Tumour evolution depends on heritable differences between cells in traits affecting cell survival or replication. It is well-established that cancer cells are genetically and phenotypically heterogeneous; however, the extent to which this phenotypic variation is heritable is far less well-explored.  Here we estimate the broad-sense heritability (H2) of two cell traits related to cancer hallmarks - cell motility and generation time - within populations of four cancer cell lines in vitro, and find that motility is strongly heritable. This heritability is stable across multiple cell generations, with heritability values at the high end of those measured for a range of traits in natural populations of animals or plants. These findings confirm a central assumption of cancer evolution, provide a first quantification of the evolvability of key traits in cancer cells, and indicate that there is ample raw material for experimental evolution in cancer cell lines. Generation time, a trait directly affecting cell fitness, shows substantially lower values of heritability than cell speed, consistent with its having been under directional selection removing heritable variation.

Experiments were conducted over 7 months to record the motility and generation time of four cancer cell lines - HeLa, HT1080, MCF7 and MDA-MB-231 - in cell culture. Cell lines were cultured in vitro on a well plate for 24 hours before being placed into the environmental chamber of a Nikon TiE Timelapse Microscope. Multiple points within every well were chosen and videoed over a period of 72 hours. These videos were then analysed and cell motility and division tracked using MtrackJ and ImageJ. Records were kept of which cells divided and the track number assigned to each daughter cell. This allowed comparison between and within cell families so the heritability of the cell traits, motility and generation time, could be estimated. Broad sense heritability was estimated as the slope parameter of an ordinary least squares regression between related cells. Three cell-cell relationships were tested, mother-daughter, sister-sister and cousin-cousin. Our results showed that cell motility has high and significant heritability for all cell lines tested whereas generation time showed little or no significant heritability for the cell lines tested. This is consistent with expectations that traits closely linked to fitness will in general have lower heritability. These results are important for cancer evolution as heritability (a prerequisite for natural selection to be able to act) has never before been measured for any traits in cancer cells.

Measurements of broad sense heritability of generation time and motility in cancer cell lines