Premature aging disorders: A clinical and genetic compendium

Progeroid disorders make up a heterogeneous group of very rare hereditary diseases characterized by clinical signs that often mimic physiological aging in a premature manner. Apart from Hutchinson‐Gilford progeria syndrome, one of the best‐investigated progeroid disorders, a wide spectrum of other premature aging phenotypes exist, which differ significantly in their clinical presentation and molecular pathogenesis. Next‐generation sequencing (NGS)‐based approaches have made it feasible to determine the molecular diagnosis in the early stages of a disease. Nevertheless, a broad clinical knowledge on these disorders and their associated symptoms is still fundamental for a comprehensive patient management and for the interpretation of variants of unknown significance from NGS data sets. This review provides a detailed overview on characteristic clinical features and underlying molecular genetics of well‐known as well as only recently identified premature aging disorders and also highlights novel findings towards future therapeutic options.

nuclear lamina proteins leading to the disruption of the nuclear envelope and to nuclear blebbing are more related to HGPS, Nestor-Guillermo progeria syndrome, restrictive dermopathy, and mandibuloacral dysplasia. Defects in DNA repair causing genomic instability and increased cancer risk are associated with RecQ helicase-mutant disorders such as Werner syndrome, Bloom syndrome and Rothmund-Thomson syndrome, but play also a role in Ataxia-telangiectasia, Cockayne syndrome, Nijmegen breakage syndrome, Seckel syndrome and xeroderma pigmentosum, among others.
Major effects of telomere attrition are for example known for dyskeratosis congenital and Hoyeraal-Hreidarsson syndrome, 3 although telomere dysfunction has been described in almost all premature aging syndromes associated with genomic instability. Likewise, mitochondrial dysfunction is frequently observed. It is a key characteristic of Fontaine progeroid syndrome and some progeroid cutis laxa syndromes, but also contributes to the pathogenesis of several other progeroid disorders such as HGPS, mandibuloacral dysplasia type B, Cockayne syndrome, ataxia-telangiectasia, and xeroderma pigmentosum. 6 Hereinafter, we give an overview of well-known and brand-new premature aging disorders, their clinical characteristics and their molecular basis. The detailed phenotypes are summarized in Table 1.
We have selected these disorders based on prevalance, awareness level, novel insights, and the authors' interest to present the disease spectrum. The described conditions result from different major molecular mechanisms such as nuclear lamina alterations, genomic instability and mitochondrial dysfunction, but also include additional premature aging phenotypes with an alternative or unknown pathogenesis. Based on these criteria, we performed a systematic search in F I G U R E 1 Hallmarks of aging disorders. This schematic illustration summarizes the nine proposed molecular and cellular mechanisms underlying progeroid diseases (adapted from López-Otín et al. 1  PubMed and OMIM database for premature aging disorders in general and separately for each condition.

