A homozygous mutation in the lamin A/C gene associated with a novel syndrome of arthropathy, tendinous calcinosis, and progeroid features

Deciphering the Clinical Presentations in LMNA-related Lipodystrophy: Report of 115 Cases and a Systematic Review

CONTEXT

Lipodystrophy syndromes are a heterogeneous group of rare genetic or acquired disorders characterized by generalized or partial loss of adipose tissue. LMNA-related lipodystrophy syndromes are classified based on the severity and distribution of adipose tissue loss.

OBJECTIVE

We aimed to annotate all clinical and metabolic features of patients with lipodystrophy syndromes carrying pathogenic LMNA variants and assess potential genotype-phenotype relationships.

METHODS

We retrospectively reviewed and analyzed all our cases (n = 115) and all published cases (n = 379) curated from 94 studies in the literature.

RESULTS

The study included 494 patients. The most common variants in our study, R482Q and R482W, were associated with similar metabolic characteristics and complications though those with the R482W variant were younger (aged 33 [24] years vs 44 [25] years; P < .001), had an earlier diabetes diagnosis (aged 27 [18] vs 40 [17] years; P < .001) and had lower body mass index levels (24 [5] vs 25 [4]; P = .037). Dyslipidemia was the earliest biochemical evidence described in 83% of all patients at a median age of 26 (10) years, while diabetes was reported in 61% of cases. Among 39 patients with an episode of acute pancreatitis, the median age at acute pancreatitis diagnosis was 20 (17) years. Patients who were reported to have diabetes had 3.2 times, while those with hypertriglyceridemia had 12.0 times, the odds of having pancreatitis compared to those who did not.

CONCLUSION

This study reports the largest number of patients with LMNA-related lipodystrophy syndromes to date. Our report helps to quantify the prevalence of the known and rare complications associated with different phenotypes and serves as a comprehensive catalog of all known cases.

Genomic instability and DNA replication defects in progeroid syndromes

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

Insight into the functional organization of nuclear lamins in health and disease

Lamins are the main component of the nuclear lamina, a protein meshwork at the inner nuclear membrane which primarily provide mechanical stability to the nucleus. Lamins, type V intermediate filament proteins, are also involved in many nuclear activities. Structural analysis of nuclei revealed that lamins form 3.5nm thick filaments often interact with nuclear pore complexes. Mutations in the LMNA gene, encoding A-type lamins, have been associated with at least 15 distinct diseases collectively termed laminopathies, including muscle, metabolic and neurological disorders, and premature aging syndrome. It is unclear how laminopathic mutations lead to such a wide array of diseases, essentially affecting almost all tissues.

Nuclear lamina genetic variants, including a truncated LAP2, in twins and siblings with nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is becoming the major chronic liver disease in many countries. Its pathogenesis is multifactorial, but twin and familial studies indicate significant heritability, which is not fully explained by currently known genetic susceptibility loci. Notably, mutations in genes encoding nuclear lamina proteins, including lamins, cause lipodystrophy syndromes that include NAFLD. We hypothesized that variants in lamina-associated proteins predispose to NAFLD and used a candidate gene-sequencing approach to test for variants in 10 nuclear lamina-related genes in a cohort of 37 twin and sibling pairs: 21 individuals with and 53 without NAFLD. Twelve heterozygous sequence variants were identified in four lamina-related genes (ZMPSTE24, TMPO, SREBF1, SREBF2). The majority of NAFLD patients (>90%) had at least one variant compared to <40% of controls (P < 0.0001). When only insertions/deletions and changes in conserved residues were considered, the difference between the groups was similarly striking (>80% versus <25%; P < 0.0001). Presence of a lamina variant segregated with NAFLD independently of the PNPLA3 I148M polymorphism. Several variants were found in TMPO, which encodes the lamina-associated polypeptide-2 (LAP2) that has not been associated with liver disease. One of these, a frameshift insertion that generates truncated LAP2, abrogated lamin-LAP2 binding, caused LAP2 mislocalization, altered endogenous lamin distribution, increased lipid droplet accumulation after oleic acid treatment in transfected cells, and led to cytoplasmic association with the ubiquitin-binding protein p62/SQSTM1.

