Identification of a mutation that causes exon skipping during collagen pre-mRNA splicing in an Ehlers-Danlos syndrome variant

Type 1 collagen: Synthesis, structure and key functions in bone mineralization

Collagen is a highly abundant protein in the extracellular matrix of humans and mammals, and it plays a critical role in maintaining the body's structural integrity. Type I collagen is the most prevalent collagen type and is essential for the structural integrity of various tissues. It is present in nearly all connective tissues and is the main constituent of the interstitial matrix. Mutations that affect collagen fiber formation, structure, and function can result in various bone pathologies, underscoring the significance of collagen in sustaining healthy bone tissue. Studies on type 1 collagen have revealed that mutations in its encoding gene can lead to diverse bone diseases, such as osteogenesis imperfecta, a disorder characterized by fragile bones that are susceptible to fractures. Knowledge of collagen's molecular structure, synthesis, assembly, and breakdown is vital for comprehending embryonic and foetal development and several aspects of human physiology. In this review, we summarize the structure, molecular biology of type 1 collagen, its biomineralization and pathologies affecting bone.

Clinical features, molecular results, and management of 12 individuals with the rare arthrochalasia Ehlers-Danlos syndrome

Arthrochalasia Ehlers-Danlos syndrome (aEDS) is a rare autosomal dominant connective tissue disorder that is characterized by congenital bilateral hip dislocations, severe generalized joint hypermobility, recurrent joint (sub)luxations, and skin hyperextensibility. To date, 42 patients with aEDS have been published. We report 12 patients with aEDS from 10 families with 6 unpublished individuals and follow-up data on 6 adult patients. The clinical features are largely comparable with patients reported in the literature. Most (n = 10) patients had variants leading to (partial) loss of exon 6 of the COL1A1 or COL1A2 genes. One patient did not have a previously reported likely pathogenic COL1A1 variant. Data regarding management were retrieved. Hip surgery was performed in 5/12 patients and 3/12 patients underwent spinal surgery. As much as 4/12 patients were wheelchair-bound or unable to walk unaided. Fractures were present in 9/12 individuals with 1 patient requiring bisphosphonate treatment. Echocardiograms were performed in 10 patients and 2 individuals showed an abnormality likely unrelated to aEDS. One patient gave birth to two affected children and went through preterm labor requiring medication but had no additional complications. Of the eight adults in our cohort, the majority entered a career. Our data point toward a genotype-phenotype relationship with individuals with aEDS due to pathogenic COL1A1 variants causing complete or partial loss of exon 6 being more severely affected regarding musculoskeletal features. There is a significant lack of knowledge with regard to management of aEDS, particularly in adulthood. As such, systematic follow-up and multidisciplinary treatment is essential.

Electron microscopy in the diagnosis of Ehlers-Danlos syndromes: correlation with clinical and genetic investigations

BACKGROUND

The Ehlers-Danlos syndromes (EDS) consist of 13 subtypes with overlapping features including joint hypermobility, skin and vascular fragility and generalized connective tissue friability. As DNA analysis has become the gold standard for investigation of EDS, transmission electron microscopy (TEM) in clinical practice is decreasing. However, owing to the use of next-generation sequencing, the frequency of variants of uncertain significance (VUS) identified using DNA analysis is increasing. We hypothesized that TEM can provide evidence for or against pathogenicity of VUS.

OBJECTIVES

The aim of this study was to evaluate the role of TEM in the diagnosis of EDS subtypes.

METHODS

Data were collected from patients who underwent a skin biopsy between October 2012 and March 2017 at the London EDS National Diagnostic Service. TEM biopsies were categorized as 'normal' or 'abnormal' according to the description and conclusion in the TEM reports. Definitive diagnoses were reached via a combination of clinical features, structural and functional studies and DNA investigations.

RESULTS

The analysis included 177 patients, comprising 30 abnormal and 147 normal TEM reports. A definitive diagnosis of monogenic EDS subtypes was made in 24 patients. Overall, 17 of these 24 patients (71%) had an abnormal biopsy report and seven (29%) had a normal biopsy report. No TEM findings were specifically associated with any EDS subtype, although collagen flowers were present in most patients with a genetically confirmed diagnosis of classical EDS.

CONCLUSIONS

TEM analysis of collagen structure may have the potential to provide evidence for or against the pathogenicity of a VUS, but more work is needed to establish a clear role for TEM in this process. What's already known about this topic? Collagen fibril abnormalities can be seen in several Ehlers-Danlos syndrome (EDS) subtypes. What does this study add? This study provides clinical data, transmission electron microscopy (TEM) data and molecular data of one of the largest groups of patients suspected to have a monogenetic EDS subtype. No TEM findings were specifically associated with an EDS subtype. There was a higher percentage (71%) of abnormal biopsy findings in patients with a definitive diagnosis of a monogenetic EDS subtype and where a class 4/5 genetic variant was present.

