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Lamins comprise a group of structural proteins, which belong to the intermediate-filament super family of proteins. Lamins polymerize to form the nuclear lamina, a mesh-like layer of intermediate filaments attached to the inner membrane of the nuclear envelope - a structure which surrounds the nucleus of cells-. The nuclear lamina is situated between chromatin and the inner nuclear membrane therefore reinforcing the nuclear mechanical strength as well as maintaining its shape and structural integrity1. Lamins are also found within the nucleus and are thought to regulate several cellular processes including DNA synthesis, RNA transcription and chromatin organization1. Finally, lamins also are of imminent importance in the disassembly and reassembly of the nuclear envelope during cell division.
Lamins are encoded by three different genes, giving rise to a total of seven different proteins. These can be further subdivided into two distinct categories termed A- type and B- type lamins respectively. The A- type lamins i.e. lamin A, lamin C, lamin AÎ” 10 and lamin C2, result from the alternative splicing of the same gene, the LMNA gene, located on chromosome 12. The group of B- type lamins consists of lamin B1, encoded by the LMNB1 gene on chromosome 5, and lamins B2 and B3 which result from alternative splicing of the LMNB2 gene, located on chromosome 193.
A- type lamins differ from B- type lamins with respect to their function during cell division and their differential expression patterns in various cellular populations. A- type lamins become soluble during mitosis whereas B- type lamins remain attached to the nuclear lamina throughout cell division. Additionally, while B- type lamins are constitutively expressed in all human cells, the expression of A- type lamins is developmentally regulated and is restricted to terminally differentiated cells. Alternatively, lamin B3 is confined to spermatocytes. The expression of B- type lamins is necessary for cell survival, normal development and maintenance of nuclear integrity. On the other hand, the expression of A- type lamins is dependent on the stage of cell differentiation and is absent in embryonic or non-differentiated cells4. These variations in their expression patterns suggest that B- type lamins are fundamental building blocks of the nuclear lamina whereas A- type lamins have a more specialized role.
Recently, the interest in nuclear lamins and their function has been intensified, especially after the discovery of a number of mutations in the LMNA gene that are associated with several inherited disorders, termed laminopathies. These disorders include progeroid syndromes, muscular dystrophies, dilated cardiomyopathy, a type of Charcot-Marie- Tooth disease and many others. LMNA knock- out experiments in mice have shown that in the absence of both lamin A and lamin C expression developmental delay and cardiac muscle dystrophy, leading to death 4-6 weeks after birth, are observed. In contrast, mice lacking only the expression of lamin C appeared to be perfectly healthy.
Gene mutations affecting the function of B- type lamins have not been associated with any disease so far. This suggests that the loss of B- type lamins function is either non significant or lethal during development.
Here, we provide a recent review of the literature regarding the structure and function of the LMNA gene as well as a summary of congenital disorders associated with LMNA deficiency.
Î™Î™. THE LMNA GENE
The human lamin A/ C gene designated LMNA, encompasses 12 exons and spans a region of 25374 base pairs on the long (q) arm of chromosome 1. Specifically it is localized within the genomic region 1q21.2 - 1q21.3 between base pairs 154,351,121 and 154,376,494. The chromosomal position of the LMNA gene is schematically depicted in Fig. 1.
Alternative splicing of the LMNA gene gives rise to four different proteins2, 5. Lamin A and lamin C are the two main gene products in most differentiated cells. Lamin A is produced by the complete mRNA transcript of the LMNA gene. Initially, a prenylated precursor of lamin A is produced. This precursor molecule, known as prelamin A, consists of 664 amino acids and undergoes post- translational modifications to yield the mature form of lamin A. This mature form of lamin A consists of 646 amino acids and has a molecular weight of 70 kDa. Lamin C, an isoform of lamin A, consists of 572 amino acids and has a molecular weight of 65 kDa. Lamin A and lamin C differ only in their carboxy terminal domains. Lamin A has a unique carboxy terminal tail consisting of 98 amino acids, whereas lamin C has a shorter carboxy terminal tail composed of only 6 amino acids. In particular, lamin C protein lacks the amino acid residues encoded by part of exon 10 and both exons 11 and 12. Two additional products are also produced by alternative splicing of the LMNA gene. These are lamin Î‘Î”10 and lamin C2. Lamin Î‘Î”10 lacks all the amino acid residues encoded by exon 10 and it is expressed both in healthy and tumor cell lines6. Lamin C2 is only expressed in spermatocytes and is presumed to have an important role in spermatocyte chromatin organisation. It is interesting to note that only lamin A has been associated with disease development in mammals.
