Involvement of xeroderma pigmentosum group A (XPA) in progeria arising from defective maturation of prelamin A

Cardiac and skeletal muscle manifestations in the G608G mouse model of Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is a rare premature aging disorder resulting from de novo mutations in the lamin A gene. Children with HGPS typically pass away in their teenage years due to cardiovascular diseases such as atherosclerosis, myocardial infarction, heart failure, and stroke. In this study, we characterized the G608G HGPS mouse model and explored cardiac and skeletal muscle function, along with senescence-associated phenotypes in fibroblasts. Homozygous G608G HGPS mice exhibited cardiac dysfunction, including decreased cardiac output and stroke volume, and impaired left ventricle relaxation. Additionally, skeletal muscle exhibited decreased isometric tetanic torque, muscle atrophy, and increased fibrosis. HGPS fibroblasts showed nuclear abnormalities, decreased proliferation, and increased expression of senescence markers. These findings provide insights into the pathophysiology of the G608G HGPS mouse model and inform potential therapeutic strategies for HGPS.

Progerin Can Induce DNA Damage in the Absence of Global Changes in Replication or Cell Proliferation

Hutchinson-Gilford Progeria Syndrome (HGPS) is a rare genetic condition characterized by features of accelerated aging, and individuals with HGPS seldom live beyond their mid-teens. The syndrome is commonly caused by a point mutation in the LMNA gene which codes for lamin A and its splice variant lamin C, components of the nuclear lamina. The mutation causing HGPS leads to production of a truncated, farnesylated form of lamin A referred to as "progerin." Progerin is also expressed at low levels in healthy individuals and appears to play a role in normal aging. HGPS is associated with an accumulation of genomic DNA double-strand breaks (DSBs) and alterations in the nature of DSB repair. The source of DSBs in HGPS is often attributed to stalling and subsequent collapse of replication forks in conjunction with faulty recruitment of repair factors to damage sites. In this work, we used a model system involving immortalized human cell lines to investigate progerin-induced genomic damage. Using an immunofluorescence approach to visualize phosphorylated histone H2AX foci which mark sites of genomic damage, we report that cells engineered to express progerin displayed a significant elevation of endogenous damage in the absence of any change in the cell cycle profile or doubling time of cells. Genomic damage was enhanced and persistent in progerin-expressing cells treated with hydroxyurea. Overexpression of wild-type lamin A did not elicit the outcomes associated with progerin expression. Our results show that DNA damage caused by progerin can occur independently from global changes in replication or cell proliferation.

Hutchinson-Gilford Progeria Syndrome: Cellular Mechanisms and Therapeutic Perspectives

In humans, aging is characterized by a gradual decline of physical and psychological functions, with the concomitant onset of chronic-degenerative diseases, which ultimately lead to death. The study of Hutchinson-Gilford progeria syndrome (HGPS), a premature aging disorder that recapitulates several features of natural aging, has provided important insights into deciphering the aging process. The genetic origin of HGPS is a de novo point mutation in the LMNA gene that drives the synthesis of progerin, mutant version of lamin A. Progerin is aberrantly anchored to the nuclear envelope disrupting a plethora of molecular processes; nonetheless, how progerin exerts a cascade of deleterious alterations at the cellular and systemic levels is not fully understood. Over the past decade, the use of different cellular and animal models for HGPS has allowed the identification of the molecular mechanisms underlying HGPS, paving the way towards the development of therapeutic treatments against the disease. In this review, we present an updated overview of the biology of HGPS, including its clinical features, description of key cellular processes affected by progerin (nuclear morphology and function, nucleolar activity, mitochondrial function, protein nucleocytoplasmic trafficking and telomere homeostasis), as well as discussion of the therapeutic strategies under development.

Corruption of DNA end-joining in mammalian chromosomes by progerin expression

Hutchinson-Gilford Progeria Syndrome (HGPS) is a rare genetic condition characterized by features of accelerated aging and a life expectancy of about 14 years. HGPS is commonly caused by a point mutation in the LMNA gene which codes for lamin A, an essential component of the nuclear lamina. The HGPS mutation alters splicing of the LMNA transcript, leading to a truncated, farnesylated form of lamin A termed "progerin." Progerin is also produced in small amounts in healthy individuals by alternative splicing of RNA and has been implicated in normal aging. HGPS is associated with an accumulation of genomic DNA double-strand breaks (DSBs), suggesting alteration of DNA repair. DSB repair normally occurs by either homologous recombination (HR), an accurate, templated form of repair, or by nonhomologous end-joining (NHEJ), a non-templated rejoining of DNA ends that can be error-prone; however a good portion of NHEJ events occurs precisely with no alteration to joined sequences. Previously, we reported that over-expression of progerin correlated with increased NHEJ relative to HR. We now report on progerin's impact on the nature of DNA end-joining. We used a model system involving a DNA end-joining reporter substrate integrated into the genome of cultured thymidine kinase-deficient mouse fibroblasts. Some cells were engineered to express progerin. Two closely spaced DSBs were induced in the integrated substrate through expression of endonuclease I-SceI, and DSB repair events were recovered through selection for thymidine kinase function. DNA sequencing revealed that progerin expression correlated with a significant shift away from precise end-joining between the two I-SceI sites and toward imprecise end-joining. Additional experiments revealed that progerin did not reduce HR fidelity. Our work suggests that progerin suppresses interactions between complementary sequences at DNA termini, thereby shifting DSB repair toward low-fidelity DNA end-joining and perhaps contributing to accelerated and normal aging through compromised genome stability.

Lonafarnib and everolimus reduce pathology in iPSC-derived tissue engineered blood vessel model of Hutchinson-Gilford Progeria Syndrome

Hutchinson-Gilford Progeria Syndrome (HGPS) is a rare, fatal genetic disease that accelerates atherosclerosis. With a limited pool of HGPS patients, clinical trials face unique challenges and require reliable preclinical testing. We previously reported a 3D tissue engineered blood vessel (TEBV) microphysiological system fabricated with iPSC-derived vascular cells from HGPS patients. HGPS TEBVs exhibit features of HGPS atherosclerosis including loss of smooth muscle cells, reduced vasoactivity, excess extracellular matrix (ECM) deposition, inflammatory marker expression, and calcification. We tested the effects of HGPS therapeutics Lonafarnib and Everolimus separately and together, currently in Phase I/II clinical trial, on HGPS TEBVs. Everolimus decreased reactive oxygen species levels, increased proliferation, reduced DNA damage in HGPS vascular cells, and improved vasoconstriction in HGPS TEBVs. Lonafarnib improved shear stress response of HGPS iPSC-derived endothelial cells (viECs) and reduced ECM deposition, inflammation, and calcification in HGPS TEBVs. Combination treatment with Lonafarnib and Everolimus produced additional benefits such as improved endothelial and smooth muscle marker expression and reduced apoptosis, as well as increased TEBV vasoconstriction and vasodilation. These results suggest that a combined trial of both drugs may provide cardiovascular benefits beyond Lonafarnib, if the Everolimus dose can be tolerated.

