A substitution at a non-glycine position in the triple-helical domain of pro alpha 2(I) collagen chains present in an individual with a variant of the Marfan syndrome

Comprehensive Genetic Analysis of 128 Candidate Genes in a Cohort With Idiopathic, Severe, or Familial Osteoporosis

Context

The genetic bases of osteoporosis (OP), a disorder with high heritability, are poorly understood at an individual level. Cases of idiopathic or familial OP have long puzzled clinicians as to whether an actionable genetic cause could be identified.

Objective

We performed a genetic analysis of 28 cases of idiopathic, severe, or familial osteoporosis using targeted massively parallel sequencing.

Design

Targeted sequencing of 128 candidate genes was performed using Illumina NextSeq. Variants of interest were confirmed by Sanger sequencing or SNP array.

Patients and Setting

Thirty-seven patients in an academic tertiary hospital participated (54% male; median age, 44 years; 86% with fractures), corresponding to 28 sporadic or familial cases.

Main Outcome Measure

The identification of rare stop-gain, indel, splice site, copy-number, or nonsynonymous variants altering protein function.

Results

Altogether, we identified 28 variants of interest, but only 3 were classified as pathogenic or likely pathogenic variants: COL1A2 p.(Arg708Gln), WNT1 p.(Gly169Asp), and IDUA p.(His82Gln). An association of variants in different genes was found in 21% of cases, including a young woman with severe OP bearing WNT1, PLS3, and NOTCH2 variants. Among genes of uncertain significance analyzed, a potential additional line of evidence has arisen for GWAS candidates GPR68 and NBR1, warranting further studies.

Conclusions

While we hope that continuing efforts to identify genetic predisposition to OP will lead to improved and personalized care in the future, the likelihood of identifying actionable pathogenic variants in intriguing cases of idiopathic or familial osteoporosis is seemingly low.

Genetics of the extracellular matrix in aortic aneurysmal diseases

Aortic aneurysms are morbid conditions that can lead to rupture or dissection and are categorized as thoracic (TAA) or abdominal aortic aneurysms (AAA) depending on their location. While AAA shares overlapping risk factors with atherosclerotic cardiovascular disease, TAA exhibits strong heritability. Human genetic studies in the past two decades have successfully identified numerous genes involved in both familial and sporadic forms of aortic aneurysm. In this review we will discuss the genetic basis of aortic aneurysm, focusing on the extracellular matrix and how insights from these studies have informed our understanding of human biology and disease pathogenesis.

Identification of a p.Arg708Gln variant in COL1A2 in atypical femoral fractures

OBJECTIVES

Long-term bisphosphonates exposure is a proven risk factor for atypical femoral fractures (AFF) but several cases occur in untreated patients. The identification of other risk factors for AFF is critical for the management of osteoporosis. We here assessed the genetic factors associated with AFF regardless of the treatment.

METHODS

Cases were identified through ICD-10 codes in 3 academic centers. Medical records were analyzed by 2 investigators that adjudicated X-rays for typical or atypical fractures. Genetic screening for ALPL, SOX9, COL1A1 and COL1A2 variants was performed after patient's information and consent.

RESULTS

A total of 389 cases were identified and 268 were ruled out according to the ASBMR Task Force recommendations. On the remaining 121, 14 (11.6%) were AFF. Anti-osteoporotic drugs were more frequent in the AFF group compared to the typical fracture group (35% vs 5%, P<0.001) but only 4 (28.6%) patients with AFF had been exposed to bisphosphonates. Genetic analysis performed in 5 patients found one with a heterozygous mutation in COL1A2 (rs72658163, NM_000089.3:c.2123G>A, p.Arg708Gln). This rare variant (Minor Allele Frequency=0.0008) causes a missense mutation that alters collagen fibrillogenesis. Eight heterozygous polymorphisms for ALPL were also found in 3 patients.

