[Maxillar osteonecrosis associated with bisphosphonate treatment and dental implants]. / Ostéonécrose maxillaire sous bisphosphonates et implants dentaires.

INTRODUCTION: Bisphosphosnates are reference products used to treat osteoporosis, malignant bone disease, Paget's disease, and hypercalcemia. However these drugs seem to induce osteonecrosis of the jaws. This osteonecrosis is frequently observed and must be evoked in patients presenting with oral ulceration under bisphosphonate therapy. OBSERVATION: We report the case of a long-term fully dental implanted patient treated by bisphosphonates who presented a maxillar ostenecrosis with no previous radiotherapy. DISCUSSION: The risk factors and mechanism of this induced osteonecrosis are described. But could long term osseo-integrated dental implants be a triggering factor?

Repair of 8-oxoguanine and Ogg1-incised apurinic sites in a CHO cell line.

The repair mechanisms involved in the removal of 8-oxo-7,8-dihydroguanine (8-oxoG) in damaged DNA have been investigated using cell-free extracts or purified proteins. However, in vivo repair assays are required to further dissect mechanisms involved in the repair of 8-oxoG in the cellular context. In this study, we analyzed the removal of 8-oxoG from plasmids that contain a single 8-oxoG.C base pair in a sequence that can be transcribed (TS) or nontranscribed (NTS) in a chinese hamster ovary (CHO) cell line. The results show that 8-oxoG located in a TS is removed faster than in a NTS, indicating transcription-coupled repair (TCR) of 8-oxoG in rodent cells. The results also show that CHO cells efficiently repair DNA molecules that contain an Ogg1-incised AP site, which is the first intermediate in the course of base excision repair of 8-oxoG.

BRCA1 and BRCA2 are necessary for the transcription-coupled repair of the oxidative 8-oxoguanine lesion in human cells.

The breast and ovarian cancer susceptibility genes, BRCA1 and BRCA2, are likely to participate in DNA lesion processing. Oxidative lesions, such as 8-oxoguanine, occur in DNA after endogenous or exogenous oxidative stress. We show that deficiency for either BRCA1 or BRCA2 in human cancer cells leads to a block of the RNA polymerase II transcription machinery at the 8-oxoguanine site and impairs the transcription-coupled repair of the lesion, leading to a high mutation rate. Expression of wild-type BRCA1 from a recombinant adenovirus fully complements the repair defect in BRCA1-deficient cells. These results represent the first demonstration of the essential contribution of BRCA1 and BRCA2 gene products in the repair of the 8-oxoguanine oxidative damage specifically located on the transcribed strand in human cells. This suggests that cells from individuals predisposed to breast and/or ovarian cancer may undergo a high rate of mutations because of the deficiency of this damage repair pathway after oxidative stress.

Transcription coupled repair of 8-oxoguanine in murine cells: the ogg1 protein is required for repair in nontranscribed sequences but not in transcribed sequences.

To assess the role of the Ogg1 DNA glycosylase in the transcription-coupled repair (TCR) of the mutagenic lesion, 7, 8-dihydro-8oxoguanine (8-OxoG), we have investigated the removal of this lesion in wild-type and ogg1(-/-) null mouse embryo fibroblast (MEF) cell lines. We used nonreplicating plasmids containing a single 8-OxoG.C base pair in a different assay that allowed us to study the removal of 8-OxoG located in a transcribed sequence (TS) or in a nontranscribed sequence (NTS). The results show that the removal of 8-OxoG in a wild-type MEF cell line is faster in the TS than in the NTS, indicating TCR of 8-OxoG in murine cells. In the homozygous ogg1(-/-) MEF cell line, 8-OxoG was not removed from the NTS whereas there was still efficient 8-OxoG repair in the TS. Expression of the mouse Ogg1 protein in the homozygous ogg1(-/-) cell line restored the ability to remove 8-OxoG in the NTS. Therefore, we have demonstrated that Ogg1 is essential for the repair of 8-OxoG in the NTS but is not required in the TS. These results indicate the existence of an Ogg1-independent pathway for the TCR of 8-OxoG in vivo.

Mutation spectra induced by replication of two vicinal oxidative DNA lesions in mammalian cells.

