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Genetic Disorders of Bone and Extracellular Matrix

Joan C. Marini, MD, PhD, Chief, Bone and Extracellular Matrix Branch
Aileen M. Barnes, MS, Research Associate
Wayne A. Cabral, AB, Chemist
Weizhong Chang, PhD, Staff Scientist
Laura Felley, BS, Postbaccalaureate Fellow
Antonella Forlino, PhD, Guest Scientist
Rusella Mirza, PhD, PostDoctoral Fellow
Adi Reich, PhD, Postdoctoral Fellow
Cathy E. Reisenberg, PhD, FN P-BC, Senior Research Assistant

In an integrated program of laboratory and clinical investigation, the Bone and Extracellular Matrix Branch (BEMB) studies the molecular biology of the heritable connective tissue disorders osteogenesis imperfecta (OI) and Ehlers-Danlos syndrome (EDS). Our objective is to elucidate the mechanisms by which the primary gene defect causes skeletal fragility and other connective tissue symptoms and then apply this knowledge to the treatment of children with these conditions. Recently, we identified the long-sought cause of recessive OI. Discoveries of defects in collagen modification generated a new paradigm for collagen-related disorders of matrix. We have established that structural defects in collagen cause dominant OI while defects in the components of a complex in the endoplasmic reticulum that modifies collagen cause recessive OI. Our challenge now is to understand the cellular and biochemical mechanisms of recessive OI. We have also generated a knockin murine model for OI with a classical collagen mutation and are using the model to study disease pathogenesis and the skeletal matrix of OI, the effects of pharmacological therapies, and approaches to gene therapy. Our clinical studies involve children with types II and IV OI, who form a longitudinal study group enrolled in age-appropriate clinical protocols for the treatment of their condition.

Recessive osteogenesis imperfecta mechanism

Severe/lethal recessive OI has been shown to be caused by deficiency in P3H1 and CRTAP, components of the collagen prolyl 3-hydroxylation complex. Deficiency in CRTAP causes type VII OI (#610862 in the OMIM database), while deficiency in P3H1 causes type VIII OI (OMIM #610915). The ER-resident 3-hydroxylation complex, of which cyclophilin B is the third member, is responsible for one of the post-translational modifications of type I collagen—the 3-hydroxylation of the Pro986 residue of the α1(I) chains. Null mutations in LEPRE1, the gene that encodes P3H1, or CRTAP cause a severe to lethal bone dysplasia whose phenotype overlaps with the severe/lethal dominant forms of OI in terms of crumpled and undertubulated long bones, disorganized bone matrix, and extreme bone fragility. However, the two recessive forms share other characteristics that are distinct from those of dominant OI; affected individuals have white or light sclerae, rather than the blue sclerae of dominant OI, small/normal rather than enlarged head circumferences and rhizomelia of long bones. Biochemically, the type I collagen produced by affected children lacks Pro986 hydroxylation, but, surprisingly, exhibits overmodification of the helical region of collagen chains by the P4H1 and LOH modification system, indicating that folding of the helix is delayed. We focused on the structure of the complex and the interactions of its components and discovered that the basis of the clinical and biochemical indistinguishability of types VII and VIII OI is the mutual protection of P3H1 and CRTAP in the 3-hydroxylation complex. P3H1 is absent and CRTAP is severely reduced in cells that have a defect in either component of the complex, although the transcript level of the normal component is not reduced. Thus, absence or reduced levels of both proteins occur in cells with a mutation in either gene. Stable transfection of CRTAP expression constructs into CRTAP-null cells restores P3H1, as well as CRTAP, in these cells. The restored complex is functional in the ER for collagen folding, as shown by the near normalization of collagen helical modification. Also, in LEPRE1-null cells, the secretion of CRTAP into conditioned media is double the level seen in normal control cells.

Recently, we completed the identification of defects causing recessive OI in all three members of the collagen 3-hydroxylation complex by identifying a mutation in PPIB, which alters the start codon of cyclophilin B (CyPB). We identified two siblings whose Senegalese parents are consanguineous; the children have moderately severe recessive OI with white sclerae and normal dentition but without rhizomelia. They are homozygous for a point mutation in the CyPB start codon, and each parent and unaffected sibling is a carrier of this mutation. As a consequence of the mutation, PPIB transcript levels are reduced to half of normal in the probands. The CyPB protein is undetectable by Western blot with three antibodies to different epitopes and by immunofluorescence microscopy. The protein is also undetectable after cells are treated with proteosomal inhibitors, indicating that it is not rapidly degraded after synthesis, and it is also undetectable in guanidinium extracts, indicated that the protein did not elude detection in Westerns because it had formed insoluble aggregates. The absence of CyPB demonstrates moderate reduction of the 3-hydroxylation complex. Although transcript levels of CRTAP and LEPRE1 are elevated, this is insufficient to maintain normal levels of the other two proteins in the complex, which are reduced to about half of normal. Thus, CyPB does provide some stabilization to the complex, although not in the mutual protection level that CRTAP and P3H1 provide each other. Even though reduced, the level of the P3H1/CRTAP complex is sufficient to fully 3-hydroxylate the Pro986 residue. Lastly, CyPB is a well-known peptidyl-prolyl cis-trans isomerase, which has been thought to be the unique isomerase catalyzing the rate-limiting step in collagen folding. Surprisingly, collagen helical modification and alpha chain gel migration are normal in proband cells despite the absence of CyPB. These data indicated that there must be redundancy for this crucial isomerization in cells.

