Genetics: FAN1 Mutations Result in Defective DNA Damage Repair and Karyomegalic Interstitial Nephritis

RESEARCH HIGHLIGHTS Nature Reviews Nephrology 8, 491 (2012); published online 24 July 2012; doi:10.1038/nrneph.2012.146

GENETICS FAN1 mutations result in defective DNA damage repair and karyomegalic interstitial nephritis utations in FAN1 lead to a independently of the Fanconi anaemia defective DNA damage response pathway. In support of this theory, the M(DDR) and karyomegalic researchers found that depletion of interstitial nephritis (KIN), according to FANCD2 in FAN1-mutant cells resulted in new research. “Our findings implicate a worsening of the mitomycin C sensitivity susceptibility to environmental genotoxins defect, suggesting additive pathways. and inadequate DNA repair as novel Most DDR pathways and NPHP-like mechanisms contributing to renal fibrosis ciliopathies are conserved in zebrafish; and [chronic kidney disease]”, state therefore, the researchers performed the researchers. morpholino oligonucleotide knockdown To identify causative genes for of fan1 in zebrafish embryos. Knockdown nephronophthisis (NPHP)-like ciliopathies, of fan1 resulted in NPHP-like and DDR Weibin Zhou, Edgar Otto et al. examined phenotypes, characterized by a shortened two affected siblings of New Zealand Maori body axis, microcephaly, microphthalmia descent. Renal histology demonstrated the and massive apoptotic cell death. Zhou presence of enlarged nuclei (karyomegaly), * et al. found increased levels of γH2AX, leading to a diagnosis of KIN—a rare indicative of activated DDR signalling. NPHP-like fibrotic kidney disease. The Suppressing apoptosis by knocking researchers performed homozygosity * down p53 function revealed phenotypes mapping and exome sequencing, revealing characteristic of NPHP-like ciliopathies, a homozygous nonsense mutation in including pronephric renal cysts. “Loss of FAN1, which encodes Fanconi anemia- fan1 function results in ciliopathy-related Zebrafish embryos with knockdown ofp53 alone at associated nuclease 1. Zhou et al. then 72 h postfertilization show normal pronephric tubules phenotypes, which are revealed when p53 sequenced FAN1 exons in DNA samples (arrowheads, upper panel) whereas knockdown of both fan1 function is inhibited”, the authors state. and p53 at 72 h postfertilization results in pronephric kidney obtained from 10 families with KIN. They cysts (asterisks, lower panel). Nature Publishing Group © The researchers also examined γH2AX identified 12 different FAN1 mutations in Zhou, W. et al. Nat. Genet. doi:10.1038/ng.2347. levels in a small sample of human biopsy nine of the 10 families; these mutations samples from kidney transplant recipients. were absent in 96 healthy controls, thereby To repair the ICL sensitivity defect, Levels of γH2AX were increased in identifying recessive mutations in FAN1 as the researchers transduced fibroblasts samples with histological evidence of an important cause of KIN. from one of the affected siblings with kidney damage, suggesting that DDR The FAN1 protein acts in a DNA repair either wild-type FAN1 cDNA or with might contribute to renal damage in adults pathway that is involved in the repair of FAN1 cDNA harbouring KIN-associated with chronic kidney disease as well as in interstrand cross-link (ICL) damage. To mutations. Wild-type FAN1 rescued the individuals with NPHP-like ciliopathies. investigate the effect of the FAN1 mutation mitomycin C sensitivity defect; however, Zhou et al. say that their study raises on genome maintenance and DDR, the none of the cDNAs carrying mutations the question of whether individuals who researchers in collaboration with Agata from individuals with KIN complemented carry hypomorphic alleles of FAN1 rather Smogorzewska, studied the response of the defect, with the exception of one than two null alleles might develop renal FAN1-mutant cells to the ICL-inducing variant that partially rescued cell survival failure later in life. “It will be important to agents mitomycin C and diepoxybutane. with exposure to mitomycin C, suggesting examine a potential role for ICL-causing Upon exposure of primary fibroblasts and that this particular variant represents a genotoxins in the absolute and relative lymphoblastoid cell lines from individuals hypomorphic allele of FAN1. Transduction increase in end-stage kidney disease”, with FAN1 mutations to mitomycin C, with a cDNA carrying a mutation known they explain. chromatid breaks and radial chromosomes to inhibit interaction of FAN1 with Susan J. Allison on metaphase spreads were observed, FANCD2, necessary for recruitment consistent with a role of FAN1 in DDR. of FAN1 to sites of ICL damage via the Both mitomycin C and diepoxybutane Fanconi anaemia pathway, fully rescued Original article Zhou, W. et al. FAN1 mutations cause reduced the survival of FAN1-mutant the mitomycin C sensitivity defect. The karyomegalic interstitial nephritis, linking chronic kidney cells, demonstrating that FAN1 deficiency researchers believe that this finding failure to defective DNA damage repair. Nat. Genet. doi:10.1038/ng.2347 causes ICL sensitivity. suggests that FAN1 has the ability to act