Molecular mechanisms and clinical manifestations of rare genetic disorders associated with type I collagen

Type I collagen is an important structural protein of bone, skin, tendon, ligament and other connective tissues. It is initially synthesized as a precursor form, procollagen, consisting of two identical pro-α1(I) and one proα2(I) chains, encoded by COL1A1 and COL1A2, respectively. The N- and C- terminal propeptides of procollagen are cleavage by N-proteinase and C-proteinase correspondingly, to form the central triple helix structure with Gly-X-Y repeat units. Mutations of COL1A1 and COL1A2 genes are associated with osteogenesis imperfecta, some types of Ehlers-Danlos syndrome, Caffey diseases, and osteogenesis imperfect/Ehlers- Danlos syndrome overlapping diseases. Clinical symptoms caused by different variations can be variable or similar, mild to lethal, and vice versa. We reviewed the relationship between clinical manifestations and type I collagen - related rare genetic disorders and their possible molecular mechanisms for different mutations and disorders.

Molecular diagnosis of genodermatoses

The progress of molecular genetics helps clinicians to prove or exclude a suspected diagnosis for a vast and yet increasing number of genodermatoses. This leads to precise genetic counselling, prenatal diagnosis and preimplantation genetic haplotyping for many inherited skin conditions. It is also helpful in such occasions as phenocopy, late onset and incomplete penetrance, uniparental disomy, mitochondrial inheritance and pigmentary mosaicism. Molecular methods of two genodermatoses are explained in detail, i.e. genodermatoses with skin fragility and neurofibromatosis type 1.

Ehlers-Danlos arthrochalasia type (VIIA-B)--expanding the phenotype: from prenatal life through adulthood

The Ehlers-Danlos syndromes (EDS) form a clinically and genetically heterogeneous group of inherited connective-tissue disorders characterized by joint hypermobility, tissue fragility and skin abnormalities. Six subtypes have been well characterized based on clinical features and molecular genetic abnormalities. The arthrochalasia type EDS (formerly types VIIA and B) is characterized by severe generalized joint hypermobility with multiple dislocations including congenital bilateral dislocation of the hips, muscular hypotonia and distinct dysmorphic features. The diagnosis of the arthrochalasia type EDS is of importance in the neonatal period because of consequences of physical disability in later life. However, the differential diagnosis may be difficult because of overlap with other hypermobility syndromes. In addition, the significant hypotonia may direct the physician toward various neuromuscular diagnoses. As patients become older, the hypotonia decreases and facial features become less distinct. In this report, we describe seven patients at different ages. Timing of diagnosis varied from prenatal life to adult age. The diagnosis of EDS type VII was confirmed by biochemical studies or mutation analysis showing characteristic mutations in COL1A1 and COL1A2. These mutations result in skipping of exon 6, which leads to defective collagen synthesis. For physicians treating patients with EDS type VII, achieving mobility for the patient is the greatest challenge and it may be impossible because of recurrent dislocations of nearly all joints in severe cases.

Ehlers-Danlos syndrome type IV

Ehlers-Danlos syndrome type IV, the vascular type of Ehlers-Danlos syndromes (EDS), is an inherited connective tissue disorder defined by characteristic facial features (acrogeria) in most patients, translucent skin with highly visible subcutaneous vessels on the trunk and lower back, easy bruising, and severe arterial, digestive and uterine complications, which are rarely, if at all, observed in the other forms of EDS. The estimated prevalence for all EDS varies between 1/10,000 and 1/25,000, EDS type IV representing approximately 5 to 10% of cases. The vascular complications may affect all anatomical areas, with a tendency toward arteries of large and medium diameter. Dissections of the vertebral arteries and the carotids in their extra- and intra-cranial segments (carotid-cavernous fistulae) are typical. There is a high risk of recurrent colonic perforations. Pregnancy increases the likelihood of a uterine or vascular rupture. EDS type IV is inherited as an autosomal dominant trait that is caused by mutations in the COL3A1 gene coding for type III procollagen. Diagnosis is based on clinical signs, non-invasive imaging, and the identification of a mutation of the COL3A1 gene. In childhood, coagulation disorders and Silverman's syndrome are the main differential diagnoses; in adulthood, the differential diagnosis includes other Ehlers-Danlos syndromes, Marfan syndrome and Loeys-Dietz syndrome. Prenatal diagnosis can be considered in families where the mutation is known. Choriocentesis or amniocentesis, however, may entail risk for the pregnant woman. In the absence of specific treatment for EDS type IV, medical intervention should be focused on symptomatic treatment and prophylactic measures. Arterial, digestive or uterine complications require immediate hospitalisation, observation in an intensive care unit. Invasive imaging techniques are contraindicated. Conservative approach is usually recommended when caring for a vascular complication in a patient suffering from EDS type IV. Surgery may, however, be required urgently to treat potentially fatal complications.

Comparative genome analysis of cortactin and HS1: the significance of the F-actin binding repeat domain

BACKGROUND

In human carcinomas, overexpression of cortactin correlates with poor prognosis. Cortactin is an F-actin-binding protein involved in cytoskeletal rearrangements and cell migration by promoting actin-related protein (Arp)2/3 mediated actin polymerization. It shares a high amino acid sequence and structural similarity to hematopoietic lineage cell-specific protein 1 (HS1) although their functions differ considerable. In this manuscript we describe the genomic organization of these two genes in a variety of species by a combination of cloning and database searches. Based on our analysis, we predict the genesis of the actin-binding repeat domain during evolution.

