A homozygousZMPSTE24null mutation in combination with a heterozygous mutation in theLMNAgene causes Hutchinson-Gilford progeria syndrome (HGPS): insights into the pathophysiology of HGPS

Defective prelamin A processing promotes unconventional necroptosis driven by nuclear RIPK1

Defects in the prelamin A processing enzyme caused by loss-of-function mutations in the ZMPSTE24 gene are responsible for a spectrum of progeroid disorders characterized by the accumulation of farnesylated prelamin A. Here we report that defective prelamin A processing triggers nuclear RIPK1-dependent signalling that leads to necroptosis and inflammation. We show that accumulated prelamin A recruits RIPK1 to the nucleus to facilitate its activation upon tumour necrosis factor stimulation in ZMPSTE24-deficient cells. Kinase-activated RIPK1 then promotes RIPK3-mediated MLKL activation in the nucleus, leading to nuclear envelope disruption and necroptosis. This signalling relies on prelamin A farnesylation, which anchors prelamin A to nuclear envelope to serve as a nucleation platform for necroptosis. Genetic inactivation of necroptosis ameliorates the progeroid phenotypes in Zmpste24-/- mice. Our findings identify an unconventional nuclear necroptosis pathway resulting from ZMPSTE24 deficiency with pathogenic consequences in progeroid disorder and suggest RIPK1 as a feasible target for prelamin A-associated progeroid disorders.

Mouse models of accelerated aging in musculoskeletal research for assessing frailty, sarcopenia, and osteoporosis - A review

Musculoskeletal aging encompasses the decline in bone and muscle function, leading to conditions such as frailty, osteoporosis, and sarcopenia. Unraveling the underlying molecular mechanisms and developing effective treatments are crucial for improving the quality of life for those affected. In this context, accelerated aging models offer valuable insights into these conditions by displaying the hallmarks of human aging. Herein, this review focuses on relevant mouse models of musculoskeletal aging with particular emphasis on frailty, osteoporosis, and sarcopenia. Among the discussed models, PolgA mice in particular exhibit hallmarks of musculoskeletal aging, presenting early-onset frailty, as well as reduced bone and muscle mass that closely resemble human musculoskeletal aging. Ultimately, findings from these models hold promise for advancing interventions targeted at age-related musculoskeletal disorders, effectively addressing the challenges posed by musculoskeletal aging and associated conditions in humans.

An exceptional biallelic N-terminal frame shift mutation in ZMPSTE24 leads to non-lethal progeria due to possible utilization of a downstream alternative start codon

Biallelic mutations in ZMPSTE24 are known to be associated with autosomal recessive mandibuloacral dysplasia with type B lipodystrophy (MADB) and lethal restrictive dermopathy (RD), respectively. Disease manifestation is depending on the remaining enzyme activity of the mutated ZMPSTE24 protein. To date, complete loss of function has exclusively been reported in RD cases. In this study, we identified a novel N-terminal homozygous frameshift mutation (c.28_29insA) in a consanguineous family segregating with MADB. An in-depth analysis of the mutated sequence revealed, that the one base pair insertion creates a novel downstream in-frame start codon, which supposedly serves as an alternative translation initiation site (TIS). This possible rescue mechanism would explain the relatively mild clinical outcome in the studied individuals. Our findings demonstrate the necessity for careful interpretation of N-terminal variants potentially effecting translation initiation.

Progerin-Induced Impairment in Wound Healing and Proliferation in Vascular Endothelial Cells

Progerin as a mutated isoform of lamin A protein was first known to induce premature atherosclerosis progression in patients with Hutchinson-Gilford progeria syndrome (HGPS), and its role in provoking an inflammatory response in vascular cells and accelerating cell senescence has been investigated recently. However, how progerin triggers endothelial dysfunction that often occurs at the early stage of atherosclerosis in a mechanical environment has not been studied intensively. Here, we generated a stable endothelial cell line that expressed progerin and examined its effects on endothelial wound repair under laminar flow. We found decreased wound healing rate in progerin-expressing ECs under higher shear stress compared with those under low shear. Furthermore, the decreased wound recovery could be due to reduced number of cells at late mitosis, suggesting potential interference by progerin with endothelial proliferation. These findings provided insights into how progerin affects endothelial mechanotransduction and may contribute to the disruption of endothelial integrity in HGPS vasculature, as we continue to examine the mechanistic effect of progerin in shear-induced endothelial functions.

Generalized lipoatrophy syndromes

Generalized lipodystrophy (GL) syndromes are a group of rare heterogenous disorders, characterized by total subcutaneous fat loss. The frequency of GL is currently assessed as approximately 0,23 cases per million of the population, in Europe - as 0,96 cases per million of the population. They can be congenital (CGL) or acquired (AGL) depending on the etiology and the time of the onset of fat loss. Both CGL and AGL are often associated with different metabolic complications, such as hypertriglyceridemia, insulin resistance and lipoatrophic diabetes mellitus, metabolically associated FLD, arterial hypertension, proteinuria, reproductive system disorders. In this review we aimed to summarize the information on all forms of generalized lipodystrophy, especially the ones of genetic etiology, their clinical manifestations and complications, the perspectives for diagnostics, treatment and further research.

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.

Mutations Involved in Premature-Ageing Syndromes

Premature-ageing syndromes are a heterogeneous group of rare genetic disorders resembling features of accelerated ageing and resulting from mutations in genes coding for proteins required for nuclear lamina architecture, DNA repair and maintenance of genome stability, mitochondrial function and other cellular processes. Hutchinson–Gilford progeria syndrome (HGPS) and Werner syndrome (WS) are two of the best-characterized progeroid syndromes referred to as childhood- and adulthood-progeria, respectively. This article provides an updated overview of the mutations leading to HGPS, WS, and to the spectrum of premature-ageing laminopathies ranging in severity from congenital restrictive dermopathy (RD) to adult-onset atypical WS, including RD-like laminopathies, typical and atypical HGPS, more and less severe forms of mandibuloacral dysplasia (MAD), Néstor-Guillermo progeria syndrome (NGPS), atypical WS, and atypical progeroid syndromes resembling features of HGPS and/or MAD but resulting from impaired DNA repair or mitochondrial functions, including mandibular hypoplasia, deafness, progeroid features, and lipodystrophy (MDPL) syndrome and mandibuloacral dysplasia associated to MTX2 (MADaM). The overlapping signs and symptoms among different premature-ageing syndromes, resulting from both a large genetic heterogeneity and shared pathological pathways underlying these conditions, require an expert clinical evaluation in specialized centers paralleled by next-generation sequencing of panels of genes involved in these disorders in order to establish as early as possible an accurate clinical and molecular diagnosis for a proper patient management.

