Maternal mutations in human NLRP7 cause pregnancies recurrent hydatidiform molar pregnancies with imprinting defects. Maternal mutations in its highly homologous neighboring gene NLRP2, cause a multi-locus imprinting disorder (MLID) that manifests as Beckwith-Wiedemann syndrome in offspring. Both are associated with abnormal DNA methylation of maternally imprinted genes. Rodents have the Nlrp2 gene, but no Nlrp7. We hypothesized that Nlrp2 may combine functions of both human homologs and generated a Nlrp2-mutant mouse model to study the mechanisms by which its maternal inactivation causes the observed offspring and placental abnormalities. We found that NLRP2 protein is a new member of the subcortical maternal complex (SCMC), a cytoplasmic complex in oocytes that persists in preimplantation embryos with a presumed role in maternal-to- zygote transition and zygotic genome activation, which are the processes by which embryos switch from reliance on maternally contributed transcripts and proteins to their own transcription and translation. We found that a maternal-effect mutation in Nlrp2 disrupts the SCMC, causing Nlrp2-null females to produce fewer and smaller litters with offspring that have birth defects, growth abnormalities, and imprinting defects. In vitro cultured embryos of these Nlrp2-null females have a more severe phenotype with early cleavage-stage arrest. These data for the first time link the SCMC to imprinting reprogramming and indicate that Nlrp2-null mice are an excellent model to study the mechanisms of this interaction. They support the overarching goal of this project, to characterize how maternal loss of NLRP2, a new SCMC protein, alters imprinting in offspring, and how this leads to a range of reproductive phenotypes. Towards this goal, for specific aim 1, we will characterize the cellular and developmental events in the zygote and preimplantation embryo that are at the origin of the reproductive and imprinting phenotypes.
For specific aim 2, we will investigate the molecular mechanisms that underlie the reproductive phenotypes of maternal inactivation of Nlrp2 by following up on other interesting early observations, which include that NLRP2 and DNA-methyltransferase 1 (DNMT1) co-localize in preimplantation embryos, and that Nlrp2-deficient oocytes have altered gene expression profiles.
For specific aim 3 we will characterize in vivo the variable later developmental abnormalities of embryos and placentas resulting from maternal loss of Nlrp2. This will increase knowledge on the reproductive disorders caused by maternal effect mutations. All phenotyping will be complemented by transcriptomics, methylomics and profiling of DNMT1 binding to fully integrate molecular and phenotypic data. We predict that this work will uncover important new principles of genome reprogramming during gametogenesis and after fertilization and will clarify how the SCMC affects this process. The long-term benefit for human health will include better understanding of multi-locus imprinting disorders and some forms of infertility and in vitro fertilization failures.
Maternal mutations in the human highly-homologous genes NLRP7 and NLRP2 cause either pregnancies with abnormal placentas or offspring with birth defects, that both are associated with abnormalities in how imprinted genes are regulated. We have generated a mouse model with mutations in Nlrp2 and propose a detailed characterization of these mice to examine how loss of function of this gene in females causes these abnormalities. This will increase general knowledge of oocyte and early embryonic development, switching of imprinting in the germline and early embryo, and how disruption of these processes causes infertility, abnormal pregnancies and children with imprinting disorders.
