Differential mutation frequency in mitochondrial DNA from thyroid tumours.

Lack of a chromatin structure and histone protection makes mitochondrial DNA susceptible to oxidative damage. Suboptimal DNA repair leads to a higher frequency of mitochondrial mutations, which are associated with aging, carcinogenesis and environmental insult. The instability of the hypervariable region II of the mitochondrial genome was investigated in radiation-associated thyroid tumours, which were diagnosed in children from Belarus after the accident at the Chernobyl nuclear power plant, and from 40 sporadic thyroid tumours from Munich. Two mutations were identified in two out of 126 tumours from Belarus, and eight mutations were found in six out of 40 tumours from Munich. All mutations were deletions or insertions of C in a poly-cytidine (C7TC6) microsatellite. The mutation frequency correlated with the age of the patients at surgery. Mutations with the typical pattern of base substitutions following oxidative DNA damage were not identified.

Instability of microsatellites in radiation-associated thyroid tumours with short latency periods.

PURPOSE: To determine the instability of microsatellite sequences in post-Chernobyl thyroid tumours from children and young adults, and to ascertain whether they correlated with the age of the patient at the time of the accident and the tumour latency period. MATERIALS AND METHODS: The stability of 26 microsatellite markers was investigated in 122 radiation-associated thyroid tumours (96 children, 26 adults) from Belarus and 39 spontaneous thyroid tumours (adults) from Munich without radiation history. RESULTS: A significant correlation between patient age at the time of the accident and the instability of microsatellite sequences was established. Also, a high instability of microsatellite sequences was found in 28 early thyroid tumours from Belarus with latency periods of 6-8 years, in contrast to a low instability of microsatellites in 94 tumours emerging 9-11 years after the accident. Microsatellite instability in the reference group from Munich proved similar to the early thyroid tumours from Belarus. CONCLUSION: Early, fast-growing and aggressive post-Chernobyl thyroid tumours are characterized by an increase in microsatellite instability.

Investigations into the molecular effects of single nucleotide polymorphism.

OBJECTIVES: DNA sequences are very rich in short repeats and their pattern can be altered by point mutations. We wanted to investigate the effect of single nucleotide polymorphism (SNP) on the pattern of short DNA repeats and its biological consequences. METHODS: Analysis of the pattern of short DNA repeats of the Thy-1 sequence with and without SNP. Searching for DNA-binding factors in any region of significance. RESULTS: Comparing the pattern of short repeats in the Thy-1 gene sequences of Turkish patients with ataxia telangiectasia (AT) with the "wild type" sequence from the DNA database, we identified a missing 8-bp repeat element due to an SNP in position 1271 (intron II) in AT-DNA sequences. Only the mutated sequence had the potential for the formation of a stem loop in DNA or pre-mRNA. In super-shift experiments we found that DNA oligomers covering the area of this SNP formed a complex with proteins amongst which we identified the proliferating cell nuclear antigen (PCNA) protein. CONCLUSION: SNPs have the potential to alter DNA or pre-mRNA conformation. Although no SNP-depeding formation of the DNA-protein complex was evident, future investigations could reveal differential molecular mechanisms of cellular regulation.

Microsatellite instability and loss of heterozygosity in radiation-associated thyroid carcinomas of Belarussian children and adults.

DNA from 129 paired thyroid tumorous and non-tumorous tissue samples of Belarussian children (102 patients; age at surgery

Regulation of the cell cycle following DNA damage in normal and Ataxia telangiectasia cells.

A proportion of the population is exposed to acute doses of ionizing radiation through medical treatment or occupational accidents, with little knowledge of the immediate effects. At the cellular level, ionizing radiation leads to the activation of a genetic program which enables the cell to increase its chances of survival and to minimize detrimental manifestations of radiation damage. Cytotoxic stress due to ionizing radiation causes genetic instability, alterations in the cell cycle, apoptosis, or necrosis. Alterations in the G1, S and G2 phases of the cell cycle coincide with improved survival and genome stability. The main cellular factors which are activated by DNA damage and interfere with the cell cycle controls are: p53, delaying the transition through the G1-S boundary; p21WAF1/CIP1, preventing the entrance into S-phase; proliferating cell nuclear antigen (PCNA) and replication protein A (RPA), blocking DNA replication; and the p53 variant protein p53 as together with the retinoblastoma protein (Rb), with less defined functions during the G2 phase of the cell cycle. By comparing a variety of radioresistant cell lines derived from radiosensitive ataxia telangiectasia cells with the parental cells, some essential mechanisms that allow cells to gain radioresistance have been identified. The results so far emphasise the importance of an adequate delay in the transition from G2 to M and the inhibition of DNA replication in the regulation of the cell cycle after exposure to ionizing radiation.

Characterization of a hybrid hamster-human cell line complemented for the ataxia telangiectasia DNA repair defects.

The fibroblast cell line AT5BIVA, from ataxia telangiectasia complementation group D, was transfected with a neo gene providing G418 resistance for subsequent selections. The G418 resistant cell line was fused with gamma-irradiated Chinese hamster ovary cells and a radioresistant hybrid, atxbc, was isolated following an X-ray selection procedure. All the cellular defects characteristic of ataxia telangiectasia were corrected to some degree; atxbc cells: (i) were confirmed to be resistant to X-rays; (ii) had regained control over DNA synthesis after DNA damage, and (iii) could overcome the radiation-induced block in the G2 phase of the cell cycle. Repetitive element polymerase chain reaction amplification of integrated hamster DNA from primers to middle repetitive elements confirmed the presence of hamster-specific sequences, suggesting that the phenotype of the cells had been corrected by integration of a normal hamster gene rather than by a reversion event.