CONCLUSION

Several variants in nuclear lamina-related genes were identified in a cohort of twins and siblings with NAFLD; one such variant, which results in a truncated LAP2 protein and a dramatic phenotype in cell culture, represents an association of TMPO/LAP2 variants with NAFLD and underscores the potential importance of the nuclear lamina in NAFLD. (Hepatology 2018;67:1710-1725).

LMNA Sequences of 60,706 Unrelated Individuals Reveal 132 Novel Missense Variants in A-Type Lamins and Suggest a Link between Variant p.G602S and Type 2 Diabetes

Mutations in LMNA, encoding nuclear intermediate filament proteins lamins A and C, cause multiple diseases ('laminopathies') including muscular dystrophy, dilated cardiomyopathy, familial partial lipodystrophy (FPLD2), insulin resistance syndrome and progeria. To assess the prevalence of LMNA missense mutations ('variants') in a broad, ethnically diverse population, we compared missense alleles found among 60,706 unrelated individuals in the ExAC cohort to those identified in 1,404 individuals in the laminopathy database (UMD-LMNA). We identified 169 variants in the ExAC cohort, of which 37 (∼22%) are disease-associated including p.I299V (allele frequency 0.0402%), p.G602S (allele frequency 0.0262%) and p.R644C (allele frequency 0.124%), suggesting certain LMNA mutations are more common than previously recognized. Independent analysis of LMNA variants via the type 2 diabetes (T2D) Knowledge Portal showed that variant p.G602S associated significantly with type 2 diabetes (p = 0.02; odds ratio = 4.58), and was more frequent in African Americans (allele frequency 0.297%). The FPLD2-associated variant I299V was most prevalent in Latinos (allele frequency 0.347%). The ExAC cohort also revealed 132 novel LMNA missense variants including p.K108E (limited to individuals with psychiatric disease; predicted to perturb coil-1B), p.R397C and p.R427C (predicted to perturb filament biogenesis), p.G638R and p.N660D (predicted to perturb prelamin A processing), and numerous Ig-fold variants predicted to perturb phenotypically characteristic protein-protein interactions. Overall, this two-pronged strategy- mining a large database for missense variants in a single gene (LMNA), coupled to knowledge about the structure, biogenesis and functions of A-type lamins- revealed an unexpected number of LMNA variants, including novel variants predicted to perturb lamin assembly or function. Interestingly, this study also correlated novel variant p.K108E with psychiatric disease, identified known variant p.I299V as a potential risk factor for metabolic disease in Latinos, linked variant p.G602 with type 2 diabetes, and identified p.G602S as a predictor of diabetes risk in African Americans.

Mandibuloacral dysplasia and LMNA A529V mutation in Turkish patients with severe skeletal changes and absent breast development

Mandibuloacral dysplasia (MAD) is an autosomal recessive disorder characterized by acroosteolysis (resorption of terminal phalanges), skin changes (hyperpigmentation), clavicular hypoplasia, craniofascial anomalies, a hook nose and prominent eyes, delayed closures of the cranial sutures, lipodystrophy, alopecia, and skeletal anomalies. MAD patients are classified according to lipodystrophy patterns: type A and type B. The vast majority of MAD cases are caused by LMNA gene mutations. MAD patients with type A lipodystrophy (MADA) have been reported to have LMNA R527H, A529V, or A529T mutations. In this report, we describe two MADA patients with progressive skeletal changes, absent breast development, and cataract in addition to the classical MAD phenotype. Both patients were found to be homozygous for the Ala529Val mutation of the LMNA gene. Our female patient is the oldest MADA patient (59 years old) who has ever been reported with the LMNA mutation and also the LMNA Ala529Val mutation. This study is the second report on MADA patients with a homozygous Ala529Val mutation.