Elevated Levels of Lamin A Promote HR and NHEJ-Mediated Repair Mechanisms in High-Grade Ovarian Serous Carcinoma Cell Line

Extensive research for the last two decades has significantly contributed to understanding the roles of lamins in the maintenance of nuclear architecture and genome organization which is drastically modified in neoplasia. It must be emphasized that alteration in lamin A/C expression and distribution is a consistent event during tumorigenesis of almost all tissues of human bodies. One of the important signatures of a cancer cell is its inability to repair DNA damage which befalls several genomic events that transform the cells to be sensitive to chemotherapeutic agents. This genomic and chromosomal instability is the most common feature found in cases of high-grade ovarian serous carcinoma. Here, we report elevated levels of lamins in OVCAR3 cells (high-grade ovarian serous carcinoma cell line) in comparison to IOSE (immortalised ovarian surface epithelial cells) and, consequently, altered damage repair machinery in OVCAR3. We have analysed the changes in global gene expression as a sequel to DNA damage induced by etoposide in ovarian carcinoma where lamin A is particularly elevated in expression and reported some differentially expressed genes associated with pathways conferring cellular proliferation and chemoresistance. We hereby establish the role of elevated lamin A in neoplastic transformation in the context of high-grade ovarian serous cancer through a combination of HR and NHEJ mechanisms.

Molecular and therapeutic insights of rapamycin: a multi-faceted drug from Streptomyces hygroscopicus

The advancement in pharmaceutical research has led to the discovery and development of new combinatorial life-saving drugs. Rapamycin is a macrolide compound produced from Streptomyces hygroscopicus. Rapamycin and its derivatives are one of the promising sources of drug with broad spectrum applications in the medical field. In recent times, rapamycin has gained significant attention as of its activity against cytokine storm in COVID-19 patients. Rapamycin and its derivatives have more potency when compared to other prevailing drugs. Initially, it has been used exclusively as an anti-fungal drug. Currently rapamycin has been widely used as an immunosuppressant. Rapamycin is a multifaceted drug; it has anti-cancer, anti-viral and anti-aging potentials. Rapamycin has its specific action on mTOR signaling pathway. mTOR has been identified as a key regulator of different pathways. There will be an increased demand for rapamycin, because it has lesser adverse effects when compared to steroids. Currently researchers are focused on the production of effective rapamycin derivatives to combat the growing demand of this wonder drug. The main focus of the current review is to explore the origin, development, molecular mechanistic action, and the current therapeutic aspects of rapamycin. Also, this review article revealed the potential of rapamycin and the progress of rapamycin research. This helps in understanding the exact potency of the drug and could facilitate further studies that could fill in the existing knowledge gaps. The study also gathers significant data pertaining to the gene clusters and biosynthetic pathways involved in the synthesis and production of this multi-faceted drug. In addition, an insight into the mechanism of action of the drug and important derivatives of rapamycin has been expounded. The fillings of the current review, aids in understanding the underlying molecular mechanism, strain improvement, optimization and production of rapamycin derivatives.

Critical DNA damaging pathways in tumorigenesis

The acquisition of DNA damage is an early driving event in tumorigenesis. Premalignant lesions show activated DNA damage responses and inactivation of DNA damage checkpoints promotes malignant transformation. However, DNA damage is also a targetable vulnerability in cancer cells. This requires a detailed understanding of the cellular and molecular mechanisms governing DNA integrity. Here, we review current work on DNA damage in tumorigenesis. We discuss DNA double strand break repair, how repair pathways contribute to tumorigenesis, and how double strand breaks are linked to the tumor microenvironment. Next, we discuss the role of oncogenes in promoting DNA damage through replication stress. Finally, we discuss our current understanding on DNA damage in micronuclei and discuss therapies targeting these DNA damage pathways.

The landscape of aging

Aging is characterized by a progressive deterioration of physiological integrity, leading to impaired functional ability and ultimately increased susceptibility to death. It is a major risk factor for chronic human diseases, including cardiovascular disease, diabetes, neurological degeneration, and cancer. Therefore, the growing emphasis on “healthy aging” raises a series of important questions in life and social sciences. In recent years, there has been unprecedented progress in aging research, particularly the discovery that the rate of aging is at least partly controlled by evolutionarily conserved genetic pathways and biological processes. In an attempt to bring full-fledged understanding to both the aging process and age-associated diseases, we review the descriptive, conceptual, and interventive aspects of the landscape of aging composed of a number of layers at the cellular, tissue, organ, organ system, and organismal levels.

Epigenetics, DNA damage, and aging

Over the course of a human lifespan, genome integrity erodes, leading to an increased abundance of several types of chromatin changes. The abundance of DNA lesions (chemical perturbations to nucleotides) increases with age, as does the number of genomic mutations and transcriptional disruptions caused by replication or transcription of those lesions, respectively. At the epigenetic level, precise DNA methylation patterns degrade, likely causing increasingly stochastic variations in gene expression. Similarly, the tight regulation of histone modifications begins to unravel. The genomic instability caused by these mechanisms allows transposon element reactivation and remobilization, further mutations, gene dysregulation, and cytoplasmic chromatin fragments. This cumulative genomic instability promotes cell signaling events that drive cell fate decisions and extracellular communications known to disrupt tissue homeostasis and regeneration. In this Review, we focus on age-related epigenetic changes and their interactions with age-related genomic changes that instigate these events.

Hutchinson-Gilford Progeria Syndrome: An Overview of the Molecular Mechanism, Pathophysiology and Therapeutic Approach

Lamin A/C encoded by the LMNA gene is an essential component for maintaining the nuclear structure. Mutation in the lamin A/C leads to a group of inherited disorders is known as laminopathies. In the human body, there are several mutations in the LMNA gene that have been identified. It can affect diverse organs or tissues or can be systemic, causing different diseases. In this review, we mainly focused on one of the most severe laminopathies, Hutchinson-Gilford progeria syndrome (HGPS). HGPS is an immensely uncommon, deadly, metameric ill-timed laminopathies caused by the abnormal splicing of the LMNA gene and production of an aberrant protein known as progerin. Here, we also presented the currently available data on the molecular mechanism, pathophysiology, available treatment, and future approaches to this deadly disease. Due to the production of progerin, an abnormal protein leads to an abnormality in nuclear structure, defects in DNA repair, shortening of telomere, and impairment in gene regulation which ultimately results in aging in the early stage of life. Now some treatment options are available for this disease, but a proper understanding of the molecular mechanism of this disease will help to develop a more appropriate treatment which makes it an emerging area of research.

A Link between Replicative Stress, Lamin Proteins, and Inflammation

Double-stranded breaks (DSB), the most toxic DNA lesions, are either a consequence of cellular metabolism, programmed as in during V(D)J recombination, or induced by anti-tumoral therapies or accidental genotoxic exposure. One origin of DSB sources is replicative stress, a major source of genome instability, especially when the integrity of the replication forks is not properly guaranteed. To complete stalled replication, restarting the fork requires complex molecular mechanisms, such as protection, remodeling, and processing. Recently, a link has been made between DNA damage accumulation and inflammation. Indeed, defects in DNA repair or in replication can lead to the release of DNA fragments in the cytosol. The recognition of this self-DNA by DNA sensors leads to the production of inflammatory factors. This beneficial response activating an innate immune response and destruction of cells bearing DNA damage may be considered as a novel part of DNA damage response. However, upon accumulation of DNA damage, a chronic inflammatory cellular microenvironment may lead to inflammatory pathologies, aging, and progression of tumor cells. Progress in understanding the molecular mechanisms of DNA damage repair, replication stress, and cytosolic DNA production would allow to propose new therapeutical strategies against cancer or inflammatory diseases associated with aging. In this review, we describe the mechanisms involved in DSB repair, the replicative stress management, and its consequences. We also focus on new emerging links between key components of the nuclear envelope, the lamins, and DNA repair, management of replicative stress, and inflammation.

