Synthetic lethality between metabolic inhibition of Nicotinamide N-methyltransferase (NNMT) and oncogenic epidermal growth factor signaling in glioblastomas

Introduction: Nicotinamide N-methyltransferase (NNMT) play a key role in single carbon metabolism by catalyzing the conversion of methyl groups from S-adenosyl-L-methionine (SAM) to nicotinamide (NAM) to generate S-adenosine hyperblastoids (SAH) and 1-methylnicotinamide (MNA). Oncogenic Epidermal growth factor receptor (EGFR) activation is a major driver of glioblastoma pathogenesis. Here, we examine the intersection between oncogenic signaling and NNMT activation in glioblastoma pathogenesis.

Methods: Informatic and laboratory study

Results: Unbiased proteomic analysis revealed that NNMT is one of the most highly activated enzymes in response to oncogenic Epidermal growth factor receptor (EGFR) activation. In parallel, we observed that NNMT expression is tightly coupled to EGF expression in independent clinical sample collections, including the TCGA. Confirming these associations, activation of wild-type EGFR by EGF or the constitutively active mutant EGFRvIII increased NNMT expression and enzyme activity in both adherent and glioblastoma stem-like cell lines. This EGFR driven NNMT expression required STAT1 and p38 mitogen-activated protein kinase pathway. Importantly, knockdown of NNMT by RNA interference or pharmacologic inhibition of NNMT induced lethality in EGFRvIII driven glioblastoma cell lines without impacting the viability of non-EGFR driven glioblastomas, suggesting synthetic lethality. The severity of this lethality is an order of magnitude higher than those we previously established for PARP or PLK inhibition. Based on these findings, we designed a novel, highly specific small molecule inhibitor of NNMT and showed its tumoricidal activity against glioblastoma xenograft models both in vitro and in vivo.

Conclusion : Our results suggest NNMT as a synthetic lethal target for classical glioblastomas dependent on EGFR oncogenic signaling and warrant exploration as a therapeutic strategy.