Genetic Diversity in the IZUMO1-JUNO Protein-Receptor Pair Involved in Human Reproduction
Authors: Allingham Jessica , Floriano Wely B. 
Published: August 2021

Fertilization in mammals begins with the union of egg and sperm, an event that starts a cascade of cellular processes. The molecular-level understanding of these processes can guide the development of new strategies for controlling and/or promoting fertilization, and inform researchers and medical professional on the best choice of interventions. The proteins encoded by the IZUMO1 and JUNO genes form a ligand-receptor protein pair involved in the recognition of sperm and egg. Due to their role in the fertilization process, these proteins are potential targets for the development of novel anti-contraceptive, as well as infertility treatments. Here we present a comprehensive analysis of these gene sequences, with the objective of identifying evolutionary patterns that may support their relevance as targets for preventing or improving fertility among humans. JUNO and IZUMO1 gene sequences were identified within the genomes of over 2,000 humans sequenced in the 1000 Genomes Project. The human sequences were subjected to analyses of nucleotide diversity, selection neutrality, population-based differentiation (FST), haplotype inference, and whole chromosome scanning for signals of positive or of balancing selection. Derived alleles were determined by comparison to archaic hominin and other primate genomes. The potential effect of common non-synonymous variants on protein-protein interaction was also assessed. IZUMO1 displays higher variability among human individuals than JUNO. Genetic differentiation between continental population pairs was within whole-genome estimates for all but the JUNO gene in the African population group with respect to the other 4 population groups (American, East Asian, South Asian, and European). Tajima’s D values demonstrated deviation from neutrality for both genes in comparison to a group of genes identified in the literature as under balancing or positive selection. Tajima’s D for IZUMO1 aligns with values calculated for genes presumed to be under balancing selection, whereas JUNO’s value aligned with genes presumed to be under positive selection. These inferences on selection are both supported by SNP density, nucleotide diversity and haplotype analysis. A JUNO haplotype carrying 3 derived alleles out of 5, one of which is a missense mutation implicated in polyspermy, was found to be significant in a population of African ancestry. Polyspermy has a disadvantageous impact on fertility and its presence in approximately 30% of the population of African ancestry may be associated to a potentially beneficial role of this haplotype. This role has not been established and may be related to a non-reproductive role of JUNO. The high degree of conservation of the JUNO sequence combined with a dominant haplotype across multiple population groups supports JUNO as a potential target for the development of contraceptive treatments. In addition to providing a detailed account of human genetic diversity across these 2 important and related genes, this study also provides a framework for large population-based studies investigating protein-protein interactions at the genome level.

AUTHOR SUMMARY

Fertilization in mammals depends on egg and sperm connecting to each other. Two proteins that are essential for this process are egg’s JUNO and sperm’s IZUMO1. These proteins are encoded by genes known as JUNO and IZUMO1, which are present in the genomes of all humans. We analyzed the publicly available genomes of over 2,000 individuals from different continental populations (African (AFR), American (AMR), East Asian (EAS), European (EUR) and South Asian (SAS)) to investigate how well-conserved these genes are among humans, and to look for any signs that genetic variation on these genes could impact the binding of their encoded proteins to one another which, in turn, could impact the fertilization process. We found that the JUNO gene in the African population sampled diverged the most compared to the other population groups, and this divergence was above what is generally observed across the genome. Around 30% of the people of African ancestry investigated here carry a mutation that is associated to the abnormal fertilization of an egg by more than one sperm (known as polyspermy). Why this mutation is so common in the African population should be further investigated. We speculate that this mutation confers a genetic advantage related to the role of this receptor-ligand pair in cells not involved in reproduction.