Heterozygosity of ALG9 in Association with Autosomal Dominant Polycystic Liver Disease

Abstract

α-1,2-mannosyltransferase (ALG9) germline variants are linked to autosomal dominant polycystic kidney disease (ADPKD). Many individuals affected with ADPKD possess polycystic livers as a common extrarenal manifestation. We performed whole exome sequencing in a female with autosomal dominant polycystic liver disease (ADPLD) without kidney cysts and established the presence of a heterozygous missense variant (c.677G>C p.(Gly226Ala)) in ALG9. In silico pathogenicity prediction and 3D protein modeling determined this variant as pathogenic. Loss of heterozygosity is regularly seen in liver cyst walls. Immunohistochemistry indicated the absence of ALG9 in liver tissue from this patient. ALG9 expression was absent in cyst wall lining from ALG9- and PRKCSH-caused ADPLD patients but present in the liver cyst lining derived from an ADPKD patient with a PKD2 variant. Thus, heterozygous pathogenic variants in ALG9 are also associated with ADPLD. Somatic loss of heterozygosity of the ALG9 enzyme was seen in the ALG9 patient but also in ADPLD patients with a different genetic background. This expanded the phenotypic spectrum of ADPLD to ALG9.

Keywords: ADPLD; ALG9; PCLD; cyst wall; polycystic liver disease (PLD); whole exome sequencing.

Figure 1

Phenotype and variant interpretation of ALG9 c.677G>C. (A,B) Transverse and coronal CT scans of the patient before surgery. On CT scans, liver cysts are displayed as homogenous, darker-gray circular shapes. The locations of the liver cysts are marked with black asterisks (*). The displayed cross-sectional images were chosen to illustrate cyst distribution and volume, not to illustrate the number of cysts. (C) Conservation analysis in 12 species with ALG9′s secondary structure (yellow: α helix; purple: 3₁₀ helix; green: coil) and transmembrane regions (blue). (D) The 3D structure of wildtype ALG9 with amino acid position 226 in red. (E) Close-up of glycine at amino acid position 226 in red. (F) Close-up of alanine at amino acid position 226 in red.

Figure 2

ALG9 expression in cystic liver tissue. (A–J) ADPLD individual with the heterozygous pathogenic missense variant in ALG9, (K–O) an ADPLD individual with a heterozygous pathogenic splice site variant in PRKCSH, and (P–Y) an ADPKD individual with PLD with a heterozygous pathogenic frameshift variant in PKD2. Green: protein of interest marker ALG9; red: cholangiocyte marker CK19; blue: DNA marker DAPI. The key features of liver cyst tissue: The epithelium lining of a liver cyst is a single-cell layer of cholangiocytes, which encapsulates the cyst fluid and is generally surrounded by a multilayer of fibrotic/connective cells (E,O,T). The epithelium lining of a bile duct is also a single-cell layer of cholangiocytes. However, the epithelial cells of the bile duct are more cuboidal shaped (J,Y). Image magnification: 63×.