Name: Andre Claude Mbouombouo Mfossa
Date: February 3, 2020 (15:00 h)
Mechanistic analysis of radiation-induced microcephaly in the mouse
Exposure to high doses of ionizing radiation during embryonic neurogenesis in humans and mice results in microcephaly. However, the underlying molecular and cellular mechanisms are not yet fully elucidated. Previous studies using E11 mice indicated that gene expression changes regulated by the P53 transcription factor in the brains of these mice shortly after irradiation might be responsible for the detrimental effects.
In this study, we aimed at documenting the role of the Trp53 gene in the etiology of radiation-induced microcephaly. For this, we generated a mouse model in which both Trp53 alleles are conditionally inactivated in the embryonic dorsal forebrain only (cKO mice), during early neurogenesis. Following irradiation at E11, the brains of cKO mice were significantly larger than those of irradiated control mice. The radiation-induced DNA-damage normally results in cell cycle arrest, followed by massive apoptosis of neuroprogenitor cells and premature neuronal differentiation, which explains the reduction in brain size. These effects were attenuated (with regard to cell cycle arrest, cell death) or absent (premature differentiation) in cKO mice.
Furthermore, we characterized the circular transcripts of some identified P53 target genes (i.e. Pvt1, Ano3, Sec14l5 and Rnf169). Whereas the expression of the linear transcripts of these genes peaks early after radiation exposure, several of the circular isoforms show prolonged induction in irradiated embryonic mouse brain, primary cortical neurons and blood. In subsequent work, we characterized D630023F18Rik, also a P53 target gene of which we have identified several new long and short splice variants. The short ones are specifically transcribed after irradiation and encompass a novel exon that is transcribed from an alternative P53-regulated promoter. The biological stability of the short transcripts was shown to be enhanced after irradiation suggesting their importance for the cellular radiation response. Both the long and short transcripts encode for polypeptides of various lengths, which were mainly found in brain regions related to the thalamocortical circuitry and migrating interneurons in the developing brain.
Taken together, this PhD thesis provides evidence that Trp53 and its encoded protein P53 is a major effector of radiation-induced microcephaly. Further experiments using pharmacological inhibition of P53 may additionally support invetigating such drugs as possible countermeasures to prevent effects of prenatal exposure to ionizing radiation. We also provide a solid basis for work aiming to investigate the potential use of circRNAs as biomarkers of brain aging as well as for radiation exposure. Another studied gene, D630023F18Rik, may play an essential role in the developing brain, supported by its particular spatiotemporal expression pattern and the levels of its short splice variants following irradiation.
- (KU Leuven) Danny Huylebroeck
- Roel Quintens
- Rafi Benotmane
Click here for a list of obtained PhD degrees.