CONCLUSION

Genetic screening for variants in only 4 genes and 5 patients with AFF resulted in the identification of genetic variants in 3 patients including a rare variant in COL1A2, suggesting a possible genetic susceptibility to AFF. This finding should encourage clinician to further genotype patients with AFF in a collaborative multicentric project.

Molecular diagnosis of hypophosphatasia and differential diagnosis by targeted Next Generation Sequencing

Hypophosphatasia (HPP) is a rare inherited skeletal dysplasia due to loss of function mutations in the ALPL gene. The disease is subject to an extremely high clinical heterogeneity ranging from a perinatal lethal form to odontohypophosphatasia affecting only teeth. Up to now genetic diagnosis of HPP is performed by sequencing the ALPL gene by Sanger methodology. Osteogenesis imperfecta (OI) and campomelic dysplasia (CD) are the main differential diagnoses of severe HPP, so that in case of negative result for ALPL mutations, OI and CD genes had often to be analyzed, lengthening the time before diagnosis. We report here our 18-month experience in testing 46 patients for HPP and differential diagnosis by targeted NGS and show that this strategy is efficient and useful. We used an array including ALPL gene, genes of differential diagnosis COL1A1 and COL1A2 that represent 90% of OI cases, SOX9, responsible for CD, and 8 potentially modifier genes of HPP. Seventeen patients were found to carry a mutation in one of these genes. Among them, only 10 out of 15 cases referred for HPP carried a mutation in ALPL and 5 carried a mutation in COL1A1 or COL1A2. Interestingly, three of these patients were adults with fractures and/or low BMD. Our results indicate that HPP and OI may be easily misdiagnosed in the prenatal stage but also in adults with mild symptoms for these diseases.

Identification of a functional proprotein convertase cleavage site in microfibril-associated glycoprotein 2

Microfibril-associated glycoprotein 2 (MAGP2) is a secreted protein associated with multiple cellular activities including the organization of elastic fibers in the extracellular matrix (ECM), angiogenesis, as well as regulating Notch and integrin signaling. Importantly, increases in MAGP2 positively correlate with poor prognosis for some ovarian cancers. It has been assumed that full-length MAGP2 is responsible for all reported effects; however, here we show MAGP2 is a substrate for the proprotein convertase (PC) family of endoproteases. Proteolytic processing of MAGP2 by PC cleavage could serve to regulate secretion and thus, activity and function as reported for other extracellular and cell-surface proteins. In support of this idea, MAGP2 contains an evolutionarily conserved PC consensus cleavage site, and amino acid sequencing of a newly identified MAGP2 C-terminal cleavage product confirmed functional PC cleavage. Additionally, mutagenesis of the MAGP2 PC consensus cleavage site or treatment with PC inhibitors prevented MAGP2 proteolytic processing. Finally, both cleaved and uncleaved MAGP2 were detected extracellularly and MAGP2 secretion appeared independent of PC cleavage, suggesting that PC processing occurs mainly outside the cell. Our characterization of alternative forms of MAGP2 present in the extracellular space not only enhances diversity of this ECM protein but also provides a previously unrecognized molecular mechanism for regulation of MAGP2 biological activity.

Genotype-phenotype correlations in autosomal dominant osteogenesis imperfecta

Osteogenesis imperfecta, discussed in Baldridge et al. 2008 is an inherited bone fragility disorder with a wide range of clinical severity that in the majority of cases is caused by mutations in COL1A1 or COL1A2, the genes that encode the two collagen type I alpha chains. Here we describe genotype-phenotype correlations in OI patients who have mutations affecting collagen type I. This paper is based on findings in a large single-centre OI population and a review of the literature.

