Two novel distinct COL1A2 mutations highlight the complexity of genotype–phenotype correlations in osteogenesis imperfecta and related connective tissue disorders

Osteogenesis imperfecta is a heritable connective tissue disorder characterized by variable symptoms including predisposition to fractures. Despite the identification of numerous mutations, a reliable genotype–phenotype correlation has remained notoriously difficult. We now describe two patients with osteogenesis imperfecta and novel, so far undescribed mutations in the COL1A2 gene, further highlighting this complexity. A 3-year-old patient presented with features reminiscent of a connective tissue disorder, with joint hypermobility, Wormian bones, streaky lucencies in the long bones and relative macrocephaly. The patient carried a heterozygous c.1316G > A (p.Gly439Asp) mutation in the COL1A2 gene located in a triple-helix region, in which glycine substitutions have been assumed to cause perinatal lethal OI (Sillence type II). A second family with type I osteogenesis imperfecta carried a heterozygous nonsense mutation c.4060C > T (p.Gln1354X) within the last exon of COL1A2. Whereas other heterozygous nonsense mutations in COL1A2 do not lead to a phenotype, in this case the mRNA is presumed to escape nonsense-mediated decay. Therefore the predicted COL1A2 propeptide lacks the last 13 C-terminal amino acids, suggesting that the OI phenotype results from decelerated assembly and overmodification of the collagen triple helix. The presented COL1A2 mutations exemplify the complexity of COL1A2 genotype–phenotype correlation in genetic counselling in OI.     

Exclusion of candidate genes in a family with arterial tortuosity syndrome

Arterial tortuosity syndrome (ATS) is a rare hereditary disorder with variable clinical presentation including tortuosity and elongation of the major arteries, often associated with pulmonary artery stenosis, pulmonary hypertension, and skin and joint laxity, suggestive of a connective tissue disorder. ATS is transmitted in an autosomal recessive mode, but the causal gene is unknown. We report an Italian pedigree with three inbred families in which five patients show signs of ATS. In particular, four adult patients present arterial tortuosity and elongation of the main arteries. Two of these patients, with the most severe degree of arterial tortuosity, also show severe peripheral stenosis of the main pulmonary artery. The fifth young patient shows a severe pulmonary valve stenosis in the absence of arterial tortuosity. All patients show signs of Ehlers-Danlos syndrome (EDS): soft skin with abundant subcutaneous tissue and joint laxity, hernias, and disorganization of the extracellular matrix (ECM) of fibronectin (FN) and of actin microfilaments in cultured skin fibroblasts. Linkage analysis of the genes involved in EDS and other connective tissue disorders, excluded COL1A1, COL1A2, COL2A1, COL3A1, COL5A1, COL5A2, COL5A3, COL6A1, COL6A2, ADAMTS2, ELN, FN1, TNXA, and TNXB as candidate genes in the family under study, thus indicating that ATS is a distinct clinical and molecular entity.     

Linkage of epidermolysis bullosa simplex to keratin gene loci.

Epidermolysis bullosa simplex (EBS) is an autosomal dominant disorder characterised by intraepidermal blistering of the skin. Two families with Weber-Cockayne EBS have been analysed for linkage to keratin gene loci. In the first family, linkage was found to chromosome 17 markers flanking the keratin 14 gene (D17S74: Zmax = +2.45, theta = 0.10; COL1A1: Zmax = +0.97, theta = 0.00) and markers near the keratin 5 gene on chromosome 12 were excluded (D12S17: Z less than -2.0, theta = 0.08; COL2A1: Z less than -2.0, theta = 0.13). In contrast, the second family showed linkage to the region containing the keratin 5 gene (D12S17: Zmax = +1.37, theta = 0.08; COL2A1: Zmax = +0.33, theta = 0.15) and was not linked to the keratin 14 gene (D17S74: Z less than -2.0, theta = 0.14). The Weber-Cockayne form of EBS is genetically heterogeneous with linkage to different keratin gene loci.     

Czech dysplasia: report of a large family and further delineation of the phenotype

Czech dysplasia (OMIM 609162) is a recently delineated COL2A1 disorder characterized by early-onset progressive pseudorheumatoid arthritis, platyspondyly, short third and fourth metatarsals, normal height, and the absence of ophthalmological problems or cleft palate. Czech dysplasia is caused by a specific missense mutation (R275C, c.823C > T) in the triple helical domain of the COL2A1 gene. We report on a large family with 11 patients with typical Czech dysplasia and sensorineural hearing loss. Hearing loss has hitherto not been considered as a major manifestation of Czech dysplasia. Mutation analysis documented the COL2A1 c.823C > T (R275C) mutation in all affected individuals. Thus, Czech dysplasia is possibly caused exclusively by the R275C mutation, which is a unique situation among the COL2A1 disorders. The family provides further evidence for the remarkably uniform manifestation of the clinical and radiological abnormalities and adds hearing loss to the list of major anomalies of Czech dysplasia.     