Alteration of genetic recombination and double-strand break repair in human cells by progerin expression

Hutchinson-Gilford Progeria Syndrome (HGPS) is a rare autosomal, dominant genetic condition characterized by many features of accelerated aging. On average, children with HGPS live to about fourteen years of age. The syndrome is commonly caused by a point mutation in the LMNA gene which normally codes for lamin A and its splice variant lamin C, components of the nuclear lamina. The LMNA mutation alters splicing, leading to production of a truncated, farnesylated form of lamin A referred to as "progerin." Progerin is also expressed at very low levels in healthy individuals and appears to play a role in normal aging. HGPS is associated with an accumulation of genomic DNA double-strand breaks (DSBs), suggesting corruption of DNA repair. In this work, we investigated the influence of progerin expression on DSB repair in the human genome at the nucleotide level. We used a model system that involves a reporter DNA substrate inserted in the genome of cultured human cells. A DSB could be induced within the substrate through exogenous expression of endonuclease I-SceI, and DSB repair events occurring via either homologous recombination (HR) or nonhomologous end-joining (NHEJ) were recoverable. Additionally, spontaneous HR events were recoverable in the absence of artificial DSB induction. We compared DSB repair and spontaneous HR in cells overexpressing progerin versus cells expressing no progerin. We report that overexpression of progerin correlated with an increase in DSB repair via NHEJ relative to HR, as well as an increased fraction of HR events occurring via gene conversion. Progerin also engendered an apparent increase in spontaneous HR events, with a highly significant shift toward gene conversion events, and an increase in DNA amplification events. Such influences of progerin on DNA transactions may impact genome stability and contribute to aging.

Lamin A involvement in ageing processes

Lamin A, a main constituent of the nuclear lamina, is the major splicing product of the LMNA gene, which also encodes lamin C, lamin A delta 10 and lamin C2. Involvement of lamin A in the ageing process became clear after the discovery that a group of progeroid syndromes, currently referred to as progeroid laminopathies, are caused by mutations in LMNA gene. Progeroid laminopathies include Hutchinson-Gilford Progeria, Mandibuloacral Dysplasia, Atypical Progeria and atypical-Werner syndrome, disabling and life-threatening diseases with accelerated ageing, bone resorption, lipodystrophy, skin abnormalities and cardiovascular disorders. Defects in lamin A post-translational maturation occur in progeroid syndromes and accumulated prelamin A affects ageing-related processes, such as mTOR signaling, epigenetic modifications, stress response, inflammation, microRNA activation and mechanosignaling. In this review, we briefly describe the role of these pathways in physiological ageing and go in deep into lamin A-dependent mechanisms that accelerate the ageing process. Finally, we propose that lamin A acts as a sensor of cell intrinsic and environmental stress through transient prelamin A accumulation, which triggers stress response mechanisms. Exacerbation of lamin A sensor activity due to stably elevated prelamin A levels contributes to the onset of a permanent stress response condition, which triggers accelerated ageing.

The functional importance of lamins, actin, myosin, spectrin and the LINC complex in DNA repair

Three major proteins in the nucleoskeleton, lamins, actin, and spectrin, play essential roles in maintenance of nuclear architecture and the integrity of the nuclear envelope, in mechanotransduction and mechanical coupling between the nucleoskeleton and cytoskeleton, and in nuclear functions such as regulation of gene expression, transcription and DNA replication. Less well known, but critically important, are the role these proteins play in DNA repair. The A-type and B-type lamins, nuclear actin and myosin, spectrin and the LINC (linker of nucleoskeleton and cytoskeleton) complex each function in repair of DNA damage utilizing various repair pathways. The lamins play a role in repair of DNA double-strand breaks (DSBs) by nonhomologous end joining (NHEJ) or homologous recombination (HR). Actin is involved in repair of DNA DSBs and interacts with myosin in facilitating relocalization of these DSBs in heterochromatin for HR repair. Nonerythroid alpha spectrin (αSpII) plays a critical role in repair of DNA interstrand cross-links (ICLs) where it acts as a scaffold in recruitment of repair proteins to sites of damage and is important in the initial damage recognition and incision steps of the repair process. The LINC complex contributes to the repair of DNA DSBs and ICLs. This review will address the important functions of these proteins in the DNA repair process, their mechanism of action, and the profound impact a defect or deficiency in these proteins has on cellular function. The critical roles of these proteins in DNA repair will be further emphasized by discussing the human disorders and the pathophysiological changes that result from or are related to deficiencies in these proteins. The demonstrated function for each of these proteins in the DNA repair process clearly indicates that there is another level of complexity that must be considered when mechanistically examining factors crucial for DNA repair.

Causes and consequences of genomic instability in laminopathies: Replication stress and interferon response

Mammalian nuclei are equipped with a framework of intermediate filaments that function as a karyoskeleton. This nuclear scaffold, formed primarily by lamins (A-type and B-type), maintains the spatial and functional organization of the genome and of sub-nuclear compartments. Over the past decade, a body of evidence has highlighted the significance of these structural nuclear proteins in the maintenance of nuclear architecture and mechanical stability, as well as genome function and integrity. The importance of these structures is now unquestioned given the wide range of degenerative diseases that stem from LMNA gene mutations, including muscular dystrophy disorders, peripheral neuropathies, lipodystrophies, and premature aging syndromes. Here, we review our knowledge about how alterations in nuclear lamins, either by mutation or reduced expression, impact cellular mechanisms that maintain genome integrity. Despite the fact that DNA replication is the major source of DNA damage and genomic instability in dividing cells, how alterations in lamins function impact replication remains minimally explored. We summarize recent studies showing that lamins play a role in DNA replication, and that the DNA damage that accumulates upon lamins dysfunction is elicited in part by deprotection of replication forks. We also discuss the emerging model that DNA damage and replication stress are “sensed” at the cytoplasm by proteins that normally survey this space in search of foreign nucleic acids. In turn, these cytosolic sensors activate innate immune responses, which are materializing as important players in aging and cancer, as well as in the response to cancer immunotherapy.

An overview of treatment strategies for Hutchinson-Gilford Progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is a sporadic, autosomal dominant disorder characterized by premature and accelerated aging symptoms leading to death at the mean age of 14.6 years usually due to cardiovascular complications. HGPS is caused by a de novo point mutation in the LMNA gene encoding the intermediate filament proteins lamins A and C which are structural components of the nuclear lamina. This mutation leads to the production of a truncated toxic form of lamin A, issued from aberrant splicing and called progerin. Progerin accumulates in HGPS cells' nuclei and is a hallmark of the disease. Small amounts of progerin are also produced during normal aging. HGPS cells and animal preclinical models have provided insights into the molecular and cellular pathways that underlie the disease and have also highlighted possible mechanisms involved in normal aging. This review reports recent medical advances and treatment approaches for patients affected with HGPS.

