Premature termination mutations in FBN1: distinct effects on differential allelic expression and on protein and clinical phenotypes

Qualitative and quantitative analysis of FBN1 mRNA from 16 patients with Marfan Syndrome

Background

Pathogenic mutations in FBN1, encoding the glycoprotein, fibrillin-1, cause Marfan syndrome (MFS) and related connective tissue disorders. In the present study, qualitative and quantitative effects of 16 mutations, identified in FBN1 in MFS patients with systematically described phenotypes, were investigated in vitro.

Methods

Qualitative analysis was performed with reverse transcription-PCR (RT-PCR) and gel electrophoresis, and quantitative analysis to determine the FBN1 mRNA levels in fibroblasts from the 16 patients with MFS was performed with real-time PCR.

Results

Qualitative analysis documented that the mutations c.4817-2delA and c.A4925G led to aberrant FBN1 mRNA splicing leading to in frame deletion of exon 39 and in exon 39, respectively. No difference in the mean FBN1 mRNA level was observed between the entire group of cases and controls, nor between the group of patients with missense mutations and controls. The mean expression levels associated with premature termination codon (PTC) and splice site mutations were significantly lower than the levels in patients with missense mutations. A high level of FBN1 mRNA in the patient with the missense mutation c.G2447T did not segregate with the mutation in three of his first degree relatives. No association was indicated between the FBN1 transcript level and specific phenotypic manifestations.

Conclusions

Abnormal FBN1 transcripts were indicated in fibroblasts from patients with the splice site mutation c.4817-2delA and the missense mutation c.A4925G. While the mean FBN1 mRNA expression level in fibroblasts from patients with splice site and PTC mutations were lower than the mean level in patients with missense mutations and controls, inter-individual variability was high. The observation that high level of FBN1 mRNA in the patient with the missense mutation c.G2447T did not segregate with the mutation in the family suggests that variable expression of the normal FBN1 allele may contribute to explain the variability in FBN1 mRNA level.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-015-0260-4) contains supplementary material, which is available to authorized users.

Wide mutation spectrum and frequent variant Ala27Thr of FBN1 identified in a large cohort of Chinese patients with sporadic TAAD

Genetic etiology in majority of patients with sporadic thoracic aortic aneurysm and dissections (STAAD) remains unknown. Recent GWAS study suggested common variant(s) in FBN1 is associated with STAAD. The present study aims to test this hypothesis and to identify mutation spectrum by targeted exome sequencing of the FBN1 gene in 146 unrelated patients with STAAD. Totally, 15.75% of FBN1 variants in STAAD were identified, including 5 disruptive and 18 missense mutations. Most of the variants were novel. Genotype-phenotype correlation analysis suggested that the maximum aortic diameter in the disruptive mutation group was significantly larger than that in the non-Cys missense mutation group. Interestingly, the variant Ala27Thr at −1 position, which is predicted to change the cleavage site of the signal peptidase of fibrillin-1, was detected in two unrelated patients. Furthermore, genotyping analysis of this variant detected 10 heterozygous Ala27Thr from additional 666 unrelated patients (1.50%), versus 7 from 1500 controls (0.47%), indicating a significant association of this variant with STAAD. Collectively, the identification of the variant Ala27Thr may represent a relatively common genetic predisposition and a novel pathogenetic mechanism for STAAD. Also, expansion of the mutation spectrum in FBN1 will be helpful in genetic counselling for Chinese patients with STAAD.

Muscle and Bone Impairment in Children With Marfan Syndrome: Correlation With Age and FBN1 Genotype

Marfan syndrome (MFS) is a rare connective tissue disorder caused by mutation in the gene encoding the extracellular matrix protein fibrillin-1 (FBN1), leading to transforming growth factor-beta (TGF-β) signaling dysregulation. Although decreased axial and peripheral bone mineral density (BMD) has been reported in adults with MFS, data about the evolution of bone mass during childhood and adolescence are limited. The aim of the present study was to evaluate bone and muscle characteristics in children, adolescents, and young adults with MFS. The study population included 48 children and young adults (22 girls) with MFS with a median age of 11.9 years (range 5.3 to 25.2 years). The axial skeleton was analyzed at the lumbar spine using dual-energy X-ray absorptiometry (DXA), whereas the appendicular skeleton (hand) was evaluated using the BoneXpert system (with the calculation of the Bone Health Index). Muscle mass was measured by DXA. Compared with healthy age-matched controls, bone mass at the axial and appendicular levels and muscle mass were decreased in children with MFS and worsened from childhood to adulthood. Vitamin D deficiency (<50 nmol/L) was found in about a quarter of patients. Serum vitamin D levels were negatively correlated with age and positively correlated with lumbar spine areal and volumetric BMD. Lean body mass (LBM) Z-scores were positively associated with total body bone mineral content (TB-BMC) Z-scores, and LBM was an independent predictor of TB-BMC values, suggesting that muscle hypoplasia could explain at least in part the bone loss in MFS. Patients with a FBN1 premature termination codon mutation had a more severe musculoskeletal phenotype than patients with an inframe mutation, suggesting the involvement of TGF-β signaling dysregulation in the pathophysiologic mechanisms. In light of these results, we recommend that measurement of bone mineral status should be part of the longitudinal clinical investigation of MFS children.

The clinical presentation of Marfan syndrome is modulated by expression of wild-type FBN1 allele

Marfan syndrome is an autosomal dominant disorder mainly caused by mutations within FBN1 gene. The disease displays large variability in age of onset or severity and very poor phenotype/genotype correlations have been demonstrated. We investigated the hypothesis that phenotype severity could be related to the variable expression level of fibrillin-1 (FBN1) synthesized from the wild-type (WT) allele. Quantitative reverse-transcription and polymerase chain reaction was used to evaluate FBN1 levels in skin fibroblasts from 80 Marfan patients with premature termination codons and in skin fibroblasts from 80 controls. Results in controls showed a 3.9-fold variation in FBN1 mRNA synthesis level between subjects. A similar 4.4-fold variation was found in the Marfan population, but the mean level of FBN1 mRNA was a half of the control population. Differential allelic expression analysis in Marfan fibroblasts showed that over 90% of FBN1 mRNA was transcribed from the wild allele and the mutated allele was not detected. In the control population, independently of the expression level of FBN1, we observed steady-state equilibrium between the two allelic-mRNAs suggesting that FBN1 expression mainly depends on trans-acting regulators. Finally, we show that a low level of residual WT FBN1 mRNA accounts for a high risk of ectopia lentis and pectus abnormality and tends to increase the risk of aortic dilatation.

