Senescence‐associated β‐galactosidase reveals the abundance of senescent CD8+ T cells in aging humans

Aging leads to a progressive functional decline of the immune system, rendering the elderly increasingly susceptible to disease and infection. The degree to which immune cell senescence contributes to this decline remains unclear, however, since markers that label immune cells with classical features of cellular senescence accurately and comprehensively have not been identified. Using a second‐generation fluorogenic substrate for β‐galactosidase and multi‐parameter flow cytometry, we demonstrate here that peripheral blood mononuclear cells (PBMCs) isolated from healthy humans increasingly display cells with high senescence‐associated β‐galactosidase (SA‐βGal) activity with advancing donor age. The greatest age‐associated increases were observed in CD8+ T‐cell populations, in which the fraction of cells with high SA‐βGal activity reached average levels of 64% in donors in their 60s. CD8+ T cells with high SA‐βGal activity, but not those with low SA‐βGal activity, were found to exhibit features of telomere dysfunction‐induced senescence and p16‐mediated senescence, were impaired in their ability to proliferate, developed in various T‐cell differentiation states, and had a gene expression signature consistent with the senescence state previously observed in human fibroblasts. Based on these results, we propose that senescent CD8+ T cells with classical features of cellular senescence accumulate to levels that are significantly higher than previously reported and additionally provide a simple yet robust method for the isolation and characterization of senescent CD8+ T cells with predictive potential for biological age.     

Oligonucleotide/oligosaccharide-binding fold proteins: a growing family of genome guardians

The maintenance of genomic stability relies on the coordinated action of a number of cellular processes, including activation of the DNA-damage checkpoint, DNA replication, DNA repair, and telomere homeostasis. Many proteins involved in these cellular processes use different types of functional modules to regulate and execute their functions. Recent studies have revealed that many DNA-damage checkpoint and DNA repair proteins in human cells possess the oligonucleotide/oligosaccharide-binding (OB) fold domains, which are known to bind single-stranded DNA in both prokaryotes and eukaryotes. Furthermore, during the DNA damage response, the OB folds of the human checkpoint and DNA repair proteins play critical roles in DNA binding, protein complex assembly, and regulating protein–protein interactions. These findings suggest that the OB fold is an evolutionarily conserved functional module that is widely used by genome guardians. In this review, we will highlight the functions of several well-characterized or newly discovered eukaryotic OB-fold proteins in the DNA damage response.     

SLX4: multitasking to maintain genome stability

The SLX4/FANCP tumor suppressor has emerged as a key player in the maintenance of genome stability, making pivotal contributions to the repair of interstrand cross-links, homologous recombination, and in response to replication stress genome-wide as well as at specific loci such as common fragile sites and telomeres. SLX4 does so in part by acting as a scaffold that controls and coordinates the XPF–ERCC1, MUS81–EME1, and SLX1 structure-specific endonucleases in different DNA repair and recombination mechanisms. It also interacts with other important DNA repair and cell cycle control factors including MSH2, PLK1, TRF2, and TOPBP1 as well as with ubiquitin and SUMO. This review aims at providing an up-to-date and comprehensive view on the key functions that SLX4 fulfills to maintain genome stability as well as to highlight and discuss areas of uncertainty and emerging concepts.     

Telomere protection and TRF2 expression are enhanced by the canonical Wnt signalling pathway

The DNA‐binding protein TRF2 is essential for telomere protection and chromosome stability in mammals. We show here that TRF2 expression is activated by the Wnt/β‐catenin signalling pathway in human cancer and normal cells as well as in mouse intestinal tissues. Furthermore, β‐catenin binds to TRF2 gene regulatory regions that are functional in a luciferase transactivating assay. Reduced β‐catenin expression in cancer cells triggers a marked increase in telomere dysfunction, which can be reversed by TRF2 overexpression. We conclude that the Wnt/β‐catenin signalling pathway maintains a level of TRF2 critical for telomere protection. This is expected to have an important role during development, adult stem cell function and oncogenesis.     

