Approximately one third of all mammalian genes are essential for life. Phenotypes resulting from mouse knockouts of these genes have provided tremendous insight into gene function and congenital disorders. As part of the International Mouse Phenotyping Consortium effort to generate and phenotypically characterize 5000 knockout mouse lines, we have identified 410 lethal genes during the production of the first 1751 unique gene knockouts. Using a standardised phenotyping platform that incorporates high-resolution 3D imaging, we identified novel phenotypes at multiple time points for previously uncharacterized genes and additional phenotypes for genes with previously reported mutant phenotypes. Unexpectedly, our analysis reveals that incomplete penetrance and variable expressivity are common even on a defined genetic background. In addition, we show that human disease genes are enriched for essential genes identified in our screen, thus providing a novel dataset that facilitates prioritization and validation of mutations identified in clinical sequencing efforts.

Adult germline stem cells are capable of self-renewal, tissue regeneration and production of large numbers of differentiated progeny. We show here that the classical mouse mutant luxoid affects adult germline stem cell self-renewal. Young homozygous luxoid mutant mice produce limited numbers of normal spermatozoa and then progressively lose their germ line after birth. Transplantation studies showed that germ cells from mutant mice did not colonize recipient testes, suggesting that the defect is intrinsic to the stem cells. We determined that the luxoid mutant contains a nonsense mutation in the gene encoding Plzf, a transcriptional repressor that regulates the epigenetic state of undifferentiated cells, and showed that Plzf is coexpressed with Oct4 in undifferentiated spermatogonia. This is the first gene shown to be required in germ cells for stem cell self-renewal in mammals.The luxoid (lu) mutation arose spontaneously in 1950 and was initially characterized for its semidominant limb abnormalities and recessive skeletal and male infertility phenotypes 1,2 . A subsequent report suggested that luxoid mutant males had limited short-term production of normal-looking sperm followed by a rapid loss of all Plzf is required in adult male germ cells for stem cell self-renewal

Geographic atrophy (GA), an untreatable advanced form of age-related macular degeneration, results from retinal pigmented epithelium (RPE) cell death. Here we show that the microRNA (miRNA)-processing enzyme DICER1 is reduced in the RPE of humans with GA, and that conditional ablation of Dicer1, but not seven other miRNA-processing enzymes, induces RPE degeneration in mice. DICER1 knockdown induces accumulation of Alu RNA in human RPE cells and Alu-like B1 and B2 RNAs in mouse RPE. Alu RNA is increased in the RPE of humans with GA, and this pathogenic RNA induces human RPE cytotoxicity and RPE degeneration in mice. Antisense oligonucleotides targeting Alu/B1/B2 RNAs prevent DICER1 depletion-induced RPE degeneration despite global miRNA downregulation. DICER1 degrades Alu RNA, and this digested Alu RNA cannot induce RPE degeneration in mice. These findings reveal a miRNA-independent cell survival function for DICER1 involving retrotransposon transcript degradation, show that Alu RNA can directly cause human pathology, and identify new targets for a major cause of blindness.

Kisspeptins are products of the Kiss1 gene, which bind to GPR54, a G protein-coupled receptor. Kisspeptins and GPR54 have been implicated in the neuroendocrine regulation of GnRH secretion. To test the hypothesis that testosterone regulates Kiss1 gene expression, we compared the expression of KiSS-1 mRNA among groups of intact, castrated, and castrated/testosterone (T)-treated male mice. In the arcuate nucleus (Arc), castration resulted in a significant increase in KiSS-1 mRNA, which was completely reversed with T replacement, whereas in the anteroventral periventricular nucleus, the results were the opposite, i.e. castration decreased and T increased KiSS-1 mRNA expression. In the Arc, the effects of T on KiSS-1 mRNA were completely mimicked by estrogen but only partially mimicked by dihydrotestosterone, a nonaromatizable androgen, suggesting that both estrogen receptor (ER) and androgen receptor (AR) play a role in T-mediated regulation of KiSS-1. Studies of the effects of T on KiSS-1 expression in mice with either a deletion of the ERalpha or a hypomorphic allele to the AR revealed that the effects of T are mediated by both ERalpha and AR pathways, which was confirmed by the presence of either ERalpha or AR coexpression in most KiSS-1 neurons in the Arc. These observations suggest that KiSS-1 neurons in the Arc, whose transcriptional activity is inhibited by T, are targets for the negative feedback regulation of GnRH secretion, whereas KiSS-1 neurons in the anteroventral periventricular nucleus, whose activity is stimulated by T, may mediate other T-dependent processes.