Novel α-1,3-Glucosyltransferase Variants and Their Broad Clinical Polycystic Liver Disease Spectrum

Abstract

Protein-truncating variants in α-1,3-glucosyltransferase (ALG8) are a risk factor for a mild cystic kidney disease phenotype. The association between these variants and liver cysts is limited. We aim to identify pathogenic ALG8 variants in our cohort of autosomal dominant polycystic liver disease (ADPLD) individuals. In order to fine-map the phenotypical spectrum of pathogenic ALG8 variant carriers, we performed targeted ALG8 screening in 478 ADPLD singletons, and exome sequencing in 48 singletons and 4 patients from two large ADPLD families. Eight novel and one previously reported pathogenic variant in ALG8 were discovered in sixteen patients. The ALG8 clinical phenotype ranges from mild to severe polycystic liver disease, and from innumerable small to multiple large hepatic cysts. The presence of <5 renal cysts that do not affect renal function is common in this population. Three-dimensional homology modeling demonstrated that six variants cause a truncated ALG8 protein with abnormal functioning, and one variant is predicted to destabilize ALG8. For the seventh variant, immunostaining of the liver tissue showed a complete loss of ALG8 in the cystic cells. ALG8-associated ADPLD has a broad clinical spectrum, including the possibility of developing a small number of renal cysts. This broadens the ADPLD genotype-phenotype spectrum and narrows the gap between liver-specific ADPLD and kidney-specific ADPKD.

Keywords: ADPLD; ALG8; clinical spectrum; next-generation sequencing; polycystic liver disease.

Figure 1

Pedigrees of the two ADPLD families with heterozygous ALG8 variants. (A) Family 1 (c.160C>T p.(Gln54*)). (B) Family 2 (c.685C>T p.(Arg229*)). Circles and squares with a vertical stripe (|) indicate individuals with a small number of hepatic cysts who do not fulfill the ADPLD criteria (hepatic cyst(s) but no PLD). A diagonal stripe (/) indicates that this individual has passed away. Arrows indicate the individuals who have undergone exome sequencing. Blue indicates that the individual carries the two common nucleotides (CC), and red indicates that the individual carries one common nucleotide and one rare nucleotide (CT) at the genomic position of interest. In individuals without annotated CC or CT, the presence of hepatic cysts is unknown and no genetic screening has been performed. ?, these individuals have been genetically screened but the presence of hepatic cysts is unknown. 5, these parents have 6 children (one boy and 5 children of unknown sex).

Figure 2

CT scans of eight ADPLD individuals with heterozygous pathogenic ALG8 variants. (A) Individuals 8826 and (B) 9173 are part of family 1, and are carriers of the same nonsense variant (c.160C>T p.(Gln54*)). (C) Individuals 3642 and (D) 8515 are carriers of frameshift variants (c.272delA p.(Asn91Metfs*5) and c.371delG p.(Cys124Serfs*33)), (E) 10027 is a carrier of a splice-site variant (c.478+3A>G p.?), and (F) 6935, (G) 23932 and (H) 11698 are carriers of different nonsense variants (c.685C>T p.(Arg229*), c.1090C>T p.(Arg364*), and c.1501delG p.(Val501*)).

Figure 3

ALG8 protein and variant localizations. (A) 3D structure of the ALG8 wildtype; (B–H) 3D structure of the truncated ALG8 proteins due to the specified nonsense and frameshift variants; (I) 3D structure of ALG8 and the amino acid position 154 in red, with (J) a close up of the glycine side-chain at position 154, and (K) a close up of the arginine side-chain at position 154. (L) Schematic interpretation of the ALG8 protein, and the localization of pathogenic variants associated with liver and kidney cysts.

Figure 3

ALG8 protein and variant localizations. (A) 3D structure of the ALG8 wildtype; (B–H) 3D structure of the truncated ALG8 proteins due to the specified nonsense and frameshift variants; (I) 3D structure of ALG8 and the amino acid position 154 in red, with (J) a close up of the glycine side-chain at position 154, and (K) a close up of the arginine side-chain at position 154. (L) Schematic interpretation of the ALG8 protein, and the localization of pathogenic variants associated with liver and kidney cysts.

Figure 4

ALG8 expression in human liver tissue. (A–F) Individual 11698, with the ALG8 nonsense variant c.1501delG p.(Val501*), and (G–I) an ADPLD individual with the PRKCSH splice-site variant c.292+1G>C p.?. (A,D,G) The protein of interest ALG8, (B,E,H) the cholangiocyte marker CK19, and (C,F,I) the hepatocyte marker HNF4α. Marker: 50 µm. The dotted arrow indicates the absence of marker staining. Solid arrows indicate the presence of marker staining. The key features of the liver cyst are displayed in Supplementary Figure S1.