The VUS Challenge in Cystic Kidney Disease: A Case-Based Review.

Genetic testing in nephrology is becoming increasingly important to diagnose patients and to provide appropriate care. This is especially true for Autosomal Dominant Polycystic Kidney Disease (ADPKD) because this is a common cause of kidney failure and genetically complex. As well as the major genes, PKD1 and PKD2, there are at least 6 minor loci, and phenotypic, and in some cases, genetic overlap with other cystic disorders. Targeted Next Generation Sequencing (tNGS), a low cost, high throughput technique, has made routine genetic testing viable in nephrology clinics. Appropriate pre- and post-testing genetic counseling is essential to the testing process. Carefully assessing variants is also critical, with the genetic report classifying variants in accordance with ACMG guidelines. However, variants of uncertain significance (VUS) may pose a significant challenge for the ordering clinician. In ADPKD, and particularly within PKD1, there is high allelic heterogeneity; no single variant is present in more than 2% of families. The Mayo/PKD Foundation variant database, a research tool, is the best current database of PKD1 and PKD2 variants containing over 2300 variants identified in individuals with PKD, but novel variants are often identified. In patients with a high pre-test probability of ADPKD based on clinical criteria, but no finding of a pathogenic (P) or likely pathogenic (LP) variant in a cystic kidney gene, additional evaluation of cystic gene VUS can be helpful. In this case-based review, we propose an algorithm for the assessment of such variants in a clinical setting and show how some can be reassigned to a diagnostic grouping. When assessing the relevance of a VUS, we consider both patient/family-specific and allele related factors using population and variant databases and available prediction tools, as well as genetic expertise. This analysis plus further family studies can aid in making a genetic diagnosis.