Elevated expression levels of lysyl oxidases protect against aortic aneurysm progression in Marfan syndrome

Patients with Marfan syndrome (MFS) are at high risk of life-threatening aortic dissections. The condition is caused by mutations in the gene encoding fibrillin-1, an essential component in the formation of elastic fibers. While experimental findings in animal models of the disease have shown the involvement of transforming growth factor-β (TGF-β)- and angiotensin II-dependent pathways, alterations in the vascular extracellular matrix (ECM) may also play a role in the onset and progression of the aortic disease. Lysyl oxidases (LOX) are extracellular enzymes, which initiates the formation of covalent cross-linking of collagens and elastin, thereby contributing to the maturation of the ECM. Here we have explored the role of LOX in the formation of aortic aneurysms in MFS. We show that aortic tissue from MFS patients and MFS mouse model (Fbn1(C1039G/+)) displayed enhanced expression of the members of the LOX family, LOX and LOX-like 1 (LOXL1), and this is associated with the formation of mature collagen fibers. Administration of a LOX inhibitor for 8weeks blocked collagen accumulation and aggravated elastic fiber impairment, and these effects correlated with the induction of a strong and rapidly progressing aortic dilatation, and with premature death in the more severe MFS mouse model, Fbn1(mgR/mgR), without any significant effect on wild type animals. This detrimental effect occurred preferentially in the ascending portion of the aorta, with little or no involvement of the aortic root, and was associated to an overactivation of both canonical and non-canonical TGF-β signaling pathways. The blockade of angiotensin II type I receptor with losartan restored TGF-β signaling activation, normalized elastic fiber impairment and prevented the aortic dilatation induced by LOX inhibition in Fbn1(C1039G/+) mice. Our data indicate that LOX enzymes and LOX-mediated collagen accumulation play a critical protective role in aneurysm formation in MFS.

GenTAC registry report: gender differences among individuals with genetically triggered thoracic aortic aneurysm and dissection

Previous data suggest women are at increased risk of death from aortic dissection. Therefore, we analyzed data from the GenTAC registry, the NIH-sponsored program that collects information about individuals with genetically triggered thoracic aortic aneurysms and cardiovascular conditions. We performed cross-sectional analyses in adults with Marfan syndrome (MFS), familial thoracic aortic aneurysm or dissection (FTAAD), bicuspid aortic valve (BAV) with thoracic aortic aneurysm or dissection, and subjects under 50 years of age with thoracic aortic aneurysm or dissection (TAAD <50 years). Women comprised 32% of 1,449 subjects and were 21% of subjects with BAV, 34% with FTAAD, 22% with TAAD <50 years, and 47% with MFS. Thoracic aortic dissections occurred with equal gender frequency yet women with BAV had more extensive dissections. Aortic size was smaller in women but was similar after controlling for BSA. Age at operation for aortic valve dysfunction, aneurysm or dissection did not differ by gender. Multivariate analysis (adjusting for age, BSA, hypertension, study site, diabetes, and subgroup diagnoses) showed that women had fewer total aortic surgeries (OR = 0.65, P < 0.01) and were less likely to receive angiotensin converting enzyme inhibitors (ACEi; OR = 0.68, P < 0.05). As in BAV, other genetically triggered aortic diseases such as FTAAD and TAAD <50 are more common in males. In women, decreased prevalence of aortic operations and less treatment with ACEi may be due to their smaller absolute aortic diameters. Longitudinal studies are needed to determine if women are at higher risk for adverse events.

The haploinsufficient Col3a1 mouse as a model for vascular Ehlers-Danlos syndrome

Vascular Ehlers-Danlos syndrome is a rare genetic disorder resulting from mutations in the α-1 chain of type III collagen (COL3A1) and manifesting as tissue fragility with spontaneous rupture of the bowel, gravid uterus, or large or medium arteries. The heterozygous Col3a1 knockout mouse was investigated as a model for this disease. The collagen content in the abdominal aorta of heterozygotes was reduced, and functional testing revealed diminishing wall strength of the aorta in these mice. Colons were grossly and histologically normal, but reduced strength and increased compliance of the wall were found in heterozygotes via pressure testing. Although mice demonstrated no life-threatening clinical signs or gross lesions of vascular subtype Ehlers-Danlos syndrome type IV, thorough histological examination of the aorta of heterozygous mice revealed the presence of a spectrum of lesions similar to those observed in human patients. Lesions increased in number and severity with age (0/5 [0%] in 2-month-old males vs 9/9 [100%] in 14-month-old males, P < .05) and were more common in male than female mice (23/26 [88.5%] vs 14/30 [46.7%] in 9- to 21-month-old animals, P < .05). Haploinsufficiency for Col3a1 in mice recapitulates features of vascular Ehlers-Danlos syndrome in humans and can be used as an experimental model.

Mutations in fibrillin-1 cause congenital scleroderma: stiff skin syndrome

The predisposition for scleroderma, defined as fibrosis and hardening of the skin, is poorly understood. We report that stiff skin syndrome (SSS), an autosomal dominant congenital form of scleroderma, is caused by mutations in the sole Arg-Gly-Asp sequence-encoding domain of fibrillin-1 that mediates integrin binding. Ordered polymers of fibrillin-1 (termed microfibrils) initiate elastic fiber assembly and bind to and regulate the activation of the profibrotic cytokine transforming growth factor-beta (TGFbeta). Altered cell-matrix interactions in SSS accompany excessive microfibrillar deposition, impaired elastogenesis, and increased TGFbeta concentration and signaling in the dermis. The observation of similar findings in systemic sclerosis, a more common acquired form of scleroderma, suggests broad pathogenic relevance.

Neuroradiologic manifestations of Loeys-Dietz syndrome type 1

BACKGROUND AND PURPOSE

Loeys-Dietz syndrome (LDS) is a recently described entity that has the triad of arterial tortuosity and aneurysms, hypertelorism, and bifid uvula or cleft palate. Its neuroradiologic manifestations have not been well delineated. We sought to describe the neuroradiologic features of LDS and to assess the manifestations that would warrant follow-up imaging.