| Insights into nuclear lamina alterations
Despite its rarity, HGPS has become one of the best-investigated segmental progeria syndromes in terms of its underlying pathogenesis, due to its striking similarities to the normal aging process. HGPS is caused The most common cause of HGPS is the de novo heterozygous silent mutation c.1824C>T (p.Gly608Gly) in LMNA. 10,11 This substitution activates a cryptic splice donor site resulting in exon skipping and subsequently the removal of 50 amino acids near the C-terminus of prelamin A, which contains the cleavage site for ZMPSTE24. As a result, the last proteolytic processing step cannot take place and the production of mature lamin A is thus blocked. The altered lamin A, also called progerin, is permanently farnesylated at the C-terminal end, which causes a stable association with the inner nuclear membrane. 12  (called nuclear blebbing), thickening of the nuclear lamina, loss of peripheral heterochromatin, and disorganization of nuclear pores. 14 In addition, they display an increased nuclear stiffness and a higher sensitivity to mechanical strain accompanied by impaired viability and increased apoptosis. Thus, in tissues with high mechanical stress such as vascular cells, the increased mechanical sensitivity might contribute to loss of smooth muscle cells and the development of arteriosclerosis in HGPS patients. 13,15,16 During mitosis, progerin also impairs chromosome maintenance by displacing the centromere protein F (CENP-F) from kinetochores, disrupting chromosome segregation, delaying nuclear envelope (NE) reformation, and trapping lamina components and inner NE proteins in the endoplasmic reticulum. All these progerin-dependent mechanisms lead to chromatin lagging, binucleated cells and genomic instability, which provoke premature senescence. 17 21 In addition, the altered interaction of progerin with transcription factors controlling adipogenesis has been discussed as a cause of lipodystrophy in HGPS. 13 Growth behavior of HGPS fibroblasts is characterized by an initial period of hyperproliferation, which is followed by rapid proliferation decline and increased apoptosis rate leading to early cellular senescence. 22 These findings suggest that proliferation control is dysregulated in HGPS and might potentially be explained by the two-step theory of cellular senescence as a result of cell cycle arrest and geroconversion. In this process, growth stimulation associated with mTOR activation first results in hyperstimulation during cell cycle arrest, which in turn causes premature senescence and loss of proliferation. 23,24 Human HGPS fibroblasts and mouse embryonic fibroblasts derived from progeroid models, such as Zmpste24-deficient mice, showed an accumulation of DNA damage and chromosomal aberrations as well as an increased sensitivity to DNA-damaging agents and a defect in DNA repair. 13,19,25,26 High levels of reactive oxygen species (ROS) in HGPS fibroblasts possibly contribute to a severely impaired capacity to repair DNA damage and premature cellular senescence. 27,28 Cellular senescence in HGPS is also accompanied by p53 hyperactivation 29 and mitochondrial dysfunction with downregulation of mitochondrial oxidative phosphorylation proteins. 30 However, the definite mechanism by which progerin induces the aging phenotype is not fully understood. Molecular and cellular findings might help to further decode the pathophysiology of aging disorders and to develop therapeutic strategies for HGPS. Telomere shortening is a well-described and important mechanism during physiological aging in human cells to maintain genomic integrity by triggering cell senescence and preventing the proliferation of cells with potential genetic alterations. 58,59 Interestingly, mitochondrial dysfunction leads to telomere shortening, while in turn telomere damage results in impaired mitochondrial biosynthesis and function, contributing to aging and a wide range of diseases. 60 Shorter telomeres have been proposed to be associated with a higher risk and mortality of atherosclerosis, cardiovascular disease, and cancer. 59,61-63 Dyskeratosis congenita and its more severe variant Hoyeraal-Hreidarsson syndrome were among the first disorders described that are caused by mutations in components of the telomerase complex leading to telomere attrition. 64 Dyskeratosis congenita cells exhibit shorter telomeres as well as decreased levels of telomerase RNA and telomerase activity compared to control cells. 64 In these primary telomeropathies, an anticipation effect of short telomere length associated with a reduced age of onset and a more severe phenotype in later generations have been discussed. 59,65 In addition, secondary telomeropathies such as ataxia-telangiectasia, Nijmegen breakage syndrome, Seckel syndrome, and RecQ helicase disorders present with overlapping phenotypes, but they are caused by mutations in DNA repair proteins contributing to telomere maintainance. 3,59 Cells derived from these patients mainly exhibit telomere aberrations and/or telomere deletion on sister chromatids, rather than a general telomere shortening as observed in dyskeratosis congenita. 59 DNA repair and telomere attrition are also altered in a wide range of other premature aging syndromes. Moreover, delayed recruitment of DNA repair factors such as 53BP1 and RAD51 and accelerated telomere shortening during proliferation have also been observed in HGPS fibroblasts, suggesting that genomic instability also contributes to the pathogenesis of laminopathies. 66 Hereinafter, we will give a clinical overview of premature aging syndromes caused by major defects in genomic maintenance.