RESULTS

Cortactin homologues exist in sponges, worms, shrimps, insects, urochordates, fishes, amphibians, birds and mammalians, whereas HS1 exists in vertebrates only, suggesting that both genes have been derived from an ancestor cortactin gene by duplication. In agreement with this, comparative genome analysis revealed very similar exon-intron structures and sequence homologies, especially over the regions that encode the characteristic highly conserved F-actin-binding repeat domain. Cortactin splice variants affecting this F-actin-binding domain were identified not only in mammalians, but also in amphibians, fishes and birds. In mammalians, cortactin is ubiquitously expressed except in hematopoietic cells, whereas HS1 is mainly expressed in hematopoietic cells. In accordance with their distinct tissue specificity, the putative promoter region of cortactin is different from HS1.

CONCLUSIONS

Comparative analysis of the genomic organization and amino acid sequences of cortactin and HS1 provides inside into their origin and evolution. Our analysis shows that both genes originated from a gene duplication event and subsequently HS1 lost two repeats, whereas cortactin gained one repeat. Our analysis genetically underscores the significance of the F-actin binding domain in cytoskeletal remodeling, which is of importance for the major role of HS1 in apoptosis and for cortactin in cell migration.

Genomic structure and functional characterisation of the promoters of human and mouse nogo/rtn4

The reticulon-family member Nogo-A is a potent neurite growth inhibitory protein in vitro and may play a role in the restriction of axonal regeneration after injury and of structural plasticity in the CNS of higher vertebrates. Of the three major isoforms of Nogo, Nogo-A is mostly expressed in the brain, Nogo-B is found in a ubiquitous pattern, and Nogo-C is most highly expressed in muscle. Seven additional splice-variants derived both from differential splicing and differential promoter usage have been identified. Analysis of the TATA-less Nogo-A/B promoter (P1) shows that conserved GC-boxes and a CCAAT-box within the first 500bp upstream of the transcription start are responsible for its regulation. No major differences in the methylation status of the P1 CpG-island in tissues expressing or not expressing Nogo-A/B could be detected, suggesting that silencer elements are involved in the regulation. The specific expression pattern of Nogo-A/B is due to differential splicing. The basal Nogo-C promoter (P2) is regulated by a proximal and a distal element. The 5'UTR of Nogo-C harbours a negative control element. These data may help to identify factors that can modulate Nogo transcription, thus offering an alternative approach for Nogo neutralisation.

Usher syndrome: from genetics to pathogenesis

Usher syndrome (USH) is defined by the association of sensorineural deafness and visual impairment due to retinitis pigmentosa. The syndrome has three distinct clinical subtypes, referred to as USH1, USH2, and USH3. Each subtype is genetically heterogeneous, and 12 loci have been detected so far. Four genes have been identified, namely, USH1B, USH1C, USH1D, and USH2A. USH1B, USH1C, and USH1D encode an unconventional myosin (myosin VIIA), a PDZ domain-containing protein (harmonin), and a cadherin-like protein (cadherin-23), respectively. Mutations of these genes cause primary defects of the sensory cells in the inner ear, and probably also in the retina. In the inner ear, the USH1 genes, I propose, are involved in the same signaling pathway, which may control development and/or maintenance of the hair bundles of sensory cells via an adhesion force (a) at the junctions between these cells and supporting cells and (b) at the level of the lateral links that interconnect the stereocilia. In contrast, the molecular pathogenesis of USH2A, which is owing to a defect of a novel extracellular matrix protein, is likely to be different from that of USH1.

Truncated typeXVII collagen expression in a patient with non-herlitz junctional epidermolysis bullosa caused by a homozygous splice-site mutation

SUMMARY

Type XVII collagen (180-kDa bullous pemphigoid antigen) is a structural component of hemidesmosomes. Mutations in the type XVII collagen gene (COL17A1) have been established to be the molecular basis of non-Herlitz junctional epidermolysis bullosa (JEB-nH), an inherited skin blistering disorder. Here we report for the first time truncated type XVII collagen expression, caused by homozygosity for a COL17A1 donor splice-site mutation (4261+1 g --> c), which was identified by PCR amplification on genomic DNA. By RT-PCR and sequencing of cDNA derived from mRNA from the patient's cultured keratinocytes, we provide evidence of cryptic splicing and exon skipping, most abundantly of exon 52. JEB-nH patients with COL17A1 splice-site mutations resulting in an exon skip often have no immunologically detectable type XVII collagen. However, in our patient with the generalized atrophic benign epidermolysis bullosa subtype, a small amount of type XVII collagen was detectable in the skin, and immunoblotting of cultured keratinocytes revealed that the 180-kDa protein was 10 kDa shorter. We hypothesize that the function of this truncated type XVII collagen polypeptide, which is expressed at low levels, is impaired, explaining the JEB-nH phenotype.

