Nuclease S1 mapping of a homozygous mutation in the carboxyl-propeptide-coding region of the pro alpha 2(I) collagen gene in a patient with osteogenesis imperfecta

Advances in Skeletal Dysplasia Genetics

Skeletal dysplasias result from disruptions in normal skeletal growth and development and are a major contributor to severe short stature. They occur in approximately 1/5,000 births, and some are lethal. Since the most recent publication of the Nosology and Classification of Genetic Skeletal Disorders, genetic causes of 56 skeletal disorders have been uncovered. This remarkable rate of discovery is largely due to the expanded use of high-throughput genomic technologies. In this review, we discuss these recent discoveries and our understanding of the molecular mechanisms behind these skeletal dysplasia phenotypes. We also cover potential therapies, unusual genetic mechanisms, and novel skeletal syndromes both with and without known genetic causes. The acceleration of skeletal dysplasia genetics is truly spectacular, and these advances hold great promise for diagnostics, risk prediction, and therapeutic design.

Phenotype and genotype analysis of Chinese patients with osteogenesis imperfecta type V

Osteogenesis imperfecta (OI) type V is an autosomal-dominant disease characterized by calcification of the forearm interosseous membrane, radial head dislocation, a subphyseal metaphyseal radiodense line, and hyperplastic callus formation. The causative mutation, c.-14C>T in the 5'-untranslated region of IFITM5, was recently discovered to be involved in this disease. However, in spite of the little genotypic variability, considerable phenotypic variability has been recognized in two cohorts of patients, the majority of whom were Caucasians. Using exome sequencing, we identified the same heterozygous mutation in four Chinese families with OI type V. This study confirms the molecular cause of OI type V and describes the phenotype of Chinese patients with this disorder. In conclusion, the phenotype of Chinese patients was generally similar to that of Caucasian patients.

Defects in GPI biosynthesis perturb Cripto signaling during forebrain development in two new mouse models of holoprosencephaly

Holoprosencephaly is the most common forebrain defect in humans. We describe two novel mouse mutants that display a holoprosencephaly-like phenotype. Both mutations disrupt genes in the glycerophosphatidyl inositol (GPI) biosynthesis pathway: gonzo disrupts Pign and beaker disrupts Pgap1. GPI anchors normally target and anchor a diverse group of proteins to lipid raft domains. Mechanistically we show that GPI anchored proteins are mislocalized in GPI biosynthesis mutants. Disruption of the GPI-anchored protein Cripto (mouse) and TDGF1 (human ortholog) have been shown to result in holoprosencephaly, leading to our hypothesis that Cripto is the key GPI anchored protein whose altered function results in an HPE-like phenotype. Cripto is an obligate Nodal co-factor involved in TGFβ signaling, and we show that TGFβ signaling is reduced both in vitro and in vivo. This work demonstrates the importance of the GPI anchor in normal forebrain development and suggests that GPI biosynthesis genes should be screened for association with human holoprosencephaly.

Comparison of the mismatch-specific endonuclease method and denaturing high-performance liquid chromatography for the identification of HBB gene mutations

Background

Beta-thalassemia is a common autosomal recessive hereditary disease in the Meditertanean, Asia and African areas. Over 600 mutations have been described in the beta-globin (HBB), of which more than 200 are associated with a beta-thalassemia phenotype.

Results

We used two highly-specific mutation screening methods, mismatch-specific endonuclease and denaturing high-performance liquid chromatography, to identify mutations in the HBB gene. The sensitivity and specificity of these two methods were compared. We successfully distinguished mutations in the HBB gene by the mismatch-specific endonuclease method without need for further assay. This technique had 100% sensitivity and specificity for the study sample.

Conclusion

Compared to the DHPLC approach, the mismatch-specific endonuclease method allows mutational screening of a large number of samples because of its speed, sensitivity and adaptability to semi-automated systems. These findings demonstrate the feasibility of using the mismatch-specific endonuclease method as a tool for mutation screening.

Transplantation of human fetal mesenchymal stem cells improves glomerulopathy in a collagen type I alpha 2-deficient mouse