Protein structural and mechanistic basis of progeroid laminopathies

Progeroid laminopathies are characterized by the premature appearance of certain signs of physiological aging in a subset of tissues. They are caused by mutations in genes coding for A-type lamins or lamin-binding proteins. Here, we review how different mutations causing progeroid laminopathies alter protein structure or protein-protein interactions and how these impact on mechanisms that protect cell viability and function. One group of progeroid laminopathies, which includes Hutchinson-Gilford progeria syndrome, is characterized by accumulation of unprocessed prelamin A or variants. These are caused by mutations in the A-type lamin gene (LMNA), altering prelamin A itself, or in ZMPSTE24, encoding an endoprotease involved in its processing. The abnormally expressed farnesylated proteins impact on various cellular processes that may contribute to progeroid phenotypes. Other LMNA mutations lead to the production of nonfarnesylated A-type lamin variants with amino acid substitutions in solvent-exposed hot spots located mainly in coil 1B and the immunoglobulin fold domain. Dominant missense mutations might reinforce interactions between lamin domains, thus giving rise to excessively stabilized filament networks. Recessive missense mutations in A-type lamins and barrier-to-autointegration factor (BAF) causing progeroid disorders are found at the interface between these interacting proteins. The amino acid changes decrease the binding affinity of A-type lamins for BAF, which may contribute to lamina disorganization, as well as defective repair of mechanically induced nuclear envelope rupture. Targeting these molecular alterations in A-type lamins and associated proteins identified through structural biology studies could facilitate the design of therapeutic strategies to treat patients with rare but severe progeroid laminopathies.

Hutchinson-Gilford Progeria syndrome: Report of the first Togolese case

The aim of this article is to describe the first case of Hutchinson-Gilford Progeria Syndrome (HGPS) in Togo and review all Africans cases. Our patient was a 12.8-year-old Togolese boy followed in our unit till he was 15-year-old for HGPS. He was the only child of non-consanguineous parents. The phenotypic findings were craniofacial dysmorphy, dwarfism, lipodystrophy, diffusely scattered hyperpigmented foci, pyriform thorax, nail dystrophy, decreased joint mobility, and camptodactyly. He had characteristic facies with prominent forehead, prominent eyes, absent ear lobule, thin nasal skin, convex nasal profile, micrognathia, and crowded teeth. Radiologicals findings were bilateral coxa valga, pyriform thorax, and acro-osteolysis. We sequenced the entire coding region of LMNA gene, and mutation analysis revealed a heterozygous mutation c.1824C>T (p.Gly608Gly). Our patient is therefore the fifth African and the fourth with classical mutation, first of Western Africa, and second of (sub-Saharan) African black race. The recurrence of HGPS is low like the cause is neomutation or germinal mosaicism.

The tripartite architecture of the eukaryotic integral membrane protein zinc metalloprotease Ste24

Ste24 enzymes, a family of eukaryotic integral membrane proteins, are zinc metalloproteases (ZMPs) originally characterized as “CAAX proteases” targeting prenylated substrates, including a‐factor mating pheromone in yeast and prelamin A in humans. Recently, Ste24 was shown to also cleave nonprenylated substrates. Reduced activity of the human ortholog, HsSte24, is linked to multiple disease states (laminopathies), including progerias and lipid disorders. Ste24 possesses a unique “α‐barrel” structure consisting of seven transmembrane (TM) α‐helices encircling a large intramembranous cavity (~14 000 ų). The catalytic zinc, coordinated via a HExxH…E/H motif characteristic of gluzincin ZMPs, is positioned at one of the cavity's bases. The interrelationship between Ste24 as a gluzincin, a long‐studied class of soluble ZMPs, and as a novel cavity‐containing integral membrane protein protease has been minimally explored to date. Informed by homology to well‐characterized soluble, gluzincin ZMPs, we develop a model of Ste24 that provides a conceptual framework for this enzyme family, suitable for development and interpretation of structure/function studies. The model consists of an interfacial, zinc‐containing “ZMP Core” module surrounded by a “ZMP Accessory” module, both capped by a TM helical “α‐barrel” module of as yet unknown function. Multiple sequence alignment of 58 Ste24 orthologs revealed 38 absolutely conserved residues, apportioned unequally among the ZMP Core (18), ZMP Accessory (13), and α‐barrel (7) modules. This Tripartite Architecture representation of Ste24 provides a unified image of this enzyme family.

Premature aging syndromes: From patients to mechanism

Aging is an inevitable consequence of human life resulting in a gradual deterioration of cell, tissue and organismal function and an increased risk to develop chronic ailments. Premature aging syndromes, also known as progeroid syndromes, recapitulate many clinical features of normal aging and offer a unique opportunity to elucidate fundamental mechanisms that contribute to human aging. Progeroid syndromes can be broadly classified into those caused by perturbations of the nuclear lamina, a meshwork of proteins located underneath the inner nuclear membrane (laminopathies); and a second group that is caused by mutations that directly impair DNA replication and repair. We will focus mainly on laminopathies caused by incorrect processing of lamin A, an intermediate filament protein that resides at the nuclear periphery. Hutchinson-Gilford Progeria (HGPS) is an accelerated aging syndrome caused by a mutation in lamin A and one of the best studied laminopathies. HGPS patients exhibit clinical characteristics of premature aging, including alopecia, aberrant pigmentation, loss of subcutaneous fat and die in their teens as a result of atherosclerosis and cardiovascular complications. Here we summarize how cell- and mouse-based disease models provided mechanistic insights into human aging and discuss experimental strategies under consideration for the treatment of these rare genetic disorders.

Mechanisms of vascular aging: What can we learn from Hutchinson-Gilford progeria syndrome?

Aging is the main risk factor for cardiovascular disease (CVD). The increased prevalence of CVD is partly due to the global increase in life expectancy. In this context, it is essential to identify the mechanisms by which aging induces CVD, with the ultimate aim of reducing its incidence. Both atherosclerosis and heart failure significantly contribute to age-associated CVD morbidity and mortality. Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic disorder caused by the synthesis of progerin, which is noted for accelerated aging and CVD. This mutant form of prelamin A induces generalised atherosclerosis, vascular calcification, and cardiac electrophysiological abnormalities, leading to premature aging and death, mainly due to myocardial infarction and stroke. This review discusses the main vascular structural and functional abnormalities during physiological and premature aging, as well as the mechanisms involved in the exacerbated CVD and accelerated aging induced by the accumulation of progerin and prelamin A. Both proteins are expressed in non-HGPS individuals, and physiological aging shares many features of progeria. Research into HGPS could therefore shed light on novel mechanisms involved in the physiological aging of the cardiovascular system.

Mandibuloacral dysplasia: A premature ageing disease with aspects of physiological ageing

Mandibuloacral dysplasia (MAD) is a rare genetic condition characterized by bone abnormalities including localized osteolysis and generalized osteoporosis, skin pigmentation, lipodystrophic signs and mildly accelerated ageing. The molecular defects associated with MAD are mutations in LMNA or ZMPSTE24 (FACE1) gene, causing type A or type B MAD, respectively. Downstream of LMNA or ZMPSTE24 mutations, the lamin A precursor, prelamin A, is accumulated in cells and affects chromatin dynamics and stress response. A new form of mandibuloacral dysplasia has been recently associated with mutations in POLD1 gene, encoding DNA polymerase delta, a major player in DNA replication. Of note, involvement of prelamin A in chromatin dynamics and recruitment of DNA repair factors has been also determined under physiological conditions, at the border between stress response and cellular senescence. Here, we review current knowledge on MAD clinical and pathogenetic aspects and highlight aspects typical of physiological ageing.

Phenotypic heterogeneity of ZMPSTE24 deficiency

A 4-year-old girl was referred to the Undiagnosed Diseases Network with a history of short stature, thin and translucent skin, macrocephaly, small hands, and camptodactyly. She had been diagnosed with possible Hallerman-Streiff syndrome. Her evaluation showed that she was mosaic for uniparental isodisomy of chromosome 1, which harbored a pathogenic c.1077dupT variant in ZMPSTE24 which predicts p.(Leu362fsX18). ZMPSTE24 is a zinc metalloproteinase that is involved in processing farnesylated proteins and pathogenic ZMPSTE24 variants cause accumulation of abnormal farnesylated forms of prelamin A. This, in turn, causes a spectrum of disease severity which is based on enzyme activity. The current patient has an intermediate form, which is a genocopy of severe Progeria.