Overexpression of the C-type natriuretic peptide (CNP) is associated with overgrowth and bone anomalies in an individual with balanced t(2;7) translocation

Longitudinal bone growth is determined by the process of endochondral ossification in the cartilaginous growth plate, which is located at both ends of vertebrae and long bones and involves many systemic hormones and local regulators. We report the molecular characterization of a de novo balanced t(2;7)(q37.1;q21.3) translocation in a young female with Marfanoid habitus and skeletal anomalies. The translocation was characterized by fluorescence in situ hybridization (FISH), checked for other abnormalities by array-comparative genomic hybridization (CGH), and finally, the breakpoints were cloned, sequenced, and compared. Biochemical dosage was applied to study the possible mechanisms that may cause the proposita's phenotype. The breakpoint on chromosome 2 disrupts the hypothetical gene MGC42174 (HUGO-approved symbol DIS3L2) and is located in the proximity of the NPPC gene coding for C-type natriuretic peptide (CNP), a molecule that regulates endochondral bone growth. CNP plasma concentration was doubled in the proband compared to five normal controls, while NPPC was substantially overexpressed in her fibroblasts. A transgenic mouse generated to target NPPC overexpression in bone showed a phenotype highly reminiscent of the patient's phenotype. The breakpoint on chromosome 7 is localized proximally at about 75 kb from the COL1A2 gene. The COL1A2 allele on the derivative chromosome was strongly underexpressed in fibroblasts, but total collagen was not significantly different from controls. Several evidences support the conclusion that the proband's abnormal phenotype is associated with C-type natriuretic peptide overexpression.

Y-position cysteine substitution in type I collagen (α1(I) R888C/p.R1066C) is associated with osteogenesis imperfecta/Ehlers-Danlos syndrome phenotype

The most common mutations in type I collagen causing types II-IV osteogenesis imperfecta (OI) result in substitution for glycine in a Gly-Xaa-Yaa triplet by another amino acid. We delineated a Y-position substitution in a small pedigree with a combined OI/Ehlers-Danlos Syndrome (EDS) phenotype, characterized by moderately decreased DEXA z-score (-1.3 to -2.6), long bone fractures, and large-joint hyperextensibility. Affected individuals have an alpha1(I)R888C (p.R1066C) substitution in one COL1A1 allele. Polyacrylamide gel electrophoresis (PAGE) of [(3)H]-proline labeled steady-state collagen reveals slight overmodification of the alpha1(I) monomer band, much less than expected for a substitution of a neighboring glycine residue, and a faint alpha1(I) dimer. Dimers form in about 10% of proband type I collagen. Dimer formation is inefficient compared to a possible 25%, probably because the SH-side chains have less proximity in this Y-position than when substituting for a glycine. Theoretical stability calculations, differential scanning calorimetry (DSC) thermograms, and thermal denaturation curves showed only weak local destabilization from the Y-position substitution in one or two chains of a collagen helix, but greater destabilization is seen in collagen containing dimers. Y-position collagen dimers cause kinking of the helix, resulting in a register shift that is propagated the full length of the helix and causes resistance to procollagen processing by N-proteinase. Collagen containing the Y-position substitution is incorporated into matrix deposited in culture, including immaturely and maturely cross-linked fractions. In vivo, proband dermal fibrils have decreased density and increased diameter compared to controls, with occasional aggregate formation. This report on Y-position substitutions in type I collagen extends the range of phenotypes caused by nonglycine substitutions and shows that, similar to X- and Y-position substitutions in types II and III collagen, the phenotypes resulting from nonglycine substitutions in type I collagen are distinct from those caused by glycine substitutions.

Three arginine to cysteine substitutions in the pro-alpha (I)-collagen chain cause Ehlers-Danlos syndrome with a propensity to arterial rupture in early adulthood

