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

Abstract

Embryonic aneuploidy from mis-segregation of chromosomes during meiosis causes pregnancy loss. Proper disjunction of homologous chromosomes requires the mismatch repair (MMR) genes MLH1 and MLH3, essential in mice for fertility. Variants in these genes can increase colorectal cancer risk, yet the reproductive impacts are unclear. To determine if MLH1/3 single nucleotide polymorphisms (SNPs) in human populations could cause reproductive abnormalities, we use computational predictions, yeast two-hybrid assays, and MMR and recombination assays in yeast, selecting nine MLH1 and MLH3 variants to model in mice via genome editing. We identify seven alleles causing reproductive defects in mice including female subfertility and male infertility. Remarkably, in females these alleles cause age-dependent decreases in litter size and increased embryo resorption, likely a consequence of fewer chiasmata that increase univalents at meiotic metaphase I. Our data suggest that hypomorphic alleles of meiotic recombination genes can predispose females to increased incidence of pregnancy loss from gamete aneuploidy.

A novel recombination protein C12ORF40/REDIC1 is required for meiotic crossover formation

“…Defects in crossover formation can result in meiotic arrest or aneuploidy, ultimately damaging human fertility 42,43 . In this study, we, for the first time, identified a homozygous frameshift mutation in C12orf40, a previously uncharacterized gene, from two genetically unrelated azoospermic men with meiotic arrest and revealed its essential role in the formation of class I crossovers.…”

Genomic instability and the link to infertility: A focus on microsatellites and genomic instability syndromes

“…In MLH3 R1230H/R1230H male mice, testicular size was reduced with complete sperm absence. Screening for MSI within mutant mice did not reveal MSI occurrences but data were not shown [93]. Previous work suggests mice that Do not have a MLH1 functional gene, have MSI and show increases in morbidity, lymphoma development, and different gastrointestinal related tumours [94].…”

“…Singh et al (ref 93), used computational analysis in different human genomic databases to investigate variants associated with MLH1 (E268G, K618E, V326A and V716M) and MLH3 (A3914T, R93Q and R1230H). These variants were then created in mice for phenotypic analysis [93]. To summarise the results, MLH3 41230H males were completely infertile due to spermatocyte arrest and reduced testicular size, whilst in female mice, MLH1 E268G , MLH1 V326A , MLH1 K618E , MLH1 K618T , and MLH1 V716M , produced offspring where the size of subsequent litters reduced over time.…”

“…MLH1 K618T/K618T females had fewer follicles, thus, suggesting chromosomal crossover events during meiosis were reduced. Further effects reported included that decreased chromosomal crossover events lead to increased aneuploidy and embryo loss, however the exact mechanism remains unanswered [93]. In MLH3 R1230H/R1230H male mice, testicular size was reduced with complete sperm absence.…”

Pericentric inversion of chromosome 6 and male fertility problems

“…Recently, Tang et al [ 2 ] reported that variants of homozygous helicase for meiosis 1 are responsible for spermatogenic failure. MLH3 single nucleotide polymorphisms in human populations can lead to male infertility [ 3 , 4 , 5 ]. The breakpoints of chromosome structural rearrangement could result in additional alterations, such as gene disruption or position effect, which ultimately lead to male infertility [ 6 ].…”