Mutations in the fibrillin gene responsible for dominant ectopia lentis and neonatal Marfan syndrome.

Mutations in the gene coding for fibrillin on chromosome 15 (FBN1) are known to cause Marfan syndrome (MFS). A related disorder, dominant ectopia lentis (EL), has also been linked genetically to this locus. We now describe ten novel mutations of FBN1 resulting in strikingly different phenotypes. In addition to classic MFS, FBN1 mutations also give rise to EL and a severe neonatal form of MFS. Interestingly, the neonatal MFS mutations are clustered in one particular region of FBN1, possibly providing new insights into genotype-phenotype comparisons.

Badly engineered fibrillin lessons from molecular studies of marfan syndrome.

Marfan syndrome (MFS) is one of the most common inherited connective tissue disorders that severely affects the cardiovascular system. Mutations in the gene encoding fibrillin-1 (FBN1) have been shown to cause MFS as well as dominant ectopia lentis and neonatal Marfan syndrome. Fibrillin-1 is the major component of elastic fiber microfibrils in the extracellular matrix of connective tissue. Recent molecular studies have brought some light into understanding the pathogenesis of MFS, but the diagnostic problems still prevail, and targeted therapy of MFS must await better dissection of the role of fibrillin-1 in tissue phenotype using different experimental systems. (Trends Cardiovasc Med 1997;7:282-288). © 1997, Elsevier Science Inc.

DNA diagnostics of the Marfan syndrome: application of amplifiable polymorphic markers.

The diagnosis of Marfan syndrome (MFS) is still based on careful clinical examination. There are, however, many factors creating problems in the firm establishment of the correct diagnosis. After the identification of the defective gene in MFS, fibrillin 1 (FBN1), several mutations in this gene have been reported. Since so far all but one of the mutations in FBN1 have been family specific, a common diagnostic DNA test for all MFS patients is not to be expected in the near future. Here, we have utilized four polymorphic markers in the diagnostics in MFS families from different populations. Two of the markers, FBN1a and a novel FBN1b, are intragenic markers of FBN1 and two others, D15S103 (G113) and CYP19, are very close to and most probably flank FBN1. The combined use of the multiallelic markers proved highly useful in MFS diagnostics providing informativeness in all analysed families.

A compound-heterozygous Marfan patient: two defective fibrillin alleles result in a lethal phenotype.

We describe here the identification of defined mutations in both alleles of the fibrillin gene (FBN1) in a compound-heterozygote Marfan syndrome (MFS) child who had a very severe form of MFS resulting in death from cardiac failure at the age of 4 mo. The nonconsanguineous parents were both affected with MFS. The father's heterozygous point mutation has earlier been reported to result in W217G substitution, the mother was here shown to carry a heterozygous point mutation resulting in G2627R substitution, and the child had inherited both these mutations. The mutant FBN1 alleles were demonstrated to be transcribed with equal efficiency compared with the normal alleles, but metabolic labeling of fibroblast cultures from the child and both parents showed reduced biosynthesis and secretion of profibrillin. Also, the respective amounts of fibrillin in cell-culture media and extracellular-matrix extracts were markedly diminished, particularly in the cell cultures from father and child. In addition, immunofluorescence analysis of the cell cultures of all three family members revealed a drastically reduced amount of microfibrils, and virtually no visible fibrils could be seen in the case of the compound-heterozygote child. These findings demonstrate incomplete dominance of fibrillin mutations and underline the fatal consequences of the complete absence of normal fibrillin molecules in the microfibrils.

A point mutation creating an extra N-glycosylation site in fibrillin-1 results in neonatal Marfan syndrome.

Fibrillin-1 is a large cysteine-rich glycoprotein of the 10-nm microfibrils in the extracellular matrix. A spectrum of mutations in the fibrillin-1 gene (FBN1) have been identified in patients with Marfan syndrome (MFS), and the majority of mutations resulting in the neonatal and often lethal form of MFS have been identified in the restricted region of exons 24-32 of the FBN1 gene. Here we report a novel point mutation in exon 25 of the FBN1 gene in a patient with lethal MFS. The mutation resulted in a molecular defect rarely encountered in human diseases, the creation of an extra consensus sequence for N-glycosylation. Metabolic labeling of the patient fibroblast culture and in vitro expression of the mutagenized cDNA construct suggest that this novel N-glycosylation site is actually utilized. Immunohistochemical and ultrastructural analyses of the fibroblast cultures of the patient show that this excessive N-glycosylation severely affects microfibril formation in vitro; this finding emphasizes the importance of correct posttranslational modifications of fibrillin molecules for correct aggregation into microfibrillar structures.