Fetal mesenchymal stem cell (fetal MSC) therapy has potential to treat genetic diseases with early onset, including those affecting the kidney and urinary tract. A collagen type I alpha 2-deficient mouse has a deletion in the alpha2 chain of the procollagen type I gene, resulting in the synthesis of abnormal alpha1(I)(3) homotrimers, which replace normal alpha 1(I)2 alpha 2(I)1 heterotrimers and a glomerulopathy. We first confirmed that col1 alpha 2-deficient homozygous mice show abnormal collagen deposition in the glomeruli, which increases in frequency and severity with postnatal age. Intrauterine transplantation of human MSCs from first trimester fetal blood led postnatally to a reduction of abnormal homotrimeric collagen type I deposition in the glomeruli of 4-12 week-old col1 alpha 2-deficient mice. Using bioluminescence imaging, in situ hybridization and immunohistochemistry in transplanted col1 alpha 2-deficient mice, we showed that the damaged kidneys preferentially recruited donor cells in glomeruli, around mesangial cells. Real-time RT-PCR demonstrated that this effect was seen at an engraftment level of 1% of total cells in the kidney, albeit higher in glomeruli. We conclude that intrauterine transplantation of human fetal MSCs improves renal glomerulopathy in a collagen type I-deficient mouse model. These data support the feasibility of prenatal treatment for hereditary renal diseases.

Selective retention and degradation of molecules with a single mutant alpha1(I) chain in the Brtl IV mouse model of OI

We investigated the secretion, matrix incorporation and interactions of molecules with one and two mutant alpha1(I) collagen chains in the Brtl IV murine model for Osteogenesis Imperfecta, carrying a Gly-349 to Cys substitution in one col1a1 allele. We detected a significant deviation from the expected 25 and 50% content of the molecules with no (37-46%) and one (26-40%) mutant chains in skin and bone as well as in fibroblast and osteoblast cell culture media. Steady-state labeling with (35)S-Cys demonstrated incomplete secretion of the mutant collagen in cell culture, particularly molecules containing one mutant chain. Pulse and pulse-chase experiments revealed slower secretion of the latter. An enlargement of endoplasmic reticulum in skin fibroblasts from Brtl IV mice, clearly visible by electron microscopy, supported the abnormal secretion identified by biochemical studies. We observed increased susceptibility of molecules with one mutant chain to proteolytic degradation in vitro, but we did not detect significant selective degradation in cell culture media. Mutant collagen molecules incorporated from the media into newly deposited fibers and into fully crosslinked and mature matrix in the same ratio as they were secreted. Specific labeling of reactive -SH demonstrated that about half of the Cys349-SH groups in the mutant molecules were exposed and potentially available for aberrant interactions with other molecules inside or outside the cells. Based on these and our previous findings, we argue that the outcome in Brtl IV may be significantly affected by cellular stress and malfunction caused by the retention and degradation of newly synthesized mutant collagen.

Changes in thermal stability and microunfolding pattern of collagen helix resulting from the loss of alpha2(I) chain in osteogenesis imperfecta murine

Homozygous mutations resulting in formation of alpha1(I)(3) homotrimers instead of normal type I collagen cause mild to severe osteogenesis imperfecta (OI) in humans and mice. Limited studies of changes in thermal stability of type I homotrimers were reported previously, but the results were not fully consistent. We revisited this question in more detail using purified tendon collagen from wild-type (alpha1(I)(2)alpha2(I) heterotrimers) and oim (alpha1(I)(3)) mice as well as artificial alpha1(I)(3) homotrimers obtained by refolding of rat-tail-tendon collagen. We found that at the same heating rate oim homotrimers completely denature at approximately 2.5deg.C higher temperature than wild-type heterotrimers, as determined by differential scanning calorimetry. At the same, constant temperature, homotrimers denature approximately 100 times slower than heterotrimers, as determined by circular dichroism. Detailed analysis of proteolytic cleavage at different temperatures revealed that microunfolding of oim homotrimers and wild-type heterotrimers occurs at similar rate but within a number of different sites. In particular, the weakest spot on the oim triple helix is located approximately 100 amino acid residues from the C-terminal end within the cyanogen bromide peptide CB6. The same microunfolding site is also present in wild-type collagen, but the weakest spot of the latter is located close to the N-terminal end of CB8. Amino acid analysis and differential gel electrophoresis showed virtually no posttranslational overmodification of oim mouse tendon collagen. Moreover, thermal stability and microunfolding of artificial rat-tail-tendon homotrimers were similar to oim homotrimers. Thus, the observed changes are associated with difference in the amino acid composition of alpha1(I) and alpha2(I) chains rather than posttranslational overmodification.

Osteogenesis imperfecta type VII maps to the short arm of chromosome 3

We have identified a novel form of autosomal recessive osteogenesis imperfecta (OI) in a small First Nations community from northern Quebec. Mutation screening of the COL1A1/COL1A2 genes revealed no detectable mutations, and type I collagen protein analyses were also normal. By linkage analysis, we mapped this unique autosomal recessive variant of osteogenesis imperfecta to chromosome 3p22-24.1. Based on the assumption of a founder effect, genome-wide screening was performed on a DNA sample pooled from seven affected individuals. Familial as well as historical recombinations identified within an extended haplotype of 19 markers localized the disease between markers D3S2324 and D3S1561, separated by <5 cM. Based on chromosomal localization to 3p22-24.1, the transforming growth factor-beta receptor 2 gene and the parathyroid hormone/parathyroid hormone-related peptide receptor were tested, but were excluded as being associated with the phenotype. This study excludes type I collagen mutations in the pathogenesis of the disease and assigns this form of OI to a locus other than the ones containing the type I collagen genes.