Genetic linkage analysis of 14 candidate gene loci in a family with autosomal dominant osteoarthritis without dysplasia.

The role of various gene loci was investigated in a family in which familial osteoarthritis (FOA), with onset at an early age, is transmitted as an autosomal dominant mendelian trait. The absence of clinical and radiographic signs of dysplasia and calcium pyrophosphate deposition disease (CPDD) indicates that the basic disease process in this family is osteoarthritis (OA). Genetic linkage analysis of 14 candidate genes resulted in the exclusion of 10 important genes (COL2A1, COL9A1, COL9A2, COL11A1, COL11A2, COMP, the CPDD region, CRTL-1, CRTM, and MMP3). Other relevant genes were not informative in this family. The candidate loci previously identified in FOA and heritable skeletal disorders associated with OA are clearly not involved in the development of the primary FOA phenotype in the family investigated, indicating genetic heterogeneity.     

Compound heterozygosity for a dominant glycine substitution and a recessive internal duplication mutation in the type XVII collagen gene results in junctional epidermolysis bullosa and abnormal dentition.

Junctional epidermolysis bullosa is a heterogeneous autosomal recessively inherited blistering skin disorder associated with fragility at the dermal-epidermal junction. Previously, mutations in this condition have been described in the three genes for the anchoring filament protein laminin 5 (LAMA3, LAMB3, and LAMC2), in the gene encoding the hemidesmosome-associated beta4 integrin (ITGB4), and in the gene for the hemidesmosomal protein type XVII collagen (COL17A1/BPAG2). In this study, we report a patient with a form of junctional epidermolysis bullosa with skin fragility and dental anomalies who is a compound heterozygote for a novel combination of mutations, ie, a glycine substitution mutation in one allele and an internal duplication in the other allele of COL17A1. The patient also has two offspring, both of whom have inherited the glycine substitution mutation, whereas the other COL17A1 allele is normal. The latter individuals show no evidence of skin fragility but have marked dental abnormalities with enamel hypoplasia and pitting. The clinical phenotype of junctional epidermolysis bullosa in the proband in this family probably arises due to a combination of the glycine substitution and the internal duplication in COL17A1, whereas the dental abnormalities of her offspring may be the result of the glycine substitution in COL17A1 alone, resulting in this dominantly inherited clinical phenotype.     

Knobloch syndrome: novel mutations in COL18A1, evidence for genetic heterogeneity, and a functionally impaired polymorphism in endostatin

Knobloch syndrome (KNO) is an autosomal recessive disorder characterized by high myopia, vitreoretinal degeneration with retinal detachment, and congenital encephalocele. Pathogenic mutations in the COL18A1 gene on 21q22.3 were recently identified in KNO families. Analysis of two unrelated KNO families from Hungary and New Zealand allowed us to confirm the involvement of COL18A1 in the pathogenesis of KNO and to demonstrate the existence of genetic heterogeneity. Two COL18A1 mutations were identified in the Hungarian family: a 1-bp insertion causing a frameshift and a premature in-frame stop codon and an amino acid substitution. This missense variant is located in a conserved amino acid of endostatin, a cleavage product of the carboxy-terminal domain of collagen alpha 1 XVIII. D1437N (D104N in endostatin) likely represents a pathogenic mutation, as we show that the endostatin N104 mutant is impaired in its affinity towards laminin. Linkage to the COL18A1 locus was excluded in the New Zealand family, providing evidence for the existence of a second KNO locus. We named the second unmapped locus for Knobloch syndrome KNO2. Mutation analysis excluded COL15A1, a member of the multiplexin collagen subfamily similar to COL18A1, as being responsible for KNO2.     

Occurrence of deletion of a COL2A1 allele as the mutation in Stickler syndrome shows that a collagen type II dosage effect underlies this syndrome

We describe a novel type of mutation in the COL2A1 gene in a family with Stickler syndrome, namely a deletion of an entire COL2A1 allele. Until now, almost all COL2A1 mutations found in this syndrome are nucleotide substitutions, small deletions, or insertions, resulting in premature translation termination. Since the phenotype in this family is not different from cases with a truncated alpha-chain, our finding supports the suggestion that a dosage effect is underlying Stickler syndrome. Moreover, in mutation screening protocols for COL2A1 one should be aware of the possibility of large deletions, which are not detected by generally used PCR-based methods.     

