction. Taken together, both computational and crystallographic analyses lend support to the prediction that the naturally occurring EGFRvIV mutants lack a kinase-inhibitory region, thereby attaining enhanced constitutive enzymatic activities.
In summary, we conclude that the C-tail distal to the kinase domain of EGFR possesses an inhibitory role, which interferes with catalytic activation in the absence of ligand-induced dimerization. Upon deletion in brain tumors, mutant EGFRs acquire basal tyrosine phosphorylation that instigates unique sets of signaling proteins, and culminates in malignant transformation (see model in ). Intriguingly, the shorter deletion mutant, namely EGFRvIVb, consistently showed higher kinase and oncogenic activities than the longer deletion mutant, EGFRvIVa. Future mutational studies will resolve these differences, along with pinpointing the exact amino acids necessary for kinase regulation.
Figure 6 A model that compares WT-EGFR and two oncogenic mutants, EGFRvIV and EGFRvIII. The red circles indicate phosphorylated tyrosine residues; the yellow rectangle marks the segment missing in the C-tail of EGFRvIV mutants. Unlike EGFRvIII, WT-EGFR and the (more ...)