The question of heterogeneity in Marfan syndrome

Mutation screening in the FBN1 gene responsible for Marfan syndrome and related disorder in Chinese families

Background

Previous studies showed that the fibrillin‐1 gene (FBN1) is responsible for Marfan sydrome (MFS) pathogenesis. This study is conducted to screen for mutations in the FBN1 gene in Chinese families with MFS.

Methods

Eight families with MFS and related disorder were recruited in this study. All available family members underwent complete physical, ophthalmic, and cardiovascular examination. Mutation screening was performed using targeted next‐generation sequencing. Candidate variants were amplified by polymerase chain reaction and verified by direct Sanger sequencing.

Results

Four novel heterozygous mutations in FBN1, including c.2861G>T (p.R954L), c.4087G>A (p.D1363N), c.4987T>G (p.C1663G), and c.5032T>G (p.Y1678D), as well as four known mutations, c.3617G>A (p.G1206D), c.4460A>G (p.D1487G), c.4588C>T (p.R1530C), and c.718C>T (p.R240C) were identified. Affected patients from each family were found to carry one of the mutations, whereas the unaffected members and 1,086 normal controls were not. Each mutation was found to be cosegregated with MFS phenotype and related disorder in each family. Multiple sequence alignment of the human fibrillin‐1 protein showed that these mutations occurred in a highly conserved region among different species.

Conclusions

Eight FBN1 mutations were identified in Chinese families with MFS and related disorder. These data expands FBN1 mutation spectrum and further emphasizes the role of FBN1 in the pathogenesis of MFS.

A Decade of Discovery in the Genetic Understanding of Thoracic Aortic Disease

Aortic aneurysms are responsible for a significant number of all deaths in Western countries. In this review we provide a perspective on the important progress made over the past decade in the understanding of the genetics of this condition, with an emphasis on the more frequent forms of vascular smooth muscle and transforming growth factor β (TGF-β) signalling alterations. For several nonsyndromic and syndromic forms of thoracic aortic disease, a genetic basis has now been identified, with 3 main pathomechanisms that have emerged: perturbation of the TGF-β signalling pathway, disruption of the vascular smooth muscle cell (VSMC) contractile apparatus, and impairment of extracellular matrix synthesis. Because smooth muscle cells and proteins of the extracellular matrix directly regulate TGF-β signalling, this latter pathway emerges as a key component of thoracic aortic disease initiation and progression. These discoveries have revolutionized our understanding of thoracic aortic disease and provided inroads toward gene-specific stratification of treatment. Last, we outline how these genetic findings are translated into novel pharmaceutical approaches for thoracic aortic disease.

Newly developed aortic dissection after aorta cannulation during mitral valve surgery in a patient with marfan syndrome

We report a case of newly developed aortic dissection after aorta cannulation during mitral valve surgery in a patient with Marfan syndrome. An unexpected fatal complication of cardiac surgery detected on postoperative imaging survey in Marfan syndrome patient and its surgical finding are described.

Critical appraisal of the revised Ghent criteria for diagnosis of Marfan syndrome

Marfan syndrome (MFS) is a connective tissue disorder with major features in cardiovascular, ocular and skeletal systems. Recently, diagnostic criteria were revised where more weight was given to the aortic root dilatation. We applied the revised Marfan nosology in an established adult Marfan population to define practical repercussions of novel criteria for clinical practice and individual patients. Out of 180 MFS patients, in 91% (n = 164) the diagnosis of MFS remained. Out of 16 patients with rejected diagnosis, four patients were diagnosed as MASS (myopia, mitral valve prolapse, borderline non-progressive aortic root dilatation, skeletal findings and striae) phenotype, three as ectopia lentis syndrome and in nine patients no alternative diagnosis was established. In 13 patients, the diagnosis was rejected because the Z-score of the aortic root was <2, although the aortic diameter was larger than 40 mm in six of them. In three other patients, the diagnosis of MFS was rejected because dural ectasia was given less weight in the revised nosology. Following the revised Marfan nosology, the diagnosis of MFS was rejected in 9% of patients, mostly because of the absence of aortic root dilatation defined as Z-score ≥2. Currently used Z-scores seem to underestimate aortic root dilatation, especially in patients with large body surface area (BSA). We recommend re-evaluation of criteria for aortic root involvement in adult patients with a suspected diagnosis of MFS.

