Homologous recombination, cancer and the ‘RAD51 paradox’

Genetic instability is a hallmark of cancer cells. Homologous recombination (HR) plays key roles in genome stability and variability due to its roles in DNA double-strand break and interstrand crosslink repair, and in the protection and resumption of arrested replication forks. HR deficiency leads to genetic instability, and, as expected, many HR genes are downregulated in cancer cells. The link between HR deficiency and cancer predisposition is exemplified by familial breast and ovarian cancers and by some subgroups of Fanconi anaemia syndromes. Surprisingly, although RAD51 plays a pivotal role in HR, i.e., homology search and in strand exchange with a homologous DNA partner, almost no inactivating mutations of RAD51 have been associated with cancer predisposition; on the contrary, overexpression of RAD51 is associated with a poor prognosis in different types of tumours. Taken together, these data highlight the fact that RAD51 differs from its HR partners with regard to cancer susceptibility and expose what we call the ‘RAD51 paradox’. Here, we catalogue the dysregulations of HR genes in human pathologies, including cancer and Fanconi anaemia or congenital mirror movement syndromes, and we discuss the RAD51 paradox.