Million-Person Genetic Study Finds Clues to Metabolic Protection, With FNIP1 Emerging as a Potential Drug Target

Some people are born with rare genetic variants that seem to turn their bodies’ energy expenditure up slightly. A million-person study published in *Nature* found that people with impaired FNIP1 function not only had more favorable fat distribution and blood glucose levels, but also had markedly lower odds of a composite of cardiometabolic diseases, pointing drug research for obesity, diabetes, and fatty liver disease toward a new pathway.

The research team analyzed exome sequencing data from 1,032,116 people across 11 cohorts in the Americas, Europe, and Asia, using the ratio of triglycerides to high-density lipoprotein cholesterol as an indicator of metabolic status. The analysis identified 59 genes with independent associations, 23 of which are already targets of approved drugs or therapies in clinical development, showing that large-scale human genetic data may help identify druggable biological pathways from naturally occurring variation.

FNIP1 was the most striking new lead. About one in every 7,000 participants carried an extremely rare protein-truncating variant. Most of these variants affected a single copy of the gene and were associated with lower liver fat and blood glucose, more favorable abdominal fat distribution, and higher lean body mass. In an analysis of 227,636 cases and 265,114 controls, carriers had approximately 60% lower odds of the composite endpoint of coronary artery disease, type 2 diabetes, metabolic dysfunction-associated steatotic liver disease, or cirrhosis. This association applies to the overall composite of diseases and cannot be interpreted as showing that the risk of each individual disease fell by the same proportion.

The protein produced by FNIP1 works with molecules including FLCN to suppress mitochondrial biogenesis, oxidative phosphorylation, and energy expenditure. After researchers used small interfering RNA to reduce FNIP1 expression in primary human hepatocytes, genes related to lipid breakdown and lysosomes were activated, providing a mechanistic explanation that these protective associations may stem from increased fat utilization.

Animal experiments took the findings a step further. In male mice fed a high-fat, high-fructose diet, simultaneously inhibiting hepatic Fnip1 and Fnip2, or inhibiting their interacting gene Flcn, reduced weight gain and liver fat accumulation and improved insulin sensitivity. This chain of evidence spanning human genetics, hepatocytes, and in vivo models is stronger than a genetic association alone, but the genetic interventions in mice are not entirely equivalent to the natural loss of function of a single FNIP1 copy in humans.

The study also delineated safety boundaries. Loss of function in both copies of FNIP1 can cause immunodeficiency, while FLCN variants are associated with susceptibility to lung cysts, pneumothorax, and kidney cancer. Another mouse study found that complete Flcn knockout in specific hepatocytes may increase the risk of liver injury and cancer development. The research team therefore proposed a precise but incomplete FNIP1 inhibition strategy focused on the liver, such as hepatocyte-targeted RNA drugs, with the aim of preserving metabolic benefits while avoiding systemic effects.

The current findings still cannot prove that inhibiting FNIP1 can treat people who already have disease, and there are not yet any human dosing data to answer questions about appropriate dosage, long-term immune effects, and liver safety. The central question for the next stage will not be whether this gene can be “turned off,” but whether it can be reduced by the appropriate degree in the correct tissue, translating the protection that a small number of people inherit into a therapy that can be used safely by the majority.