Factors influencing prognosis in patients with marfan syndrome after aortic surgery

OBJECTIVE

Aortic aneurysm formation leading eventually to aortic rupture or dissection in early adult life is a fatal outcome of Marfan syndrome (MFS). Advances in the treatment of the syndrome have improved prognosis, but the long-term reoperation rate is still high. It remains unknown which factors influence the long-term prognosis, including the reoperation and mortality rates, in surgically treated Chinese patients with MFS. The authors studied 125 such patients to investigate factors influencing prognosis after aortic surgery.

DESIGN

A retrospective clinical investigation.

SETTING

An academic medical center.

SUBJECTS

One hundred twenty-five Marfan patients who had undergone aortic surgery.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The indications for aortic surgery were aortic aneurysm and/or dissection in the 125 Marfan patients. The most commonly performed procedure was the Bentall in 92 patients. Sixteen patients underwent total arch replacement combined with stented elephant trunk implantation. Ten patients underwent the David procedure. Overall in-hospital and 30-day mortality rate was 1.6%. The survival rate was 97.5%, 91.4%, and 74.2% at 1, 5, and 10 years after surgery, respectively. The reoperation rate was 2.5%, 12.9%, and 32.9% at 1, 5, and 10 years after surgery, respectively. Multivariate analysis revealed that increased systolic blood pressure (Sys BP) was the predictor of death (p < 0.05), and body mass index and smoking were significant predictors of reoperation (p < 0.05).

CONCLUSIONS

The present findings report the factors influencing the prognosis of Chinese patients with MFS after aortic surgical procedures. Managing these risk factors may enable health care professionals to improve the prognosis of MFS patients after aortic surgical procedures.

Genetic testing in aortic aneurysm disease: PRO

Thoracic aortic aneurysms leading to type A dissections (TAAD) are the major diseases affecting the aorta. A genetic predisposition for TAAD can occur as part of a genetic syndrome. It can be inherited in an autosomal dominant manner with decreased penetrance and variable expression. Genetic heterogeneity for familial TAAD has been demonstrated with the identification of four genes leading to TAAD. Genetic testing for TAAD and the phenotype and management of patients harboring mutations in these genes are addressed in this article.

Marfan syndrome. Part 1: pathophysiology and diagnosis

Marfan syndrome is a connective-tissue disease inherited in an autosomal dominant manner and caused mainly by mutations in the gene FBN1. This gene encodes fibrillin-1, a glycoprotein that is the main constituent of the microfibrils of the extracellular matrix. Most mutations are unique and affect a single amino acid of the protein. Reduced or abnormal fibrillin-1 leads to tissue weakness, increased transforming growth factor beta signaling, loss of cell-matrix interactions, and, finally, to the different phenotypic manifestations of Marfan syndrome. Since the description of FBN1 as the gene affected in patients with this disorder, great advances have been made in the understanding of its pathogenesis. The development of several mouse models has also been crucial to our increased understanding of this disease, which is likely to change the treatment and the prognosis of patients in the coming years. Among the many different clinical manifestations of Marfan syndrome, cardiovascular involvement deserves special consideration, owing to its impact on prognosis. However, the diagnosis of patients with Marfan syndrome should be made according to Ghent criteria and requires a comprehensive clinical assessment of multiple organ systems. Genetic testing can be useful in the diagnosis of selected cases.

Transforming growth factor-beta signaling in thoracic aortic aneurysm development: a paradox in pathogenesis

Thoracic aortic aneurysms (TAAs) are potentially devastating, and due to their asymptomatic behavior, pose a serious health risk characterized by the lack of medical treatment options and high rates of surgical morbidity and mortality. Independent of the inciting stimuli (biochemical/mechanical), TAA development proceeds by a multifactorial process influenced by both cellular and extracellular mechanisms, resulting in alterations of the structure and composition of the vascular extracellular matrix (ECM). While the role of enhanced ECM proteolysis in TAA formation remains undisputed, little attention has been focused on the upstream signaling events that drive the remodeling process. Recent evidence highlighting the dysregulation of transforming growth factor-beta (TGF-beta) signaling in ascending TAAs from Marfan syndrome patients has stimulated an interest in this intracellular signaling pathway. However, paradoxical discoveries have implicated both enhanced TGF-beta signaling and loss of function TGF-beta receptor mutations, in aneurysm formation; obfuscating a clear functional role for TGF-beta in aneurysm development. In an effort to elucidate this subject, TGF-beta signaling and its role in vascular remodeling and pathology will be reviewed, with the aim of identifying potential mechanisms of how TGF-beta signaling may contribute to the formation and progression of TAA.

