Let's talk about sex chromosomes

Working with iPS cells has numerous advantages compared with other methods. For one thing, you use adult cells to make iPS cells, so you no longer need embryos. Furthermore, the reprogrammed cells have the patient's DNA, so the body will be less likely to reject them.

"Cell reprogramming is revolutionising medicine, yet surprisingly little is known about how it actually works," says stem cell expert Vincent Pasque, assistant professor at KU Leuven, Belgium.

Together with Kathrin Plath from UCLA, Vincent Pasque from KU Leuven led an international study into how adult cells reprogram to iPS cells. To investigate the different stages of the reprogramming process, the researchers separated male and female cells to examine them side by side.

The researchers discovered that female and male cells behave differently after the reprogramming process and that this is due to their different number of X chromosomes -- two in female cells and one in male cells.

Vincent Pasque explains: "In a normal situation, one of the two X chromosomes in female cells is inactive. But when these cells are reprogrammed into iPS cells, the inactive X becomes active. So the female iPS cells now have two active X chromosomes, while males have only one. We now know that this explains the different behaviour of male and female cells after reprogramming because, over time, female cells lose one of the two X chromosomes and start behaving more or less like male iPS cells."

The differences between male and female iPS cells are most pronounced for one of the key processes in the field of epigenetics: DNA methylation, a modification that changes the activity of a DNA segment without changing its sequence. The researchers found that DNA methylations are erased in female iPS cells but not in male cells.

"Our study also indicates that the path towards iPS cells involves the use of specific control switches on DNA called enhancers," adds Dr Constantinos Chronis from UCLA.

Vincent Pasque concludes: "Our results show that studying male and female cells separately is key to a better understanding of how iPS cells are made. And we really need to understand the process if we want to create better disease models and to help the millions of patients waiting for more effective treatments."