Ionizing radiations often induce multiple and clustered DNA lesions at the site of DNA interaction. As a model, we have studied the toxicity and the mutagenicity of two adjacent oxidative bases as clustered DNA lesions in mammalian cells using shuttle vectors. The chosen oxidative lesions were 8-oxo-7,8-dihydroguanine, the formylamine residue resulting from the oxidation of a pyrimidine base and the tandem lesion 8-oxo-7,8-dihydroguanine/formylamine where both modifications are located at a vicinal position. A single-stranded DNA shuttle vector carrying a unique DNA lesion was constructed, transfected into simian COS7 cells and mutations induced after replication in mammalian cells were screened in bacteria. 8-oxo-7,8-dihydroguanine, as expected, does not affect greatly survival (70% bypass) whereas formylamine and the tandem lesions are blocking alterations, DNA polymerase bypass being of 45% and 17%, respectively. Base insertion opposite the lesion was studied. Under our experimental conditions, replication of 8-oxo-7, 8-dihydroguanine finally gives rise to guanine:cytosine pairing, rendering this lesion only slightly mutagenic. This is not the case for the formylamine that codes preferentially for adenine (71%). In addition, one-base deletions were observed targeted to the site to the lesion. Cytosine and thymine were inserted opposite the lesion with similar but low frequencies. Thus, coding properties of the formylamine render this residue very mutagenic when coming from the oxidative alteration of a cytosine. The coding properties of the tandem damage are a combination of the contribution of the two isolated lesions with a very high percentage of adenine insertion (94%) opposite the formylamine residue of the tandem lesion. The toxicity as well as the mutation spectrum of the tandem lesion allow us to speculate about the molecular mechanism with which the DNA polymerase replicates these two lesions.

Transcription-coupled repair of 8-oxoguanine: requirement for XPG, TFIIH, and CSB and implications for Cockayne syndrome.

Analysis of transcription-coupled repair (TCR) of oxidative lesions here reveals strand-specific removal of 8-oxo-guanine (8-oxoG) and thymine glycol both in normal human cells and xeroderma pigmentosum (XP) cells defective in nucleotide excision repair. In contrast, Cockayne syndrome (CS) cells including CS-B, XP-B/CS, XP-D/CS, and XP-G/CS not only lack TCR but cannot remove 8-oxoG in a transcribed sequence, despite its proficient repair when not transcribed. The XP-G/CS defect uniquely slows lesion removal in nontranscribed sequences. Defective TCR leads to a mutation frequency at 8-oxoG of 30%-40% compared to the normal 1%-4%. Surprisingly, unrepaired 8-oxoG blocks transcription by RNA polymerase II. These data imply that TCR is required for polymerase release to allow repair and that CS results from defects in TCR of oxidative lesions.

Repair and mutagenesis survey of 8-hydroxyguanine in bacteria and human cells.

8-Hydroxyguanine is one of the major products formed by the reactive oxygen species which are generated in living cells as a consequence of either the normal metabolic pathways or an exogeneous chemical or physical stress. The production of the oxidative damage is described and the different repair pathways of the oxidative lesions are analyzed from bacteria to human cells. Analysis of repair in human cells harboring different deficiencies in the nucleotide excision repair mechanism such as xeroderma pigmentosum cells from different complementation groups and cells from Cockayne's syndrome patients allows us to emphasize the possibility of the intervention of this repair mechanism on the elimination of oxidative damages. Finally, a repair model of oxidative lesions is proposed.

Repair and mutagenic potency of 8-oxoG:A and 8-oxoG:C base pairs in mammalian cells.

Replication of the oxidative lesion 8-oxo-7,8-dihydroguanine (GO) leads to the formation of both 8-oxo-7,8-dihydroguanine:adenine (GO:A) and 8-oxo-7,8-di-hydroguanine:cytosine (GO:C) pairs. The repair and mutagenic potency of these two kinds of base pairs were studied in simian COS7 and human MRC5V1 cells using the shuttle vector technology. Shuttle vectors carrying a unique GO residue opposite either a C or an A were constructed, then transfected into recipient mammalian cells. DNA repair resulting in G:C pairs and mutation frequency, were determined using resistance to digestion by the Ngo MI restriction enzyme for screening and DNA sequencing of suspect mutants. Results showed that the GO:C mismatch was well repaired since almost no mutations were detected in the plasmid progeny obtained 72 h after cell transfection. The GO:A pair was poorly repaired since only 32-34% of the plasmid progeny contained G:C whereas two thirds contained A:T at the original site. Repair kinetics measured with a non-replicating vector deleted by 13 bp at the SV40 replication origin, showed that GO:A was slowly repaired. Only 30% of the mispairs were corrected in 12 h. During this time 100% of the plasmids containing GO:A pairs were replicated as seen by the replication kinetics in a vector with an intact SV40 replication origin. These results show that, under our experimental conditions, replication is occurring before completion of DNA repair which explains the high mutagenic potency of the GO:A mispair.

