The Matrix Biochemistry Section focuses its research on the functions of five major noncollagenous proteins first found associated with the mineralized matrix of bones and teeth but that we later showed are also made by many metabolically active ductal epithelial cells. The five proteins are bone sialoprotein (BSP), osteopontin (OPN), dentin matrix protein-1 (DMP1), dentin sialophosphoprotein (DSPP), and matrix extracellular phosphoglycoprotein (MEPE). We have made a strong case for the genetic relatedness of these seemingly different proteins and there is increasing acceptance of the SIBLING (Small Integrin-Binding LIgand, N-linked Glycoprotein) family concept. The genes encoding these proteins are all clustered in a tandem fashion within a short (400,000 base pairs) region of human chromosome 4 and similarly on all other mammals studied to date. After comparing the intron-exon structures and conserved motifs of their respective protein-encoding exons, we proposed that the five genes might be the result of ancient gene duplication and subsequent divergence. We and others have shown that all known cases of nonsyndromic dentin dysplasia (DD) and dentinogenesis imperfecta (DGI) are the result of either a variety of point mutations at the very beginning of the DSPP gene or later deletions that result in frameshift mutations within the repeat domain. In the past we proposed that all known mutations have dominant negative effects (mutations in a single copy of the gene cause the diseases but complete loss of one copy does not) but the mechanisms for this remained unexplored. This year we have shown that all known mutations (except Y6D) cause the retention of the mutant DSPP protein in the endoplasmic reticulum of our model system. Furthermore, we have shown that the retained mutant proteins cause a dose-dependent loss in the DSPP protein made from the normal allele. Mutations causing the least amount of the normal DSPP protein to be secreted out of the cells cause the more severe disease, DGI. Our current research involves a focus on the trafficking of both normal and mutated acidic proteins within the endoplasmic reticulum as well as their movements through the Golgi and out of the cell.
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