Uncovering the molecular basis of how germline mutations of the synaptonemal complex lead to human infertility

Centre/Institute: Wellcome Centre for Cell Biology

Background

How is the chromosome number halved during meiosis to create haploid spermatozoa and oocytes that form healthy diploid zygotes upon fertilisation? At the heart of this process is the synaptonemal complex (SC), a zipper-like protein assembly that binds together homologous chromosome pairs and enables their genetic exchange by crossing-over prior to segregation. The SC’s supramolecular structure imposes a 100-nm separation between homologous chromosomes, and is continuous along the chromosome axis (up to 25-m), making it one of the largest individual protein structures in a cell. The SC is essential for meiosis, and its defects lead to human infertility (affecting 10% of couples), recurrent miscarriage (affecting 5% of couples) and germline genetic disorders such as Down’s syndrome. Importantly, structural defects of meiosis, including those affecting SC structure and recombination, account for 25-30% of cases of infertility.

The human SC has eight known protein components (SYCP1-3, SYCE1-3, SIX6OS1 and TEX12), which perform distinct and critical roles in its molecular structure and function. We have previously determined how SYCP1 assembles into an SC-like lattice (Dunce et al 2018; Crichton et al 2023), SYCP3 forms a paracrystalline array that mediates chromatin looping and compaction, SYCE2-TEX12 assembles forms intermediate filament-like fibres that mediate SC extension along the chromosome length (Dunce et al 2021), and how SYCE1 and SIX6OS1 forms a multivalent complex that stabilizes SC structure (Sánchez-Sáez et al 2020). We now seek to understand how germline mutations affect the structure and function of the SC and thus uncover the molecular basis of how they lead to human infertility and miscarriage.

Aims

This project aims to uncover how clinical SC mutations affect the structure, assembly and interactions of SC proteins, and thus how they lead to infertility and recurrent miscarriage. This project builds on our previous work on the structure and assembly of the human SC (see references below). We will determine how clinical SC mutations affect the structure, assembly and interactions of SC proteins through structural biology and biophysics. In parallel, we will employ computational approaches to predict the structure, stability and interactions of mutants SC proteins (based on our existing structures) in silico. Our biochemical and computational findings will predict the meiotic consequence of clinical mutations, which we will test by generating and analysing the meiotic phenotypes of mice harbouring homozygous mutations. Together, these integrated approaches will reveal the precise molecular mechanism whereby individual clinical SC mutations lead to infertility/miscarriage.

We will analyze established causative SC mutations of infertility/miscarriage (including SYCP3 and SYCE1) to establish their aetiology, and candidate SC mutations (identified by collaborators) to establish whether they are causative in infertility/miscarriage. Ultimately, this will provide a prognostic tool by determining causative genetic blocks in individual patients through determining the consequence of SC variants identified by genome sequencing. This will enable us to direct patients to the most appropriate fertility treatment, and importantly prevent futile treatments in patients for which they are inappropriate. Further, it will guide the development of new treatments based on genetic cause that will benefit those for whom there are no current options.

Training Outcomes

This project will involve interdisciplinary training in structural biology, computational modelling and mouse genetics. Structural biology training will include molecular biology, protein biochemistry, biophysics (MALS, SAXS and interaction studies), X-ray crystallography and cryo-electron microscopy. Computational modelling will include structural modelling, molecular dynamics and machine-learning methods. Mouse genetics work will include the generation of mutant mice, analysis by histology and immunofluorescence (SIM and widefield), and quantitative image analysis

References

Crichton et al, NSMB, 2023: https://doi.org/10.1038/s41594-022-00909-1

Dunce et al, NSMB, 2021: https://doi.org/10.1038/s41594-021-00636-z

Sánchez-Sáez et al, Science Advances 2020: https://doi.org/10.1126/sciadv.abb1660

Dunce et al, NSMB, 2018: https://doi.org/10.1038/s41594-018-0078-9