| Ataxia-Telangiectasia (MIM #208900)
Ataxia-telangiectasia is an autosomal recessive neurodegenerative disorder with the mostly early-childhood onset and an estimated prevalence of 1:40 000-1:100 000 live births. 67 Apart from premature aging signs, characteristic clinical features include progressive cerebellar ataxia, oculomotor apraxia, oculocutaneous telangiectasia, immunodeficiency, recurrent infections, and hypersensitivity to radiation with increased risk for malignancy (especially leukemia and lymphoma). 67,68 Patients can also present with endocrine abnormalities such as growth retardation, reproductive dysfunction, and diabetes. 69 Ataxiatelangiectasia is due to biallelic mutations in ATM (ataxia-telangiectasia mutated), which are mainly truncations. 70 In addition, further recurrent founder mutations are known from other populations. 78 Recently, biallelic mutations in TOP3A, RMI1, and RMI2 genes have been identified in a few patients to cause a clinical phenotype that overlaps with Bloom syndrome and is accompanied by an elevated number of sister chromatid exchanges on cytogenetic analysis. 84,85 Although multiple café-au-lait patches were observed, the classical ery- Cockayne syndrome is a rare developmental and neurodegenerative disorder with more than 120 genetically confirmed patients worldwide. 89 The annual incidence is estimated at approximately 1:250 000 and the prevalence at approximately 2.5 per million. 90 The typical phenotype is characterized by cachectic dwarfism with sunken eyes and progressive microcephaly with structural brain anomalies, severe developmental delay and mental retardation. The clinical spectrum also includes cutaneous photosensitivity, pigmentary retinopathy, cataracts, sensorineural hearing loss, atherosclerosis, feeding difficulties, and dental anomalies. Most patients die from progressive multiorgan degeneration in the first or second decade with a mean age of death of 12 years. 90,91 Two genes have been identified to be mainly associated with the disease in an autosomal recessive inheritance pattern: Cockayne syndrome type A is linked to mutations in ERCC8 (one-third of the patients) and Cockayne syndrome type B to mutations in ERCC6 (twothirds of the patients). 89   Rothmund-Thomson syndrome manifests typically by a sun-sensitive erythema of the face at the mean age of 3 to 6 months, which spreads over time and develops into chronic reticulated pigmentation, telangiectases and areas of punctate atrophy known as poikiloderma. 127 Additional clinical features of Rothmund-Thomson syndrome include short stature, sparse hair, increased risk for malignancies (eg, osteosarcomas and skin cancers), juvenile cataracts, and skeletal, dental, and nail abnormalities. 128,129 Rothmund-Thomson syndrome is subdivided into two different types. The majority of more than 300 reported cases belong to Rothmund-Thomson syndrome type 2, which is due to biallelic truncating and missense mutations in RECQL4 gene [130][131][132] and is associated more often with increased cancer risk and skeletal anomalies. 133 RECQL4 is coding for a DNA helicase of the RecQ family and is linked to genomic instability, carcinogenesis and aging processes. 134 Biallelic mutations in RECQL4 have also been identified in patients with Rapadilino syndrome (MIM #266280) and Baller-Gerold syndrome (MIM #218600), 135

which share clinical features with
Rothmund-Thomson syndrome such as growth retardation, skeletal anomalies and increased cancer risk. 133 Recently, biallelic mutations in the ANAPC1 gene were described as cause of Rothmund-Thomson syndrome type 1, which is characterized by the presence of juvenile cataracts. All 10 patients reported so far exhibit a deep intronic splicing mutation in ANAPC1 either in homozygous state or compound heterozygous with another truncating mutation. 133 ANAPC1 belongs to an anaphase-promoting complex/cyclosome that is known to be involved in DNA replication and repair, cell differentiation, cell senescence metabolism and neuronal function. 136

| Ruijs-Aalfs Syndrome (MIM #616200)
Until now, only three patients from two unrelated families have been described with Ruijs-Aalfs syndrome, who presented with hepatocellular carcinoma at an early age as well as low body weight, muscular atrophy, lipodystrophy, delayed bone age and mild joint restrictions.
One patient also showed short stature, bilateral cataracts and premature hair graying. 137