West African LEPRE1 allele for type VIII OI

Our initial studies of probands with type VIII OI identified a recurring mutation, IVS5+1G—>T, in African Americans and West African immigrants to the United States. The mutation was subsequently identified by other investigators in individuals of African origin but not in any other racial group. The mutation results in five alternative spliced forms of the transcript, each leading to PTCs (papillary thyroid cancers). Homozygotes for the mutation die within months of birth, while compound heterozygotes can survive into their teen years but with extremely severe bone disease. We hypothesize that this is a West African founder mutation, which was transported to the Caribbean and the Americas with the Atlantic slave trade. To investigate the incidence of mutation carriers and the molecular anthropology of the disease, we used a custom SNP (single nucleotide polymorphism) assay to screen contemporary African Americans from Maryland, Pennsylvania, and Washington, DC as well as contemporary Africans. In the three African American cohorts, the incidence of carriers was 1/200–300 individuals, typical of a rare metabolic disorder and predicting a frequency of homozygous type VIII OI among African-Americans of about 1/200,000 births. For West African studies, we collaborated with Charles Rotimi, who shared over 1,200 DNA samples from contemporary Ghanaians and Nigerians. To our surprise, we found that the carrier frequency for this lethal bone dysplasia in West Africa is over 1.5%. This particularly high incidence of carriers predicts that the rate of recessive OI in West Africa will equal the rate of dominant OI. To determine whether the mutation is pan-African rather than limited to West Africa, we collaborated with Sarah Tishkoff and Timothy Rebbeck to screen samples from Cameroon, Chad, CAR, and Senegal—countries surrounding Nigeria and Ghana. No carriers were found in these samples. Haplotype analysis of African American pedigrees and West African triads on the 4.2 MB region surrounding the LEPRE1 gene yielded a 450kb conserved region. Calculations based on these data estimate that the mutation first occurred about 600 years ago, prior to the Atlantic slave trade, consistent with our hypothesis. We are interested in determining the forces responsible for maintaining the elevated carrier frequency of this lethal mutation in contemporary West Africans.

C-propeptide cleavage site mutations increase bone mineralization.

Type I procollagen is processed to mature collagen by the removal of both N- and C-terminal propeptides. Investigations on the role of the C-propeptide date back over two decades. The C-propeptide is cleaved at the Ala-Asp peptide bond between the telopeptide and the C-propeptide of each chain by C-proteinase/BMP-1; cleavage is enhanced by C-proteinase enhancer protein (PCPE-1). Probands with substitutions at three of the four cleavage site residues have been identified; we are studying two of those probands. One child is the patient of Katarina Lindahl, our Swedish collaborator, and the other is an NIH patient. Surprisingly, the two children have elevated bone density DEXA z-scores, although their facture history, radiographs, and bone histomorphometry is typical of mild OI. The only abnormality in collagen biochemistry in these children is delayed cleavage of the C-propeptide. This led us to investigate matrix mineralization of these children. In collaboration with Adele Boskey, we found that FTIR (Fourier transform infrared spectroscopy) demonstrated a higher mineral:matrix ratio in both the trabecular and cortical bone of each patient than in either age-matched normal or classical OI controls, as well as more marked maturation of collagen cross-links, although crystallinity and mineral composition were typical of classical OI. We extended the investigation of mineralization with BMDD (bone mineral density distribution) and BEI (backscattered electron imaging) to show that, in the α2(I) cleavage site mutation, the bone had a uniformly higher mineral density, while in the α1(I) mutation, the average mineral density was typical of classical OI but markedly more heterogeneous, with areas of very high and low bone density. Modeling of the collagen fibril has indicated that the C-terminal end is located on the surface of the fibril. If the C-propeptide is not cleaved, it will be located on the surface of the fibril where it apparently generates an initiating locus for increased mineralization. These studies have shown that mutations in the C-propeptide cleavage site cause a distinctive form of OI and have revealed important basic information about the propeptide's role in bone mineralization.