The roles of two novel FBN1 gene mutations in the genotype-phenotype correlations of Marfan syndrome and ectopia lentis patients with marfanoid habitus

Mutations in the fibrillin-1 (FBN1) gene have been identified in patients with Marfan syndrome (MFS) and Marfan-like connective tissue disorders. In this study, two Chinese families were recruited. The patients in family 1 were well characterized with MFS, while those in family 2 displayed Marfan-like disorders such as ectopia lentis (EL) and marfanoid habitus, but did not develop cardiovascular diseases. We aimed to analyze the pathogenic mutations and their relationships with phenotypes in these two Chinese families. All participants underwent complete physical, ophthalmic, and cardiovascular examinations. The 65 exons and flanking intronic sequences of FBN1 were amplified by polymerase chain reaction, and screened for mutations by denaturing high-performance liquid chromatography and sequencing. One hundred and fifteen unrelated controls were analyzed using the same methods to confirm the mutations. In family 1, we identified the mutation p.C499S in the calcium-binding epidermal growth factor (cbEGF)-like domain 3 of FBN1. In family 2, the mutation p.C908Y was identified in an interdomain region of the hybrid motif 2 linked to the cbEGF-like domain 10. It can be concluded that FBN1 mutations involving cysteine substitutions are usually associated with MFS and EL with some MFS features. Moreover, pathology seemed more serious when the mutations disrupted the three disulfide bridges in the cbEGF-like domains, which was more likely to cause typical MFS than if the mutations occurred in the hybrid motifs. Our data preliminarily establish a genotype-phenotype correlation in the diagnostic process of MFS and predominant EL with Marfan-like features.

A comparison of genetic chromosomal loci for intracranial, thoracic aortic, and abdominal aortic aneurysms in search of common genetic risk factors

BACKGROUND

Genetic factors are likely to be involved in the pathogenesis of intracranial, ascending thoracic aorta, and infrarenal aortic abdominal aneurysms. Common genetic risk factors for these three types of aneurysms have been suggested. This review describes the results of whole-genome linkage studies on intracranial, thoracic aorta, and aortic abdominal aneurysms, and compares the genomic loci identified in these studies in search of possible common genetic risk factors for the three aneurysmal types.

METHODS

A literature search of all whole-genome linkage studies performed on intracranial, thoracic aorta, and aortic abdominal aneurysms was performed. The genomic loci identified in these studies were described and compared in search of similarities between them.

RESULTS

Five chromosomal regions on 3p24-25, 4q32-34, 5q, 11q24, and 19q that may play a role in the pathogenesis of two or more aneurysmal types were identified: 3p24-25 for thoracic aorta and intracranial aneurysms; 4q32-34 for aortic abdominal and intracranial aneurysms; 5q for thoracic aorta and intracranial aneurysms; 11q24 for thoracic aorta, aortic abdominal, and intracranial aneurysms; and 19q for aortic abdominal and intracranial aneurysms.

CONCLUSIONS

Five chromosomal regions that may include common genetic factors for intracranial, thoracic aorta, and aortic abdominal aneurysms were identified. Further studies are needed to explore these chromosomal regions in different aneurysm patient groups and may further help to unravel the disease pathogenesis of aneurysms in general.

Severe aortic and arterial aneurysms associated with a TGFBR2 mutation

BACKGROUND

A 24-year-old man presented with previously diagnosed Marfan's syndrome. Since the age of 9 years, he had undergone eight cardiovascular procedures to treat rapidly progressive aneurysms, dissection and tortuous vascular disease involving the aortic root and arch, the thoracoabdominal aorta, and brachiocephalic, vertebral, internal thoracic and superior mesenteric arteries. Throughout this extensive series of cardiovascular surgical repairs, he recovered without stroke, paraplegia or renal impairment.

