“Superior Genome” May Allow Down Syndrome Fetuses to Survive Pregnancy with Average Life Expectancy of 60 Years–Authors Propose That Embryonic Selection for Reduced Burden of Slightly Deleterious Variants Permits Survival of Extremely Deleterious Variant

Trisomy 21 is a serious genetic disorder, with four pregnancies out of five not reaching term naturally if the fetus is affected. However, 20% of conceptuses with Down's syndrome are born live, grow up and many live into their 60s and 70s (the average life span is 60). How is this possible? Researchers from the Universities of Geneva (UNIGE) and Lausanne (UNIL) have found that children born with Down's syndrome have an “excellent” genome – “better” in terms of certain measurable criteria (gene variation, gene regulation, gene expression), in fact, than the average genome of people without the genetic abnormality. It is possible that this “higher-quality” genome offsets the disabilities caused by the extra chromosome, helping the fetus to survive and the child to grow and develop. In their abstract of an article published in the January 2018 issue of Genome Research, the researchers say the following. “Here, we investigate if the survival probability of aneuploid fetuses is affected by the genome-wide burden of slightly deleterious variants. We analyzed two cohorts of live-born Down syndrome individuals (388 genotyped samples and 16 fibroblast transcriptomes) and observed a deficit of slightly deleterious variants on chromosome 21 and decreased transcriptome-wide variation in the expression level of highly constrained genes. We interpret these results as signatures of embryonic selection, and propose a genetic handicap model whereby an individual bearing an extremely severe deleterious variant (such as aneuploidy) could escape embryonic lethality if the genome-wide burden of slightly deleterious variants is sufficiently low.
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