Cardiac arrhythmia and late-onset muscle weakness caused by a myofibrillar myopathy with unusual histopathological features due to a novel missense mutation in FLNC

Myofibrillar myopathies (MFM) are mostly adult-onset diseases characterized by progressive morphological alterations of the muscle fibers beginning in the Z-disk and the presence of protein aggregates in the sarcoplasm. They are mostly caused by mutations in different genes that encode Z-disk proteins, including DES, CRYAB, LDB3, MYOT, FLNC and BAG3. A large family of French origin, presenting an autosomal dominant pattern, characterized by cardiac arrhythmia associated to late-onset muscle weakness, was evaluated to clarify clinical, morphological and genetic diagnosis. Muscle weakness began during adult life (over 30 years of age), and had a proximal distribution. Histology showed clear signs of a myofibrillar myopathy, but with unusual, large inclusions. Subsequently, genetic testing was performed in MFM genes available for screening at the time of clinical/histological diagnosis, and desmin (DES), αB-crystallin (CRYAB), myotilin (MYOT) and ZASP (LDB3), were excluded. LMNA gene screening found the p.R296C variant which did not co-segregate with the disease. Genome wide scan revealed linkage to 7q.32, containing the FLNC gene. FLNC direct sequencing revealed a heterozygous c.3646T>A p.Tyr1216Asn change, co-segregating with the disease, in a highly conserved amino acid of the protein. Normal filamin C levels were detected by Western-blot analysis in patient muscle biopsies and expression of the mutant protein in NIH3T3 showed filamin C aggregates. This is an original FLNC mutation in a MFM family with an atypical clinical and histopathological presentation, given the presence of significantly focal lesions and prominent sarcoplasmic masses in muscle biopsies and the constant heart involvement preceding significantly the onset of the myopathy. Though a rare etiology, FLNC gene should not be excluded in early-onset arrhythmia, even in the absence of myopathy, which occurs later in the disease course.

Osteogenic differentiation of human lens epithelial cells might contribute to lens calcification

Calcification of the human lens has been described in senile cataracts and in young patients with congenital cataract or chronic uveitis. Lens calcification is also a major complication of cataract surgery and plays a role in the opacification of intraocular lenses. A cell-mediated process has been suggested in the background of lens calcification, but so far the exact mechanism remained unexplored. Lens calcification shares remarkable similarities with vascular calcification; in both pathological processes hydroxyapatite accumulates in the soft tissue. Vascular calcification is a regulated, cell-mediated process in which vascular cells undergo osteogenic differentiation. Our objective was to investigate whether human lens epithelial cells (HuLECs) can undergo osteogenic transition in vitro, and whether this process contributes to lens calcification. We used inorganic phosphate (Pi) and Ca to stimulate osteogenic differentiation of HuLECs. Osteogenic stimuli (2.5mmol/L Pi and 1.2mmol/L Ca) induced extracellular matrix mineralization and Ca deposition in HuLECs with the critical involvement of active Pi uptake. Osteogenic stimuli almost doubled mRNA expressions of osteo-/chondrogenic transcription factors Runx2 and Sox9, which was accompanied by a 1.9-fold increase in Runx2 and a 5.5-fold increase in Sox9 protein expressions. Osteogenic stimuli induced mRNA and protein expressions of alkaline phosphatase and osteocalcin in HuLEC. Ca content was higher in human cataractous lenses, compared to non-cataractous controls (n=10). Osteocalcin, an osteoblast-specific protein, was expressed in 2 out of 10 cataractous lenses. We conclude that osteogenic stimuli induce osteogenic differentiation of HuLECs and propose that this mechanism might play a role in lens calcification.

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.