INVESTIGATIONS

CT scans, arteriogram, genetic mutation screening of transforming growth factor beta receptors 1 and 2.

DIAGNOSIS

Diffuse and rapidly progressing vascular disease in a patient who met the diagnostic criteria for Marfan's syndrome, but was later rediagnosed with Loeys-Dietz syndrome. Genetic testing also revealed a de novo mutation in transforming growth factor beta receptor 2.

MANAGEMENT

Regular cardiovascular surveillance for aneurysms and dissections, and aggressive surgical treatment of vascular disease.

Genetic Basis of Thoracic Aortic Aneurysms and Dissections: Potential Relevance to Abdominal Aortic Aneurysms

Ascending thoracic aortic aneurysms leading to type A dissections (TAAD) have long been known to occur in association with a genetic syndrome such as Marfan syndrome (MFS). More recently, TAAD has also been demonstrated to occur as an autosomal dominant disorder in the absence of syndromic features, termed familial TAAD. Familial TAAD demonstrates genetic heterogeneity, and linkage studies have identified TAAD loci at 5q13-14 (TAAD1), 11q23 (FAA1), 3p24-25 (TAAD2), and 16p12.2-13.13. The genetic heterogeneity of TAAD is reflected by variation in disease in terms of the age of onset, progression, penetrance, and association with additional cardiac and vascular features. The underlying genetic heterogeneity of TAAD is reflected in the phenotypic variation associated with familial TAAD with respect to age of onset, progression, penetrance, and association with additional cardiac and vascular features. Mutations in the TGFBR2 gene have been identified as the cause of disease linked to the 3p24-25 locus, implicating dysregulation of TGF-beta signaling in TAAD. Mutations in myosin heavy chain (MYH11), a smooth muscle cell-specific contractile protein, have been identified in familial TAAD associated with patent ductus arteriosus (PDA) linked to 16p12.2-12.13. The identification of these novel disease pathways has led to new directions for future research addressing the pathology and treatment of TAAD.

Recent progress in genetics of Marfan syndrome and Marfan-associated disorders

Marfan syndrome (MFS, OMIM #154700) is a hereditary connective tissue disorder, clinically presenting with cardinal features of skeletal, ocular, and cardiovascular systems. In classical MFS, changes in connective tissue integrity can be explained by defects in fibrillin-1, a major component of extracellular microfibrils. However, some of the clinical manifestations of MFS cannot be explained by mechanical properties alone. Recent studies manipulating mouse Fbn1 have provided new insights into the molecular pathogenesis of MFS. Dysregulation of transforming growth factor beta (TGFbeta) signaling in lung, mitral valve and aortic tissues has been implicated in mouse models of MFS. TGFBR2 and TGFBR1 mutations were identified in a subset of patients with MFS (MFS2, OMIM #154705) and other MFS-related disorders, including Loeys-Dietz syndrome (LDS, #OMIM 609192) and familial thoracic aortic aneurysms and dissections (TAAD2, #OMIM 608987). These data indicate that genetic heterogeneity exists in MFS and its related conditions and that regulation of TGFbeta signaling plays a significant role in these disorders.

Genetic basis of thoracic aortic aneurysms and aortic dissections

Ascending thoracic aortic aneurysms leading to type A dissections (TAAD) can occur in association with a genetic syndrome, such as Marfan syndrome (MFS), or as an autosomal dominant disorder in the absence of syndromic features, termed familial TAAD. Familial TAAD demonstrates genetic heterogeneity, and linkage studies have identified three TAAD loci at 5q13-14 (TAAD1), 11q23 (FAA1), and 3p24-25 (TAAD2). The underlying genetic heterogeneity of TAAD is reflected in the phenotypic variation associated with familial TAAD with respect to age of onset, progression, penetrance, and association with additional cardiac and vascular features. Recently, mutations in the TGFBR2 gene have been identified as the cause of disease linked to the TAAD2 locus, supporting the hypothesis that dysregulation of TGFbeta signaling is a mechanism leading to aneurysms and dissections. The recent identification of the TGFbeta pathway as a key target in the molecular pathogenesis of TAAD has opened new avenues for future genetic and therapeutic research.

