Intrinsic DNA damage repair deficiency results in progressive microglia loss and replacement.

The DNA excision repair protein Ercc1 is important for nucleotide excision, double strand DNA break, and interstrand DNA crosslink repair. In constitutive Ercc1-knockout mice, microglia display increased phagocytosis, proliferation and an enhanced responsiveness to lipopolysaccharide (LPS)-induced peripheral inflammation. However, the intrinsic effects of Ercc1-deficiency on microglia are unclear. In this study, Ercc1 was specifically deleted from Cx3cr1-expressing cells and changes in microglia morphology and immune responses at different times after deletion were determined. Microglia numbers were reduced with approximately 50% at 2-12 months after Ercc1 deletion. Larger and more ramified microglia were observed following Ercc1 deletion both in vivo and in organotypic hippocampal slice cultures. Ercc1-deficient microglia were progressively lost, and during this period, microglia proliferation was transiently increased. Ercc1-deficient microglia were gradually replaced by nondeficient microglia carrying a functional Ercc1 allele. In contrast to constitutive Ercc1-deficient mice, microglia-specific deletion of Ercc1 did not induce microglia activation or increase their responsiveness to a systemic LPS challenge. Gene expression analysis suggested that Ercc1 deletion in microglia induced a transient aging signature, which was different from a priming or disease-associated microglia gene expression profile.