Mechanism of MTARC1 Gene Variant Protecting Against Fatty Liver Fibrosis Revealed via Liver Organoids and Prime Editing
Background
Metabolic dysfunction-associated steatotic liver disease (MASLD) is a representative chronic liver disease affecting approximately 30% of the global population. It is a multifactorial disease that begins with simple fat accumulation in hepatocytes and progresses to metabolic dysfunction-associated steatohepatitis (MASH), fibrosis, cirrhosis, and eventually liver cancer. The patterns of onset and progression are strongly influenced by individual genetic factors. Genome-wide association studies (GWAS) have reported various single-nucleotide polymorphisms (SNPs) that increase or decrease disease risk.
The rs2642438 (p.A165T) variant of the Mitochondrial amidoxime reducing component 1 (MTARC1) gene has been identified as a protective factor that significantly reduces the risk of fatty liver and cirrhosis in patient cohorts. While the epidemiological link between the genotype and disease suppression was confirmed, it remained unclear what metabolic mechanism MTARC1 employs within hepatocytes and how a single variant alters cellular phenotypes. This was due to the difficulty of proving pure causality of a single variant using patient-derived cells with diverse genomic backgrounds or simple 2D in vitro culture models.
Key Findings
Researchers solved this problem by combining prime editing, a next-generation gene editing technology, with a 3D organoid culture system. Using prime editors at the human embryonic stem cell (hESC) stage, they precisely introduced the MTARC1 rs2642438 variant and then differentiated them into human liver organoids (HLOs). This allowed for a comparative experiment design where control organoids maintaining the reference sequence and organoids possessing only the single nucleotide variant were produced side-by-side within the same genomic background.
When free fatty acids and inflammatory stimuli were applied to the differentiated HLOs to induce fat accumulation and fibrotic damage, clear phenotypic differences emerged. HLOs with the rs2642438 variant showed significantly lower MTARC1 protein expression and a substantial reduction in intracellular triglyceride accumulation compared to reference sequence HLOs. They also exhibited a distinct pattern of protection from fatty liver lesions and fibrotic responses even under lipotoxicity and inflammatory stimuli. The disease-suppressing effects observed in the patient cohort were successfully reproduced in a 3D human tissue model.
Tracing the mechanism of triglyceride reduction revealed that the primary driver was the direct inhibition of the intracellular de novo lipogenesis (DNL) pathway, rather than the promotion of mitochondrial Ξ²-oxidation. As the amount of MTARC1 protein decreased, the metabolic signals inducing fatty acid synthesis were blocked.
Interesting data were also obtained from drug response evaluations. When resmetirom, an FDA-approved thyroid hormone receptor-beta (THRB) agonist, was administered, triglyceride levels significantly decreased in the reference sequence HLOs under fatty liver injury conditions. Conversely, in HLOs already possessing the rs2642438 protective variant, triglyceride levels did not decrease further upon additional treatment with resmetirom. This is interpreted as the drug's action being redundant, as the protective variant itself already substantially blocks the fat accumulation pathway.
Significance and Outlook
This study demonstrates that stem cell-derived organoids generated via prime editing serve as a powerful preclinical platform for establishing the causality of disease-associated genetic variants. It is noted that this approach confirmed at the molecular level how minute genetic differences at the single-nucleotide level induce disease resistance in human tissue models.
These findings strongly support the potential of MTARC1 inhibitors as promising new drug targets from a therapeutic strategy perspective. There is growing expectation that antisense oligonucleotides (ASOs) or small-molecule compounds that artificially suppress MTARC1 expression or function may confer liver-protective effects similar to those observed with genetic protective variants.
The resmetirom responsiveness data highlights the need for patient-specific precision medicine. This is because for MASH patients carrying specific protective variants, the expected effect of resmetirom treatment may be limited. In future clinical trial designs for new drugs, stratified analysis according to the patient's MTAR1 genotype is likely to serve as a crucial variable.
A challenge to overcome is that stem cell-derived liver organoids may not fully reflect the immune cell interactions or microvascular environments of actual adult liver tissue. Subsequent verification using immune cell co-culture systems or liver microfluidic chip-based models will be necessary to evaluate their impact on long-term in vivo metabolic homeostasis.
BACKGROUND: Metabolic dysfunction-associated steatotic liver disease (MASLD) is the most prevalent cause of chronic liver disease worldwide. This progressive, multifactorial condition is characterized by hepatic steatosis, which can evolve into steatohepatitis, fibrosis, cirrhosis, and liver cancer. Genetic factors influence the development, progression, and complications in MASLD, and genome-wide association studies (GWAS) have identified single-nucleotide polymorphisms (SNPs) associated with altered risk. Mitochondrial amidoxime reducing component 1 (MTARC1) rs2642438 (p.A165T) variant has been identified as protective, but the role of MTARC1 and the impact of this variant in hepatocytes remains poorly understood. METHODS: We applied prime editing to create the rs2642438 variant in human embryonic stem cells (hESCs) before differentiation into human liver organoids (HLOs) to investigate the effect of the variant under conditions of steatotic and fibrotic injury. RESULTS: Compared with HLOs formed from hESCs containing the MTARC1 reference sequence, HLOs with the rs2642438 variant show lower levels of MTARC1 protein and triglycerides and are protected from steatotic and fibrotic injury, as predicted by the phenotype observed in patients carrying the variant. The observed decrease in triglyceride levels with the variant appears to be driven more by suppression of de novo lipogenesis than stimulation of Ξ²-oxidation in the HLO model. While resmetirom, the thyroid hormone receptor-beta (THRB) agonist approved to treat patients with metabolic dysfunction-associated steatohepatitis (MASH), was effective in reducing triglyceride levels in the setting of steatotic injury in HLOs with the reference sequence, HLOs containing the variant did not show further reduction in triglyceride levels with exposure to resmetirom. CONCLUSIONS: Collectively, these findings establish a platform to model disease-associated SNPs in HLOs and provide insight into the activity of the MTARC1 variant.
This research provides a direct standard for establishing patient-specific treatment strategies in the development of MASH therapeutics. By pre-screening for the presence of the MTARC1 rs2642438 genotype, the priority of administering specific mechanism-based therapies such as resmetirom can be adjusted, or patient groups with a high probability of response can be selected. Meanwhile, the pharmaceutical and biotechnology industry has gained momentum to accelerate the development of MTARC1 inhibitors as a new target for controlling lipid accumulation in hepatocytes. Designing gene therapy pipelines that use siRNA or ASO to lower MTARC1 protein levels in hepatocytes is emerging as a promising alternative.