Identification of two novel pathogenic variants of the NR1H4 gene in intrahepatic cholestasis of pregnancy patients

Background: Intrahepatic cholestasis of pregnancy (ICP) can cause adverse pregnancy outcomes, such as spontaneous preterm delivery and stillbirth. It is a complex disease influenced by multiple factors, including genetics and the environment. Previous studies have reported that functioning nuclear receptor subfamily 1 group H member 4 (NR1H4) plays an essential role in bile acid (BA) homeostasis. However, some novel variants and their pathogenesis have not been fully elucidated. Therefore, this research aimed to investigate the genetic characteristics of the NR1H4 gene in ICP.

Methods: In this study, we sequenced the entire coding region of NR1H4 in 197 pregnant women with ICP disease. SIFT and PolyPhen2 were used to predict protein changes. Protein structure modelling and comparisons between NR1H4 reference and modified protein structures were performed by SWISS-MODEL and Chimera 1.14rc, respectively. T-tests were used to analyse the potential significant differences between NR1H4 mutations and wild types for 29 clinical features. Fisher's test was conducted to test the significance of differences in mutation frequencies between ICP and the three databases.

Results: We identified four mutations: two novel missense mutations, p.S145F and p.M185L; rs180957965 (A230S); and rs147030757 (N275N). The two novel missense mutations were absent in 1029 controls and three databases, including the 1000 Genomes Project (1000G_ALL), Exome Aggregation Consortium (ExAC) and ChinaMAP. Two web-available tools, SIFT and PolyPhen2, predicted that these mutations are harmful to the function of the protein. Moreover, compared to the wild-type protein structure, the NR1H4 p.S145F and p.M185L protein structure showed a slight change in the chemical bond in two zinc finger structures. Combined clinical data indicate that the mutation group had higher levels of total bile acid (TBA) than the wild-type group. Therefore, we hypothesized that these two mutations altered the protein structure of NR1H4, which impaired the function of NR1H4 itself and its target gene and caused an increase in TBA.

Conclusions: To our knowledge, this is the first study to identify the novel p.S145F and p.M185L mutations in 197 ICP patients. Our present study provides new insights into the genetic architecture of ICP involving the two novel NR1H4 mutations.