Scientists Discover HELZ2 Controls Fatty Liver and Atherosclerosis by Degrading ApoB mRNA

Apolipoprotein B (APOB) is the structural backbone of VLDL and LDL particles, making it central to both liver fat export and cardiovascular disease risk. While APOB protein degradation during lipoprotein assembly has been studied extensively, whether and how APOB mRNA stability is regulated remained unknown. This study published in Circulation fills that gap by identifying HELZ2 — a dual-domain RNA helicase/exonuclease — as a physiological regulator of APOB mRNA turnover in hepatocytes, with direct consequences for fatty liver and atherosclerosis.

The discovery began with a large-scale ENU (N-ethyl-N-nitrosourea) forward genetic screen in C57BL/6J mice fed a high-fat diet. One mutant line, named 'Colby,' exhibited striking hepatic lipid accumulation without any change in body weight. Whole-genome sequencing mapped the causative mutation to Helz2 — a single nucleotide transition (Chr2: 180,874,995, A>G, GRCm39) converting leucine 1833 to proline (L1833P). Critically, this mutation resides in HELZ2's helicase domain and represents a gain-of-function allele: Colby-mutant HELZ2 showed enhanced ATPase and RNA unwinding activity compared to wild-type HELZ2 in biochemical assays.

Mechanistically, the team demonstrated that HELZ2 physically binds APOB mRNA — particularly within a 30-nucleotide region in the 5'-UTR — and promotes its degradation through helicase activity. RNA immunoprecipitation (RIP) assays confirmed specific HELZ2-APOB mRNA interaction, while deletion mapping pinpointed the 5'-UTR binding element. The Colby mutation dramatically increased this degradative activity, resulting in markedly reduced hepatic Apob mRNA and protein levels. Because less APOB means fewer VLDL particles can be assembled and secreted, triglycerides accumulate in the liver. Hepatic triglyceride secretion assays confirmed impaired VLDL-TG output in Colby mice.

The reciprocal experiment used Helz2 knockout mice fed a high-fat diet. These mice showed significantly elevated hepatic Apob mRNA and reduced hepatic triglyceride accumulation compared to wild-type littermates, confirming that endogenous HELZ2 normally limits APOB mRNA levels. A doxycycline-inducible, liver-specific HELZ2 overexpression model (TRE-Helz2; Alb-rtTA) phenocopied the Colby state — even modest hepatocyte-restricted HELZ2 induction was sufficient to decrease Apob mRNA and alter lipid handling — providing strong evidence for a gain-of-function mechanism rather than a neomorphic effect.

For cardiovascular implications, heterozygous Helz2Colby/+ mice were crossed into both Apoe−/− and Ldlr−/− atherosclerosis-prone backgrounds and fed a high-cholesterol diet. In both models, a single copy of the Colby allele conferred significant protection against atherosclerotic plaque formation, assessed by Oil Red O staining of the aortic sinus and en face aorta morphometry. FPLC lipoprotein profiling showed reduced VLDL and LDL cholesterol fractions in Colby heterozygotes, consistent with lower hepatic APOB output. These findings establish a clear mechanistic chain: enhanced HELZ2 activity → reduced APOB mRNA → less VLDL secretion → lower circulating APOB-containing lipoproteins → reduced atherosclerosis.

The study also confirmed conservation of the mechanism in human hepatocyte cell lines (Huh-7 and HepG2), where overexpression of wild-type or Colby-mutant human HELZ2 reduced endogenous APOB mRNA and protein. The human APOB 5'-UTR contains an analogous HELZ2-binding element, suggesting translational relevance. Together, these findings reframe HELZ2 as a druggable RNA-level regulator of lipoprotein biology, offering a novel therapeutic axis — distinct from statins or PCSK9 inhibitors — for MASLD and cardiovascular disease.