The immune system's mutation machine finally gets its safety manual

To fight infections, your B cells deliberately mutate their own DNA. The enzyme AID rewrites antibody genes to build better antibodies — a dangerous trick that has kept scientists guessing for more than 20 years: how does the enzyme hit the right genes while sparing thousands of others? Now a team at the Montreal Clinical Research Institute, led by Université de Montréal professor Javier Di Noia, has found the gatekeepers: two proteins, MLLT1 and MLLT3, that gather AID into tiny molecular compartments right where it is needed. Regions rich in the two proteins matched precisely where the mutations landed, in both animal models and human cells. Removing both proteins made the mutations virtually disappear — even though the enzyme was still there. Published in Nature, the discovery also opens a therapeutic path: AID misfires can drive lymphomas and leukemias, and molecules that inhibit MLLT1 and MLLT3 reduced cancer-linked mutations in experiments. "Only regions of the genome that accumulate sufficient levels of MLLT1 and MLLT3 can concentrate AID enough to enable efficient mutation," said Di Noia.