Genetic Basis of Human Complement C8α-γ Deficiency

Deficiency of the α-γ subunit of the eighth component of complement (C8α-γD) is frequently associated with recurrent neisserial infections, especially meningitis caused by Neisseria meningitidis. We here report the molecular basis of C8α-γD in two unrelated Japanese subjects. Screening all 11 exons of the C8α gene and all 7 exons of the C8γ gene and their boundaries by exon-specific PCR/single-strand conformation polymorphism demonstrated aberrant single-stranded DNA fragments in exon 2 of C8α gene in case 1 and in exons 2 and 9 of C8α gene in case 2. Nucleotide sequencing of the amplified DNA fragments in case 1 revealed a homozygous single-point mutation at the second exon-intron boundary, inactivating the universally conserved 5′ splice site consensus sequence of the second intron (IVS2+1G→T). Case 2 was a compound heterozygote for the splice junction mutation, IVS2+1G→T, and a nonsense mutation at Arg394 (R394X). R394X was caused by a C to T transition at nucleotide 1407, the first nucleotide of the codon CGA for Arg394, leading to a stop codon TGA. No mutations were detected in the C8γ gene by our method. Our results indicate that the pathogenesis of C8α-γD might be caused by heterogeneous molecular defects in the C8α gene.

Differential splicing of type II procollagen mRNA in canine retina

Canine retina expressed two forms of Type II procollagen mRNA, IIA and IIB, due to alternate splicing of exon 2 at the N-propeptide region. The N-propeptide region of Type II procollagen showed a high degree of homology with human, approximately 96% both at the nucleotide and the amino acid levels. Expression of IIA and IIB forms of Type II procollagen from early stage of development to adult life suggests the important role of both forms for the development as well as maintenance of the ocular structure.

Point mutations and an intragenic deletion in LIS1, the lissencephaly causative gene in isolated lissencephaly sequence and Miller-Dieker syndrome

Classical lissencephaly (smooth brain) or generalized agyria-pachygyria is a severe brain malformation which results from an arrest of neuronal migration at 9-13 weeks gestation. It has been observed in several malformation syndromes including Miller-Dieker syndrome (MDS) and isolated lissencephaly sequence (ILS). A gene containing beta-transducin like repeats, now known as LIS1, was previously mapped to the ILS/MDS chromosome region on 17p13.3. We recently localized the classical lissencephaly critical region to the LIS1 gene locus by molecular analysis of key ILS and MDS patients. We have now characterized the structure of LIS1, which consists of 11 exons, and have searched for the presence of subtle mutations in 19 ILS patients who showed no gross rearrangements of LIS1. Single strand conformational polymorphism (SSCP) analysis revealed band-shifts for three patients, each involving a different coding exon, which were not observed in their respective parental DNAs. Sequence analysis identified these de novo mutations as dA --> dG transition in exon VI at nucleotide 446, a dC --> dT transition in exon VIII at nucleotide 817, and a 22 bp deletion at the exon IX-intron 9 junction from nucleotide 988 to 1,002+7, which causes skipping of exon IX in the mature LIS1 transcript. These changes are predicted to result in an H149R amino acid substitution, an R273X premature translation termination, and abolition of amino acids 301-334, in the respective LIS1 proteins. These data thus confirm LIS1 as the gene responsible for classical lissencephaly in ILS and MDS.

Ehlers-Danlos syndrome: recent advances and current understanding of the clinical and genetic heterogeneity

The Ehlers-Danlos syndrome (EDS) is a heterogeneous group of generalized connective tissue disorders, the major manifestations of which are skin fragility, skin hyperextensibility, and joint hypermobility. The clinical and molecular definition of more than ten types of EDS has, more than ever, emphasized the importance of correct diagnosis because the natural history and mode of inheritance differ among the types. Unfortunately, much of the older literature does not differentiate among the types clearly, and in some instances the life-threatening complications of EDS type IV may be cited as characteristic of the syndrome as a whole. Heterogeneity in EDS began to be appreciated about 30 years ago when the core of the modern classification was developed by Barabas. Since then it was extended by Beighton et al, and then amplified with the insights provided by biochemical and molecular genetic studies. Clinical studies were sufficient initially to distinguish five types of EDS; biochemical studies identified four additional types, EDS type VI, EDS type VII, EDS type IX, and EDS type X, whereas clinical criteria distinguished EDS type VIII. In the following sections, the clinical manifestations of the different forms of EDS are summarized and the known biochemical and molecular genetic features are emphasized.

Ehlers-Danlos syndrome type VII: phenotype and genotype

A patient suffering from a severe form of Ehlers-Danlos syndrome is presented (EDS type VII). The presence of bilateral congenital hip dislocation, generalized joint hypermobility and a soft hyperelastic skin with abnormal scarring suggested a specific collagen type I defect. SDS-PAGE analysis of collagens secreted into the medium of fibroblast cultures showed a retarded migration of more than half of the alpha 2(I) chains. CNBr peptide mapping of the HPLC-purified altered chain localized the mutant locus to the N-terminal region of the protein. cDNA analysis of the corresponding gene COL1A2 revealed, in addition to the expected collagen sequence, a transcript missing the entire exon 6. This exon encodes a major crosslinking site within collagen fibres as well as the N-propeptidase cleavage site. The skipping of exon 6 is caused by a splice site mutation substituting an A for a G at the first nucleotide of intron 6.

