The Role of MEKK1 and MKK4 Mutation in Breast Cancer

Abstract

MEKK1 and MKK4 are signalling protein kinases that activate the JNK and p38 MAP kinases. Furthermore, these proteins make up the upstream MAP3 kinase and intermediate MAP2 kinase components of a canonical MAP kinase pathway, regulating MAP kinase activation. Conversely, phosphatases negatively regulate MAPK activation as part of negative feedback processes.Large scale tumour characterisation studies have identified recurrent mutation of the MEKK1 and MKK4 genes in breast cancer. Mutation of MEKK1 and MKK4 tends to be mutually exclusive, indicating the deregulation of a common signalling pathway. Furthermore, the frequency of MEKK1 and MKK4 mutation indicates an important role in breast cancer biology. However, the precise molecular and phenotypic impact of MEKK1 and MKK4 mutation in breast cancer has not been elucidated.In my project, I used publically available luminal A breast tumour data to identify the gene expression and protein expression characteristics, associated with the subset of tumours harbouring MEKK1 and MKK4 mutations. I show that dual specificity phosphatases DUSP8 and DUSP10 are commonly downregulated in MEKK1 and MKK4 mutant breast cancer. Furthermore, I show that ERK1/2 MAPK phosphorylation is upregulated in MEKK1 mutant breast cancer. In an approach to functionally characterise MEKK1 mutations, I show that truncation mutations result in loss-of-function, though in a complex manner. In contrast, the majority of missense MEKK1 mutations analysed do not alter the capacity for MEKK1 to induce downstream signalling. Truncation mutations make up the vast majority of MEKK1 mutations found in breast cancer, thus I show that MEKK1 mutations predominantly result in inactivation.Furthermore, the use of CRISPR-Cas9 knockout approaches allowed me to study the oncogenic transformation of mammary epithelial cells, in the context of MEKK1 and MKK4 inactivation. In this study, I show that MKK4 is required for PIK3CAH1047R driven growth factor independent proliferation. However, MEKK1 and MKK4 loss does not influence oncogenic transformation in this cellular context.Together, the findings I present here indicate MEKK1 and MKK4 mutation may result in deregulation of MAPK negative feedback mechanisms. This may result in increased ERK1/2 MAPK activation. Therefore, this study provides novel insight into the molecular impact of MEKK1 and MKK4 mutation, opening up new avenues of investigation to better understand MEKK1 and MKK4 mutant breast cancer biology.

Date of Award	1 Aug 2017
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Wolfgang Breitwieser (Supervisor) & Nicholas Jones (Supervisor)