| The role of mitochondrial dysfunction
MtDNA is exposed to a wide range of endogenous and exogenous agents, which lead to reduced cellular viability and contribute to the pathogenesis of several genetic disorders. A major source of endogenous damage are ROS generated during oxidative phosphorylation. 162 Apart from faithful mtDNA replication, several pathways play a significant role in maintaining mitochondrial integrity such as mtDNA repair and degeneration, ROS scavenging, mitochondrial morphology regulation by fission and fusion, and whole mitochondrial removal by mitophagy ( Figure 3). Since mtDNA is not coding for any gene contributing to DNA maintenance, mitochondrial repair proteins are derived from nuclear genome. 163 Mitochondria possess several DNA repair pathways similar to those of the nucleus such as BER, MMR, HR, and NHEJ, while highly mutagenized und unrepairable mtDNA will be removed by degradation. 164 Moreover, overwhelming ROS production during oxidative phosphorylation is compensated by endogenous antioxidants to reduce oxidative stress. 165 In addition, the dynamic process of mitochondrial fusion and fission plays also an essential role in preserving a healthy pool of mitochondria. Fusion of two mitochondria helps to distribute damaged mtDNA, while fission is necessary to create new mitochondria by organelle division. Severe mitochondrial damage provokes an autophagic elimination of the whole dysfunctional organelle called mitophagy, rather than fusion and fission. 166 It is well known that the aging process in human is associated with a progressive decline in mitochondrial function. For more than 60 years, it has been proposed that accumulation of ROS during aging induces molecular damage disrupting mitochondrial function and that this process is causative of the development of age-related disorders. 167 More recent considerations state that ROS are not a direct trigger of aging; by modulating stress response pathways, high levels of ROS contribute to the aging phenotype. 168 Apart from oxidative damage induced by ROS, the accumulation of mainly point mutations and deletions in mtDNA with advanced age leads to impaired mitochondrial integrity and function, which is accompanied by reduced respiratory chain activity, ATP production, and activity of metabolic enzymes. 169,170 Thereby, mitochondrial dysfunction is involved in the pathogenesis of various age-related disorders such as metabolic syndrome, cancer, neurodegenerative and cardiovascular diseases. [171][172][173][174] Mitochondrial defects are also related to several progeroid disorders that are caused either by lamin alteration or by impaired DNA repair. For example, downregulation of mitochondrial oxidative phosphorylation proteins was observed in HGPS cells and was accompanied by mitochondrial dysfunction. 30 For mandibuloacral dysplasia type B, siRNA-mediated knockdown of LMNA and ZMPSTE24 in human fibroblasts resulted in alterations of mitochondrial membrane potential, mitochondrial respiration, and cell proliferation. 175,176 In cell models for Cockayne syndrome, 177 ataxia-telangiectasia 178 and xeroderma pigmentosum 179 similar alterations have been observed such as lower levels of mitophagy, increased mitochondrial content, higher membrane potential, and higher ATP consumption. 6 The majority of progeroid cutis laxa syndromes is caused by biallelic mutations in ALDH18A1 and PYCR1, encoding mitochondrial proteins involved in proline de novo biosynthesis. [180][181][182][183] Analyses of PYCR1-deficient patient fibroblasts revealed abnormal mitochondrial morphology as well as altered mitochondrial membrane potential and higher apoptosis rate upon oxidative stress. 184,185 Similarly, mitochondrial swelling was observed in fibroblasts derived from a patient carrying homozygous deletions in ALDH18A1. 186 Dysfunction of pyrroline-5-carboxylate synthase impacts on various mitochondria-associated pathways including oxidative phosphorylation and lipid metabolism. 187 Surprisingly, gerodermia osteodysplastica with broad clinical overlap to ALDH18A1-and PYCR1-related cutis laxa syndromes is due to biallelic mutations in GORAB, encoding a Golgi protein involved in vesicular transport processes. [181][182][183] In addition, defects in v-ATPase subunits encoded by ATP6V0A2, ATP6V1E1, and ATP6V1A were observed in progeroid cutis laxa patients. Fibroblasts from these patients displayed swelling and fragmentation of the Golgi apparatus as well as delayed vesicular trafficking between the Golgi apparatus and endoplasmic reticulum. 188,189 These results indicate that not only an impaired mitochondrial function, but also defects in the Golgi apparatus and vesicular trafficking are involved in the pathogenesis of progeroid cutis laxa syndromes.
Mitochondrial dysfunction has also been identified in the recently described Fontaine progeroid syndrome, which encompasses Fontaine syndrome and Gorlin-Chaudhry-Moss syndrome as two initially separately classified phenotypes. 190 Both entities are due to missense mutations affecting the same amino acid position in the SLC25A24 gene. 6,191 SLC25A24 belongs to the solute carrier 25 (SLC25) family of nuclear genes and encodes an ATP-Mg/P i carrier that is responsible for the exchange of ATP-Mg or ADP-Mg for phosphate across the mitochondrial inner membrane. 192,193 SLC25A24-mutant cells exhibit altered mitochondrial morphology (Figure 4), slower proliferation rate, impaired mitochondrial ATP synthesis and increased mitochondrial membrane potential. 6,191 Interestingly, SLC25A24-mutant cells also showed a higher sensitivity to oxidative stress indicated by lower mitochondrial ATP levels and mitochondrial swelling. Based on this finding, it was assumed that the sensitivity to oxidative stress might be the potential link between mitochondrial dysfunction and the aging phenotype in Fontaine progeroid syndrome. 191

