Melanoma and the RAS-RAF-MEK-ERK Signalling Pathway

Our group aims to develop new therapeutic strategies that are tailored for individual cancer patients based on improved understanding of cancer biology.  This approach is called “personalised medicine” because treatment is tailored to each patient’s tumour, rather than using a one size fits all approach.  Thus, our aim is to implement a personalised medicine approach for melanoma to improve treatment outcomes for patients and we anticipate that the lessons we learn in melanoma will be applicable to other cancer types.

We have found that the different genetic forms of melanoma display different biochemical properties.  For example, the anti-diabetic drug metformin drives the growth of melanoma in which BRAF is mutated, but inhibits the growth of melanoma in which NRAS is mutated.  This is because metformin increases the production of vascular endothelial growth factor (VEGF) in BRAF mutant, but not in NRAS mutant melanoma cells.  Critically, VEGF encourages the development of new blood vessels into the growing tumours, increasing the flow of oxygen and nutrients and allowing the tumours to grow more rapidly.  Agents that target VEGF overcome this effect and cooperate with metformin to suppress the growth of the BRAF tumours.  These data highlight the importance of understanding the biology of each tumour if effective therapies are to be developed for individual patients. 

The most common form of melanoma occurs on hair bearing skin that is intermittently exposed to UV light (recreational sun exposure) and we have developed transgenic models for melanoma that allow us to investigate gene-gene and gene-environment interactions that drive melanoma development.  We are currently using these models to identify the genes that interact with BRAF and NRAS to drive melanoma development and to examine the role of UV light.

As an alternative approach to improving our knowledge of melanoma, we are also using next generation sequencing to reveal the landscape of mutations that occur in individual human melanoma samples.  Over the last year, we have focussed on acral melanoma in particular, a rare form of melanoma that develops on the non-hair bearing skin of the hands and feet.  These sites were thought to be protected from the damaging effects of UV light, but our sequencing revealed that some acral melanomas present a UV light DNA-damage “signature”.  This suggests that UV light also appears to play a role in the aetiology of some acral melanomas.  We are also expanding our studies to examine the genetics of other rare forms of melanoma to allow us to gain further insight into melanoma biology so that we can develop new therapeutic strategies for the treatment of all forms of this disease.

A major breakthrough in melanoma treatment occurred with the development of drugs that inhibit BRAF.  These drugs inhibit the RAS-RAF-MEK-ERK pathway in cells in which BRAF is mutated.  Importantly, these drugs can achieve impressive clinical responses in patients whose tumours express the mutant forms of BRAF, but are ineffective in patients whose tumours express wild-type BRAF.  Curiously, one of the unexpected side-effects of BRAF drugs is that they activate rather than inhibit the RAS-RAF-MEK-ERK pathway when RAS is mutated.  This effect occurs because RAF drugs stabilise the formation of complexes between ARAF, BRAF and CRAF in the presence of active RAS.  The complexes contain drug-bound, and drug-free RAF molecules and the drug-bound partners hyper-activate the drug-free partners, thereby hyper-activating the signalling pathway and stimulating the growth of the cells (Figure 2).  Thus, while BRAF drugs inhibit the signalling pathway in cells in which BRAF is mutated, they activate the pathway in cells when NRAS is mutated and this effect is called the “RAF-inhibitor paradox”.

We have shown that the RAF-inhibitor paradox underlies the development of non-melanoma skin lesions (keratoacanthomas and squamous cell carcinomas) in about a third of patients treated with these drugs.  Notably, this is not because the BRAF drugs act as tumour promoters per se; rather they act by accelerating the growth of pre-existing, pre-malignant tumours in susceptible patients.  The drugs in effect place a growth-selective advantage on these pre-existing tumours, and this knowledge led us to discover that anti-proliferative agents such as 5-fluorouracil can be used to treat these lesions in patients for whom surgery is not an option.

Although BRAF drugs have provided a paradigm shift in the treatment of melanoma, unfortunately the responses to these drugs are generally short-lived and most patients will fail on treatment after a relatively short period of disease control.  Furthermore, about 20% of patients do not respond to BRAF drugs despite the presence of a BRAF mutation.  We have therefore continued to develop new BRAF drugs and are testing if these are effective in patients whose tumours are resistant to the pre-existing drugs.  We are also studying how resistance develops in patients undergoing treatment.

 
Figure 1: The RAS-RAF-MEK-ERK pathway is depicted.  NRAS is activated downstream of receptor tyrosine kinases (RTK) and it activates BRAF, which in turn activates MEK and MEK activates ERK, driving cell growth and survival.

 
Figure 2: The RAF paradox. (A) In the presence of mutated BRAF, the MEK/ERK pathway is hyper activated even though RAS is not active.  (B) In BRAF mutant cells, BRAF drugs block BRAF activity and inhibit the activity of MEK and ERK.  (C)  When cells in which NRAS is mutated are treated with BRAF drugs, although BRAF is inhibited, it is driven into a complex with CRAF and hyper-activates CRAF, thereby driving paradoxical hyper-activation of MEK and ERK.