III. LAMIN STRUCTURE AND FUNCTION
Similar to all members of the intermediate filament protein family, lamins consist of three distinct domains: a globular amino- terminal head, a central rod like Î±- helical mid section domain consisting of four coiled coil areas and a globular carboxy- terminal tail. The four coiled coil regions are interconnected by linker segments7. A schematic representation of the structure of lamin proteins is shown in figure 2.
A- type lamins are characterized by the presence of a nuclear localization signal (NLS) within their carboxy- terminal domain which acts as the required tag for their nuclear import. This NLS tag ensures that A- type lamins are transported into the nucleus after their post- translational modifications in the endoplasmic reticulum. In addition, the carboxy- terminal domain of A- type lamins contains a chromatin-binding region5.
Interestingly both B- type lamins and lamin A (excluding lamin C) are the only members of the intermediate-filament protein super- family that possess the CAAX (Cysteine, Aliphatic amino acid, Aliphatic amino acid, Any amino acid) carboxy-terminal motif. The CAAX motif is the target for a sequence of post- translational modifications. Lamins are initially produced as prelamin molecules that contain the CAAX motif and are then altered through a complex series of post- translational modifications. These prelamin molecules are at first farnesylated in a process catalyzed by the enzyme farnesyl- transferase. Farnesylation includes the addition of three isoprene units at the SH group of the cysteine residue and the methylation of the free carboxyl group. Finally, the three amino acid residues (AAX) of the CAAX motif are proteolytically cleaved and the cysteine molecule undergoes methyl esterification.
These post- translational modifications are essential for the targeting of both B- type lamins and lamin A inside the nucleus as well as for their incorporation into the nuclear membrane8. As has been aforementioned, the lamin C protein does not contain the CAAX carboxy terminal motif. Therefore, the targeting of lamin C to the nucleus and its incorporation into the nuclear lamina depends solely on lamin A.
Once B- type lamins and lamin A are incorporated into the nuclear lamina, they undergo different processing. B- type lamins remain farnesylated for the rest of the cells' lifetime. In contrast, 15 additional amino acid residues are cleaved from Lamin A, upstream of the cysteine residue at the carboxy terminal domain9, 10. This cleavage, which yields the mature form of lamin A, is catalysed by the enzyme ZMPSTE24, a zing metalloproteinase. The same enzyme is also responsible for the cleavage of the amino acid residues of the CAAX motif mentioned before11.
Lamin filament organization
Contrary to the cytoplasmic intermediate- filament proteins, nuclear lamins are organised in a two dimensional structure. Lamins initially form dimers. It has not been clarified yet whether lamins form homo- or hetero- dimers, but recent evidence suggests that lamins form mostly homodimers consisting either of A- type or B- type lamins. Lamin dimer formation is mediated through the interaction between the coiled coil areas of the lamin rod domain. These dimers are then arranged in head- to- tail associations to form protofilaments which in turn form anti-parallel associations to yield lamin tetramers. Lamin tetramers are formed in such a way so that they have overlapping regions in both their carboxy and amino terminal domains. The final step of lamin polymerization is marked by the formation of either paracrystalline structures or filaments with a diameter of 10nm (intermediate filaments)12. A schematic representation of lamin organization into filaments is shown in figure 3.
Lamin behavior during cell cycle
During cell division, lamins provide support to the nuclear membrane and help maintaining the nuclear shape. This notion is supported by experiments in both LMNA knock-down cells and gene inactivation studies in mice. In particular, the nucleus of cells derived from LMNA deficient animals has been reported to be abnormally elongated and fragile13. Moreover, transfection of healthy cells with the mutant form of lamin was shown to interrupt lamin organization in both in vitro and in vivo experiments, resulting in nuclear shape alteration and reduced nucleus stability14.