The gene structure of B-type nuclear lamins of Xenopus laevis: implications for the evolution of the vertebrate lamin family

The gene structure of the Xenopus laevis lamin B1 gene is presented, together with a partial analysis of the lamin B2 gene of the same species. The intron/exon pattern of lamin B1 is strikingly similar to other vertebrate lamin genes and is completely identical to that of Xenopus lamin B3. An additional intron present in the murine lamin B2 gene has a counterpart in the orthologue gene of Xenopus. These data complete a comparison of the genomic organization of all vertebrate lamin types known so far. They allow the conclusion that the genes encoding B1 and B3 probably reflect the ancestral lamin gene organization and that the B2-type lamins are derived from them. A non-conforming splice junction sequence is found in the Xenopus lamin B1 gene. A GC is found instead of the canonical GT dinucleotide at the 5' end of intron IX. Comparison with other unusual splice sites indicates that the lamin B1 gene represents a functional gene.

The clinical features of Ehlers-Danlos syndrome type VIIB resulting from a base substitution at the splice acceptor site of intron 5 of the COL1A2 gene

The features of a 32 year old woman with Ehlers-Danlos syndrome type VIIB and affected members of her family, resulting from a mutation in one COL1A2 allele, were studied. Her dermal type I collagen contained alpha 2(I) chains and mutant pN-alpha 2(I) chains in which the amino-terminal propeptide remained attached to the alpha 2(I) chain. She was heterozygous for an AG-->AC mutation at the splice acceptor site of intron 5 of the COL1A2 gene. The mutation activated a cryptic AG splice acceptor site corresponding to positions +14 and +15 of exon 6 of the COL1A2 gene. In contrast to previous reports only five, rather than all 18, amino acids encoded by exon 6 were deleted in the proband. The deleted peptide removed the amino-proteinase cleavage site, but not the nearby lysine cross linking site in the amino-telopeptide of the alpha 2(I) chain. She was born with bilateral hip dislocations, knee subluxations, and generalised joint hypermobility. Bilateral inguinal herniae and an umbilical hernia were present at birth. Facial features included a depressed nasal bridge with prominent paranasal folds. The skin was soft, moderately hyperelastic, and sagged over the face. Skin fragility and easy bruising were apparent from childhood. Skin wounds healed slowly and with broad, paper thin scars. Throughout her life, she had multiple fractures of the small bones of her hands and feet following moderate trauma. Electron microscopy of the proband's dermis as well as deep fascia and hip joint capsule from her affected brother showed that collagen fibrils in transverse section were nearly circular but with irregular margins. Light microscopy of bone from her affected brother and son showed normal Haversian systems and lamellar bone. All of these tissues contained approximately equal amounts of the normal and mutant alpha2(I) chains. The findings of this study confirm that loss of the amino-proteinase cleavage site of the pro alpha2(I) collagen chains, owing to anomalous splicing of exon 6 sequences in the conversion of pre-mRNA to mRNA, produces the clinical features of Ehlers-Danlos syndrome type VIIB. The history of frequent fractures found in this family is atypical and indicates an overlap with osteogenesis imperfecta.

Collagen genes: mutations affecting collagen structure and expression

It is to be expected that more collagen genes will be identified and that additional heritable connective tissue diseases will be shown to arise from collagen mutations. Further progress will be fostered by the coordinated study of naturally occurring and induced heritable connective tissues diseases. In some instances, human mutations will be studied in more detail using transgenic mice, while in others, transgenic studies will be used to determine the type of human phenotype that is likely to result from mutations of a given collagen gene. Further studies of transcriptional regulation of the collagen genes will provide the prospect for therapeutic control of expression of specific collagen genes in patients with genetically determined collagen disorders as well as in a wide range of common human diseases in which abnormal formation of the connective tissues is a feature.

Further evidence that the failure to cleave the aminopropeptide of type I procollagen is the cause of Ehlers-Danlos syndrome type VII

Dermal fibroblasts from a Chinese Ehlers-Danlos syndrome type VII patient synthesized approximately equal amounts of normal pro-alpha 2(I) chains of type I procollagen and abnormal ones with electrophoretic mobility of pN alpha 2(I) chains, in which the amino-propeptide (N-propeptide) was retained. Reverse-transcriptase PCR analysis of the proband's RNA showed outsplicing of the 54 base exon 6 in half of the pro-alpha 2(I) mRNAs. Exon 6 encodes 18 amino acids of the N-telopeptide which contains the procollagen N-proteinase cleavage site and a cross-link precursor lysine. Loss of these sequences would result in failure to cleave the amino-propeptide of pro-alpha 2(I) and the accumulation of pN-alpha 2(I) chains. Nucleotide sequencing analyses of the proband's COL1A2 gene showed the presence of a T to C transition at position +2 of intron 6 in one allele and the proband is heterozygous for the defect. This mutation which destroyed the consensus GT dinucleotide at the 5' splice donor site of the intron is responsible for the loss of exon 6 by exon skipping. Electron microscopic analysis of the patient's dermis showed the presence of abnormal collagen I fibrils of irregular diameter and circularity. This mutation in COL1A2 in an EDS VII patient is the first reported case in the Chinese population and is identical to one reported for another EDS-VII (Libyan) patient. The occurrence of an identical mutation in two probands of different ethnic origin is direct evidence that the mutant genotype is the cause of the EDS VII phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