Mutations in the COL1A1 and COL1A2 genes, encoding the proalpha1 and 2 chains of type I collagen, cause osteogenesis imperfecta (OI) or Ehlers-Danlos syndrome (EDS) arthrochalasis type. Although the majority of missense mutations in the collagen type I triple helix affect glycine residues in the Gly-Xaa-Yaa repeat, few nonglycine substitutions have been reported. Two arginine-to-cysteine substitutions in the alpha1(I)-collagen chain are associated with classic EDS [R134C (p.R312C)] or autosomal dominant Caffey disease with mild EDS features [R836C (p.R1014C)]. Here we show alpha1(I) R-to-C substitutions in three unrelated patients who developed iliac or femoral dissection in early adulthood. In addition, manifestations of classic EDS in Patient 1 [c.1053C>T; R134C (p.R312C); X-position] or osteopenia in Patients 2 [c.1839C>T; R396C (p.R574C); Y-position] and 3 [c.3396C>T; R915C (p.R1093C); Y-position] are seen. Dermal fibroblasts from the patients produced disulfide-bonded alpha1(I)-dimers in approximately 20% of type I collagen, which were efficiently secreted into the medium in case of the R396C and R915C substitution. Theoretical stability calculations of the collagen type I heterotrimer and thermal denaturation curves of monomeric mutant alpha1(I)-collagen chains showed minor destabilization of the collagen helix. However, dimers were shown to be highly unstable. The R134C and R396C caused delayed procollagen processing by N-proteinase. Ultrastructural findings showed collagen fibrils with variable diameter and irregular interfibrillar spaces, suggesting disturbed collagen fibrillogenesis. Our findings demonstrate that R-to-C substitutions in the alpha1(I) chain may result in a phenotype with propensity to arterial rupture in early adulthood. This broadens the phenotypic range of nonglycine substitutions in collagen type I and has important implications for genetic counseling and follow-up of patients carrying this type of mutation.

Potential modifier role of the R618Q variant of proalpha2(I)collagen in type I collagen fibrillogenesis: in vitro assembly analysis

An arginine to glutamine substitution in the triple helix of proalpha2(I)collagen (R618Q) was first reported in a patient with a variant of Marfan syndrome and later identified in conjunction with a second mutation in a patient with osteogenesis imperfecta (OI). The presence of the R618Q proalpha2(I)collagen allele in unaffected or mildly affected family members suggests that the R618Q allele is either a non-affecting polymorphism or a potential genetic modifier. Conservation of arginine618 across species and fibrillar collagen types suggests it is functionally significant. To investigate the functional significance of the R618Q proalpha2(I)collagen allele, we isolated type I collagen from cultured dermal fibroblasts of control and two unrelated individuals heterozygous for the R618Q proalpha2(I)collagen allele and evaluated helical stability and fibrillar assembly. Type I collagen thermal stability analyzed by protease susceptibility and CD spectroscopy demonstrated no statistical difference between control and R618Q containing collagen molecules. In vitro fibril assembly analyses demonstrated that R618Q containing collagen exhibits rapid fibrillar growth with minimal fibril nucleation phase. Further, electron microscopy demonstrated that the diameter of assembled R618Q containing collagen fibrils was approximately 20% of control collagen fibrils. These findings suggest the R618Q variant does not impact triple helical stability but has a role in collagen fibril assembly, supporting the hypothesis that the R618Q proalpha2(I)collagen variant is a modifier of connective tissue structure/function and is potentially involved in disease pathogenesis.

Variation in the vitreous phenotype of Stickler syndrome can be caused by different amino acid substitutions in the X position of the type II collagen Gly-X-Y triple helix

Stickler syndrome is a dominantly inherited disorder characterized by arthropathy, midline clefting, hearing loss, midfacial hypoplasia, myopia, and retinal detachment. These features are highly variable both between and within families. Mutations causing the disorder have been found in the COL2A1 and COL11A1 genes. Premature termination codons in COL2A1 that result in haploinsufficiency of type II collagen are a common finding. These produce a characteristic congenital "membranous" anomaly of the vitreous of all affected individuals. Experience has shown that vitreous slit-lamp biomicroscopy can distinguish between patients with COL2A1 mutations and those with dominant negative mutations in COL11A1, who produce a different "beaded" vitreous phenotype. Here we characterize novel dominant negative mutations in COL2A1 that result in Stickler syndrome. Both alter amino acids in the X position of the Gly-X-Y triple-helical region. A recurrent R365C mutation occurred in two unrelated sporadic cases and resulted in the membranous vitreous anomaly associated with haploinsufficiency. In a large family with linkage to COL2A1, with a LOD score of 2.8, a unique L467F mutation produced a novel "afibrillar" vitreous gel devoid of all normal lamella structure. These data extend the mutation spectrum of the COL2A1 gene and help explain the basis for the different vitreous phenotypes seen in Stickler syndrome.