In addition, nuclear lamins disassemble and then reassemble in every cell cycle. The nuclear lamina disassembly and reformation is controlled by lamin phosphorylation and dephosphorylation carried out by protein kinases12, 15. Nuclear lamins are phosphorylated during prophase. Lamin phosphorylation results in a conformational change in the lamin protein molecule resulting in filament disorganization and breakdown of the nuclear envelope. A- type lamins are solubilized during mitosis whereas B- type lamins remain associated to the nuclear lamina throughout cell division. Lamin de- phosphorylation during telophase results in filament reconstruction and reformation of the nuclear envelope12. Some researchers support that lamins are essential for the initiation of nuclear envelope assembly via polymer formation and interaction with other proteins16. Other researchers however support that lamins are not essential for the formation of the nuclear envelope but are only transferred there after the reassembly of the nuclear envelope 17.
Lamin involvement in DNA transcription
Nuclear lamins are also involved in mRNA synthesis. DNA trancription requires correct lamin organization since experiments involving cells lacking normal nucleus shape revealed that these cells fail to perform DNA transcription properly14, 18. Lamin involvement in DNA transcription could be limited to maintaining nuclear shape and allowing the accumulation of transcription factors inside the nucleus. Other groups support that lamin involvement in DNA transcription is more direct as they are purported to provide a scaffold for the accumulation of several proteins involved in transcription1. Lamins accumulate in nuclear areas where RNA polymerase II is active. Experiments have revealed that the removal of the amino-terminal domain of lamin A disregulates nuclear lamina organization, RNA polymerase II activity and hence transcription19. The regulation of expression of various genes is thought to be dependent on the affinity of lamins A/C for chromatin since the inactive genes seem to accumulate in the nuclear peripheral area20, 21.
Lamins and apoptosis
Nuclear lamins also have an important role in apoptosis, one form of programmed cell death. Apoptosis involves nuclear shrinking and chromatin condensation and/ or breakdown. During apoptosis, lamins are broken down by caspases22. Lamin breakdown compromises the integrity of the nuclear envelope as well as the association of lamins with chromatin thus contributing to the apoptotic process. This notion is also supported by experiments with cells in which only the mutant form of lamin A is expressed. As a result of the presence of this mutant form of lamin A in these cells causes a delay in the apoptotic pathway 23.
More than 200 mutations (278 mutations reported on the NCBI database website http://www.ncbi.nlm.nih.gov ) in the LMNA gene have been associated with the development of a spectrum of disorders collectively described as laminopathies. These disorders involve defects affecting striated muscle, adipose tissue distribution, peripheral nervous system as well as progeroid syndromes. Mutations resulting in the development of laminopathies have been identified in nearly all exons of the LMNA gene. These mutations include point, frame shift and deletion mutations. Interestingly, the phenotypic abnormalities caused by the same LMNA mutation as well as their clinical course, differs even among members of the same family. Therefore grouping of the different laminopathies into distinct categories is difficult as they exhibit many phenotypical similarities and may have overlapping clinical signs. It has hence been suggested that laminopathies may represent the same disease with a variation in the degree of penetrance 2.
The mechanism by which mutations in the LMNA gene affect nuclear function and cause disease has not yet been fully elucidated. The fact that lamins affect both the stability and the mechanical integrity of the nucleus has given rise to many different hypotheses regarding the pathogenic pathway of LMNA mutations24. The two main hypotheses are "the mechanical stress hypothesis" and "the gene expression hypothesis". The "mechanical stress hypothesis" states that the production of truncated lamin A, as a result of LMNA mutations, compromises the structural integrity of the nuclear lamina thus leading to increased susceptibility to cellular damage caused by mechanical stress. Regarding muscle cells, nucleus instability causes cell dissociation leading to cell death. The "gene expression mechanism" relates to the role of lamin proteins in tissue specific gene expression. According to this hypothesis mutations in the LMNA gene can deregulate tissue specific gene expression either directly or indirectly, through epigenetic modifications11. An overview of reported laminopathies, the responsible LMNA mutations, the resulting protein defect and a comparative overview of the clinical spectrum is shown in table I.