The consequence of translesional replication of unique UV-induced photoproducts.

The consequence of translesional replication of unique UV-induced photoproducts is reviewed here. Mutagenesis induced by unique UV-induced lesions, the thymine-thymine dimer (TT) and the thymine-thymine pyrimidine pyrimidone (6-4), [T(6-4)T] carried on single-stranded DNA vectors and replicated in bacteria, yeast and mammalian cells have been considered. It has been found that in all of the three species the (TT) dimers induce a low mutation frequency compared to the (6-4) photoproduct. The molecular analysis of the mutations induced is reported, showing specific differences depending on the species considered.

Mutagenicity of a unique thymine-thymine dimer or thymine-thymine pyrimidine pyrimidone (6-4) photoproduct in mammalian cells.

The mutagenic properties of UV-induced photoproducts, both the cis-syn thymine-thymine dimer (TT) and the thymine-thymine pyrimidine pyrimidone (6-4) photoproduct [T(6-4)T] were studied in mammalian cells using shuttle vectors. A shuttle vector able to replicate in both mammalian cells and bacteria was produced in its single-stranded DNA form. A unique photoproduct was inserted at a single restriction site and after recircularization of the single-stranded DNA vector, this latter was transfected into simian COS7 cells. After DNA replication the vector was extracted from cells and used to transform bacteria. Amplified DNA was finally analyzed without any selective screening, DNA from randomly picked bacterial colonies being directly sequenced. Our results show clearly that both lesions are mutagenic, but at different levels. Mutation frequencies of 2 and 60% respectively were observed with the TT dimer and the T(6-4)T. With the TT dimer the mutations were targeted on the 3'-T. With the T(6-4)T a large variety of mutations were observed. A majority of G-->T transversions were semi-targeted to the base before the 5'-T of the photoproduct. These kinds of mutations were not observed when the same plasmid was transfected directly into SOS-induced JM105 bacteria or when the T(6-4)T oligonucleotide inserted in a different plasmid was replicated in SOS-induced SMH10 Escherichia coil bacteria. These semi-targeted mutations are therefore the specific result of bypass of the T(6-4)T lesion in COS7 cells by one of the eukaryotic DNA polymerases.

Mutagenicity of a unique 8-oxoguanine in a human Ha-ras sequence in mammalian cells.

The processing of a unique 8-oxoguanine residue in DNA has been studied in mammalian cells using a single-stranded shuttle vector. A fragment of human Ha-ras carrying the lesion on the first (G1) or the second guanine (G2) of codon 12 was inserted in a shuttle plasmid. Extrachromosomal DNA is replicated in animal cells, extracted and used to transform bacteria to be amplified and individualized. DNA sequencing of bacterial clones showed the mutagenic potency of 8-oxoguanine in vivo to be approximately 4%. The presence of the 8-oxoguanine does not greatly affect survival of the progeny. No significant difference was observed between the mutation frequencies induced by 8-oxoguanine located either at the G1 or G2 position. The majority of the mutations, targeted at the lesion level, are G to T transversions. These base substitutions induced respectively glycine to cysteine (G1) or valine (G2) change in the P21ras protein. These mutations may contribute to activation of the protooncogene, leading to spontaneous tumorigenesis.

Coding properties of a unique apurinic/apyrimidinic site replicated in mammalian cells.

The coding properties of abasic sites have been studied in mammalian cells using a single-stranded shuttle vector carrying a unique abasic site. The probe was produced by digestion with the uracil DNA glycosylase of a uracil-containing oligonucleotide which was inserted in the single-stranded vector. After replication in monkey COS7 cells able to support DNA replication of the vector, the plasmid progeny were isolated in bacteria. DNA sequencing of rescued plasmids showed that replication of abasic sites does not lead to preferential insertion of a given base opposite the non-coding site. The four bases were inserted with a frequency which was not statistically different from a random distribution. It appears therefore that the "A rule insertion" opposite a unique abasic site does not apply, at least with the sequence we used, for an exogenous single-stranded DNA replicated in mammalian cells. It was not necessary to induce SOS-like conditions by pretreatment of host cells, in order to replicate abasic sites.