Hereditary Dentin Defects

A Novel Mutation in the DSPP Gene Associated with Dentinogenesis Imperfecta Type II

Hereditary dentin defects are divided into dentinogenesis imperfecta and dentin dysplasia. We identified a family segregating severe dentinogenesis imperfecta. The kindred spanned four generations and showed an autosomal-dominant pattern of inheritance. The proband was a child presenting with a severely affected primary dentition, with wide-open pulp chambers and multiple pulp exposures, resembling a DGI type III (DGI-III) pattern. We hypothesized that a mutation in the DSPP gene is responsible for this severe phenotype. Mutational analyses revealed a novel mutation (c.53T>A, p.V18D) near the intron-exon boundary in the third exon of the DSPP gene. We analyzed the effect of the mutation by means of an in vitro splicing assay, which revealed that the mutation did not affect pre-mRNA splicing. Further studies are needed for a better understanding of the nature of the disease and the development of an appropriate treatment strategy.

A dentin sialophosphoprotein mutation that partially disrupts a splice acceptor site causes type II dentin dysplasia

The dentin sialophosphoprotein (DSPP) gene on chromosome 4q21.3 encodes the major noncollagenous protein in tooth dentin. DSPP mutations are the principal cause of dentin dysplasia type II, dentinogenesis imperfecta type II, and dentinogenesis imperfecta type III. We have identified a DSPP splice junction mutation (IVS2-6T>G) in a family with dentin dysplasia type II. The primary dentition is discolored brown with severe attrition. The mildly discolored permanent dentition has thistle-shaped pulp chambers, pulp stones, and eventual pulp obliteration. The mutation is in the sixth nucleotide from the end of intron 2, perfectly segregates with the disease phenotype, and is absent in 200 normal control chromosomes. An in vitro splicing assay shows that pre-mRNA splicing of the mutant allele generates wild-type mRNA and mRNA lacking exon 3 in approximately equal amounts. Skipping exon 3 might interfere with signal peptide cleavage, causing endoplasmic reticulum stress, and also reduce DSPP secretion, leading to haploinsufficiency.

A comprehensive analysis of normal variation and disease-causing mutations in the human DSPP gene

Within nine dentin dysplasia (DD) (type II) and dentinogenesis imperfecta (type II and III) patient/families, seven have 1 of 4 net -1 deletions within the approximately 2-kb coding repeat domain of the DSPP gene while the remaining two patients have splice-site mutations. All frameshift mutations are predicted to change the highly soluble DSPP protein into proteins with long hydrophobic amino acid repeats that could interfere with processing of normal DSPP and/or other secreted matrix proteins. We propose that all previously reported missense, nonsense, and splice-site DSPP mutations (all associated with exons 2 and 3) result in dominant phenotypes due to disruption of signal peptide-processing and/or related biochemical events that also result in interference with protein processing. This would bring the currently known dominant forms of the human disease phenotype in agreement with the normal phenotype of the heterozygous null Dspp (-/+) mice. A study of 188 normal human chromosomes revealed a hypervariable DSPP repeat domain with extraordinary rates of change including 20 slip-replication indel events and 37 predominantly C-to-T transition SNPs. The most frequent transition in the primordial 9-basepair (bp) DNA repeat was a sense-strand CpG site while a CpNpG (CAG) transition was the second most frequent SNP. Bisulfite-sequencing of genomic DNA showed that the DSPP repeat can be methylated at both motifs. This suggests that, like plants and some animals, humans methylate some CpNpG sequences. Analysis of 37 haplotypes of the highly variable DSPP gene from geographically diverse people suggests it may be a useful autosomal marker in human migration studies.