COL4A5 splice site mutation and alpha 5(IV) collagen mRNA in Alport syndrome

Mutations affecting the COL4A5 gene encoding the alpha 5 chain of type IV collagen, are involved in the pathogenesis of X-linked Alport syndrome. We used denaturing gradient gel electrophoresis (DGGE) to screen PCR amplified exons of COL4A5 for point mutations in a set of 18 Alport patients previously characterized by Southern blotting. One sequence variant was identified in the exon 38 region of a male Alport patient. Sequence analysis revealed a G to C transversion in the 5' intron splice donor site downstream from exon 38 (GT to CT). To determine the effect of the mutation on mRNA splicing, alpha 5(IV) cDNA was generated from total RNA of peripheral blood lymphocytes. Subsequent cDNA PCR yielded a product 81 base pairs shorter in the affected Alport patient, compared to normal controls. The absence of exon 38 from the alpha 5(IV) cDNA was confirmed by sequence analysis. The results demonstrated that the mutation leads to skipping of exon 38 in the processing of alpha 5(IV) pre-mRNA. The shortened transcript lacked 27 codons encoding a Gly-X-Y-repeat sequence with a preserved reading frame, enabling the translation of codons further downstream. Clinically, the patient presented with juvenile onset Alport syndrome, end-stage renal failure, and deafness. He had no ocular lesions. Typical ultrastructural changes of the glomerular basement membrane (GBM) were shown on electron microscopy. The patient developed anti-GBM antibodies after renal transplantation, however, renal function deteriorated only moderately.

Quantitative comparison of initiation and mutation phenotypes in hepatocytes of the analbuminemic rat

The potential relationship between mutagenesis and carcinogenesis has been examined in the Nagase analbuminemic rat treated with a single dose of benzo[a]pyrene, an incomplete liver carcinogen. The apparent mutation rate at the albumin locus was calculated by determining the number of hepatocytes expressing a cross-reactive product of albumin in analbuminemic rats treated with benzo[a]pyrene. The rate of initiation, the first stage in carcinogenesis, was determined by assessing the number of hepatocytes expressing the placental isozyme of glutathione S-transferase (PGST) after administration of benzo[a]pyrene. Since the expression of PGST may represent hepatocellular changes independent of initiation, promotion with phenobarbital was employed to clonally expand those putatively initiated hepatocytes expressing PGST. With immunohistochemical measures to assess changes in albumin expression, a threefold increase in the number of hepatocytes expressing albumin was detected after administration of benzo[a]pyrene in Nagase analbuminemic rats. A more than five-fold increase in altered hepatic foci (AHF) exhibiting increased PGST expression was observed in animals given benzo[a]pyrene treatment followed by phenobarbital, compared with those given benzo[a]pyrene alone. The number of albumin-expressing single hepatocytes detected was of the same order of magnitude as the number of individual hepatocytes and AHF expressing PGST, suggesting that similar events may be involved in their formation. Since 3 x 10(6) single hepatocytes expressing albumin were found in the analbuminemic rat liver after a single administration of benzo[a]pyrene, while less than 2 x 10(4) AHF expressing PGST were observed, formation of individual hepatocytes expressing albumin was a far more frequent event than clonal expansion of initiated hepatocytes in the Nagase analbuminemic rat. However, the number of loci of PGST expression including AHF and single hepatocytes is comparable to that of single hepatocytes expressing albumin.

An intronic deletion leading to skipping of exon 21 of COL1A2 in a boy with mild osteogenesis imperfecta

A mild form of osteogenesis imperfecta was diagnosed in a 5-year-old boy with short stature, osteoporosis, blueish sclerae, dentinogenesis imperfecta, hyperextensible joints and bruisable skin. His skin fibroblasts synthesized both normal and shortened pro-alpha 2(I) collagen chains. Procollagen I molecules containing the shortened pro-alpha 2 chains were unstable and thus virtually excluded from secretion at 37 degrees C. Secretion was only partially restored at 30 degrees C. Cyanogen bromide mapping confined the defect to peptide alpha 2(I)CB4. PCR amplification of cDNA showed that the 108 nucleotides corresponding to exon 21 (coding for residues 274 to 309 of the helical domain) were missing in about half of the COL1A2 mRNA. Genomic DNA analysis showed that both exons 21 of COL1A2 were intact, but nucleotides +2 to +40 in intron 21 were deleted on one allele. The intronic deletion altered the conserved nucleotides at position +2 and +5 of the splicing donor site and apparently caused skipping of exon 21 during mRNA splicing. The mild phenotype associated with this COL1A2 mutation may be explained by very poor secretion of the structurally defective procollagen I molecules, which minimizes their deleterious effects on extracellular matrix formation.

