Abstract 698: Variability in xenograft growth rates can be explained by intra-tumor evolutionary dynamics
Authors: Zopf Christopher J. , Chen Andrew , Patel Mayank , Palani Santhosh , Bandi Syamala , Blair Derek , Shyu Wen Chyi , Chakravarty Arijit 
Published: May 2015

Abstract
Growing evidence frames cancer as a disease of somatic Darwinian evolution. Recent reports establish both inter- and intra-tumor genetic heterogeneity within patients, and evolution in response to treatment. Cell lines and xenografts also display genetic heterogeneity arising from chromosomal instability, and evolve in response to experimental conditions. Although selection pressures in culture or in a mouse differ from those in a patient, studying the evolutionary process pre-clinically may yield insights useful in the design of experimental systems of cancer.
Using the soft agar colony growth assay, we previously showed that individual colonies in soft agar have widely divergent growth rates that are mostly inherited over experimental timescales. In this work we ask what the impact of that divergence is on xenograft growth kinetics, and examine the implications for the development of resistance.
First, we applied a stochastic, dynamic model of tumor kinetics to simulate clonal heterogeneity in xenograft growth rates. Each simulated “virtual tumor” consisted of a set of independently growing subclone cells, whose growth rates were bootstrapped from a distribution derived from 104 individual HCT-116 colonies in the soft agar colony growth assay. Both the sampling distribution variance and the number of subclones sampled to found each xenograft were varied to test the effect of intercell heterogeneity and starting population size on overall xenograft growth rate. We simulated tumor growth profiles over both a two-week implant and a three-week observation period to reflect experimental conditions. Three stochastic simulation scenarios were considered for possible cellular events: birth only; birth and death; and birth, death, and mutation.
We compared the simulation results to an experimentally determined distribution derived from 308 individual HCT-116 xenograft tumors. Our findings suggest the variability of whole tumors is related to the clonal growth rate diversity within the HCT-116 cell line. In particular, the observed xenograft growth rate heterogeneity can be explained entirely by a scenario where many orders of magnitude fewer cells survive to initiate each tumor than the millions of cells implanted experimentally. This finding is consistent with previously published experimental results that have been interpreted to suggest the existence of cancer stem cells. We demonstrate the small number of founder cells in each xenograft leads to evolutionary drift under treatment conditions, when a resistant mutant may come to dominate stochastically due to the small population size.We then extend this work to ask questions about experimental design - how to develop a xenograft model system that is either more reproducible (less heterogeneity) or more heterogeneous (less reproducible). The latter system may be particularly valuable in studying the emergence of resistance.
Citation Format: Christopher J. Zopf, Andrew Chen, Mayank Patel, Santhosh Palani, Syamala Bandi, Derek Blair, Wen Chyi Shyu, Arijit Chakravarty. Variability in xenograft growth rates can be explained by intra-tumor evolutionary dynamics. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 698. doi:10.1158/1538-7445.AM2014-698