Osteogenesis imperfecta type VII: an autosomal recessive form of brittle bone disease

Osteogenesis imperfecta (OI) is a heritable disease of bone with low bone mass and bone fragility. The disease is generally classified into four types based on clinical features and disease severity, although recently fifth and sixth forms have also been reported. Most forms of OI are autosomal dominant. Rarely, autosomal recessive disease has been described. We report the clinical, radiological, and histological features of four children (age 3.9-8.6 years at last follow-up; all girls) and four adults (age 28-33 years; two women) with a novel form of autosomal recessive OI living in an isolated First Nations community in northern Quebec. In keeping with the established numeric classification for OI forms, we have called this form of the disease OI type VII. The phenotype is moderate to severe, characterized by fractures at birth, bluish sclerae, early deformity of the lower extremities, coxa vara, and osteopenia. Rhizomelia is a prominent clinical feature. Histomorphometric analyses of iliac crest bone samples revealed findings similar to OI type I, with decreased cortical width and trabecular number, increased bone turnover, and preservation of the birefringent pattern of lamellar bone. The disease has subsequently been localized to chromosome 3p22-24.1, which is outside the loci for type I collagen genes. The underlying genetic basis for the disease remains to be determined.

Osteogenesis imperfecta murine: interaction between type I collagen homotrimers

Types I, II, and III collagens are believed to have evolved from the same homotrimer ancestor and they have substantial sequence homology, but type I molecules are alpha1(I)(2)alpha2(I) heterotrimers, unlike homotrimeric types II and III. It is believed that the alpha2(I) chain first appeared in lower vertebrates and that it plays a particularly important role in bone formation. For instance, spontaneous mutations resulting in non- functional alpha2 chains and formation of type I homotrimers cause severe bone pathology (osteogenesis imperfecta) in humans and in animals. However, the exact role of the alpha2 chain is not known. Here, we report measurements of intermolecular forces between collagen helices in native and reconstituted fibers composed of type I homotrimers, heterotrimers and their mix. For comparison, we report forces between type II homotrimers in reconstituted fibers. In agreement with previous studies, we find that the absence of the alpha2 chain reduces temperature-favored attraction between collagen helices, either because of the difference in amino acid sequence of the alpha1 and alpha2 chains or because of more extensive post-translational modification of homotrimers. We find that forces between helices in fibers from type I (as well as type II) homotrimers are not sensitive to pH between pH 6 and 7.5, in contrast to type I heterotrimers. Apparently, the effect of pH is related to extra histidine residues present on alpha2 chains but not on alpha1 chains. Finally, our measurements indicate that the alpha2 chain is responsible for binding some soluble compound(s), possibly glycosaminoglycans, whose displacement results, e.g., in the loss of tendon crystallinity. The ability of the alpha2 chain to bind non-collagen matrix components may be particularly important for bone matrix formation and mineralization.

Production of recombinant human type I procollagen homotrimer in the mammary gland of transgenic mice

The large scale production of recombinant collagen for use in biomaterials requires an efficient expression system capable of processing a large (> 400 Kd) multisubunit protein requiring post-translational modifications. To investigate whether the mammary gland of transgenic animals fulfills these requirements, transgenic mice were generated containing the alpha S1-casein mammary gland-specific promoter operatively linked to 37 Kb of the human alpha 1(I) procollagen structural gene and 3' flanking region. The frequency of transgenic lines established was 12%. High levels of soluble triple helical homotrimeric [(alpha 1)3] type I procollagen were detected (up to 8 mg/ml) exclusively in the milk of six out of 9 lines of lactating transgenic mice. The transgene-derived human procollagen chains underwent efficient assembly into a triple helical structure. Although proline or lysine hydroxylation has never been described for any milk protein, procollagen was detected with these post-translational modifications. The procollagen was stable in milk; minimal degradation was observed. These results show that the mammary gland is capable of expressing a large procollagen gene construct, efficiently assembling the individual polypeptide chains into a stable triple helix, and secreting the intact molecule into the milk.