An inherited LMNA gene mutation in atypical Progeria syndrome

Hutchinson-Gilford Progeria syndrome (HGPS) is a rare genetic disorder, characterized by several clinical features that begin in early childhood, recalling an accelerated aging process. The diagnosis of HGPS is based on the recognition of common clinical features and detection of the recurrent heterozygous c.1824C>T (p.Gly608Gly) mutation within exon 11 in the Lamin A/C encoding gene (LMNA). Besides "typical HGPS," several "atypical progeria" syndromes (APS) have been described, in a clinical spectrum ranging from mandibuloacral dysplasia to atypical Werner syndrome. These patients's clinical features include progeroid manifestations, such as short stature, prominent nose, premature graying of hair, partial alopecia, skin atrophy, lipodystrophy, skeletal anomalies, such as mandibular hypoplasia and acroosteolyses, and in some cases severe atherosclerosis with metabolic complications. APS are due in several cases to de novo heterozygous LMNA mutations other than the p.Gly608Gly, or due to homozygous BAFN1 mutations in Nestor-Guillermo Progeria syndrome (NGPS). We report here and discuss the observation of a non-consanguineous Moroccan patient presenting with atypical progeria. The molecular studies showed the heterozygous mutation c.412G>A (p.Glu138Lys) of the LMNA gene. This mutation, previously reported as a de novo mutation, was inherited from the apparently healthy father who showed a somatic cell mosaicism.

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.

Atypical progeroid syndrome due to heterozygous missense LMNA mutations

CONTEXT

Hutchinson-Gilford progeria syndrome (HGPS) and mandibuloacral dysplasia are well-recognized allelic autosomal dominant and recessive progeroid disorders, respectively, due to mutations in lamin A/C (LMNA) gene. Heterozygous LMNA mutations have also been reported in a small number of patients with a less well-characterized atypical progeroid syndrome (APS).

OBJECTIVE

The objective of the study was to investigate the underlying genetic and molecular basis of the phenotype of patients presenting with APS.

RESULTS

We report 11 patients with APS from nine families, many with novel heterozygous missense LMNA mutations, such as, P4R, E111K, D136H, E159K, and C588R. These and previously reported patients now reveal a spectrum of clinical features including progeroid manifestations such as short stature, beaked nose, premature graying, partial alopecia, high-pitched voice, skin atrophy over the hands and feet, partial and generalized lipodystrophy with metabolic complications, and skeletal anomalies such as mandibular hypoplasia and mild acroosteolysis. Skin fibroblasts from these patients when assessed for lamin A/C expression using epifluorescence microscopy revealed variable nuclear morphological abnormalities similar to those observed in patients with HGPS. However, these nuclear abnormalities in APS patients could not be rescued with 48 h treatment with farnesyl transferase inhibitors, geranylgeranyl transferase inhibitors or trichostatin-A, a histone deacetylase inhibitor. Immunoblots of cell lysates from fibroblasts did not reveal prelamin A accumulation in any of these patients.

CONCLUSIONS

APS patients have a few overlapping but some distinct clinical features as compared with HGPS and mandibuloacral dysplasia. The pathogenesis of clinical manifestations in APS patients seems not to be related to accumulation of mutant farnesylated prelamin A.

Severe mandibuloacral dysplasia caused by novel compound heterozygous ZMPSTE24 mutations in two Japanese siblings

Mandibuloacral dysplasia (MAD) is a rare autosomal recessive progeroid syndrome, characterized by mandibular hypoplasia, acroosteolysis affecting distal phalanges and clavicles, delayed closure of the cranial sutures, atrophic skin, and lipodystrophy. Recently, mutations in lamin A/C (LMNA) and zinc metalloprotease (ZMPSTE24), involved in post-translational processing of prelamin A to mature lamin A, have been identified in MAD kindreds. We now report novel compound heterozygous mutations in exon 1 (c.121C>T; p.Q41X) and exon 6 (c.743C>T; p.P248L) in ZMPSTE24 in two Japanese sisters, 7- and 3-year old, with severe MAD and characteristic facies and atrophic skin. The older sister had lipodystrophy affecting the chest and thighs but sparing abdomen. Their parents and a brother, who were healthy, had heterozygous mutations. The missense mutation, P248L, was not found in 100 normal subjects of Japanese origin. The mutant Q41X was inactive in a yeast halo assay; however, the mutant P248L retained near normal ZMPSTE24 activity. Immunoblots demonstrated accumulation of prelamin A in the patients' cell lysates from lymphoblasts. The lymphoblasts from the patients also revealed less intense staining for lamin A/C on immunofluorescence. We conclude that ZMPSTE24 deficiency results in accumulation of farnesylated prelamin A, which may be responsible for cellular toxicity and the MAD phenotype.

