Comparison of Tug-of-War Models Assuming Moran versus Branching Process Population Dynamics

Mutations arising during cancer evolution are typically categorized as either 'drivers' or 'passengers', depending on whether they increase the cell fitness. Recently, McFarland et al. introduced the tug-of-war model for the joint effect of rare advantageous drivers and frequent but deleterious passengers. We examine this model under two common but distinct frameworks, the Moran model and the branching process. We show that frequently used statistics are similar between a version of the Moran model and the branching process conditioned on the final cell count, under different selection scenarios. We infer the selection coefficients for three breast cancer samples, resulting in good fits of the shape of their Site Frequency Spectra. All fitted values for the selective disadvantage of passenger mutations are nonzero, supporting the view that they exert deleterious selection during tumorigenesis that driver mutations must compensate.