Expression of collagen alpha1(I) mRNA variants during tooth and bone formation in the rat

Collagen alpha1(I) mRNA is composed of two variants of 5 and 6 kb, differing in the length of the 3' untranslated region. In this work, the nucleotide sequences of the two rat mRNA variants were compared, and their expression pattern in cells forming bone, dentin, and cementum were analyzed. The sequences were determined from cDNA inserts of tooth and bone libraries plus directly from PCR fragments, obtained from bone. A total of 5721 bases of the rat collagen alpha1(I) sequence from cDNA of tooth and bone was determined. All sequences of the short variant were represented in the long variant. Only the alternatively poly-A additions gave rise to the variants in hard tissue. Two oligonucleotides were chosen as probes, one of which recognized, on Northern blots, the two bands of 5 and 6 kb, and the other the 6-kb variant only. The oligonucleotides were used in in situ hybridization experiments, for study of the distribution of the variants in different extracellular matrix-forming cells. Osteoblasts, odontoblasts, and cementum-associated cells were closely examined in sections from rat maxillae from 2 to 25 days of age. A similar or identical pattern of mRNA expression was observed with both oligonucleotides, indicating that the two mRNA variants were co-expressed in all cases.

Analysis of the COL1A1 and COL1A2 genes by PCR amplification and scanning by conformation-sensitive gel electrophoresis identifies only COL1A1 mutations in 15 patients with osteogenesis imperfecta type I: identification of common sequences of null-allele mutations

Although >90% of patients with osteogenesis imperfecta (OI) have been estimated to have mutations in the COL1A1 and COL1A2 genes for type I procollagen, mutations have been difficult to detect in all patients with the mildest forms of the disease (i.e., type I). In this study, we first searched for mutations in type I procollagen by analyses of protein and mRNA in fibroblasts from 10 patients with mild OI; no evidence of a mutation was found in 2 of the patients by the protein analyses, and no evidence of a mutation was found in 5 of the patients by the RNA analyses. We then searched for mutations in the original 10 patients and in 5 additional patients with mild OI, by analysis of genomic DNA. To assay the genomic DNA, we established a consensus sequence for the first 12 kb of the COL1A1 gene and for 30 kb of new sequences of the 38-kb COL1A2 gene. The sequences were then used to develop primers for PCR for the 103 exons and exon boundaries of the two genes. The PCR products were first scanned for heteroduplexes by conformation-sensitive gel electrophoresis, and then products containing heteroduplexes were sequenced. The results detected disease-causing mutations in 13 of the 15 patients and detected two additional probable disease-causing mutations in the remaining 2 patients. Analysis of the data developed in this study and elsewhere revealed common sequences for mutations causing null alleles.

Aberrant O-glycosylation in the collagenous domain of pro alpha2(I) procollagen subunits synthesized by chemically transformed hamster fibroblasts

Chemically transformed Syrian hamster embryo fibroblasts (NQT-SHE) do not synthesize the pro alpha1(I) subunit of type I collagen, but they secrete two forms of the pro alpha2(I) subunit (N33 and N50) with abnormal post-translational modifications localized in the alpha2CB3,5 cyanogen bromide peptide of the collagenous domain (B. Peterkofsky and W. Prather (1992) J. Biol. Chem. 267 5388-5395). Isoelectric focusing and treatment of the modified chains with glycosidases and biotinylated Jacalin lectin identified the modifications as Gal beta1,3-GalNAc-O-Ser/Thr with or without a terminal sialic acid in an alpha2,6 linkage. Unhydroxylated N33 alpha-chains also reacted with Jacalin, confirming that the abnormal modification was O-glycosylation and not hyperhydroxylation of proline or lysine. Cells were treated with benzyl GalNAc, a competitive inhibitor of galactosyl transferase that prevents addition of Gal to GalNAc-O-Ser/Thr and thus blocks elongation of O-glycosyl chains. Treated cells secreted pro alpha2(I) chains containing GalNAc-O-Ser/Thr but no galactose or sialic acid, which suggested that Gal addition takes place before sialylation. Treatment of NQT-SHE cells with monensin and brefeldin A inhibited secretion and led to intracellular accumulation of pro alpha2(I) chains that contained only GalNAc. Therefore, it appears that GalNAc addition to pro alpha2(I) chains in NQT-SHE cells occurs in the cis-Golgi, while sialic acid and galactose are added in the trans-Golgi network. The pro alpha2(I) chains produced by NQT-SHE cells most likely are modified because they are in the denatured state, and thus potential O-glycosylation sites become available that would not be exposed in normal triple helical procollagen.

