Aging is accompanied by a progressive decline in cellular and physiological functions, increasing susceptibility to diseases and mortality. A hallmark of aging is genomic instability due to the accumulation of DNA damage, linked to mutations, altered gene expression, and cognitive decline. Impaired DNA repair pathways are implicated in premature aging and neurological symptoms. Conversely, upregulation of DNA repair pathways is associated with longer lifespans in some species. Understanding the regulation of DNA damage and repair in the brain is crucial for developing interventions against aging and neurodegenerative diseases. Impaired DNA repair is a feature of several neurodegenerative conditions, including Alzheimer's disease (AD). Post-mortem AD brains show increased DNA double-strand breaks and reduced expression of repair factors, indicating impaired repair capacity. In mouse models of AD, elevated DNA damage is observed even before the onset of neurological symptoms, suggesting a causal role for DNA damage in neurodegeneration. Enhancing DNA repair mechanisms could potentially mitigate the functional decline associated with aging and neurodegeneration. Histone deacetylases (HDACs) are enzymes that regulate various cellular processes, including transcription, chromatin remodeling, and DNA repair. HDAC1, a class I HDAC, has been previously shown to maintain genomic integrity in neurons. It is recruited to DNA double-strand breaks to deacetylate histones, promoting repair via non-homologous end joining (NHEJ). However, the role of HDAC1 in other DNA repair pathways, and its broader impact on gene expression and brain function during aging, remained unclear. This study investigates the effects of conditional HDAC1 deletion in neurons and astrocytes on brain function and DNA repair.

The literature extensively documents the link between DNA damage accumulation and age-related cognitive decline and neurodegeneration. Studies have shown that mutations in DNA repair genes and impaired DNA repair pathways are associated with premature aging and neurological symptoms. Conversely, enhanced DNA repair mechanisms have been observed in long-lived species, and interventions aimed at reducing DNA damage have shown promise in improving outcomes in model systems. In Alzheimer's disease, post-mortem studies reveal elevated levels of DNA damage, particularly double-strand breaks, alongside reduced expression of DNA repair factors. These findings highlight the crucial role of DNA repair in maintaining brain health and preventing age-related cognitive decline.

To investigate the role of HDAC1 in brain aging and neurodegeneration, the researchers generated brain-specific conditional knockout (*Hdac1* cKO) mice by crossing *Hdac1^fl/fl* mice with those expressing Nestin-Cre. This resulted in HDAC1 deletion in neurons and astrocytes. The researchers then examined these mice at various ages (young and aged) for various phenotypes. The researchers employed a variety of techniques to assess the effects of HDAC1 deficiency: * **Immunohistochemistry:** To analyze the distribution and abundance of different cell types (neurons, astrocytes, microglia) in the hippocampus and to assess for astrogliosis (indicated by increased GFAP immunoreactivity and astrocyte hypertrophy). * **Comet Assay:** To measure DNA damage in hippocampal tissue. This assay detects DNA strand breaks, allowing quantification of DNA damage levels. A modified comet assay using formamidopyrimidine DNA glycosylase (FPG) was used to specifically detect 8-oxoG lesions. * **Behavioral Tests:** To evaluate cognitive function. The researchers used contextual fear conditioning (to assess hippocampal-dependent spatial memory) and the Morris water maze (to assess spatial learning and memory). * **Electrophysiology:** To measure long-term potentiation (LTP) in hippocampal slices, a measure of synaptic plasticity crucial for learning and memory. * **RNA-sequencing (RNA-seq):** To analyze gene expression changes in the hippocampus of *Hdac1* cKO mice compared to controls at both young and aged stages. This allowed identification of differentially expressed genes (DEGs) and functional analysis through Gene Ontology (GO) analysis to determine affected cellular pathways. * **Chromatin Immunoprecipitation (ChIP) followed by qPCR and sequencing (ChIP-seq):** To analyze HDAC1 binding sites in the genome and to detect 8-oxoG accumulation at specific gene promoters. * **In vitro assays:** To examine the interaction between HDAC1 and OGG1, including in vitro acetylation and deacetylation assays and an OGG1 cleavage assay to measure OGG1 activity. Mass spectrometry was used to identify acetylation sites on OGG1. * **In vivo drug administration:** Exifone, an HDAC1 activator, was administered to both wild-type and 5XFAD mice to assess its effects on 8-oxoG levels, cognitive function, and LTP. LC/MS/MS was used to measure exifone concentrations in brain tissue. AAV mediated knockdown of HDAC1 in the hippocampus was also used. The 5XFAD mouse model, expressing human amyloid precursor protein (APP) and presenilin 1 (PSEN1) mutations, was used to investigate the role of HDAC1 in an Alzheimer's disease context. The study also included experiments in cultured neurons and astrocytes to investigate cellular mechanisms in more detail.