High efficiency of mutation detection in type 1 stickler syndrome using a two-stage approach: vitreoretinal assessment coupled with exon sequencing for screening COL2A1

Stickler syndrome is a genetically heterogeneous disorder that affects the ocular, skeletal, and auditory systems. To date three genes, COL2A1, COL11A1, and COL11A2, encoding the heterotypic type II/XI collagen fibrils present in vitreous and cartilage have been shown to have mutations that result in Stickler syndrome. As systemic features in this disorder are variable we have used an ophthalmic examination to differentiate those patients with a membranous vitreous phenotype associated with mutations in COL2A1, from other patients who may have mutations in other genes. Gene amplification and exon sequencing was used to screen 50 families or sporadic cases with this membranous phenotype, for mutations in COL2A1. Mutations were detected in 47 (94%) cases consisting of 166 affected and 78 unaffected individuals. We also demonstrate that the predominantly ocular form of type 1 Stickler syndrome is not confined to mutations in the alternatively spliced exon 2. Using splicing reporter constructs we demonstrate that a mutant GC donor splice site in intron 51 can be spliced normally; this contributed to the predominantly ocular phenotype in the family in which it occurred.     

Prenatal prediction of osteogenesis imperfecta (OI type IV): exclusion of inheritance using a collagen gene probe.

Autosomal dominant osteogenesis imperfecta is caused by mutations in the COL1A2 and COL1A1 genes of type I collagen. In a family with OI type IV genetically linked to the COL1A2 gene, we attempted prenatal diagnosis in a pregnancy at risk by genotyping the DNA of the fetus for a COL1A2 gene associated RFLP. Our results showed that the fetus inherited the normal COL1A2 allele from her affected parent. Linkage analysis can thus be used in the prenatal diagnosis of dominantly inherited osteogenesis imperfecta.     

A single base mutation in the type II procollagen gene (COL2A1) that converts glycine α1-247 to serine in a family with late-onset spondyloepiphyseal dysplasia

A search for mutations in the gene for type II procollagen (COLZA 1) was carried out in a family with late-onset Spondyloepiphyseal dysplasia resulting in short stature, restricted mobility and severe pain in joints, deforming arthritis in the hips, and claudication. Analysis of the HindIII and VNTR polymorphisms at the COL2A1 gene in the family raised the possibility that the gene cosegregated with the disease. Screening for mutations in the COL2A1 gene using PCR-denaturing gradient get electro-phoresis suggested a sequence variation in exon 19 of one allele of the COL2A1 gene in the proband. Direct sequencing of the PCR products for exon 19 revealed a single base mutation that converted the codon of -GGT- for glycine at αl-247 to -AGT-, a codon for serine. The mutant COL2A1 allele was present in all affected family members, but absent in nonaffected members and in a group of 50 unrelated healthy individuals. It was also absent in 20 unrelated patients with chondrodysplasias and 30 unrelated patients with early-onset osteoarthritis. © 1994 Wiley-Liss, Inc.     

Mutational spectrum of type I collagen genes in Korean patients with osteogenesis imperfecta

Mutations in the type I collagen genes COL1A1 and COL1A2 are responsible for the dominantly inherited connective tissue disorder osteogenesis imperfecta (OI). The severity of OI is diverse, ranging from perinatal lethality to a very mild phenotype that is characterized by normal stature and the absence of deformities. Although there have been several studies on the mutational spectra of COL1A1 and/or COL1A2 in Western populations, very few cases have been reported from Asia. In this study, we investigated 67 unrelated Korean probands with OI and used nucleotide sequence analysis to detect COL1A1 and COL1A2 mutations. Thirty-five different mutations were identified in the two genes, including 24 novel mutations. Among the 35 kinds of detected mutations, 15 were glycine substitutions (seven in COL1A1 and eight in COL1A2), one was a nonsense mutation, four were frameshift mutations in COL1A1, three were in-frame duplications in COL1A2, and 12 were splice site mutations (seven in COL1A1 and five in COL1A2). Until now, mutations in the COL1A1 and COL1A2 genes known to cause OI were unique and rarely repeated in other families. Interestingly, the c.982G>A (p.Gly328Ser) mutation in COL1A2 was found recurrently and was the causative mutation in five independent OI probands. Haplotype analysis of the COL1A2 gene revealed that four probands from five independent OI probands with c.982G>A (p.Gly328Ser) had a common haplotype. Our clinical data showed the heterogeneity even within a specific genotype, which suggested the complex expression of this disease.     