Hutchinson-Gilford Progeria Syndrome: Challenges at Bench and Bedside

The structural nuclear proteins known as "lamins" (A-type and B-type) provide a scaffold for the compartmentalization of genome function that is important to maintain genome stability. Mutations in the LMNA gene -encoding for A-type lamins- are associated with over a dozen of degenerative disorders termed laminopathies, which include muscular dystrophies, lipodystrophies, neuropathies, and premature ageing diseases such as Hutchinson Gilford Progeria Syndrome (HGPS). This devastating disease is caused by the expression of a truncated lamin A protein named "progerin". To date, there is no effective treatment for HGPS patients, who die in their teens from cardiovascular disease. At a cellular level, progerin expression impacts nuclear architecture, chromatin organization, response to mechanical stress, and DNA transactions such as transcription, replication and repair. However, the current view is that key mechanisms behind progerin toxicity still remain to be discovered. Here, we discuss new findings about pathological mechanisms in HGPS, especially the contribution of replication stress to cellular decline, and therapeutic strategies to ameliorate progerin toxicity. In particular, we present evidence for retinoids and calcitriol (hormonal vitamin D metabolite) being among the most potent compounds to ameliorate HGPS cellular phenotypes in vitro, providing the rationale for testing these compounds in preclinical models of the disease in the near term, and in patients in the future.

Genomic instability and DNA replication defects in progeroid syndromes

Progeroid syndromes induced by mutations in lamin A or in its interactors – named progeroid laminopathies – are model systems for the dissection of the molecular pathways causing physiological and premature aging. A large amount of data, based mainly on the Hutchinson Gilford Progeria syndrome (HGPS), one of the best characterized progeroid laminopathy, has highlighted the role of lamins in multiple DNA activities, including replication, repair, chromatin organization and telomere function. On the other hand, the phenotypes generated by mutations affecting genes directly acting on DNA function, as mutations in the helicases WRN and BLM or in the polymerase polδ, share many of the traits of progeroid laminopathies. These evidences support the hypothesis of a concerted implication of DNA function and lamins in aging. We focus here on these aspects to contribute to the comprehension of the driving forces acting in progeroid syndromes and premature aging.

Involvement of the nuclear structural proteins in aging-related responses of human skin to the environmental stress

Human skin is a stratified endocrine organ with primary roles in protection against detrimental biochemical and biophysical factors in the environment. Environmental stress causes gradual accumulation of the macromolecular damage and clinical manifestations consistent with chronic inflammatory conditions and premature aging of the skin. Structural proteins of cell nucleus, the nuclear lamins and lamina-associated proteins, play an important role in the regulation of a number of signal transduction pathways associated with stress. The nuclear lamina proteins have been implicated in a number of degenerative disorders with frequent clinical manifestations of the skin conditions related to premature aging. Analysis of the molecular signatures in response of the skin to a range of damaging factors not only points at the likely involvement of the nuclear lamina in transmission of the signals between the environment and cell nucleus but also defines skin's sensitivity to stress, and therefore the capacities to counteract external damage in aging.

Progerin impairs vascular smooth muscle cell growth via the DNA damage response pathway

Mutations of the lamin A gene cause various premature aging syndromes, including Hutchinson-Gilford progeria syndrome (HGPS) and atypical Werner syndrome. In HGPS (but not atypical Werner syndrome), extensive loss of vascular smooth muscle cells leads to myocardial infarction with premature death. The underlying mechanisms how single gene mutations can cause various phenotypes are largely unknown. We performed an interactome analysis using mutant forms of lamin A involved in progeroid syndromes. We found that the mutant lamin A responsible for HGPS, known as progerin, could not bind to proteins related to the DNA damage response, including DNA-dependent protein kinase (DNA-PK). In contrast, wild-type lamin A and lamin A mutants causing atypical Werner syndrome were able to bind to these molecules. We also found that forced expression of progerin in vascular smooth muscle cells led to activation of DNA-PK and cellular growth arrest, while knockdown of DNA-PK attenuated this. Deletion of p53 also improved the inhibition of cell growth due to forced expression of progerin. These findings suggested that progerin activates the DNA damage response pathway and that dysregulation of this pathway may be responsible for the development of cardiovascular pathology in patients with HGPS.

p53 isoforms regulate premature aging in human cells

Cellular senescence is a hallmark of normal aging and aging-related syndromes, including the premature aging disorder Hutchinson-Gilford Progeria Syndrome (HGPS), a rare genetic disorder caused by a single mutation in the LMNA gene that results in the constitutive expression of a truncated splicing mutant of lamin A known as progerin. Progerin accumulation leads to increased cellular stresses including unrepaired DNA damage, activation of the p53 signaling pathway and accelerated senescence. We previously established that the p53 isoforms Δ133p53 and p53β regulate senescence in normal human cells. However, their role in premature aging is unknown. Here, we report that p53 isoforms are expressed in primary fibroblasts derived from HGPS patients, are associated with their accelerated senescence and that their manipulation can restore the replication capacity of HGPS fibroblasts. We found that in near-senescent HGPS fibroblasts, which exhibit low levels of Δ133p53 and high levels of p53β, restoration of Δ133p53 expression was sufficient to extend replicative lifespan and delay senescence, despite progerin levels and abnormal nuclear morphology remaining unchanged. Conversely, Δ133p53 depletion or p53β overexpression accelerated the onset of senescence in otherwise proliferative HGPS fibroblasts. Our data indicate that Δ133p53 exerts its role by modulating full-length p53 (FLp53) signaling to extend the replicative lifespan and promotes the repair of spontaneous progerin-induced DNA double strand breaks (DSBs). We showed that Δ133p53 dominant-negative inhibition of FLp53 occurs directly at the p21/CDKN1A and miR-34a promoters, two p53-senescence associated genes. In addition, Δ133p53 expression increased expression of the DNA repair RAD51, likely through upregulation of E2F1, a transcription factor that activates RAD51, to promote repair of DSBs. In summary, our data indicate that Δ133p53 modulates p53 signaling to repress progerin-induced early onset of senescence in HGPS cells. Therefore, restoration of Δ133p53 expression may be a novel therapeutic strategy to treat aging-associated phenotypes of HGPS in vivo.

Progerin impairs chromosome maintenance by depleting CENP-F from metaphase kinetochores in Hutchinson-Gilford progeria fibroblasts

Hutchinson-Gilford progeria syndrome (HGPS, OMIM 176670) is a rare premature aging disorder that leads to death at an average age of 14.7 years due to myocardial infarction or stroke. The most common mutation in HGPS is at position G608G (GGC>GGT) within exon 11 of the LMNA gene. This mutation results in the deletion of 50 amino acids at the carboxyl-terminal tail of prelamin A, producing a truncated farnesylated protein called progerin. Lamins play important roles in the organization and structure of the nucleus. The nuclear build-up of progerin causes severe morphological and functional changes in interphase HGPS cells. In this study, we investigated whether progerin elicits spatiotemporal deviations in mitotic processes in HGPS fibroblasts. We analyzed the nuclear distribution of endogenous progerin during mitosis in relation to components of the nuclear lamina, nuclear envelope (NE) and nuclear pores. We found that progerin caused defects in chromosome segregation as early as metaphase, delayed NE reformation and trapped lamina components and inner NE proteins in the endoplasmic reticulum at the end of mitosis. Progerin displaced the centromere protein F (CENP-F) from metaphase chromosome kinetochores, which caused increased chromatin lagging, binucleated cells and genomic instability. This accumulation of progerin-dependent defects with each round of mitosis predisposes cells to premature senescence.