Genotype-phenotype correlation in congenital heart disease

PURPOSE OF REVIEW

The understanding of the etiology of congenital cardiac lesions is rapidly progressing from the recognition of embryologic origins to insight into the genetic basis for these disorders. Concurrently, in this era, great effort is being expended to gather data that will generate clinically useful genotype-phenotype correlation. This rapidly evolving area of inquiry, in which the clinical implications of mutation status are fully explored, makes available information applicable to those involved in all aspects of congenital cardiac disease.

RECENT FINDINGS

Three syndromes with cardiovascular phenotypes were selected for review. Each has received a great deal of attention in the recent past based on improved understanding of the range of mutations expressed and the relation of these mutations to clinical findings. These three syndromes--Noonan, Marfan, and long QT syndrome--span the range of congenital heart disease and provide examples of genotype-phenotype correlation.

SUMMARY

Better understanding of the clinical implications of specific mutations should allow not only for more sensitive and specific diagnoses to be made but also for improvements in therapeutic options and efficacy.

Evidence for a critical contribution of haploinsufficiency in the complex pathogenesis of Marfan syndrome

Marfan syndrome is a connective tissue disorder caused by mutations in the gene encoding fibrillin-1 (FBN1). A dominant-negative mechanism has been inferred based upon dominant inheritance, mulitimerization of monomers to form microfibrils, and the dramatic paucity of matrix-incorporated fibrillin-1 seen in heterozygous patient samples. Yeast artificial chromosome-based transgenesis was used to overexpress a disease-associated mutant form of human fibrillin-1 (C1663R) on a normal mouse background. Remarkably, these mice failed to show any abnormalities of cellular or clinical phenotype despite regulated overexpression of mutant protein in relevant tissues and developmental stages and direct evidence that mouse and human fibrillin-1 interact with high efficiency. Immunostaining with a human-specific mAb provides what we believe to be the first demonstration that mutant fibrillin-1 can participate in productive microfibrillar assembly. Informatively, use of homologous recombination to generate mice heterozygous for a comparable missense mutation (C1039G) revealed impaired microfibrillar deposition, skeletal deformity, and progressive deterioration of aortic wall architecture, comparable to characteristics of the human condition. These data are consistent with a model that invokes haploinsufficiency for WT fibrillin-1, rather than production of mutant protein, as the primary determinant of failed microfibrillar assembly. In keeping with this model, introduction of a WT FBN1 transgene on a heterozygous C1039G background rescues aortic phenotype.

Comprehensive molecular screening of theFBN1gene favors locus homogeneity of classical Marfan syndrome

In order to estimate the contribution of mutations at the fibrillin-1 locus (FBN1) to classical Marfan syndrome (MFS) and to study possible phenotypic differences between patients with an FBN1 mutation vs. without, a comprehensive molecular study of the FBN1 gene in a cohort of 93 MFS patients fulfilling the clinical diagnosis of MFS according to the Ghent nosology was performed. The initial mutation screening by CSGE/SSCP allowed identification of an FBN1-mutation in 73 patients. Next, sequencing of all FBN1-exons was performed in 11 mutation-negative patients, while in nine others, DHPLC was used. This allowed identification of seven and five additional mutations, respectively. Southern blot analysis revealed an abnormal hybridization pattern in one more patient. A total of 23 out of the 85 mutations identified here are reported for the first time. Phenotypic comparison of MFS patients with cysteine-involving mutations vs. premature termination mutations revealed significant differences in ocular and skeletal involvement. The phenotype of the eight patients without proven FBN1 mutation did not differ from the others with respect to the presence of major cardiac, ocular, and skeletal manifestations or positive familial history. Most likely, a portion of FBN1-mutations remains undetected because of technical limitations. In conclusion, the involvement of the FBN1-gene could be demonstrated in at least 91% of all MFS patients (85/93), which strongly suggests that this gene is the predominant, if not the sole, locus for MFS.