Beneficial Outcome of Losartan Therapy Depends on Type of FBN1 Mutation in Marfan Syndrome

BACKGROUND

It has been shown that losartan reduces aortic dilatation in patients with Marfan syndrome. However, treatment response is highly variable. This study investigates losartan effectiveness in genetically classified subgroups.

METHODS AND RESULTS

In this predefined substudy of COMPARE, Marfan patients were randomized to daily receive losartan 100 mg or no losartan. Aortic root dimensions were measured by MRI at baseline and after 3 years. FBN1 mutations were classified based on fibrillin-1 protein effect into (1) haploinsufficiency, decreased amount of normal fibrillin-1, or (2) dominant negative, normal fibrillin-1 abundance with mutant fibrillin-1 incorporated in the matrix. A pathogenic FBN1 mutation was found in 117 patients, of whom 79 patients were positive for a dominant negative mutation (67.5%) and 38 for a mutation causing haploinsufficiency (32.5%). Baseline characteristics between treatment groups were similar. Overall, losartan significantly reduced aortic root dilatation rate (no losartan, 1.3±1.5 mm/3 years, n=59 versus losartan, 0.8±1.4 mm/3 years, n=58; P=0.009). However, losartan reduced only aortic root dilatation rate in haploinsufficient patients (no losartan, 1.8±1.5 mm/3 years, n=21 versus losartan 0.5±0.8 mm/3 years, n=17; P=0.001) and not in dominant negative patients (no losartan, 1.2±1.7 mm/3 years, n=38 versus losartan 0.8±1.3 mm/3 years, n=41; P=0.197).

CONCLUSIONS

Marfan patients with haploinsufficient FBN1 mutations seem to be more responsive to losartan therapy for inhibition of aortic root dilatation rate compared with dominant negative patients. Additional treatment strategies are needed in Marfan patients with dominant negative FBN1 mutations.

CLINICAL TRIAL REGISTRATION

http://www.trialregister.nl/trialreg/index.asp; Unique Identifier: NTR1423.

Increased frequency of FBN1 truncating and splicing variants in Marfan syndrome patients with aortic events

PURPOSE

Marfan syndrome is a systemic disorder that typically involves FBN1 mutations and cardiovascular manifestations. We investigated FBN1 genotype-phenotype correlations with aortic events (aortic dissection and prophylactic aortic surgery) in patients with Marfan syndrome.

METHODS

Genotype and phenotype information from probands (n = 179) with an FBN1 pathogenic or likely pathogenic variant were assessed.

RESULTS

A higher frequency of truncating or splicing FBN1 variants was observed in Ghent criteria-positive patients with an aortic event (n = 34) as compared with all other probands (n = 145) without a reported aortic event (79 vs. 39%; P < 0.0001), as well as Ghent criteria-positive probands (n = 54) without an aortic event (79 vs. 48%; P = 0.0039). Most probands with an early aortic event had a truncating or splicing variant (100% (n = 12) and 95% (n = 21) of patients younger than 30 and 40 years old, respectively). Aortic events occurred at a younger median age in patients with truncating/splicing variants (29 years) as compared with those with missense variants (51 years). A trend toward a higher frequency of truncating/splicing variants in patients with aortic dissection (n = 21) versus prophylactic surgery (n = 13) (85.7 vs. 69.3%; not significant) was observed.

CONCLUSION

These aortic event- and age-associated findings may have important implications for the management of Marfan syndrome patients with FBN1 truncating and splicing variants.Genet Med 17 3, 177-187.

Nonsense-mediated decay in genetic disease: friend or foe?

Eukaryotic cells utilize various RNA quality control mechanisms to ensure high fidelity of gene expression, thus protecting against the accumulation of nonfunctional RNA and the subsequent production of abnormal peptides. Messenger RNAs (mRNAs) are largely responsible for protein production, and mRNA quality control is particularly important for protecting the cell against the downstream effects of genetic mutations. Nonsense-mediated decay (NMD) is an evolutionarily conserved mRNA quality control system in all eukaryotes that degrades transcripts containing premature termination codons (PTCs). By degrading these aberrant transcripts, NMD acts to prevent the production of truncated proteins that could otherwise harm the cell through various insults, such as dominant negative effects or the ER stress response. Although NMD functions to protect the cell against the deleterious effects of aberrant mRNA, there is a growing body of evidence that mutation-, codon-, gene-, cell-, and tissue-specific differences in NMD efficiency can alter the underlying pathology of genetic disease. In addition, the protective role that NMD plays in genetic disease can undermine current therapeutic strategies aimed at increasing the production of full-length functional protein from genes harboring nonsense mutations. Here, we review the normal function of this RNA surveillance pathway and how it is regulated, provide current evidence for the role that it plays in modulating genetic disease phenotypes, and how NMD can be used as a therapeutic target.

Exome sequencing identified new mutations in a Marfan syndrome family

Marfan syndrome is a common autosomal dominant hereditary connective tissue disorder. There is no cure for Marfan syndrome currently. Next-generation sequencing (NGS) technology is efficient to identify genetic lesions at the exome level. Here we carried out exome sequencing of two Marfan syndrome patients. Further Sanger sequencing validation in other five members from the same family was also implemented to confirm new variants which may contribute to the pathogenesis of the disease. Two new variants, including one nonsense SNP in the Marfan syndrome gene FBN1 and one missense mutation in exon 15 of LRP1, which may be related to the phenotype of the patients were identified. The exome sequencing analysis provides us a new insight into the molecular events governing pathogenesis of Marfan syndrome.

Relation between genotype and left-ventricular dilatation in patients with Marfan syndrome

Cardiovascular manifestations in patients with Marfan syndrome (MFS) are related to aortic and valvular abnormalities. However, dilatation of the left ventricle (LV) can occur, even in the absence of aortic surgery or valvular abnormalities. We evaluated genetic characteristics of patients with MFS with LV dilatation. One hundred eighty-two patients fulfilling the MFS criteria, without valvular abnormalities or previous aortic surgery, with a complete FBN1 analysis, were studied. FBN1 mutations were identified in over 81% of patients. Twenty-nine patients (16%) demonstrated LV dilatation (LV end diastolic diameter corrected for age and body surface area >112%). FBN1-positive patients carrying a non-missense mutation more often had LV dilatation than missense mutation carriers (14/74 versus 5/75; p<0.05). Finally, FBN1-negative MFS patients significantly more often demonstrated LV dilatation than FBN1-positive patients (10/33 versus 19/149; p<0.05). It is concluded that LV dilatation in MFS patients is more often seen in patients with a non-missense mutation and in those patients without an FBN1 mutation. Therefore physicians should be aware of the possibility of LV dilatation in these patients even in the absence of valvular pathology.