Shared molecular and cellular mechanisms of premature ageing and ageing-associated diseases

Ageing is the predominant risk factor for many common diseases. Human premature ageing diseases are powerful model systems to identify and characterize cellular mechanisms that underpin physiological ageing. Their study also leads to a better understanding of the causes, drivers and potential therapeutic strategies of common diseases associated with ageing, including neurological disorders, diabetes, cardiovascular diseases and cancer. Using the rare premature ageing disorder Hutchinson-Gilford progeria syndrome as a paradigm, we discuss here the shared mechanisms between premature ageing and ageing-associated diseases, including defects in genetic, epigenetic and metabolic pathways; mitochondrial and protein homeostasis; cell cycle; and stem cell-regenerative capacity.

Reprogramming progeria fibroblasts re-establishes a normal epigenetic landscape

Ideally, disease modeling using patient‐derived induced pluripotent stem cells (iPSCs) enables analysis of disease initiation and progression. This requires any pathological features of the patient cells used for reprogramming to be eliminated during iPSC generation. Hutchinson–Gilford progeria syndrome (HGPS) is a segmental premature aging disorder caused by the accumulation of the truncated form of Lamin A known as Progerin within the nuclear lamina. Cellular hallmarks of HGPS include nuclear blebbing, loss of peripheral heterochromatin, defective epigenetic inheritance, altered gene expression, and senescence. To model HGPS using iPSCs, detailed genome‐wide and structural analysis of the epigenetic landscape is required to assess the initiation and progression of the disease. We generated a library of iPSC lines from fibroblasts of patients with HGPS and controls, including one family trio. HGPS patient‐derived iPSCs are nearly indistinguishable from controls in terms of pluripotency, nuclear membrane integrity, as well as transcriptional and epigenetic profiles, and can differentiate into affected cell lineages recapitulating disease progression, despite the nuclear aberrations, altered gene expression, and epigenetic landscape inherent to the donor fibroblasts. These analyses demonstrate the power of iPSC reprogramming to reset the epigenetic landscape to a revitalized pluripotent state in the face of widespread epigenetic defects, validating their use to model the initiation and progression of disease in affected cell lineages.

Intermittent treatment with farnesyltransferase inhibitor and sulforaphane improves cellular homeostasis in Hutchinson-Gilford progeria fibroblasts

Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic condition associated with mutations in the LMNA gene. This disease recapitulates some aspects of normal aging, such as hair loss, thin skin, joint stiffness, and atherosclerosis. The latter leads to heart attack or stroke that causes death at an average age of 14.6 years in children with HGPS. The typical LMNA mutation results in the production of a truncated prelamin A protein, progerin, that remains permanently farnesylated and abnormally associated with the nuclear envelope. Farnesyltransferase inhibitors (FTIs) reverse nuclear structure abnormalities that are characteristic of HGPS cells. The first clinical trial using the FTI, Ionafarnib, demonstrated some improvements in HGPS children and, in particular, showed a decrease in arterial stiffness. Recently, we reported that sulforaphane, an antioxidant derived from cruciferous vegetables, efficiently stimulates autophagy and enhances progerin clearance in HGPS fibroblasts. In the present study, we investigated the effect of combined lonafarnib and sulforaphane treartment in HGPS fibroblast cultures. We report that co-administration of both drugs exerts a synergistic and additive positive effect on autophagy activity but was cytotoxic to HGPS cells. In contrast, intermittent treatment with lonafarnib followed by sulforaphane separately and in repeated cycles rescued the HGPS cellular phenotype. We propose that intermittent treatment with FTI and SFN separately might be a promising therapeutic avenue for children with HGPS.

Progerin sequestration of PCNA promotes replication fork collapse and mislocalization of XPA in laminopathy-related progeroid syndromes

Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic disorder that is caused by a point mutation in the LMNA gene, resulting in production of a truncated farnesylated-prelamin A protein (progerin). We previously reported that XPA mislocalized to the progerin-induced DNA double-strand break (DSB) sites, blocking DSB repair, which led to DSB accumulation, DNA damage responses, and early replication arrest in HGPS. In this study, the XPA mislocalization to DSBs occurred at stalled or collapsed replication forks, concurrent with a significant loss of PCNA at the forks, whereas PCNA efficiently bound to progerin. This PCNA sequestration likely exposed ds-ssDNA junctions at replication forks for XPA binding. Depletion of XPA or progerin each significantly restored PCNA at replication forks. Our results suggest that although PCNA is much more competitive than XPA in binding replication forks, PCNA sequestration by progerin may shift the equilibrium to favor XPA binding. Furthermore, we demonstrated that progerin-induced apoptosis could be rescued by XPA, suggesting that XPA-replication fork binding may prevent apoptosis in HGPS cells. Our results propose a mechanism for progerin-induced genome instability and accelerated replicative senescence in HGPS.-Hilton, B. A., Liu, J., Cartwright, B. M., Liu, Y., Breitman, M., Wang, Y., Jones, R., Tang, H., Rusinol, A., Musich, P. R., Zou, Y. Progerin sequestration of PCNA promotes replication fork collapse and mislocalization of XPA in laminopathy-related progeroid syndromes.

Disruption of PCNA-lamins A/C interactions by prelamin A induces DNA replication fork stalling

The accumulation of prelamin A is linked to disruption of cellular homeostasis, tissue degeneration and aging. Its expression is implicated in compromised genome stability and increased levels of DNA damage, but to date there is no complete explanation for how prelamin A exerts its toxic effects. As the nuclear lamina is important for DNA replication we wanted to investigate the relationship between prelamin A expression and DNA replication fork stability. In this study we report that the expression of prelamin A in U2OS cells induced both mono-ubiquitination of proliferating cell nuclear antigen (PCNA) and subsequent induction of Pol η, two hallmarks of DNA replication fork stalling. Immunofluorescence microscopy revealed that cells expressing prelamin A presented with high levels of colocalisation between PCNA and γH2AX, indicating collapse of stalled DNA replication forks into DNA double-strand breaks. Subsequent protein-protein interaction assays showed prelamin A interacted with PCNA and that its presence mitigated interactions between PCNA and the mature nuclear lamina. Thus, we propose that the cytotoxicity of prelamin A arises in part, from it actively competing against mature lamin A to bind PCNA and that this destabilises DNA replication to induce fork stalling which in turn contributes to genomic instability.

Current insights into LMNA cardiomyopathies: Existing models and missing LINCs

The nuclear lamina is a critical structural domain for the maintenance of genomic stability and whole-cell mechanics. Mutations in the LMNA gene, which encodes nuclear A-type lamins lead to the disruption of these key cellular functions, resulting in a number of devastating diseases known as laminopathies. Cardiomyopathy is a common laminopathy and is highly penetrant with poor prognosis. To date, cell mechanical instability and dysregulation of gene expression have been proposed as the main mechanisms driving cardiac dysfunction, and indeed discoveries in these areas have provided some promising leads in terms of therapeutics. However, important questions remain unanswered regarding the role of lamin A dysfunction in the heart, including a potential role for the toxicity of lamin A precursors in LMNA cardiomyopathy, which has yet to be rigorously investigated.