Chromothripsis: Breakage-fusion-bridge over and over again

The acquisition of massive but localized chromosome translocations, a phenomenon termed chromothripsis, has received widespread attention since its discovery over a year ago. Until recently, chromothripsis was believed to originate from a single catastrophic event, but the molecular mechanisms leading to this event are yet to be uncovered. Because a thorough interpretation of the data are missing, the phenomenon itself has wrongly acquired the status of a mechanism used to justify many kinds of complex rearrangements. Although the assumption that all translocations in chromothripsis originate from a single event has met with criticism, satisfactory explanations for the intense but localized nature of this phenomenon are still missing. Here, we show why the data used to describe massive catastrophic rearrangements are incompatible with a model comprising a single event only and propose a molecular mechanism in which a combination of known cellular pathways accounts for chromothripsis. Instead of a single traumatic event, the protection of undamaged chromosomes by telomeres can limit repetitive breakage-fusion-bridge events to a single chromosome arm. Ultimately, common properties of chromosomal instability, such as aneuploidy and centromere fission, might establish the complex genetic pattern observed in this genomic state.     

Overcoming resistance to rapalogs in gliomas by combinatory therapies

Glioblastoma is the most common and aggressive brain tumor type, with a mean patient survival of approximately 1 year. Many previous analyses of the glioma kinome have identified key deregulated pathways that converge and activate mammalian target of rapamycin (mTOR). Following the identification and characterization of mTOR-promoting activity in gliomagenesis, data from preclinical studies suggested the targeting of mTOR by rapamycin or its analogs (rapalogs) as a promising therapeutic approach. However, clinical trials with rapalogs have shown very limited efficacy on glioma due to the development of resistance mechanisms. Analysis of rapalog-insensitive glioma cells has revealed increased activity of growth and survival pathways compensating for mTOR inhibition by rapalogs that are suitable for therapeutic intervention. In addition, recently developed mTOR inhibitors show high anti-glioma activity. In this review, we recapitulate the regulation of mTOR signaling and its involvement in gliomagenesis, discuss mechanisms resulting in resistance to rapalogs, and speculate on strategies to overcome resistance. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).     

Characterization of the first adult de novo case of 46,X,der(Y)t(X;Y)(p22.3;q11.2)

Herein, we describe a case of an infertile man detected in postnatal diagnosis with FISH characterization and array-CGH used for genome-wide screening which allowed the identification of a complex rearrangement involving sex chromosomes, apparently without severe phenotypic consequences. The deletion detected in our patient has been compared with previously reported cases leading us to propose a hypothetical diagnostic algorithm that would be useful in similar clinical situations, with imperative multi disciplinary approach integrated with genetic counseling. Our patient, uniquely of reproductive age, is one of six reported cases of duplication of Xp22.3 (~ 8.4 Mb) segment and contemporary deletion of Yq (~ 42.9 Mb) with final karyotype as follows:

46,X,der(Y),t(X;Y)(Ypter → Yq11.221::Xp22.33 → Xpter).ish der(Y) (Yptel+,Ycen+,RP11-529I21+,RP11-506M9-Yqtel −,Xptel +). arrXp22.33p22.31(702–8,395,963, 8,408,289x1), Yq11.221q12 (14,569,317x1, 14,587,321–57,440,839x0)     

End‐joining inhibition at telomeres requires the translocase and polySUMO‐dependent ubiquitin ligase Uls1

In eukaryotes, permanent inhibition of the non‐homologous end joining (NHEJ) repair pathway at telomeres ensures that chromosome ends do not fuse. In budding yeast, binding of Rap1 to telomere repeats establishes NHEJ inhibition. Here, we show that the Uls1 protein is required for the maintenance of NHEJ inhibition at telomeres. Uls1 protein is a non‐essential Swi2/Snf2‐related translocase and a Small Ubiquitin‐related Modifier (SUMO)‐Targeted Ubiquitin Ligase (STUbL) with unknown targets. Loss of Uls1 results in telomere–telomere fusions. Uls1 requirement is alleviated by the absence of poly‐SUMO chains and by rap1 alleles lacking SUMOylation sites. Furthermore, Uls1 limits the accumulation of Rap1 poly‐SUMO conjugates. We propose that one of Uls1 functions is to clear non‐functional poly‐SUMOylated Rap1 molecules from telomeres to ensure the continuous efficiency of NHEJ inhibition. Since Uls1 is the only known STUbL with a translocase activity, it can be the general molecular sweeper for the clearance of poly‐SUMOylated proteins on DNA in eukaryotes.