An overview of mutation detection methods in genetic disorders

GENETIC DISORDERS ARE TRADITIONALLY CATEGORIZED INTO THREE MAIN GROUPS: single-gene, chromosomal, and multifactorial disorders. Single gene or Mendelian disorders result from errors in DNA sequence of a gene and include autosomal dominant (AD), autosomal recessive (AR), X-linked recessive (XR), X-linked dominant and Y-linked (holandric) disorders. Chromosomal disorders are due to chromosomal aberrations including numerical and structural damages. Molecular and cytogenetic techniques have been applied to identify genetic mutations leading to diseases. Accurate diagnosis of diseases is essential for appropriate treatment of patients, genetic counseling and prevention strategies. Characteristic features of patterns of inheritance are briefly reviewed and a short description of chromosomal disorders is also presented. In addition, applications of cytogenetic and molecular techniques and different types of mutations are discussed for genetic diagnosis of the pediatric genetic diseases. The purpose is to make pediatricians familiar with the applications of cytogenetic and molecular techniques and tools used for genetic diagnosis.

Dural ectasia and FBN1 mutation screening of 40 patients with Marfan syndrome and related disorders: role of dural ectasia for the diagnosis

Marfan syndrome is an autosomal dominant disorder of connective tissue caused by mutations in the gene encoding fibrillin-1 (FBN1), a matrix component of microfibrils. Dural ectasia, i.e. enlargement of the neural canal mainly located in the lower lumbar and sacral region, frequently occurs in Marfan patients. The aim of our study was to investigate the role of dural ectasia in raising the diagnosis of Marfan syndrome and its association with FBN1 mutations. We studied 40 unrelated patients suspected for MFS, who underwent magnetic resonance imaging searching for dural ectasia. In all of them FBN1 gene analysis was also performed. Thirty-seven patients resulted affected by Marfan syndrome according to the '96 Ghent criteria; in 30 of them the diagnosis was confirmed when revaluated by the recently revised criteria (2010). Thirty-six patients resulted positive for dural ectasia. The degree of dural ectasia was grade 1 in 19 patients, grade 2 in 11 patients, and grade 3 in 6 patients. In 7 (24%) patients, the presence of dural ectasia allowed to reach a positive score for systemic feature criterion. Twenty-four patients carried an FBN1 mutation, that were represented by 13 missense (54%), and 11 (46%) mutations generating a premature termination codon (PTC, frameshifts and stop codons). No mutation was detected in the remaining 16 (6 patients with MFS and 10 with related disorders according to revised Ghent criteria). The prevalence of severe (grade 2 and grade 3) involvement of dura mater was higher in patients harbouring premature termination codon (PTC) mutations than those carrying missense-mutations (8/11 vs 2/13, P = 0.0111). Our data emphasizes the importance of dural ectasia screening to reach the diagnosis of Marfan syndrome especially when it is uncertain and indicates an association between PTC mutations and severe dural ectasia in Marfan patients.

Two novel mutations of fibrillin-1 gene correlate with different phenotypes of Marfan syndrome in Chinese families

PURPOSE

To identify the causative mutations in two Chinese families with autosomal dominant Marfan syndrome and to describe the associated phenotypes.

METHODS

Complete physical, ophthalmic, and cardiovascular examinations were given to the patients and unaffected individuals in the two families. Exclusive linkage mapping was performed for transforming growth factor beta receptor II (TGFBR2) and fibrillin-1 (FBN1) loci in both families. The entire coding region and flanking splice sites of the FBN1 gene were screened for mutations in the two families with Sanger sequencing. The potential mutations of FBN1 were tested in 100 normal controls.

RESULTS

Lens dislocation was observed in two out of ten patients in the MF1 family and all patients in the MF2 family. However, the MF1 family displayed more severe cardiovascular and skeletal system involvement compared with the MF2 family. The transforming growth factor beta receptor II locus was excluded in both families by linkage analysis. A maximum multipoint lod score score of 2.83 was obtained for marker D15S992 (located in the FBN1 gene) in the MF1 family and 1.51 for the same marker in the MF2 family. Two novel mutations of FBN1, p.C271* and p.C637Y, were identified in the MF1 and MF2 families, respectively.

CONCLUSIONS

Genotype-phenotype correlations in this study indicate that nonsense mutations of FBN1 may correlate with relatively severe systemic phenotypes when compared with cysteine substitutions, the most common type of FBN1 mutations. Genetic diagnosis for patients with Marfan syndrome would help with genetic counseling, clinical intervention, and prognosis.

Identification of a novel FBN1 gene mutation in a large Pakistani family with Marfan syndrome

PURPOSE

To describe a novel mutation in the fibrillin-1 (FBN1) gene in a large Pakistani family with autosomal dominant Marfan syndrome (MFS).

METHODS

Blood samples were collected of 11 family members affected with Marfan syndrome, and DNA was isolated by phenol-extraction. The coding exons of FBN1 were analyzed by polymerase chain reaction (PCR) and direct sequencing. One hundred-thirty controls were screened for a mutation in the FBN1 gene that was identified in this family by restriction fragment length polymorphism (RFLP) analysis.

RESULTS

A novel heterozygous missense mutation c.2368T>A; p.Cys790Ser was observed in exon 19. This mutation substitutes a highly conserved cysteine residue by serine in a calcium binding epidermal growth factor-like domain (cbEGF) of FBN1. This mutation was present in all affected members and absent from unaffected individuals of the family in addition to 130 healthy Pakistani controls. Interestingly all affected family members presented with ectopia lentis, myopia and glaucoma, but lacked the cardinal cardiovascular features of MFS.

CONCLUSIONS

This is a first report of a mutation in FBN1 in MFS patients of Pakistani origin. The identification of a FBN1 mutation in this family confirms the diagnosis of MFS patients and expands the worldwide spectrum of FBN1 mutations.

A new novel mutation in FBN1 causes autosomal dominant Marfan syndrome in a Chinese family

PURPOSE

Screening of mutations in the fibrillin-1 (FBN1) gene in a Chinese family with autosomal dominant Marfan syndrome (MFS).

