In the human, the contribution of the sexes to the genetic load is dissimilar. Especially for point mutations, expanded simple tandem repeats and structural chromosome mutations, the contribution of the male germline is dominant. Far less is known about the male germ cell stage(s) that are most vulnerable to mutation contraction. For the understanding of de novo mutation induction in the germline, mechanistic insight of DNA repair in the zygote is mandatory. At the onset of embryonic development, the parental chromatin sets occupy one pronucleus (PN) each and DNA repair can be regarded as a maternal trait, depending on proteins and mRNAs provided by the oocyte. Repair of DNA double-strand breaks (DSBs) is executed by non-homologous end joining (NHEJ) and homologous recombination (HR). Differentiated somatic cells often resolve DSBs by NHEJ, whereas embryonic stem cells preferably use HR. We show NHEJ and HR to be both functional during the zygotic cell cycle. NHEJ is already active during replacement of sperm protamines by nucleosomes. The kinetics of G1 repair is influenced by DNA-PK(cs) hypomorphic activity. Both HR and NHEJ are operative in S-phase, HR being more active in the male PN. DNA-PK(cs) deficiency upregulates the HR activity. Both after sperm remodeling and at first mitosis, spontaneous levels of gammaH2AX foci (marker for DSBs) are high. All immunoflurescent indices of DNA damage and DNA repair point at greater spontaneous damage and induced repair activity in paternal chromatin in the zygote.

DNA double-strand break repair in parental chromatin of mouse zygotes, the first cell cycle as an origin of de novo mutation

https://www.cell.com/cancer-cell/pdf/S1535-6108(05)00196-0.pdf

A mouse model for cyclin E-dependent genetic instability and tumorigenesis

Inv dup(15) is a clinically significant bisatellited derivative of chromosome 15. Five unrelated patients with this abnormality are described and compared with ten confirmed and nine suspected cases in the literature. Mental and developmental retardation, hypotonia, behavioral disturbances, seizures, abnormal dermatoglyphics, and mild somatic anomalies were the most consistent findings. The extra chromosomes in our patients were identified with the aid of various techniques, including distamycin A/DAPI banding. A comparison of satellite polymorphisms suggested that the rearrangements frequently arose by meiotic nonsister chromatid exchange and second-division nondisjunction. A maternal origin was indicated in two cases, and parental ages were distinctly elevated.

Cytogenetic and clinical studies in five cases of inv dup(15).

Previous analyses by fluorescence in situ hybridization of structures present 20-30 cell generations after the primary events of mammalian gene amplification have shown that tens of megabases of DNA separate each copy of the selected gene in chromosomal arrays that contain up to 15 copies. Since these structures are very unstable, it is necessary to study amplified DNA as soon as possible after it has been formed to relate the structures observed to the primary mechanisms that generated them. Previously, new amplifications of the CAD gene were analyzed in colonies of 10(5) N-(phosphonoacetyl)-L-aspartate-resistant Syrian hamster BHK cells. CAD is on the p arm of chromosome B9 and the amplified genes were usually found in large extensions of B9p, with one copy in its normal position. We now report that dividing drug-resistant cells have been physically separated from static drug-sensitive cells, to allow the amplified structures to be observed only a few cell generations after they have been formed. The most informative results are that about one-third of the newly formed chromosomes carrying amplified CAD genes are dicentric and that about half of these carry two B9q arms. These observations reveal that recombination between the p telomeric regions of two B9 sister chromatids is an important primary event of amplification in this system. The resulting dicentric chromosomes can then enter bridge-breakage-fusion cycles that provide the means to increase the number of CAD genes per cell in successive generations by an asymmetric distribution at each cell division.

Fusions near telomeres occur very early in the amplification of CAD genes in Syrian hamster cells.

Bisphenol A (BPA) is a synthetic monomer widely used to polymerize polycarbonate plastics and resins. It is shown in vitro to interfere with microtubules, producing aberations in mitotic and meiotic spindles. An increase of meiotic abnormalities in untreated female mice from an experimental colony was temporally correlated with the accidental release of BPA from polycarbonate cages and bottles damaged by inadvertent treatment with harsh alkaline detergents [P.A. Hunt, K.E. Koehler, M. Susiarjo, C.A. Hodges, A. Ilagan, R.C. Voigt, S. Thomas, B.F. Thomas, T.J. Hassold, Bisphenol A exposure causes meiotic aneuploidy in the female mouse, Curr. Biol. 13 (2003) 546-553]. In the present study, potential aneugenic effects of BPA on mouse male and female germ cells and bone marrow cells have been evaluated after acute, sub-chronic or chronic in vivo exposure. Female mice were orally treated with a single BPA dose, with 7 daily administrations or exposed for 7 weeks to BPA in drinking water. No significant induction of hyperploidy or polyploidy was observed in oocytes and zygotes at any treatment condition. The only detectable effect was a significant increase of metaphase II oocytes with prematurely separated chromatids after chronic exposure; this effect, however, had no irreversible consequence upon the fidelity of chromosome segregation during the second meiotic division, as demonstrated by the normal chromosome constitution of zygotes under the same exposure condition. With male mice, no delay of meiotic divisions was found after six daily oral doses of BPA with the BrdU assay. Similarly, no induction of hyperploidy and polyploidy was shown in epydidimal sperm hybrized with probes for chromosomes 8, X and Y, 22 days after six daily oral BPA doses. Finally, two daily oral BPA doses did not induce any increase of micronucleus frequencies in polychromatic erythrocytes of mouse bone marrow. In conclusion, our results do not add evidence to the suspected aneugenic activity of BPA and suggest that other factors or co-factors should be considered to explain the unexpected burst of meiotic abnormalities previously attributed to accidental BPA exposure.

F Salamanca

Papers

C bands in human metaphase chromosomes treated by barium hydroxide.