Genetic Disorders of Bone and Extracellular Matrix

Eunice Kennedy Shriver National Institute Of Child Health & Human Development

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Abstract

In an integrated program of laboratory and clinical investigation, we study the molecular biology of the heritable connective tissue disorders osteogenesis imperfecta (OI). Our objective is to elucidate the mechanisms by which the primary gene defect causes skeletal fragility and then apply the knowledge gained from our studies to the treatment of children with these conditions. Structural defects of type I collagen molecule are well known to cause the dominant bone disorder OI. A dozen years ago, we identified defects in two components of the collagen prolyl 3-hydroxylation complex, CRTAP and P3H1 as the cause of recessive OI. Our work generated a new paradigm for collagen-related disorders of matrix, in which structural defects in collagen cause dominant OI, while defects in proteins that interact with collagen cause the rare forms of OI. Recessive OI is now a major area of investigation for the SHDBEM. We delineated a mutation in IFITM5, which encodes the transmembrane protein BRIL, that establishes a connection between types V and VI OI. The BRIL S40L substitution results in minimal expression and secretion of PEDF by mutant FB and osteoblasts. In contrast to the gain-of-function BRIL mutation that causes type V OI, the BRIL S40L causes decreased mineralization and expression of bone markers. Only type I collagen shows similar expression pattern in both mutations, with decreased expression, secretion and matrix incorporation. We have generated a murine model for this mutation, in which both heterozygotes and homozygotes are viable, which is currently being characterized. The first publication on this mouse focuses on bone material properties. Although the type I collagen in this model has no structural or modification abnormalities, it has reduced DXA bone density associated with increased mineral concentration in bone tissue. The cellular pathology appears to focus on osteocytes, which are increased in density, decreased in size and have reduced canicular connections. Examination of bone collagen organization by second harmonic generation revealed dramatically disordered collagen fibrils. We are focusing on the mechanism by which a defect in an osteoblast plasma transmembrane protein can affect matrix organization Because type V and VI OI are connected by a poorly described pathway, we are also studying the PEDF knock-out mouse as a model for type VI OI. We focused on calvarial osteoblasts to demonstrate that absence of PEDF delays osteoblast maturation and matrix mineralization. Bone perfusion revealed a significant increase in periosteal vessel density in knock-out mice, reinforced by transcriptome analysis. Interestingly, TGF-b signaling is activated in type VI OI cells, and PEDF attenuated TGFb-induced expression of pro-angiogenic factors. These data point to an antagonism between PEDF and TGFb in the control of osteogenesis and bone vascularization. Type XIV OI is a moderately severe form of OI which was identified in 2013. It is caused by recessive defects in TMEM38B, which encodes TRIC-B, an ER cation channel. In probands with recessive null defects in TRIC-B, we demonstrated impaired ER calcium flux, resulting in ER-stress along the ATF4 pathway. TRIC-deficiency was shown to be collagen-related, impairing collagen synthesis and assembly at multiple steps. Collagen helical lysine hydroxylation was reduced, although the levels of LH1 protein were increased. We investigated the unique aspects of TMEM38B deficiency in 8 patients with type XIV OI in collaboration with colleagues in the UK and Vienna. There is striking variability of OI severity even between siblings. Like other OI types, type XIV patients have low DXA BMD. However, unlike other OI types, bone mineralization is not increased and nanoporosity is low. Low bone turnover in type XIV OI is likely explained by an intrinsic defect in osteoclasts, which normally express TMEM38B. We have investigated type XIV patient osteoblasts using a multi-omics approach and compared their functional characteristics with murine osteoblasts in a model generated by collaborators in Italy. There were two major novel discoveries for OI in the patient osteoblasts. The first was that pathways for cell-cell adhesion were strongly downregulated, decreasing cell proliferation and altering the interaction of mutant cells with each other and with extracellular matrix. The second unanticipated finding concerned the mitochondria of TMEM38B mutant osteoblasts which were strikingly elongated and had decreased fission and fusion markers, demonstrating that ER stress in these cells can alter mitochondrial function via the ER-mitochondrial contact sites. The murine Tric-B null model revealed an imbalance in osteoblast mitochondrial calcium flux. These cells have impaired SMAD signalling, with reduced SMAD phosphorylation and nuclear translocation. Decreased SMAD signalling was also demonstrated in osteoblasts of patients with type XIV OI. The altered murine SMAD signalling was mainly caused by altered Ca-calmodulin kinase II-mediated signalling, and could only be partially rescued by treatment with TGF-b. Recently, we delineated the first X-linked recessive form of OI, made more exciting by its novel bone mechanism. X-linked OI is moderately severe with pre- and post-natal fractures of ribs and long bones, dysplastic bone with bowing and crumpling. It is caused by missense mutations in MBTPS2, which encodes Site-2 protease (S2P). S2P is a critical component of Regulated Intramembrane Proteolysis (RIP), a process in which S1P and S2P, located in the Golgi Membrane, sequentially cleave regulatory proteins transported from the ER membrane in times of cell stress or sterol metabolite deficiency. The levels of mutant S2P transcripts and protein are normal, but RIP function on substrates OASIS, ATF6 and SREBP are impaired. At the bone tissue level, hydroxylation of type I collagen K87 residues is half normal, altering collagen crosslinking in bone. Osteoblasts with S2P defects have a differentiation defect. The impact of S2P mutations on various arms of the RIP pathway and the distinction of the OI mutations from the S2P mutations causing cholesterol defects is being investigated.