METHODS

It has been reported that FBN1 mutations account for approximately 90% of Autosomal Dominant MFS. FBN1 mutations were analyzed in a Chinese family of 36 members including 13 MFS patients. The genomic DNAs from blood leukocytes of the patients and their relatives were isolated and the entire coding region of FBN1 was amplified by PCR. The sequence of FBN1 was dertermined with an ABI 3100 Genetic Analyzer.

RESULTS

A previously unreported the missense mutation G214S (caused by a 640 A→G heterozygous change) in FBN1 was identified in the Chinese family. The mutation was associated with the disease phenotype in patients, but not detected in their relatives or in the 100 normal controls.

CONCLUSIONS

This is the first report of molecular characterization of FBN1 in the MFS family of Chinese origin. Our results expand the spectrum of FBN1 mutations causing MFS and further confirm the role of FBN1 in the pathogenesis of MFS. Direct sequencing of the mutation in FBN1 may be used for diagnosis of MFS.

Identification of a novel FBN1 mutation in a Chinese family with isolated ectopia lentis

PURPOSE

To identify the genetic defect in a Chinese family with autosomal dominant inherited ectopia lentis.

METHODS

twenty-one family members, including seven patients underwent general physical and fully ophthalmic examinations. Genomic DNA was extracted from leukocytes of venous blood of these individuals in the family. Polymerase chain reaction (PCR) amplification and direct sequencing of all 65 coding exons of the fibrillin-1 gene (FBN1) were analyzed.

RESULTS

Mutation screening in FBN1 identified a T>C transition at nucleotide position c,1759 leading to substitution of Cysteine for Arginine at codon 587 (C587R). This nucleotide substitution was not seen in any unaffected member of the family.

CONCLUSIONS

We detected a novel mutation in FBN1. Our result expands the mutation spectrum of FBN1 and help in the study of the molecular pathogenesis of Marfan syndrome and Marfan-related diseases.

Skeletogenic phenotype of human Marfan embryonic stem cells faithfully phenocopied by patient-specific induced-pluripotent stem cells

Marfan syndrome (MFS) is a heritable connective tissue disorder caused by mutations in the gene coding for FIBRILLIN-1 (FBN1), an extracellular matrix protein. MFS is inherited as an autosomal dominant trait and displays major manifestations in the ocular, skeletal, and cardiovascular systems. Here we report molecular and phenotypic profiles of skeletogenesis in tissues differentiated from human embryonic stem cells and induced pluripotent stem cells that carry a heritable mutation in FBN1. We demonstrate that, as a biological consequence of the activation of TGF-β signaling, osteogenic differentiation of embryonic stem cells with a FBN1 mutation is inhibited; osteogenesis is rescued by inhibition of TGF-β signaling. In contrast, chondrogenesis is not perturbated and occurs in a TGF-β cell-autonomous fashion. Importantly, skeletal phenotypes observed in human embryonic stem cells carrying the monogenic FBN1 mutation (MFS cells) are faithfully phenocopied by cells differentiated from induced pluripotent-stem cells derived independently from MFS patient fibroblasts. Results indicate a unique phenotype uncovered by examination of mutant pluripotent stem cells and further demonstrate the faithful alignment of phenotypes in differentiated cells obtained from both human embryonic stem cells and induced pluripotent-stem cells, providing complementary and powerful tools to gain further insights into human molecular pathogenesis, especially of MFS.

[Genotypic characterization of a Portuguese population of Marfan syndrome patients]

INTRODUCTION

The diagnosis of Marfan syndrome (MFS) depends on a multidisciplinary clinical evaluation. Molecular study to identify mutations in the FBN1 gene can establish a definitive diagnosis even with atypical or «incomplete» phenotypes and enable earlier diagnosis in asymptomatic patients.

OBJECTIVES

The aim of the present work was to evaluate the frequency and type of FBN1 gene mutations in a population of Marfan syndrome patients referred to a tertiary care center with cardiothoracic surgery.

METHODS

Our sample included 30 individuals with MFS (from 14 families), evaluated in cardiology, rheumatology and ophthalmology consultations. In all patients, DNA was extracted from a peripheral blood sample and mutation screening of the entire coding sequence of the FBN1 gene was then performed, using the polymerase chain reaction.

RESULTS

We identified 12 different mutations in the 14 families studied. Of these, only two had been previously described in the literature, while the other 10 were found to be new mutations; 36% of patients carried a missense mutation and 50% carried a mutation leading to a premature termination codon.

CONCLUSIONS

To the best of our knowledge this is the first genotypic description of Portuguese patients with MFS. In this study, we highlight the need for comprehensive clinical evaluation of these patients and the value of FBN1 mutation analysis in selected cases. By describing 10 new mutations, we have also helped broaden the spectrum of known FBN1 mutations associated with MFS.

Osteosarcoma in a Marfan patient with a novel premature termination codon in the FBN1 gene

Osteosarcoma is a malignant neoplasm of mesenchymal origin that is presumed to arise from osteoblasts. Considered a rare tumor, approximately 1000 cases of osteosarcoma are diagnosed in the United States each year, and osteosarcoma of the foot is rarer still. Marfan syndrome (MFS) is a rare genetic disorder that affects 1 in 5000 individuals and is caused by mutations in the fibrillin 1 (FBN1) gene. MFS phenotype affects several body systems, including soft connective tissue and bone. Here we report, for the first time, an individual with MFS that was treated for osteosarcoma. Surgically resected tissue was used to initiate an osteosarcoma cell line (PSU-OS-M) that exhibits attachment-independent growth and loss of contact inhibition in vitro. Genomic DNA was isolated from the tumor cells, and primers that anneal to intronic regions were used to amplify and sequence all 65 coding exons of the FBN1 gene. A two base pair insertion that results in a novel premature termination codon (PTC) was found in exon 52. Protein from the normal allele is detectable in PSU-OS-M cell-conditioned medium, but protein from the mutant allele was not detectable. Immunofluorescent microscopy demonstrates that PSU-OS-M cells can assemble fibrillin 1 microfibrils in culture, and fibronectin assembly is normal. As such, the PSU-OS-M cell line is to our knowledge the first oncogenically transformed cell line with a mutant fibrillin gene and may serve as a useful tool for studying molecular mechanisms of MFS and nonsense-mediated decay.

