Dr Nick Dibb

Kinase inhibitors, stem cells and microRNA

Current research interests:

Kinase inhibitors of the PDGF receptor family
The PDGF receptor family consists of a set of five closely related tyrosine kinase receptors: KIT (SCF); FMS (CSF-1 or M-CSF); PDGF α and β PDGF) and FLT3 (FLT3) that are activated by the specific growth factors shown in brackets. Oncogenic mutations of these receptors contribute to a wide range of cancers including leukaemia and stomach cancer. These oncogenic mutations either disrupt the juxtamembrane region (JMR) of the receptors or they specifically mutate an aspartate residue within the activation segment of the kinase domain. We were involved in the discovery of the kinase domain mutation (Glover et al., 1995) and in collaboration with Cliff Mol we have proposed a molecular explanation as to why mutations of this aspartate residue cause illicit kinase activation (Dibb et al., 2004; figure 1). The KIT receptor can be inhibited by the ATP analogue Imatinib, which is now used successfully to treat those patients with stomach cancer that have mutations of the JMR of KIT. Unfortunately, mutations of the asparate residue 816 of the kinase domain of KIT make the receptor resistant to Imatinib and so this drug cannot be used to treat patients with mast cell leukaemia, who usually have an Asp 816 mutation. We have recently proven that imatinib and dasatinib are also effective inhibitors of the FMS receptor (Taylor et al., 2006; Brownlow et al., 2009). FMS inhibitors have great potential for the treatment of both cancerous and inflammatory disease and we are actively pursuing this area of research.

KIT kinase

Figure 1. Displacement of Val 559 by Asp 816 during the activation of the kinase domain of the KIT receptor. The two most commonly mutated single amino acids of the KIT receptor in cancer are Asp816 then Val559. Fig 1 shows a small but critical part of the inactive and active kinase domain structures of KIT (solved by Cliff Mol) in order to illustrate that Asp816 moves from a hydrophilic environment into the exact hydrophobic position vacated by Val 559 during kinase activation. This demonstrates a structural connection between Asp816 and Val559 of KIT and indicates that the reason why Asp816 frequently mutates to Val in cancer is because it much easier for a Val at position 816 to take over or stabilise the rather hydrophobic and tightly packed position previously occupied by Val 559. This model can also explain why Val559 is frequently mutated to the charged residues Asp or Glu in cancer (Dibb et al., 2004).

Receptor downregulation
Receptor downregulation is an important mechanism that regulates cell signaling by causing the internalisation and degradation of the ligand or the entire ligand:receptor complex. Receptors that are defective for downregulation are oncogenic, furthermore, receptor overexpression is one of the most consistent features of cancerous cells. Overexpression of M-CSF and FMS is causally implicated in the development of the majority of uterine, ovarian and breast cancers. We have concluded that the downregulation of FMS can occur independently of receptor kinase activity and is instead initiated by movement of the activation segment of the kinase domain as a result of ligand binding (Uden et al., 1999). We are also investigating ligand-independent mechanisms of FMS downregulation.

Genomics
The ongoing genome sequencing projects are generating a mass of information of relevance to gene evolution. We have recently discovered that the exon junction sequences that flank some actin introns can function as cryptic splice sites and that the required splicing information is contained within exons not introns. These cryptic splice sites were most probably targets for the insertion of actin introns during evolution. These results raise the possibility that splicing information within exons can be recognised by the splicing machinery independently of sequence information within introns and that this information is functional and predates introns. We are currently collaborating with Dr Yuri Kapustin (NCBI) in order to develop these results through a bioinformatics approach.

MicroRNAs are newly discovered regulators of the translation of perhaps most mRNAs and are encoded by an extremely large gene family. In collaboration with Attila Molnar and David Baulcombe at Cambridge University we have recently cloned and sequenced microRNAs expressed by human mesenchymal and embryonic stem cells. In collaboration with Dr Wei Cui at the IRDB, we are currently investigating the role of microRNAs in stem cell proliferation and differentiation.