An alpha2(I) glycine to aspartate substitution is responsible for the presence of a kink in type I collagen in a lethal case of osteogenesis imperfecta

Type I collagen synthesized by cultured skin fibroblasts was analyzed biochemically and molecularly to characterize the defect in a patient affected by lethal Osteogenesis Imperfecta. The SDS-Urea-PAGE of procollagen and collagen revealed a broad alpha1(I) band, a normal alpha2(I) and another alpha2(I) band migrating equidistant between alpha1 and alpha2. When synthesized in the presence of alphaalpha'-dipyridyl, an inhibitor of prolyl and lysyl hydroxylation, procollagen and collagen of media and cell layers contained both normal and slower alpha2(I), but only normal alpha1(I). The persistence of the two forms of alpha2(I) chains suggested a mutation in a COL1A2 gene. CNBr cleavage of collagen yielded overmodified alpha1(I) CB3 and CB7 peptides and delayed migration of the alpha2(I) CB3-5 peptide. A delayed CB3-5 was also found after alpha,alpha'-dipyridyl treatment. These data localized the mutation between aa 353 and 551 in alpha2(I) (CB3-5). Sequencing the subcloned alleles in this region revealed a G-->A transition at nt 1671 in one allele, changing Gly 421 to Asp in an alpha2(I) chain. The mutation was demonstrated to occur on the paternally derived allele, using a common C-->A polymorphism at alpha2(I) nt 1585 and by the presence of a rare variant, Arg618-->Gln (Phillips et al., 1990), in the paternal genomic DNA and the proband's mutant allele. Procollagen processing was normal. The Tm of the slow alpha2(I) collagen was 2 degrees C lower than the control, indicating decreased triple helix stability. Mutant collagen was incorporated in the extracellular matrix deposited by cultured fibroblasts. The dramatic delay in alpha2(I) electrophoretic mobility must be induced by the Gly-->Asp substitution, since the Arg-->Gln variant causes only mild electrophoretic delay. Substantial delay in gel mobility even in the absence of overmodification suggested the presence of a kink in the mutated alpha2(I) chains. Rotary shadowing electron microscopy of secreted fibroblast procollagen confirmed the presence of a kink in the region of the helix containing the glycine substitution. The kinking of the collagen helix occurs in the absence of dimer formation. Kinking may interfere with normal helix folding, as well as with the interactions of collagen fibrils with the collagenous and non-collagenous extracellular matrix proteins.

The human type I collagen mutation database

Type I collagen is the most abundant and ubiquitously distributed of the collagen family of proteins. It is a heterotrimer comprising two alpha1(I) chains and one alpha2(I) chain which are encoded by the unlinked loci COL1A1 and COL1A2 respectively. Mutations at these loci result primarily in the connective tissue disorders osteogenesis imperfecta and Ehlers-Danlos syndrome types VIIA and VIIB. Two instances of osteoporosis and a single instance of Marfan syndrome are also the result of mutations at these loci. The mutation data are accessible on the world wide web at http://www.le.ac.uk/depts/ge/collagen/collagen.html

Mutation analysis of coding sequences for type I procollagen in individuals with low bone density