Prognosis factors in probands with an FBN1 mutation diagnosed before the age of 1 year

Marfan syndrome (MFS) is an autosomal dominant connective tissue disorder. Diagnostic criteria of neonatal MFS (nMFS), the most severe form, are still debated. The aim of our study was to search for clinical and molecular prognostic factors that could be associated with length of survival. Probands ascertained via the framework of the Universal Marfan database-FBN1, diagnosed before the age of 1 y and presenting with cardiovascular features (aortic root dilatation or valvular insufficiency) were included in this study. Clinical and molecular data were correlated to survival. Among the 60 individuals, 38 had died, 82% died before the age of 1 y, mostly because of congestive heart failure. Three probands reached adulthood. Valvular insufficiencies and diaphragmatic hernia were predictive of shorter life expectancy. Two FBN1 mutations were found outside of the exon 24-32 region (in exons 4 and 21). Mutations in exons 25-26 were overrepresented and were associated with shorter survival (p = 0.03). We report the largest genotyped series of probands with MFS diagnosed before 1 y of life. In this population, factors significantly associated with shorter survival are presence of valvular insufficiencies or diaphragmatic hernia in addition to a mutation in exons 25 or 26.

Paucity of skeletal manifestations in Hispanic families with FBN1 mutations

Marfan syndrome (MFS) is an autosomal dominant condition with pleiotropic manifestations involving the skeletal, ocular, and cardiovascular systems. The diagnosis is based primarily on clinical involvement of these and other systems, referred to as the Ghent criteria. We have identified three Hispanic families from Mexico with cardiovascular and ocular manifestations due to novel FBN1 mutations but with paucity of skeletal features. The largest family, hMFS001, had a frameshift mutation in exon 24 (3075delC) identified as the cause of aortic disease in the family. Assessment of eight affected adults revealed no major skeletal manifestation of MFS. Family hMFS002 had a missense mutation (R1530C) in exon 37. Four members fulfilled the criteria for ocular and cardiovascular phenotype but lacked skeletal manifestations. Family hMFS003 had two consecutive missense FBN1 mutations (C515W and R516G) in exon 12. Eight members fulfilled the ocular criteria for MFS and two members had major cardiovascular manifestations, however none of them met criteria for skeletal system. These data suggest that individuals of Hispanic descent with FBN1 mutations may not manifest skeletal features of the MFS to the same extent as Caucasians. We recommend that echocardiogram, ocular examination and FBN1 molecular testing be considered for any patients with possible MFS even in the absence of skeletal features, including Hispanic patients.

Quantitative sequence analysis of FBN1 premature termination codons provides evidence for incomplete NMD in leukocytes

We improved, evaluated, and used Sanger sequencing for quantification of single nucleotide polymorphism (SNP) variants in transcripts and gDNA samples. This improved assay resulted in highly reproducible relative allele frequencies (e.g., for a heterozygous gDNA 50.0+/-1.4%, and for a missense mutation-bearing transcript 46.9+/-3.7%) with a lower detection limit of 3-9%. It provided excellent accuracy and linear correlation between expected and observed relative allele frequencies. This sequencing assay, which can also be used for the quantification of copy number variations (CNVs), methylations, mosaicisms, and DNA pools, enabled us to analyze transcripts of the FBN1 gene in fibroblasts and blood samples of patients with suspected Marfan syndrome not only qualitatively but also quantitatively. We report a total of 18 novel and 19 known FBN1 sequence variants leading to a premature termination codon (PTC), 26 of which we analyzed by quantitative sequencing both at gDNA and cDNA levels. The relative amounts of PTC-containing FBN1 transcripts in fresh and PAXgene-stabilized blood samples were significantly higher (33.0+/-3.9% to 80.0+/-7.2%) than those detected in affected fibroblasts with inhibition of nonsense-mediated mRNA decay (NMD) (11.0+/-2.1% to 25.0+/-1.8%), whereas in fibroblasts without NMD inhibition no mutant alleles could be detected. These results provide evidence for incomplete NMD in leukocytes and have particular importance for RNA-based analyses not only in FBN1 but also in other genes.

Extracellular microfibrils: contextual platforms for TGFbeta and BMP signaling

The extracellular matrix plays a key role in organ formation and tissue homeostasis. Recent studies have revealed that fibrillin assemblies (microfibrils) confer both tissue integrity and regulate signaling events that instruct cell performance and that perturbation of either function manifests in disease. These analyses have also indicated that fibrillin assemblies impart contextual specificity to TGFbeta and BMP signaling. Moreover, correlative evidence suggests functional coupling between cell-directed assembly of microfibrils and targeting of TGFbeta and BMP complexes to fibrillins. Hence, the emerging view is that fibrillin-rich microfibrils are molecular integrators of structural and instructive signals with TGFbetas and BMPs as nodal points that convert extracellular inputs into discrete and context-dependent cellular responses.

Human Splicing Finder: an online bioinformatics tool to predict splicing signals

Thousands of mutations are identified yearly. Although many directly affect protein expression, an increasing proportion of mutations is now believed to influence mRNA splicing. They mostly affect existing splice sites, but synonymous, non-synonymous or nonsense mutations can also create or disrupt splice sites or auxiliary cis-splicing sequences. To facilitate the analysis of the different mutations, we designed Human Splicing Finder (HSF), a tool to predict the effects of mutations on splicing signals or to identify splicing motifs in any human sequence. It contains all available matrices for auxiliary sequence prediction as well as new ones for binding sites of the 9G8 and Tra2-β Serine-Arginine proteins and the hnRNP A1 ribonucleoprotein. We also developed new Position Weight Matrices to assess the strength of 5′ and 3′ splice sites and branch points. We evaluated HSF efficiency using a set of 83 intronic and 35 exonic mutations known to result in splicing defects. We showed that the mutation effect was correctly predicted in almost all cases. HSF could thus represent a valuable resource for research, diagnostic and therapeutic (e.g. therapeutic exon skipping) purposes as well as for global studies, such as the GEN2PHEN European Project or the Human Variome Project.

Therapy of Marfan Syndrome

Marfan syndrome is a common inherited disorder of connective tissue caused by deficiency of the matrix protein fibrillin-1. Effective surgical therapy for the most life-threatening manifestation, aortic root aneurysm, has led to a nearly normal lifespan for affected individuals who are appropriately recognized and treated. Traditional medical therapies, such as beta-adrenergic receptor blockade, are used to slow pathologic aortic growth and decrease the risk of aortic dissection by decreasing hemodynamic stress. New insights regarding the pathogenesis of Marfan syndrome have developed from investigation of murine models of this disorder. Fibrillin-1 deficiency is associated with excess signaling by transforming growth factor beta (TGFbeta). TGFbeta antagonists have shown great success in improving or preventing several manifestations of Marfan syndrome in these mice, including aortic aneurysm. These results highlight the potential for development of targeted therapies based on discovery of disease genes and interrogation of pathogenesis in murine models.