Defective pro alpha 2(I) collagen synthesis in a recessive mutation in mice: a model of human osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable disorder of connective tissue associated with fractures, osteopenia, and short stature. OI results from mutations affecting the pro alpha 1 or pro alpha 2 gene of type I collagen. We describe a strain of mice with a nonlethal recessively inherited mutation (oim) that results in phenotypic and biochemical features that simulate moderate to severe human OI. The phenotype of homozygous oim mice includes skeletal fractures, limb deformities, generalized osteopenia, and small body size. Their femurs are smaller and demonstrate marked cortical thinning and fewer medullary trabeculae than those of wild-type mice. Breeding studies show the mutation is inherited in most crosses as a single recessive gene on chromosome 6, near the murine Cola-2 gene. Biochemical analysis of skin and bone, as well as isolated dermal fibroblast cultures, demonstrate that alpha 1(I) homotrimeric collagen accumulates in these tissues and is secreted by fibroblasts. Short labeling studies in fibroblasts demonstrate an absence of pro alpha 2(I) collagen chains. Nucleotide sequencing of the cDNA encoding the COOH-propeptide reveals a G deletion at pro alpha 2(I) nucleotide 3983; this results in an alteration of the sequence of the last 48 amino acids. The oim mouse will facilitate the study of type I collagen-related skeletal disease.

Paternal mosaicism for a COL1A1 dominant mutation (alpha 1 Ser-415) causes recurrent osteogenesis imperfecta

We describe a dominant point mutation in the COL1A1 gene causing extremely severe osteogenesis imperfecta (OI type II/III) which was detected in the dermal fibroblasts of a proband, diagnosed by ultrasonography at 24 weeks of gestation. Type I collagen secretion was reduced and pro alpha 1(I) chains were overmodified. The mutation was localised in one COL1A1 allele by chemical cleavage of mismatched bases in normal cDNA/proband's mRNA heteroduplexes, and identified by cloning and sequencing. A G-to-A transition which causes the substitution of Gly-415 with serine in the alpha 1(I) triple helical domain was found. The same mutation was detected in the father's spermatozoa and lymphocytes. Mosaicism in the father's germline explains the occurrence in the family of two additional OI pregnancies, which were documented by X-ray and ultrasound investigations.

Stop codon in the procollagen II gene (COL2A1) in a family with the Stickler syndrome (arthro-ophthalmopathy)

Linkage analysis with restriction fragment length polymorphisms for the gene for type II procollagen (COL2A1) was carried out in a family with the Stickler syndrome, or arthro-ophthalmopathy, an autosomal dominant disorder that affects the eyes, ears, joints, and skeleton. The analysis demonstrated linkage of the disease and COL2A1 with a logarithm-of-odds score of 1.51 at zero recombination. A newly developed procedure for preparing cosmid clones was employed to isolate the allele for type II procollagen that was linked to the disease. Analysis of over 7000 nucleotides of the gene revealed a single base mutation that altered a CG dinucleotide and converted the codon CGA for arginine at amino acid position alpha 1-732 to TGA, a stop codon. From previous work on procollagen biosynthesis, it is apparent that the truncated polypeptide synthesized from an allele with a stop codon at alpha 1-732 cannot participate in the assembly of type II procollagen, and therefore that the mutation would decrease synthesis of type II procollagen. It was not apparent, however, why the mutation produced marked changes in the eye, which contains only small amounts of type II collagen, but relatively mild effects on the many cartilaginous structures of the body that are rich in the same protein.

Mutation in a gene for type I procollagen (COL1A2) in a woman with postmenopausal osteoporosis: evidence for phenotypic and genotypic overlap with mild osteogenesis imperfecta

Mutations in the two genes for type I collagen (COL1A1 or COL1A2) cause osteogenesis imperfecta (OI), a heritable disease characterized by moderate to extreme brittleness of bone early in life. Here we show that a 52-year-old postmenopausal woman with severe osteopenia and a compression fracture of a thoracic vertebra had a mutation in the gene for the alpha 2(I) chain of type I collagen (COL1A2) similar to mutations that cause OI. cDNA was prepared from the woman's skin fibroblast RNA and assayed for the presence of a mutation by treating DNA heteroduplexes with carbodiimide. The results indicated a sequence variation in the region encoding amino acid residues 660-667 of the alpha 2(I) chain. Further analysis demonstrated a single-base mutation that caused a serine-for-glycine substitution at position 661 of the alpha 2(I) triple-helical domain. The substitution produced posttranslational overmodification of the collagen triple helix, as is seen with most glycine substitutions that cause OI. The patient had a history of five previous fractures, slightly blue sclerae, and slight hearing loss. Therefore, the results suggest that there may be phenotypic and genotypic overlap between mild osteogenesis imperfecta and postmenopausal osteoporosis, and that a subset of women with postmenopausal osteoporosis may have mutations in the genes for type I procollagen.

