Direct sequencing of PCR products derived from cDNAs for the pro alpha 1 and pro alpha 2 chains of type I procollagen as a screening method to detect mutations in patients with osteogenesis imperfecta

Expanding the genetic and clinical spectrum of osteogenesis imperfecta: identification of novel rare pathogenic variants in type I collagen-encoding genes

Introduction

Osteogenesis imperfecta (OI) is a clinically and genetically heterogeneous skeletal disorder. The majority of affected cases are attributed to autosomal dominant pathogenic variants (PVs) found in the COL1A1 and COL1A2 genes, which encode type I collagen. However, PVs in other genes involved in collagen posttranslational modification, processing, crosslinking, osteoblast differentiation, and bone mineralization have also been associated with OI.

Methods

In this study, we present the results of next-generation sequencing (NGS) analysis using a custom panel of 11 genes known to be associated with OI. This clinical study enrolled a total of 10 patients, comprising 7 male and 3 female patients from 7 families, all from the Puglia Region in South Italy, providing a detailed overview of their age, gender, family history, OI type, and non-skeletal features.

Results

The genetic analysis revealed 5 PVs in the COL1A1 gene and 2 PVs in the COL1A2 gene. Importantly, three of these PVs have not been previously reported in the literature. These include two novel heterozygous frameshift PVs in COL1A1 (c.2890_2893del and c.3887del) and one novel heterozygous missense PV in COL1A2 (c.596G>T).

Discussion

The identification of these previously unreported PVs expands the variant spectrum of the COL1A1 and COL1A2 genes and may have implications for accurate diagnosis, genetic counselling, and potential therapeutic interventions in affected individuals and their families.

Confirmation of the pathogenicity of a mutation p.G337C in the COL1A2 gene associated with osteogenesis imperfecta

Mutation analysis as the gold standard is particularly important in diagnosis of osteogenesis imperfecta (OI) and it may be preventable upon early diagnosis. In this study, we aimed to analyze the clinical and genetic materials of an OI pedigree as well as to confirm the deleterious property of the mutation.A pedigree with OI was identified. All family members received careful clinical examinations and blood was drawn for genetic analyses. Genes implicated in OI were screened for mutation. The function and structure of the mutant protein were predicted using bioinformatics analysis.The proband, a 9-month fetus, showed abnormal sonographic images. Disproportionately short and triangular face with blue sclera was noticed at birth. She can barely walk and suffered multiple fractures till 2-year old. Her mother appeared small stature, frequent fractures, blue sclera, and deformity of extremities. A heterozygous missense mutation c.1009G>T (p.G337C) in the COL1A2 gene was identified in her mother and her. Bioinformatics analysis showed p.G337 was well-conserved among multiple species and the mutation probably changed the structure and damaged the function of collagen.We suggest that the mutation p.G337C in the COL1A2 gene is pathogenic for OI by affecting the protein structure and the function of collagen.

Two novel COL1A1 mutations in patients with osteogenesis imperfecta (OI) affect the stability of the collagen type I triple-helix

Osteogenesis imperfecta (OI) is a bone dysplasia caused by mutations in the COL1A1 and COL1A2 genes. Although the condition has been intensely studied for over 25 years and recently over 800 novel mutations have been published, the relation between the location of mutations and clinical manifestation is poorly understood. Here we report missense mutations in COL1A1 of several OI patients. Two novel mutations were found in the D1 period. One caused a substitution of glycine 200 by valine at the N-terminus of D1 in OI type I/IV, lowering collagen stability by 50% at 34 degrees C. The other one was a substitution of valine 349 by phenylalanine at the C-terminus of D1 in OI type I, lowering collagen stability at 37.5 degrees C. Two other mutations, reported before, changed amino residues in D4. One was a lethal substitution changing glycine 866 to serine in genetically identical twins with OI type II. That mutated amino acid was near the border of D3 and D4. The second mutation changed glycine 1040 to serine located at the border of D4 and D0.4, in a proband manifesting OI type III, and lowered collagen stability at 39 degrees C (2 degrees C lower than normal). Our results confirm the hypothesis on a critical role of the D1 and D4 regions in stabilization of the collagen triple-helix. The defect in D1 seemed to produce a milder clinical type of OI, whereas the defect in the C-terminal end of collagen type caused the more severe or lethal types of OI.