Genetic linkage and association studies in bipolar affective disorder: a time for optimism

Genetic research on complex diseases is beginning to bear fruit, with the successful identification of candidate susceptibility genes in diabetes, asthma, and other illnesses. Similar success is on the horizon for bipolar affective disorder (BPAD), but significant challenges remain. In this review, we outline the basic concepts of linkage and association mapping for complex phenotypes. We point out important caveats inherent in both approaches, and review guidelines on the interpretation of linkage statistics and significance thresholds. We then apply these concepts to an evaluation of the present status of genetic linkage and association studies in BPAD. The challenges posed by locus heterogeneity, phenotype definition, and sample size requirements are given a detailed treatment. Despite these challenges, we argue that the way ahead remains firmly rooted in linkage studies, complemented by association studies in linked regions. This is the only truly genome-wide approach currently available; it has succeeded in other complex phenotypes, and it is the surest strategy for mapping susceptibility genes in BPAD. Once these genes are identified, genetic mapping methods will yield to the other methods of 21st-century molecular biology as we begin to elucidate the pathophysiology of BPAD.

Mapping a locus for familial thoracic aortic aneurysms and dissections (TAAD2) to 3p24-25

BACKGROUND

Familial thoracic aortic aneurysms and dissections (TAAD) occur as part of known syndromes such as Marfan syndrome but can also be inherited in families in an autosomal dominant manner as an isolated condition. Previous studies have mapped genes causing nonsyndromic familial TAAD to 5q13-15 (TAAD1) and 11q23.2-q24 (FAA1). Further genetic heterogeneity for the condition was evident by the presence of TAAD in some families not linked to these known loci.

METHODS AND RESULTS

A 4-generation family with dominant mode of inheritance of TAAD was studied. Affected status was determined by dilation of the ascending aorta, surgical repair of an aneurysm or dissection, or death as the result of aortic dissection. None of the family members evaluated met the diagnostic criteria for Marfan syndrome. After exclusion of known loci for familial TAAD, a genome-wide scan was carried out to map the defective gene causing the disease in the family. A locus was mapped to a 25-cM region on 3p24-25 with a maximum multipoint logarithm of the odds score of 4.28.

CONCLUSIONS

A third locus for nonsyndromic TAAD was mapped to 3p24-25 and termed the TAAD2 locus. This locus overlaps a previously mapped second locus for Marfan syndrome, termed the MFS2 locus. Future characterization of the TAAD2 gene will determine if TAAD2 is allelic to MFS2. In addition, identification of the TAAD2 gene will improve the presymptomatic diagnosis of individuals with this life-threatening genetic syndrome and provide information concerning the pathogenesis of the disease.

Characterization of microsatellite markers flanking FBN1: utility in the diagnostic evaluation for Marfan syndrome

Marfan syndrome (MFS) is an autosomal dominant disorder of connective tissue with marked interfamilial and intrafamilial variation in phenotype. The primary defect in affected patients resides in the gene for fibrillin-1 (FBN1) on 15q21. Linkage analysis has shown no locus heterogeneity in the classic phenotype, although substantial allelic heterogeneity exists. Recently it has been shown that the size of the gene is approximately 200 kb. These and other factors have precluded routine mutation screening for presymptomatic and prenatal diagnosis. Previously we described four intragenic microsatellite polymorphisms that can be used for haplotype segregation analysis. The utility of this approach is limited because the markers do not fully span the gene and show incomplete informativeness, with 16% homozygosity for the most common haplotype. We have now identified and localized highly polymorphic microsatellite markers that fall within 1 Mb of FBN1. Complete haplotype heterozygosity was observed in a population of 50 unrelated control individuals when the flanking markers and existing intragenic polymorphisms were used in combination. We demonstrate the utility of haplotype segregation analysis in the presymptomatic diagnosis and counseling of families showing atypical or equivocal manifestations of MFS.

The Marfan syndrome

The Marfan syndrome (MFS), initially described just over 100 years ago, was among the first conditions classified as a heritable disorder of connective tissue. MFS lies at one end of a phenotypic continuum, with people in the general population who have one or another of the features of MFS at the other end, and those with a variety of other conditions in between. Diagnosis of MFS and these other conditions remains based on clinical features. Mutations in FBN1, the gene that encodes fibrillin-1, are responsible for MFS and (in a few patients) other disorders in the continuum. In addition to skeletal, ocular, and cardiovascular features, patients with MFS have involvement of the skin, integument, lungs, and muscle tissue. Over the past 30 years, evolution of aggressive medical and surgical management of the cardiovascular problems, especially mitral valve prolapse, aortic dilatation, and aortic dissection, has resulted in considerable improvement in life expectancy.