Inherited disorders of collagen gene structure and expression

As a result of investigations completed during the last 15 years, the molecular bases of most form of osteogenesis imperfecta (OI) and of some forms of the Ehlers-Danlos syndrome (EDS) are now known. Most forms of OI result from point mutations in the genes (COL1A1 and COL1A2) that encode the chains of type I procollagen or mutations that affect the expression of these genes. Less frequently, mutations that affect the size of the chain can also result in these phenotypes. The phenotypic presentation appears to be determined by the nature of the mutation, the chain in which it occurs, and, for point mutations, the position of the substitution and the nature of the substituting amino acid in the protein product. Similar mutations in the gene (COL3A1) that encodes the chains of type III procollagen result in the EDS type IV phenotype. Mutations which result in deletion of the cleavage site for the aminoterminal procollagen protease result in the EDS type VII phenotype and other mutations which affect the structure of the triple-helical domain by deletions and alter the conformation of the substrate at the site of proteolytic conversion can produce mixed phenotypes. Alterations in post-translational processing of collagenous proteins can result in the EDS type VI and EDS type IX phenotypes. Linkage analysis and study of type II collagen proteins from individuals with a variety of skeletal dysplasias suggest that similar mutations in these genes also result in clinically apparent phenotypes. Mutations in the majority of the 20 known collagen genes have not yet been identified.

Increased expression of the gene for the pro alpha 1(IV) chain of basement-membrane procollagen in cultured skin fibroblasts from two variants of osteogenesis imperfecta

Fibroblasts from two lethal variants of osteogenesis imperfecta were shown to synthesize increased amounts of type IV procollagen. Previous studies established that one of these variants had a non-functional allele for the pro alpha 2 chain of type I procollagen, whereas the other pro alpha 2(I) allele contained a mutation leading to synthesis of shortened pro alpha 2(I) chains. In the two variants, the relative level of mRNA for pro alpha 1(IV) was 31 and 42% of the level of mRNA for pro alpha 1(I) chains. A value of less than 2% was found for a third lethal and four non-lethal variants of osteogenesis imperfecta. Immunofluorescent staining of fibroblasts from the two variants synthesizing increased amounts of type IV procollagen indicated that a homogeneous population of cells synthesized both type IV and type I procollagen. The results suggest that mutations in the type I procollagen genes that result in osteogenesis imperfecta can be associated with increased expression of the genes for type IV procollagen.

Thalassemia: molecular pathology and management

Recent advances in molecular biology have allowed us to develop an almost complete picture of the molecular pathology of the thalassemia syndromes. The different classes of mutations that are responsible for the thalassemia syndromes will be discussed along with the special insights they have provided into the controls of eukaryotic gene expression. While management of these disorders has not kept pace with our understanding of their cause, there have been notable advances in treatment. Perhaps even more exciting is what the future holds, as the continued march of molecular biology is melded with novel approaches to the definitive treatment of thalassemias.

Existence of malfunctioning pro alpha2(I) collagen genes in a patient with a pro alpha 2(I)-chain-defective variant of Ehlers-Danlos syndrome

Collagen synthesis was examined in skin fibroblasts from a patient with a variant of Ehlers-Danlos syndrome. The relative rate of collagen synthesis to total protein synthesis in the patient's fibroblasts was always one-half of that in fibroblasts from normal controls. Total collagen synthesis, as assessed by quantification of total hydroxyproline, was also significantly lower than that of controls, indicating that the rate of collagen synthesis by the patient's fibroblasts was decreased compared with that by normal fibroblasts. Analysis of procollagen and collagen components showed the absence of the pro alpha 2(I) chain and its derivatives. Dot-blot and Northern-blot analyses showed the patient's fibroblasts to contain less than 10% of the mRNAs for pro alpha 2(I) found in control fibroblasts. In spite of these results, Southern blot analysis of genomic DNA indicated the presence of the same number of genes for the pro alpha 2(I) collagen chain in the patient's fibroblasts as in control fibroblasts, suggesting malfunctioning pro alpha 2(I) collagen genes as the cause for failure of the patient's fibroblasts to synthesize pro alpha 2(I) collagen chains.

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.

Osteonectin content in human osteogenesis imperfecta bone shows a range similar to that of two bovine models of OI

Samples of human osteogenesis imperfecta (OI) bone were analyzed for osteonectin content by SDS gel electrophoresis and immunodetection on Western blots. The OI bone osteonectin content varied from normal to severely depressed. Previously, we showed that two clinically identical but genetically unrelated bovine models of OI were differentiated biochemically by their bone osteonectin content: one OI model had normal bone osteonectin while the other was severely depressed in this parameter. The data in this pilot study suggest that further investigation of bone osteonectin content may prove useful in the clinical assessment of human OI cases.