MATERIALS AND METHODS

Two neuroradiologists retrospectively reviewed CT angiography (CTA), MR imaging, and plain film studies related to the head and neck in 25 patients ranging from 1 to 55 years of age, all of whom had positive genetic testing and clinical characteristics of LDS. Arterial tortuosity was evaluated by subjective assessment of 2D and 3D volumetric CTA and MR angiography data. Craniosynostosis and spinal manifestations were assessed by using plain films and CT images. MR images mostly of the head were reviewed for associated findings such as hydrocephalus, Chiari malformation, etc. Clinical manifestations were collated from the electronic patient record.

RESULTS

All patients had extreme arterial tortuosity, which is characteristic of this syndrome. Thirteen patients had scoliosis, 12 had craniosynostosis, 8 had intracranial aneurysms, 6 had spinal instability, 3 had dissections of the carotid and vertebrobasilar arteries, 3 had hydrocephalus, 4 had dural ectasia, 2 had a Chiari malformation, and 1 had intracranial hemorrhage as a complication of vascular dissection.

CONCLUSIONS

Significant neuroradiologic manifestations are associated with LDS, predominantly arterial tortuosity. Most of the patients in this series were young and, therefore, may require serial CTA monitoring for development of intra- and extracranial dissections and aneurysms, on the basis of the fact that most of the patients with pseudoaneurysms and dissection were older at the time of imaging. Other findings of LDS such as craniosynostosis, Chiari malformation, and spinal instability may also need to be addressed.

Extracellular microfibrils in development and disease

Fibrillins are the structural components of extracellular microfibrils that impart physical properties to tissues, alone or together with elastin as elastic fibers. Genetic studies in mice have revealed that fibrillin-rich microfibrils are also involved in regulating developmental programs and homeostatic processes through the modulation of TGF-beta/BMP signaling events. A new paradigm has thus emerged whereby the spatiotemporal organization of microfibrils dictates both the cellular activities and physical properties of connective tissues. These observations have paved the way to novel therapeutic approaches aimed at counteracting the life-threatening complications in human conditions caused by dysfunctions of fibrillin-rich microfibrils.

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.

Endothelial dysfunction and compromised eNOS/Akt signaling in the thoracic aorta during the progression of Marfan syndrome

BACKGROUND AND PURPOSE

Aortic complications account for the major mortality in Marfan syndrome (MFS), a connective tissue disorder caused by mutations in FBN1 encoding fibrillin-1. We hypothesized that MFS impaired endothelial function and nitric oxide (NO) production in the aorta.

EXPERIMENTAL APPROACH

Mice (at 3, 6, 9 and 12 months of age) heterozygous for the Fbn1 allele encoding a cysteine substitution (Fbn1 (C1039G/+), Marfan mice, n=75), the most common class of mutation in MFS, were compared with age-matched control littermates (n=75). Thoracic and abdominal aortas from the two groups were studied.

KEY RESULTS

Isometric force measurements revealed that relaxation to ACh (but not to sodium nitroprusside) was diminished in the phenylephrine-precontracted Marfan thoracic aorta at 6 months of age (pEC(50)=6.12+/-0.22; maximal response, E(max)=52.7+/-6.8%; control: pEC(50)=7.34+/-0.19; E(max)=84.8+/-2.2%). At one year, both inhibition of NO production with N(omega)-nitro-L-arginine methyl ester, or denudation of endothelium increased the phenylephrine-stimulated contraction in the control thoracic aorta by 35%, but had no effect in the Marfan aorta, indicating a loss of basal NO production in the Marfan vessel. From 6 months, a reduced phosphorylation of endothelial NOS (eNOS)(Ser1177) and Akt(Thr308) detected by Western blotting was observed in the Marfan thoracic aorta, which was accompanied by decreased levels of cGMP. Expressions of Akt and eNOS in the abdominal aorta were not different between the two groups.

CONCLUSIONS AND IMPLICATIONS

MFS impairs endothelial function and signaling of NO production in the thoracic aorta, suggesting the importance of NO in the age-related progression of thoracic aortic manifestations.

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.

Interaction between krit1 and icap1alpha infers perturbation of integrin beta1-mediated angiogenesis in the pathogenesis of cerebral cavernous malformation

Cerebral cavernous malformation (CCM) is a common autosomal dominant disorder characterized by venous sinusoids that predispose to intracranial hemorrhage. CCM is genetically heterogeneous, with loci at 7q, 7p and 3q. Mutations in KRIT1 account for all cases linked to 7q (CCM1), but the pathogenesis of CCM is not understood. Krev Interaction Trapped 1 (krit1) was originally identified through its interaction with the Ras-family GTPase krev1/rap1a in a two-hybrid screen, inferring a role in GTPase signaling cascades. We demonstrated additional 5'-coding exons for krit1, extending the N-terminus by 207 amino acids compared to the previously reported protein. Remarkably, by two-hybrid analysis and co-immunoprecipitation, full-length krit1 fails to interact with krev1/rap1a but shows strong interaction with integrin cytoplasmic domain-associated protein-1 (icap1). Icap1 binds to a NPXY motif in the cytoplasmic domain of beta1 integrin and participates in beta1-mediated cell adhesion and migration. The novel N-terminus of krit1 contains a NPXY motif that it is required for icap1 interaction. Like beta1 integrin, krit1 interacts with the 200 amino acid isoform of icap1 (icap1alpha), but not a 150 amino acid form that results from alternative splicing (icap1beta). In a competition assay, induced expression of krit1 diminishes the interaction between icap1alpha and beta1 integrin. Taken together, these data suggest that beta1 integrin and krit1 compete for the same site on icap1alpha, perhaps constituting a regulatory mechanism. Loss-of-function KRIT1 mutations, as observed in CCM1, would shift the balance with predicted consequences for endothelial cell performance during integrin beta1-dependent angiogenesis.

When the message goes awry: disease-producing mutations that influence mRNA content and performance

Mutations that cause disease commonly occur in the coding sequence and directly influence protein structure and function. However, many diseases result from mutations that influence various aspects of mRNA metabolism, including processing, export, stability, and translational control.

Familial thoracic aortic aneurysms and dissections: genetic heterogeneity with a major locus mapping to 5q13-14

BACKGROUND

Aneurysms and dissections affecting the ascending aorta are associated primarily with degeneration of the aortic media, called medial necrosis. Families identified with dominant inheritance of thoracic aortic aneurysms and dissections (TAA/dissections) indicate that single gene mutations can cause medial necrosis in the absence of an associated syndrome.