| Fontaine Progeroid Syndrome (MIM #612289)
Fontaine syndrome and Gorlin-Chaudhry-Moss syndrome are two overlapping phenotypes that have been recently summarized as Fontaine progeroid syndrome with overall 11 genetically confirmed F I G U R E 3 Mechanisms maintaining mitochondrial integrity. Several pathways are involved to preserve mitochondrial integrity and biogenesis. They include mitochondrial DNA (mtDNA) replication, repair and degeneration, but also reactive oxygen species (ROS) scavenging and mitochondrial morphology regulation by fission-fusion-processes and whole organelle removal by mitophagy cases reported in the literature. 6,190,191,194 The with microcephaly, triangular face, blue sclerae, cataract, hypotonia, brain anomalies, and skeletal anomalies. 185,195,196 ARCL2B is caused by biallelic mutations in PYCR1 which are predominantly missense or splice site mutations clustering in exons 4 to 6. 185 PYCR1 encodes for the mitochondrial protein pyrroline-5-carboxylate reductase 1 which is involved in proline de novo biosynthesis. 197 The De Barsy syndrome is classified as autosomal recessive cutis laxa type III (ARCL3) and shares many clinical features with ARCL2B, albeit in a more severe form, especially growth retardation, hypotonia, mental retardation, and movement disorders. 186,196,198 In addition, corneal dystrophy is a distinguishing hallmark. De Barsy syndrome is divided into two subtypes: ARCL3A is due to biallelic mutations in ALDH18A1, 186,199 whereas ARCL3B is caused by biallelic mutations in

| Alternative or unknown molecular mechanisms
The vast majority of premature aging disorders can be related to nuclear architecture alterations, DNA repair defects, telomere attrition, and mitochondrial impairment in a highly intertwined manner.
However, other premature aging phenotypes are caused by mutations in genes involved in metabolism, growth, and connective tissue, among others. The molecular mechanism leading to premature senescence in most of these cases is not fully enlightened. Moreover, even despite many whole-exome and whole-genome sequencing efforts, the genetic cause of a number of premature aging disorders, such as Hallermann-Streiff syndrome, has remained elusive.