Identification of Two Homozygous Sequence Variants in the COL7A1 Gene Underlying Dystrophic Epidermolysis Bullosa by Whole‐Exome Analysis in a Consanguineous Family

Dystrophic epidermolysis bullosa (DEB) is an inherited skin disorder with variable severity and heterogeneous genetic involvement. Diagnostic approaches for this condition include clinical evaluations and electron microscopy of patients’ skin biopsies, followed by Sanger sequencing (SS) of a large gene (118 exons) that encodes the alpha chain of type VII collagen (COL7A1) located on Chromosome 3p21.1. However, the use of SS may hinder diagnostic efficiency and lead to delays because it is costly and time‐consuming. We evaluated a 5‐generation consanguineous family with 3 affected individuals presenting the severe generalised DEB phenotype. Human whole‐exome sequencing (WES) revealed 2 homozygous sequence variants: the previously reported variant p.Arg578* in exon 13 and a novel variant p.Arg2063Gln in exon 74 of the COL7A1 gene. Validation by SS, performed on all family members, confirmed the cosegregation of the 2 variants with the disease phenotype. To the best of our knowledge, 2 homozygous COL7A1 variants have never been simultaneously reported in DEB patients; however, the upstream protein truncation variant is more likely to be disease‐causing than the novel missense variant. WES can be used as an efficient molecular diagnostic tool for evaluating autosomal recessive forms of DEB.     

Stickler syndrome: further mutations in COL11A1 and evidence for additional locus heterogeneity

Stickler syndrome (hereditary arthro-ophthalmopathy) is a dominantly inherited connective tissue disorder with ocular, oro-facial, auditory and skeletal manifestations. It is genetically and phenotypically heterogeneous with the majority of families having mutations in the gene encoding type II collagen (COL2A1) and exhibiting a characteristic 'membranous' or type 1 vitreous phenotype. More recently a novel mutation in the gene encoding the alpha1 chain of type XI collagen (COL11A1) was reported in a Stickler syndrome pedigree with a different 'beaded' or type 2 vitreous phenotype. In the present study five more families with the type 2 vitreous phenotype were examined for linkage to four candidate genes: COL2A1, COL5A2, COL11A1 and COL11A2. Two families were linked to COL11A1 and sequencing identified mutations resulting in shortened alphal(XI) collagen chains, one via exon skipping and the other via a multiexon deletion. One of the families showed weak linkage to COL5A2 but sequencing the open reading frame failed to identify a mutation. In the remaining two families all four loci were excluded by linkage analysis. These data confirm that mutations in COL11A1 cause Stickler syndrome with the type2 vitreous phenotype and also reveal further locus heterogeneity.     

Linkage analysis of five fibrillar collagen loci in a large French Marfan syndrome family.

Marfan syndrome consists of a group of dominantly inherited disorders of connective tissue with wide clinical variability. Using the candidate gene approach, we have attempted to map the gene defect in a large French Marfan syndrome family with no ocular manifestations. We performed linkage studies with polymorphic probes for five structural procollagen genes. The data obtained exclude linkage of Marfan syndrome to the two major fibrillar collagen (COL1A1, COL1A2, and COL2A1) genes. These results confirm previously published data obtained from smaller pedigrees. A small positive lod score (Z = 0.99, theta = 0.00) was obtained for the COL3A1-COL5A2 gene cluster located on chromosome 2.     

一个新的COL4A5基因的剪接位点突变引起Alport综合征

目的 对一个Alport综合征家系进行研究,期望找到导致该家系发病的遗传基础.方法 对家系成员采样并提取DNA,对家系中的先证者和1名正常对照进行COL4A5基因全部编码区域的突变检测,限制性片段长度多态件分析技术对家系中所有成员和200名正常对照进行验证.结果 在该Alport综合征家系中发现一个新的COL4A5基因的剪接位点突变c.1517-1G＞T,而在家系的未患病成员,以及对照人群中未能检测到该突变.结论 发现了一个新的COL4A5基因的剪接位点改变c.1517-IG＞T,该突变可导致Alport综合征,该发现丰富了引起Alport综合征的COL4A5基因的突变谱.     

