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We recently demonstrated the presence of spontaneous cholemia with high total bile acids (H-TBA) in a subset (∼5%–25%) of all C57BL/6 mice obtained from various commercial breeders.¹ Cholemic/H-TBA mice are predisposed to develop accelerated liver cancer with elevated biliary damage (increased serum alkaline phosphatase and biliary expansion) and fibrosis on feeding of a high-caloric diet (eg, Western diet) but are resistant to obesity and metabolic dysfunction compared with littermate low-TBA control mice. Therefore, we proposed that investigators should exclude cholemic/H-TBA C57BL/6 mice in future studies to prevent potential bias and to avoid inconsistent or perplexing findings.

In line with our data, it was recently demonstrated that a subset (∼25%) of C57BL/6JOlaHsd mice fed from birth a semisynthetic diet (AIN-93G) displayed elevated TBAs.² These mice with elevated TBAs displayed smaller livers but showed increased serum liver damage markers and bile duct hyperplasia and increased immune infiltration, liver fibrosis, and hepatocyte proliferation. Mice with smaller livers fed the semisynthetic diet also showed lower serum total cholesterol and triglyceride levels similar to Western diet feeding. Overall, this highlights that the occurrence of cholemic mice with H-TBA is a common phenomenon of C57BL/6 mice and that experimenters should be aware of this fact when conducting metabolic or cancer studies.

As postulated by Ronda and colleagues, we agree that a possible cause for the cholemic phenotype is the presence of congenital portosystemic shunts (PSS), which in humans is also termed the Abernethy malformation (AM) or congenital absence of the portal vein. Interestingly, it was shown that approximately 25% of C57BL/6 mice exhibit sporadic congenital PSS.³ Importantly, patients with PSS/AM are also predisposed to develop liver masses including hepatocellular carcinoma.^4,5

Patients with PSS/AM have altered hepatic vascular dynamics, which results in dysregulated hepatic metabolism of intestinal products including bile acids. It is very likely that this dysregulated metabolism contributes to the pronounced liver damage, fibrosis, and inflammation observed in cholemic/H-TBA mice on high-caloric diet (Western diet or choline-deficient high-fat diet) or semisynthetic diet feeding.^1,2

Furthermore, we observed that the toxin carbon tetrachloride (CCl₄), which induces liver injury and fibrosis, did not exacerbate fibrosis in cholemic/H-TBA mice.¹ This suggests that cholemic/H-TBA mice may only be susceptible to dietary stress, which causes metabolic dysregulation and consequently liver damage, as opposed to all forms of liver injury.

Further studies should aim to unravel the mechanistic basis for spontaneous cholemia/H-TBA phenotype in C57BL/6 mice. It seems possible that genetic and epigenetic factors contribute to this disease phenotype. We observed spontaneous cholemia only in the C57BL/6 strain, but not in B6-FVB/N-129-mixed mice or in DBA/2 mice.¹ Whole-exome sequencing and selection for genetic alterations unique to C57BL/6 cholemic mice may therefore aid in the identification of genetic alterations driving this phenotype. Ultimately, it would be important to compare the genetic signature of cholemic/H-TBA mice with PSS/MA patients to determine whether similar genetic alterations are responsible for this abnormality in mice and humans.