Emergency prevention of extinction of a transgenic allele in a less-fertile transgenic mouse line by crossing with an inbred or outbred mouse strain coupled with assisted reproductive technologies

Loss of maternal ATRX results in centromere instability and aneuploidy in the mammalian oocyte and pre-implantation embryo

The α-thalassemia/mental retardation X-linked protein (ATRX) is a chromatin-remodeling factor known to regulate DNA methylation at repetitive sequences of the human genome. We have previously demonstrated that ATRX binds to pericentric heterochromatin domains in mouse oocytes at the metaphase II stage where it is involved in mediating chromosome alignment at the meiotic spindle. However, the role of ATRX in the functional differentiation of chromatin structure during meiosis is not known. To test ATRX function in the germ line, we developed an oocyte-specific transgenic RNAi knockdown mouse model. Our results demonstrate that ATRX is required for heterochromatin formation and maintenance of chromosome stability during meiosis. During prophase I arrest, ATRX is necessary to recruit the transcriptional regulator DAXX (death domain associated protein) to pericentric heterochromatin. At the metaphase II stage, transgenic ATRX-RNAi oocytes exhibit abnormal chromosome morphology associated with reduced phosphorylation of histone 3 at serine 10 as well as chromosome segregation defects leading to aneuploidy and severely reduced fertility. Notably, a large proportion of ATRX-depleted oocytes and 1-cell stage embryos exhibit chromosome fragments and centromeric DNA–containing micronuclei. Our results provide novel evidence indicating that ATRX is required for centromere stability and the epigenetic control of heterochromatin function during meiosis and the transition to the first mitosis.

The transmission of an abnormal chromosome complement from the gametes to the early embryo, a condition called aneuploidy, is a major cause of congenital birth defects and pregnancy loss. Human embryos are particularly susceptible to aneuploidy, which in the majority of cases is the result of abnormal meiosis in the female gamete. However, the molecular mechanisms involved in the onset of aneuploidy in mammalian oocytes are not fully understood. We show here that, the α-thalassemia/mental retardation X-linked protein (ATRX) is essential for the maintenance of chromosome stability during female meiosis. ATRX is required to recruit the transcriptional regulator DAXX to pericentric heterochromatin at prophase I of meiosis. Notably, lack of ATRX function at the metaphase II stage interferes with the establishment of chromatin modifications associated with chromosome condensation leading to segregation defects, chromosome fragmentation, and severely reduced fertility. Our results provide direct evidence for a role of ATRX in the regulation of pericentric heterochromatin structure and function in mammalian oocytes and have important implications for our understanding of the epigenetic factors contributing to the onset of aneuploidy in the female gamete.