Polycomb proteins form chromatin-modifying complexes that implement transcriptional silencing in higher eukaryotes. Hundreds of genes are silenced by Polycomb proteins, including dozens of genes that encode crucial developmental regulators in organisms ranging from plants to humans. Two main families of complexes, called Polycomb repressive complex 1 (PRC1) and PRC2, are targeted to repressed regions. Recent studies have advanced our understanding of these complexes, including their potential mechanisms of gene silencing, the roles of chromatin modifications, their means of delivery to target genes and the functional distinctions among variant complexes. Emerging concepts include the existence of a Polycomb barrier to transcription elongation and the involvement of non-coding RNAs in the targeting of Polycomb complexes. These findings have an impact on the epigenetic programming of gene expression in many biological systems.

Polycomb group (PcG) proteins maintain transcriptional repression during development, likely by creating repressive chromatin states. The Extra Sex Combs (ESC) and Enhancer of Zeste [E(Z)] proteins are partners in an essential PcG complex, but its full composition and biochemical activities are not known. A SET domain in E(Z) suggests this complex might methylate histones. We purified an ESC-E(Z) complex from Drosophila embryos and found four major subunits: ESC, E(Z), NURF-55, and the PcG repressor, SU(Z)12. A recombinant complex reconstituted from these four subunits methylates lysine-27 of histone H3. Mutations in the E(Z) SET domain disrupt methyltransferase activity in vitro and HOX gene repression in vivo. These results identify E(Z) as a PcG protein with enzymatic activity and implicate histone methylation in PcG-mediated silencing.

Sunlight is a carcinogen to which everyone is exposed. Its UV component is the major epidemiologic risk factor for squamous cell carcinoma of the skin. Of the multiple steps in tumor progression, those that are sunlight-related would be revealed if they contained mutations specific to UV. In a series of New England and Swedish patients, we find that 14/24 (58%) of invasive squamous cell carcinomas of the skin contain mutations in the p53 tumor suppressor gene, each altering the amino acid sequence. Involvement of UV light in these p53 mutations is indicated by the presence in three of the tumors of a CC --TT double-base change, which is only known to be induced by UV. UV is also implicated by a UV-like occurrence of mutations exclusively at dipyrimidine sites, including a high frequency of C --T substitutions. p53 mutations in internal malignancies do not show these UVspecific mutations. The dipyrimidine specificity also implicates dipyrimidine photoproducts containing cytosine as oncogenic photoproducts. We believe these results identify a carcinogenrelated step in a gene involved in the subsequent human cancer.The frequency of skin cancers induced by sunlight in the United States approaches that of all other cancers combined and is doubling each decade (1-3). Ninety-five percent of these are non-melanoma skin cancers, resulting in one-third as many deaths as melanoma (4).Epidemiology has identified causal agents for many human cancers, including skin cancer (5), and reagents such as retroviruses have revealed genes that become oncogenic when mutated (6). Yet, the events between a human carcinogen and the human tumor mutations are unknown. Several questions central to oncology converge on these missing events. In the case of squamous cell carcinoma of the skin, they include the wavelength oflight, the gene that absorbs the photon, the DNA photoproduct, the contribution of mutagenesis versus systemic effects of sunlight such as immunosuppression, the type of mutation, possible hotspot sequences, and confidence that the genetic alterations observed in a tumor participated in tumor formation.Squamous cell carcinoma of the skin is an ideal cancer for determining which of the multiple steps in tumorigenesis are carcinogen-related for the following reasons.(i) The carcinogen is known; in lightly pigmented individuals of all races, the majority of skin cancers are due to sunlight. Of these, squamous cell carcinoma is more sunlightdependent than basal cell carcinoma or melanoma (1, 7). This carcinogen is physically well-defined, whereas agents such as tobacco smoke are complex mixtures.(ii) UV light produces distinctive mutations, leaving a "signature" in the DNA. Mutations due to direct absorption of UV light by DNA are predominantly C --T transitions at dipyrimidine sites, including CC --TT double-base mutations, in organisms from viral to human (refs. 8-12 and references therein). Because CC -* TT base changes are only known to be caused by UV, their presence identifies UV as the mutagen. The appearance of C...

Squamous cell carcinoma of the skin (SCC) can progress by stages: sun-damaged epidermis, with individual disordered keratinocytes; actinic keratosis (AK), spontaneously regressing keratinized patches having aberrant cell differentiation and proliferation; carcinoma in situ; SCC and metastasis. To understand how sunlight acts as a carcinogen, we determined the stage at which sunlight mutates the p53 tumour-suppressor gene and identified a function for p53 in skin. The p53 mutations induced by ultraviolet radiation and found in > 90% of human SCCs were present in AKs. Inactivating p53 in mouse skin reduced the appearance of sunburn cells, apoptotic keratinocytes generated by overexposure to ultraviolet. Skin thus appears to possess a p53-dependent 'guardian-of-the-tissue' response to DNA damage which aborts precancerous cells. If this response is reduced in a single cell by a prior p53 mutation, sunburn can select for clonal expansion of the p53-mutated cell into the AK. Sunlight can act twice: as tumour initiator and tumour promoter.