METHODS AND RESULTS

Fifteen families were identified with multiple members with TAAs/dissections. DNA from affected members from 2 of the families was used for a genome-wide search for the location of the defective gene by use of random polymorphic markers. The data were analyzed by the affected-pedigree-member method of linkage analysis. This analysis revealed 3 chromosomal loci with multiple markers demonstrating evidence of linkage to the phenotype. Linkage analysis using further markers in these regions and DNA from 15 families confirmed linkage of some of the families to 5q13-14. Genetic heterogeneity for the condition was confirmed by a heterogeneity test. Data from 9 families with the highest conditional probability of being linked to 5q were used to calculate the pairwise and multipoint logarithm of the odds (LOD) scores, with a maximum LOD of 4.74, with no recombination being obtained for the marker D5S2029. In 6 families, the phenotype was not linked to the 5q locus.

CONCLUSIONS

A major locus for familial TAAs and dissections maps to 5q13-14, with the majority (9 of 15) of the families identified demonstrating evidence of linkage to this locus. The condition is genetically heterogeneous, with 6 families not demonstrating evidence of linkage to any loci previously associated with aneurysm formation.

A strategy for disease gene identification through nonsense-mediated mRNA decay inhibition

Premature termination codons (PTCs) have been shown to initiate degradation of mutant transcripts through the nonsense-mediated messenger RNA (mRNA) decay (NMD) pathway. We report a strategy, termed gene identification by NMD inhibition (GINI), to identify genes harboring nonsense codons that underlie human diseases. In this strategy, the NMD pathway is pharmacologically inhibited in cultured patient cells, resulting in stabilization of nonsense transcripts. To distinguish stabilized nonsense transcripts from background transcripts upregulated by drug treatment, drug-induced expression changes are measured in control and disease cell lines with complementary DNA (cDNA) microarrays. Transcripts are ranked by a nonsense enrichment index (NEI), which relates expression changes for a given transcript in NMD-inhibited control and patient cell lines. The most promising candidates can be selected using information such as map location or biological function; however, an important advantage of the GINI strategy is that a priori information is not essential for disease gene identification. GINI was tested on colon cancer and Sandhoff disease cell lines, which contained previously characterized nonsense mutations in the MutL homolog 1 (MLH1) and hexosaminidase B (HEXB) genes, respectively. A list of genes was produced in which the MLH1 and HEXB genes were among the top 1% of candidates, thus validating the strategy.

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.

Phenotypic alteration of vascular smooth muscle cells precedes elastolysis in a mouse model of Marfan syndrome

Marfan syndrome is associated with early death due to aortic aneurysm. The condition is caused by mutations in the gene (FBN1) encoding fibrillin-1, a major constituent of extracellular microfibrils. Prior observations suggested that a deficiency of microfibrils causes failure of elastic fiber assembly during late fetal development. Mice homozygous for a targeted hypomorphic allele (mgR) of Fbn1 revealed a predictable sequence of abnormalities in the vessel wall including elastic fiber calcification, excessive deposition of matrix elements, elastolysis, and intimal hyperplasia. Here we describe previously unrecognized concordant findings in elastic vessels from patients with Marfan syndrome. Furthermore, ultrastructural analysis of mgR mice revealed cellular events that initiate destructive changes. The first detectable abnormality was an unusually smooth surface of elastic laminae, manifesting the loss of cell attachments that are normally mediated by fibrillin-1. Adjacent cells adopted alteration in their expression profile accompanied by morphological changes but retained expression of vascular smooth muscle cell markers. The abnormal synthetic repertoire of these morphologically abnormal smooth muscle cells in early vascular lesions included elastin, among other matrix elements, and matrix metalloproteinase 9, a known mediator of elastolysis. Ultimately, cell processes associated with zones of elastic fiber thinning and fragmentation. These data suggest that the loss of cell attachments signals a nonproductive program to synthesize and remodel an elastic matrix. This refined understanding of the pathogenesis of vascular disease in Marfan syndrome will facilitate the development of therapeutic strategies.

Familial Tetralogy of Fallot caused by mutation in the jagged1 gene

Tetralogy of Fallot (ToF) is the most common form of complex congenital heart disease, occurring in approximately 1 in 3000 live births. Evaluation of candidate loci in a large kindred segregating autosomal dominant ToF with reduced penetrance culminated in identification of a missense mutation (G274D) in JAG1, the gene encoding jagged1, a Notch ligand expressed in the developing right heart. Nine of eleven mutation carriers manifested cardiac disease, including classic ToF, ventricular septal defect with aortic dextroposition and isolated peripheral pulmonic stenosis (PPS). All forms of ToF were represented, including variants with pulmonic stenosis, pulmonic atresia and absent pulmonary valve. No individual within this family met diagnostic criteria for any previously described clinical syndrome, including Alagille syndrome (AGS), caused by haploinsufficiency for jagged1. All mutation carriers had characteristic but variable facial features, including long, narrow and upslanting palpebral fissures, prominent nasal bridge, square dental arch and broad, prominent chin. This appearance was distinct from that of unaffected family members and typical AGS patients. The glycine corresponding to position 274 is highly conserved in other epidermal growth factor-like domains of jagged1 and in those of other proteins. Its substitution in other proteins has been associated with mild or atypical variants of disease. These data support either a relative loss-of-function or a gain-of-function pathogenetic mechanism in this family and suggest that JAG1 mutations may contribute significantly to common variants of right heart obstructive disease.

Rent1, a trans-effector of nonsense-mediated mRNA decay, is essential for mammalian embryonic viability

The ability to detect and degrade transcripts that lack full coding potential is ubiquitous but non-essential in lower eukaryotes, leaving in question the evolutionary basis for complete maintenance of this function. One hypothesis holds that nonsense-mediated RNA decay (NMD) protects the organism by preventing the translation of truncated peptides with dominant negative or deleterious gain-of-function potential. All organisms studied to date that are competent for NMD express a structural homolog of Saccharomyces cerevisiae Upf1p. We have now explored the consequences of loss of NMD function in vertebrates through targeted disruption of the Rent1 gene in murine embryonic stem cells which encodes a mammalian ortholog of Upf1p. Mice heterozygous for the targeted allele showed no apparent phenotypic abnormalities but homozygosity was never observed, demonstrating that Rent1 is essential for embryonic viability. Homozygous targeted embryos show complete loss of NMD and are viable in the pre-implantation period, but resorb shortly after implantation. Furthermore, Rent1(-/-) blastocysts isolated at 3.5 days post-coitum undergo apoptosis in culture following a brief phase of cellular expansion. These data suggest that NMD is essential for mammalian cellular viability and support a critical role for the pathway in the regulated expression of selected physiologic transcripts.

