Fibroblasts are an essential cell population for human skin architecture and function. While fibroblast heterogeneity is well established, this phenomenon has not been analyzed systematically yet. We have used single-cell RNA sequencing to analyze the transcriptomes of more than 5,000 fibroblasts from a sun-protected area in healthy human donors. Our results define four main subpopulations that can be spatially localized and show differential secretory, mesenchymal and pro-inflammatory functional annotations. Importantly, we found that this fibroblast 'priming' becomes reduced with age. We also show that aging causes a substantial reduction in the predicted interactions between dermal fibroblasts and other skin cells, including undifferentiated keratinocytes at the dermal-epidermal junction. Our work thus provides evidence for a functional specialization of human dermal fibroblasts and identifies the partial loss of cellular identity as an important age-related change in the human dermis. These findings have important implications for understanding human skin aging and its associated phenotypes.

Mutations and expression changes of epigenetic modifiers are pervasive in human tumours, making epigenetic factors attractive anti-tumour targets. The open-versus-closed chromatin state within cancer cells-of-origin correlates with the uneven distribution of mutations. However, the long-term effect on mutability of targeting epigenetic modifiers in cancer patients is unclear. Here we show that increasing chromatin accessibility by deleting histone H3 lysine 9 (H3K9) methyltransferase G9a in murine epidermis does not alter the singlenucleotide variants (SNVs) burden or global genomic distribution in chemical mutagen-induced squamous tumours. G9a-depleted tumours developed after a prolonged latency compared to their wild-type counterparts but were more aggressive and had an expanded cancer progenitor pool, pronounced genomic instability and frequent loss-of-function p53 mutations. Thus, we call for caution when assessing long-term therapeutic benefits of chromatin modifier inhibitors, which may promote more aggressive disease.