A biopsy-proven case report of karyomegalic interstitial nephritis resulting from the recurrent deletion 15q13.3 and a likely pathogenic variant in FAN1.

Abstract: Karyomegalic interstitial nephritis (KIN) is a rare cause of chronic interstitial nephritis. Affected patients typically present with progressive chronic kidney disease, leading to end-stage renal disease by early adulthood. Diagnosis is most often established by renal biopsy, which reveals characteristic karyomegalic tubular epithelial cells in the setting of chronic interstitial nephritis. Although these histopathological findings are highly specific for KIN, we emphasize the critical role of genetic testing, in conjunction with renal biopsy, to confirm the diagnosis and to enable accurate genetic counseling. Here, we report a noteworthy case of KIN featuring a compound genetic alteration, consisting of (i) the likely pathogenic variant (c. 2741₂742insCC, p. (Ser915Leufs*75) ) on one allele and (ii) the recurrent deletion at 15q13. 2–q13. 3 affecting the FAN1 locus on the other allele.