Hereditary dentine disorders: dentinogenesis imperfecta and dentine dysplasia

The hereditary dentine disorders, dentinogenesis imperfecta (DGI) and dentine dysplasia (DD), comprise a group of autosomal dominant genetic conditions characterised by abnormal dentine structure affecting either the primary or both the primary and secondary dentitions. DGI is reported to have an incidence of 1 in 6,000 to 1 in 8,000, whereas that of DD type 1 is 1 in 100,000. Clinically, the teeth are discoloured and show structural defects such as bulbous crowns and small pulp chambers radiographically. The underlying defect of mineralisation often results in shearing of the overlying enamel leaving exposed weakened dentine which is prone to wear. Currently, three sub-types of DGI and two sub-types of DD are recognised but this categorisation may change when other causative mutations are found. DGI type I is inherited with osteogenesis imperfecta and recent genetic studies have shown that mutations in the genes encoding collagen type 1, COL1A1 and COL1A2, underlie this condition. All other forms of DGI and DD, except DD-1, appear to result from mutations in the gene encoding dentine sialophosphoprotein (DSPP), suggesting that these conditions are allelic. Diagnosis is based on family history, pedigree construction and detailed clinical examination, while genetic diagnosis may become useful in the future once sufficient disease-causing mutations have been discovered. Differential diagnoses include hypocalcified forms of amelogenesis imperfecta, congenital erythropoietic porphyria, conditions leading to early tooth loss (Kostmann's disease, cyclic neutropenia, Chediak-Hegashi syndrome, histiocytosis X, Papillon-Lefevre syndrome), permanent teeth discolouration due to tetracyclines, Vitamin D-dependent and vitamin D-resistant rickets. Treatment involves removal of sources of infection or pain, improvement of aesthetics and protection of the posterior teeth from wear. Beginning in infancy, treatment usually continues into adulthood with a number of options including the use of crowns, over-dentures and dental implants depending on the age of the patient and the condition of the dentition. Where diagnosis occurs early in life and treatment follows the outlined recommendations, good aesthetics and function can be obtained.

The role of dentists in diagnosing osteogenesis imperfecta in patients with dentinogenesis imperfecta

BACKGROUND

Osteogenesis imperfecta (OI), also known as "brittle bone disease," can be difficult to diagnose in its mild form. The authors describe a clinical case of a diagnosis of dentinogenesis imperfecta (DI), in which a literature review combined with an analysis of dental alterations led to indications of OI involvement.

CASE DESCRIPTION

Since DI can be associated with OI, the authors reviewed correlated studies and obtained a new medical history from the patient. They then conducted a radiographic and clinical examination of the dentition and submitted an affected third molar to scanning electron microscopy analysis. They compared their findings with descriptions of OI type I dental alterations in the literature and confirmed their diagnosis by means of a medical evaluation.

CLINICAL IMPLICATIONS

In cases in which DI is diagnosed, patients should be examined carefully and the occurrence of OI should be considered since, in its mild form, it might be misdiagnosed.

Periodontal status of patients with dentin dysplasia type I: report of three cases within a family

BACKGROUND

Dentin dysplasia type I (DDI) is a rare hereditary disturbance of dentin formation. It is characterized by clinically normal-appearing crowns; obliteration of pulp chambers; and short, blunted and malformed roots that are commonly associated with periodontal attachment loss (PAL). In this context, we report three cases within a family with similar clinical and radiographic features of DDI but with differing microbiologic and periodontal conditions.

METHODS

A 42-year-old white female and her two daughters (25 and 10 years of age) presented with a diagnosis of DDI. Probing depth (PD), clinical attachment level (CAL), visible plaque, and bleeding on probing (BOP) were recorded. Subgingival biofilm samples were randomly collected and analyzed by checkerboard DNA-DNA hybridization.

RESULTS

The mother presented 34.9% of sites with PD > or =4 mm, 41.3% of sites with CAL > or =4 mm, and 57% of sites with BOP; both daughters presented no sites with PD or CAL >3 mm and <10% of sites with BOP. Microbiologic analysis detected Gemella morbillorum, Neisseria mucosa, and Staphylococcus aureus in > or =50% of the mother's samples. The daughters showed high levels (>10(4) bacterial cells) of some periodontopathic bacteria, including members of the red (Porphyromonas gingivalis) and orange (Fusobacterium periodonticum and F. nucleatum polymorphum) complexes and beneficial species of the yellow (Streptococcus gordonii) and purple (Veillonella parvula) complexes. The mother presented high mean levels only for four tested species (N. mucosa, Prevotella melaninogenica, Treponema denticola, and V. parvula).

