UPMC Children’s Nephrology Research Team Uncovers Roles of the DNA Repair Protein FAN1 in Kidney Tubular Repair and Regeneration

UPMC Children’s Hospital of Pittsburgh Division of Pediatric Nephrology researchers in the Airik Laboratory have uncovered the role that the FAN1 protein plays in the kidney tubular epithelial cells during kidney repair and regeneration after injury. FAN1 is a DNA repair enzyme and mutations in FAN1 have been shown to lead to the development of chronic kidney disease (CKD) in humans. However, the molecular basis for FAN1 cytoprotection in the kidney tubular cells has remained elusive. The study was published in the journal Kidney International in August 2022. Rannar Airik, PhD, was the study's senior author, and Merlin Airik, PhD, was the first author of the new investigation.

The Airik Lab for the Investigation of Causes of Chronic Kidney Disease focuses on identifying the genetic and molecular mechanisms of chronic kidney disease, including renal ciliopathies that may be targetable by therapy. To achieve this goal the lab employs transgenic mouse models that are orthologous to human chronic kidney disease combined with kidney-derived cell culture models that are highly useful for exploring the mechanistic underpinnings of CKD at a molecular and cellular level. The Airik Lab has previously demonstrated that impaired repair of damaged DNA in the kidney tubular epithelial cells can lead to kidney tubular injury and precipitate progressive fibrotic changes in the kidney, culminating in the loss of kidney function. Investigating the role of intact DNA repair pathways in normal tubular maintenance and assessing how their deficiency underpins chronic kidney pathology is a central area of investigation in the lab.

New Study – Overview, Highlights, and Future Clinical Implications

In their new investigation, Dr. Airik and colleagues used a mouse model in which the Fan1 gene is inactivated in the tubular epithelial cells in the kidney.  When Fan1 knockout mice were subjected to various kinds of kidney injury, including administration of the genotoxic agent cisplatin or surgical ureteral obstruction, the researchers were able to recapitulate the kidney pathology observed in humans with mutations in the FAN1 gene. This demonstrated that the mice can be reliably used to investigate the mechanistic processes underlying FAN1-deficient kidney disease.

For example, the research team found that when mice with kidney-specific Fan1 inactivation were subjected to cisplatin, the tubular epithelial cells showed a failure to correct the DNA lesions while coincidentally induced new rounds of DNA replication without intervening mitosis. This cell cycle modification, known as endoreplication, inadvertently leads to polyploidization or the presence of extra copies of chromosomes in the cell. While endoreplication is a normal process in some tissues, such as liver, it is a hallmark of tissue pathogenesis in the kidney and associated with karyomegalic interstitial nephritis.

"Essentially, what happens is that the tubular cells deficient in FAN1, when subjected to injury through cisplatin, fail to regenerate normally," says Dr. Airik. "These cells accumulate DNA damage that prevents them from carrying out their normal functions, they contain abnormally enlarged nuclei (karyomegaly) due to endoreplication, which increases their metabolic activity, secretion of proinflammatory signals, and over time leads to fibrotic injury and chronic kidney disease."

Additional studies using human kidney proximal tubule cells in which the FAN1 gene was deleted led to similar abnormalities in chromosome ploidy and provided further mechanistic insights to the underlying processes. “It appears that FAN1 function is almost uniquely essential for kidney tubular cell homeostasis and repair, since mutations in FAN1 rarely result in other clinical phenotypes than chronic kidney disease in humans,” says Dr. Airik.

Additionally, the team performed a separate battery of experiments that introduced the use of roscovitine, a selective CDK inhibitor, prior to inducing injury with cisplatin. In these experiments, the DNA damage – endoreplication and polyploidization – were greatly reduced along with injury to the tubular cells.

Further studies on the DNA damage response and FAN1 protein are in progress in the Airik Lab. Also, further work will need to progress to better understand the potential use of roscovitine to blunt possible kidney injury in the setting of geno- or nephrotoxic agents or exposures that may impair the DNA damage response.

“Right now, our basic science findings are important for clinicians in a couple of ways,” says Dr. Airik.  “First, while karyomegalic interstitial nephritis is rare, patients with the disease likely harbor mutation in the FAN1 gene should be counseled on the matter, and potential issues should they encounter a genotoxic agent like cisplatin. Second, our findings also point to further studies that need to happen for individuals with FAN1 mutations who may have, for example, a form of cancer that is typically treated with an agent like cisplatin. Based on our findings, exposure to chemotherapy may potentially lead to kidney injury in a situation like this."

Read the complete study and its findings using the reference link below.

Reference

Airik M, Phua YL, Huynh AB, McCourt BT, Rush BM, Tan RJ, Vockley J, Murray SL, Dorman A, Conlon PJ, Airik R. Persistent DNA Damage Underlies Tubular Cell Polyploidization and Progression to Chronic Kidney Disease in Kidneys Deficient in the DNA Repair Protein FAN1. Kidney Int. 2022 Aug 2. Online ahead of print.