<i>ALG9</i> Mutation Carriers Develop Kidney and Liver Cysts.

Abstract

Mutations in <i>PKD1</i> or <i>PKD2</i> cause typical autosomal dominant polycystic kidney disease (ADPKD), the most common monogenic kidney disease. Dominantly inherited polycystic kidney and liver diseases on the ADPKD spectrum are also caused by mutations in at least six other genes required for protein biogenesis in the endoplasmic reticulum, the loss of which results in defective production of the <i>PKD1</i> gene product, the membrane protein polycystin-1 (PC1). We used whole-exome sequencing in a cohort of 122 patients with genetically unresolved clinical diagnosis of ADPKD or polycystic liver disease to identify a candidate gene, <i>ALG9</i>, and <i>in vitro</i> cell-based assays of PC1 protein maturation to functionally validate it. For further validation, we identified carriers of <i>ALG9</i> loss-of-function mutations and noncarrier matched controls in a large exome-sequenced population-based cohort and evaluated the occurrence of polycystic phenotypes in both groups. Two patients in the clinically defined cohort had rare loss-of-function variants in <i>ALG9</i>, which encodes a protein required for addition of specific mannose molecules to the assembling N-glycan precursors in the endoplasmic reticulum lumen. <i>In vitro</i> assays showed that inactivation of <i>Alg9</i> results in impaired maturation and defective glycosylation of PC1. Seven of the eight (88%) cases selected from the population-based cohort based on <i>ALG9</i> mutation carrier state who had abdominal imaging after age 50; seven (88%) had at least four kidney cysts, compared with none in matched controls without <i>ALG9</i> mutations. <i>ALG9</i> is a novel disease gene in the genetically heterogeneous ADPKD spectrum. This study supports the utility of phenotype characterization in genetically-defined cohorts to validate novel disease genes, and provide much-needed genotype-phenotype correlations.