Trafficking defects and loss of ligand binding are the underlying causes of all reported DDR2 missense mutations found in SMED-SL patients.
Spondylo-meta-epiphyseal dysplasia (SMED) with short limbs and abnormal calcifications (SMED-SL) is a rare, autosomal recessive human growth disorder, characterized by disproportionate short stature, short limbs, short broad fingers, abnormal metaphyses and epiphyses, platyspondyly and premature calcifications. Recently, three missense mutations and one splice-site mutation in the DDR2 gene were identified as causative genetic defects for SMED-SL, but the underlying cellular and biochemical mechanisms were not explored. Here we report a novel DDR2 missense mutation, c.337G>A (p.E113K), that causes SMED-SL in two siblings in the United Arab Emirates. Another DDR2 missense mutation, c.2254C>T (p.R752C), matching one of the previously reported SMED-SL mutations, was found in a second affected family. DDR2 is a plasma membrane receptor tyrosine kinase that functions as a collagen receptor. We expressed DDR2 constructs with the identified point mutations in human cell lines and evaluated their localization and functional properties. We found that all SMED-SL missense mutants were defective in collagen-induced receptor activation and that the three previously reported mutants (p.T713I, p.I726R and p.R752C) were retained in the endoplasmic reticulum. The novel mutant (p.E113K), in contrast, trafficked normally, like wild-type DDR2, but failed to bind collagen. This finding is in agreement with our recent structural data identifying Glu113 as an important amino acid in the DDR2 ligand-binding site. Our data thus demonstrate that SMED-SL can result from at least two different loss-of-function mechanisms: namely defects in DDR2 targeting to the plasma membrane or the loss of its ligand-binding activity.
Faculty of Medicine and Health Sciences, United Arab Emirates University, Al-Ain, United Arab Emirates.
Bassam R
BR
Ali
Huifang
H
Xu
Nadia A
NA
Akawi
Anne
A
John
Noushad S
NS
Karuvantevida
Ruth
R
Langer
Lihadh
L
Al-Gazali
Birgit
B
Leitinger
D001483:Base Sequence; D002648:Child; D017931:DNA Primers / Q000235:genetics; D000070818:Discoidin Domain Receptors; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D007150:Immunohistochemistry; D008297:Male; D008958:Models, Molecular; D008969:Molecular Sequence Data; D020125:Mutation, Missense / Q000235:genetics; D010009:Osteochondrodysplasias / Q000000981:diagnostic imaging / Q000235:genetics* / Q000378:metabolism*; D010375:Pedigree; D011485:Protein Binding / Q000235:genetics; D021381:Protein Transport / Q000235:genetics / Q000502:physiology; D011859:Radiography; D020794:Receptor Protein-Tyrosine Kinases / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D038981:Receptors, Collagen / Q000378:metabolism; D011975:Receptors, Mitogen / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D017422:Sequence Analysis, DNA; D015398:Signal Transduction / Q000235:genetics / Q000502:physiology; D014162:Transfection; D014479:United Arab Emirates
D017931:DNA Primers; D038981:Receptors, Collagen; D011975:Receptors, Mitogen; D000070818:Discoidin Domain Receptors; D020794:Receptor Protein-Tyrosine Kinases