IV. Î‘. Laminopathies expressed as progeroid syndromes.
IV. Î‘. 1. Hutchinson- Gilford Progeria Syndrome
The Hutchinson- Gilford progeria syndrome (HGPS, OMIM #176670), also known as "progeria in childhood" is a rare genetic disorder characterized by accelerated and premature aging experienced by children in their early life25. Affected infants appear normal at birth but usually develop profound growth delay within the first year of their life. Diagnosis is usually confirmed by the age of two. The clinical phenotype of the disorder is characterized by severe growth retardation, skeletal alterations, progressive hair loss leading to alopecia and lipodystrophy (a redistribution of body fat). Particularly, a marked reduction of subcutaneous fat is observed. As the disease progresses affected individuals experience joint stiffness and progressive atherosclerosis whereas in the late stages of the disease patients have thin, wrinkled, brown spotted skin and suffer form hypertension as a result of atherosclerosis. Death occurs at a mean age of 13 years (ranging from 8- 20 years) usually from myocardial infarction or congestive heart failure, either one resulting from severe atherosclerosis.
The estimated incidence of HGPS is 1 in 8 million births. In the majority of incidents the index case is the only member of the family with the disease even within families with a large number of siblings. The disease is generally caused in an autosomal dominant fashion even though rare cases of autosomal recessive inheritance have been reported. These could either have resulted from a misdiagnosis of other diseases that clinically resemble HGPS or could be the result of mosaicism25.
The molecular basis of HGPS remained unclear until it was discovered to be the result of a heterozygous mutation in the LMNA gene and was therefore classified as a laminopathy26. The mutation causes a single base transition (cytosineâ†’ thymine) at position 1824 of exon 11 thus activating a cryptic splice donor site leading to an mRNA transcript short of 150 nucleotides. This results in the production of an aberrant protein known as progerin/ LAÎ”50, a molecule which lacks 50 amino acids near its carboxy terminal end. This faulty protein cannot undergo the normal post translational modifications (such as prelamin A) and therefore cannot be processed to its mature lamin A form. Progerin/ LAÎ”50 is initially farnesylated just like prelamin A, but because it lacks the proteolytic cleavage site which removes the CAAX motif, it remains permanently farnesylated thereby blocking the rest of the process. The normal process of prelamin A maturation and how this is disturbed in Hutchinson- Gilford progeria syndrome is shown in Figure 4.
Accumulation of progerin/ LAÎ”50 within cell nuclei causes progressive alterations of nuclear architecture (nuclear lamina enlargement, loss of peripheral heterochromatin and nuclear pore aggregations). Moreover, when healthy fibroblast cells are transfected with progerin, their nuclei progressively resemble those of fibroblast cells from HGPS patients27. Progerin/ LAÎ”50 expression in cells also results in genome instability, faulty DNA repairing and progressively altered post translational modifications (i.e. histone methylation)28.
Î™V. Î‘. 2. Restrictive dermopathy
Restrictive dermopathy (RD, OMIM # 275210) is another premature aging syndrome. It is a lethal rarely occurring disorder with a phenotype that resembles HGPS phenotype but is much more severe. It is characterized by intrauterine growth retardation, tight and rigid skin, prominent superficial vessels, characteristic facial features (small face, micrognathism), joint contractures and premature neonatal death, within the first week of life. In most cases there is premature delivery as a result of reduced or no fetal mobility7, 29.
RD is associated with mutations in two genes: the LMNA gene and the ZMPSTE24 gene, also known as FACE- 1 in humans29. This gene encodes an enzyme responsible for the correct posttranslational processing of prelamin A. Mutations in the LMNA gene are mostly splicing mutations causing partial or total absence of exon 11, resulting in the production of an abnormal prelamin A protein molecule which cannot be properly post- translationally processed to the mature lamin A form.
