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-ß (TGF-ß)- 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-ß signaling pathways. The blockade of angiotensin II type I receptor with losartan restored TGF-ß 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.

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.

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.

A missense mutation in type VII collagen in two affected siblings with recessive dystrophic epidermolysis bullosa.

Recessive dystrophic epidermolysis bullosa is a severe mutilating genodermatosis. Previous ultrastructural demonstrations of altered anchoring fibrils, and recent genetic linkage analyses have suggested that type VII collagen, the major component of anchoring fibrils, is a candidate gene. We have identified a homozygous methionine-to-lysine mutation in two affected siblings, while their unaffected mother and half-brother are heterozygous carriers. The mutation resides in a highly conserved region of the C-terminus of type VII collagen, strongly suggesting that it is the cause of the disease in this family.

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.

Cardiovascular Outcomes in Aortopathy: GenTAC Registry of Genetically Triggered Aortic Aneurysms and Related Conditions.

BACKGROUND: The GenTAC (Genetically Triggered Thoracic Aortic Aneurysm and Cardiovascular Conditions) Registry enrolled patients with genetic aortopathies between 2007 and 2016. OBJECTIVES: The purpose of this study was to compare age distribution and probability of elective surgery for proximal aortic aneurysm, any dissection surgery, and cardiovascular mortality among aortopathy etiologies. METHODS: The GenTAC study had a retrospective/prospective design. Participants with bicuspid aortic valve (BAV) with aneurysm (n = 879), Marfan syndrome (MFS) (n = 861), nonsyndromic heritable thoracic aortic disease (nsHTAD) (n = 378), Turner syndrome (TS) (n = 298), vascular Ehlers-Danlos syndrome (vEDS) (n = 149), and Loeys-Dietz syndrome (LDS) (n = 121) were analyzed. RESULTS: The 25% probability of elective proximal aortic aneurysm surgery was 30 years for LDS (95% CI: 18-37 years), followed by MFS (34 years; 95% CI: 32-36 years), nsHTAD (52 years; 95% CI: 48-56 years), and BAV (55 years; 95% CI: 53-58 years). Any dissection surgery 25% probability was highest in LDS (38 years; 95% CI: 33-53 years) followed by MFS (51 years; 95% CI: 46-57 years) and nsHTAD (54 years; 95% CI: 51-61 years). BAV experienced the largest relative frequency of elective surgery to any dissection surgery (254/33 = 7.7), compared with MFS (273/112 = 2.4), LDS (35/16 = 2.2), or nsHTAD (82/76 = 1.1). With MFS as the reference population, risk of any dissection surgery or cardiovascular mortality was lowest in BAV patients (HR: 0.13; 95% CI: 0.08-0.18; HR: 0.13; 95%: CI: 0.06-0.27, respectively). The greatest risk of mortality was seen in patients with vEDS. CONCLUSIONS: Marfan and LDS cohorts demonstrate age and event profiles congruent with the current understanding of syndromic aortopathies. BAV events weigh toward elective replacement with relatively few dissection surgeries. Nonsyndromic HTAD patients experience near equal probability of dissection vs prophylactic surgery, possibly because of failure of early diagnosis.

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.

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.

The skipping of constitutive exons in vivo induced by nonsense mutations.

Nonsense mutations create a premature signal for the termination of translation of messenger RNA. Such mutations have been observed to cause a severe reduction in the amount of mutant allele transcript or to generate a peptide truncated at the carboxyl end. Analysis of fibrillin transcript from a patient with Marfan syndrome revealed the skipping of a constitutive exon containing a nonsense mutation. Similar results were observed for two nonsense mutations in the gene encoding ornithine delta-aminotransferase from patients with gyrate atrophy. All genomic DNA sequences flanking these exons that are known to influence RNA splicing were unaltered, which suggests that nonsense mutations can alter splice site selection in vivo.

Quantitative differences in biosynthesis and extracellular deposition of fibrillin in cultured fibroblasts distinguish five groups of Marfan syndrome patients and suggest distinct pathogenetic mechanisms.

Pulse-chase studies of [35S]cysteine-labeled fibrillin were performed on fibroblast strains from 55 patients with Marfan syndrome (MFS), including 13 with identified mutations in the fibrillin-1 gene and 10 controls. Quantitation of the soluble intracellular and insoluble extracellular fibrillin allowed discrimination of five groups. Groups I (n = 8) and II (n = 19) synthesize reduced amounts of normal-sized fibrillin, while synthesis is normal in groups III (n = 6), IV (n = 18), and V (n = 4). When extracellular fibrillin deposition is measured, groups I and III deposit between 35 and 70% of control values, groups II and IV < 35%, and group V > 70%. A deletion mutant with a low transcript level from the mutant allele and seven additional patients have the group I protein phenotype. Disease in these patients is caused by a reduction in microfibrils associated with either a null allele, an unstable transcript, or an altered fibrillin product synthesized in low amounts. In 68% of the MFS individuals (groups II and IV), a dominant negative effect is invoked as the main pathogenetic mechanism. Products made by the mutant allele in these fibroblasts are proposed to interfere with microfibril formation. Insertion, deletion, and exon skipping mutations, resulting in smaller fibrillin products, exhibit the group II phenotype. A truncated form of fibrillin of 60 kD was identified with specific fibrillin antibodies in one of the group II cell culture media. Seven of the nine known missense mutations, giving rise to abnormal, but normal-sized fibrillin molecules, are in group IV.

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.

A mutation in the surfactant protein B gene responsible for fatal neonatal respiratory disease in multiple kindreds.

To determine the molecular defect accounting for the deficiency of pulmonary surfactant protein B (SP-B) in full-term neonates who died from respiratory failure associated with alveolar proteinosis, the sequence of the SP-B transcript in affected infants was ascertained. A frameshift mutation consisting of a substitution of GAA for C in codon 121 of the SP-B cDNA was identified. The three affected infants in the index family were homozygous for this mutation, which segregated in a fashion consistent with autosomal recessive inheritance of disease. The same mutation was found in two other unrelated infants who died from alveolar proteinosis, one of whom was also homozygous, and in the parents of an additional unrelated, affected infant, but was not observed in 50 control subjects. We conclude that this mutation is responsible for SP-B deficiency and neonatal alveolar proteinosis in multiple families and speculate that the disorder is more common than was recognized previously.

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.

Marfan phenotype variability in a family segregating a missense mutation in the epidermal growth factor-like motif of the fibrillin gene.

To examine the associations among fibrillin gene mutations, protein function, and Marfan syndrome phenotype, we screened for alterations in the fibrillin coding sequence in patients with a range of manifestations and clinical severity. A cysteine to serine substitution at codon 1409 (C1409S) was identified in an epidermal growth factor (EGF)-like motif from one fibrillin allele which segregates with the disease phenotype through three generations of a family affected with the Marfan syndrome. This alteration was not observed in 60 probands from other families or in 88 unrelated normal individuals. The altered cysteine is completely conserved in all EGF-like motifs identified in fibrillin, and in all proteins that contain this motif. These observations strongly indicate that C1409S is the disease-producing mutation in this family. The phenotype of individuals carrying C1409S varied widely with respect to onset of disease, organ-system involvement, and clinical severity; certain affected adults were unaware of their status before being diagnosed through this investigation. We conclude that fibrillin gene defects cause familial Marfan syndrome, that mutations in the EGF-like motif of the fibrillin gene are not uniformly associated with severe disease, and that fibrillin genotype is not the sole determinant of Marfan phenotype.

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.

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.

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.

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.