A rare form of spondylometaphyseal dysplasia-type A4

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

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.

Spondylometaphyseal dysplasia with cone-rod dystrophy

The co-occurrence of skeletal dysplasia and ophthalmologic abnormality is extremely rare. We report on a boy of a unique form of spondylometaphyseal dysplasia associated with cone-rod dystrophy. He presented with postnatal severe short stature, progressive lower limb deformity with rhizomelic shortening of the long bones, prominent joints with limited mobility in knees and elbows, mild thoracic scoliosis, and vision impairment due to cone dystrophy. Correction of deformity and simultaneous limb lengthening was performed in bilateral femora and tibiae without major complications. Skeletal manifestations in addition to comprehensive ophthalmologic examinations were described in this patient who had been followed from infancy to 16 years of age.

Spondylometaphyseal dysplasia, east-African type: a new form of early, severe SMD with rounded vertebrae

Spondylometaphyseal dysplasias (SMD) are a heterogeneous group of bone dysplasias characterized by vertebral and metaphyseal changes of various severities. We report two unrelated patients of east African origin with a skeletal disorder consisting of 1) severe metaphyseal dysplasia of early onset, sparing hand bones, with bracket-shaped metaphyses; 2) dysplastic pelvis with irregular iliac rim; and 3) oval-shaped vertebral bodies. Contrasting with most types of SMD, the spinal dysplasia is limited to mild changes in the vertebral body shape that tend to soften with time, whereas the iliac rims have a striking lacy appearance. Except for the most common types (Kozlowski type and Schmidt type), most of the literature on SMD deals with single case reports, without longitudinal data, for which molecular definition is still lacking and classification remains unclear. These two patients could belongs to the A4 group in the classification of Maroteaux and Spranger [1991: Pediatr Radiol 2l:293-297], and illustrate the difficulties of a clinical classification of SMD.