Cloning of the murine Krit1 cDNA reveals novel mammalian 5' coding exons

Human KRIT1 (Krev interaction trapped 1), a defective gene product in cerebral cavernous malformation, was cloned from a HeLa cell cDNA library by virtue of its interaction with Krev/rap1A, a small ras-family GTPase. We have now characterized the full-length cDNA for the murine orthologue that encodes a predicted protein of 736 amino acids, 207 amino acids longer than the previously reported human protein. 5' Rapid amplification of cDNA ends analysis of mouse mRNA demonstrated a single transcriptional start site. The putative initiator codon was found within a context that conformed well to the Kozak consensus sequence and was preceded by an in-frame termination codon. BLAST analysis revealed that conceptual translation of a fragment of human genomic DNA upstream of the 5' end of the reported KRIT1 coding sequence predicts extension of the human open reading frame by 207 codons with 95% amino acid identity between the novel putative human and murine amino termini. This block of coding sequence was divided among four exons that are flanked by consensus splice site sequences. The extreme evolutionary conservation of this region, including a putative nuclear localization signal, indicates functional importance. These data have immediate relevance to mutation screening efforts in cerebral cavernous malformation and may contribute to our understanding of the normal biology of KRIT1 and the pathogenesis of this disorder.

Novel Upf2p orthologues suggest a functional link between translation initiation and nonsense surveillance complexes

Transcripts harboring premature signals for translation termination are recognized and rapidly degraded by eukaryotic cells through a pathway known as nonsense-mediated mRNA decay (NMD). In addition to protecting cells by preventing the translation of potentially deleterious truncated peptides, studies have suggested that NMD plays a broader role in the regulation of the steady-state levels of physiologic transcripts. In Saccharomyces cerevisiae, three trans-acting factors (Upf1p to Upf3p) are required for NMD. Orthologues of Upf1p have been identified in numerous species, showing that the NMD machinery, at least in part, is conserved through evolution. In this study, we demonstrate additional functional conservation of the NMD pathway through the identification of Upf2p homologues in Schizosaccharomyces pombe and humans (rent2). Disruption of S. pombe UPF2 established that this gene is required for NMD in fission yeast. rent2 was demonstrated to interact directly with rent1, a known trans-effector of NMD in mammalian cells. Additionally, fragments of rent2 were shown to possess nuclear targeting activity, although the native protein localizes to the cytoplasmic compartment. Finally, novel functional domains of Upf2p and rent2 with homology to eukaryotic initiation factor 4G (eIF4G) and other translational regulatory proteins were identified. Directed mutations within these so-called eIF4G homology (4GH) domains were sufficient to abolish the function of S. pombe Upf2p. Furthermore, using the two-hybrid system, we obtained evidence for direct interaction between rent2 and human eIF4AI and Sui1, both components of the translation initiation complex. Based on these findings, a novel model in which Upf2p and rent2 effects decreased translation and accelerated decay of nonsense transcripts through competitive interactions with eIF4G-binding partners is proposed.

Mouse models of genetic diseases resulting from mutations in elastic fiber proteins

The inability to study appropriate human tissues at various stages of development has precluded the elaboration of a thorough understanding of the pathogenic mechanisms leading to diseases linked to mutations in genes for elastic fiber proteins. Recently, new insights have been gained by studying mice harboring targeted mutations in the genes that encode fibrillin-1 and elastin. These genes have been linked to Marfan syndrome (MFS) and supravalvular aortic stenosis (SVAS), respectively. For fibrillin-1, mouse models have revealed that phenotype is determined by the degree of functional impairment. The haploinsufficiency state or the expression of low levels of a product with dominant-negative potential from one allele is associated with mild phenotypes with a predominance of skeletal features. Exuberant expression of a dominant-negative-acting protein leads to the more severe MFS phenotype. Mice harboring targeted deletion of the elastin gene (ELN) show many of the features of SVAS in humans, including abnormalities in the vascular wall and altered hemodynamics associated with changes in wall compliance. The genetically altered mice suggest that SVAS is predominantly a disease of haploinsufficiency. These studies have underscored the prominent role of the elastic matrix in the morphogenesis and homeostasis of the vessel wall.

Nonsense-mediated mRNA decay in health and disease

All eukaryotes possess the ability to detect and degrade transcripts harboring premature signals for the termination of translation. Despite the ubiquitous nature of nonsense-mediated mRNA decay (NMD) and its demonstrated role in the modulation of phenotypes resulting from selected nonsense alleles, very little is known regarding its basic mechanism or the selective pressure for complete evolutionary conservation of this function. This review will present the current models of NMD that have been generated during the study of model organisms and mammalian cells. The physiological burden of nonsense transcripts and the emerging view that NMD plays a broad and critical role in the regulation of gene expression will also be discussed. Such issues are relevant to the proposal that pharmacological manipulation of NMD will find therapeutic application.

Enzymatic mutation detection (EMD) of novel mutations (R565X and R1523X) in the FBN1 gene of patients with Marfan syndrome using T4 endonuclease VII

The Enzymatic Mutation Detection (EMDtrade mark) method is a streamlined and improved version of the original Enzymatic Cleavage of Mismatch (EMC) method. EMD is a fully homogeneous, rapid four step procedure that allows for detection and localization of mismatched or unmatched nucleotides within heteroduplex DNA. To test the utility of EMD for use in the screening of large and complex genes, the fibrillin 1 (FBN1) gene was scanned in a cohort of six patients diagnosed with connective tissue disorders. Four of the six patients were diagnosed with classic Marfan syndrome (MFS). The results were compared with a previous MDEtrade mark scanning of the same patient cohort. Two causative mutations, R565X and R1523X, were detected by EMD that were not detected by MDE. In both cases, the mutation resulted in premature termination of translation. In addition, several polymorphisms were detected by the enzymatic approach that failed detection by heteroduplex analysis. We propose that the EMD method is a sensitive and rapid approach to mutation detection in large genes such as FBN1.