Co-occurrence of autosomal dominant polycystic kidney disease and Marfan syndrome in a kindred

Several reports exist of the co-occurrence of autosomal dominant polycystic kidney disease (ADPKD) and Marfan syndrome, including a report of ADPKD and "overlap" connective tissue disorder in a family with linkage to the PKD1 locus. We report the results of clinical and linkage investigations of an ADPKD family in whom several affected subjects also had aortic vascular complications as well as features of Marfan syndrome. Detailed clinical assessment and linkage analysis were performed with polymorphic microsatellite markers closely linked to the PKD1 and FBN1 loci. Survival data were compared with 10 geographically matched PKD1 families. Although several subjects had features of both ADPKD and Marfan syndrome, detailed clinical examination of the extended family indicated that the two conditions had converged within the kindred. For those with ADPKD, linkage was established to the PKD1 locus (lod score, 6.04). Among those with features of Marfan syndrome, linkage was confirmed to the FBN1 locus (lod score, 1.87). Five of six subjects with both ADPKD and the high-risk FBN1 haplotype had associated vascular complications. In contrast, among the remaining nine individuals with PKD1 alone, seven had aortic assessments, and none were found to have aortic complications. Our experience suggests that when prominent features of connective tissue disease or vascular complications are found in ADPKD patients, alternative additional diagnoses should be considered, including the possibility of a coinherited FBN1 mutation responsible for Marfan syndrome or, alternatively, an associated milder FBN1 phenotype in the absence of sufficient other clinical features to allow Marfan syndrome to be diagnosed.

The molecular genetics of Marfan syndrome and related microfibrillopathies

Mutations in the gene for fibrillin-1 (FBN1) have been shown to cause Marfan syndrome, an autosomal dominant disorder of connective tissue characterised by pleiotropic manifestations involving primarily the ocular, skeletal, and cardiovascular systems. Fibrillin-1 is a major component of the 10-12 nm microfibrils, which are thought to play a role in tropoelastin deposition and elastic fibre formation in addition to possessing an anchoring function in some tissues. Fibrillin-1 mutations have also been found in patients who do not fulfil clinical criteria for the diagnosis of Marfan syndrome, but have related disorders of connective tissue, such as isolated ectopia lentis, familial aortic aneurysm, and Marfan-like skeletal abnormalities, so that Marfan syndrome may be regarded as one of a range of type 1 fibrillinopathies. There appear to be no particular hot spots since mutations are found throughout the entire fibrillin-1 gene. However, a clustering of mutations associated with the most severe form of Marfan syndrome, neonatal Marfan syndrome, has been noted in a region encompassing exons 24 to 32. The gene for fibrillin-2 (FBN2) is highly homologous to FBN1, and mutations in FBN2 have been shown to cause a phenotypically related disorder termed congenital contractural arachnodactyly. Since mutations in the fibrillin genes are likely to affect the global function of the microfibrils, the term microfibrillopathy may be the most appropriate to designate the spectrum of disease associated with dysfunction of these molecules. The understanding of the global and the molecular functions of the fibrillin containing microfibrils is still incomplete and, correspondingly, no comprehensive theory of the pathogenesis of Marfan syndrome has emerged to date. Many, but not all, fibrillin-1 gene mutations are expected to exert a dominant negative effect, whereby mutant fibrillin monomers impair the global function of the microfibrils. In this paper we review the molecular physiology and pathophysiology of Marfan syndrome and related microfibrillopathies.

