Baat-Deficient Mice Recapitulate Elevated 7α-Hydroxy-3-Oxo-4-Cholestenoic Acid Observed in a Japanese Patient With...

Bile acid Coenzyme A: amino acid N-acyltransferase (BAAT) catalyzes the conjugation of bile acids with taurine or glycine, a process essential for bile acid solubility and intestinal lipid absorption. Mutations in BAAT cause an inborn error of bile acid metabolism, typically characterized by reduced conjugated bile acids and fat-soluble vitamin deficiency. However, several clinical features of BAAT deficiency cannot be fully explained by impaired conjugation alone, suggesting the presence of broader metabolic disturbances. In this study, we analyzed serum bile acid and intermediate profiles in a Japanese patient with genetically confirmed BAAT deficiency and identified a marked elevation of 7α-hydroxy-3-oxo-4-cholestenoic acid (7-HOCA), a key intermediate in bile acid synthesis. To investigate the metabolic consequences of BAAT loss, we generated a hepatic Baat knockdown mouse model using adeno-associated virus serotype 8-mediated delivery of Nme2Cas9 and Baat-targeting sgRNA. This model faithfully recapitulated the accumulation of 7-HOCA observed in the patient, together with a significant reduction in amino acid-conjugated bile acids. Integrated analyses combining bile acid profiling, quantitative PCR, and stable-isotope tracing of cholesterol demonstrated enhanced cholesterol flux toward bile acid biosynthetic pathways and upregulation of bile acid synthetic enzymes in Baat-deficient livers. These findings indicate that BAAT deficiency leads to dysregulated bile acid synthesis in addition to defective conjugation. Together, our results reveal a previously unrecognized metabolic phenotype of BAAT deficiency and provide mechanistic insight into its pathophysiology, establishing a novel in vivo model for studying bile acid metabolism beyond simple conjugation defects.