Abstract Body (Do not enter title and authors here): Introduction
Dysregulated lipid metabolism underlies both metabolic dysfunction-associated steatotic liver disease (MASLD) and atherosclerosis. Apolipoprotein B (APOB), essential for hepatic very low-density lipoprotein (VLDL) secretion, is central to both pathologies. While APOB protein turnover has been extensively studied, it remains unknown whether and how APOB mRNA stability is regulated under physiological or pathological conditions.

Research Question
Can large-scale, phenotype-driven forward genetic screen uncover novel regulators of lipid metabolism, potentially revealing unanticipated mechanisms of hepatic lipid control and cardiovascular risk?

Methods
We screened over 30,000 N-ethyl-N-nitrosourea (ENU)-mutagenized mice, covering ~20% of autosomal genes, to identify spontaneous metabolic phenotypes. This led to the discovery of a rare, dominant gain-of-function mutation (named Colby) in helicase with zinc finger 2 (Helz2), associated with severe hepatic steatosis. We validated causality using CRISPR-engineered Helz2^Colby knock-in and Helz2 knockout mice, and generated epitope-tagged alleles to track endogenous HELZ2 protein. Functional studies included lipid and lipoprotein profiling, VLDL secretion assays, mRNA-protein interaction mapping, RIP-seq, and domain-specific mutagenesis in murine and human hepatocytes. Atherosclerosis was evaluated in Apoe^-/- and Ldlr-/- mice. Human GWAS datasets were analyzed for translational relevance.

Results
HELZ2 was identified as a conserved RNA helicase that binds the 5'UTR of APOB mRNA and promotes its degradation. The Colby mutation enhances this helicase activity, leading to markedly reduced APOB expression, impaired hepatic lipid export, and steatosis without weight gain. In contrast, Helz2 knockout mice exhibitd increased Apob mRNA and were resistant to diet-induced steatosis. Notably, a single Helz2^Colby allele normalized plasma APOB-containing lipoproteins and significantly reduced atherosclerosis in both Apoe^-/- and Ldlr^-/- mice. A nearby human HELZ2 variant correlates with circulating VLDL levels, supporting translational significance.

Conclusions
This unbiased genetic screen uncovered HELZ2 as a novel, evolutionarily conserved regulator of APOB mRNA stability. This previously unrecognized mechanism operates upstream of known APOB protein regulation and links hepatic RNA control to systemic lipid balance and vascular disease. HELZ2 thus represents a promising therapeutic target for MASLD and atherosclerosis.