DNA Methylation Alterations Exhibit Intraindividual Stability and Interindividual Heterogeneity in Prostate Cancer Metastases

Prostate tumors develop a unique epigenetic DNA methylation signature that is clonally maintained in disseminated metastases. Surveying the DNA Methylation “Cityscape” of Prostate Cancer Alterations in DNA methylation are a hallmark of human cancers, including prostate cancer. Understanding which of these alterations “drive” cancer initiation, progression, and metastasis, and which of these are merely “passengers” not involved in the chain of causation, is a major translational challenge. To tackle this challenge in the context of metastatic prostate cancer, Aryee et al. carried out genome-scale analyses of DNA methylation alterations in multiple metastases from each of 13 men who had died of metastatic prostate cancer. To visualize both the frequency of each methylation alteration in the metastases and the consistency with which each alteration was maintained across all metastases from an individual, the authors created DNA methylation “cityscape” plots. These analyses revealed that each individual developed a unique DNA methylation signature that was largely maintained across all metastases within that individual. Additionally, a set of DNA “hypermethylation” alterations, defined as regions that were normally unmethylated but acquired cancer-specific DNA methylation, were enriched for prostate cancer “drivers. ” Such DNA hypermethylation alterations are attractive potential targets for development of longitudinal markers and therapeutic strategies for prostate cancer management. Human cancers almost ubiquitously harbor epigenetic alterations. Although such alterations in epigenetic marks, including DNA methylation, are potentially heritable, they can also be dynamically altered. Given this potential for plasticity, the degree to which epigenetic changes can be subject to selection and act as drivers of neoplasia has been questioned. We carried out genome-scale analyses of DNA methylation alterations in lethal metastatic prostate cancer and created DNA methylation “cityscape” plots to visualize these complex data. We show that somatic DNA methylation alterations, despite showing marked interindividual heterogeneity among men with lethal metastatic prostate cancer, were maintained across all metastases within the same individual. The overall extent of maintenance in DNA methylation changes was comparable to that of genetic copy number alterations. Regions that were frequently hypermethylated across individuals were markedly enriched for cancer- and development/differentiation-related genes. Additionally, regions exhibiting high consistency of hypermethylation across metastases within individuals, even if variably hypermethylated across individuals, showed enrichment for cancer-related genes. Whereas some regions showed intraindividual metastatic tumor heterogeneity in promoter methylation, such methylation alterations were generally not correlated with gene expression. This was despite a general tendency for promoter methylation patterns to be strongly correlated with gene expression, particularly at regions that were variably methylated across individuals. These findings suggest that DNA methylation alterations have the potential for producing selectable driver events in carcinogenesis and disease progression and highlight the possibility of targeting such epigenome alterations for development of longitudinal markers and therapeutic strategies.