Two bovine models of osteogenesis imperfecta exhibit decreased apatite crystal size

In recent years advances have been made in detailing the changes in both collagen and noncollagenous proteins caused by a variety of mutations leading to osteogenesis imperfecta. Much less, however, is known about the mineral phase in the affected bone. In this report, we measured the crystallinity of the apatite in bovine OI bone. Line broadening of the 002 reflection (which estimates changes in the long or c axis of the crystals) and of the 310 reflection (which estimates changes in the thickness of the crystals) both show large decreases (30 and 35% respectively). Transmission electron micrograph measurements indicate that these changes were most probably a result of smaller crystals. No decrease in the ash weight of the bone was observed.

Genetic disorders of collagen

Osteogenesis imperfecta, Ehlers-Danlos syndrome, and Marfan syndrome form a group of genetic disorders of connective tissue. These disorders exhibit remarkable clinical heterogeneity which reflects their underlying biochemical and molecular differences. Defects in collagen types I and III have been found in all three syndromes.

Osteogenesis imperfecta type IV. Biochemical confirmation of genetic linkage to the pro alpha 2(I) gene of type I collagen

Fibroblasts from two affected members of a large pedigree in which osteogenesis imperfecta (OI) type IV is genetically linked to the pro alpha 2(I) gene of type I collagen synthesize two populations of pro alpha 2(I) chains. One population is normal; the second population appears to have a deletion of about 10 amino acid residues from the middle of the triple helical domain. The mutation in pro alpha 2(I) causes increased posttranslational modification in the amino-terminal half of some pro alpha 1(I) chains, lowers the melting temperature of type I collagen molecules that incorporate a mutant pro alpha 2(I) chain, and prevents or delays the secretion of those molecules from fibroblasts in cell culture. On the basis of this study and linkage studies in additional families, it appears that the OI type IV phenotype is often the result of heterozygosity for mutations in pro alpha 2(I) that alter the triple helical structure of type I collagen.

Osteogenesis imperfecta type IV: evidence of abnormal triple helical structure of type I collagen

Skin fibroblasts from a patient with mild osteogenesis imperfecta (OI) type IV synthesize two populations of type I procollagen molecules. One population contains pro alpha 1(I) and pro alpha 2(I) chains that migrate normally in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and a second population contains only slower migrating pro alpha 1(I) and pro alpha 2(I) chains. The total amount of type I procollagen made by OI cells and the ratio of pro alpha 1(I):pro alpha 2(I) is normal. When labeled under conditions that inhibit post-translational modification of pro alpha chains, the OI cells produce only single populations of pro alpha 1(I) and pro alpha 2(I) chains indicating that the apparent increased molecular weight of some OI pro alpha chains is due to excessive post-translational modification rather than peptidyl insertions. Peptide maps indicate that excessive post-translational modification occurs along the entire triple helical segment of some alpha 1(I) and alpha 2(I) chains produced by OI cells. The effect of the mutation is to lower the melting temperature of the molecules containing slow migrating alpha 1(I) and alpha 2(I) chains to 39.5 degrees C (compared to 41.5 degrees C for control), and to delay secretion of the over-modified type I procollagen from OI cells. These data are consistent with a mutation near the carboxyl-terminal end of the triple helical domain which delays triple helical formation and renders all chains available for further post-translational modification amino-terminal to the mutation. Such alterations in triple helical structure, thermal stability, and secretion previously associated only with the lethal OI type II phenotype are thus also seen in the mild OI type IV phenotype.

Use of molecular haplotypes specific for the human pro alpha 2(I) collagen gene in linkage analysis of the mild autosomal dominant forms of osteogenesis imperfecta

Autosomal dominant osteogenesis imperfecta (OI) is a heterogeneous group of disorders. Molecular haplotypes associated with the pro alpha 2(I) gene of human type I procollagen were used for genetic linkage studies in a group of 10 families with OI. The clinical phenotypes of the families studied were those of OI type I and OI type IV. Evidence for linkage was highly suggestive in the four families with OI type IV (Z = 3.91 for theta = 0). In contrast, little or no indication for linkage was found in the six families with OI type I (Z = .055 for theta = .415). Heterogeneity between the two groups of families was highly significant (chi 2 = 11.14, P = .0008), suggesting that at least two separate gene defects may be the cause of the autosomal dominant forms of OI.

