To our knowledge, this is the first cohort study reporting the prevalence, characteristics, and clinical significance of GD in PBC patients. We found a high prevalence of GD both in non-cirrhotic and cirrhotic PBC patients regardless of sex. PBC patients with GD were older, more often coexistent type 2 diabetes, and associated with a more severe liver disease at baseline than those without GD. More importantly, Cox regression analysis showed that concomitant GD was an independent risk factor that was associated with worse outcomes in non-cirrhotic PBC patients after adjusting for other important variables.
We found a significant higher prevalence of GD in PBC patients (26.2%) than that in general population, which was reported as 2.3–6.5% in Chinese adults[13]. Not surprisingly, a high prevalence of GD (33.6%) was shown in cirrhotic PBC patients, which could be mainly explained by cirrhosis and a high proportion of females, two well-known risk factors for developing GD[14, 15]. However, we also identified a high prevalence of GD in non-cirrhotic and male PBC patients, indicating that there might be other potential mechanisms for gallstone formation in these particular populations. The following factors may be explanations for the high prevalence of GD in PBC patients.
Firstly, the reduced bile acids pool due to cholestasis broke the balance of bile contents. Normally, cholesterol, lecithin, and cholic acids maintain homeostasis by micelles formation[16]. Under the circumstances of cholestasis, part of the bile acids will be transported into the systemic circulation instead of into the bile canaliculus due to excretion impairment[17], resulting in the reduction of the bile acid pool and cholesterol supersaturation. At the same time, the free bilirubin solubility in bile also depends on bile acids concentration, the decrease of the latter can lead to the formation of pigment stones.
Secondly, the altered biliary bile acids composition in PBC patients favored the gallstone formation. Studies have proved that chenodeoxycholic acid (CDCA) can lower the cholesterol saturation in bile and dissolve cholesterol stones by suppressing cholesterol synthesis and secretion[18, 19]. In contrast, cholic acid (CA) can increase the intestinal cholesterol absorption[20]. In a study that enrolled 98 treatment-naïve PBC patients and 14 healthy controls, Combes B et al.[21] found an increased CA and decreased CDCA in bile in PBC patients, which means that increased CA to CDCA ratio in PBC patients provided a favorable condition for gallstones formation.
Thirdly, the biliary infection caused by the imbalance of cholestasis-bile acids-intestinal microbiota triangle also played a crucial role. Nowadays, the crosstalk between bile acids and gut microbiota in the pathogenesis of PBC has arisen great interest in investigators. On the one hand, gut microbiota can regulate secondary bile acids metabolism and modulate hepatic bile acid synthesis[22]; on another hand, bile acids can shape gut microbiota by inhibiting the overgrowth of certain bacteria[23]. In PBC, the reduced bile acids secretion, altered gut microbiota[24] leads to bacterial translocation and biliary infection, eventually contributing to the formation of diverse types of gallstones.
Finally, impaired gallbladder mobility caused by constant stimulation of hydrophobic bile acids due to cholestasis might be another factor[25]. Besides, treatment with fibrates might promote cholesterol gallstone formation since PPARα reduces the transcription of CYP7A1, a rate-limiting enzyme regulating bile acids synthesis from cholesterol[26]. Putting together, the high prevalence of GD in PBC patients might be explained by reduced total bile acid pool, altered biliary bile acids composition, imbalance of gut microbiota, impaired gallbladder mobility, and drug treatment like fibrates.
Another interesting finding in the present study was that concomitant GD was independently associated with worse outcomes in non-cirrhotic PBC patients. Since cirrhosis is the most important risk factor associated with prognosis and GD is more prevalent in cirrhotic patients, we analyzed the impact of GD on prognosis stratified by cirrhosis. For non-cirrhotic PBC patients, only four risk factors including age, male sex, bilirubin, together with GD remain significant in the multivariate Cox regression analysis. The significance of the other three parameters was widely reported in many cohorts[27, 28], while none of the studies focused on the impact of GD on the prognosis of PBC. In our cohort, although patients with GD were older and more often coexistent metabolism-related disease at baseline than those without in non-cirrhotic PBC patients, the disease severity at baseline was comparable between the two groups (supplementary table 1). Besides, considering that variables that might be associated with prognosis were basically all involved in the univariate Cox analysis and GD remains significant after adjusting for age and other important variables, we believe the results were reliable. For cirrhotic PBC patients, concomitant GD was not significant in univariate Cox analysis.
The mechanism that GD only affected the prognosis of non-cirrhotic patients was unknown. One hypothesis was that concomitant GD in the early stage of the disease means that the patients had a more disordered biliary and intestinal microenvironment which results in a worse prognosis. In turn, the physical presence of gallstones can cause the inflammation of the gallbladder wall and impair gallbladder mobility, further aggravating cholestasis[29]. Once patients entered an advanced stage, the bile acids metabolism and intestinal microenvironment were already disturbed due to cirrhosis no matter with or without gallstones. This further suggests that early intervention is more beneficial to the disease. Indeed, the first-line and second-line drugs for the treatment of PBC including UDCA and obeticholic acid (OCA) are also effective drugs for the treatment of gallstones.
Our study suggested that early prevention or treatment of gallstones might improve the prognosis of non-cirrhotic PBC patients, providing a novel way of thinking for therapy. Lifestyle modification and management of metabolism-related diseases might be the first choice since there was no other effective medication. Other potential therapy targeting gut-liver axis like gut microbiota or gallbladder mobility might another choice. Multiple works waiting to be done in the future, what are the categories of gallstones in non-cirrhotic and cirrhotic PBC patients? Is GD a risk factor for other cholestatic liver diseases like primary sclerosing cholangitis? The answers remain to be investigated.
There are some limitations in the present study. First, the diagnosis of GD was made by other radiologists in routine abdominal ultrasonography instead of the investigators of the study. However, we believe the results were reliable because of the availability and operability of the ultrasonography. Second, we were not able to directly compare the prevalence between PBC patients and healthy controls since we didn’t include sex- and age-matched healthy populations. Third, some of the laboratory data were missing because of the retrospective nature.
In conclusion, a high prevalence of GD was found in PBC patients regardless of sex, concomitant GD was an independent risk factor for a worse prognosis in non-cirrhotic PBC patients.