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1.  Trafficking defects and loss of ligand binding are the underlying causes of all reported DDR2 missense mutations found in SMED-SL patients 
Human Molecular Genetics  2010;19(11):2239-2250.
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.
doi:10.1093/hmg/ddq103
PMCID: PMC2865377  PMID: 20223752
2.  Defective cellular trafficking of missense NPR-B mutants is the major mechanism underlying acromesomelic dysplasia-type Maroteaux 
Human Molecular Genetics  2008;18(2):267-277.
Natriuretic peptides (NPs) comprise a family of structurally related but genetically distinct hormones that regulate a variety of physiological processes such as cardiac growth, blood pressure, axonal pathfinding and endochondral ossification leading to the formation of vertebrae and long bones. The biological actions of NPs are mediated by natriuretic peptide receptors (NPRs) A, B and C that are located on the cell surface. Mutations in NPR-B have been shown to cause acromesomelic dysplasia-type Maroteaux (AMDM), a growth disorder in humans and severe dwarfism in mice. We hypothesized that missense mutations of NPR-B associated with AMDM primarily affect NPR-B function by the arrest of receptor trafficking at the endoplasmic reticulum (ER), due to conformational change, rather than an impairment of ligand binding, transmission of signal through the membrane or catalytic activity. Twelve missense mutations found in AMDM patients and cn/cn mice were generated by site-directed mutagenesis and transiently overexpressed in HeLa cells. Confocal microscopy revealed that 11 out of 12 mutants were retained in the ER. Determination of the ligand-dependent cGMP response confirmed that ER-retained NPR-B mutants are non-functional. Meanwhile, the only cell surface-targeted NPR-B missense mutant (D176E) displayed greatly reduced enzymatic activity due to impaired ligand binding. Thus, in the majority of cases of AMDM associated with missense NPR-B mutation, disease appears to result from defects in the targeting of the ER receptor to the plasma membrane.
doi:10.1093/hmg/ddn354
PMCID: PMC2638773  PMID: 18945719

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