Background

Metabolic dysfunction-associated steatotic liver disease (MASLD), a major cause of chronic liver disease in modern medicine, is a global health challenge. This disease, which begins with simple fat accumulation and progresses to fibrosis and hepatocellular carcinoma (HCC), is caused by various genetic and environmental factors. Among these, the C-to-G base mutation at the rs738409 locus of the patatin-like phospholipase domain-containing protein 3 (PNPLA3) gene, i.e., the I148M mutation, is considered a key genetic factor determining the progression of fatty liver.

However, a precise analysis to elucidate how this potent mutation disrupts the physiological processes within hepatocytes remains incomplete. Previous studies mainly used animal models with artificially overexpressed PNPLA3 mutations or compared hepatocytes obtained from different patients. This approach has the problem of being unable to completely eliminate the noise caused by random genetic background differences between individuals. There was a need for the development of a precise cellular model that could trace the causal effect of increasing the copy number of the mutation on hepatocyte physiology on the same genetic background.

Key Findings

The researchers directly addressed this problem by using prime editing (PE), a next-generation gene editing technology. PE is a tool that enables precise base conversion by combining reverse transcriptase without completely cleaving the DNA double strand. To increase experimental efficiency and identify edited cells, the researchers designed an 'all-in-one' PE vector system incorporating the green fluorescent protein (GFP) gene. This system was designed to deliver PE protein, guide RNA (pegRNA), and nicking RNA (ngRNA) simultaneously to the HepG2 hepatocyte-derived cell line using a single plasmid.

Cells were sorted using a fluorescence-activated cell sorter (FACS), a device that separates cells based on fluorescence activation, resulting in highly purified cells with strong GFP signals. These cells were cultured at the single-cell level to establish isogenic HepG2 clone lines that share the same genetic background but differ only in the number of PNPLA3 I148M mutations. They successfully created wild-type (C/C), heterozygous mutant (C/G), and homozygous mutant (G/G) clones, effectively mapping the function of the mutation.

In this isogenic cell line model, experiments were conducted to induce lipid accumulation by administering free fatty acids (FFA). The results showed that G/G mutant clones exhibited significantly higher intracellular triglyceride accumulation compared to C/C wild-type clones under the same conditions. Furthermore, these cells showed not only lipid accumulation but also a distinct induction of endoplasmic reticulum stress response or abnormal inhibition of lipid secretion metabolic pathways. This is a remarkable achievement in directly observing the physical process by which a genetic mutation disrupts hepatocyte homeostasis.

Significance and Prospects

This study is of great academic value in that it created a disease model with perfectly eliminated genetic background noise using gene editing technology, demonstrating the single-factor causal relationship of complex metabolic diseases. It has provided a reliable human cell-based testbed for elucidating the mechanisms of fatty liver and liver fibrosis caused by the PNPLA3 I148M mutation. It is expected to serve as a foundation for the design of precision therapies that can break the chain of metabolic diseases leading to liver cancer.

However, the limitation is that this model is based on HepG2, an immortalized cancer cell line. Liver cancer cells differ from normal human hepatocytes in terms of metabolic activity and signaling pathways. Therefore, in the future, it is essential to apply this prime editing technique to iPSC-derived hepatocytes or a 3D liver organoid system to conduct further validation in a model closer to actual human tissue. The integration with drug delivery technology, which safely delivers therapeutic substances to target tissues in the patient's body, should also be a priority.