Cell-Autonomous Progeroid Changes in Conditional Mouse Models for Repair Endonuclease XPG Deficiency

Abstract

As part of the Nucleotide Excision Repair (NER) process, the endonuclease XPG is involved in repair of helix-distorting DNA lesions, but the protein has also been implicated in several other DNA repair systems, complicating genotype-phenotype relationship in XPG patients. Defects in XPG can cause either the cancer-prone condition xeroderma pigmentosum (XP) alone, or XP combined with the severe neurodevelopmental disorder Cockayne Syndrome (CS), or the infantile lethal cerebro-oculo-facio-skeletal (COFS) syndrome, characterized by dramatic growth failure, progressive neurodevelopmental abnormalities and greatly reduced life expectancy. Here, we present a novel (conditional) Xpg−/− mouse model which -in a C57BL6/FVB F1 hybrid genetic background- displays many progeroid features, including cessation of growth, loss of subcutaneous fat, kyphosis, osteoporosis, retinal photoreceptor loss, liver aging, extensive neurodegeneration, and a short lifespan of 4–5 months. We show that deletion of XPG specifically in the liver reproduces the progeroid features in the liver, yet abolishes the effect on growth or lifespan. In addition, specific XPG deletion in neurons and glia of the forebrain creates a progressive neurodegenerative phenotype that shows many characteristics of human XPG deficiency. Our findings therefore exclude that both the liver as well as the neurological phenotype are a secondary consequence of derailment in other cell types, organs or tissues (e.g. vascular abnormalities) and support a cell-autonomous origin caused by the DNA repair defect itself. In addition they allow the dissection of the complex aging process in tissue- and cell-type-specific components. Moreover, our data highlight the critical importance of genetic background in mouse aging studies, establish the Xpg−/− mouse as a valid model for the severe form of human XPG patients and segmental accelerated aging, and strengthen the link between DNA damage and aging.

Non-catalytic Roles for XPG with BRCA1 and BRCA2 in Homologous Recombination and Genome Stability

“…Large numbers of γH2AX foci and increased γH2AX were also observed in SV40-transformed XPCS1RO cells from an XP-G/CS patient (Ellison et al, 1998), but not in SV40-transformed WT (VA13) cells (Figures 1G, S1F, lane 2 vs. 1). Importantly, we similarly observed elevated γH2AX in primary dermal fibroblasts derived from xpg −/−mice (Barnhoorn et al, 2014) compared to WT littermate controls (Figures 1H, S1G). …”

“…Data from both mouse models and human patients establish that while XPG is required for normal postnatal development, its endonuclease activity is not. In both cases, inactivating point mutations cause only UV sensitivity, whereas knockouts (mouse) or truncations (patients) cause the severe CS phenotype and very early death (Barnhoorn et al, 2014). …”

“…Mouse models recapitulate the patient phenotypes. The original Xpg knockout resulted in death before weaning (Harada et al, 1999), but a recent conditional knockout mouse in a hybrid strain background survives to 15–18 weeks and displays many progressive progeroid features, including early cessation of growth, cachexia, kyphosis, and extensive neurodegeneration (Barnhoorn et al, 2014). In striking contrast, mice with point mutations inactivating XPG enzymatic activity are UV sensitive but otherwise normal (Shiomi et al, 2004; Tian et al, 2004), similar to XP-G patients.…”

Restricted diet delays accelerated ageing and genomic stress in DNA-repair-deficient mice

“…DR animals retained 50% more neurons and maintained full motoric function, even far beyond the lifespan of ad libitum (AL) animals. Repair-deficient, progeroid Xpg −/− mice, a Cockayne syndrome model 6 , responded similarly, extending this observation to other repair mutants. The DR response in Ercc1 Δ/− mice closely resembled DR in wild type animals.…”

DNA Damage: From Chronic Inflammation to Age-Related Deterioration

“…Transcriptome analysis in cells lacking a functional CSB protein revealed an NF-κB-dependent pro-inflammatory response. The latter is thought to be responsible for the extraordinary neurodegenerative and wasting symptoms of this and other NER progeroid disorders (Newman et al, 2006; de Waard et al, 2010; Goss et al, 2011; Jaarsma et al, 2011; de Graaf et al, 2013; Barnhoorn et al, 2014). In other instances, DNA damage-driven inflammation may trigger tissue-specific degenerative changes leading to systemic metabolic abnormalities.…”