Architecture of the canine IDUA gene and mutation underlying canine mucopolysaccharidosis I

Mucopolysaccharidosis I (MPS I) is a lysosomal storage disease caused by deficiency of alpha-L-iduronidase. In addition to the well-known human forms (Hurler, Hurler/Scheie, and Scheie syndromes), there exists a canine model of the disease. By using previously described canine cDNA encoding alpha-L-iduronidase as a probe, the canine IDUA gene has been cloned and characterized. It contains 14 exons spread over 13 kb. An unusual GC dinucleotide was found at the donor splice site of intron 11. A transcriptional start site was identified by primer extension 177 bp upstream of the initiator AUG codon. The upstream region was found to be similar to the promoter region of many housekeeping genes: it is GC rich and has seven potential Sp1 binding sites but no TATA box or CAAT motif. The mutation in canine MPS I was localized to the area of intron 1 by RT-PCR, identified by sequence analysis of amplified genomic DNA, and confirmed by restriction analysis; it is a G-->A transition in the donor splice site of intron 1. The mutation causes retention of intron 1 in the RNA and creates a premature termination codon at the exon-intron junction.

Expression of mutant alpha (I)-procollagen in osteoblast and fibroblast cultures from a proband with osteogenesis imperfecta type IV

This study compares the synthesis of mutant type I collagen in cultured dermal fibroblasts and trabecular osteoblasts that were isolated from a patient with moderately severe osteogenesis imperfecta (type IV). Previous study of this patient's dermal fibroblasts revealed a 2000 dalton deletion located in cyanogen bromide peptide 4 of alpha 2(I)-collagen. The phenotype of the bone cell cultures was defined by a 3-4 day logarithmic phase doubling time, predominantly type I collagen production over type III and alkaline phosphatase activity 13.5 times dermal fibroblast levels. The current study revealed that both fibroblasts and osteoblasts synthesized a normal and a shortened alpha 2(I) chain, each as the product of separate alleles. Following pepsin treatment of the procollagens, a shortened alpha 1(I) chain was also seen in both cell types. Cyanogen bromide peptide mapping of osteoblast alpha-chains demonstrated the same deletions in the cyanogen bromide peptide 4 as observed in the fibroblast cyanogen bromide maps. PAGE analysis of oligonucleotide-specific cDNA that was reverse transcribed from RNA isolated from fibroblasts and osteoblasts also demonstrated the presence of two bands, one the normal size of alpha 2(I) cDNA and a second species that was smaller by 54 base pairs. Sequencing of polymerase chain reaction-amplified cDNA fragments revealed an in-frame deletion of exon 12. This finding was confirmed by the RNase protection method. Genomic DNA sequencing detected a T----G point mutation in the second position of the 5' splice donor site of intron 12. Therefore, in this patient with osteogenesis imperfecta there was no qualitative alteration in the osteoblast-specific expression of this mutant alpha 2(I)-collagen allele compared to dermal fibroblasts.

A null mutation in the rhodopsin gene causes rod photoreceptor dysfunction and autosomal recessive retinitis pigmentosa

Mutations within the rhodopsin gene are known to give rise to autosomal dominant retinitis pigmentosa (RP), a common hereditary form of retinal degeneration. We now describe a patient with autosomal recessive RP who is homozygous for a nonsense mutation at codon 249 within exon 4 of the rhodopsin gene. This null mutation, the first gene defect identified in autosomal recessive retinitis pigmentosa, should result in a functionally inactive rhodopsin protein that is missing the sixth and seventh transmembrane domains including the 11-cis-retinal attachment site. We also found a different null mutation carried heterozygously by an unrelated unaffected individual. Heterozygous carriers of either mutation had normal ophthalmologic examinations but their electroretinograms revealed an abnormality in rod photoreceptor function.

Genetic linkage of the Marfan syndrome, ectopia lentis, and congenital contractural arachnodactyly to the fibrillin genes on chromosomes 15 and 5. The International Marfan Syndrome Collaborative Study

BACKGROUND

The large glycoprotein fibrillin is a structural component of elastin-containing microfibrils found in many tissues. The Marfan syndrome has been linked to the fibrillin gene on chromosome 15, but congenital contractural arachnodactyly, which shares some of the physical features of the syndrome, has been linked to the fibrillin gene on chromosome 5.

METHODS

Using specific markers for the fibrillin genes, we performed genetic linkage analysis in 28 families with the Marfan syndrome and 8 families with four phenotypically related disorders--congenital contractural arachnodactyly (3 families), ectopia lentis (2), mitral-valve prolapse syndrome (2), and annuloaortic ectasia (1).

