A novel mutation in DDR2 causing spondylo-meta-epiphyseal dysplasia with short limbs and abnormal calcifications (SMED-SL) results in defective intra-cellular trafficking

Abstract

BackgroundThe rare autosomal genetic disorder, Spondylo-meta-epiphyseal dysplasia with short limbs and abnormal calcifications (SMED-SL), is reported to be caused by missense or splice site mutations in the human discoidin domain receptor 2 (DDR2) gene. Previously our group has established that trafficking defects and loss of ligand binding are the underlying cellular mechanisms of several SMED-SL causing mutations. Here we report the clinical characteristics of two siblings of consanguineous marriage with suspected SMED-SL and identification of a novel disease-causing mutation in the DDR2 gene.MethodsClinical evaluation and radiography were performed to evaluate the patients. All the coding exons and splice sites of the DDR2 gene were sequenced by Sanger sequencing. Subcellular localization of the mutated DDR2 protein was determined by confocal microscopy, deglycosylation assay and Western blotting. DDR2 activity was measured by collagen activation and Western analysis.ResultsIn addition to the typical features of SMED-SL, one of the patients has an eye phenotype including visual impairment due to optic atrophy. DNA sequencing revealed a novel homozygous dinucleotide deletion mutation (c.2468_2469delCT) on exon 18 of the DDR2 gene in both patients. The mutation resulted in a frameshift leading to an amino acid change at position S823 and a predicted premature termination of translation (p.S823Cfs*2). Subcellular localization of the mutant protein was analyzed in mammalian cell lines, and it was found to be largely retained in the endoplasmic reticulum (ER), which was further supported by its N-glycosylation profile. In keeping with its cellular mis-localization, the mutant protein was found to be deficient in collagen-induced receptor activation, suggesting protein trafficking defects as the major cellular mechanism underlying the loss of DDR2 function in our patients.ConclusionsOur results indicate that the novel mutation results in defective trafficking of the DDR2 protein leading to loss of function and disease. This confirms our previous findings that DDR2 missense mutations occurring at the kinase domain result in retention of the mutant protein in the ER.

Discoidin Receptor 2 Controls Bone Formation and Marrow Adipogenesis

“…First, human DDR2 mutations cause spondylometa-epiphyseal dysplasia (SMED), a skeletal disorder associated with dwarfism, short fingers, bowing of long bones, abnormal calcifications, and craniofacial abnormalities. (10,11) In addition, polymorphisms in DDR2 are associated with low bone mineral density (BMD) and fracture risk in a Han Chinese population. (12) Mice harboring deletions in the Ddr2 locus have a SMED-like phenotype characterized by dwarfism and reduction in total BMD.…”

Novel DDR2 mutation identified by whole exome sequencing in a Moroccan patient with spondylo‐meta‐epiphyseal dysplasia, short limb‐abnormal calcification type

“…Only seven mutations have been revealed in DDR2 , four missense, two deletions, and one splice‐site mutations. These mutations lead to loss‐of‐function by at least two different mechanisms, namely, defects in DDR2 targeting the plasma membrane and its ligand‐binding activity [Bargal et al, ; Ali et al, ; Al‐Kindi et al, ] (Table ).…”

Nrp1, a Neuronal Regulator, Enhances DDR2-ERK-Runx2 Cascade in Osteoblast Differentiation via Suppression of DDR2 Degradation

“…Loss of function of human or mouse Ddr2 gene leads to dysplasia in bones [20,21,22,23]. Therefore, the regulation of DDR2 in osteoblast differentiation is of great importance.…”

“…Ddr2 knockout mice and slie mice (mouse colony with spontaneous autosomal-recessive mutation in the Ddr2 locus) exhibit short long bones and irregular growth of flat bones [20]. Human Ddr2 gene mutation causes a rare form of dwarfism, spondylo-meta-epiphyseal dysplasia short limb-hand type (SMED-SL) [21,22,23]. Our and other's studies also demonstrated that DDR2 plays essential roles in osteoblast differentiation and chondrocyte maturation by modulating the phosphorylation and transactivity of Runx2 in an ERK MAPK-dependent manner [24,25].…”