Bilateral Glomerulocystic Kidney Disease With Extensive Embryonal Hyperplasia in a Setting of <i>HNF1B</i> Mutation

Glomerulocystic renal disease has numerous etiologies, including <i>HNF1B</i> mutations. In addition to cysts, morphologic renal findings in a setting of <i>HNF1B</i> mutations include cystic renal dysplasia, solitary functioning kidney, horseshoe kidney, and oligomeganephronia. Embryonal hyperplasia resembling nephrogenic rests has been reported in rare cases of glomerulocystic disease, but none have been genetically characterized. We report a case of bilateral glomerulocystic kidney disease (GCKD) showing extensive embryonal hyperplasia in a setting of germline <i>HNF1B</i> mutation with progressive renal failure. Explant showed numerous epithelial proliferations throughout the intervening stroma. GCKD may be seen in a setting of <i>HNF1B</i> mutation; however, the additional finding of extensive embryonal hyperplasia in a case with a known mutation has never been reported. Reports of similar embryonal hyperplasia, associated with either cystic kidney disease or other disease processes, appear to represent a heterogeneous population of presentations and etiologies, though there is sufficient evidence to suggest that this is a finding, that is, recurrently associated with cystic kidney diseases. The underlying pathobiology of embryonal hyperplasia and the neoplastic potential in this setting is unknown. We report this case to highlight a novel combination of morphologic and genetic findings in GCKD and to raise awareness of this rare finding.