DNA regions that do not code for proteins, previously termed 'junk DNA', constitute the majority of the mammalian genome and were previously considered non-essential for organism survival. However, new research published in the journal Cell identified that a specific type of non-coding DNA, called a 'transposon', is an essential gene regulator implicated in mouse survival and embryo implantation.
'A lot of interesting studies illustrate that transposons are a driving force of human genome evolution. Yet, this is the first example that I know of where deletion of a piece of junk DNA leads to a lethal phenotype, demonstrating that the function of specific transposons can be essential', said the senior co-author of the study Professor Ting Wang at Washington University, Missouri.
Transposons represent ancient viral DNA that has been incorporated into mammalian DNA. In the study, the researchers identified a specific transposon, MT2B2, that acts as a promoter for the mouse gene Cdk2ap1, whose expression in turn increases cell proliferation and speeds the timing of embryo implantation in the uterus. By disabling the MT2B2 promoter, the Cdk2ap1 gene only existed in its longer protein form, which resulted in embryonic lethality in about half of cases due to impaired implantation, often in the wrong part of the uterus.
While transposons are generally specific to individual species, the researchers found that different species-specific transposon families were turned on briefly before implantation in eight mammals – human, rhesus monkey, marmoset, mouse, goat, cow, pig and opossum.
'What's amazing is that different species have largely different transposons that are expressed in preimplantation embryos, but the global expression profiles of these transposons are nearly identical among all the mammalian species,' commented the senior co-author Professor Lin He at University of California, Berkeley, suggesting that viral DNA could play an important role in early embryonic development in all mammals.
These findings could have implications for human infertility. Nearly half of miscarriages do not have a clear genetic component, but perhaps transposons might play a role in human reproduction.