Insight from the Brtl mouse model for OI

The Brtl mouse model for OI, which was generated by the BEMB, is a knock-in mouse that contains a Gly349Cys substitution in the α1(I) chain. Brtl was modeled on a type IV OI child and accurately reproduces most features of type IV OI. Brtl has provided important insights into both potential OI treatments and the mechanism of OI. First, we conducted a controlled treatment trial of the bisphosphonate alendronate in Brtl and wild-type littermates, focusing on the effect of bisphosphonate on the OI femur, which cannot be examined directly in children in treatment trials. We found that bone density and bone volume improved with treatment, specifically with a dramatic increase in trabecular number. The femora also achieved increased loading before fracture. However, during the timeframe in which Brtl BMD and femur loading improved, important detrimental side effects appeared. Retention of mineralized cartilage and decline in the strength of bone material likely account for the paradoxical increase in bone fractures seen after prolonged pamidronate treatment. There was also a toxic effect on osteoblast morphology—previously cuboidal cells became flat lining cells. These results reinforce the conclusion of the pediatric trial to limit the duration of bisphosphonate treatment. Collaborative studies with Ken Kozloff at the University of Michigan using a fluorescent bisphosphonate analog have validated FRFP as an accurate biomarker of bisphosphonate deposition and retention (Kozloff et al., J Bone Min Res. 2010;25:1748) in preparation for site-specific deposition studies to understand the decreased material quality associated with bisphosphonate deposition. A second therapeutic trial involving Brtl has been an in utero cell transplantation study using normal mesenchymal stem cells (MSC) tagged with GFP. Uptake of normal bone-forming cells into Brtl bone was clearly demonstrated. Surprisingly, despite the low-level of cell uptake into bone, the day-one lethality outcome for 30% of Brtl mice was almost entirely rescued and the biomechanics of Brtl femora at two months was significantly improved. These results coincide with our previous data on two human mosaic carriers who have less than 30% mutant bone cells and a very mild phenotype.

Brtl has also provided important information about the cellular function in OI. Comparison of lethal and surviving Brtl pups found that ER-stress was significantly increased in the lethal pups. Gadd153 (CHOP) expression was increased 2–3 fold in bone cells of lethal pups and was present in Western blots; CHOP is a component of an apoptosis pathway. Conversely, αβ-crystalline, a small heat shock protein that protects against apoptosis, is elevated in osteoblasts of surviving Brtl cells. Other studies of surviving Brtl mice have shown the importance of osteoclasts to the OI phenotype. Osteoclast surface in Brtl is increased and is uncoupled from osteoblast surface, resulting in a decline in bone formation rate (BFR). Surprisingly, the number is TRAP⁺ cells is elevated in Brtl marrow cultures, suggesting elevated numbers of precursors. We demonstrated that this increase is independent of the RANKL/OPG signaling pathway and we are currently exploring other osteoclast-stimulating pathways. This information could provide important targets for treatment of OI. Finally, a collaborative study with Joseph Wallace and Mark Banaszak Holl demonstrated that the nanoscale morphology of type I collagen fibrils is altered in Brtl bone (Wallace et al., J Structural Biol. 2010;Aug 7. [Epub ahead of print]). Altered fibrils structure may be detected by osteoblasts as part of OI pathology; additional studies may link the morphological observations to nanoscale mechanical integrity.

Bisphosphonate treatment of children with types III and IV OI

Our randomized controlled trial of bisphosphonate in children with types III and IV OI was the first randomized trial in the United States and one of four worldwide. Our trial of pamidronate examined both direct skeletal gains and secondary gains reported in uncontrolled trials. For the primary skeletal outcomes, we found increased BMD z-scores, and improved vertebral area and compressions. We were the first to note, however, that improvement in vertebral BMD tapered off after one to two years of treatment, a conclusion later extended by histomorphometry from the Shriner's Hospital Montreal trial, which showed that almost all bone histology gains occurred in the first 2–3 years of treatment. We also found that the treatment group did not experience a decrease in long-bone fractures, which again coincided with the lack of fracture improvement or equivocal improvement in fractures in other controlled trials. The BEMB controlled trial did not support the secondary gains claimed in observational trials; treated children did not experience significant improvement in ambulation level, or lower-extremity strength, or alleviation of pain, suggesting that previously reported gains in these parameters were a placebo effect. Our current treatment protocol recommends treatment for 2–3 years, with subsequent follow-up of bone status. Furthermore, we are now engaged in a dose comparison trial, in which the dose from our first trial is compared with a lower dose, achieved by increasing the cycle interval but giving the same dose/kg/cycle. Given the decade-long half-life and side effects of bisphosphonate on normal as well as dysplastic bone, including osteonecrosis of the jaw (ONJ), bone healing, bone modeling, and decline in the quality of the bone material, it is important to determine the lowest cumulative dose that will provide vertebral benefits. Preliminary analysis indicates that OI children obtain benefits from lower pamidronate doses that are comparable to the benefits from higher doses. We are also focusing on the important secondary features of OI, including hearing loss, pulmonary function, and cardiac abnormalities, to determine their incidence and progression in our study population.

