A variant inSMOC2,inhibiting BMP signaling by competitively binding to BMPR1B, causes multiple epiphyseal dysplasia

Abstract

Abstract： Previously study showed that SMOC, a matricellular protein, inhibits BMP 20 signaling downstream of its receptor via activation of mitogen-activated protein kinase 21 (MAPK) signaling. In our study, exome sequencing revealed a missense mutation 22 (c.1076T>G, p.Leu359Arg) in EC domain of SMOC2 in a Chinese family with 23 multiple epiphyseal disease (MED). The pathogenicity of this SMOC2 variant was 24 verified by Smoc2 L359R/L359R knock-in mice. Of note, decreasing phosphorylation of 25 SMAD1/5/9 was detected in growth plates and primary chondrocytes from 26Smoc2 L359R/L359R mice. Furthermore, binding affinity of mutant SMOC2 with collagen 27 IX and HSPG in the extracellular matrix of cartilage were reduced while binding 28 affinity with BMPRIB was intact. In addition, in contrast to previously results, that 29 SMOC2 cannot antagonize BMP activity in the presence of a constitutively activated 30 BMP receptor. These results support that SMOC2 with p.Leu359Arg variant act as an 31 antagonist of canonical BMP pathway by competitively binding with BMP receptors. 32 33 Introduction 34Multiple epiphyseal dysplasia (MED, MIM 132400) is a genotypically and 35 phenotypically heterogeneous skeletal dysplasia and chondrodysplasia that affects 36 epiphysis of long bones [1, 2] . MED is characterized by early-onset arthritis, especially in 37 hip and knee joints, which results in joint pain and stiffness, waddling gait in early 38 childhood, and mild to moderate shortness of stature. Eight disease genes, which may 39 be inherited in an autosomal dominant or recessive pattern, have been identified. 40Autosomal dominant variants include collagen oligomeric matrix protein (COMP) [3, 4] , 41 collagen type IX α-1 (COL9A1) [5] , collagen type IX α-2 (COL9A2) [6] , collagen type IX 42 α-3 (COL9A3) [7] , matrilin-3 (MATN3) [8] , and collagen type II α-1 (COL2A1) [9] . 43Autosomal recessive variants are the sulfate transporter gene (SLC26A2) and 44 calcium-activated nucleotidase-1 (CANT1) [10, 11] . All proteins encoded by the known 45 MED-associated genes are involved in maintaining the structural integrity of the 46 cartilage extracellular matrix (ECM). All variants in these genes account for the 47 molecular basis of about 70% of MED cases [11] . However, a number of MED cases 48 have no identifiable genetic mutation, and additional genetic etiologies of MED remain 49 to be identified. 50 SPARC-related modular calcium binding 2 (SMOC2) and SMOC1, its closest 51 homolog, are members of the protein family BM-40 (also known as secreted protein 52 acidic and rich in cysteines [SPARC] or osteonectin) [12, 13] . BM-40 is a prototypic 53 collagen-binding matricellular protein that participates in regulating cell-matrix 54 4 interactions, in particular influencing bone mineralization, wound repair and other 55 biological functions. The BM-40 family of modular extracellular proteins is 56 characterized by a follistatin-like (FS) domain as well as an extracellular 57 calcium-binding (EC) domain with two EF-hand calcium-b...