RESULTS

Genetic linkage was established between the Marfan syndrome and only the fibrillin gene on chromosome 15, with a maximum lod score of 25.6 (odds for linkage, 10(25.6):1). Ectopia lentis was also linked to the fibrillin gene on chromosome 15, whereas congenital contractural arachnodactyly was linked to the fibrillin gene on chromosome 5. There was no linkage of mitral-valve prolapse to the fibrillin gene on chromosome 5; studies of chromosome 15 were not informative. Annuloaortic ectasia was not linked to either fibrillin gene.

CONCLUSIONS

The Marfan syndrome appears to be caused by mutations in a single fibrillin gene on chromosome 15. Diagnosis of the Marfan syndrome by genetic linkage and analysis is now feasible in many families.

Invariant exon skipping in the human alpha-galactosidase A pre-mRNA: Ag+1 to t substitution in a 5'-splice site causing Fabry disease

Fabry disease, an inborn error of glycosphingolipid catabolism, results from lesions in the X-linked gene encoding the human lysosomal hydrolase, alpha-galactosidase A (alpha-D-galactoside galactohydrolase; EC 3.2.1.22). To detect alpha-galactosidase A RNA processing or stability defects causing Fabry disease, Northern hybridization analyses were performed with poly(A)+ RNA isolated from cultured lymphoblasts from unrelated Fabry hemizygotes. Using a riboprobe complimentary to the normal 1.45-kb alpha-galactosidase A mRNA, a single 1.25-kb transcript was identified in three classically affected brothers from a Japanese Fabry family. Densitometric analysis revealed that the 1.25-kb transcripts were present at 50 to 60% of normal amounts. RNase A analysis identified a deletion of about 200 bp that appeared to include the entire 198 bp of exon 6. Amplification and direct sequencing of a genomic region containing exon 6 from an affected hemizygote revealed a g+1 to t transversion in the invariant gt consensus 5'-splice site of intron 6, which resulted in the deletion of the entire exon 6 sequence. This novel splicing lesion causing Fabry disease is the first g+1 to t transversion of a mammalian 5'-splice site that consistently eliminates the preceding exon.

The molecular defect in a family with mild atypical osteogenesis imperfecta and extreme joint hypermobility: exon skipping caused by an 11-bp deletion from an intron in one COL1A2 allele

We have investigated a family with an autosomal dominantly inherited connective-tissue defect causing extreme joint hypermobility, premature osteoporosis and late-onset fractures. Analysis of collagenous proteins from affected individuals showed a deletion in some alpha 2(I) chains. Peptide mapping localized this to the CB peptide alpha 2CB4, which covers the N-terminal one-third of the protein chain. Polymerase chain reaction amplification and sequencing of cDNA derived from this region of the mRNA identified a heterozygous deletion of the 54 bp comprising exon 9. Similar analysis of the genomic DNA revealed an 11-bp deletion from bp3 to bp13 of IVS-9. This disrupts the consensus 5' splice signal (GTAAGT) and leads to exon skipping. In a family study of 13 affected and unaffected family members using both heteroduplex formation and direct analysis for the deletion, all of the affected, but no unaffected individuals, were found to carry the deletion. This generated a positive Lod score of 2.6 with the Liped programme.

Deletion of exon 8 causes glycosylasparaginase deficiency in an African American aspartylglucosaminuria (AGU) patient

We have indentified a GT-to-TT transversion at the splice donor site of intron 8 in the glycosylasparaginase gene from an African American aspartylglucosaminuria (AGU) patient. This mutation causes abnormal splicing of glycosylasparaginase pre-mRNA by joining exon 7 to 9 and excluding 134 bp exon 8. The effect of the mutation is compounded by a frame shift that occurs after the deletion site resulting in premature translational termination. The truncated AGU protein was neither catalytically active nor processed into mature alpha and beta subunits. Both this and a previously characterized Finnish AGU mutation appear to affect folding of the single-chain precursor of glycosylasparaginase and thereby prevent transport of the enzyme to lysosomes.

Osteogenesis imperfecta: translation of mutation to phenotype

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Ehlers-Danlos syndrome type VII: a single base change that causes exon skipping in the type I collagen alpha 2(I) chain

We have examined the procollagens and collagens produced by skin fibroblasts from a patient with Ehlers-Danlos syndrome type VII. The patient was heterozygous for an abnormal alpha 2(I) chain migrating with the approximate size of pN alpha 2(I) chains after pepsin digestion. Peptide mapping suggested that the abnormality was located at the amino-terminus of the alpha 2(I) chain. Quantitative analysis of the alpha 2(I) mRNA indicated loss of the exon 6 sequences, and subsequent polymerase chain reaction amplification of cDNA demonstrated a deletion of the 54 bp of exon 6 from some of the alpha 2(I) mRNA. Analysis of genomic DNA from the patient revealed a single base change in one COL1A2 allele, substituting an A for a G as the first base of intron 6. This change mutates the obligate GT-dinulceotide splicing signal to AT and leads to exon skipping with splicing from exon 5 to exon 7. Loss of exon 6 sequences results in the loss of the procollagen-N-propeptidase cleavage site and a lysine residue that normally participates in covalent intermolecular crosslinking within collagen fibres.