An accurate method for comparing transcript levels of two alleles or highly homologous genes: application to fibrillin transcripts in Marfan patients' fibroblasts.

We introduce here a novel and generally applicable, solid-phase minisequencing-based approach for rapid estimation of relative levels of transcripts with high sequence homology. This study was undertaken to screen for the consequences of different fibrillin-1 mutations on the transcript levels in patients with the Marfan syndrome (MFS). This dominantly inherited, connective tissue disorder is characterized by pleiotrophic symptoms in cardiovascular, skeletal, and ocular systems. A spectrum of disease mutations in the gene encoding fibrillin-1 (FBN1), a glycoprotein component of extracellular matrix microfibrils, has been identified in MFS patients, but the mechanisms by which mutations result in different phenotypic manifestations are still unknown to a large extent. Our data from the quantitation of FBN1 transcripts provide support for the hypothesis that mutations causing premature stop codons result in a milder phenotype than classical MFS by reducing the stability of the mutant transcript and, consequently, decreasing the interference of mutant polypeptide in the formation of fibrillin fibers. We also applied this mRNA quantitation method to determine the relative ratio between transcripts from the genes coding for two highly homologous microfibrillar components, FBN1 and FBN2, in control fibroblast cultures as well as in fibroblasts from MFS patients. Interestingly, these data show large variations between the levels of the two transcripts in fibroblast cultures, but these variations do not correlate either with the nature of the disease mutation or to the clinical MFS phenotype.

Prenatal diagnosis of Marfan syndrome: identification of a fibrillin-1 mutation in chorionic villus sample.

Marfan syndrome (MFS) is one of the most common heritable connective tissue disorders and is caused by mutations in a gene coding for fibrillin-1. All but one of over 30 published mutations have been unique and specific prenatal diagnostics can only be provided to families with a previously established mutation. We have earlier identified a 366 bp deletion of fibrillin mRNA in a three-generation British Marfan family. An affected female in the family together with her husband sought prenatal diagnosis. Chorionic villus sampling was performed at 11.5 weeks of gestation and total RNA was directly extracted from the sample. After reverse transcription and polymerase chain reaction (PCR) of the cDNA, the same deletion was identified in the chorionic villus sample (CVS) and the mother's sample in agarose gel electrophoresis. The fetal origin of the CVS was confirmed with polymorphic markers. In addition to the mutation analysis, CVS cells of the proband and a control fetus were cultured for biochemical studies of fibrillin polypeptides. The results of the biochemical investigation were in concordance with the molecular analysis.

A gene for inherited cutaneous venous anomalies ("glomangiomas") localizes to chromosome 1p21-22.

Venous malformations (VMs) are localized defects of vascular morphogenesis. They can occur in every organ system, most commonly in skin and muscle. They can cause pain and bleeding, and in some critical locations they can be life threatening. Usually venous anomalies occur sporadically, but families with dominant inheritance have been identified. Using linkage analysis, we have established in earlier reports that some families with inherited VMs show linkage to chromosome 9p21; the mutation causes ligand-independent activation of an endothelial cell-specific receptor tyrosine kinase, TIE-2. Here we show that VMs with glomus cells (known as "glomangiomas"), inherited as an autosomal dominant trait in five families, are not linked to 9p21 but, instead, link to a new locus, on 1p21-p22, called "VMGLOM" (LOD score 12.70 at recombination fraction.00). We exclude three known positional candidate genes, DR1 (depressor of transcription 1), TGFBR3 (transforming growth factor-beta receptor, type 3), and TFA (tissue factor). We hypothesize that cutaneous venous anomalies (i.e., glomangiomas) are caused by mutations in a novel gene that may act to regulate angiogenesis, in concert with the TIE-2 signaling pathway.

Marfan Database (second edition): software and database for the analysis of mutations in the human FBN1 gene.

Fibrillin is the major component of extracellular microfibrils. Mutations in the fibrillin gene on chromosome 15 (FBN1) were described at first in the heritable connective tissue disorder, Marfan syndrome (MFS). More recently, FBN1 has also been shown to harbor mutations related to a spectrum of conditions phenotypically related to MFS. These mutations are private, essentially missense, generally non-recurrent and widely distributed throughout the gene. To date no clear genotype/phenotype relationship has been observed excepted for the localization of neonatal mutations in a cluster between exons 24 and 32. The second version of the computerized Marfan database contains 89 entries. The software has been modified to accomodate new functions and routines.