Hardman, E. | bioRxiv Science

bioRxiv2020

Maternal iron deficiency perturbs embryonic cardiovascular development

Congenital heart disease (CHD) is the most common type of birth defect, with a global prevalence of 0.9% of live births1. Most research in the last 30 years has focused on finding genetic causes of CHD. However, despite the association of over 100 genes with CHD, mutations in these genes only explain ~30% of cases2. Many of the remaining cases of CHD are caused by in utero exposure to environmental factors3. Here we have identified a completely new environmental teratogen causing CHD: maternal iron deficiency. In humans, iron deficiency anaemia is a major global health problem. 38% of pregnant women worldwide are anaemic4, and at least half of these are due to iron deficiency, the most prevalent micronutrient deficiency. We describe a mouse model of maternal iron deficiency anaemia that causes severe cardiovascular defects in her offspring. We show that these defects likely arise from increased retinoic acid signalling in iron deficient embryos, probably due to reduced activity of the iron-dependent retinoic acid catabolic CYP26 enzymes. The defects can be prevented by maternal iron administration early in pregnancy, and are also greatly reduced in offspring of mothers deficient in both iron and the retinoic acid precursor vitamin A. Finally, one puzzling feature of many genetic forms of CHD in humans is the considerable variation in penetrance and severity of defects. We show that maternal iron deficiency acts as a significant modifier of heart and craniofacial phenotype in a mouse model of Down syndrome. Given the high incidence of maternal iron deficiency, peri-conceptional iron monitoring and supplementation could be a viable strategy to reduce the prevalence and severity of CHD in human populations worldwide.

developmental biology↗

bioRxiv2020

Frequent loss-of-heterozygosity in CRISPR-Cas9-edited early human embryos

CRISPR-Cas9 genome editing is a promising technique for clinical applications, such as the correction of disease-associated alleles in somatic cells. The use of this approach has also been discussed in the context of heritable editing of the human germline. However, studies assessing gene correction in early human embryos report low efficiency of mutation repair, high rates of mosaicism and the possibility of unintended editing outcomes that may have pathologic consequences. We developed computational pipelines to assess single-cell genomics and transcriptomics datasets from OCT4 (POU5F1) CRISPR-Cas9-targeted and control human preimplantation embryos. This allowed us to evaluate on-target mutations that would be missed by more conventional genotyping techniques. We observed loss-of-heterozygosity in edited cells that spanned regions beyond the POU5F1 on-target locus, as well as segmental loss and gain of chromosome 6, on which the POU5F1 gene is located. Unintended genome editing outcomes were present in approximately 16% of the human embryo cells analysed and spanned 4 to 20kb. Our observations are consistent with recent findings indicating complexity at on-target sites following CRISPR-Cas9 genome editing. Our work underscores the importance of further basic research to assess the safety of genome editing techniques in human embryos, which will inform debates about the potential clinical use of this technology.

developmental biology↗