Mutations in Fibronectin Cause a Subtype of Spondylometaphyseal Dysplasia with “Corner Fractures”

Abstract

Fibronectin is a master organizer of extracellular matrices (ECMs) and promotes the assembly of collagens, fibrillin-1, and other proteins. It is also known to play roles in skeletal tissues through its secretion by osteoblasts, chondrocytes, and mesenchymal cells. Spondylometaphyseal dysplasias (SMDs) comprise a diverse group of skeletal dysplasias and often manifest as short stature, growth-plate irregularities, and vertebral anomalies, such as scoliosis. By comparing the exomes of individuals with SMD with the radiographic appearance of ''corner fractures'' at metaphyses, we identified three individuals with fibronectin (FN1) variants affecting highly conserved residues. Furthermore, using matching tools and the SkelDys emailing list, we identified other individuals with de novo FN1 variants and a similar phenotype. The severe scoliosis in most individuals and rare developmental coxa vara distinguish individuals with FN1 mutations from those with classical Sutcliffe-type SMD. To study functional consequences of these FN1 mutations on the protein level, we introduced three disease-associated missense variants (p.Cys87Phe [c.260G>T], p.Tyr240Asp [c.718T>G], and p.Cys260Gly [c.778T>G]) into a recombinant secreted N-terminal 70 kDa fragment (rF70K) and the full-length fibronectin (rFN). The wild-type rF70K and rFN were secreted into the culture medium, whereas all mutant proteins were either not secreted or secreted at significantly lower amounts. Immunofluorescence analysis demonstrated increased intracellular retention of the mutant proteins. In summary, FN1 mutations that cause defective fibronectin secretion are found in SMD, and we thus provide additional evidence for a critical function of fibronectin in cartilage and bone.

Mechanistic insights into the cellular effects of a novel FN1 variant associated with a spondylometaphyseal dysplasia

“…Features in common with the patient described here include multiple vertebral changes, which contribute to abnormalities of the spine and chest, and metaphyseal changes that radiographically resemble corner fractures. As in other cases of SMD with corner fractures, metaphyseal changes disappeared upon reaching skeletal maturity, reinforcing that these changes are most probably secondary ossification centers that are developmental in nature and not true fractures. Metaphyseal changes in the proximal humerus and developmental coxa vera are common in other cases of SMD linked to FN1 mutations, but were not observed in this patient.…”

“…As in other cases of SMD with corner fractures, metaphyseal changes disappeared upon reaching skeletal maturity, reinforcing that these changes are most probably secondary ossification centers that are developmental in nature and not true fractures. Metaphyseal changes in the proximal humerus and developmental coxa vera are common in other cases of SMD linked to FN1 mutations, but were not observed in this patient. This patient also presents unique changes in the elbow, manifest as bilateral avascular necrosis, intra‐articular loose body formation, and restricted range of motion.…”

“…Our results suggest two potential mechanisms, not mutually exclusive, to explain the FN matrix deficiencies. The mutation identified here as well as other FN mutations related to SMD resides in the N‐terminal assembly domain. FN interactions involving this domain are essential for fibril formation and may be altered in mutant FN.…”

“…The second mechanism derives from our evidence showing intracellular retention of mutant FN and a decrease in secreted FN by primary dermal fibroblasts. Secretion of mutant recombinant FN fragments by cells transduced with other FN1 point mutations was also decreased . Retention and secretion defects suggest that reduced matrix levels may be due to a reduced abundance of extracellular FN.…”

“…Mutations in type II collagen or FN have been identified in patients with SMD with corner fractures. Features in common with the patient described here include multiple vertebral changes, which contribute to abnormalities of the spine and chest, and metaphyseal changes that radiographically resemble corner fractures.…”