Influence of low doses of ionizing radiation on the mammalian female germ cells

Background

Radioadaptation is a phenomenon whereby cells exposed to a low dose (conditioning dose) of ionizing radiation are more resistant to a dose in the order of a few Gy (challenging dose) delivered some time thereafter (from a few hours up to 1 month or even more). The induction of an adaptive state in the cell is part of a general stress response to DNA damaging agents. It can possibly be explained by a stimulation of the DNA repair machinery.

Compared to somatic cells, germ cells are of greater relevance for the evaluation of genetic hazard because the DNA or chromosome damage induced in germ cells may be transmitted to the next generation where it can cause adverse heritable effects. So far, there have been no studies on the potential existence of an adaptive response in female germ cells exposed to low doses of ionising radiation.

Recently developed methods allow in vitro development of mouse oocyte/follicle complexes from the mid-growth stage (1-2 layers of follicle cells) to the pre-ovulatory stage (multilayered Graafian follicle). Upon appropriate hormonal stimulation, Graafian follicles can then be ovulated in vitro, producing a high yield of fertilizable metaphase II oocytes. With this system, oocyte/follicle complexes can be exposed to either acute or chronic irradiation at/from well defined stages and the effects of the treatments can be analyzed at various levels, i.e. morphological (follicle and oocyte growth), functional (hormonal secretion by the follicle, formation of antrum and acquisition of meiotic competence, fertilizability), cytogenetic (chromosome aberrations in MI or MII oocytes), cell or molecular biological (apoptosis, gene expression….). Therefore, such an in vitro system has determinant advantages for radiobiological studies on female germ cells in various conditions.

Research

BacteriaThe system has been used in our laboratory, in the frame of a contract with EDF (Electricité de France) to investigate the potential of low doses of ionizing radiation to induce an adaptive response in oocytes at two different stages of growth and maturation. Prior irradiation of oocytes with a conditioning dose of 50 mGy led to a reduction in the yield of chromosome aberrations when irradiation with a challenging dose of 2 or 4 Gy occurred on day 12 of the culture (preovulatory stage) but not on day 0 (mid-growth stage).

This study will now be completed by an analysis of gene expression in oocytes irradiated by low doses of x-rays, enabling us to correlate changes in the levels of radiation-induced chromosome aberrations with a stimulation or repression of particular genes.

Chemo and/or radiotherapy applied to young cancer patients most often have severe effects upon female fertility. Today, few options are available to protect ovarian function in females. However, these options are either ineffective, belong to the field of experimental research or/and are not applicable to all patients. Drugs that could protect the oocyte and its surrounding feeder cells from damage can be of great importance. Therefore, a part of our experiments is also devoted to the influence of natural antioxydants on the radiation sensitivity of the female germ cells, with the aim of protecting female cancer patients from premature ovarian failure.

BacteriaOther investigations performed in our laboratory suggested that the guinea-pig oocyte represents one of the best models for studies on the genetic effects of radiation in the human female germ cells. For this reason, efforts are also devoted to the development of an in vitro system for the culture of guinea-pig oocytes, similar to that already used in the mouse. This will allow precise comparisons of the radiation sensitivity of oocytes of these two species, irradiated at comparable stages and under identical conditions. Moreover, it will help to decipher the mechanisms responsible for the interspecies differences of radiation sensitivity, ultimately leading to a better definition of the genetic hazard associated to an exposure of women to radiation.

People: Dr. Paul Jacquet , Lic. Iris Adriaens, J. BusetMieke Neefs

References

Jacquet, P., J. Buset, J. Vankerkom, S. Baatout, L. de Saint-Georges, L. Baugnet-Mahieu and C. Desaintes (2001), Radiation-induced chromosome aberrations in guinea-pig growing oocytes, and their relation to follicular atresia, Mutation Res., 473, 249-254.

Adriaens, I., R. Cortvrindt, and J. Smitz (2004), Differential FSH exposure in preantral follicle culture has marked effects on folliculogenesis and oocyte developmental competence, Human Reproduction, 19, 398-408.

Jacquet, P., I. Adriaens, J. Buset, M. Neefs and J. Vankerkom (2005), Cytogenetic studies in mouse oocytes irradiated in vitro at different stages of maturation, by use of an early preantral follicle culture system, Mutation Res., 583, 168-177.

Adriaens, I., P. Jacquet, R. Cortvrindt, K. Janssen and J. Smitz (2006), Melatonin has dose-dependent effects on folliculogenesis, oocyte maturation capacity and steroidogenesis, Toxicology, 228, 333-343.

Jacquet, P., J. Buset, M. Neefs and J. Vankerkom (2008), Studies on the adaptive response in mouse female germ cells x-irradiated in vitro at two different stages of maturation, In vivo, in the press.