Mark A Lemmon, PhD, FRS

Signaling Mechanisms of Receptor Tyrosine Kinases

A major focus of the Lemmon lab is to understand transmembrane signaling by growth factor receptor tyrosine kinases (RTKs), of which there are 58 in the human proteome – separated into 20 different families. Mutations in almost all of these RTKs – some activating, some inactivating – cause cancer or other diseases, and RTKs are important therapeutic targets. We are interested in understanding how these receptors signal, and – importantly – how RTK mutations seen in afflicted patients affect receptor activity. Understanding these mutations provides an important window into molecular mechanism, but also allows us to use our mechanistic understanding to advance development and application of targeted therapeutics. To achieve this, we combine cellular, biochemical, biophysical, and structural approaches – and collaborate closely with geneticists and clinical investigators. Our goal is to link detailed mechanistic understanding to biology in the intact organism (or patient). Although we are interested in all RTKs, and in RTK signaling in general – at the organismal, cellular, and molecular levels (and use a wide variety of approaches) – we are currently focused on the following families (or groups of families) or RTKs:

1. The Epidermal Growth Factor Receptor (EGFR) family
Often considered a ‘prototypic’ RTK, the EGFR has been a major focus of our work for the past decade or so. Combining crystallographic, cellular, and other approaches we have developed a sophisticated understanding of how growth factor binding promotes dimerization of the extracellular region of the receptor, how EGFR is regulated allosterically, and how its intracellular juxtamembrane region contributes to activation. We have also defined the molecular function of the extracellular EGFR inhibitor Argos, from which we hope to extract clues for developing new EGFR inhibitory approaches in cancer.

A second key area in our EGFR work is to understand how different activating ligands can promote distinct modes of signaling through this single receptor. EGF, TGF-alpha, betacellulin, HB-EGF, epiregulin, epigen, and amphiregulin all signal through EGFR - but with subtly different consequences. Our most recent work is revealing some unexpected structural origins for these differences that are impacting how we think about the EGFR. This work is also causing us to appreciate how kinetic aspects of RTK signaling may be very important in defining signaling specificity. We are extending this concept to other receptors, including several involved in immune cell regulation that may be important in immuno-oncology.

2. Understanding activating mutations in RTKs, and how they affect inhibitor response in cancer patients
Working with ALK mutations seen in neuroblastoma patients and EGFR mutations seen in lung cancer patients, we are trying to understand how to define which new mutations seen in patients are activating - in terms of their signaling activity - and how they respond to available ALK or EGFR inhibitors that are being used in the clinic. Working with Ravi Radhakrishnan in Penn Bioengineering, we are also trying to develop algorithms for predicting whether newly identified mutations are activating and inhibitor-sensitive, which we hope will one day guide clinical treatment.

3. RTKs that bind to ligands in the Wnt family
It is now known that several orphan RTKs, namely PTK7/CCK4 (called Lemon in Hydra!), Ror1/2, Ryk, and MuSK are involved in ‘non-canonical’ Wnt signaling. This is a new arena for RTKs, and how they are involved remains unclear. Moreover, PTK7/CCK4, Ror1/2 and Ryk all have so-called pseudokinases in their intracellular region. That is, they look like RTKs, but appear to have ‘dead’ tyrosine kinases. We are working hard to understand how these unusual RTKs mediate signaling by these unexpected ligands, combining biochemical, cellular and structural studies with a collaboration with Peter Klein’s lab at Penn in Xenopus. This work is likely to open new paradigms in RTK signaling and – we hope – to illuminate new therapeutic avenues. We also hope that these studies will shed new light on how pseudokinases function more broadly.

Receptor tyrosine kinases