Aging in the Cardiovascular System: Lessons from Hutchinson-Gilford Progeria Syndrome

Aging, the main risk factor for cardiovascular disease (CVD), is becoming progressively more prevalent in our societies. A better understanding of how aging promotes CVD is therefore urgently needed to develop new strategies to reduce disease burden. Atherosclerosis and heart failure contribute significantly to age-associated CVD-related morbimortality. CVD and aging are both accelerated in patients suffering from Hutchinson-Gilford progeria syndrome (HGPS), a rare genetic disorder caused by the prelamin A mutant progerin. Progerin causes extensive atherosclerosis and cardiac electrophysiological alterations that invariably lead to premature aging and death. This review summarizes the main structural and functional alterations to the cardiovascular system during physiological and premature aging and discusses the mechanisms underlying exaggerated CVD and aging induced by prelamin A and progerin. Because both proteins are expressed in normally aging non-HGPS individuals, and most hallmarks of normal aging occur in progeria, research on HGPS can identify mechanisms underlying physiological aging.

Prelamin A impairs 53BP1 nuclear entry by mislocalizing NUP153 and disrupting the Ran gradient

The nuclear lamina is essential for the proper structure and organization of the nucleus. Deregulation of A-type lamins can compromise genomic stability, alter chromatin organization and cause premature vascular aging. Here, we show that accumulation of the lamin A precursor, prelamin A, inhibits 53BP1 recruitment to sites of DNA damage and increases basal levels of DNA damage in aged vascular smooth muscle cells. We identify that this genome instability arises through defective nuclear import of 53BP1 as a consequence of abnormal topological arrangement of nucleoporin NUP153. We show for the first time that this nucleoporin is important for the nuclear localization of Ran and that the deregulated Ran gradient is likely to be compromising the nuclear import of 53BP1. Importantly, many of the defects associated with prelamin A expression were significantly reduced upon treatment with Remodelin, a small molecule recently reported to reverse deficiencies associated with abnormal nuclear lamina.

Failure of ossification of the occipital bone in mandibuloacral dysplasia type B

Mandibuloacral dysplasia with type B lipodystrophy is a rare autosomal recessive disease characterized by atrophic skin, lipodystrophy, and skeletal features. It is caused by mutations in ZMPSTE24, a gene encoding a zinc metalloproteinase involved in the post-translational modification of lamin. Nine distinct pathogenic variants have been identified in 11 patients from nine unrelated families with this disorder. We report a 12-year-old boy with mandibuloacral dysplasia with type B lipodystrophy and a novel homozygous c.1196A>G; p.(Tyr399Cys) mutation in ZMPSTE24. The patient had typical dermatological and skeletal features of mandibuloacral dysplasia with type B lipodystrophy, sparse hair, short stature, mild microcephaly, facial dysmorphism, and a striking failure of ossification of the interparietal region of the occipital bone, up to the position where transverse occipital suture can be observed. Newly recognized signs for mandibuloacral dysplasia with type B lipodystrophy were gaze palsy and ptosis. Delayed closure of cranial sutures and Wormian bones have been described in three patients, but an ossification failure strictly limited to the occipital bone, as seen in the present patient, appears to be unique for mandibuloacral dysplasia with type B lipodystrophy. This observation illustrates that ZMPSTE24 could play a specific role in membranous ossification in the interparietal part of the squama (Inca bone) but not in the intracartilaginous ossification of the supraoccipital. This failure of ossification in the squama appears to be a useful feature for the radiological diagnosis of mandibuloacral dysplasia with type B lipodystrophy. © 2016 Wiley Periodicals, Inc.

Mandibular hypoplasia, deafness, progeroid features and lipodystrophy (MDPL) syndrome in the context of inherited lipodystrophies

BACKGROUND

Lipodystrophies are a large heterogeneous group of genetic or acquired disorders characterized by generalized or partial fat loss, usually associated with metabolic complications such as diabetes mellitus, hypertriglyceridemia and hepatic steatosis. Many efforts have been made in the last years in identifying the genetic etiologies of several lipodystrophy forms, although some remain to be elucidated.

METHODS

We report here the clinical description of a woman with a rare severe lipodystrophic and progeroid syndrome associated with hypertriglyceridemia and diabetes whose genetic bases have been clarified through whole-exome sequencing (WES) analysis.

RESULTS

This article reports the 5th MDPL (Mandibular hypoplasia, deafness, progeroid features, and lipodystrophy syndrome) patient with the same de novo p.S605del mutation in POLD1. We provided further genetic evidence that this is a disease-causing mutation along with a plausible molecular mechanism responsible for this recurring event. Moreover we overviewed the current classification of the inherited forms of lipodystrophy, along with their underlying molecular basis.

CONCLUSIONS

Progress in the identification of lipodystrophy genes will help in better understanding the role of the pathways involved in the complex physiology of fat. This will lead to new targets towards develop innovative therapeutic strategies for treating the disorder and its metabolic complications, as well as more common forms of adipose tissue redistribution as observed in the metabolic syndrome and type 2 diabetes.

Comparison of senescence-associated miRNAs in primary skin and lung fibroblasts

MicroRNAs are non-coding RNAs with roles in many cellular processes. Tissue-specific miRNA profiles associated with senescence have been described for several cell and tissue types. We aimed to characterise miRNAs involved in core, rather than tissue-specific, senescence pathways by assessment of common miRNA expression differences in two different cell types, with follow-up of predicted targets in human peripheral blood. MicroRNAs were profiled in early and late passage primary lung and skin fibroblasts to identify commonly-deregulated miRNAs. Expression changes of their bioinformatically-predicted mRNA targets were then assessed in both cell types and in human peripheral blood from elderly participants in the InCHIANTI study. 57/178 and 26/492 microRNAs were altered in late passage skin and lung cells respectively. Three miRNAs (miR-92a, miR-15b and miR-125a-3p) were altered in both tissues. 14 mRNA targets of the common miRNAs were expressed in lung and skin fibroblasts, of which two demonstrated up-regulation in late passage skin and lung cells (LYST; p = 0.02 [skin] and 0.02 [lung] INMT; p = 0.03 [skin] and 0.04 [lung]). ZMPSTE24 and LHFPL2 demonstrated altered expression in late passage skin cells only (p = 0.01 and 0.05 respectively). LHFPL2 was also positively correlated with age in peripheral blood (p value = 6.6 × 10(-5)). We find that the majority of senescence-associated miRNAs demonstrate tissue-specific effects. However, miRNAs showing common effects across tissue types may represent those associated with core, rather than tissue-specific senescence processes.