Mutations in one of the two genes encoding type I procollagen (COL1A1 and COL1A2) are frequently the cause of osteogenesis imperfecta (OI), a disorder characterized by brittle bones. Here we tested whether patients with low bone density also have mutations in these genes. The 26 patients studied had no apparent metabolic bone disease, but most had a positive family history of osteopenia or osteoporosis. Although a diagnosis of OI was considered by the clinician in some cases, the clinical criteria for OI were not satisfied. Our strategy for mutation analysis consisted of PCR amplification of cDNA made to fibroblast mRNA using primers specific for the coding regions of COL1A1 and COL1A2. The PCR products were then sequenced directly with primers located within each PCR product. We found that 3 of 26 patients had mutations that altered the encoded amino acid. One mutation, at position alpha 2(I)-661 has been reported (Spotila et al. 1991 Proc Natl Acad Sci USA PNAS 88:5423). The other 2 patients, who were not related to each other, had a mutation that altered the proline codon at alpha 1(I)-27 to alanine. This mutation was not found in 81 normal individuals or in 37 additional osteopenic individuals. However, its effect on the biologic function of type I collagen, as well as its role in osteopenia, is uncertain. In addition to the two mutations, we found a polymorphism in codon alpha 2(I)-459. Although this polymorphism involved an amino acid substitution, it was present with equal frequency in the patient and the normal population. By analyzing this and previously reported neutral sequence variants in the COL1A2 gene, we determined that all patients expressed both alleles of the COL1A2 gene. The 12 patients who were heterozygous for a COL1A1 neutral sequence variant also expressed both alleles. Here we present all PCR primer and sequencing primer information. The results suggest that surveying a larger group of similarly selected individuals may reveal additional mutations in the COL1A1 or COL1A2 genes.

Identification of type I collagen gene polymorphisms: tolerance of sequence variation at an alpha 2(I) helix Y position

This study has examined the frequency and distribution of polymorphisms in the type I collagen coding sequences. RNA from a group of human skin fibroblast cell lines was analyzed by the chemical cleavage mismatch detection method using hydroxylamine, a reagent specific for C-base mismatches, and overlapping cDNA probes covering the entire prepro alpha 1(I) and prepro alpha 2(I) coding regions. Mismatches were detected at only two nucleotide positions, one in each of the type I collagen sequences, suggesting that polymorphisms are relatively rare within these cDNAs. cDNA sequence analysis demonstrated that the prepro alpha 1(I) mismatch, detected in only one cell line, was due to a sequence polymorphism involving the wobble position of the codon for arginine residue 59 within the amino-propeptide globular subdomain of the pro alpha 1(I) chain and not resulting in a change in the polypeptide primary structure. In contrast, the prepro alpha 2(I) mismatch, detected in 6 of the 16 cell lines, was shown to arise from a sequence polymorphism affecting the identity of Y-position residue 459 of the alpha 2(I) triple helical domain, resulting in an alanine/proline dimorphism at this position. This study is the first to identify a type I collagen coding sequence polymorphism resulting in an alteration at the level of the amino acid sequence. The data suggest that at least some alpha 1(I) and alpha 2(I) helix Y positions may be tolerant of sequence variation, particularly if the replacing amino acid is proline, a residue involved in stabilizing the collagen triple helix.

The molecular basis of Marfan syndrome

The Marfan syndrome is an inherited, autosomal dominant disorder that affects the skeletal, ocular, and cardiovascular systems. Recent biochemical and genetic studies have demonstrated that this deadly genetic disorder arises from defects in the connective tissue protein fibrillin. Fibrillin is a component of microfibrils, structures found in the extracellular matrices of most tissues, including those affected in Marfan patients. The appearance of microfibrils in the matrix produced by Marfan patient fibroblasts is different from that of normal cells. Genetic linkage between the fibrillin gene and the Marfan phenotype has been established and the gene mapped to the same chromosomal position as the disease locus. In several instances, the disease has been associated with mutations in the fibrillin gene, confirming that defects in fibrillin cause the Marfan syndrome.