Two new recurrent nucleotide mutations in the COL1A1 gene in four patients with osteogenesis imperfecta: about one-fifth are recurrent

Previous observations on mutations causing osteogenesis imperfecta (OI) suggested that unrelated patients had private mutations. Here preliminary studies on two patients with type I OI indicated that some mutations in the COL1A1 gene for type I procollagen cannot be detected by analyses of cDNAs. Therefore, we developed a protocol whereby 43 exon and exon flanking sequences of the COL1A1 gene can be amplified by PCR and scanned for mutations by denaturing gradient gel electrophoresis. Two new recurrent nucleotide mutations in the gene were found in four apparently unrelated patients with OI. Analysis of previous publications indicated that up to one-fifth of the mutations causing OI are recurrent in the sense that they were identical in apparently unrelated probands. About 80% of these identical mutations were in CpG dinucleotide sequences.

Use of the Cre/lox recombination system to develop a non-lethal knock-in murine model for osteogenesis imperfecta with an alpha1(I) G349C substitution. Variability in phenotype in BrtlIV mice

We utilized the Cre/lox recombination system to develop the first knock-in murine model for osteogenesis imperfecta (OI). The moderately severe OI phenotype was obtained from an alpha1(I) Gly(349) --> Cys substitution in type I collagen, reproducing the mutation in a type IV OI child. We introduced four single nucleotide (nt) changes into murine col1a1 exon 23: the disease causing G-->T transversion (nt 1546), an adjacent G-->T change (nt 1551) to generate a GUC ribozyme cleavage site, and two transversions (nt 1567 C-->A and nt 1569 C-->G) to cause a Leu --> Met substitution. We also introduced a 3.2-kilobase pair transcription/translation stop cassette in intron 22, flanked by directly repeating lox recombination sites. After homologous recombination in ES cells, two male chimeras were obtained. Chimeras were mated with transgenic females expressing Cre recombinase to remove the stop cassette from a portion of the progeny's cells. To generate mice with full expression of the Gly(349) --> Cys mutation, these offspring were then mated with wild-type females. Skeletal staining and bone histology of the F2 revealed a classical OI phenotype with deformity, fragility, osteoporosis and disorganized trabecular structure. We designate these mice BrtlIV (Brittle IV). BrtlIV mice have phenotypic variability ranging from perinatal lethality to long term survival with reproductive success. The phenotypic variability is not associated with differences in expression levels of the mutant allele in total RNA derived from tissue extracts. Expression of the mutant protein is also equivalent in different phenotypes. Thus, these mice are an excellent model for delineation of the modifying factors postulated to affect human OI phenotypes. In addition, we generated knock-in mice carrying an "intronic" inclusion by mating chimeras with wild-type females. Alternative splicing involving the stop cassette results in retention of non-collagenous sequences. These mice reproduce the lethal phenotype of similar human mutations and are designated BrtlII.

Phenotypic comparison of an osteogenesis imperfecta type IV proband with a de novo alpha2(I) Gly922 --> Ser substitution in type I collagen and an unrelated patient with an identical mutation

We examined the type I collagen synthesized by cultured dermal fibroblasts from a patient affected with osteogenesis imperfecta (OI) type IV. Both normal and abnormal trimers were produced. The mutant collagen molecules were excessively modified intracellularly, had a melting temperature 4 degrees C lower than the control, were secreted at a reduced rate, and underwent delayed processing to mature alpha chains.Molecular investigations identified a G --> A transition in one COL1A2 allele, resulting in a Gly922 --> Ser substitution in the alpha2(I) chain. The proband's mutation was demonstrated to arise "de novo" by the absence of the mutant allele restriction enzyme pattern from parental genomic DNA.We analyzed the insoluble extracellular matrix deposited by long-term cultured fibroblasts from our patient and from a previously described unrelated individual who carries an identical substitution. In both cases, the mutant chain constituted 10-15% of the total alpha chains deposited.We also present here the first detailed comparison of phenotype between unrelated OI patients with an identical collagen mutation. These two patients are both Caucasian females, ages 8 and 9 years, each diagnosed as type IV OI by the Sillence classification. They have a similar phenotype including moderate skeletal fragility with several femur fractures, dentinogenesis imperfecta, wormian bone, and reduced height and weight. We conclude that this phenotype is related both to the location of this mutation and to the similar extent of matrix incorporation by the mutant chains. Molecular and biochemical studies of unrelated individuals with identical amino acid substitutions in type I collagen resulting in either similar or dissimilar clinical outcomes will make a significant contribution to identifying the factors involved in the modulation of the OI phenotype.