New ZMPSTE24 (FACE1) mutations in patients affected with restrictive dermopathy or related progeroid syndromes and mutation update

Restrictive dermopathy (RD) is a rare and extremely severe congenital genodermatosis, characterized by a tight rigid skin with erosions at flexure sites, multiple joint contractures, low bone density and pulmonary insufficiency generally leading to death in the perinatal period. RD is caused in most patients by compound heterozygous or homozygous ZMPSTE24 null mutations. This gene encodes a metalloprotease specifically involved in lamin A post-translational processing. Here, we report a total of 16 families for whom diagnosis and molecular defects were clearly established. Among them, we report seven new ZMPSTE24 mutations, identified in classical RD or Mandibulo-acral dysplasia (MAD) affected patients. We also report nine families with one or two affected children carrying the common, homozygous thymine insertion in exon 9 and demonstrate the lack of a founder effect. In addition, we describe several new ZMPSTE24 variants identified in unaffected controls or in patients affected with non-classical progeroid syndromes. In addition, this mutation update includes a comprehensive search of the literature on previously described ZMPSTE24 mutations and associated phenotypes. Our comprehensive analysis of the molecular pathology supported the general rule: complete loss-of-function of ZMPSTE24 leads to RD, whereas other less severe phenotypes are associated with at least one haploinsufficient allele.

Laminopathy-inducing mutations reduce nuclear import of expressed prelamin A

Lamins are structural components of the nuclear lamina and integral parts of the nucleoplasm. The tripartite domain structure partitions the molecule into an amino-terminal head, central rod and a carboxy-terminal tail domain. The tail domain contains a nuclear localization sequence and in most lamins an additional CaaX motif, which is necessary to post-translationally process prelamin to mature lamin. As players of nuclear and cellular integrity, lamins must possess unrestrained access to the nucleus. To study whether nuclear trafficking of lamins is compromised in laminopathies, we determined relative nuclear import activities between expressed prelamin A and selected laminopathy-inducing mutants thereof. Furthermore, the impact of inhibition of maturation on nuclear import of expressed prelamin A was examined. To perform quantitative transport measurements, import competent but lamina incorporation-deficient GFP- or DsRed-tagged prelamin A deletion mutants were used, which lacked the head and rod domain (ΔHR-prelamin A). Nuclear accumulation of ΔHR-prelamin A carrying the lipodystrophy and metabolic syndrome-inducing mutations R419C and L421P or progeria-causing deletions was significantly reduced, but that of the maturation-deficient mutant ΔHR-prelamin A SSIM was significantly increased. In the case of the full length prelamin A mutants R419C and L421P altered subcellular localization and reduced lamina incorporation were detected, with the prelamin A-binding protein Narf being redistributed into R419-containing aggregates. The results suggest that impaired nuclear transport of certain prelamin A mutants may represent a contributing factor in the pathogenesis of certain laminopathies.

Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease

Some individuals with a particular disease-causing mutation or genotype fail to express most if not all features of the disease in question, a phenomenon that is known as 'reduced (or incomplete) penetrance'. Reduced penetrance is not uncommon; indeed, there are many known examples of 'disease-causing mutations' that fail to cause disease in at least a proportion of the individuals who carry them. Reduced penetrance may therefore explain not only why genetic diseases are occasionally transmitted through unaffected parents, but also why healthy individuals can harbour quite large numbers of potentially disadvantageous variants in their genomes without suffering any obvious ill effects. Reduced penetrance can be a function of the specific mutation(s) involved or of allele dosage. It may also result from differential allelic expression, copy number variation or the modulating influence of additional genetic variants in cis or in trans. The penetrance of some pathogenic genotypes is known to be age- and/or sex-dependent. Variable penetrance may also reflect the action of unlinked modifier genes, epigenetic changes or environmental factors. At least in some cases, complete penetrance appears to require the presence of one or more genetic variants at other loci. In this review, we summarize the evidence for reduced penetrance being a widespread phenomenon in human genetics and explore some of the molecular mechanisms that may help to explain this enigmatic characteristic of human inherited disease.

Restrictive dermopathy: report of two siblings

Restrictive dermopathy (RD) is a rare and lethal autosomal recessive syndrome characterized by very tight, thin, and easily eroded skin and contracture of joints. We present two siblings in a family. Case 1, a female neonate, showed mild characteristic presentations of RD and survived for 16 days, and Case 2, a male neonate, was stillborn with typical severe features of RD. His skin biopsy showed typical histological findings, and genetic study revealed a homozygous nonsense mutation on the exon 6 of zinc metalloproteinase STE24 (ZMPSTE24). The exact pathogenic mechanism of RD remains poorly understood. The most recent studies on mutations in lamin A and/or ZMPSTE24 have shed some light on the pathophysiology of RD and may help direct the development of future therapeutic approaches.

LMNA-associated cardiocutaneous progeria: an inherited autosomal dominant premature aging syndrome with late onset

Hutchinson-Gilford Progeria Syndrome (HGPS) is a premature aging disorder caused by mutations in LMNA, which encodes the nuclear scaffold proteins lamin A and C. In HGPS and related progerias, processing of prelamin A is blocked at a critical step mediated by the zinc metalloprotease ZMPSTE24. LMNA-linked progerias can be grouped into two classes: (1) the processing-deficient, early onset "typical" progerias (e.g., HGPS), and (2) the processing-proficient "atypical" progeria syndromes (APS) that are later in onset. Here we describe a previously unrecognized progeria syndrome with prominent cutaneous and cardiovascular manifestations belonging to the second class. We suggest the name LMNA-associated cardiocutaneous progeria syndrome (LCPS) for this disorder. Affected patients are normal at birth but undergo progressive cutaneous changes in childhood and die in middle age of cardiovascular complications, including accelerated atherosclerosis, calcific valve disease, and cardiomyopathy. In addition, the proband demonstrated cancer susceptibility, a phenotype rarely described for LMNA-based progeria disorders. The LMNA mutation that caused LCPS in this family is a heterozygous c.899A>G (p.D300G) mutation predicted to alter the coiled-coil domain of lamin A/C. In skin fibroblasts isolated from the proband, the processing and levels of lamin A and C are normal. However, nuclear morphology is aberrant and rescued by treatment with farnesyltransferase inhibitors, as is also the case for HGPS and other laminopathies. Our findings advance knowledge of human LMNA progeria syndromes, and raise the possibility that typical and atypical progerias may converge upon a common mechanism to cause premature aging disease.

Prelamin A accelerates vascular calcification via activation of the DNA damage response and senescence-associated secretory phenotype in vascular smooth muscle cells

RATIONALE

Vascular calcification is prevalent in the aging population, yet little is known of the mechanisms driving age-associated vascular smooth muscle cell (VSMC) phenotypic change.

OBJECTIVE

To investigate the role of nuclear lamina disruption, a specific hallmark of VSMC aging, in driving VSMC osteogenic differentiation.

METHODS AND RESULTS

Prelamin A, the unprocessed form of the nuclear lamina protein lamin A, accumulated in calcifying human VSMCs in vitro and in vivo, and its overexpression promoted VSMC osteogenic differentiation and mineralization. During VSMC aging in vitro, prelamin A accumulation occurred concomitantly with increased p16 expression and osteogenic differentiation and was associated with increased levels of DNA damage. Microarray analysis showed that DNA damage repair pathways were significantly impaired in VSMCs expressing prelamin A and that chemical inhibition and siRNA depletion of the DNA damage response kinases ataxia-telangiectasia mutated/ataxia-telangiectasia- and Rad3-related effectively blocked VSMC osteogenic differentiation and mineralization. In coculture experiments, prelamin A-expressing VSMCs induced alkaline phosphatase activity in mesenchymal progenitor cells, and this was abrogated by inhibition of ataxia-telangiectasia-mutated signaling, suggesting that DNA damage induces the secretion of pro-osteogenic factors by VSMCs. Cytokine array analysis identified several ataxia-telangiectasia mutated-dependent senescence-associated secretory phenotype factors/cytokines released by prelamin A-positive VSMCs, including the calcification regulators bone morphogenetic protein 2, osteoprotegerin, and interleukin 6.

