Perspectives for the use of structural information and chemical genetics to develop inhibitors of Janus kinases

Abstract

Gain-of-function mutations in the genes encoding Janus kinases have been discovered in various haematologic diseases. Jaks are composed of a FERM domain, an SH2 domain, a pseudokinase domain and a kinase domain, and a complex interplay of the Jak domains is involved in regulation of catalytic activity and association to cytokine receptors. Most activating mutations are found in the pseudokinase domain. Here we present recently discovered mutations in the context of our structural models of the respective domains. We describe two structural hotspots in the pseudokinase domain of Jak2 that seem to be associated either to myeloproliferation or to lymphoblastic leukaemia, pointing at the involvement of distinct signalling complexes in these disease settings. The different domains of Jaks are discussed as potential drug targets. We present currently available inhibitors targeting Jaks and indicate structural differences in the kinase domains of the different Jaks that may be exploited in the development of specific inhibitors. Moreover, we discuss recent chemical genetic approaches which can be applied to Jaks to better understand the role of these kinases in their biological settings and as drug targets.

Fig 1

Domain structure of Janus kinases and general functions of the different domains. Model structures of the Jak1-FERM, -SH2 and pseudokinase domain as well as the solved crystal structure of the Jak2 kinase domain (PDB entry code: 2B7A) are represented.

Fig 2

Principle of the specific inhibition of analogue-sensitive kinases. (A) Top view and (B) side view of the catalytic cleft of the Jak2 kinase domain with the bulky ATP-analogue N6-benzyl-ADP (highlighted in orange). The sterical clash between the benzyl group of the bulky inhibitor and the gatekeeper residue M929 in the wild-type Jak2 kinase domain is highlighted as a red frame (A) or surface (B). Mutation of the gatekeeper residue to glycine (Jak2-M929G) extends the catalytic pocket and allows the accommodation of the bulky compound. The representation was generated using the solved crystal structure of the Jak2 kinase domain (PDB entry code 2B7A).

Fig 3

Model structure of the pseudokinase domain of Jak2 with the predicted locations of patient-derived mutations with validated activating effect. Residues for which point mutations were reported in patients are represented as green stick models with spheres indicating the Van-der-Waals radii of atoms. Regions carrying insertions and/or deletions are indicated by a green coloured backbone without stick models (black frames). The structural mutation hotspots I and II are highlighted in red. The model structures of the FERM, SH2 and pseudokinase domains of Jak1, Jak2 and Jak3 shown in this review were generated as described in the Supporting Information. For molecular modelling and graphic representation of all protein structures in this review, the programs WHAT IF [275] and Pymol [DeLano, WL (2002) The PyMOL Molecular Graphics System. DeLano Scientific, San Carlos, CA, USA] were used.