Mutations in ZMPSTE24 have been identified in RD patients displaying no gene defects in LMNA. All patients investigated were found to carry single mutations which cause the insertion of a thymine nucleotide (c1085_1086insT) in a specific region of exon 9 of the ZMPSTE24 gene. This mutation introduces a premature termination codon for the gene's transcription30. These patients lack both the mature form of lamin A and the ZMPSTE24 enzyme. All ZMPSTE24 mutations identified were found to be heterozygous and not sufficient to cause the phenotype of RD by themselves (i.e. complete absence of ZMPSTE24 activity). This suggested the existence of an additional molecular defect in these patients. A further study revealed that the complete absence of ZMPSTE24 activity can either result from the presence of the above mentioned ZMPSTE24 mutation in either a homozygous state (for seven out of 10 patients with RD) or in a compound heterozygous state with a mutation in the second ZMPSTE24 allele (for 3 of the 10 patients with RD). These findings indicate that RD can either be considered as a primary laminopathy resulting from dominant LMNA mutations or as a secondary laminopathy caused by recessive null ZMPSTE24 mutations. In both cases there is accumulation of either normal or truncated prelamin A causing morphological and stability related alterations of the affected cells.
Î™V. Î‘. 3. Atypical Werner syndrome
Werner syndrome (WS, OMIM # 277700) is another greatly rare disorder characterized by premature aging. It is caused by mutations in the WRN gene which encodes a multifunctional nuclear protein, possessing both helicase and exonuclease function, belonging to the RECQ family of DNA helicases. Symptoms typically manifest after the first decade of life and diagnosis is confirmed later, in adult life. The main clinical features include sclerodermal skin changes, hypogonadism, bilateral cataract, type 2 diabetes mellitus, short stature, prematurely aged face and premature appearance of grey hair. A large group of patients reported with WS do not have any WRN mutation and are therefore designated as 'atypical' (non WRN) Werner patients.
Due to the fact that the phenotype of the disease is similar to phenotypes presented by some laminopathies, it has been investigated whether LMNA mutations are present in a group of atypical WS patients31. Four out of ten patients with 'atypical' WS were found to be heterozygous for three de novo missense mutations in the LMNA gene. These mutations are R133L, L140R and A57P and they all cause a base substitution resulting in an amino acid change in the encoded protein. All LMNA mutations associated with the 'atypical' WS are dominant and cause a more severe phenotype than the classic WS.
IV. A. 4. Therapeutic approach for the progeroid syndromes
To date there is no cure for the progeroid syndromes but the research towards this end has revealed some hopeful results. The most promising therapeutic approach involves the use of farnesyl transferase inhibitors (FTI's). The use of FTI's on fibroblasts of HGPS patients prevents the farnesylation of prelamin A and the accumulation of progerin in the nucleus of these cells thus reversing the nuclear morphological changes caused by this accumulation32.
An alternative therapeutic approach involves gene therapy. Using gene therapy techniques Fong et al (2006) investigated the interplay between prelamin A, lamin A and lamin C. It was mad apparent by the results that prelamin A and lamin A were not involved in the pathology of progeroid syndromes since mutant mice (ZMPSTE24-/-) seemed to recover after an LMNALCO copy was introduced. This is indicative of the fact that even a single copy of lamin C is sufficient to correct the disease's phenotype without lamin A presence is not required33.
Furthermore, in the case of HGPS fibroblasts it could be demonstrated that the accumulation of progerin / LAÎ”50 and a reversal of the pathologic cellular phenotype could be achieved with the use of a modified oligonucleotide. This modified oligonucleotide prevents the activation of the cryptic splice site involved in HGPS34.
IV. B. Laminopathies expressed with muscular defects
Î™V. Î’. 1. Emery- Dreifuss Muscular Dystrophy (EDMD)
Emery- Dreifuss muscular dystrophy is a rare disorder affecting both skeletal and cardiac muscle. It is characterized by early muscle and joint contractures, progressive weakness and waste of humeroperoneal muscles and cardiomyopathy with conduction defects which is the most serious and life threatening clinical manifestation of the disease7, 35. EDMD was initially described as an X- linked disorder but autosomal dominant and autosomal recessive traits of inheritance were also described.
