Karyomegalic interstitial nephritis as a rare cause of kidney graft dysfunction: case report and review of literature

A rare multisystemic disorder with chronic kidney disease: Karyomegalic interstitial nephritis due to homozygous FAN1 c.2260C>T variant

Karyomegalic interstitial nephritis (KIN) is a rare entity associated with biallelic FAN1 (FANCD2/FANCI-Associated Nuclease 1) gene variants. In FAN1-related KIN, abnormal liver function tests and respiratory involvement are common, in addition to chronic kidney disease. Karyomegalic changes have also been reported in many other organs in patients with FAN1-related KIN in various studies. We report the case of a 35-year-old male with chronic kidney disease of unknown aetiology, concurrent recurrent upper and lower respiratory tract infections, and elevated liver function test results with unidentified aetiology. The patient's family history was remarkable for consanguineous parent marriage and history of kidney transplantation in his aunt. A kidney biopsy was performed, which was consistent with KIN. Clinical exome sequencing revealed a homozygous nonsense variant NM_014967.5 (FAN1): c. 2260C > T (p.Arg754Ter). According to the American College of Medical Genetics (ACMG) criteria, this variant is pathogenic and, to the best of our knowledge, has not been previously reported, homozygously. Therefore, the histopathological and clinical diagnoses of KIN were confirmed by genetic studies in our patient. This case report expands the genetic spectrum of FAN1-related KIN, and briefly reviews the current literature data.

Phenotypic and Genotypic Features of the FAN1 Mutation-Related Disease in a Large Hungarian Family

Pathogenic variants in the FAN1 gene lead to a systemic disease with karyomegalic interstitial nephritis (KIN) at the forefront clinically. The phenotypic-genotypic features of a FAN1 mutation-related disease involving five members of a Hungarian Caucasian family are presented. Each had adult-onset chronic kidney disease of unknown cause treated with renal replacement therapy and elevated liver enzymes. Short stature, emaciation, latte-colored skin, freckles, and a hawk-like nose in four patients, a limited intellect in two patients, and chronic restrictive lung disease in one patient completed the phenotype. Severe infections occurred in four patients. All five patients had ceased. Four patients underwent autopsy. KIN and extrarenal karyomegaly were observed histologically; the livers showed no specific abnormality. The genotyping using formalin-fixed tissue samples detected a hitherto undescribed homozygous FAN1 mutation (c.1673_1674insT/p.Met558lfs*4; exon 5) in three of these patients and a heterozygous FAN1 mutation in one patient. The reason for the heterozygosity is discussed. In addition, 56 family members consented to the screening for FAN1 mutation from which 17 individuals proved to be heterozygous carriers; a blood chemistry evaluation of their kidney and liver function did not find any abnormality. The clinical presentation of FAN1-related disease was multifaceted, and not yet described manifestations were observed besides kidney and liver disease. Mutation in this gene should be suspected in adults with small kidneys of unknown cause, elevated liver enzymes, and recurrent infections, even without a family history.

Polyploid tubular cells: a shortcut to stress adaptation

Tubular epithelial cells (TCs) compose the majority of kidney parenchyma and play fundamental roles in maintaining homeostasis. Like other tissues, mostly immature TC with progenitor capabilities are able to replace TC lost during injury via clonal expansion and differentiation. In contrast, differentiated TC lack this capacity. However, as the kidney is frequently exposed to toxic injuries, evolution positively selected a response program that endows differentiated TC to maintain residual kidney function during kidney injury. Recently, we and others have described polyploidization of differentiated TC, a mechanism to augment the function of remnant TC after injury by rapid hypertrophy. Polyploidy is a condition characterized by >2 complete sets of chromosomes. Polyploid cells often display an increased functional capacity and are generally more resilient to stress as evidenced by being conserved across many plants and eukaryote species from flies to mammals. Here, we discuss the occurrence of TC polyploidy in different contexts and conditions and how this integrates into existing concepts of kidney cell responses to injury. Collectively, we aim at stimulating the acquisition of novel knowledge in the kidney field as well as accelerating the translation of this basic response mechanism to the clinical sphere.