Table 1. Hallmarks of aging in comparison with Werner syndrome.

Aging hallmarks	Brief description	Werner syndrome (WS)	Reference for WS
Genome instability	Alteration to the genetic information over time due to DNA damage and defective DNA repair mechanisms. Genomic instability affects overall functions of the cell.	Patient cells show gross genomic instability. WRN-deficient cells display large deletions.	Salk et al. 8 Chen et al. 9
Telomere attrition	Progressive decrease in telomere length over multiple cell divisions. Telomere attrition mainly occurs owing to the end-replication problem and the lack of telomerase enzyme.	WRN interacts with Pot1 and TRF2 components of the shelterin complex to promote telomere maintenance. Telomere length in older patients with WS (40–60 years) is markedly shorter than in younger patients with WS (~30 years) and age-matched non-WS individuals.	Opresko et al. 11 Ishikawa et al. 10 Tokita et al. 72
Epigenetic alterations	Involves alterations in the DNA methylation patterns, post-translational modification of histones, and chromatin remodeling	Patients with WS show an increased DNA methylation age with an average of 6.4 years. WRN interacts with methylation complex consisting of SUV39H1, HP1α, and LAP2β, which is responsible for the epigenetic histone mark H3K9 trimethylation (H3K9me3). In response to DNA damage, WRN recruits chromatin assembly factor 1 (CAF-1) to alter chromatin structure.	Maierhofer et al. 12 Jiao et al. 78 Zhang et al. 14
Loss of proteostasis	Impairment of protein homeostasis due to accumulation of misfolded proteins and deregulation of proteolytic system. Chronic expression of misfolded, unfolded, or aggregation of proteins contributes to the development of age- related pathologies such as Alzheimer’s disease and cataracts.	Cataracts are one of the most common features observed in patients with WS. WRN expression is severely affected by promoter hypermethylation in age- related cataract lens cells.	Zhu et al. 15
Mitochondrial dysfunction	Reduction in the biogenesis of mitochondria and mitophagy. Reduced ATP production coupled with increased electron leakage. Oxidation of mitochondrial proteins.	WS cells show increased reactive oxygen species (ROS) production. Hepatocytes of Wrn (Δhel/Δhel) mice have decreased mitochondria and show altered mitochondrial functions.	Cogger et al. 16
Cellular senescence	Stable arrest of the cell cycle coupled with stereotyped phenotypic changes such as the accumulation of persistent DNA damage, senescence-associated β-galactosidase, p16 INK4A, and/or telomere shortening	Cellular senescence is a striking feature of WS patient cells. WRN deficiency increased the accumulation of persistent DNA damage, p16, and senescence- associated β-galactosidase.	Norwood et al. 73 Lu et al. 13
Deregulated nutrient sensing	Somatotropic axis essentially consisting of growth hormone, insulin-like growth factors (IGF-1 and II), and their carrier proteins and receptors regulates metabolism in mammals. In addition to insulin–IGF-1 (IIS) signaling pathway, which senses glucose, three interconnected nutrient sensing systems are associated with aging. The mechanistic target of rapamycin (mTOR) senses high amino acid concentrations, AMPK (5′- adenosine monophosphate [AMP]-activated protein kinase) senses low-energy states by detecting high AMP levels, and sirtuins sense low-energy states by detecting high NAD + levels. With aging, IIS pathway decreases, mTOR activity increases, AMPK upregulates in skeletal muscles, and sirtuins are downregulated.	WRN protects against starvation-induced autophagy. Further research is required to elaborate the role of WRN in regulating nutrient-sensing mechanisms.	Maity et al. 17
Stem cell exhaustion	A decline in the proliferation of stem and progenitor cells, which are required for tissue regeneration	WRN-deficient mesenchymal stem cells showed progressive disorganization of heterochromatin and premature senescence.	Zhang et al. 14
Altered intercellular communication	Enhanced activation of nuclear factor kappa B (NF-κB) and increased production of tumor necrosis factor (TNF), interleukin-1 beta (IL-1β), and cytokines resulting in age-associated alteration in intercellular communication. Accumulation of pro-inflammatory tissue damage, failure of immune system to clear pathogens and dysfunctional host cells, and occurrence of defective autophagy response. Bystander effect in which senescent cells induce senescence in neighboring cells via gap junction–mediated cell-cell contacts and ROS.	Patients with WS have elevated levels of inflammation-driven aging- associated cytokines (IL-4, IL-6, IL-10, granulocyte macrophage colony- stimulating factor [GM-CSF], IL-2, TNF- α, interferon gamma [IFNγ], monocyte chemoattractant protein-1 [MCP-1], and granulocyte colony-stimulating factor [G- CSF]) compared with normal individuals.	Goto et al. 18