Mutations in KRIT1 in familial cerebral cavernous malformations

OBJECTIVES

The recognition of six unrelated Hispanic-American families in which cerebral cavernous malformations (CCM) segregated as an autosomal dominant trait established a genetic basis for this disease. Linkage analysis subsequently identified locus heterogeneity with disease genes for CCM at chromosomal regions 7q, 7p, and 3q. Recently, mutations in KRIT1, a gene on 7q at the CCM1 locus, were identified in French and Hispanic-American families with CCM. This study confirms the identity the KRIT1 founder mutation in Hispanic-Americans and reports a novel KRIT1 mutation in a Caucasian family.

METHODS

Oligonucleotide primers were designed to allow amplification of genomic DNA sequences from four Hispanic-American families and five non-Hispanic families for all 12 exons of the KRIT1 gene using the polymerase chain reaction (PCR). The amplified DNA was then screened using single strand conformation polymorphism analysis (SSCP) and sequencing. The expression pattern of KRIT1 was analyzed by Northern blotting.

RESULTS

Analysis of the KRIT1 gene revealed a point mutation in exon 6 that predicts the substitution of a premature termination codon for glutamine at codon 248 in all four Hispanic-American families, confirming previous findings. SSCP analysis and sequencing revealed an 11 base pair duplication in exon 7 leading to a premature termination codon in one Caucasian family. Northern analysis demonstrated widespread expression of this gene, however, the highest level of expression was in the brain.

CONCLUSION

The common KRIT1 mutation causing the majority of CCM in Hispanic-Americans has been identified and independently confirmed, allowing efficient presymptomatic molecular diagnosis. In keeping with prior results, both newly identified mutations create a premature termination codon and are predicted to initiate degradation of the mutant mRNA through the nonsense-mediated mRNA decay pathway. These data strongly suggest loss of function as the relevant patho-genetic mechanism.

Pathogenetic sequence for aneurysm revealed in mice underexpressing fibrillin-1

Dissecting aortic aneurysm is the hallmark of Marfan syndrome (MFS) and the result of mutations in fibrillin-1, the major constituent of elastin-associated extracellular microfibrils. It is yet to be established whether dysfunction of fibrillin-1 perturbs the ability of the elastic vessel wall to sustain hemodynamic stress by disrupting microfibrillar assembly, by impairing the homeostasis of established elastic fibers, or by a combination of both mechanisms. The pathogenic sequence responsible for the mechanical collapse of the elastic lamellae in the aortic wall is also unknown. Targeted mutation of the mouse fibrillin-1 gene has recently suggested that deficiency of fibrillin-1 reduces tissue homeostasis rather than elastic fiber formation. Here we describe another gene-targeting mutation, mgR, which shows that underexpression of fibrillin-1 similarly leads to MFS-like manifestations. Histopathological analysis of mgR/mgR specimens implicates medial calcification, the inflammatory-fibroproliferative response, and inflammation-mediated elastolysis in the natural history of dissecting aneurysm. More generally, the phenotypic severity associated with various combinations of normal and mutant fibrillin-1 alleles suggests a threshold phenomenon for the functional collapse of the vessel wall that is based on the level and the integrity of microfibrils.

Revised genomic organization of FBN1 and significance for regulated gene expression

FBN1 encodes fibrillin-1, an extracellular matrix protein that is defective in Marfan syndrome. This gene is divided into 65 exons and was previously reported to be approximately 110 kb in length. The existence of 3 exons upstream of the exon containing the putative initiating methionine left open the possibility of alternative fibrillin-1 isoforms that vary at their N-termini. Detailed examination of YACs containing human FBN1 reveal that the gene is 200 kb, almost twice as large as previously thought. Characterization of the porcine FBN1 cDNA and 5' flanking sequence demonstrates extreme conservation between the pig and the human predicted proteins and argues against the possibility of alternative amino-terminal coding sequence. These data further our understanding of the regulatory requirements for gene expression and establish a framework for recombinant expression of fibrillin-1.

Multiple molecular mechanisms underlying subdiagnostic variants of Marfan syndrome

Mutations in the FBN1 gene, which encodes fibrillin-1, cause Marfan syndrome (MFS) and have been associated with a wide range of milder, overlap phenotypes. The factors that modulate phenotypic severity, both between and within families, remain to be determined. This study examines the relationship between the FBN1 genotype and phenotype in families with extremely mild phenotypes and in those that show striking clinical variation among apparently affected individuals. In one family, clinically similar but etiologically distinct disorders are segregating independently. In another, somatic mosaicism for a mutant FBN1 allele is associated with subdiagnostic manifestations, whereas germ-line transmission of the identical mutation causes severe and rapidly progressive disease. A third family cosegregates mild mitral valve prolapse syndrome with a mutation in FBN1 that can be functionally distinguished from those associated with the classic MFS phenotype. These data have immediate relevance for the diagnostic and prognostic counseling of patients and their family members.

Restoration of open reading frame resulting from skipping of an exon with an internal deletion in the COL7A1 gene

Restoration of open reading frame resulting from exon skipping has been documented as a mechanism of rescuing an mRNA transcript containing mutations. In this study, we describe a mutation in a family with dystrophic epidermolysis bullosa consisting of a 16-bp deletion within exon 87 of the type VII collagen gene (COL7A1) and predicted to lead to a frameshift and downstream premature termination codon. Unexpectedly, mRNA analysis revealed instead that the intraexonic deletion led to skipping of exon 87 and subsequent restoration of the open reading frame. The phenotypes and patterns of inheritance observed in this family arise from three different mutations, all of which affect RNA processing. Restoration of open reading frame by exon skipping represents a previously undescribed mechanism of action of intraexonic deletion mutations.