The X- linked form of the disease (EDMD1, OMIM #310300) is associated with mutations in the EMD gene which encodes for the protein emerin, a building component of the nuclear membrane. The EMD gene is located on the X- chromosome at position q 28. EMD gene mutations associated with EDMD involve non- sense, missense, frame shift and splicing mutations resulting in partial or total emerin loss.
Linkage analysis studies in a large French family mapped the locus associated with autosomal dominant EDMD (EDMD2, OMIM # 181350) to the region 1q11-1q23 of chromosome 1, the same area encompassing the LMNA gene. In the same pedigree four LMNA mutations were identified to be co- segregating with the disease phenotype. These mutations (one non- sense and three missense mutations) result in one amino acid substitution in the highly conserved regions of the rod domain and the carboxy- terminal domain of the lamin protein. These alterations affect the polymerization process of lamins thus disturbing the nuclear lamina's architecture35. Additional LMNA mutations causing EDMD phenotypes were identified by the same research group. These were distributed along the full length of the LMNA gene resulting in a variety of phenotypes, which differ even among members of the same family36. The locations of the EDMD related mutations identified in the LMNA gene are illustrated in figure 5. Furthermore, the same group reported the first case of autosomal recessive inheritance relating to the disease. The affected patient was found to be homozygous for the C644T mutation causing an amino acid substitution at position 622 (Î-622Î¥). This was the only mutation in the LMNA gene identified in this patient and was the result of consanguinity since his parents were first cousins and heterozygous carriers of the same mutation36.
IV. B. 2 Limb Girdle Muscular Dystrophy- type 1B (LGMD1B)
Limb Girdle muscular dystrophies represent a genetically heterogeneous group of muscular disorders characterized by progressive proximal muscle weakness and waste. These disorders can be related to autosomal dominant and autosomal recessive mode of inheritance.
LGMD1B (OMIM # 159001) is a form of the disease inherited in an autosomal dominant manner. In addition to the phenotypic characteristics typical for LGMD, this form of the disease is further characterized by age related atrioventricular cardiac conduction defects and dilated cardiomyopathy but without early onset muscle contractures. The genetic locus related to LGMD1B was mapped to chromosome 1, in the region 1q11-1q21. This suggested that LGMD1B and EDMD are both allelic disorders37.
Three mutations in the LMNA gene have been associated with LGMD1B in three different families. The first mutation causes an in frame deletion within exon 3 (delK208), the second mutation is a missense mutation in exon 6 (R377H) and the third mutation causes a transversion in the consensus splice donor site of intron 9. The first two mutations mentioned above affect amino acids in the highly conserved areas of the rod domain of nuclear lamins and therefore affect both lamin dimerization and the normal assembly of the nuclear lamina. The third mutation gives rise to a truncated lamin molecule lacking half of the globular tail domain of lamins A/C37.
IV. B. 3. Dilated Cardiomyopathy with Conduction Defects (DCM- CD)
Dilated cardiomyopathy is a cardiac muscle defect characterized by dilatation of the cardiac muscle, reduced systolic contraction and arrhythmias progressively leading to reduced cardiac function and cardiac failure. Since cardiac dilatation is a widely expressed symptom in several laminopathies the LMNA gene was suspected to carry mutations associated with the DCM-CD phenotype.
Five LMNA mutations associated with disease development were identified in patients from 11 different families38. One of these mutations causes an amino acid substitution Arg571Ser within the carboxy- terminal of lamin C causing a milder phenotype. The remaining four mutations cause an amino acid substitution either in exon 1 (Arg60Gly, Leu85Arg) or in exon 3 (Asn195Lys, Gln203Gly), thus affecting the protein's rod domain. These mutations and their location within the LMNA gene are depicted in figure 6. Patients with dilated cardiomyopathy carrying mutations in the LMNA gene have a much more severe phenotype than patients with dilated cardiomyopathy without any mutations in the LMNA gene.
