Autosomal Dominant Polycystic Kidney Disease: Presence of Hypomorphic Alleles in PKD1 Gene.

Autosomal dominant polycystic kidney disease is characterized by multiple cysts in both kidneys manifesting in adult life. In general, the disorder is caused by a pathogenic variant in one allele of PKD1 or PKD2 genes, while the other allele is normal. Pathogenic variants in both the alleles are rare and have variable phenotypes, from lethal or perinatal presentation to a mild form in later adulthood, depending on the type of variant. Here, we describe a proband with two variants (p.Thr1773Ile and p.Ala1871Thr in trans ) in PKD1 gene, who presented with disease at age 24 years. Both the parents and one brother had a variant in one allele, the other being wild type only and had normal ultrasound findings. Segregation studies suggest that both the variants may act as "hypomorphic" or "incompletely penetrant" alleles and acting together resulted in haploinsufficiency of protein PC1 in renal cells, leading to cystogenesis in the proband. The consequences of the presence of two hypomorphic variants have been poorly documented in literature. We reviewed the few published cases having two hypomorphic variants and the data conform to the conclusions that we reached by study of the family described. It is emphasized that to resolve the significance of suspected hypomorphic variants, segregation studies in the parents and siblings are essential.