Expression of progerin does not result in an increased mutation rate

In the premature ageing disease Hutchinson-Gilford progeria syndrome (HGPS), the underlying genetic defect in the lamin A gene leads to accumulation at the nuclear lamina of progerin—a mutant form of lamin A that cannot be correctly processed. This has been reported to result in defects in the DNA damage response and in DNA repair, leading to the hypothesis that, as in normal ageing and in other progeroid syndromes caused by mutation of genes of the DNA repair and DNA damage response pathways, increased DNA damage may be responsible for the premature ageing phenotypes in HGPS patients. However, this hypothesis is based upon the study of markers of the DNA damage response, rather than measurement of DNA damage per se or the consequences of unrepaired DNA damage—mutation. Here, using a mutation reporter cell line, we directly compared the inherent and induced mutation rates in cells expressing wild-type lamin A or progerin. We find no evidence for an elevated mutation rate in progerin-expressing cells. We conclude that the cellular defect in HGPS cells does not lie in the repair of DNA damage per se.

Xeroderma Pigmentosa Group A (XPA), Nucleotide Excision Repair and Regulation by ATR in Response to Ultraviolet Irradiation

The sensitivity of Xeroderma pigmentosa (XP) patients to sunlight has spurred the discovery and genetic and biochemical analysis of the eight XP gene products (XPA-XPG plus XPV) responsible for this disorder. These studies also have served to elucidate the nucleotide excision repair (NER) process, especially the critical role played by the XPA protein. More recent studies have shown that NER also involves numerous other proteins normally employed in DNA metabolism and cell cycle regulation. Central among these is ataxia telangiectasia and Rad3-related (ATR), a protein kinase involved in intracellular signaling in response to DNA damage, especially DNA damage-induced replicative stresses. This review summarizes recent findings on the interplay between ATR as a DNA damage signaling kinase and as a novel ligand for intrinsic cell death proteins to delay damage-induced apoptosis, and on ATR's regulation of XPA and the NER process for repair of UV-induced DNA adducts. ATR's regulatory role in the cytosolic-to-nuclear translocation of XPA will be discussed. In addition, recent findings elucidating a non-NER role for XPA in DNA metabolism and genome stabilization at ds-ssDNA junctions, as exemplified in prematurely aging progeroid cells, also will be reviewed.

Hutchinson-Gilford Progeria Syndrome: A premature aging disease caused by LMNA gene mutations

Products of the LMNA gene, primarily lamin A and C, are key components of the nuclear lamina, a proteinaceous meshwork that underlies the inner nuclear membrane and is essential for proper nuclear architecture. Alterations in lamin A and C that disrupt the integrity of the nuclear lamina affect a whole repertoire of nuclear functions, causing cellular decline. In humans, hundreds of mutations in the LMNA gene have been identified and correlated with over a dozen degenerative disorders, referred to as laminopathies. These diseases include neuropathies, muscular dystrophies, lipodystrophies, and premature aging diseases. This review focuses on one of the most severe laminopathies, Hutchinson-Gilford Progeria Syndrome (HGPS), which is caused by aberrant splicing of the LMNA gene and expression of a mutant product called progerin. Here, we discuss current views about the molecular mechanisms that contribute to the pathophysiology of this devastating disease, as well as the strategies being tested in vitro and in vivo to counteract progerin toxicity. In particular, progerin accumulation elicits nuclear morphological abnormalities, misregulated gene expression, defects in DNA repair, telomere shortening, and genomic instability, all of which limit cellular proliferative capacity. In patients harboring this mutation, a severe premature aging disease develops during childhood. Interestingly, progerin is also produced in senescent cells and cells from old individuals, suggesting that progerin accumulation might be a factor in physiological aging. Deciphering the molecular mechanisms whereby progerin expression leads to HGPS is an emergent area of research, which could bring us closer to understanding the pathology of aging.

Loss of H3K9me3 Correlates with ATM Activation and Histone H2AX Phosphorylation Deficiencies in Hutchinson-Gilford Progeria Syndrome

Compelling evidence suggests that defective DNA damage response (DDR) plays a key role in the premature aging phenotypes in Hutchinson-Gilford progeria syndrome (HGPS). Studies document widespread alterations in histone modifications in HGPS cells, especially, the global loss of histone H3 trimethylated on lysine 9 (H3K9me3). In this study, we explore the potential connection(s) between H3K9me3 loss and the impaired DDR in HGPS. When cells are exposed to a DNA-damaging agent Doxorubicin (Dox), double strand breaks (DSBs) are generated that result in the phosphorylation of histone H2A variant H2AX (gammaH2AX) within an hour. We find that the intensities of gammaH2AX foci appear significantly weaker in the G0/G1 phase HGPS cells compared to control cells. This reduction is associated with a delay in the recruitment of essential DDR factors. We further demonstrate that ataxia-telangiectasia mutated (ATM) is responsible for the amplification of gammaH2AX signals at DSBs during G0/G1 phase, and its activation is inhibited in the HGPS cells that display significant loss of H3K9me3. Moreover, methylene (MB) blue treatment, which is known to save heterochromatin loss in HGPS, restores H3K9me3, stimulates ATM activity, increases gammaH2AX signals and rescues deficient DDR. In summary, this study demonstrates an early DDR defect of attenuated gammaH2AX signals in G0/G1 phase HGPS cells and provides a plausible connection between H3K9me3 loss and DDR deficiency.

All-trans retinoic acid and rapamycin normalize Hutchinson Gilford progeria fibroblast phenotype

Hutchinson Gilford progeria syndrome is a fatal disorder characterized by accelerated aging, bone resorption and atherosclerosis, caused by a LMNA mutation which produces progerin, a mutant lamin A precursor. Progeria cells display progerin and prelamin A nuclear accumulation, altered histone methylation pattern, heterochromatin loss, increased DNA damage and cell cycle alterations. Since the LMNA promoter contains a retinoic acid responsive element, we investigated if all-trans retinoic acid administration could lower progerin levels in cultured fibroblasts. We also evaluated the effect of associating rapamycin, which induces autophagic degradation of progerin and prelamin A. We demonstrate that all-trans retinoic acid acts synergistically with low-dosage rapamycin reducing progerin and prelamin A, via transcriptional downregulation associated with protein degradation, and increasing the lamin A to progerin ratio. These effects rescue cell dynamics and cellular proliferation through recovery of DNA damage response factor PARP1 and chromatin-associated nuclear envelope proteins LAP2α and BAF. The combined all-trans retinoic acid-rapamycin treatment is dramatically efficient, highly reproducible, represents a promising new approach in Hutchinson-Gilford Progeria therapy and deserves investigation in ageing-associated disorders.

A high-content imaging-based screening pipeline for the systematic identification of anti-progeroid compounds

Hutchinson-Gilford Progeria Syndrome (HGPS) is an early onset lethal premature aging disorder caused by constitutive production of progerin, a mutant form of the nuclear architectural protein lamin A. The presence of progerin causes extensive morphological, epigenetic and DNA damage related nuclear defects that ultimately disrupt tissue and organismal functions. Hypothesis-driven approaches focused on HGPS affected pathways have been used in attempts to identify druggable targets with anti-progeroid effects. Here, we report an unbiased discovery approach to HGPS by implementation of a high-throughput, high-content imaging based screening method that enables systematic identification of small molecules that prevent the formation of multiple progerin-induced aging defects. Screening a library of 2816 FDA approved drugs, we identified retinoids as a novel class of compounds that reverses aging defects in HGPS patient skin fibroblasts. These findings establish a novel approach to anti-progeroid drug discovery.

