| Summary: | Autosomal dominant polycystic kidney disease (ADPKD) is the most common genetic cause of kidney failure in adult life. Rarely, ADPKD can be diagnosed in utero or in infancy, and the genetic mechanism underlying such severe presentation has been shown to be related to reduced gene dosage. Biallelic <i>PKD1</i> variants are often identified in early onset ADPKD, with one main pathogenic variant and a modifier hypomorphic variant showing an in trans configuration. We describe two unrelated individuals with early onset cystic kidney disease and unaffected parents, where a combination of next-generation sequencing of cystic genes including <i>PKHD1</i>, <i>HNF1B</i> and <i>PKD1</i> allowed the identification of biallelic <i>PKD1</i> variants. Furthermore, we review the medical literature in order to report likely <i>PKD1</i> hypomorphic variants reported to date and estimate a minimal allele frequency of 1/130 for this category of variants taken as a group. This figure could help to orient genetic counseling, although the interpretation and the real clinical impact of rare <i>PKD1</i> missense variants, especially if previously unreported, remain challenging.
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