A mutated human homologue to yeast Upf1 protein has a dominant-negative effect on the decay of nonsense-containing mRNAs in mammalian cells

All eukaryotic cells analyzed have developed mechanisms to eliminate the production of mRNAs that prematurely terminate translation. The mechanisms are thought to exist to protect cells from the deleterious effects of in-frame nonsense codons that are generated by routine inefficiencies and inaccuracies in RNA metabolism such as pre-mRNA splicing. Depending on the particular mRNA and how it is produced, nonsense codons can mediate a reduction in mRNA abundance either (i) before its release from an association with nuclei into the cytoplasm, presumably but not certainly while the mRNA is being exported to the cytoplasm and translated by cytoplasmic ribosomes, or (ii) in the cytoplasm. Here, we provide evidence for a factor that functions to eliminate the production of nonsense-containing RNAs in mammalian cells. The factor, variously referred to as Rent1 (regulator of nonsense transcripts) or HUPF1 (human Upf1 protein), was identified by isolating cDNA for a human homologue to Saccharomyces cerevisiae Upf1p, which is a group I RNA helicase that functions in the nonsense-mediated decay of mRNA in yeast. Using monkey COS cells and human HeLa cells, we demonstrate that expression of human Upf1 protein harboring an arginine-to-cysteine mutation at residue 844 within the RNA helicase domain acts in a dominant-negative fashion to abrogate the decay of nonsense-containing mRNA that takes place (i) in association with nuclei or (ii) in the cytoplasm. These findings provide evidence that nonsense-mediated mRNA decay is related mechanistically in yeast and in mammalian cells, regardless of the cellular site of decay.

The surveillance complex interacts with the translation release factors to enhance termination and degrade aberrant mRNAs

The nonsense-mediated mRNA decay pathway is an example of an evolutionarily conserved surveillance pathway that rids the cell of transcripts that contain nonsense mutations. The product of the UPF1 gene is a necessary component of the putative surveillance complex that recognizes and degrades aberrant mRNAs. Recent results indicate that the Upf1p also enhances translation termination at a nonsense codon. The results presented here demonstrate that the yeast and human forms of the Upf1p interact with both eukaryotic translation termination factors eRF1 and eRF3. Consistent with Upf1p interacting with the eRFs, the Upf1p is found in the prion-like aggregates that contain eRF1 and eRF3 observed in yeast [PSI+] strains. These results suggest that interaction of the Upf1p with the peptidyl release factors may be a key event in the assembly of the putative surveillance complex that enhances translation termination, monitors whether termination has occurred prematurely, and promotes degradation of aberrant transcripts.

Preimplantation genetic diagnosis of human embryos for Marfan's syndrome

PURPOSE

Single-cell nested polymerase chain reaction (PCR) and Ddel endonuclease digestion were used to detect the presence of a Marfan's syndrome mutation in human preimplantation embryos derived from in vitro fertilization (IVF). These procedures were conducted to eliminate the possibility of transmission of the affected allele from the father to his offspring. The mutation on chromosome 15 is transmitted as an autosomal dominant trait, and the chance of having a child affected with the disease is 50%.

METHODS

A couple presented to the Program for In Vitro Fertilization, Reproductive Surgery and Infertility for preimplantation genetic diagnosis. IVF was performed and embryo biopsy was done on day 3 embryos. Single blastomeres were removed from embryos and subjected to nested PCR analysis and endonuclease digestion to detect a Marfan's syndrome mutation located on chromosome 15 inherited from the father.

RESULTS

Thirteen oocytes were injected with spermatozoa using intracytoplasmic sperm injection, and nine fertilized normally. Following embryo biopsy and polymerase chain reaction amplification-Ddel endonuclease digestion, five embryos were detected that were positive for the mutation. The four non-affected embryos were transferred to the uterus, resulting in a healthy and normal ongoing pregnancy.

Targetting of the gene encoding fibrillin-1 recapitulates the vascular aspect of Marfan syndrome

Aortic aneurysm and dissection account for about 2% of all deaths in industrialized countries; they are also components of several genetic diseases, including Marfan syndrome (MFS). The vascular phenotype of MFS results from mutations in fibrillin-1 (FBN1), the major constituent of extracellular microfibrils. Microfibrils, either associated with or devoid of elastin, give rise to a variety of extracellular networks in elastic and non-elastic tissues. It is believed that microfibrils regulate elastic fibre formation by guiding tropo-elastin deposition during embryogenesis and early post-natal life. Hence, vascular disease in MFS is thought to result when FBN1 mutations preclude elastic fibre maturation by disrupting microfibrillar assembly. Here we report a gene-targetting experiment in mice that indicates that fibrillin-1 microfibrils are predominantly engaged in tissue homeostasis rather than elastic matrix assembly. This finding, in turn, suggests that aortic dilation is due primarily to the failure by the microfibrillar array of the adventitia to sustain physiological haemodynamic stress, and that disruption of the elastic network of the media is a secondary event.

Inhibition of fibrillin 1 expression using U1 snRNA as a vehicle for the presentation of antisense targeting sequence

This study examines whether the mimicking of selected properties of naturally occurring antisense RNAs in prokaryotes allows efficient inhibition of gene expression by in situ-expressed recombinant molecules in mammalian cells. Prokaryotic regulatory transcripts are expressed at high levels and have hairpin structures at their termini, features reminiscent of small nuclear RNAs (snRNAs) which are abundant and stable in the nucleus of all mammalian cells. A sequence complementary to fibrillin-1 (FBN1) mRNA, interrupted in its center by a hammerhead ribozyme, was substituted for the Sm protein binding site between the stem-loop structures of U1 snRNA. Expression of the chimeric antisense RNA resulted in dramatic inhibition of expression of fibrillin-1 message and protein in stably transfected cultured cells. The inhibitory effect was localized to the nucleus. The biological properties of U1 snRNA may provide a widely applicable vehicle for the in vivo delivery of antisense targeting sequences.