Collagens and their abnormalities in a wide spectrum of diseases

Collagens are a family of extracellular matrix proteins that play a dominant role in maintaining the structural integrity of various tissues. Nineteen collagen types containing altogether more than 30 distinct polypeptide chains have now been identified, and their genes have been found to be dispersed among at least 12 chromosomes. Mutations in collagen genes or deficiencies in the activities of specific post-translational enzymes of collagen synthesis have been characterized in many heritable disorders such as osteogenesis imperfecta, several chondrodysplasias, several subtypes of the Ehlers-Danlos syndrome, the X-linked Alport syndrome and dystrophic forms of epidermolysis bullosa. In addition, collagen mutations have been found in certain common diseases, namely osteoporosis, osteoarthrosis and aortic aneurysms, and it is now evident that subsets of patients with these diseases have defects in types I, II or III collagen, respectively, as a predisposing factor. Mutations have so far been identified in only six of the more than 30 collagen genes, and thus research into collagen defects is only in its early stages. Transgenic mice have been shown to offer an excellent tool for investigating the consequences of mutations in collagen genes and identifying additional diseases caused by collagen defects. Excessive collagen accumulation also poses a common problem in medicine, leading to fibrosis with impairment of the normal functioning of the affected tissue. This has prompted attempts to develop drugs which inhibit collagen synthesis. Prolyl 4-hydroxylase would seem a particularly suitable target for antifibrotic therapy, and several compounds are now known that inhibit this enzyme. In particular, derivatives of pyridine 2,4-dicarboxylate have been shown to inhibit hepatic collagen accumulation in rats with two models of liver fibrosis.

Mutations in the COL1A2 gene of type I collagen that result in nonlethal forms of osteogenesis imperfecta

Although virtually all mutations that result in osteogenesis imperfecta (OI) affect the genes that encode the chains of type I procollagen, the effects of mutations in the COL1A2 gene have received less attention than those in the COL1A1 gene. We have characterized mutations in 4 families that give rise to different OI phenotypes. In three families substitutions of glycine residues by cysteine in the triple helical domain (a single example at position 259 and 2 families in which substitution of glycine at 646 by cysteine) have been identified, and in the fourth a G for A transition at position +4 in intron 33 led to use of an alternative splice site and inclusion of 6 amino acids (val-gly-arg-ile-leu-phe) between residues 585 and 586 of the normal triple helix. The relation between position of substitution of glycine by cysteine in the COL1A2 gene does not follow the pattern developed in the COL1A1 gene. To determine how COL1A2 mutations produce OI phenotypes, we have produced a full-length mouse cDNA into which we plan to place mutations and examine their effects in stably transfected osteogenic cells and in transgenic animals.

Elucidation of the gene defect in Marfan syndrome. Success by two complementary research strategies

Marfan syndrome, which is characterized by manifestations in the skeletal, ocular and cardiovascular systems, is one of the most common inherited connective-tissue disorders. The independently performed genetic assignment of the Marfan locus and classical biochemical and immunohistochemical analyses complemented each other in the search for the Marfan gene defect and in 1991 the fibrillin gene in chromosome 15 was identified as the Marfan gene. So far, three mutations leading to the Marfan phenotype have been reported in this gene coding for a microfibrillar protein. The available data suggests a wide spectrum of different mutations of fibrillin and although mutations of the fibrillin gene account for the majority of Marfan cases, evidence also exists for locus heterogeneity in a minority of Marfan cases.

Osteogenesis imperfecta due to recurrent point mutations at CpG dinucleotides in the COL1A1 gene of type I collagen

Most individuals with osteogenesis imperfecta (OI) are heterozygous for dominant mutations in one of the genes that encode the chains of type I collagen. Each of the more than 30 mutations characterized to date has been unique to the affected member(s) of the family. We have determined that two individuals with a progressive deforming variety of OI, OI type III, have the same new dominant mutation [alpha 1(I)gly154 to arg] and that two unrelated infants with perinatal lethal OI, OI type II, share a second new dominant mutation [alpha 1(I)gly1003 to ser]. These mutations occurred at CpG dinucleotides, in a manner consistent with deamination of a methylated cytosine residue, and raise the possibility that CpG dinucleotides are common sites of recurrent mutations in collagen genes. Further, these findings confirm that the OI type-III phenotype, previously thought to be inherited in an autosomal recessive manner, can result from new dominant mutations in the COL1A1 gene of type-I collagen.