Disrupted Minor Intron Splicing Activates Reductive Carboxylation-mediated Lipogenesis to Drive MASH Progression

Aberrant RNA splicing is tightly linked to diseases, including metabolic dysfunction-associated steatotic liver disease (MASLD). Here, we revealed that minor intron splicing, a unique and conserved RNA processing event, is largely disrupted upon the progression of metabolic dysfunction-associated steatohepatitis (MASH) in mice and humans. We demonstrated deficiency of minor intron splicing in the liver induces MASH transition upon obesity-induced insulin resistance and LXR activation. Mechanistically, inactivation of minor intron splicing leads to minor intron retention of Insig1 and Insig2, resulting in premature termination of translation, which drives proteolytic activation of SREBP1c. This mechanism is conserved in human patients with MASH. Notably, disrupted minor intron splicing activates glutamine reductive metabolism for de novo lipogenesis through the induction of Idh1, which causes the accumulation of ammonia in the liver, thereby initiating hepatic fibrosis upon LXR activation. Ammonia clearance or IDH1 inhibition blocks hepatic fibrogenesis and mitigates MASH progression. More importantly, the overexpression of Zrsr1 restored minor intron retention and ameliorated the development of MASH, indicating that dysfunctional minor intron splicing is an emerging pathogenic mechanism that drives MASH progression. Additionally, reductive carboxylation flux triggered by minor intron retention in hepatocytes serves as a crucial checkpoint and potential target for MASH therapy.