CONCLUSIONS

Prelamin A promotes VSMC calcification and aging by inducing persistent DNA damage signaling, which acts upstream of VSMC osteogenic differentiation and the senescence-associated secretory phenotype. Agents that target the DNA damage response and prelamin A toxicity may be potential therapies for the treatment of vascular calcification.

The structural basis of ZMPSTE24-dependent laminopathies

Mutations in the nuclear membrane zinc metalloprotease ZMPSTE24 lead to diseases of lamin processing (laminopathies), such as the premature aging disease progeria and metabolic disorders. ZMPSTE24 processes prelamin A, a component of the nuclear lamina intermediate filaments, by cleaving it at two sites. Failure of this processing results in accumulation of farnesylated, membrane-associated prelamin A. The 3.4 angstrom crystal structure of human ZMPSTE24 has a seven transmembrane α-helical barrel structure, surrounding a large, water-filled, intramembrane chamber, capped by a zinc metalloprotease domain with the catalytic site facing into the chamber. The 3.8 angstrom structure of a complex with a CSIM tetrapeptide showed that the mode of binding of the substrate resembles that of an insect metalloprotease inhibitor in thermolysin. Laminopathy-associated mutations predicted to reduce ZMPSTE24 activity map to the zinc metalloprotease peptide-binding site and to the bottom of the chamber.

Nuclear lamin functions and disease

Recent studies have shown that premature cellular senescence and normal organ development and function depend on the type V intermediate filament proteins, the lamins, which are major structural proteins of the nucleus. This review presents an up-to-date summary of the literature describing new findings on lamin functions in various cellular processes and emphasizes the relationship between the lamins and devastating diseases ranging from premature aging to cancer. Recent insights into the structure and function of the A- and B- type lamins in normal cells and their dysfunctions in diseased cells are providing novel targets for the development of new diagnostic procedures and disease intervention. We summarize these recent findings, focusing on data from mice and humans, and highlight the expanding knowledge of these proteins in both healthy and diseased cells.

Human ZMPSTE24 disease mutations: residual proteolytic activity correlates with disease severity

The zinc metalloprotease ZMPSTE24 plays a critical role in nuclear lamin biology by cleaving the prenylated and carboxylmethylated 15-amino acid tail from the C-terminus of prelamin A to yield mature lamin A. A defect in this proteolytic event, caused by a mutation in the lamin A gene (LMNA) that eliminates the ZMPSTE24 cleavage site, underlies the premature aging disease Hutchinson-Gilford Progeria Syndrome (HGPS). Likewise, mutations in the ZMPSTE24 gene that result in decreased enzyme function cause a spectrum of diseases that share certain features of premature aging. Twenty human ZMPSTE24 alleles have been identified that are associated with three disease categories of increasing severity: mandibuloacral dysplasia type B (MAD-B), severe progeria (atypical 'HGPS') and restrictive dermopathy (RD). To determine whether a correlation exists between decreasing ZMPSTE24 protease activity and increasing disease severity, we expressed mutant alleles of ZMPSTE24 in yeast and optimized in vivo yeast mating assays to directly compare the activity of alleles associated with each disease category. We also measured the activity of yeast crude membranes containing the ZMPSTE24 mutant proteins in vitro. We determined that, in general, the residual activity of ZMPSTE24 patient alleles correlates with disease severity. Complete loss-of-function alleles are associated with RD, whereas retention of partial, measureable activity results in MAD-B or severe progeria. Importantly, our assays can discriminate small differences in activity among the mutants, confirming that the methods presented here will be useful for characterizing any new ZMPSTE24 mutations that are discovered.

Premature aging syndrome

Hutchinson-Gilford progeria syndrome and Werner syndrome are two of the best characterized human progeroid diseases with clinical features mimicking physiological aging at an early age. Both disorders have been the focus of intense research in recent years since they might provide insights into the pathology of normal human aging. The chapter contains a detailed description of the clinical features of both disorders and then it focuses on the genetics, the resulting biochemical alterations at the protein level and the most recent findings and hypotheses concerning the molecular basis of the premature aging phenotypes. A description of available diagnostic and therapeutic approaches is included.

The role of DNA damage in laminopathy progeroid syndromes

Progeroid laminopathies are characterized by the abnormal processing of lamin A, the appearance of misshapen nuclei, and the accumulation and persistence of DNA damage. In the present article, I consider the contribution of defective DNA damage pathways to the pathology of progeroid laminopathies. Defects in DNA repair pathways appear to be caused by a combination of factors. These include abnormal epigenetic modifications of chromatin that are required to recruit DNA repair pathways to sites of DNA damage, abnormal recruitment of DNA excision repair proteins to sites of DNA double-strand breaks, and unrepairable ROS (reactive oxygen species)-induced DNA damage. At least two of these defective processes offer the potential for novel therapeutic approaches.

Human matrix metalloproteinases: an ubiquitarian class of enzymes involved in several pathological processes

Human matrix metalloproteinases (MMPs) belong to the M10 family of the MA clan of endopeptidases. They are ubiquitarian enzymes, structurally characterized by an active site where a Zn(2+) atom, coordinated by three histidines, plays the catalytic role, assisted by a glutamic acid as a general base. Various MMPs display different domain composition, which is very important for macromolecular substrates recognition. Substrate specificity is very different among MMPs, being often associated to their cellular compartmentalization and/or cellular type where they are expressed. An extensive review of the different MMPs structural and functional features is integrated with their pathological role in several types of diseases, spanning from cancer to cardiovascular diseases and to neurodegeneration. It emerges a very complex and crucial role played by these enzymes in many physiological and pathological processes.

[Laminopathies: one gene, several diseases]

Lamins A and C, encoded by the LMNA gene, are nuclear proteins expressed in all post-mitotic cells. Together with B-type lamins, they form a meshwork of proteins beneath the inner nuclear membrane, the lamina, in connection with the cytoskeleton. Lamins A/C also interact with chromatin and numerous proteins, including transcription factors. Mutations in LMNA are responsible for more than ten different disorders, commonly called "laminopathies". These diseases affect tissues in a specific (striated muscle, adipose tissue, peripheral nerve) or in a systemic manner (premature ageing syndromes). This wide spectrum of phenotypes is associated to a wide variety of mutations. This large clinical and genetic heterogeneity, unique to the LMNA gene, makes genotype-phenotype relations particularly difficult to establish. However, correlations have been obtained in several cases. Hence, LMNA mutations identified in premature ageing syndromes lead to the accumulation of immature proteins with a toxic effect for cells. Mutations in laminopathies of the adipose tissue mainly localize in the Ig-like domain of the proteins, potentially affecting the interaction with the SREBP-1 transcription factor. In laminopathies of the striated muscles, the mutations are spread throughout the gene. These mutations are thought to induce structural modifications of the proteins, thereby affecting their polymerization into nuclear lamina. Such defect would lead to a mechanical weakness of the nuclear lamina and of the cells, particularly in striated muscles continuously stretching. The exploration of pathophysiological mechanisms of LMNA mutations largely benefits from the numerous mouse models created, which have been widely used to analyze affected molecular pathways and to test putative therapeutic treatments.