Glanzmann thrombasthenia. Cooperation between sequence variants in cis during splice site selection

Glanzmann thrombasthenia (GT), an autosomal recessive bleeding disorder, results from abnormalities in the platelet fibrinogen receptor, GP(IIb)-IIIa (integrin alpha(IIb)beta3). A patient with GT was identified as homozygous for a G-->A mutation 6 bp upstream of the GP(IIIa) exon 9 splice donor site. Patient platelet GP(IIIa) transcripts lacked exon 9 despite normal DNA sequence in all of the cis-acting sequences known to regulate splice site selection. In vitro analysis of transcripts generated from mini-gene constructs demonstrated that exon skipping occurred only when the G-->A mutation was cis to a polymorphism 116 bp upstream, providing precedence that two sequence variations in the same exon which do not alter consensus splice sites and do not generate missense or nonsense mutations, can affect splice site selection. The mutant transcript resulted from utilization of a cryptic splice acceptor site and returned the open reading frame. These data support the hypothesis that pre-mRNA secondary structure and allelic sequence variants can influence splicing and provide new insight into the regulated control of RNA processing. In addition, haplotype analysis suggested that the patient has two identical copies of chromosome 17. Markers studied on three other chromosomes suggested this finding was not due to consanguinity. The restricted phenotype in this patient may provide information regarding the expression of potentially imprinted genes on chromosome 17.

Mammalian orthologues of a yeast regulator of nonsense transcript stability

All eukaryotes that have been studied to date possess the ability to detect and degrade transcripts that contain a premature signal for the termination of translation. This process of nonsense-mediated RNA decay has been most comprehensively studied in the yeast Saccharomyces cerevisiae where at least three trans-acting factors (Upf1p through Upf3P) are required. We have cloned cDNAs encoding human and murine homologues of Upf1p, termed rent1 (regulator of nonsense transcripts). Rent1 is the first identified mammalian protein that contains all of the putative functional elements in Upf1p including zinc finger-like and NTPase domains, as well as all motifs common to members of helicase superfamily I. Moreover, expression of a chimeric protein, N and C termini of Upf1p, complements the Upf1p-deficient phenotype in yeast. Thus, despite apparent differences between yeast and mammalian nonsense-mediated RNA decay, these data suggest that the two pathways use functionally related machinery.

Both the wild type and a functional isoform of CFTR are expressed in kidney

The cystic fibrosis transmembrane conductance regulator (CFTR) consists of five domains, two transmembrane-spanning domains, each composed of six transmembrane segments, a regulatory domain, and two nucleotide-binding domains (NBDs). CFTR is expressed in kidney, but its role in overall renal function is not well understood, because mutations in CFTR found in patients with cystic fibrosis are not associated with renal dysfunction. To learn more about the distribution and functional forms of CFTR in kidney, we used a combination of molecular, cell biological, and electrophysiological approaches. These include an evaluation of CFTR mRNA and protein expression, as well as both two-electrode and patch clamping of CFTR expressed either in Xenopus oocytes or mammalian cells. In addition to wild-type CFTR mRNA, an alternate form containing only the first transmembrane domain (TMD), the first NBD, and the regulatory domain (TNR-CFTR) is expressed in kidney. Although missing the second set of TMDs and the second NBD, when expressed in Xenopus oocytes, TNR-CFTR has cAMP-dependent protein kinase A (PKA)-stimulated single Cl- channel characteristics and regulation of PKA activation of outwardly rectifying Cl- channels that are very similar to those of wild-type CFTR. TNR-CFTR mRNA is produced by an unusual mRNA processing mechanism and is expressed in a tissue-specific manner primarily in renal medulla.

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.

Mutation in fibrillin-1 and the Marfanoid-craniosynostosis (Shprintzen-Goldberg) syndrome

Recent reports have described a distinct and recurrent pattern of systemic malformation that associates craniosynostosis and neurodevelopmental abnormalities with many clinical features of the Marfan syndrome (MFS), an autosomal dominant disorder of the extracellular microfibril caused by defects in the gene encoding fibrillin-1, FBN1 (ref. 8). Additional common findings include other craniofacial anomalies, hypotonia, obstructive apnea, foot deformity, and congenital weakness of the abdominal wall. So far, only 11 cases have been reported precluding the assignment of definitive diagnostic criteria. While it remains unclear whether these cases represent a discrete clinical entity with a single aetiology, they have been pragmatically grouped under the rubric Marfanoid-craniosynostosis or Shprintzen-Goldberg syndrome (SGS). Because of the significant clinical overlap between MFS and SGS, we proposed that they may be caused by allelic mutations. We now report two SGS patients who harbour mutations in FBN1. While it remains unclear whether these mutations are sufficient for the clinical expression of the entire SGS phenotype, these data suggest a role for fibrillin-1 in early craniofacial and central nervous system development. Our recent observation that FBN1 transcript is expressed as early as the 8-cell stage of human embryogenesis is consistent with this hypothesis.

Evaluation of inhA gene and catalase-peroxidase gene among isoniazid-sensitive and resistant Mycobacterium tuberculosis isolates

The katG gene and the inhA gene of 30 INH-resistant (INH-R) and 28 INH-sensitive (INH-S) isolates of M. tuberculosis from Haiti and Maryland were analysed by PCR to establish the presence and frequency of two postulated mechanisms of INH-resistance, total katG gene deletion and inhA Ser94 to Ala94 amino acid substitution. Only two of 30 INH-R isolates (3%) appear to have total katG gene deletions. All 28 INH-S isolates (100%) produced a PCR product at both the 5' and the 3' ends of the katG gene. Gene deletion of katG is a rare mechanism of INH resistance. Allele specific oligonucleotide hybridisation analysis of the inhA PCR products from the same 58 isolates revealed no mutation at amino acid 94 or directly surrounding it. Other inhA gene mutations may be responsible for INH resistance in M. tuberculosis. Diagnostic strategies using katG gene deletion or inhA Ser94 mutations would fail to detect almost all INH-R isolates.