Another DCM- CD linked LMNA mutation was identified in a female patient with familial DCM-CD. This mutation causes a base pair deletion at positions 908 and 909 of exon 5 in the LMNA gene (c908_909delCT) and is predicted to disrupt the reading frame leading to the production of truncated proteins (lamin A and lamin C)39.
A different mutation within the LMNA gene was found to be common amongst Finnish DCM patients. DCM patients from six different Finnish families were found to carry a novel mutation (S543P) which compromises the integrity of the nuclear envelope. The mutation was found in all DCM patients from the six different families but not in the healthy population that was screened. This excluded the speculation of a common polymorphism in the Finnish population and combined with the results from haplotype analysis suggested that this is a founder mutation40.
IV. C. Laminopathies expressed as lipodystrophies
Î™V. C. 1. Familial Partial Lipodystrophy Dunnigan type (FPLD)
The lipodystrophies comprise a group of disorders characterized by comparable phenotypes which result from different genotypes. Familial partial type II lipodystrophy (OMIM #151660), also known as Dunnigan type lipodystrophy is characterized by the absence or reduction of subcutaneous adipose tissue of the body and the extremities while the excess fat can be deposited within their face and neck. FPLD patients might also develop profound insulin resistance leading to diabetes mellitus. Dyslipidaemia and coronary heart disease can also be observed. FPLD is a rare autosomal dominant disease and has been associated with mutations in the LMNA gene.
The most frequent FPLD- linked LMNA mutation was characterized in 5 different Canadian families. All patients were found to be heterozygous for the mutation R482Q. This mutation is caused by a guanine to adenosine transition within exon 8 of the LMNA gene resulting in an amino acid substitution41. The mutation appears in all FPLD subjects thus suggesting the possibility of founder effect for this mutation in Canadian FLPD subjects. The presence of a common haplotype in all affected subjects further supports the theory of common ancestry for all these patients.
Linkage of the R482Q mutation with FLPD was confirmed by another study with FLPD subjects from 10 different families42. In addition two more missense mutations in codon 482 within exon 8 were found to be associated with FLPD. The most frequently encountered mutation results from the substitution of the arginine residue with a tryptophan residue (R482W). The second mutation results from the substitution of the arginine residue with an asparagine residue (R482L). Two more FLPD linked mutations were characterized at position 486 resulting in the substitution of the lysine residue at position 486 with an asparagine residue42. These mutations are present in all FLPD individuals but are absent in healthy control subjects.
All the above mentioned mutations cause a substitution of a highly conserved positively charged amino acid residue in the lamin protein molecule. Even though these mutations bring about an alteration in the overall protein charge they do not appear to affect the protein's structure, its nuclear localization nor its ability to interact with emerin and other lamin protein molecules. It is more likely that the pathogenic mechanism of these mutations involves reducing the ability of lamin interaction with adipocyte- specific nuclear proteins43.
FPLD families with other mutations have also been reported. For example, one family was found to have the R582H mutation in exon 11 of the LMNA gene. This mutation only affects the structure of lamin A thus causing a less severe disease phenotype44.
Î™V. C. 2. Mandibuloacral dysplasia
Mandibuloacral Dysplasia (MAD, OMIM # 248370) is a rarely occurring autosomal recessive syndrome that has been linked to mutations in LMNA. It is characterized by postnatal growth retardation, joint contractures resulting in craniofacial anomalies, mandibular and clavicular hypoplasia, acroosteolysis, delayed closure of cranial suture as well as partial lipodystrophy associated with insulin resistance and diabetes mellitus. Since patients with MAD often have symptoms resembling those of FLPD subjects, it was assumed that MAD could be the result of mutations in the LMNA gene.
Linkage of MAD to the LMNA locus and subsequently a mutation in LMNA associated with the disease were identified in patients from five consanguineous families. This mutation causes a guanine to adenosine transition at position 1580 of exon 2 of the LMNA gene resulting in the substitution of the arginine residue at position 527 of the protein with a histidine residue. The mutation was found in a homozygous state in all patients examined while their parents were asymptomatic heterozygotes of the exact same mutation45. The arginine amino acid residue at position 527 is located in the carboxy -terminal domain, common to lamin A and lamin C, and thus its substitution would change the surface structure of the protein resulting in the alteration of the proteins binding fundamental sites. As a result the correct assembly of nuclear lamina is disturbed leading to disease phenotype.