Comparison of heteroduplex analysis, direct sequencing, and enzyme mismatch cleavage for detecting mutations in a large gene, FBN1

Analysis of large genes for mutations of clinical relevance is complicated by intragenic heterogeneity, sensitivity, and cost of the methods available, and in the case of many conditions, specificity of the genetic alterations detected. We examined the FBN1 gene for mutations in people who had Marfan syndrome using three methods: single-chain polymorphism analysis (SSCP) with heteroduplex (HA) analysis, enzyme-mediated cleavage (EMC) of heteroduplexes, and direct sequencing. We also used these methods to search for mutations in the P53 gene in patients with hepatocellular carcinoma. The results showed that EMC was most efficient for detecting mutations. However, the cost favored SSCP with heteroduplex analysis, provided conditions did not need to be optimized to detect a mutation. Until more cost-effective and sensitive methods are developed to detect unknown mutations in large genes, diagnosis of many genetic disorders will depend on the willingness of an investigator who is studying a particular disorder to perform clinical molecular testing and have the laboratory accredited.

Advances in the molecular genetics of congenital structural heart disease

Molecular genetic analyses have generated significant advances in our understanding of congenital heart disease. Techniques of genetic mapping with polymorphic microsatellites and fluorescence in situ hybridization (FISH) have provided informative tools for localization and identification of disease genes. Some cardiovascular diseases have proven to result from single gene defects. Others relate to more complex etiologies involving several genes and their interactions. Elucidation of the molecular genetic etiologies of congenital heart disease prompts consideration of DNA testing for cardiac disorders. Future integration of these diagnostic modalities with improved treatments may ultimately decrease morbidity and mortality from congenital heart diseases.

Identifying the right stop: determining how the surveillance complex recognizes and degrades an aberrant mRNA

The nonsense-mediated mRNA decay (NMD) pathway functions by checking whether translation termination has occurred prematurely and subsequently degrading the aberrant mRNAs. In Saccharomyces cerevisiae, it has been proposed that a surveillance complex scans 3' of the premature termination codon and searches for the downstream element (DSE), whose recognition by the complex identifies the transcript as aberrant and promotes its rapid decay. The results presented here suggest that translation termination is important for assembly of the surveillance complex. Neither the activity of the initiation ternary complex after premature translation termination has occurred nor the elongation phase of translation are essential for the activity of the NMD pathway. Once assembled, the surveillance complex is active for searching and recognizing a DSE for approximately 200 nt 3' of the stop codon. We have also identified a stabilizer sequence (STE) in the GCN4 leader region that inactivates the NMD pathway. Inactivation of the NMD pathway, as a consequence of either the DSE being too far from a stop codon or the presence of the STE, can be circumvented by inserting sequences containing a new translation initiation/termination cycle immediately 5' of the DSE. Further, the results indicate that the STE functions in the context of the GCN4 transcript to inactivate the NMD pathway.

Fibrillln mutations in Marfan syndrome and related phenotypes

A casual association has been established between mutations in the fibrillin 1 gene and Marfan syndrome and related phenotypes. Analysis of mutations in these disease types has provided new insights into microfibril assembly and function. These include evidence for a mutation in a fibrillin 1 domain associated with severe phenotype; indication of profibrillin processing by a furin-like endoprotease; linkage between extracellular processing and fibrillin 1 polymerization; and involvement of calcium binding in monomer stabilization and microfibril assembly. Identification of intragenic DNA polymorphisms and determination of intron/exon junction sequences have significantly improved our ability to diagnose Marfan syndrome and to detect fibrillin 1 mutations. Additional work has provided strong evidence for structural and functional heterogeneity of microfibrillin. The evidence includes the identification of fibrillin 2, a microfibrillar component structurally related to fibrillin 1; the differential pattern of gene expression of the two fibrillin; and the association of fibrillin 2 mutations with congenital contractural arachnodactyly.

Revised diagnostic criteria for the Marfan syndrome

In 1986, the diagnosis of the Marfan syndrome was codified on the basis of clinical criteria in the Berlin nosology [Beighton et al., 1988]. Over time, weaknesses have emerged in these criteria, a problem accentuated by the advent of molecular testing. In this paper, we propose a revision of diagnostic criteria for Marfan syndrome and related conditions. Most notable are: more stringent requirements for diagnosis of the Marfan syndrome in relatives of an unequivocally affected individual; skeletal involvement as a major criterion if at least 4 of 8 typical skeletal manifestations are present; potential contribution of molecular analysis to the diagnosis of Marfan syndrome; and delineation of initial criteria for diagnosis of other heritable conditions with partially overlapping phenotypes.