Study On The Mutation Of DSPP Gene In Chinese Patients With Dentiogenesis Imperfecta Type â…¡

Background: Dentinogenesis Imperfecta Type II(DGI-II), also called Hereditary Opalescent Dentin (MIM#125490) , is an autosomal dominant disorder in which both primary and permanent teeth are affected. It occurs with an incidence of l:6000~ 1:8000 live births. The teeth exhibit discoloration ranging in hue from grayish-blue to brown, the pulp chamber being obliterated by abnormal dentine. The enamel, although unaffected, tend to fracture, which makes dentine undergo rapid attrition, leading to a marked shortening of the teeth. Radiographically, the teeth show short roots, bulbous crowns that constricts at the cervix, and pulpal obliteration. Histopathologically, the dentine is characterized by a dysplastic appearance with amorphous areas void of dentine tubules, irregular dentinal tubules, embedded cells,and occasionally interglobular dentine.Folowing mapping by linkage analysis to chromosome 4q21, theDGI-II-causing gene was identified as the dentin sialophosphoprotein(DSPP) gene. The gene product is a precursor protein that is cleaved into two dentine matrix proteins, dentine siaprotein(DSP) and dentine phosphoprotein(DPP).The human DSPP gene contains five exons and four introns. Exon 1-4 encode the N-terminus of the DSP, and exon 5 encodes the C-terminus of DSP and DPP. DSP is a glycoprotein with a relatively high sialic acid content and accounts for 5-8% of dentine extracellular matrix(DECM). Neither its three-dimensional structure nor its function is known. DPP is the major non-collagenous extracellular matrix protein. It is extremely acidic and rich in aspartic acid(D) and serine(S), but its three-dimensional structure is unknown. When predentine is transformed to dentin, the odontoblast cells secrete DPP to the mineralization front, and DPP may therefore be involved in the initial mineralization of dentin matrix collage. DPP is extensively phosphorylated and may bind a large amount of calcium and have a important role in the nucleation of hydroxyapatite.Through detailed examination, 6 point mutations were found in DSPP gene in patients with DGI-II.Objective: To investigate mutations of DSPP gene in Chinese patients with dentinogenesis imperfecta type II (DGI— II), also further illuminate the genetic basis of DGI- II in China at molecular level. Methods: (1) Transparent electron microscope (TEM) was employed to observe the ultrastructure of odontoblasts of the proband in family A. (2) Genomic DNA was extracted from venous blood of members of two Chineasefamilies with DGI- II. Polymerase chain reaction and DNA direct sequencing were employed to analyze exons 1-4 of DSPP and their flanking sequences in members of two Chinease families with DGI- II.Results: (1) The mitochondrial ridges were found disappearing in the patient' s pulp cell, and endoplasmic reticulums were expanded to vesieles.(2) In family A, the affected individuals carry an A—T transversion at nt 6026 while the unaffected members do not have such changes. The affected individuals in family B carry a T-*G transversion at nt 6013 while the unaffected members do not have such changes.Conclusion These are novel mutations .The results of this study expand the spectrum of DSPP gene mutations associated with DGI- II. Detection the mutant DSPP proto-oncogene carriers may be useful for genetic counseling of potential risk for DGI-II in the affected families.