Methods to Monitor DNA Repair Defects and Genomic Instability in the Context of a Disrupted Nuclear Lamina

The organization of the genome within the nuclear space is viewed as an additional level of regulation of genome function, as well as a means to ensure genome integrity. Structural proteins associated with the nuclear envelope, in particular lamins (A- and B-type) and lamin-associated proteins, play an important role in genome organization. Interestingly, there is a whole body of evidence that links disruptions of the nuclear lamina with DNA repair defects and genomic instability. Here, we describe a few standard techniques that have been successfully utilized to identify mechanisms behind DNA repair defects and genomic instability in cells with an altered nuclear lamina. In particular, we describe protocols to monitor changes in the expression of DNA repair factors (Western blot) and their recruitment to sites of DNA damage (immunofluorescence); kinetics of DNA double-strand break repair after ionizing radiation (neutral comet assays); frequency of chromosomal aberrations (FISH, fluorescence in situ hybridization); and alterations in telomere homeostasis (Quantitative-FISH). These techniques have allowed us to shed some light onto molecular mechanisms by which alterations in A-type lamins induce genomic instability, which could contribute to the pathophysiology of aging and aging-related diseases.

Genome Integrity in Aging: Human Syndromes, Mouse Models, and Therapeutic Options

Human syndromes and mouse mutants that exhibit accelerated but bona fide aging in multiple organs and tissues have been invaluable for the identification of nine denominators of aging: telomere attrition, genome instability, epigenetic alterations, mitochondrial dysfunction, deregulated nutrient sensing, altered intercellular communication, loss of proteostasis, cellular senescence and adult stem cell exhaustion. However, whether and how these instigators of aging interrelate or whether they have one root cause is currently largely unknown. Rare human progeroid syndromes and corresponding mouse mutants with resolved genetic defects highlight the dominant importance of genome maintenance for aging. A second class of aging-related disorders reveals a cross connection with metabolism. As genome maintenance and metabolism are closely interconnected, they may constitute the main underlying biology of aging. This review focuses on the role of genome stability in aging, its crosstalk with metabolism, and options for nutritional and/or pharmaceutical interventions that delay age-related pathology.

Hutchinson-Gilford progeria syndrome as a model for vascular aging

Hutchinson-Gilford progeria syndrome (HGPS) is a premature aging disorder caused by a de novo genetic mutation that leads to the accumulation of a splicing isoform of lamin A termed progerin. Progerin expression alters the organization of the nuclear lamina and chromatin. The life expectancy of HGPS patients is severely reduced due to critical cardiovascular defects. Progerin also accumulates in an age-dependent manner in the vascular cells of adults that do not carry genetic mutations associated with HGPS. The molecular mechanisms that lead to vascular dysfunction in HGPS may therefore also play a role in vascular aging. The vascular phenotypic and molecular changes observed in HGPS are strikingly similar to those seen with age, including increased senescence, altered mechanotransduction and stem cell exhaustion. This article discusses the similarities and differences between age-dependent and HGPS-related vascular aging to highlight the relevance of HGPS as a model for vascular aging. Induced pluripotent stem cells derived from HGPS patients are suggested as an attractive model to study vascular aging in order to develop novel approaches to treat cardiovascular disease.

DNA repair defects and genome instability in Hutchinson-Gilford Progeria Syndrome

The integrity of the nuclear lamina has emerged as an important factor in the maintenance of genome stability. In particular, mutations in the LMNA gene, encoding A-type lamins (lamin A/C), alter nuclear morphology and function, and cause genomic instability. LMNA gene mutations are associated with a variety of degenerative diseases and devastating premature aging syndromes such as Hutchinson-Gilford Progeria Syndrome (HGPS) and Restrictive Dermopathy (RD). HGPS is a severe laminopathy, with patients dying in their teens from myocardial infarction or stroke. HGPS patient-derived cells exhibit nuclear shape abnormalities, changes in epigenetic regulation and gene expression, telomere shortening, genome instability, and premature senescence. This review highlights recent advances in identifying molecular mechanisms that contribute to the pathophysiology of HGPS, with a special emphasis on DNA repair defects and genome instability.

Regulation of homologous recombinational repair by lamin B1 in radiation-induced DNA damage

DNA double-strand breaks (DSBs) are the major lethal lesion induced by ionizing radiation (IR). RAD51-dependent homologous recombination (HR) is one of the most important pathways in DSB repair and genome integrity maintenance. However, the mechanism of HR regulation by RAD51 remains unclear. To understand the mechanism of RAD51-dependent HR, we searched for interacting partners of RAD51 by a proteomics analysis and identified lamin B1 in human cells. Lamins are nuclear lamina proteins that play important roles in the structural organization of the nucleus and the regulation of chromosome functions. Immunoblotting analyses revealed that siRNA-mediated lamin B1 depletion repressed the DNA damage-dependent increase of RAD51 after IR. The repression was abolished by the proteasome inhibitor MG132, suggesting that lamin B1 stabilizes RAD51 by preventing proteasome-mediated degradation in cells with IR-induced DNA damage. We also showed that lamin B1 depletion repressed RAD51 focus formation and decreased the survival rates after IR. On the basis of these results, we propose that lamin B1 promotes DSB repair and cell survival by maintaining the RAD51 protein levels for HR upon DSB induction after IR.

Role of lamin b1 in chromatin instability

Nuclear lamins play important roles in the organization and structure of the nucleus; however, the specific mechanisms linking lamin structure to nuclear functions are poorly defined. We demonstrate that reducing nuclear lamin B1 expression by short hairpin RNA-mediated silencing in cancer cell lines to approximately 50% of normal levels causes a delay in the cell cycle and accumulation of cells in early S phase. The S phase delay appears to be due to the stalling and collapse of replication forks. The double-strand DNA breaks resulting from replication fork collapse were inefficiently repaired, causing persistent DNA damage signaling and the assembly of extensive repair foci on chromatin. The expression of multiple factors involved in DNA replication and repair by both nonhomologous end joining and homologous repair is misregulated when lamin B1 levels are reduced. We further demonstrate that lamin B1 interacts directly with the promoters of some genes associated with DNA damage response and repair, including BRCA1 and RAD51. Taken together, the results suggest that the maintenance of lamin B1 levels is required for DNA replication and repair through regulation of the expression of key factors involved in these essential nuclear functions.

Lamins: nuclear intermediate filament proteins with fundamental functions in nuclear mechanics and genome regulation

Lamins are intermediate filament proteins that form a scaffold, termed nuclear lamina, at the nuclear periphery. A small fraction of lamins also localize throughout the nucleoplasm. Lamins bind to a growing number of nuclear protein complexes and are implicated in both nuclear and cytoskeletal organization, mechanical stability, chromatin organization, gene regulation, genome stability, differentiation, and tissue-specific functions. The lamin-based complexes and their specific functions also provide insights into possible disease mechanisms for human laminopathies, ranging from muscular dystrophy to accelerated aging, as observed in Hutchinson-Gilford progeria and atypical Werner syndromes.

Sulforaphane enhances progerin clearance in Hutchinson-Gilford progeria fibroblasts

Hutchinson-Gilford progeria syndrome (HGPS, OMIM 176670) is a rare multisystem childhood premature aging disorder linked to mutations in the LMNA gene. The most common HGPS mutation is found at position G608G within exon 11 of the LMNA gene. This mutation results in the deletion of 50 amino acids at the carboxyl-terminal tail of prelamin A, and the truncated protein is called progerin. Progerin only undergoes a subset of the normal post-translational modifications and remains permanently farnesylated. Several attempts to rescue the normal cellular phenotype with farnesyltransferase inhibitors (FTIs) and other compounds have resulted in partial cellular recovery. Using proteomics, we report here that progerin induces changes in the composition of the HGPS nuclear proteome, including alterations to several components of the protein degradation pathways. Consequently, proteasome activity and autophagy are impaired in HGPS cells. To restore protein clearance in HGPS cells, we treated HGPS cultures with sulforaphane (SFN), an antioxidant derived from cruciferous vegetables. We determined that SFN stimulates proteasome activity and autophagy in normal and HGPS fibroblast cultures. Specifically, SFN enhances progerin clearance by autophagy and reverses the phenotypic changes that are the hallmarks of HGPS. Therefore, SFN is a promising therapeutic avenue for children with HGPS.