| GAPO Syndrome (MIM #230740)
GAPO syndrome was first described in 1947 and later defined by its acronym consisting of growth retardation, alopecia, pseudoanodontia, and optic atrophy. 207  PIK3R1 encodes a regulatory subunit of phosphatidyl inositol-3 kinase of class IA (PI3K), which is involved in the AKT-mTOR pathway and thereby important for cellular proliferation and growth. 240,242 Mutations in PIK3R1 seem also to disrupt the insulin signaling pathway, which explains the additional predisposition to insulin resistance and diabetes in patients with SHORT syndrome. 241  In recent years, a broad spectrum of therapeutic approaches mainly focusing on HGPS has been proposed. One of the first promising strategies was the blockade of farnesylation by farnesyltransferase inhibitors (FTIs) such as lonafarnib to obviate the conversion of prelamin A to progerin. FTIs reduced nuclear bleebing in murine and human HGPS fibroblasts. [244][245][246] In transgenic mouse models of HGPS, treatment with FTIs resulted in increased body weight, bone mineralization and lifespan, 247,248 but not all progeria symptoms could be improved by FTIs. 249 In a clinical trial, HGPS patients were treated with the FTI lonafarnib for a minimum of 2 years, which enhanced vascular stiffness, skeletal rigidity and sensorineural hearing. 250 Recently, it has been reported that HGPS patients treated with lonafarnib also have a lower mortality rate after 2.2 years of follow up. 251 Although FTI monotherapy improved some aspects of cardiovascular and bone phenotype, the therapeutic effect is still limited and associated with a number of side effects including diarrhea, fatigue, nausea, vomiting, anorexia, transiently elevated liver enzymes, and depressed serum hemoglobin. 252,253 In an approach focusing on autophagy as a mechanism to improve progerin clearance, the immunosuppressive agent and TOR pathway inhibitor rapamycin was proposed as a potential therapeutic agent. In in vitro analysis, rapamycin or its analog everolimus reduced nuclear blebbing, delayed cellular senescence and enhanced degradation of progerin in HGPS fibroblasts. [254][255][256] A phase I/II trial of everolimus in combination with lonafarnib in patients with HGPS and other progeroid laminopathies was started in 2015; the first results are still outstanding and expected for 2020. 66 Sulforaphane, an antioxidant derived from cruciferous vegetables, also stimulated progerin clearance by autophagy in HGPS fibroblasts. 257 Combined treatment of lonafarnib and sulforaphane showed cytotoxic effects in HGPS fibroblasts, whereas intermittent and separate application of the two components in repeated cycles enhanced progerin clearance, normalized nuclear shape and reduced DNA damage in HGPS fibroblasts. 258 Since the LMNA gene promotor contains retinoic acid-responsive elements, retinoids have also come into focus as therapeutic agents. In HGPS fibroblasts, treatment with retinoids alone and in combination with rapamycin improved progerin clearance and several aging cell defects. 259,260 Apart from FTIs and autophagy-activating agents a number of other therapeutic strategies focusing on other potential molecular mechanisms have been investigated in vitro for their potential use in HGPS. 66,253 The ROS scavenger N-acetyl cysteine reduced the level of unrepaired ROS-induced DNA damage and enhanced cell growth. 261 Treatment with methylene blue rescued mitochondrial defects and improved nuclear abnormalities. 262 Activation of vitamin D receptor by 1α,25-dihydroxyvitamin D3 reduced progerin production. 263 Blocking the aberrant LMNA splicing site by morpholino antisense oligonucleotides decreased the expression of progerin. 264,265 In the future, gene editing by CRISPR-Cas9 may offer novel treat-

| CONCLUSION
In the era of whole-exome and whole-genome sequencing, it has become much easier to uncover the causative mutation(s) in early stages of diseases and, consequently, to establish a distinct diagnosis in patients with premature aging phenotypes. However, a broad clinical knowledge of the associated symptoms and the underlying molecular cause as provided in this review is still fundamental to interpreting variants of unknown significance and, even more significant, to delivering comprehensive patient management including detailed information of the patients, planning of a specific surveillance program and in future hopefully also targeted therapy approaches. Treatment of patients with rare diseases such as premature aging disorders requires a close collaboration between specialists from a variety of disciplines as implemented by Centres for Rare Disorders worldwide. The focus of multidisciplinary patient care is to establish an accurate clinical and molecular diagnosis as early as possible and to provide appropriate management based on novel research advances and the expertise from different specializations.

CONFLICT OF INTEREST
The authors declare no potential conflict of interest.

PEER REVIEW
The peer review history for this article is available at https://publons. com/publon/10.1111/cge.13837.

DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no new datasets were generated or analyzed in this study.