The transposon LINE1 is an essential regulator in early human development, researchers have found.

Transposons are DNA transposable elements known as 'jumping genes', for they can move from one location to another on the genome, and can potentially cause mutations when they land inside a gene. However, a study published in Developmental Cell has found that LINE1 transposable elements are essential to regulate a fundamental transition in early human development, not allowing human embryonic stem cells to reverse to a previous developmental stage.

'Here we have discovered that these elements are not mere genomic parasites but are essential for early development', said senior co-author Dr Miguel Ramalho-Santos, senior investigator at the Lunenfeld-Tanenbaum Research Institute in Toronto, Canada.

The researchers inhibited the expression of LINE1 in cultured human embryonic stem cells (hESC) and found that these cells reversed to a more primitive eight-cell stage. At this stage, the eight cells are identical and totipotent, meaning they can develop into both the embryo and placenta. Conversely, beyond this stage, hESCs can still form all fetal cells, but become less able to contribute to the placenta.

The authors also uncovered that LINE1 RNA and PRC2, a multiprotein complex essential for embryonic development, have a role in organising the DNA in the 3D space of the cell's nucleus. They help move chromosome 19, where key genes for the eight-cell stage are located, to a gene-silencing region of the nucleus, ensuring the embryo can progress smoothly to the subsequent stages.

'We show that LINE1 regulates gene expression at a crucial turning point where the embryo starts to specialise its cells for various functions. Our results indicate that this not an accidental occurrence but a vital evolutionary mechanism', said Dr Juan Zhang, a senior co-author and postdoctoral fellow at the Lunenfeld-Tanenbaum Research Institute.

This study sheds light on the role of LINE1 in developmental progression and its importance in early human growth, contrasting with its typical behaviour of causing harmful mutations that lead to diseases, such as haemophilia and cancer.  Thus, these findings could have implications in fertility treatments and in the use of stem cells in regenerative medicine.