A new structural insight into XPA-DNA interactions

XPA (xeroderma pigmentosum group A) protein is an essential factor for NER (nucleotide excision repair) which is believed to be involved in DNA damage recognition/verification, NER factor recruiting and stabilization of repair intermediates. Past studies on the structure of XPA have focused primarily on XPA interaction with damaged DNA. However, how XPA interacts with other DNA structures remains unknown though recent evidence suggest that these structures could be important for its roles in both NER and non-NER activities. Previously, we reported that XPA recognizes undamaged DNA ds/ssDNA (double-strand/single-strandDNA) junctions with a binding affinity much higher than its ability to bind bulky DNA damage. To understand how this interaction occurs biochemically we implemented a structural determination of the interaction using a MS-based protein footprinting method and limited proteolysis. By monitoring surface accessibility of XPA lysines to NHS-biotin modification in the free protein and the DNA junction-bound complex we show that XPA physically interacts with the DNA junctions via two lysines, K168 and K179, located in the previously known XPA(98–219) DBD (DNA-binding domain). Importantly, we also uncovered new lysine residues, outside of the known DBD, involved in the binding. We found that residues K221, K222, K224 and K236 in the C-terminal domain are involved in DNA binding. Limited proteolysis analysis of XPA–DNA interactions further confirmed this observation. Structural modelling with these data suggests a clamp-like DBD for the XPA binding to ds/ssDNA junctions. Our results provide a novel structure-function view of XPA–DNA junction interactions.

The RASSF1A tumor suppressor regulates XPA-mediated DNA repair

RASSF1A may be the most frequently inactivated tumor suppressor identified in human cancer so far. It is a proapoptotic Ras effector and plays an important role in the apoptotic DNA damage response (DDR). We now show that in addition to DDR regulation, RASSF1A also plays a key role in the DNA repair process itself. We show that RASSF1A forms a DNA damage-regulated complex with the key DNA repair protein xeroderma pigmentosum A (XPA). XPA requires RASSF1A to exert full repair activity, and RASSF1A-deficient cells exhibit an impaired ability to repair DNA. Moreover, a cancer-associated RASSF1A single-nucleotide polymorphism (SNP) variant exhibits differential XPA binding and inhibits DNA repair. The interaction of XPA with other components of the repair complex, such as replication protein A (RPA), is controlled in part by a dynamic acetylation/deacetylation cycle. We found that RASSF1A and its SNP variant differentially regulate XPA protein acetylation, and the SNP variant hyperstabilizes the XPA-RPA70 complex. Thus, we identify two novel functions for RASSF1A in the control of DNA repair and protein acetylation. As RASSF1A modulates both apoptotic DDR and DNA repair, it may play an important and unanticipated role in coordinating the balance between repair and death after DNA damage.

NuMA promotes homologous recombination repair by regulating the accumulation of the ISWI ATPase SNF2h at DNA breaks

Chromatin remodeling factors play an active role in the DNA damage response by shaping chromatin to facilitate the repair process. The spatiotemporal regulation of these factors is key to their function, yet poorly understood. We report that the structural nuclear protein NuMA accumulates at sites of DNA damage in a poly[ADP-ribose]ylation-dependent manner and functionally interacts with the ISWI ATPase SNF2h/SMARCA5, a chromatin remodeler that facilitates DNA repair. NuMA coimmunoprecipitates with SNF2h, regulates its diffusion in the nucleoplasm and controls its accumulation at DNA breaks. Consistent with NuMA enabling SNF2h function, cells with silenced NuMA exhibit reduced chromatin decompaction after DNA cleavage, lesser focal recruitment of homologous recombination repair factors, impaired DNA double-strand break repair in chromosomal (but not in episomal) contexts and increased sensitivity to DNA cross-linking agents. These findings reveal a structural basis for the orchestration of chromatin remodeling whereby a scaffold protein promotes genome maintenance by directing a remodeler to DNA breaks.

HP1α mediates defective heterochromatin repair and accelerates senescence in Zmpste24-deficient cells

Heterochromatin protein 1 (HP1) interacts with various proteins, including lamins, to play versatile functions within nuclei, such as chromatin remodeling and DNA repair. Accumulation of prelamin A leads to misshapen nuclei, heterochromatin disorganization, genomic instability, and premature aging in Zmpste24-null mice. Here, we investigated the effects of prelamin A on HP1α homeostasis, subcellular distribution, phosphorylation, and their contribution to accelerated senescence in mouse embryonic fibroblasts (MEFs) derived from Zmpste24(-/-) mice. The results showed that the level of HP1α was significantly increased in Zmpste24(-/-) cells. Although prelamin A interacted with HP1α in a manner similar to lamin A, HP1α associated with the nuclease-resistant nuclear matrix fraction was remarkably increased in Zmpste24(-/-) MEFs compared with that in wild-type littermate controls. In wild-type cells, HP1α was phosphorylated at Thr50, and the phosphorylation was maximized around 30 min, gradually dispersed 2 h after DNA damage induced by camptothecin. However, the peak of HP1α phosphorylation was significantly compromised and appeared until 2 h, which is correlated with the delayed maximal formation of γ-H2AX foci in Zmpste24(-/-) MEFs. Furthermore, knocking down HP1α by siRNA alleviated the delayed DNA damage response and accelerated senescence in Zmpste24(-/-) MEFs, evidenced by the rescue of the delayed γ-H2AX foci formation, downregulation of p16, and reduction of senescence-associated β-galactosidase activity. Taken together, these findings establish a functional link between prelamin A, HP1α, chromatin remodeling, DNA repair, and early senescence in Zmpste24-deficient mice, suggesting a potential therapeutic strategy for laminopathy-based premature aging via the intervention of HP1α.

DNA damage and lamins

The spatial and temporal organization of the genome has emerged as an additional level of regulation of nuclear functions. Structural proteins associated with the nuclear envelope play important roles in the organization of the genome. The nuclear lamina, a polymeric meshwork formed by lamins (A- and B-type) and lamin-associated proteins, is viewed as a scaffold for tethering chromatin and protein complexes regulating a variety of nuclear functions. Alterations in lamins function impact DNA transactions such as transcription, replication, and repair, as well as epigenetic modifications that change chromatin structure. These data, and the association of defective lamins with a whole variety of degenerative disorders, premature aging syndromes, and cancer, provide evidence for these proteins operating as caretakers of the genome. In this chapter, we summarize current knowledge about the function of lamins in the maintenance of genome integrity, with special emphasis on the role of A-type lamins in the maintenance of telomere homeostasis and mechanisms of DNA damage repair. These findings have begun to shed some light onto molecular mechanisms by which alterations in A-type lamins induce genomic instability and contribute to the pathophysiology of aging and aging-related diseases, especially cancer.