The Marfan syndrome and the cardiovascular surgeon

The authors present the current status of surgery for the cardiovascular manifestations of the Marfan syndrome. In addition, a brief review of current Marfan genetic research is presented. Data on all Marfan patients undergoing aortic root replacement at the Johns Hopkins Hospital (September 1976-June 1995) were analyzed. Survival and event-free curves were calculated and risk factors for early and late death were determined by univariate and multivariate analysis. Two hundred twelve Marfan patients underwent aortic root replacement using composite graft (202), homograft (8) or valve-sparing procedures (2). One hundred eighty-five patients underwent elective repair with no 30-day mortality. Twenty-seven patients underwent urgent surgery, primarily for acute dissection; two patients with aortic rupture died in the operating room. Actuarial survival of the 212 patients was 88% at 5 years, 78% at 10 years and 71% at 14 years. By multivariate analysis, only poor NYHA class, male gender and urgent surgery emerged as significant independent predictors of early or late mortality. Histologic examination of excised Marfan aortic leaflets by immunofluorescent staining for fibrillin showed fragmentation of elastin-associated microfibrils. These studies suggest cautious use of valve-sparing procedures in Marfan patients. Over the last 5 years significant progress has been made in identifying mutant genes that code for defective fibrillin microfibrils in Marfan patients. Attempts are underway to develop animal models of Marfan disease for study of possible gene therapy. Aortic root replacement can be performed in Marfan patients with operative risk under 5%. Long-term results are gratifying. At present, valve-sparing procedures should be used cautiously in Marfan patients because of fibrillin abnormalities in the preserved aortic valve leaflets.

Mutations in the human gene for fibrillin-1 (FBN1) in the Marfan syndrome and related disorders

The extracellular microfibril, 10-14 nm in diameter, performs a number of functions, including serving as the scaffolding for deposition of tropoelastin to form elastic fibers. A variety of proteins compose the structure of microfibrils, the most prominent of which are the two fibrillins. Fibrillin-1 is encoded by FBN1 on human chromosome 15q21 and fibrillin-2 is encoded by FBN2 on 5q23. Each fibrillin monomer contains a large number of epidermal growth factor-like motifs, most capable of binding calcium ions, and a few motifs resembling the binding protein for transforming growth factor beta. In vitro polymerization of fibrillin monomers produces 'beads on a string' structures that look on electron microscopy much like microfibrils purified from the extracellular matrices of a variety of tissues. Mutations in FBN1 produce Marfan syndrome, a pleiotropic autosomal dominant connective tissue disorder with prominent manifestations in the skeleton, eye and cardiovascular system. A number of conditions related to Marfan syndrome are also due to FBN1 mutations. Contractural arachnodactyly is due to mutations in FBN2. In this paper we review the published mutations in these genes, preliminary results of genotype-phenotype correlations, and speculations regarding molecular pathogenesis.

Marfan syndrome as a paradigm for transcript-targeted preimplantation diagnosis of heterozygous mutations

Among the many clinical applications of the polymerase chain reaction (PCR) is its potential use in preimplantation diagnosis of genetic disorders. Performing PCR on single blastomeres from early cleavage stage (six- to eight-cell) human embryos should, in principle, enable reliable determination of disease status for certain inherited conditions. However, reports of misdiagnoses using this technique have diminished enthusiasm for its widespread clinical use. One principal source of error is the propensity for genome-targeted PCR to exclusively amplify one allele in reactions assaying a single heterozygous diploid cell. Complete reaction failure is also common. Employing the Marfan syndrome (MFS) as a paradigm, we have developed a reliable, reverse transcription-PCR-based method of genotyping single cells that overcomes these obstacles. The technique should facilitate accurate preimplantation diagnosis of MFS and other selected genetic diseases caused by heterozygous or compound-heterozygous mutations.

Fifteen novel FBN1 mutations causing Marfan syndrome detected by heteroduplex analysis of genomic amplicons

Mutations in the gene encoding fibrillin-1 (FBN1), a component of the extracellular microfibril, cause the Marfan syndrome (MFS). This statement is supported by the observations that the classic Marfan phenotype cosegregates with intragenic and/or flanking marker alleles in all families tested and that a significant number of FBN1 mutations have been identified in affected individuals. We have now devised a method to screen the entire coding sequence and flanking splice junctions of FBN1. On completion for a panel of nine probands with classic MFS, six new mutations were identified that accounted for disease in seven (78%) of nine patients. Nine additional new mutations have been characterized in the early stages of a larger screening project. These 15 mutations were equally distributed throughout the gene and, with one exception, were specific to single families. One-third of mutations created premature termination codons, and 6 of 15 substituted residues with putative significance for calcium binding to epidermal growth factor (EGF)-like domains. Mutations causing severe and rapidly progressive disease that presents in the neonatal period can occur in a larger region of the gene than previously demonstrated, and the nature of the mutation is as important a determinant as its location, in predisposing to this phenotype.

Expression of a mutant human fibrillin allele upon a normal human or murine genetic background recapitulates a Marfan cellular phenotype

The Marfan syndrome (MFS) is a connective tissue disorder inherited as an autosomal dominant trait and caused by mutations in the gene encoding fibrillin, a 350-kD glycoprotein that multimerizes to form extracellular microfibrils. It has been unclear whether disease results from a relative deficiency of wild-type fibrillin; from a dominant-negative effect, in which mutant fibrillin monomers disrupt the function of the wild-type protein encoded by the normal allele; or from a dynamic and variable interplay between these two pathogenetic mechanisms. We have now addressed this issue in a cell culture system. A mutant fibrillin allele from a patient with severe MFS was expressed in normal human and murine fibroblasts by stable transfection. Immunohistochemical analysis of the resultant cell lines revealed markedly diminished fibrillin deposition and disorganized microfibrillar architecture. Pulse-chase studies demonstrated normal levels of fibrillin synthesis but substantially reduced deposition into the extracellular matrix. These data illustrate that expression of a mutant fibrillin allele, on a background of two normal alleles, is sufficient to disrupt normal microfibrillar assembly and reproduce the MFS cellular phenotype. This underscores the importance of the fibrillin amino-terminus in normal microfibrillar assembly and suggests that expression of the human extreme 5' fibrillin coding sequence may be sufficient, in isolation, to produce an animal model of MFS. Lastly, this substantiation of a dominant-negative effect offers mutant allele knockout as a potential strategy for gene therapy.

Maintenance of an open reading frame as an additional level of scrutiny during splice site selection

Although nonsense mutations have been associated with the skipping of specific constitutively spliced exons in selected genes, notably the fibrillin gene, the basis for this association is unclear. Now, using chimaeric constructs in a model in vivo expression system, premature termination codons are identified as determinants of splice site selection. Nonsense codon recognition prior to RNA splicing necessitates the ability to read the frame of precursor mRNA in the nucleus. We propose that maintenance of an open reading frame can serve as an additional level of scrutiny during exon definition. This process may have pathogenic and evolutionary significance.