OI Mutation Consortium

The BEMB assembled and leads an international consortium of connective tissue laboratories for the compilation and analysis of a database of mutations in type I collagen that cause OI. The Consortium Database assembled double the previously available number of collagen mutations, and when the first analysis of the database was published in 2007, it listed over 830 mutations, including 682 glycine substitutions and 150 splice site defects. Genotype-phenotype modeling revealed distinct functions for each alpha chain of type I collagen, including the occurrence of exclusively lethal mutations in the Major Ligand Binding Regions (MLBR) of the α1(I) chain on the collagen monomer and the overlapping of the regularly spaced clusters of lethal mutations along the α2(I) chain with the proteoglycan binding sites on the collagen fibril. The modeling for α2(I) supports the Regional Model for mutation that was first proposed by the BEMB over 15 years ago and now correctly predicts 86% of clinical outcomes. The Consortium Database has provided the basis for biophysical mapping of the melting domains, which is being conducted by the Leikin laboratory; the lab deduced two flexible regions important for collagen fibril assembly and ligand binding. The Database also provided crucial material for James San Antonio and Joseph Orgel to model functional domains in terms of the cell and matrix interactions of the collagen fibril. The Consortium Database has been in an active assembly phase over the past year and now contains over 1,570 mutations from 9 international laboratories, including 1,253 glycine substitutions and 326 exon splicing defects. An upcoming analysis of this database will add examination of the effects of interchain salt bridges and re-nucleation residues C-terminal to the substituted glycine to the features correlated in the previous analysis.

Publications

Barnes AM, Carter EM, Cabral WA, Weis M, Chang W, Makareeva E, Leikin S, Rotimi CN, Eyre DR, Raggio CL, Marini JC. Lack of cyclophilin B in osteogenesis imperfecta with normal collagen folding. N Engl J Med. 2010;362:521-528.
Chang W, Barnes AM, Cabral WA, Bodurtha JN, Marini JC. Prolyl 3-hydroxylase 1 and CRTAP are mutually stabilizing in the endoplasmic reticulum collagen prolyl 3-hydroxylation complex. Hum Mol Genet. 2009;19(2):223-234.
Marini JC, Cabral WA, Barnes AM. Null mutations in LEPRE1 and CRTAP cause severe recessive osteogenesis imperfecta. Cell Tissue Res 2009;339(1):59-70.
Panaroni C, Gioia R, Lupi A, Besio R, Goldstein SA, Kreider J, Leikin S, Vera JC, Mertz EL, Perilli E, Baruffaldi F, Villa I, Farina A, Casasco M, Cetta G, Rossi A, Frattini A, Marini JC, Vezzoni P, Forlino A. In utero transplantation of adult bone marrow decreases perinatal lethality and rescues the bone phenotype in the knockin murine model for classical, dominant osteogenesis imperfecta. Blood. 2009;114:459-468.
Uveges TE, Kozloff KM, Ty JM, Ledgard F, Raggio CL, Gronowicz G, Goldstein SA, Marini JC. Alendronate treatment of the brtl osteogenesis imperfecta mouse improves femoral geometry and load response before fracture but decreases predicted material properties and has detrimental effects on osteoblasts and bone formation. J Bone Miner Res. 2009;24:849-859.

Collaborators

Mark M. Banaszak Holl, PhD, University of Michigan, Ann Arbor, MI
Adele Boskey, PhD, Weill Medical College of Cornell University, New York City
David Eyre, PhD, University of Washington, Seattle, WA
Steven Goldstein, PhD, University of Michigan, Ann Arbor, MI
Kenneth Kozloff, PhD, University of Michigan, Ann Arbor, MI
Sergey Leikin, PhD, Section on Physical Biochemistry, NICHD, Bethesda, MD
Katarina Lindahl, MD, Uppsala University, Sweden
Joseph Orgel, PhD, Illinois Institute of Technology, Chicago, IL
Philip Osdoby, PhD, Washington University, St. Louis, MO
Scott Paul, MD, Rehabilitation Medicine, Clinical Center, NIH, Bethesda, MD
Timothy Rebbeck, PhD, University of Pennsylvania, Philadelphia, PA
Charles N. Rotimi, PhD, NIH Center for Research on Genomics and Global Health, NHGRI, Bethesda, MD
James San Antonio, PhD, Jefferson University, Philadelphia, PA
Sarah Tishkoff, PhD, University of Pennsylvania, Philadelphia, PA
Joseph Wallace, PhD, University of Michigan, Ann Arbor, MI
The OI Mutation Consortium, NICHD, Bethesda, MD

Contact

For more information, email marinij@mail.nih.gov.

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