Molecular characterisation of paediatric ALL and new therapeutic strategies

Despite high cure rates achieved in the last two decades, treatment resistance and therapy-associated toxicity is still observed in a significant proportion of children with ALL, warranting alternative therapeutic approaches. Results obtained in our laboratory suggest that transcriptional upregulation of prosurvival pathways plays a key role in paediatric B-precursor ALL chemoresistance, thus providing a strong rationale for the inhibition of prosurvival gene transcription. In this study, I investigated the role and feasibility of the epigenetics-regulating BET family of proteins as therapeutic targets. I showed that BRD4 is ubiquitously expressed in paediatric ALL tumours and that treatment with the BET protein inhibitor JQ1 led to potent in vitro sensitisation of ALL tumour cells regardless of cellular phenotype, as well as in vivo sensitisation using xenograft models of ALL. I observed strong in vitro synergy in ALL cell lines and primary tumours treated with a combination of JQ1 and dexamethasone. Using microarray technology I confirmed BET protein inhibition involves transcriptional downregulation of prosurvival pathways and identified potential biomarkers predictive of JQ1 sensitivity. I characterised the cellular effects of BET protein inhibition in pre-B ALL and showed this was associated with inhibition of cell cycle progression, downregulation of c-Myc protein, direct inhibition of DNA replication and induction of caspase-dependent apoptosis that was independent of p53 activity. Thus, pharmacological inhibition of BET proteins provides an alternative strategy by which to downregulate prosurvival signaling and target B-precursor ALL. Finally, I explored PIM kinases that co-operate with BET proteins during the regulation of transcription as alternative therapeutic targets in B-precursor ALL. I was able to show this therapeutic approach holds promise, and warrants further study