Laura J. Niedernhofer

The liver changes with age, leading to an impaired ability to respond to hepatic insults and increased incidence of liver disease in the elderly. Therefore, there is critical need for rapid model systems to study aging-related liver changes. One potential opportunity is murine models of human progerias or diseases of accelerated aging. Ercc1 −/Δ mice model a rare human progeroid syndrome caused by inherited defects in DNA repair. To determine whether hepatic changes that occur with normal aging occur prematurely in Ercc1 −/Δ mice, we systematically compared liver from 5-month-old progeroid Ercc1 −/Δ mice to old (24-36-month-old) wild-type (WT) mice. Both displayed areas of necrosis, foci of hepatocellular degeneration, and acute inflammation. Loss of hepatic architecture, fibrosis, steatosis, pseudocapillarization, and anisokaryosis were more dramatic in Ercc1 −/Δ mice than in old WT mice. Liver enzymes were significantly elevated in serum of Ercc1 −/Δ mice and old WT mice, whereas albumin was reduced, demonstrating liver damage and dysfunction. The regenerative capacity of Ercc1 −/Δ liver after partial hepatectomy was significantly reduced. There was evidence of increased oxidative damage in Ercc1 −/Δ and old WT liver, including lipofuscin, lipid hydroperoxides and acrolein, as well as increased hepatocellular senescence. There was a highly significant correlation in genome-wide transcriptional changes between old WT and 16-month-old, but not 5-week-old, Ercc1 −/Δ mice, emphasizing that the Ercc1 −/Δ mice acquire an aging profile in early adulthood. Conclusion: There are strong functional, regulatory, and histopathological parallels between accelerated aging driven by a DNA repair defect and normal aging. This supports a role for DNA damage in driving aging and validates a murine model for rapidly testing hypotheses about causes and treatment for aging-related hepatic changes. (Hepatology 2012)

Xeroderma pigmentosum (XP) is caused by defects in the nucleotide excision repair (NER) pathway. NER removes helix-distorting DNA lesions, such as UV–induced photodimers, from the genome. Patients suffering from XP exhibit exquisite sun sensitivity, high incidence of skin cancer, and in some cases neurodegeneration. The severity of XP varies tremendously depending upon which NER gene is mutated and how severely the mutation affects DNA repair capacity. XPF-ERCC1 is a structure-specific endonuclease essential for incising the damaged strand of DNA in NER. Missense mutations in XPF can result not only in XP, but also XPF-ERCC1 (XFE) progeroid syndrome, a disease of accelerated aging. In an attempt to determine how mutations in XPF can lead to such diverse symptoms, the effects of a progeria-causing mutation (XPF^R153P) were compared to an XP–causing mutation (XPF^R799W) in vitro and in vivo. Recombinant XPF harboring either mutation was purified in a complex with ERCC1 and tested for its ability to incise a stem-loop structure in vitro. Both mutant complexes nicked the substrate indicating that neither mutation obviates catalytic activity of the nuclease. Surprisingly, differential immunostaining and fractionation of cells from an XFE progeroid patient revealed that XPF-ERCC1 is abundant in the cytoplasm. This was confirmed by fluorescent detection of XPF^R153P-YFP expressed in Xpf mutant cells. In addition, microinjection of XPF^R153P-ERCC1 into the nucleus of XPF–deficient human cells restored nucleotide excision repair of UV–induced DNA damage. Intriguingly, in all XPF mutant cell lines examined, XPF-ERCC1 was detected in the cytoplasm of a fraction of cells. This demonstrates that at least part of the DNA repair defect and symptoms associated with mutations in XPF are due to mislocalization of XPF-ERCC1 into the cytoplasm of cells, likely due to protein misfolding. Analysis of these patient cells therefore reveals a novel mechanism to potentially regulate a cell's capacity for DNA repair: by manipulating nuclear localization of XPF-ERCC1. XPF-ERCC1 is a nuclease that plays a critical role in DNA repair. Mutations in XPF are linked to xeroderma pigmentosum, characterized by sun sensitivity, high incidence of skin cancer, and neurodegeneration, or XFE progeroid syndrome, a disease of accelerated aging. Herein we report the unexpected finding that mutations in XPF cause mislocalization of XPF-ERCC1 to the cytoplasm. Recombinant mutant XPF-ERCC1 derived from XP– and XFE–causing alleles are catalytically active and if delivered to the nucleus of cells restore DNA repair. This demonstrates that protein mislocalization contributes to defective DNA repair and disease arising as a consequence of mutations in XPF. It also illustrates a novel mechanism of regulating a cell's capacity for DNA repair: by manipulating nuclear localization of XPF-ERCC1 to enhance or inhibit repair and to prevent cancer or tumor resistance to chemotherapy, respectively.

Interstrand cross-links (ICLs) prevent DNA strand separation and, therefore, transcription and replication, making them extremely cytotoxic. The precise mechanism by which ICLs are removed from mammalian genomes largely remains elusive. Genetic evidence implicates ATR, the Fanconi anemia proteins, proteins required for homologous recombination, translesion synthesis, and at least two endonucleases, MUS81-EME1 and XPF-ERCC1. ICLs cause replication-dependent DNA double-strand breaks (DSBs), and MUS81-EME1 facilitates DSB formation. The subsequent repair of these DSBs occurs via homologous recombination after the ICL is unhooked by XPF-ERCC1. Here, we examined the effect of the loss of either nuclease on FANCD2 monoubiquitination to determine if the nucleolytic processing of ICLs is required for the activation of the Fanconi anemia pathway. FANCD2 was monoubiquitinated in Mus81^−/−, Ercc1^−/−, and XPF-deficient human, mouse, and hamster cells exposed to cross-linking agents. However, the monoubiquitinated form of FANCD2 persisted longer in XPF-ERCC1-deficient cells than in wild-type cells. Moreover, the levels of chromatin-bound FANCD2 were dramatically reduced and the number of ICL-induced FANCD2 foci significantly lower in XPF-ERCC1-deficient cells. These data demonstrate that the unhooking of an ICL by XPF-ERCC1 is necessary for the stable localization of FANCD2 to the chromatin and subsequent homologous recombination-mediated DSB repair.

The high incidence of resistance to DNA-damaging chemotherapeutic drugs and severe side effects of chemotherapy have led to a search for biomarkers able to predict which patients are most likely to respond to therapy. ERCC1-XPF nuclease is required for nucleotide excision repair of helix-distorting DNA damage and the repair of DNA interstrand crosslinks. Thus, it is essential for several pathways of repair of DNA damage by cisplatin and related drugs, which are widely used in the treatment of non–small cell lung carcinoma and other late-stage tumors. Consequently, there is tremendous interest in measuring ERCC1-XPF expression in tumor samples. Many immunohistochemistry studies have been done, but the antibodies for ERCC1-XPF were not rigorously tested for antigen specificity. Herein, we survey a battery of antibodies raised against human ERCC1 or XPF for their specificity using ERCC1-XPF–deficient cells as a negative control. Antibodies were tested for the following applications: immunoblotting, immunoprecipitation from cell extracts, immunofluorescence detection in fixed cells, colocalization of ERCC1-XPF with UV radiation–induced DNA damage in fixed cells, and immunohistochemistry in paraffin-embedded samples. Although several commercially available antibodies are suitable for immunodetection of ERCC1-XPF in some applications, only a select subset is appropriate for detection of this repair complex in fixed specimens. The most commonly used antibody, 8F1, is not suitable for immunodetection in tissue. The results with validated antibodies reveal marked differences in ERCC1-XPF protein levels between samples and cell types. [Cancer Res 2009;69(17):6831–8]