HGPS and related premature aging disorders: from genomic identification to the first therapeutic approaches

Progeroid syndromes are heritable human disorders displaying features that recall premature ageing. In these syndromes, premature aging is defined as "segmental" since only some of its features are accelerated. A number of cellular biological pathways have been linked to aging, including regulation of the insulin/growth hormone axis, pathways involving ROS metabolism, caloric restriction, and DNA repair. The number of identified genes associated with progeroid syndromes has increased in recent years, possibly shedding light as well on mechanisms underlying ageing in general. Among these, premature aging syndromes related to alterations of the LMNA gene have recently been identified. This review focuses on Hutchinson-Gilford Progeria syndrome and Restrictive Dermopathy, two well-characterized Lamin-associated premature aging syndromes, pointing out the current knowledge concerning their pathophysiology and the development of possible therapeutic approaches.

New metabolic phenotypes in laminopathies: LMNA mutations in patients with severe metabolic syndrome

CONTEXT

Mutations in the LMNA gene are responsible for several laminopathies, including lipodystrophies, with complex genotype/phenotype relationships. OBJECTIVE, DESIGN, SETTING, AND PATIENTS: Sequencing of the LMNA coding regions in 277 unrelated adults investigated for lipodystrophy and/or insulin resistance revealed 17 patients with substitutions at codon 482 observed in typical Dunnigan's familial partial lipodystrophy and 10 patients with other mutations. We report here the phenotypes of the patients with non-codon 482 mutations and compare them with those of 11 patients with codon 482 mutations. We also studied skin fibroblasts or lymphocytes from seven patients.

RESULTS

LMNA mutations found in nine patients studied here affected the three protein domains. Eight of them were novel. The 10 patients with non-codon 482-associated mutations fulfilled the International Diabetes Federation diagnosis criteria for metabolic syndrome. Most of them lacked the typical lipoatrophy observed in Dunnigan's familial partial lipodystrophy. However, the severity of insulin resistance, altered glucose tolerance, and hypertriglyceridemia and the alterations of cell nuclei were similar in patients with codon 482- and non-codon 482-associated mutations. Calf hypertrophy, myalgia, and muscle cramps or weakness were present in nine patients and cardiac conduction disturbances in two patients with non-codon 482 LMNA mutations.

CONCLUSIONS

We describe here new phenotypes of metabolic laminopathy associated with non-codon 482 LMNA mutations and characterized, in the absence of obvious clinical lipoatrophy, by severe metabolic alterations and frequent muscle signs (muscular hypertrophy, myalgias, or weakness). Dual-energy x-ray absorptiometry and/or cross-sectional abdominal and thigh imaging can help diagnosis by revealing subclinical lipodystrophy. The prevalence and pathophysiology of metabolic laminopathies need to be studied further.

The neonatal progeroid syndrome (Wiedemann-Rautenstrauch): a model for the study of human aging?

The Wiedemann-Rautenstrauch syndrome (WRS) characterises a premature aging syndrome in which several features of human aging are apparent at birth therefore allowing their grouping as a neonatal progeroid condition. This differentiates WRS from other progeroid entities such as Hutchinson-Gilford progeria syndrome (HGPS) in which characteristics of premature aging become apparent some time after birth. The etiology of WRS remains unknown. Some studies have observed an autosomal recessive mode of inheritance. Several studies analysing telomere length and lamin A gene have not revealed any alterations. However, mutations in LMNA have been reported in several other atypical progeroid syndromes. Based on these observations, several hypothesis could be withdrawn concerning the etiology of WRS. The study of genes associated with lamin A metabolism, such as Zmpste24, and the metabolic pathways associated with insulin, such as protein kinase B or AKT, are of particular interest. We believe that WRS characteristics indicate that discovery of the gene and the metabolic pathway associated with this syndrome will most likely lead to new knowledge about the physiopathology of human aging.