Quantitative nucleolar proteomics reveals nuclear re-organization during stress- induced senescence in mouse fibroblast

Background

Nucleolus is the most prominent mammalian organelle within the nucleus which is also the site for ribosomal biogenesis. There have been many reports indicating the involvement of nucleolus in the process of aging. Several proteins related to aging have been shown to localize in the nucleolus, which suggests the role of this organelle in senescence.

Results

In this study, we used quantitative mass spectrometry to map the flux of proteins into and out of the nucleolus during the induction of senescence in cultured mammalian cells. Changes in the abundance of 344 nucleolar proteins in sodium butyrate-induced senescence in NIH3T3 cells were studied by SILAC (stable isotope labeling by amino acids in cell culture)-based mass spectrometry. Biochemically, we have validated the proteomic results and confirmed that B23 (nucleophosmin) protein was down-regulated, while poly (ADP-ribose) polymerase (PARP) and nuclear DNA helicase II (NDH II/DHX9/RHA) were up-regulated in the nucleolus upon treatment with sodium butyrate. Accumulation of chromatin in the nucleolus was also observed, by both proteomics and microscopy, in sodium butyrate-treated cells. Similar observations were found in other models of senescence, namely, in mitoxantrone- (MTX) treated cells and primary fibroblasts from the Lamin A knockout mice.

Conclusion

Our data indicate an extensive nuclear organization during senescence and suggest that the redistribution of B23 protein and chromatin can be used as an important marker for senescence.

The accumulation of un-repairable DNA damage in laminopathy progeria fibroblasts is caused by ROS generation and is prevented by treatment with N-acetyl cysteine

Fibroblasts from patients with the severe laminopathy diseases, restrictive dermopathy (RD) and Hutchinson Gilford progeria syndrome (HGPS), are characterized by poor growth in culture, the presence of abnormally shaped nuclei and the accumulation of DNA double-strand breaks (DSB). Here we show that the accumulation of DSB and poor growth of the fibroblasts but not the presence of abnormally shaped nuclei are caused by elevated levels of reactive oxygen species (ROS) and greater sensitivity to oxidative stress. Basal levels of ROS and sensitivity to H(2)O(2) were compared in fibroblasts from normal, RD and HGPS individuals using fluorescence activated cell sorting-based assays. Basal levels of ROS and stimulated levels of ROS were both 5-fold higher in the progeria fibroblasts. Elevated levels of ROS were correlated with lower proliferation indices but not with the presence of abnormally shaped nuclei. DSB induced by etoposide were repaired efficiently in normal, RD and HGPS fibroblasts. In contrast, DSB induced by ROS were repaired efficiently in normal fibroblasts, but in RD and HGPS fibroblasts many ROS-induced DSB were un-repairable. The accumulation of ROS-induced DSB appeared to cause the poor growth of RD and HGPS fibroblasts, since culture in the presence of the ROS scavenger N-acetyl cysteine (NAC) reduced the basal levels of DSB, eliminated un-repairable ROS-induced DSB and greatly improved population-doubling times. Our findings suggest that un-repaired ROS-induced DSB contribute significantly to the RD and HGPS phenotypes and that inclusion of NAC in a combinatorial therapy might prove beneficial to HGPS patients.

Early onset mandibuloacral dysplasia due to compound heterozygous mutations in ZMPSTE24

Mandibuloacral dysplasia (MAD) is an autosomal recessive disorder characterized by hypoplasia of the mandible and clavicles, acro-osteolysis, and lipodystrophy due to mutations in LMNA or ZMPSTE24. Only six MAD patients are reported so far with ZMPSTE24 mutations and limited phenotypic data are available for them. Here, we report on two brothers (4 years and 9-month old) with early onset MAD due to ZMPSTE24 mutations in whom thin skin was noted as early as 5 months of age. Both had micrognathia, mottled hyperpigmentation, and enlarged fontanelles but little evidence of lipodystrophy. There was no delay of mental development. The older brother had small pinched nose, short clavicles, acro-osteolysis, stunted growth, joint stiffness, and repeated fractures. There was no evidence of renal disease. Both patients were compound heterozygotes harboring a previously reported missense ZMPSTE24 mutation, p.Pro248Leu, and a novel null mutation, p.Trp450stop. These patients and the review of literature reveal that compared to MAD patients with LMNA mutations, those with ZMPSTE24 mutations develop manifestations earlier in life. Other distinguishing features in MAD due to ZMPSTE24 mutations may include premature birth, renal disease, calcified skin nodules, and lack of acanthosis nigricans. We conclude that in patients with MAD due to ZMPSTE24 mutations, the onset of disease manifestations such as thin skin and micrognathia occurs as early as 5 months of age. In these patients, skeletal phenotype presents earlier whereas lipodystrophy and renal disease may occur later in life.

Type B mandibuloacral dysplasia with congenital myopathy due to homozygous ZMPSTE24 missense mutation

Mutation in ZMPSTE24 gene, encoding a major metalloprotease, leads to defective prelamin A processing and causes type B mandibuloacral dysplasia, as well as the lethal neonatal restrictive dermopathy syndrome. Phenotype severity is correlated with the residual enzyme activity of ZMPSTE24 and accumulation of prelamin A. We had previously demonstrated that a complete loss of function in ZMPSTE24 was lethal in the neonatal period, whereas compound heterozygous mutations including one PTC and one missense mutation were associated with type B mandibuloacral dysplasia. In this study, we report a 30-year longitudinal clinical survey of a patient harboring a novel severe and complex phenotype, combining an early-onset progeroid syndrome and a congenital myopathy with fiber-type disproportion. A unique homozygous missense ZMPSTE24 mutation (c.281T>C, p.Leu94Pro) was identified and predicted to produce two possible ZMPSTE24 conformations, leading to a partial loss of function. Western blot analysis revealed a major reduction of ZMPSTE24, together with the presence of unprocessed prelamin A and decreased levels of lamin A, in the patient's primary skin fibroblasts. These cells exhibited significant reductions in lifespan associated with major abnormalities of the nuclear shape and structure. This is the first report of MAD presenting with confirmed myopathic abnormalities associated with ZMPSTE24 defects, extending the clinical spectrum of ZMPSTE24 gene mutations. Moreover, our results suggest that defective prelamin A processing affects muscle regeneration and development, thus providing new insights into the disease mechanism of prelamin A-defective associated syndromes in general.

Homozygous null mutations in ZMPSTE24 in restrictive dermopathy: evidence of genetic heterogeneity

Restrictive dermopathy (RD) results in stillbirth or early neonatal death. RD is characterized by prematurity, intrauterine growth retardation, fixed facial expression, micrognathia, mouth in the 'o' position, rigid and tense skin with erosions and denudations and multiple joint contractures. Nearly all 25 previously reported neonates with RD had homozygous or compound heterozygous null mutations in the ZMPSTE24 gene. Here, we report three new cases of RD; all died within 3 weeks of birth. One of them had a previously reported homozygous c.1085dupT (p.Leu362PhefsX19) mutation, the second case had a novel homozygous c.1020G>A (p.Trp340X) null mutation in ZMPSTE24, but the third case, a stillborn with features of RD except for the presence of tapering rather than rounded, bulbous digits, harbored no disease-causing mutations in LMNA or ZMPSTE24. In the newborn with a novel ZMPSTE24 mutation, unique features included butterfly-shaped thoracic 5 vertebra and the bulbous appearance of the distal clavicles. Skin biopsies from both the stillborn fetus and the newborn with c.1020G>A ZMPSTE24 mutation showed absence of elastic fibers throughout the dermis. This report provides evidence of genetic heterogeneity among RD and concludes that there may be an additional locus for RD which remains to be identified.

