Abstract
The placenta invades the adjacent uterus and controls the maternal immune system, like a cancer invades surrounding organs and suppresses the local immune response. Intriguingly, placental and cancer cells are globally hypomethylated and share an epigenetic phenomenon that is not well understood - they fail to silence repetitive DNA sequences (retrotransposons) that are silenced (methylated) in healthy somatic cells. In the placenta, hypomethylation of retrotransposons has facilitated the evolution of new genes essential for placental function. In cancer, hypomethylation is thought to contribute to activation of oncogenes, genomic instability, and retrotransposon unsilencing; the latter, we postulate, is possibly the most important consequence. Activation of placental retrotransposon-derived genes in cancer underpins our hypothesis that hypomethylation of these genes drives cancer cell invasion. This alludes to an interesting paradox, that while placental retrotransposon-derived genes are essential for promoting early hominid life, the same genes promote disease-susceptibility and death through cancer.