A Marfan syndrome gene expression phenotype in cultured skin fibroblasts

BACKGROUND

Marfan syndrome (MFS) is a heritable connective tissue disorder caused by mutations in the fibrillin-1 gene. This syndrome constitutes a significant identifiable subtype of aortic aneurysmal disease, accounting for over 5% of ascending and thoracic aortic aneurysms.

RESULTS

We used spotted membrane DNA macroarrays to identify genes whose altered expression levels may contribute to the phenotype of the disease. Our analysis of 4132 genes identified a subset with significant expression differences between skin fibroblast cultures from unaffected controls versus cultures from affected individuals with known fibrillin-1 mutations. Subsequently, 10 genes were chosen for validation by quantitative RT-PCR.

CONCLUSION

Differential expression of many of the validated genes was associated with MFS samples when an additional group of unaffected and MFS affected subjects were analyzed (p-value < 3 x 10-6 under the null hypothesis that expression levels in cultured fibroblasts are unaffected by MFS status). An unexpected observation was the range of individual gene expression. In unaffected control subjects, expression ranges exceeding 10 fold were seen in many of the genes selected for qRT-PCR validation. The variation in expression in the MFS affected subjects was even greater.

Effect of mutation type and location on clinical outcome in 1,013 probands with Marfan syndrome or related phenotypes and FBN1 mutations: an international study

Mutations in the fibrillin-1 (FBN1) gene cause Marfan syndrome (MFS) and have been associated with a wide range of overlapping phenotypes. Clinical care is complicated by variable age at onset and the wide range of severity of aortic features. The factors that modulate phenotypical severity, both among and within families, remain to be determined. The availability of international FBN1 mutation Universal Mutation Database (UMD-FBN1) has allowed us to perform the largest collaborative study ever reported, to investigate the correlation between the FBN1 genotype and the nature and severity of the clinical phenotype. A range of qualitative and quantitative clinical parameters (skeletal, cardiovascular, ophthalmologic, skin, pulmonary, and dural) was compared for different classes of mutation (types and locations) in 1,013 probands with a pathogenic FBN1 mutation. A higher probability of ectopia lentis was found for patients with a missense mutation substituting or producing a cysteine, when compared with other missense mutations. Patients with an FBN1 premature termination codon had a more severe skeletal and skin phenotype than did patients with an inframe mutation. Mutations in exons 24-32 were associated with a more severe and complete phenotype, including younger age at diagnosis of type I fibrillinopathy and higher probability of developing ectopia lentis, ascending aortic dilatation, aortic surgery, mitral valve abnormalities, scoliosis, and shorter survival; the majority of these results were replicated even when cases of neonatal MFS were excluded. These correlations, found between different mutation types and clinical manifestations, might be explained by different underlying genetic mechanisms (dominant negative versus haploinsufficiency) and by consideration of the two main physiological functions of fibrillin-1 (structural versus mediator of TGF beta signalling). Exon 24-32 mutations define a high-risk group for cardiac manifestations associated with severe prognosis at all ages.

Homozygosity for a FBN1 missense mutation: clinical and molecular evidence for recessive Marfan syndrome

Marfan syndrome (MFS) is known as an autosomal-dominant connective tissue disorder (MIM 154,700), involving primarily the skeletal, ocular and cardiovascular systems, and caused by mutations in the gene for fibrillin1 (FBN1). Here, we report on two cousins from a consanguineous family with a homozygous c.1,453C>T FBN1 mutation (p.Arg485Cys) and MFS. All four healthy parents were heterozygous for the c.1,453C>T FBN1 mutation and none fulfilled the Ghent criteria for MFS. This family is the first molecularly confirmed recessive MFS. The demonstration of recessive cases of MFS has obvious implications for genetic counselling as well as for molecular diagnosis.

Large genomic fibrillin-1 (FBN1) gene deletions provide evidence for true haploinsufficiency in Marfan syndrome

Mutations in the FBN1 gene are the major cause of Marfan syndrome (MFS), an autosomal dominant connective tissue disorder, which displays variable manifestations in the cardiovascular, ocular, and skeletal systems. Current molecular genetic testing of FBN1 may miss mutations in the promoter region or in other noncoding sequences as well as partial or complete gene deletions and duplications. In this study, we tested for copy number variations by successively applying multiplex ligation-dependent probe amplification (MLPA) and the Affymetrix Human Mapping 500 K Array Set, which contains probes for approximately 500,000 single-nucleotide polymorphisms (SNPs) across the genome. By analyzing genomic DNA of 101 unrelated individuals with MFS or related phenotypes in whom standard genetic testing detected no mutation, we identified FBN1 deletions in two patients with MFS. Our high-resolution approach narrowed down the deletion breakpoints. Subsequent sequencing of the junctional fragments revealed the deletion sizes of 26,887 and 302,580 bp, respectively. Surprisingly, both deletions affect the putative regulatory and promoter region of the FBN1 gene, strongly indicating that they abolish transcription of the deleted allele. This expectation of complete loss of function of one allele, i.e. true haploinsufficiency, was confirmed by transcript analyses. Our findings not only emphasize the importance of screening for large genomic rearrangements in comprehensive genetic testing of FBN1 but, importantly, also extend the molecular etiology of MFS by providing hitherto unreported evidence that true haploinsufficiency is sufficient to cause MFS.

Cellular and molecular studies of Marfan syndrome mutations identify co-operative protein folding in the cbEGF12-13 region of fibrillin-1

Human fibrillin-1 is an extra-cellular matrix glycoprotein with a modular organisation that includes 43 calcium-binding epidermal growth factor-like (cbEGF) domains arranged as multiple tandem repeats interspersed with transforming growth factor beta binding protein-like (TB) domains. We have studied Marfan syndrome-causing mutations which affect calcium binding to cbEGF13, and demonstrate that in human fibroblast cells they cause unexpected endoplasmic reticulum retention, indicative of a folding defect. Biochemical and biophysical studies of in vitro refolded fragments from the TB3-cbEGF14 region indicate long-range and unidirectional effects of these substitutions on the adjacent N-terminal domain cbEGF12. In contrast, only short-range effects of a pathogenic mutation affecting calcium binding to cbEGF19 are observed, and secretion of this mutant protein occurs. Further NMR studies on wild-type cbEGF12-13 and cbEGF12-14 identify a co-operative dependence of domain folding where calcium binding to cbEGF13 is required before cbEGF12 can adopt a native Ca(2+)-dependent fold. These data demonstrate that during biosynthesis of fibrillin-1, multiple tandem repeats of cbEGF domains may not necessarily fold independently and therefore missense mutations resulting in identical substitutions may have different effects on the fate of the mutant protein. Complex folding of modular proteins should therefore be considered when interpreting the molecular pathology of single-gene disorders.