Absence of progeria-like disease phenotypes in knock-in mice expressing a non-farnesylated version of progerin

Hutchinson-Gilford progeria syndrome (HGPS) is caused by a mutant prelamin A, progerin, that terminates with a farnesylcysteine. HGPS knock-in mice (Lmna(HG/+)) develop severe progeria-like disease phenotypes. These phenotypes can be ameliorated with a protein farnesyltransferase inhibitor (FTI), suggesting that progerin's farnesyl lipid is important for disease pathogenesis and raising the possibility that FTIs could be useful for treating humans with HGPS. Subsequent studies showed that mice expressing non-farnesylated progerin (Lmna(nHG/+) mice, in which progerin's carboxyl-terminal -CSIM motif was changed to -SSIM) also develop severe progeria, raising doubts about whether any treatment targeting protein prenylation would be particularly effective. We suspected that those doubts might be premature and hypothesized that the persistent disease in Lmna(nHG/+) mice could be an unanticipated consequence of the cysteine-to-serine substitution that was used to eliminate farnesylation. To test this hypothesis, we generated a second knock-in allele yielding non-farnesylated progerin (Lmna(csmHG)) in which the carboxyl-terminal -CSIM motif was changed to -CSM. We then compared disease phenotypes in mice harboring the Lmna(nHG) or Lmna(csmHG) allele. As expected, Lmna(nHG/+) and Lmna(nHG/nHG) mice developed severe progeria-like disease phenotypes, including osteolytic lesions and rib fractures, osteoporosis, slow growth and reduced survival. In contrast, Lmna(csmHG/+) and Lmna(csmHG/csmHG) mice exhibited no bone disease and displayed entirely normal body weights and survival. The frequencies of misshapen cell nuclei were lower in Lmna(csmHG/+) and Lmna(csmHG/csmHG) fibroblasts. These studies show that the ability of non-farnesylated progerin to elicit disease depends on the carboxyl-terminal mutation used to eliminate protein prenylation.

Polymorphisms in LMNA and near a SERPINA gene cluster are associated with cognitive function in older people

A recent genome-wide association (GWA) study of late-onset Alzheimer's disease (LOAD) identified 15 novel single nucleotide polymorphisms (SNPs) independent of ApoE. We hypothesised that variants associated with LOAD are also associated with poor cognitive function in elderly populations. We measured additive associations between the five most strongly associated LOAD SNPs and grouped Mini Mental State Examination (MMSE) scores. Variants were genotyped in respondents (mean age 79 years) from the Oxford Healthy Ageing project (OHAP) and other sites of the MRC Cognitive Function and Ageing Study (MRC-CFAS). In adjusted ordinal logistic models, two variants were associated with poorer cognitive function: rs11622883 (OR=1.14, 95% CI: 1.01-1.28, p=0.040) and rs505058 (OR=1.29, 95% CI: 1.02-1.64, p=0.036). These SNPs are close to a SERPINA gene cluster and within LMNA, respectively. The mechanisms underlying the associations with cognitive impairment and LOAD require further elucidation, but both genes are interesting candidates for involvement in age-related cognitive impairment.

Prelamin A acts to accelerate smooth muscle cell senescence and is a novel biomarker of human vascular aging

BACKGROUND

Hutchinson-Gilford progeria syndrome is a rare inherited disorder of premature aging caused by mutations in LMNA or Zmpste24 that disrupt nuclear lamin A processing, leading to the accumulation of prelamin A. Patients develop severe premature arteriosclerosis characterized by vascular smooth muscle cell (VSMC) calcification and attrition.

METHODS AND RESULTS

To determine whether defective lamin A processing is associated with vascular aging in the normal population, we examined the profile of lamin A expression in normal and aged VSMCs. In vitro, aged VSMCs rapidly accumulated prelamin A coincidently with nuclear morphology defects, and these defects were reversible by treatment with farnesylation inhibitors and statins. In human arteries, prelamin A accumulation was not observed in young healthy vessels but was prevalent in medial VSMCs from aged individuals and in atherosclerotic lesions, where it often colocalized with senescent and degenerate VSMCs. Prelamin A accumulation correlated with downregulation of the lamin A processing enzyme Zmpste24/FACE1, and FACE1 mRNA and protein levels were reduced in response to oxidative stress. Small interfering RNA knockdown of FACE1 reiterated the prelamin A-induced nuclear morphology defects characteristic of aged VSMCs, and overexpression of prelamin A accelerated VSMC senescence. We show that prelamin A acts to disrupt mitosis and induce DNA damage in VSMCs, leading to mitotic failure, genomic instability, and premature senescence.

CONCLUSIONS

This study shows that prelamin A is a novel biomarker of VSMC aging and disease that acts to accelerate senescence. It therefore represents a novel target to ameliorate the effects of age-induced vascular dysfunction.

Mammalian SUN protein interaction networks at the inner nuclear membrane and their role in laminopathy disease processes

The nuclear envelope (NE) LINC complex, in mammals comprised of SUN domain and nesprin proteins, provides a direct connection between the nuclear lamina and the cytoskeleton, which contributes to nuclear positioning and cellular rigidity. SUN1 and SUN2 interact with lamin A, but lamin A is only required for NE localization of SUN2, and it remains unclear how SUN1 is anchored. Here, we identify emerin and short nesprin-2 isoforms as novel nucleoplasmic binding partners of SUN1/2. These have overlapping binding sites distinct from the lamin A binding site. However, we demonstrate that tight association of SUN1 with the nuclear lamina depends upon a short motif within residues 209-228, a region that does not interact significantly with known SUN1 binding partners. Moreover, SUN1 localizes correctly in cells lacking emerin. Importantly then, the major determinant of SUN1 NE localization has yet to be identified. We further find that a subset of lamin A mutations, associated with laminopathies Emery-Dreifuss muscular dystrophy (EDMD) and Hutchinson-Gilford progeria syndrome (HGPS), disrupt lamin A interaction with SUN1 and SUN2. Despite this, NE localization of SUN1 and SUN2 is not impaired in cell lines from either class of patients. Intriguingly, SUN1 expression at the NE is instead enhanced in a significant proportion of HGPS but not EDMD cells and strongly correlates with pre-lamin A accumulation due to preferential interaction of SUN1 with pre-lamin A. We propose that these different perturbations in lamin A-SUN protein interactions may underlie the opposing effects of EDMD and HGPS mutations on nuclear and cellular mechanics.

Lamin A-linked progerias: is farnesylation the be all and end all?

HGPS (Hutchinson–Gilford progeria syndrome) is a severe childhood disorder that appears to mimic an accelerated aging process. The disease is most commonly caused by gene mutations that disrupt the normal post-translational processing of lamin A, a structural component of the nuclear envelope. Impaired processing results in aberrant retention of a farnesyl group at the C-terminus of lamin A, leading to altered membrane dynamics. It has been widely proposed that persistence of the farnesyl moiety is the major factor responsible for the disease, prompting clinical trials of farnesyltransferase inhibitors to prevent lamin A farnesylation in children afflicted with HGPS. Although there is evidence implicating farnesylation in causing some of the cellular defects of HGPS, results of several recent studies suggest that aberrant lamin A farnesylation is not the only determinant of the disease. These findings have important implications for the design of treatments for this devastating disease.

