Paralog dependency indirectly affects the robustness of human cells

Gene duplicates provide protection against loss-of-function mutations. This protective redundancy partly relies on the fact that paralogs carry their functions independently, i.e. the inactivation of one gene copy does not impair the function of the other copy. However, a significant fraction of paralogous proteins may form functionally dependent pairs, for instance through heteromerization. As a consequence, one could expect these heteromeric paralogs to be less protective against deleterious mutations. To test this hypothesis, we examined the fitness landscape of gene loss-of-function by CRISPR-Cas9 in more than 450 human cell lines. Our analysis revealed a robustness landscape of human cells showing regions of higher vulnerability to gene inactivation as a function of key paralog properties. We find that heteromerizing paralogs are indeed less protective than non-heteromeric ones, but this association is largely due to their higher abundance and their larger number of protein-protein interaction partners.