A study of zebrafish has helped identify a susceptibility gene for biliary atresia, as reported in the May issue of Gastroenterology.
Biliary atresia is a progressive fibro-inflammatory disorder of infants that involves the extrahepatic and intrahepatic biliary tree and causes obliteration of the ducts, leading to cholestasis, fibrosis, and cirrhosis. If uncorrected, it causes death within the first 2 years of life.
The causes of this disease are unknown, but are believed to involve exposure of a genetically susceptible individual to specific environmental factors.
Biliary atresia occurs exclusively in neonates, so variants of genes expressed during hepatobiliary development could affect susceptibility. Genome-wide association studies previously identified a region of interest at 2q37.
Shuang Cui et al. took a closer look at the chromosome region, searching for copy number variants that were increased among 61 children with the disease, compared with 5088 healthy individuals.
They found that the patients had a significant increase in deletions at 2q37.3 that resulted in deletion of one copy of GPC1. This gene encodes glypican 1—a heparan sulfate proteoglycan that regulates hepatogenesis, Hedgehog signaling, and inflammation.
To learn how deletion of GPC1 could affect biliary development, they reduced its expression in zebrafish. Zebrafish are useful for studying development—embryos develop rapidly ex utero, and their hepatobiliary development and anatomy are similar to those of mammals. By 5 days after fertilization, the zebrafish liver has distinct hepatocytes and cholangiocytes, with an interconnecting duct network. Organs in embryonic zebrafish can be imaged in vivo, and gene expression can be reduced using morpholino antisense oligonucleotide-mediated knockdown techniques.
Cui et al. found that knockdown of gpc1 in developing zebrafish led to intrahepatic biliary and gallbladder defects, observed at the late larval stage (5 days after fertilization). During normal embryogenesis, zebrafish at this stage have developed intrahepatic main ducts, interconnecting branches, and terminal ductules. The knockdown fish had fewer cholangiocytes, resulting in a less complex architecture than controls (see below figure).
Gpc1 knockdown zebrafish (gpc MO) have a decreased number and complexity of ducts, compared with controls. Cells are indicated by white dots.
Bile secretion was also significantly reduced in gpc1 knockdown fish.
Activity of the signaling protein Hedgehog is increased in patients with biliary atresia, and GPCs are known to regulate its activity. Cui et al. compared expression of genes known to be regulated by Hedgehog (gli2a, ptch1, floxl1, znf697, and ccnd1), and found it to be increased in livers from gpc1 knockout embryos.
Exposure of the gpc1 knockout fish to cyclopamine, a Hedgehog antagonist, partially reversed the biliary defects. On the other hand, injection a Hedgehog ligand led to biliary defects similar to those of the gpc1 knockdowns.
The authors propose that these findings are consistent with a model in which glypicans modulate Hedgehog activity by acting as a sink, decreasing the availability of ligand—the absence of glypican increases Hedgehog signaling.
Cui also found that liver samples from patients with BA had reduced levels of apical GPC1 in cholangiocytes, compared with samples from controls.
This is the first study to identify a potential risk gene in patients with biliary atresia and to show functional defects in the biliary system in a model organism. These findings also support a role for Hedgehog signaling in the pathogenesis of BA.
In an editorial that accompanies the article, Alexander Miethke and Stacey Huppert state that Cui et al. have shown that zebrafish are a useful tool to assign biological significance to results of genomic studies.
More Information on Biliary Atresia
Read the article online.
Cui S, Leyva–Vega M, Tsai EA, et al. Evidence from human and zebrafish that gpc1 is a biliary atresia susceptibility gene. Gastroenterology 2013;144:1107−1115.e3.
Read the accompanying editorial.
Miethke AG, Huppert SS. Fishing for biliary atresia susceptibility genes. Gastroenterology 2013;144:878−881.