CONCLUSION

A combination of radiographs, microbiologic analysis, and preventive professional monitoring care is important to avoid PAL and to provide oral health in patients with DDI.

Expression and processing of small integrin-binding ligand N-linked glycoproteins in mouse odontoblastic cells

OBJECTIVE

Small integrin-binding ligand N-linked glycoproteins (SIBLINGs) are expressed in dentin and believed to control dentinogenesis. Five members of SIBLING family include bone sialoprotein (BSP), osteopontin (OPN), matrix extracellular phosphoglycoprotein (MEPE), dentin matrix protein 1 (DMP1) and dentin sialophosphoprotein (DSPP). These genes are clustered on chromosome 4q in humans and share similar biological features. DSPP and DMP1 are processed into given structural/functional fragments in rat and porcine. It still remains unclear whether these evidences occur in mouse and other SIBLING members are also processed into given fragments from their parent precursors. The aim of this study was to identify expression and processing of the five proteins in two mouse odontoblastic cell lines.

DESIGN

Two mouse odontoblastic cells were used to study expression and processing of the five SIBLING proteins by immunohistochemistry and Western blot analyses.

RESULTS

Immunohistochemistry study showed that all of the five SIBLING members were expressed within the cytoplasm and cellular processes in the mouse odontoblastic cell lines. Expression levels of DMP1 and DSPP were higher in differentiated mouse odontoblasts than undifferentiated mouse odontoblasts. Immunolabelling signal of DSP and MEPE was also detected within the nucleus in the two cell lines. Western blot assay indicated that all five members were processed into at least two fragments in these cells.

CONCLUSIONS

These results suggest that different processed products and expression levels of the SIBLING proteins may play distinct biological functions in tooth development and mineralisation.

SCPP gene evolution and the dental mineralization continuum

Many genes critical to vertebrate skeletal mineralization are members of the secretory calcium-binding phosphoprotein (SCPP) gene family, which has evolved by gene duplication from a single ancestral gene. In humans, mutations in some of these SCPP genes have been associated with various diseases related to dentin or enamel hypoplasia. Recently, systematic searches for SCPP genes of various species have allowed us to investigate the history of phylogenetically variable dental tissues as a whole. One important conclusion is that not all disease-associated SCPP genes are present in tetrapods, and teleost fish probably have none, even in toothed species, having acquired their complement of SCPP genes through an independent duplication history. Here, we review comparative analyses of mineralized dental tissues, with particular emphasis on the use of SCPPs, within and between tetrapods and teleosts. Current knowledge suggests a close relationship among bone, dentin, teleost fish enameloid (enamel-like hard tissue), and tetrapod enamel. These tissues thus form a mineralized-tissue continuum. Contemporary dental tissues have evolved from an ancestral continuum through lineage-specific modifications.

Porcine dentin sialophosphoprotein: length polymorphisms, glycosylation, phosphorylation, and stability

Dentin sialophosphoprotein (DSPP) is critical for proper mineralization of tooth dentin, and mutations in DSPP cause inherited dentin defects. Dentin phosphoprotein (DPP) is the C-terminal cleavage product of DSPP that binds collagen and induces intrafibrillar mineralization. We isolated DPP from individual pigs and determined that its N-terminal and C-terminal domains are glycosylated and that DPP averages 155 phosphates per molecule. Porcine DPP is unstable at low pH and high temperatures, and complexing with collagen improves its stability. Surprisingly, we observed DPP size variations on SDS-PAGE for DPP isolated from individual pigs. These variations are not caused by differences in proteolytic processing or degrees of phosphorylation or glycosylation, but rather to allelic variations in Dspp. Characterization of the DPP coding region identified 4 allelic variants. Among the 4 alleles, 27 sequence variations were identified, including 16 length polymorphisms ranging from 3 to 63 nucleotides. None of the length variations shifted the reading frame, and all localized to the highly redundant region of the DPP code. The 4 alleles encode DPP domains having 551, 575, 589, or 594 amino acids and completely explain the DPP size variations. DPP length variations are polymorphic and are not associated with dentin defects.