The RNA-binding proteins TIAR and TIA1 have been reported to affect beta cell insulin production and viability. The missense E384K TIA1 autosomal dominant mutation is known to cause Welander distal myopathy. The aim of this study was to study the effects of the TIA1 E384K mutation in human insulin-producing EndoC-βH1 cells. The prime editing technique was used to generate EndoC-βH1 cell clones with the homozygous E384K TIA1 mutation. The E384K TIA1 mutation did not affect high glucose + palmitate-induced stress granule formation and cell death. Instead, the mutated cells respired and proliferated faster than wild-type cells. This was paralleled by a higher MYC mRNA and protein level, a profoundly reduced GLP-1 receptor mRNA expression, increased expression of “disallowed” beta cell genes, a proinsulin-to-insulin processing defect, a decreased insulin content and release, a decreased PAX4/ARX mRNA ratio, and an increased glucagon production. The TIA1 mutation reduced MYC mRNA binding to TIA1. Downregulation of MYC mRNA levels normalized insulin/glucagon and PAX4/ARX mRNA ratios. Long-term treatment of TIA1-mutated cells with the GLP-1R agonist liraglutide restored insulin production and reversed beta cell dedifferentiation. It is concluded that the TIA1 E384K mutation, via increased MYC levels and cell proliferation rates, causes beta cell dedifferentiation. Thus, dysfunction of RNA-binding proteins may, at least in certain cases, contribute to the impaired insulin production observed in diabetes. A better understanding of RNA-binding protein-mediated control of beta cell differentiation, and the protective impact of GLP-1 receptor agonism, could facilitate the development of new treatment strategies in diabetes.