MLH1/3variants causing aneuploidy, pregnancy loss, and premature reproductive aging

Abstract

Most spontaneous pregnancy losses are a result of embryonic aneuploidy stemming from mis-segregation of chromosomes during meiosis. Proper disjunction of homologous chromosomes is dependent upon precise control of crossing-over, a process requiring the mismatch repair (MMR) genes MLH1 and MLH3. Both are required for fertility and completion of meiosis in mice. People inheriting variants in these genes are often at high risk for colorectal cancer and Lynch syndrome, yet the potential impacts of variants upon reproduction are unclear. To determine if MLH1/3 variants (namely single nucleotide polymorphisms, or SNPs) in human populations can cause reproductive abnormalities, we used a combination of computational predictions, yeast two-hybrid assays, and assays of MMR and recombination in yeast to select nine MLH1 and MLH3 variants for modeling in mice via genome editing. We identified 7 alleles that caused reproductive defects in mice including subfertility in females, male infertility, reduced sperm counts, and increased spermatocyte apoptosis. Remarkably, these alleles in females caused age-dependent decreases in litter size, and increased resorption of embryos during pregnancy. These outcomes were likely a consequence of reduced meiotic chiasmata, in turn causing an increase in misaligned chromosomes and univalents in meiotic metaphase I (MI). Our data indicate that segregating hypomorphic alleles of meiotic recombination genes in populations can predispose females to increased incidence of pregnancy loss from gamete aneuploidy.

In vivoversusin silicoassessment of potentially pathogenic missense variants in human reproductive genes

“…Using this approach, we tested several missense variants in genes important for meiosis, finding that a substantial fraction had no clear impact on fertility [5][6][7][8] . To increase the likelihood that missense variants selected for mouse modeling impact fertility, we added two in vitro pre-screens to our selection pipeline [9][10][11] . The first prescreen was to prioritize VUS that disrupt a known protein-protein interaction (PPI), because human disease mutations are overrepresented for such variants 12 , and the second was to assess protein stability defects in cultured cells 10 .…”