Molecular modelling of MYC hyperfunction in plasma cell differentiation

c-MYC (MYC) is a major regulator of transcription and a critical transcription factor in the germinal centre (GC). Long-lived plasma cells (PCs) are generated mainly from a GC response and their differentiation requires MYC downregulation. MYC can also act as a potent oncogene when deregulated being involved in multiple mature B cell cancers. Tumour cells overexpressing MYC can also frequently deregulate the anti-apoptotic protein BCL2 for their survival. Thus, MYC and BCL2 oncogenes collaborate in cancers of the B lineage. The goal of this study was to investigate the impact of MYC overexpression on human PC differentiation. A previously established system was utilised to differentiate human memory B cells into PCs in vitro. MYC and BCL2, protecting from MYC-mediated apoptosis, were overexpressed utilising retroviral vectors. This experimental setting allowed the development of a novel model system where the effect of enforced MYC and BCL2 expression was assessed, for the first time to our knowledge, under conditions permissive for PC differentiation.

Acute overexpression of MYC and BCL2 at the stage of activated memory B cells did not result in transformation and PC differentiation arrest. MYC-BCL2 overexpressing cells acquired an abnormal plasmablast-like phenotype with impaired antibody secretion. In addition, MYC overexpression drove metabolic over secretory transcriptional reprogramming as differentiation progressed. Within the transactivation domain at the N-terminus of MYC, deletion of the MBI domain showed marginal differences to the MYCwt condition suggesting its dispensable role in MYC hyperfunction. On the contrary, deletion of the MB0 weakened the MYC hyperfunction effect while deletion of the MBII resulted in an almost non-functional MYC upon its overexpression. Thus, both the MBII and MB0 domains of MYC are required for its hyperfunction-mediated effect under conditions permissive for PC differentiation. Within MBII the results demonstrated a critical dependence on the DCMW motif and W135.