Collagen genes and proteins in osteogenesis imperfecta

Type I collagen is a heteropolymer of alpha 1(I) and alpha 2(I) chains, each of which is a separate product of genes localised to chromosomes 17 and 7 respectively. Molecular defects of type I collagen produce a group of inherited disorders of connective tissue primarily affecting bones, which are easily broken and collagen depleted (osteogenesis imperfecta). Sillence classifies these diseases into four groups, two of which are autosomal dominant and relatively mild, the others being either genetic lethals or responsible for very severe progressive disease. Here we describe two specific molecular abnormalities of type I collagen. One, a cysteine substitution in alpha 1(I) collagen, causes a mild Sillence type I disease, the other, a four base deletion in the C terminal extension of alpha 2(I) collagen, causes progressive Sillence type III disease in the homozygously affected patient and mild premature osteoporosis in his clinically symptomless parents. We have briefly reviewed a variety of other similar mutations causing various OI syndromes, which are tabulated, including various helical and non-helical deletions and a variety of structural protein changes. Several restriction fragment length polymorphisms for alpha 2(I) and alpha 1(II) collagens have also been described, and 5' EcoRI and 3' MspI polymorphisms for alpha 2(I) collagen segregate with Sillence type IV OI.

Regulation of type I collagen synthesis. Total pro alpha 1(I) and pro alpha 2(I) mRNAs are maintained in a 2:1 ratio under varying rates of collagen synthesis

The type I collagen molecule contains two alpha 1(I) chains and one alpha 2(I) chain. Previous investigations, using embryonic chick calvaria, have indicated that the two chains are synthesized in a 2:1 ratio which is controlled at a pretranslational level, since the cells contain twice as much translatable pro alpha 1(I) mRNA as pro alpha 2(I) mRNA. The present report describes hybridization analyses of the cellular levels of total cellular RNAs coding for the pro alpha 1(I) and pro alpha 2(I) chains, using as probes two cloned cDNAs complementary to chick pro alpha 1(I) and pro alpha 2(I) mRNA, respectively. Total cellular RNA was extracted from embryonic chick calvaria, pro alpha 1(I) and pro alpha 2(I) RNA sequences were quantified by Northern hybridization using conditions ensuring that hybridization efficiency and specific radioactivity were the same for the two probes. Similar analyses were carried out on RNA extracted from calvaria with different levels of collagen synthesis after culture in the presence or absence of ascorbic acid. The results for all samples analyzed indicate that total cellular pro alpha 1(I) and pro alpha 2(I) mRNAs are present in a 2:1 ratio which is maintained even during variations in collagen synthesis rate. There is no evidence for regulation mediated by different rates of processing of mRNA precursors, although preferential degradation of the pro alpha 2(I) gene transcript cannot be excluded. Thus, the synthesis of type I procollagen chains is presumably coordinated by transcriptional control.

Altered helical structure of a homotrimer of alpha 1(I)chains synthesized by fibroblasts from a variant of osteogenesis imperfecta

Cultured skin fibroblasts from a variant of osteogenesis imperfecta were previously shown to synthesize a type I procollagen which was a homotrimer of pro alpha 1(I) chains. Trimers of alpha 1(I) collagen were isolated by pepsin digestion of culture medium from these fibroblasts. The amino acid composition of the isolated protein indicated that it contained an increased amount of hydroxylysine, apparently because of post-translational over-modification. The thermal stability of the alpha 1(I) trimers was examined by circular dichroism. We found no consistent difference in the melting curve of the alpha 1(I) trimers compared to control type I collagen. We next examined the thermal stability of the alpha 1(I) trimers using digestion with a combination of trypsin and alpha-chymotrypsin as an alternative probe of helical stability. When enzymatic digestions were carried out at 36 degrees to 40 degrees C, the alpha 1(I) chains in the trimers were cleaved to polypeptides which were shortened by approximately 100 amino acids. Vertebrate collagenase digestion of the shortened molecules indicated that the 100 amino acid segment removed from each alpha 1(I) chain was located at the carboxyl-terminus. The decreased thermal stability of the alpha 1(I) trimers was probably explained by the absence of alpha 2(I) chains in the molecules. The results, however, did not exclude the possibility that the post-translational over-modification of the alpha 1(I) chains contributed to the altered helical structure.

Isolation and partial characterization of the entire human pro alpha 1(II) collagen gene

Using a cDNA probe specific for the bovine Type II procollagen, a series of overlapping genomic clones containing 45 kb of contiguous human DNA have been isolated. Sequencing of a 54 bp exon, number 29, provided direct evidence that the recombinant clones bear human Type II collagen sequences. Localization of the 5' and 3' ends of the gene indicated that the human Type II collagen gene is 30 kb in size. This value is significantly higher than that of the homologous avian gene. The segregation of a polymorphic restriction site in informative families conclusively demonstrated that the Type II gene is found in a single copy in the human haploid genome. Finally, sequencing of a triple helical domain exon has confirmed that a rearrangement leading to the fusion of two exons occurred in the pro alpha 1(I) gene, following the divergence of the fibrillar collagens.