Due to the fact that the mutation was detected in individuals (patients and asymptomatic carriers) originating from a sparsely populated area in central Italy but not in any of the healthy individuals screened it was presumed to be a founder mutation for MAD in the Italian population.
Î™V. D. Laminopathies expressed as neuropathies
Î™V. D. 1. Charcot- Marie- Tooth disease type 2B1 (CMT2B1)
Charcot-Marie-Tooth (CMT) disorders are a group of clinically and genetically heterogeneous motor and sensory neuropathies. They are the most commonly occurring hereditary disorders of the peripheral nervous system affecting 1 in 2500 individuals. Clinically, CMT is characterized by progressive muscular weakness and waste, foot deformities as well as sensory loss at distal extremities. CMT disorders are subdivided into two distinct groups based on histopathological and electrophysiological data: demyelinating CMT (CMT-1) and axonal (non- demyelinating) CMT (CMT-2).
Axonal CMT (CMT-2) follows autosomal recessive inheritance and three genetic loci have been linked to three different disease subtypes. The CMT2Î’1 subtype (OMIM #605588) is an autosomal recessive axonal CMT subtype that has been linked with LMNA mutations as the genetic locus associated with it has been mapped to chromosome 1 (1q21.2- 1q21.3) in a large Moroccan family46.
The first LMNA mutation that was associated to CMT2B1 subtype was characterized in three consanguineous Algerian families that were affected with autosomal recessive CMT2. This mutation causes a cytosine to thymine transition at position 892 in exon 5 of the LMNA gene, resulting in an amino acid substitution at position 298 (R298C) of the rod domain of lamins A/ C. The mutation therefore affects all known lamin isoforms resulting from the LMNA gene47.
This mutation was proven to co- segregate with the disease and has only been found in affected individuals originating from a restricted region in North Western Africa. These data indicate the possibility of a founder effect for this mutation. This notion is also supported by recent results confirming the presence of the same mutation, in a homozygous state, in patients from another Moroccan family also affected with the CMT2Î’1 subtype48. The founder effect hypothesis was finally confirmed by another study in which researchers used SNP and STR detection analysis in CMT2B1 affected families also originating from the same region of North Western Africa. These results confirmed that all affected individuals share a common ancestral haplotype in a region of about 1.0 Mb (1 cM) and that the most recent common ancestor would have lived about 800-900 years ago49.
From all the aforementioned information it is evident that nuclear lamins have a fundamental role regarding physiological cell function. They are involved in processes implicated both with the integrity and stability of the cells' nuclei as well as with other cells' processes which are indispensable for cellular preservation. It is also apparent that any mutation within the LMNA gene, causing reduced or altered lamin expression, leads to a plethora of diseases characterized by phenotypic heterogeneity. It is a very rare phenomenon that mutations in a single gene lead to such a vast range of diseases.
The wide range of symptoms and phenotypes caused by mutations in the LMNA gene suggests that each one of the different mutations affects a specific area of the lamin protein molecule, thus compromising the integrity and survival of the cells in the affected tissue.
Even though laminopathies are caused by mutations in the same gene, a different pathogenic mechanism underlies each one of them. In consequence by studying the differences between diseases we may deduct which function of the protein is affected by each mutation and by which mechanism this leads to disease development. Moreover, it should be examined whether mutations in the LMNA gene are not the direct cause of disease themselves, but lead to a disease by affecting the expression or function of other proteins.
In addition, the study and discovery of additional molecules which interact with lamin molecules and affect their function should be identified. The probability of involving such molecules towards the development of therapeutic approaches for laminopathies could be one of the major objects of future research approaches.
Finally, after identifying the disease causing mechanism underlying each disease, the probability of developing specific therapeutic approaches for each and every one of the laminopathies is also a challenging future research area. The therapeutic approaches to be developed should be based on the disease causing mechanism.
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