The extent to which cells in normal tissues accumulate mutations throughout life is poorly understood. Some mutant cells expand into clones that can be detected by genome sequencing. We mapped mutant clones in normal esophageal epithelium from nine donors (age range 20 to 75 years). Somatic mutations accumulated with age and were mainly caused by intrinsic mutational processes. We found strong positive selection of clones carrying mutations in 14 cancer genes, with tens to hundreds of clones per square centimeter. In middle-aged and elderly donors, clones with cancer-associated mutations covered much of the epithelium, with NOTCH1 and TP53 mutations affecting 12 to 80% and 2 to 37% of cells, respectively. Unexpectedly, the prevalence of NOTCH1 mutations in normal esophagus was several times higher than in esophageal cancers. These findings have implications for our understanding of cancer and ageing.

SUMMARY DNA hydroxylation catalyzed by Tet dioxygenases occurs abundantly in embryonic stem cells and neurons in mammals. However, its biological function in vivo is largely unknown. Here we demonstrate that Tet1 plays an important role in regulating neural progenitor cell proliferation in adult mouse brain. Mice lacking Tet1 exhibit impaired hippocampal neurogenesis accompanied by poor learning and memory. In adult neural progenitor cells deficient in Tet1, a cohort of genes involved in progenitor proliferation were hypermethylated and down-regulated. Our results indicate that Tet1 is positively involved in the epigenetic regulation of neural progenitor cell proliferation in the adult brain.

miR-137 is a brain-enriched microRNA. Its role in neural development remains unknown. Here we show that miR-137 plays an essential role in controlling embryonic neural stem cell fate determination. miR-137 negatively regulates cell proliferation and accelerates neural differentiation of embryonic neural stem cells. In addition, we show that histone demethylase LSD1, a transcriptional co-repressor of nuclear receptor TLX, is a downstream target of miR-137. In utero electroporation of miR-137 in embryonic mouse brains led to premature differentiation and outward migration of the transfected cells. Introducing a LSD1 expression vector lacking the miR-137 recognition site rescued miR-137-induced precocious differentiation. Furthermore, we demonstrate that TLX, an essential regulator of neural stem cell self-renewal, represses the expression of miR-137 by recruiting LSD1 to the genomic regions of miR-137. Thus, miR-137 forms a feedback regulatory loop with TLX and LSD1 to control the dynamics between neural stem cell proliferation and differentiation during neural development.

The repressor 8EF1 was discovered by its action on the DC5 fragment of the lens-specific 81-crystallin enhancer. C-proximal zinc fingers of 8EF1 were found responsible for binding to the DC5 fragment and had specificity to CACCT as revealed by selection of high-affinity binding sequences from a random oligonucleotide pool. CACCT is present not only in DC5 but also in the E2 box (CACCTG) elements which are the binding sites of various basic helix-loop-helix activators and also the target of an unidentified repressor, raising the possibility that 8EF1 accounts for the E2 box repressor activity. 8EF1 competed with E47 for binding to an E2 box sequence in vitro. In lymphoid cells, endogenous 8EF1 activity as a repressor was detectable, and exogenous 8EF1 repressed immunoglobulin K enhancer by binding to the KE2 site. Moreover, bEF1 repressed MyoD-dependent activation of the muscle creatine kinase enhancer and MyoD-induced myogenesis of 1OT1/2 cells. Thus, 8lEF1 counteracts basic helix-loop-helix activators through binding site competition and fulfills the conditions of the E2 box repressor. In embryonic tissues, the most prominent site of &;EF1 expression is the myotome. Myotomal expression as well as the above results argues for a significant contribution of 6EF1 in regulation of embryonic myogenesis through the modulation of the actions of MyoD family proteins.Cumulative evidence has indicated that repressors interacting with activators play crucial roles in developmental gene regulation. In the best-studied cases of transcriptional regulatory elements generating developmental specificities, it is found that a binding site of a repressor overlaps with a binding site of an activator so that the regulators of the opposite effects compete for occupancy of the same site (16,28,29,32). In general, the repressor is more widely distributed in spatial and temporal terms than the activators. Thus, in the majority of cell types the repressor occupies the element and shuts off expression of the gene, and only under conditions in which binding of the activator to the element dominates over that of the repressor is expression of the gene turned on. This seems one of the basic mechanisms to elicit stage-specific or cell-typespecific gene expression.An example is found in immunoglobulin enhancers in which E2 box activator binding sites with the core sequence of CACCTG have been identified. A group of basic helix-loophelix (bHLH) activator proteins (25) which are encoded by E2A and E2-2 genes (3) and bind to the E2 site of the immunoglobulin K enhancer (and the ,uE5 site of the immunoglobulin heavy-chain enhancer as well) are in competition with a repressor, and only in differentiated lymphoid cells is the action of the activator effectuated and immunoglobulin K enhancer activated (29). The same scenario holds true for regulatory elements of myocyte-specific genes such as AChRb in which activators containing myogenic MyoD family proteins as their component bind to an E2-box-activating element (32). In these cases, action of the r...

SummaryAging human tissues, such as sun-exposed epidermis, accumulate a high burden of progenitor cells that carry oncogenic mutations. However, most progenitors carrying such mutations colonize and persist in normal tissue without forming tumors. Here, we investigated tissue-level constraints on clonal progenitor behavior by inducing a single-allele p53 mutation (Trp53R245W; p53∗/wt), prevalent in normal human epidermis and squamous cell carcinoma, in transgenic mouse epidermis. p53∗/wt progenitors initially outcompeted wild-type cells due to enhanced proliferation, but subsequently reverted toward normal dynamics and homeostasis. Physiological doses of UV light accelerated short-term expansion of p53∗/wt clones, but their frequency decreased with protracted irradiation, possibly due to displacement by UV-induced mutant clones with higher competitive fitness. These results suggest multiple mechanisms restrain the proliferation of p53∗/wt progenitors, thereby maintaining epidermal integrity.