The linker histone H1.0 generates epigenetic and functional intratumor heterogeneity.

INTRODUCTION
Cancer arises from clonal expansion of a single cell. Yet, most human cancers are characterized by extensive intratumor heterogeneity and comprise various subpopulations of cells with distinct phenotypes and biological properties. Intratumor heterogeneity poses major challenges in understanding cancers, managing patients, and designing effective treatment strategies. Functional heterogeneity within individual tumors is partly due to the presence of genetically distinct subclonal cell populations. Furthermore, interactions between cancer cells and the tumor microenvironment can alter the phenotype of cancer cells via nongenetic mechanisms. The combination of cell-intrinsic and cell-extrinsic changes occurring during tumor growth generates functionally distinct subsets of cells that differentially contribute to tumor maintenance.

RATIONALE
In many cancers, phenotypic and functional heterogeneity can be mapped to distinct differentiation states, suggesting that cellular hierarchies established during tumor growth may affect the long-term proliferative potential of cancer cells. To shed light on the mechanisms responsible for the generation of these hierarchies, we searched for epigenetic mechanisms that determine which cancer cells can preserve unlimited proliferative potential, and thus the ability to drive long-term tumor growth, and which cells lose this ability through a differentiation process.

RESULTS
We found that, in several cancer types, individual tumors exhibit high heterogeneity of the major chromatin protein linker histone H1.0, showing strongly reduced H1.0 levels in cells characterized by long-term self-renewal ability and tumorigenic potential and higher levels in nontumorigenic cells. Combined analysis of pan-cancer patient data sets and experimental alteration of the H1F0 locus in tumor cells revealed that heterogeneous H1.0 expression patterns are partly due to differential methylation of an enhancer region that dynamically modulates H1.0 expression within tumors. Using a controlled system to model functional intratumor heterogeneity, we showed that maintenance of cell tumorigenic potential required silencing of H1.0 to avoid loss of unlimited proliferative capacity through differentiation. Mechanistically, absence of H1.0 led to destabilization of nucleosome-DNA interactions in AT-rich genomic regions and coordinated derepression of large sets of neighboring genes, resulting in activation of transcriptional programs that support cancer cell self-renewal. Gene expression changes induced by H1.0 loss were reversible, and epigenetic states restricting cell proliferative potential were reestablished upon H1.0 reexpression. In multiple cancer types, in agreement with the observed inhibition of cancer cell self-renewal by H1.0, patients expressing overall strongly reduced levels of H1.0 showed a significantly worse outcome than patients expressing higher H1.0 levels.

CONCLUSION
Intratumor heterogeneity has emerged as a general feature of cancer, but the molecular features underlying functionally diverse cellular phenotypes have been elusive. Our results uncover epigenetic determinants of tumor-maintaining cells and identify an integral component of chromatin as an important regulator of cell differentiation states within tumors. We propose that only cells insensitive to extracellular differentiation cues, capable of permanently silencing H1.0, can act as self-renewing tumor-maintaining cells and that such a mechanism supports maintenance of several types of cancer. Our results suggest that intervention aimed at restoring high levels of H1.0 in all cancer cells may enhance the differentiation process that naturally occurs during tumor growth and may be beneficial for therapeutic purposes.