Building a limb is as much about switching genes off as switching them on. Early in limb formation, a first wave of genes must fire to launch the process, then fall silent quickly so that later developmental programmes can unfold. A study published in the Proceedings of the National Academy of Sciences by Marie Kmita, professor of medicine at Université de Montréal and researcher at the Montreal Clinical Research Institute (IRCM), identifies the machinery that applies these genetic brakes.
Working in mice, Kmita's team examined two Polycomb complexes, PRC1 and PRC2, which cooperate to silence genes. Disrupting either system alone was enough to produce abnormal gene expression in the developing limb. Disrupting both at once was far more consequential: the early genes stayed switched on past their allotted time and normal limb development was severely compromised, demonstrating that the two complexes act as redundant, cooperating safeguards.
The finding illustrates a broader principle of embryonic life: cells must not only activate the right genes at the right moment but also remember which genes to keep off. Errors in this silencing machinery could help explain the origin of some congenital malformations. The team's next step is to identify the signals that guide Polycomb complexes to the correct genes at the correct times, a question with implications well beyond the limb.