成骨不全一家系的COL1A1基因突变分析

目的探讨一个成骨不全家系的COL1A1基因的突变位点及其与临床特征的关系。方法收集一个成骨不全家系的临床资料，采用聚合酶链反应以及直接测序法对家系内成员进行COL1A1基因突变位点检测，同时对50名无血缘关系健康对照者的该位点进行限制性内切酶分析。结果该家系中成骨不全患者均存在COL1A1基因的第2461位点G→A突变（17．821S），但其临床特征不一致。而在家系内非患者及正常对照者中均未发现该突变。结论COL1A1基因突变是中国人群中成骨不全致病原因之一。成骨不全的表型不仅与基因型有关，还与遗传背景有关。     

Stickler syndrome caused by COL2A1 mutations: genotype–phenotype correlation in a series of 100 patients

Stickler syndrome is an autosomal dominant connective tissue disorder caused by mutations in different collagen genes. The aim of our study was to define more precisely the phenotype and genotype of Stickler syndrome type 1 by investigating a large series of patients with a heterozygous mutation in COL2A1. In 188 probands with the clinical diagnosis of Stickler syndrome, the COL2A1 gene was analyzed by either a mutation scanning technique or bidirectional fluorescent DNA sequencing. The effect of splice site alterations was investigated by analyzing mRNA. Multiplex ligation-dependent amplification analysis was used for the detection of intragenic deletions. We identified 77 different COL2A1 mutations in 100 affected individuals. Analysis of the splice site mutations showed unusual RNA isoforms, most of which contained a premature stop codon. Vitreous anomalies and retinal detachments were found more frequently in patients with a COL2A1 mutation compared with the mutation-negative group (P<0.01). Overall, 20 of 23 sporadic patients with a COL2A1 mutation had either a cleft palate or retinal detachment with vitreous anomalies. The presence of vitreous anomalies, retinal tears or detachments, cleft palate and a positive family history were shown to be good indicators for a COL2A1 defect. In conclusion, we confirm that Stickler syndrome type 1 is predominantly caused by loss-of-function mutations in the COL2A1 gene as >90% of the mutations were predicted to result in nonsense-mediated decay. On the basis of binary regression analysis, we developed a scoring system that may be useful when evaluating patients with Stickler syndrome.     

A family with Stickler syndrome type 2 has a mutation in the COL11A1 gene resulting in the substitution of glycine 97 by valine in alpha 1 (XI) collagen

Stickler syndrome (hereditary arthro-ophthalmopathy) is the commonest inherited cause of retinal detachment and one of the commonest autosomal dominant connective tissue dysplasias. There is clinical and locus heterogeneity with about two thirds of families linked to the gene encoding type II procollagen (COL2A1). Families with Sticklers syndrome type 1 have a characteristic congenital vitreous anomaly and are linked without recombination to markers at the COL2A1 locus. In contrast families with the type 2 variety have a different vitreo- retinal phenotype and are not linked to the COL2A1 gene. Type XI collagen is a quantitatively minor fibrillar collagen related to type V collagen and associated with the more abundant type II collagen fibrils. A mutation in COL11A2, the gene for alpha 2 (XI) procollagen, has recently been found in a family described as having Stickler syndrome, although there was no ocular involvement. Here we show for the first time that a family with the full Type 2 Stickler syndrome including vitreous and retinal abnormalities is linked to the COL11A1 gene and characterise the mutation as a Glycine to Valine substitution at position 97 of the triple helical domain caused by a single base G-- >T mutation. These results are the first to provide confirmation that type XI collagen is an important structural component of human vitreous. They also support previous work suggesting that mutations in the genes encoding collagen XI can give rise to some manifestations of Stickler syndrome, but of these, only mutations in COL11A1 will give the full syndrome including the vitreo-retinal features.      

Czech dysplasia metatarsal type: another type II collagen disorder

Czech dysplasia metatarsal type is an autosomal-dominant disorder characterized by an early-onset, progressive spondyloarthropathy with normal stature. Shortness of third and/or fourth toes is a frequently observed clinical feature. Similarities between individuals with this dysplasia and patients with an R275C mutation in the COL2A1 gene, prompted us to analyze the COL2A1 gene in the original families reported with Czech dysplasia. Targeted sequencing of exon 13 of the COL2A1 gene was performed, followed by sequencing of the remaining exons in case the R275C mutation was not identified. We identified the R275C substitution in two of the original patients reported with Czech dysplasia and three additional patients. All affected individuals had a similar phenotype characterized by normal height, spondyloarthropathy, short postaxial toes and absence of ocular and orofacial anomalies. The R275C mutation was excluded in a third patient reported with Czech dysplasia. However, the identification of the Y1391C mutation in this patient with disproportionate short stature made the diagnosis of spondyloperipheral dysplasia (SPD) more probable. The Y1391C mutation is located in the C-propeptide of the procollagen chain and has been reported before in a patient with the Torrance type of lethal platyspondylic skeletal dysplasia (PLSD-T). Our observation of the same Y1391C mutation in an additional unrelated patient with SPD further supports the evidence that PLSD-T and SPD represent a phenotypic continuum. The R275C mutation in the COL2A1 gene causes a specific type II collagen disorder that was recently delineated as Czech dysplasia.