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.

Xenopus fibrillin regulates directed convergence and extension

Fibrillin-based human diseases such as Marfan syndrome and congenital contractural arachnodactyly implicate fibrillins in the function and homeostasis of multiple adult tissues. Fibrillins are also expressed in embryos, but no early developmental role has been described for these proteins. We use three independent methods to reveal a role for Xenopus fibrillin (XF) at gastrulation. First, expressing truncated forms of XF in the embryo leads to failure of gastrulation concomitant with a dominant-negative effect on native fibrillin fibril assembly. Expressing truncated XF also inhibits normal progression of the patterned, polarized cell motility that drives convergence and extension at gastrulation and perturbs directed extension in cultured explants of dorsal mesoderm. Second, injection of a synthetic peptide encoding a cell-binding domain of XF into midgastrula embryos causes acute failure of gastrulation associated with defective fibrillin fibril assembly. These injections also reveal a critical role for this peptide in the fibril assembly process. Third, morpholino-mediated knockdown of translation of XF in the embryo also perturbs normal gastrulation and directed extension. Together, these data show that native Xenopus fibrillin is essential for the process of directed convergent extension in presumptive notochord at gastrulation.

Identification of 29 novel and nine recurrent fibrillin-1 (FBN1) mutations and genotype-phenotype correlations in 76 patients with Marfan syndrome

Marfan syndrome (MFS) is an autosomal-dominant disorder of the fibrous connective tissue that is typically caused by mutations in the gene coding for fibrillin-1 (FBN1), a major component of extracellular microfibrils. The clinical spectrum of MFS is highly variable and includes involvement of the cardiovascular, skeletal, ocular, and other organ systems; however, the genotype-phenotype correlations have not been well developed. Various screening methods have led to the identification of about 600 different mutations (FBN1-UMD database; www.umd.be). In this study we performed SSCP and/or direct sequencing to analyze all 65 exons of the FBN1 gene in 116 patients presenting with classic MFS or related phenotypes. Twenty-nine novel and nine recurrent mutations were identified in 38 of the analyzed patients. The mutations comprised 18 missense (47%), eight nonsense (21%), and five splice site (13%) mutations. Seven further mutations (18%) resulted from deletion, insertion, or duplication events, six of which led to a frameshift and subsequent premature termination. Additionally, we describe new polymorphisms and sequence variants. On the basis of the data presented here and in a previous study, we were able to establish highly significant correlations between the FBN1 mutation type and the MFS phenotype in a group of 76 mutation-positive patients for whom comprehensive clinical data were available. Most strikingly, there was a significantly lower incidence of ectopia lentis in patients who carried a mutation that led to a premature termination codon (PTC) or a missense mutation without cysteine involvement in FBN1, as compared to patients whose mutations involved a cysteine substitution or splice site alteration.

A bioinformatics framework for genotype–phenotype correlation in humans with Marfan syndrome caused by FBN1 gene mutations

Mutations in the human FBN1 gene are known to be associated with the Marfan syndrome, an autosomal dominant inherited multi-systemic connective tissue disorder. However, in the absence of solid genotype-phenotype correlations, the identification of an FBN1 mutation has only little prognostic value. We propose a bioinformatics framework for the mutated FBN1 gene which comprises the collection, management, and analysis of mutation data identified by molecular genetic analysis (DHPLC) and data of the clinical phenotype. To query our database at different levels of information, a relational data model, describing mutational events at the cDNA and protein levels, and the disease's phenotypic expression from two alternative views, was implemented. For database similarity requests, a query model which uses a distance measure based on log-likelihood weights for each clinical manifestation, was introduced. A data mining strategy for discovering diagnostic markers, classification and clustering of phenotypic expressions was provided which enabled us to confirm some known and to identify some new genotype-phenotype correlations.

The molecular genetics of Marfan syndrome and related disorders

Marfan syndrome (MFS), a relatively common autosomal dominant hereditary disorder of connective tissue with prominent manifestations in the skeletal, ocular, and cardiovascular systems, is caused by mutations in the gene for fibrillin-1 (FBN1). The leading cause of premature death in untreated individuals with MFS is acute aortic dissection, which often follows a period of progressive dilatation of the ascending aorta. Recent research on the molecular physiology of fibrillin and the pathophysiology of MFS and related disorders has changed our understanding of this disorder by demonstrating changes in growth factor signalling and in matrix-cell interactions. The purpose of this review is to provide a comprehensive overview of recent advances in the molecular biology of fibrillin and fibrillin-rich microfibrils. Mutations in FBN1 and other genes found in MFS and related disorders will be discussed, and novel concepts concerning the complex and multiple mechanisms of the pathogenesis of MFS will be explained.

Analysis of nonsense-mediated mRNA decay in mutant alleles identified in Spanish Gaucher disease patients

Most of the mutations described in the GBA gene as responsible for Gaucher disease are missense mutations. Nevertheless, other alterations, including nonsense and frameshift mutations, have been reported. These mutations generate premature termination codons (PTC) that could trigger the degradation of mRNA through a mechanism known as nonsense-mediated decay (NMD). It has been established that NMD requires the presence of at least one intron downstream of the PTC, and that this PTC should be at least 50-55 nucleotides upstream of the 3'-most exon-exon junction. In this study, we analyse four GBA truncating mutations - c.108G > A (W(-4)X; HGVS recommended nomenclature: p.W36X), c.886C > T (R257X; HGVS: p.R296X), c.1098_1099insA and c.1451_1452delAC - found in Spanish Gaucher disease patients in order to determine whether they undergo mRNA decay and, if so, whether this occurs via the NMD pathway. RT-PCR showed a clear reduction of RNA for three of the alleles: W(-4)X, R257X and c.1098_1099insA. After treatment with cycloheximide (CHX), a known inhibitor of both protein synthesis and NMD, two of the mutant alleles, R257X and c.1098_1099insA, showed a partial recovery of the amount of mRNA. The third mutation, W(-4)X, did not show any significant CHX-induced recovery, while allele c.1451_1452delAC did not show mRNA decay at all. Real-time PCR confirmed these results and allowed the decay and recovery to be quantified. Finally, the protein truncation test was performed to detect the corresponding proteins. Expected products for alleles R257X, c.1451_1452delAC and c.1098_1099insA, but not for W(-4)X, were observed.

