Translational genetics identifies a novel target to treat fatty liver disease

Abstract: Fatty liver disease (FLD) is rapidly prevailing as the most common liver disease worldwide, with an estimated of one-quarter of the global population affected. Environmental and genetic determinants contribute to FLD susceptibility. The application of human and molecular genetics to drug discovery has leveraged effective molecules to treat human disease. In this thesis, we describe for the first time a genetic variant in PSD3, conferring protection against the entire spectrum of FLD. In addition, we characterized a 3D in vitro spheroid system composed of hepatocytes and hepatic stellate cells. This model allowed us to understand the molecular genetics underlaying the genetic association of the PSD3 variant. FLD progresses from steatosis to fibrosis that culminates in cirrhosis, and this progression is not well understood. Although animal models are widely used in preclinical studies on FLD, they have limitations making the translation to humans challenging. In Paper I, we created a multilineage 3D organoid system composed of immortalized human hepatocytes (HepG2) and hepatic stellate cells, (LX-2). Upon exposure to fatty acids, they acquire a phenotype of steatosis in turn resulting in fibrosis. This phenotype was rescued when treated with drug compounds, thus establishing an in vitro model able to mimic human FLD. In Paper II, we used a candidate gene approach and identified a genetic variant in the PSD3 gene (rs71519934) that results in leucine to threonine substitution at position 186 of the protein protecting against the entire spectrum of fatty liver disease (FLD). Downregulation of PSD3 with short interfering RNA (siRNA) lowered intracellular triglycerides in primary human hepatocytes when cultured both in 2D and 3D spheroids and resulted in lower de novo triglyceride synthesis and apolipoprotein secretion in rat and human hepatoma cells. Moreover, downregulation of Psd3 by antisense oligonucleotides (ASO) protected mice fed a non-alcoholic steatohepatitis (NASH)-inducing diet against FLD In conclusion, this thesis reports the identification and validation of the PSD3 genetic variant and its downregulation that confer protection against FLD. The validation was performed with several tools, including our 3D spheroid models, which offer an advantage over classical 2D culture systems. This has broadened our knowledge and understanding of the pathophysiology of FLD and of PSD3 as a potential therapeutic target to treat FLD.