Morphological analysis of 13 LMNA variants identified in a cohort of 324 unrelated patients with idiopathic or familial dilated cardiomyopathy

BACKGROUND

Mutations in the LMNA gene, encoding lamins A/C, represent a significant cause of dilated cardiomyopathy. We recently identified 18 protein-altering LMNA variants in a cohort of 324 unrelated patients with dilated cardiomyopathy. However, at least one family member with dilated cardiomyopathy in each of 6 pedigrees lacked the LMNA mutation (nonsegregation), whereas small sizes of 5 additional families precluded definitive determinations of segregation, raising questions regarding contributions by those variants to disease.

METHODS AND RESULTS

We have consequently expressed, in COS7 cells, GFP-prelamin A (GFPLaA) fusion constructs incorporating the 6 variants in pedigrees with nonsegregation (R101P, A318T, R388H, R399C, S437Hfsx1, and R654X), the 4 variants in pedigrees with unknown segregation (R89L, R166P [in 2 families], I210S, R471H), and 3 additional missense variants (R190Q, E203K, and L215P) that segregated with disease. Confocal immunofluorescence microscopy was used to characterize GFP-lamin A localization and nuclear morphology. Abnormal phenotypes were observed for 10 of 13 (77%) variants (R89L, R101P, R166P, R190Q, E203K, I210S, L215P, R388H, S437Hfsx1, and R654X), including 4 of 6 showing nonsegregation and 3 of 4 with uncertain segregation. All 7 variants affecting coil 1B and the lamin A-only mutation, R654X, exhibited membrane-bound GFP-lamin A aggregates and nuclear shape abnormalities. Unexpectedly, R388H largely restricted GFP-lamin A to the cytoplasm. Equally unexpected were unique streaked aggregates with S437Hfsx1 and giant aggregates with both S437Hfsx1 and R654X.

CONCLUSIONS

This work expands the recognized spectrum of lamin A localization abnormalities in dilated cardiomyopathy. It also provides evidence supporting pathogenicity of 10 of 13 tested LMNA variants, including some with uncertain or nonsegregation.

The posttranslational processing of prelamin A and disease

Human geneticists have shown that some progeroid syndromes are caused by mutations that interfere with the conversion of farnesyl-prelamin A to mature lamin A. For example, Hutchinson-Gilford progeria syndrome is caused by LMNA mutations that lead to the accumulation of a farnesylated version of prelamin A. In this review, we discuss the posttranslational modifications of prelamin A and their relevance to the pathogenesis and treatment of progeroid syndromes.

Ectopic expression of prelamin A in early Xenopus embryos induces apoptosis

Lamin proteins are components of metazoan cell nuclei. During evolution, two classes of lamin proteins evolved, A- and B-type lamins. B-type lamins are expressed in nearly all cell types and in all developmental stages and are thought to be indispensable for cellular survival. In contrast, A-type lamins have a more restricted expression pattern. They are expressed in differentiated cells and appear late in embryogenesis. In the earliest steps of mammalian development, A-type lamins are present in oocytes, pronuclei and during the first cleavage stages of the developing embryo. But latest after the 16-cell stage, A-type lamin proteins are not any longer detectable in embryonic cells. Amphibian oocytes and early embryos do not express lamin A. Moreover, extracts of Xenopus oocytes and eggs have the ability to selectively remove A-type lamins from somatic nuclei. This observation and the restricted expression pattern suggest that the presence of lamin A might interfere with developmental processes in the early phase of embryogenesis. To test this, we ectopically expressed lamin A during early embryonic development of Xenopus laevis by microinjection of synthetic mRNA. Here, we show that introducing mature lamin A does not interfere with normal development. However, expression of prelamin A or lamin A variants that cannot be fully processed cause severe disturbances and lead to apoptosis during gastrulation. The toxic effect is due to lack of the conversion of prenylated prelamin A to its mature form. Remarkably, even a cytoplasmic prelamin A variant that is excluded from the nucleus drives embryos into apoptosis.

ZMPSTE24, an integral membrane zinc metalloprotease with a connection to progeroid disorders

ZMPSTE24 is an integral membrane zinc metalloprotease originally discovered in yeast as an enzyme (called Ste24p) required for maturation of the mating pheromone a-factor. Surprisingly, ZMPSTE24 has recently emerged as a key protease involved in human progeroid disorders. ZMPSTE24 has only one identified mammalian substrate, the precursor of the nuclear scaffold protein lamin A. ZMPSTE24 performs a critical endoproteolytic cleavage step that removes the hydrophobic farnesyl-modified tail of prelamin A. Failure to do so has drastic consequences for human health and longevity. Here, we discuss the discovery of the yeast and mammalian ZMPSTE24 orthologs and review the unexpected connection between ZMPSTE24 and premature aging.

Laminopathies and the long strange trip from basic cell biology to therapy

The main function of the nuclear lamina, an intermediate filament meshwork lying primarily beneath the inner nuclear membrane, is to provide structural scaffolding for the cell nucleus. However, the lamina also serves other functions, such as having a role in chromatin organization, connecting the nucleus to the cytoplasm, gene transcription, and mitosis. In somatic cells, the main protein constituents of the nuclear lamina are lamins A, C, B1, and B2. Interest in the nuclear lamins increased dramatically in recent years with the realization that mutations in LMNA, the gene encoding lamins A and C, cause a panoply of human diseases ("laminopathies"), including muscular dystrophy, cardiomyopathy, partial lipodystrophy, and progeroid syndromes. Here, we review the laminopathies and the long strange trip from basic cell biology to therapeutic approaches for these diseases.

Microcephalia with mandibular and dental dysplasia in adult Zmpste24-deficient mice

ZMPSTE24 (also called FACE-1) is a zinc-metalloprotease involved in the post-translational processing of prelamin A to mature lamin A, a major component of the nuclear envelope. Mutations in the ZMPSTE24 gene or in that encoding its substrate prelamin A (LMNA) result in a series of human inherited diseases known collectively as laminopathies and showing regional or systemic manifestations (i.e. the Hutchinson-Gilford progeria syndrome). Typically, patients suffering some laminopathies show craniofacial or mandible anomalies, aberrant dentition or facial features characteristic of aged persons. To analyse whether Zmpste24(-/-) mice reproduce the cranial phenotype observed in humans due to mutations in ZMPSTE24 or LMNA, we conducted a craniometric study based on micro-computer tomography (microCT) images. Furthermore, using simple radiology, microCT, microCT-densitometry and scanning electron microscopy, we analysed the mandible and the teeth from Zmpste24(-/-) mice. Finally, the structure of the lower incisor was investigated using an H&E technique. The results demonstrate that Zmpste24(-/-) mice are microcephalic and show mandibular and dental dysplasia affecting only the mandible teeth. In all cases, the lower incisor of mice lacking Zmpste24 was smaller than in control animals, showed cylindrical morphology and a transverse fissure at the incisal edge, and the pulpal cavity was severely reduced. Structurally, the dental layers were normally arranged but cellular layers were disorganized. The inferior molars showed a reduced cusp size. Taken together, these data strongly suggest that Zmpste24(-/-) mice represent a good model to analyse the craniofacial and teeth malformations characteristic of lamin-related pathologies, and might contribute to a better understanding of the molecular events underlying these diseases.