Every day, your liver assembles billions of tiny cholesterol-carrying particles and ships them into your blood. How many it makes depends, in large part, on a single protein that scientists at UT Southwestern Medical Center have only now identified. The protein, called HELZ2, controls how quickly the liver destroys the genetic instructions for building those particles. When HELZ2 is active, fewer particles get made. When it is missing, the liver floods the bloodstream with cholesterol. The discovery, published in Circulation in early 2026, could reshape how researchers think about preventing heart disease, though it also reveals a biological catch that will complicate any attempt to turn the finding into a drug.
A genetic screen that started with a mouse named Colby
The discovery began with a forward genetic screen, a method in which researchers breed mice carrying random mutations and then hunt for animals whose blood chemistry looks unusual. One mouse line stood out: its members had strikingly low plasma cholesterol and triglycerides. The team traced the trait to a gain-of-function mutation in the gene encoding HELZ2 and nicknamed the line “Colby.”
HELZ2 turned out to be an exoribonuclease, an enzyme that chews up RNA molecules from one end. Its specific target in the liver is the messenger RNA for apolipoprotein B (APOB), the structural protein that packages cholesterol and triglycerides into very-low-density lipoproteins (VLDL) before they are released into circulation. When HELZ2 is hyperactive, as in the Colby mice, APOB mRNA gets degraded faster, fewer apoB proteins are produced, and the liver exports far less cholesterol. When the researchers knocked HELZ2 out entirely, the opposite happened: APOB mRNA persisted longer, apoB production rose, and plasma lipid levels climbed.
The downstream consequences were dramatic. Colby mice developed significantly less atherosclerotic plaque than controls, while HELZ2-deficient animals showed accelerated plaque buildup. The UT Southwestern research team described HELZ2 as a control point for liver release of cholesterol-carrying lipoproteins, a characterization supported by the metabolic phenotyping, lipoprotein profiling, and atherosclerosis measurements reported in the paper.
Why apolipoprotein B matters so much
HELZ2’s significance comes from the outsized role that apoB plays in cardiovascular disease. ApoB is the protein backbone of every VLDL, intermediate-density lipoprotein, and LDL particle in the blood. Each of those particles carries one apoB molecule, which means measuring apoB gives a direct count of the atherogenic particles circulating in the body. Large prospective studies, including analyses of the UK Biobank and other population cohorts, have consistently shown that apoB particle concentration predicts coronary artery disease risk independently of standard LDL cholesterol readings.
At the extreme end, mutations in the APOB gene itself cause familial hypercholesterolemia, an autosomal dominant condition recognized by the NIH Genetic Testing Registry in which a single pathogenic variant can drive dangerously high LDL cholesterol from birth. Conversely, people who inherit loss-of-function APOB variants tend to carry fewer atherogenic particles and face lower cardiovascular risk across their lifetimes.
HELZ2 sits upstream of all of this. Rather than altering the apoB protein itself or intercepting LDL particles after they have entered the bloodstream (the way statins and PCSK9 inhibitors work), HELZ2 determines how much APOB mRNA the liver keeps around in the first place. That makes it a fundamentally different kind of target, one that governs the production rate of apoB-containing particles at the level of RNA stability.
The liver-fat trade-off
There is a catch, and it is not a small one. The liver uses VLDL particles to export not just cholesterol but also triglycerides. When apoB-mediated export drops, triglycerides can accumulate inside liver cells. Research on inherited forms of low apoB, including familial hypobetalipoproteinemia, has documented that people with chronically suppressed lipoprotein export can develop nonalcoholic fatty liver disease, steatohepatitis, and deficiencies of fat-soluble vitamins.
The UT Southwestern mouse data fit this pattern. Animals with hyperactive HELZ2 had lower plasma lipids and less atherosclerosis but accumulated more fat in their livers. The implication is stark: dialing down apoB production may protect arteries while stressing the liver. No existing cholesterol-lowering therapy has had to navigate this particular trade-off so directly. Statins reduce cholesterol synthesis inside the cell. PCSK9 inhibitors increase the recycling of LDL receptors on the cell surface. Neither fundamentally restricts the liver’s ability to package and export triglycerides the way suppressing APOB mRNA does.
This tension is not unique to HELZ2. Mipomersen, an antisense oligonucleotide that also targets APOB mRNA, was approved by the FDA in 2013 for homozygous familial hypercholesterolemia but came with boxed warnings about hepatotoxicity and liver fat accumulation. Any future therapy aimed at HELZ2 would face the same biological constraint and would need to demonstrate that it can reduce cardiovascular risk without tipping the liver into disease.
What the study does not yet show
Several important gaps remain. The Circulation paper is built entirely on mouse models. No human hepatocyte experiments, no induced-pluripotent-stem-cell validation, and no clinical data on HELZ2 modulation in people have been reported. The institutional and primary sources contain no mention of HELZ2 inhibitor compounds, dosing studies, or safety screening in larger mammals. The distance between a mouse genetic finding and a drug that patients could take is typically measured in years.
Selectivity is another open question. HELZ2 belongs to a family of interferon-regulated enzymes. Earlier biochemical work characterized it as a 3′-to-5′ exoribonuclease of the RNB family, capable of degrading structured RNAs broadly, not just APOB transcripts. If HELZ2 acts on a wide range of messenger RNAs in the liver, blocking or boosting its activity could trigger unintended changes in gene expression or activate interferon-related inflammatory pathways. The full catalog of RNAs that HELZ2 binds and degrades in living hepatocytes has not yet been mapped.
Perhaps most critically, no genome-wide association studies or biobank analyses have yet linked common HELZ2 variants in humans to plasma apoB levels, liver enzyme markers, or incident coronary disease. Until that kind of population-level genetic evidence arrives, the case for HELZ2 as a drug target rests on animal data and mechanistic reasoning. Both are necessary foundations, but neither is sufficient on its own.
Why the path from mouse genetics to a cholesterol drug is still long
The next steps are already taking shape. Human geneticists can scan existing biobanks for associations between HELZ2 variants and lipid traits or liver disease. Cell biologists can define the full substrate range of HELZ2 in hepatocytes, clarifying whether APOB mRNA is a privileged target or simply one of many. And if the risk-benefit profile holds up, drug discovery teams can begin the slow work of designing small molecules or biologics that modulate HELZ2 activity with enough precision to be therapeutically useful.
For now, HELZ2 is not a treatment. It is a newly identified lever in a system that scientists have studied for decades without finding all the moving parts. The discovery confirms that the biology of cholesterol transport still has surprises in it and that each new mechanism comes with its own set of trade-offs. Reducing the flood of cholesterol-rich particles that drive heart attacks and strokes remains one of the most consequential goals in medicine. HELZ2 offers a fresh route toward that goal. Whether it can be safely exploited will depend on years of work that has not yet begun.
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*This article was researched with the help of AI, with human editors creating the final content.
