Using Robertsonian Translocations to Produce Mouse Aneuploidy Models

As many as 30% of human fertilisations result in an embryo with an incorrect number of chromosomes (aneuploidy). This number is consistent between embryos generated from in vitro fertilisation (IVF) and from natural conception. Aneuploid embryos are a leading cause of early embryo termination and miscarriage, and their incidence increases with maternal age. As it is older individuals who are more likely to seek assisted reproductive techniques (ART) to realise their dream of a family, the high number of aneuploid embryos generated are a significant challenge to fertility clinics.

Research into aneuploid embryos and their detection in an IVF setting is limited by most forms being embryonic lethal, limiting the availability of standard animal models. However, early categorisation of aneuploid mouse embryo phenotypes in the 1980s resulted from a model of selective breeding, where a mouse carrying a Robertsonian (Rb) translocation was mated with a wildtype mouse line, producing a range of aneuploid embryos. A further refinement of this protocol showed that specific aneuploidies were able to be pre-determined by selecting a mouse which was homologous for two Rb translocations with one arm in common (monobrachial homology; MRb). My research used this breeding programme, together for the first time with IVF to produce mouse embryos with one of four aneuploidies involving chromosomes 4, 10, 11 or 15.

The four MRb sperm stocks were combined with wildtype ova in an IVF system. All four sperm stocks successfully fertilised ova, and a subset of these progressed through multiple cleavage events to develop to morula or blastocyst embryos (8, 13, 9 and 2% from MRb4, 10, 11 and 15 sperm stocks, respectively). These embryos were photographed prior to processing, and each was given a grading based on its morphology using the Gardner scale. Embryos were processed through an outsourced next generation sequencing protocol to identify those with aneuploidy. Two out of fourteen embryos were identified as aneuploid, but not for the expected chromosomes. However, one may have been passed on via one of the Rb translocations present in the parental strain. Neither of these embryos had morphology markers that identified them to be aneuploid. Moreover, there was no correlation between DNA quality and aneuploidy.

The proven fertilisation capacity of all sperm stocks using IVF revealed in this study opens an opportunity to further explore this model. In particular, the resultant aneuploid embryos may be used to develop alternate non-invasive aneuploidy detection methodologies.