Somatic evolution of cancer genes in sex chromosomes

Genes on sex chromosomes have higher evolutionary rates than those on autosomes. However, this does not necessarily apply to somatic evolution in cancer. Many dominant mutations have been described in the so-called proto-oncogenes (OGs), while recessive mutations are typically described in tumor-suppressor genes (TSGs). Evidence indicates that mutations in X-chromosome TSGs are more likely to contribute to cancer than those in autosomal TSGs. Here, we formalize this in several dynamic models and predict, as expected, that mutations spread faster in TSGs located on the X chromosome than on autosomes (faster-X effect). Conversely, mutations in OGs spread faster on autosomes than on the X chromosome, but under high selective pressure, this difference is negligible. Published genomic screenings of cancer samples show evidence of the faster-X effect in TSGs. This pattern is observed in both sexes, suggesting that the maintenance of X-chromosome inactivation during cancer progression plays an important role in the evolution of TSGs. Strikingly, the relative mutation incidence in X-linked TSGs among females across individual studies is bimodal, with one group of studies showing a faster-X effect and another group showing similar incidences for X-linked and autosomal TSGs. This differentiation between cancer samples is not associated with the specific type of cancer or the tissue of origin. This may indicate that X-chromosome inactivation plays a differential role in the involvement of X-linked TSGs across individual cancers.