Mislocalization of prelamin A Tyr646Phe mutant to the nuclear pore complex in human embryonic kidney 293 cells

Mature lamin A is formed after post-translational processing of prelamin A, which includes prenylation and carboxymethylation of cysteine 661 in the CaaX motif, followed by two proteolytic cleavages by zinc metalloprotease (ZMPSTE24). We expressed several prelamin A mutants, C661S (defective in prenylation), Y646F (designed to undergo prenylation but not second proteolytic cleavage), double mutant, Y646F/C661S and Y646X (mature lamin A), and the wild-type construct in human embryonic kidney (HEK-293) cells. Only the Y646F mutant co-localized with nuclear pore complex proteins, including Nup53 and Nup98, whereas the other mutants localized to the nuclear envelope rim. The cells expressing Y646F mutant also revealed abnormal nuclear morphology which was partially rescued with the farnesyl transferase inhibitors. These data suggest that the unprenylated prelamin A is not toxic to the cells. The toxicity of prenylated prelamin A may be due to its association and/or accumulation at the nuclear pore complex which could be partially reversed by farnesyl transferase inhibitors.

Human laminopathies: nuclei gone genetically awry

Few genes have generated as much recent interest as LMNA, LMNB1 and LMNB2, which encode the components of the nuclear lamina. Over 180 mutations in these genes are associated with at least 13 known diseases--the laminopathies. In particular, the study of LMNA, its products and the phenotypes that result from its mutation have provided important insights into subjects ranging from transcriptional regulation, the cell biology of the nuclear lamina and mechanisms of ageing. Recent studies have begun the difficult task of correlating the genotypes of laminopathies with their phenotypes, and potential therapeutic strategies using existing drugs, modified oligonucleotides and RNAi are showing real promise for the treatment of these diseases.

The laminopathies: a clinical review

The laminopathies are a diverse group of conditions caused by mutations in the LMNA gene (MIM*150330). LMNA encodes the nuclear envelope proteins lamin A and lamin C by utilization of an alternative splice site in exon 10. The human LMNA gene was identified in 1986 but it was another 13 years before it was found to be the causative gene for a disease, namely Emery Dreifuss muscular dystrophy. Since then, a further eight clearly defined phenotypes have been associated with LMNA mutations. The diversity of these phenotypes is striking with features such as premature ageing, axonal neuropathy, lipodystrophy and myopathy being seen. These phenotypes and the emerging genotype/phenotype correlations are the subject of this review.

A benign congenital myopathy in an inbred Samaritan family

We describe a novel form of myopathy in a mother and her two daughters from an inbred Samaritan family. The patients displayed severe neonatal hypotonia, lethargy and dysmorphic features. Motor milestones were delayed; however, the hypotonia and muscle weakness gradually improved during the first 2 years of life and independent walking was achieved by 18 months. The mother at the age of 23 years shows myopathic facies and minimal proximal weakness. Her intelligence is normal. Her muscle biopsy revealed central nuclei and disruption of the intermyofibrillary network with moth eaten and spiral fibers. Mutations in SMN, MTM1 and the myotonic dystrophy genes were excluded. We suggest this is a new benign form of congenital myopathy. Inheritance is probably autosomal recessive.

Laminopathies: multisystem dystrophy syndromes

Laminopathies are a heterogeneous group of genetic disorders due to abnormalities in type A lamins and can manifest varied clinical features affecting many organs including the skeletal and cardiac muscle, adipose tissue, nervous system, cutaneous tissue, and bone. Mutations in the gene encoding lamins A and C (LMNA) cause primary laminopathies, including various types of lipodystrophies, muscular dystrophies and progeroid syndromes, mandibuloacral dysplasia, dilated cardiomyopathies, and restrictive dermopathy. The secondary laminopathies are due to mutations in ZMPSTE24 gene which encodes for a zinc metalloproteinase involved in processing of prelamin A into mature lamin A and cause mandibuloacral dysplasia and restrictive dermopathy. Skin fibroblast cells from many patients with laminopathies show a range of abnormal nuclear morphology including bleb formation, honeycombing, and presence of multi-lobulated nuclei. The mechanisms by which mutations in LMNA gene cause multisystem dystrophy are an active area of current investigation. Further studies are needed to understand the underlying mechanisms of marked pleiotropy in laminopathies.