The position of premature termination codons in the hepatocyte nuclear factor -1 beta gene determines susceptibility to nonsense-mediated decay

The nonsense-mediated decay (NMD) pathway is an mRNA surveillance mechanism that detects and degrades transcripts containing premature termination codons. The position of a truncating mutation can govern the resulting phenotype as mutations in the last exon evade NMD. In this study we investigated the susceptibility to NMD of six truncating HNF-1beta mutations by allele-specific quantitative real-time PCR using transformed lymphoblastoid cell lines. Four of six mutations (R181X, Q243fsdelC, P328L329fsdelCCTCT and A373fsdel29) showed evidence of NMD with levels of mutant transcript at 71% (p=0.009), 24% (p=0.008), 22% (p=0.008) and 3% (p=0.016) of the wild-type allele respectively. Comparable results were derived from lymphoblastoid cells and renal tubule cells isolated from a patient's overnight urine confirming that cell lines provide a good model for mRNA analysis. Two mutations (H69fsdelAC and P159fsdelT) produced transcripts unexpectedly immune to NMD. We conclude that truncating mutant transcripts of the HNF-1beta gene do not conform to the known rules governing NMD susceptibility, but instead demonstrate a previously unreported 5' to 3' polarity. We hypothesise that this may be due to reinitiation of translation downstream of the premature termination codon. Our study suggests that reinitiation of translation may be an important mechanism in the evasion of NMD, but that other factors such as the distance from the native initiation codon may also play a part.

Nonsense-mediated and nonstop decay of ribosomal protein S19 mRNA in Diamond-Blackfan anemia

Mutations in the ribosomal protein (RP)S19 gene have been found in about 25% of the cases of Diamond-Blackfan anemia (DBA), a rare congenital hypoplastic anemia that includes variable physical malformations. Various mutations have been identified in the RPS19 gene, but no investigations regarding the effect of these alterations on RPS19 mRNA levels have been performed. It is well established that mutated mRNA containing a premature stop codon (PTC) or lacking a stop codon can be rapidly degraded by specific mechanisms called nonsense mediated decay (NMD) and nonstop decay. To study the involvement of such mechanisms in DBA, we analyzed immortalized lymphoblastoid cells and primary fibroblasts from patients presenting different kinds of mutations in the RPS19 gene, generating allelic deletion, missense, nonsense, and nonstop messengers. We found that RPS19 mRNA levels are decreased in the cells with allelic deletion and, to a variable extent, also in all the cell lines with PTC or nonstop mutations. Further analysis showed that translation inhibition causes a stabilization of the mutated RPS19 mRNA. Our findings indicate that NMD and nonstop decay affect the expression of mutated RPS19 genes; this may help to clarify genotype-phenotype correlations in DBA.

Molecular genetics of Marfan syndrome

PURPOSE OF REVIEW

Marfan syndrome, the founding member of connective tissue disorders, is characterized by involvement of three major systems (skeletal, ocular, and cardiovascular) due to alteration in microfibrils. FBN1 at 15q21.1 was found to cause Marfan syndrome in 1991, and in 2004 TGFBR2 at 3p24.1 was newly identified as the Marfan syndrome type II gene. Several studies implied that fibrillin-1 and transforming growth factor-beta (TGF-beta) signaling are functionally related in extracellular matrix. Identification of TGFBR2 mutations in Marfan syndrome type II provided the direct evidence of the relation in humans.

RECENT FINDINGS

More than 500 FBN1 mutations have been found in Marfan syndrome, tentative genotype - phenotype correlations have emerged, and mouse models are providing insight into pathogenic mechanisms. TGFBR2 mutations are still limited, however, in 2005 were also reported to cause a new aneurysm syndrome. Functional association between fibrillin-1 and TGF-beta signaling in extracellular matrix has been presented.

SUMMARY

This review focuses on recent molecular genetics advances in Marfan syndrome and overlapping connective tissue disorders. Mutation spectrum of FBN1 and TGFBR2 in relation to phenotype is presented. Functional relation between fibrillin-1 and TGF-beta signaling is discussed. Future prospects in the study of Marfan syndrome are presented.

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.

Molecular mechanism for distinct neurological phenotypes conveyed by allelic truncating mutations

The molecular mechanisms by which different mutations in the same gene can result in distinct disease phenotypes remain largely unknown. Truncating mutations of SOX10 cause either a complex neurocristopathy designated PCWH or a more restricted phenotype known as Waardenburg-Shah syndrome (WS4; OMIM 277580). Here we report that although all nonsense and frameshift mutations that cause premature termination of translation generate truncated SOX10 proteins with potent dominant-negative activity, the more severe disease phenotype, PCWH, is realized only when the mutant mRNAs escape the nonsense-mediated decay (NMD) pathway. We observe similar results for truncating mutations of MPZ that convey distinct myelinopathies. Our experiments show that triggering NMD and escaping NMD may cause distinct neurological phenotypes.

Messenger RNA transcripts of the hepatocyte nuclear factor-1alpha gene containing premature termination codons are subject to nonsense-mediated decay

Mutations in the hepatocyte nuclear factor-1alpha (HNF-1a) gene cause maturity-onset diabetes of the young (MODY). Approximately 30% of these mutations generate mRNA transcripts harboring premature termination codons (PTCs). Degradation of such transcripts by the nonsense-mediated decay (NMD) pathway has been reported for many genes. To determine whether PTC mutant transcripts of the HNF-1alpha gene elicit NMD, we have developed a novel quantitative RT-PCR assay. We performed quantification of ectopically expressed mutant transcripts relative to normal transcripts in lymphoblastoid cell lines using a coding single nucleotide polymorphism (cSNP) as a marker. The nonsense mutations R171X, I414G415ATCG-->CCA, and P291fsinsC showed reduced mutant mRNA expression to 40% (P = 0.009), <0.01% (P </= 0.0001), and 6% (P = 0.001), respectively, of the normal allele. Transcript levels were restored using the translation inhibitor cycloheximide, indicating that the instability arises from NMD. The missense mutations G207D and R229P did not show NMD although R229P exhibited moderate RNA instability. This study provides the first evidence that HNF-1alpha PTC mutations may be subject to NMD. Mutations that result in significant reduction of protein levels due to NMD will not have dominant-negative activity in vivo. Haploinsufficiency is therefore likely to be the most important mutational mechanism of HNF-1alpha mutations causing MODY.