Arabidopsis CHROMOSOME TRANSMISSION FIDELITY 7 (AtCTF7/ECO1) is required for DNA repair, mitosis and meiosis.

The proper transmission of DNA in dividing cells is crucial for the survival of eukaryotic organisms. During cell division, faithful segregation of replicated chromosomes requires their tight attachment, known as sister chromatid cohesion, until anaphase. Sister chromatid cohesion is established during S-phase in a process requiring an acetyltransferase that in yeast is known as Establishment of cohesion 1 (Eco1). Inactivation of Eco1 typically disrupts chromosome segregation and homologous recombination-dependent DNA repair in dividing cells, ultimately resulting in lethality. We report here the isolation and detailed characterization of two homozygous T-DNA insertion mutants for the Arabidopsis thaliana Eco1 homolog, CHROMOSOME TRANSMISSION FIDELITY 7/ESTABLISHMENT OF COHESION 1 (CTF7/ECO1), called ctf7-1 and ctf7-2. Mutants exhibited dwarfism, poor anther development and sterility. Analysis of somatic tissues by flow cytometry, scanning electron microscopy and quantitative real-time PCR identified defects in DNA repair and cell division, including an increase in the area of leaf epidermal cells, an increase in DNA content and the upregulation of genes involved in DNA repair including BRCA1 and PARP2. No significant change was observed in the expression of genes that influence entry into the endocycle. Analysis of meiocytes identified changes in chromosome morphology and defective segregation; the abundance of chromosomal-bound cohesion subunits was also reduced. Transcript levels for several meiotic genes, including the recombinase genes DMC1 and RAD51C and the S-phase licensing factor CDC45 were elevated in mutant anthers. Taken together our results demonstrate that Arabidopsis CTF7/ECO1 plays important roles in the preservation of genome integrity and meiosis.

Chromatid and chromosome type breakage-fusion-bridge cycles in wheat (Triticum aestivum L.).

During the development of disomic additions of rye (Secale cereale L.) chromosomes to wheat (Triticum aestivum L.), two reverse tandem duplications on wheat chromosomes 3D and 4A were isolated. By virtue of their meiotic pairing, the reverse tandem duplications initiated the chromatid type of the breakage-fusion-bridge (BFB) cycle. This BFB cycle continued through pollen mitoses and in the early endosperm divisions, but no clear evidence of its presence in embryo mitoses was found. The chromosome type of BFB cycle was initiated by fusion of two broken chromosome ends resulting in a dicentric or a ring chromosome. Chromosome type BFB cycles were detected in embryo mitoses and in root tips, but they did not persist until the next meiosis and were not transmitted to the progeny. Active BFB cycles induced breakage of other wheat chromosomes that resulted in additional reverse tandem duplications and dicentric and ring chromosomes. Four loci, on chromosome arms 2BS, 3DS, 4AL, and most likely on 7DL, were particularly susceptible to breakage. The BFB cycles produced high frequency of variegation for pigmentation of the aleurone layer of kernels and somatic chimeras for a morphological marker. With the exception of low mutation rate, the observed phenomena are consistent with the activity of a Ds-like element. However, it is not clear whether such an element, if indeed present, was of wheat or rye origin.

Hyperthermia specifically inhibits bivalent chromosome disjunction in maturing mouse oocytes.

The effect of hyperthermia on mammalian oocyte maturation was studied by allowing preovulatory mouse oocytes to mature spontaneously for 17 h in vitro under controlled temperature conditions. At the end of culture, oocytes were screened for their maturation stage and for chromosome morphology and number. Mild hyperthermic conditions (38.5-40.0 degrees C) during maturation specifically disturbed the process of bivalent chromosome disjunction, but not other maturation steps, by blocking oocytes at the metaphase I stage and preventing cells from entering subsequent maturation steps. Some oocytes that had reached metaphase II under hyperthermic conditions had chromosome imbalance. Oocytes matured at 40.0 degrees C displayed chromosome morphological abnormalities, including altered sister chromatid separation and nucleus/nuclei formation, at a frequency significantly higher than oocytes matured at 37.0-39.0 degrees C. When incubation temperature was raised above 40.0 degrees C, increasing fractions of oocytes were inhibited from entering initial maturation steps. We conclude that hyperthermia during mammalian oocyte maturation specifically damages the process of bivalent chromosome disjunction and induces the appearance of chromosome structural defects and imbalance in unfertilized eggs.

The search for relevant short term bioassays for chemical carcinogens: the tribulation of a modern Sisyphus.

Based on a good correlation between carcinogenicity and mutagenic activity several rapid microbial bioassays for chemical carcinogens have been recently developed. We would like to suggest, that these microbial tests should be followed by bioassays using cultured human cells of the "average" man, and of persons with elevated cancer risk or increased susceptibility to carcinogenic agents. The main objective of using DNA repair (unscheduled uptake of 3HTdR) and DNA fragmentation (shift in sedimentation profiles) of cultured human cells was to design a test system that can simulate conditions found in man and thus provide information relevant to the human population. A trial on 98 different carcinogens, precarcinogens and noncarcinogens showed the suitability of DNA repair synthesis as a rapid, economic and relevant assay for detection of chemical carcinogens. To check the adaptability of DNA repair synthesis of human cells as a bioassay for chemical carcinogens we examined carcinogenic nitrosation products which are formed from the interaction of nitrite and nitrosatable compounds, carcinogenic or mutagenic photosensitizing chemicals, and the effect of complex interactions. Organotropic carcinogens can be detected by measuring DNA fragmentation and DNA repair in various target organs following the in vivo application of chemical carcinogens. The pros and cons of several bioassays and their usefulness in judging a carcinogenic or mutagenic hazard to human populations is discussed.