Aged *Hdac1* cKO mice displayed astrogliosis (increased astrocyte number and hypertrophy) and significantly increased DNA damage compared to controls, as measured by the comet assay. These mice also exhibited age-dependent cognitive decline in contextual fear conditioning and Morris water maze tests, and impaired hippocampal LTP induction. RNA-seq analysis revealed that, surprisingly, most differentially expressed genes (DEGs) in aged *Hdac1* cKO mice were downregulated, contrary to the expectation that HDAC1 primarily represses gene expression. Gene ontology analysis indicated that downregulated genes were enriched for functions related to ion transport, response to external stimulus, proteolysis, and aging, suggesting a functional impact on brain processes. Analysis of gene promoters revealed that downregulated genes were enriched for guanine-rich motifs, known to be susceptible to oxidative DNA damage. ChIP-qPCR showed increased 8-oxoG accumulation at the promoters of these downregulated genes in aged *Hdac1* cKO mice. The modified comet assay using FPG confirmed increased global 8-oxoG levels in aged *Hdac1* cKO mice, evident in both neurons and astrocytes. In vitro assays demonstrated that HDAC1 interacts with and deacetylates OGG1, a DNA glycosylase responsible for 8-oxoG removal, and that this deacetylation enhances OGG1 activity. In *Hdac1* cKO mice, reduced OGG1 activity was observed alongside increased OGG1 acetylation, linking HDAC1 deficiency to impaired 8-oxoG repair. In 5XFAD mice, increased 8-oxoG levels and downregulation of genes with guanine-rich promoters were observed, with a significant overlap in affected genes with those in aged *Hdac1* cKO mice. HDAC1 activity was reduced in 5XFAD mice. In *Hdac1* cKO; 5XFAD mice, 8-oxoG accumulation was further exacerbated. Pharmacological activation of HDAC1 using exifone reduced 8-oxoG lesions in aged wild-type and 5XFAD mice and improved cognitive performance (contextual fear conditioning and Morris water maze) and hippocampal LTP in 5XFAD mice. These effects were confirmed using both intraperitoneal and intracerebroventricular (ICV) administration of exifone. Finally, neuronal-specific knockdown of HDAC1 in aged wild-type mice was sufficient to cause cognitive deficits, highlighting the importance of neuronal HDAC1 function in memory.

This study reveals a novel mechanism linking HDAC1 to oxidative DNA damage repair and its implications for brain aging and neurodegeneration. The findings demonstrate that HDAC1 deficiency leads to impaired OGG1 activity, 8-oxoG accumulation, and transcriptional repression of genes crucial for brain function. The observation that many downregulated genes in HDAC1-deficient mice are also downregulated in AD patients suggests a significant functional consequence. The strong overlap between the gene expression changes observed in the *Hdac1* cKO mice and the 5XFAD AD model strongly supports a role for HDAC1 dysfunction in AD pathogenesis. The successful use of exifone, an HDAC1 activator, to alleviate the detrimental effects of 8-oxoG accumulation and improve cognitive function in 5XFAD mice provides compelling evidence for the therapeutic potential of HDAC1 activation in neurodegenerative diseases. The astrogliosis observed in *Hdac1* cKO mice could represent a secondary effect due to the impaired DNA repair or a direct consequence of HDAC1 deficiency in astrocytes. Further investigation is needed to determine the precise contribution of astrocytes to the observed phenotype.

This study establishes a critical role for HDAC1 in modulating OGG1-mediated oxidative DNA damage repair in the aging brain and Alzheimer's disease. HDAC1 deficiency leads to impaired OGG1 activity, 8-oxoG accumulation at guanine-rich promoter regions, and subsequent transcriptional repression of genes critical for brain function. Pharmacological activation of HDAC1 shows promise as a therapeutic strategy to counteract the detrimental effects of oxidative DNA damage, improve cognitive function, and potentially delay or mitigate neurodegeneration. Future research should focus on elucidating the precise molecular mechanisms involved and further investigating the therapeutic potential of HDAC1 activators in clinical settings.

The study primarily used mouse models. While these models provide valuable insights into the mechanisms of HDAC1 function and its interaction with DNA repair pathways, the findings may not directly translate to human physiology. The study focused primarily on the hippocampus; further research may be needed to determine the impact of HDAC1 dysfunction in other brain regions. While the effects of exifone are promising, further investigation is required to assess its long-term effects and potential side effects, as well as to confirm its specificity to HDAC1.