CRISPR, animals, and FDA oversight: Building a path to success.

Technological advances, such as genome editing and specifically CRISPR, offer exciting promise for the creation of products that address public health concerns, such as disease transmission and a sustainable food supply and enable production of human therapeutics, such as organs and tissues for xenotransplantation or recombinant human proteins to treat disease. The Food and Drug Administration recognizes the need for such innovative solutions and plays a key role in bringing safe and effective animal biotechnology products to the marketplace. In this article, we (the Food and Drug Administration/Center for Veterinary Medicine) describe the current state of the science, including advances in technology as well as scientific limitations and considerations for how researchers and commercial developers working to create intentional genomic alterations in animals can work within these limitations. We also describe our risk-based approach and how it strikes a balance between our regulatory responsibilities and the need to get innovative products to market efficiently. We continue to seek input from our stakeholders and hope to use this feedback to improve the transparency, predictability, and efficiency of our process. We think that working together, using appropriate science- and risk-based oversight, is the foundation to a successful path forward.

Author Correction: Template plasmid integration in germline genome-edited cattle.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Base-induced reversible H2 addition to a single Sn(ii) centre.

A range of amines catalyse the oxidative addition (OA) of H2 to [(Me3Si)2CH]2Sn (1), forming [(Me3Si)2CH]2SnH2 (2). Experimental and computational studies point to 'frustrated Lewis pair' mechanisms in which 1 acts as a Lewis acid and involve unusual late transition states; this is supported by the observation of a kinetic isotope effect for Et3N. When DBU is used the energetics of H2 activation are altered, allowing an equilibrium between 1, 2 and adduct [1·DBU] to be established, thus demonstrating reversible oxidative addition/reductive elimination (RE) of H2 at a single main group centre.

BRD4 inhibitors block telomere elongation.

Cancer cells maintain telomere length equilibrium to avoid senescence and apoptosis induced by short telomeres, which trigger the DNA damage response. Limiting the potential for telomere maintenance in cancer cells has been long been proposed as a therapeutic target. Using an unbiased shRNA screen targeting known kinases, we identified bromodomain-containing protein 4 (BRD4) as a telomere length regulator. Four independent BRD4 inhibitors blocked telomere elongation, in a dose-dependent manner, in mouse cells overexpressing telomerase. Long-term treatment with BRD4 inhibitors caused telomere shortening in both mouse and human cells, suggesting BRD4 plays a role in telomere maintenance in vivo. Telomerase enzymatic activity was not directly affected by BRD4 inhibition. BRD4 is in clinical trials for a number of cancers, but its effects on telomere maintenance have not been previously investigated.

Development of a micro-neutralization assay for ebolaviruses using a replication-competent vesicular stomatitis hybrid virus and a quantitative PCR readout.

Development of vaccines against highly pathogenic viruses that could also be used as agents of bioterrorism is both a public health issue and a national security priority. Methods that can quantify neutralizing antibodies will likely be crucial in demonstrating vaccine effectiveness, as most licensed viral vaccines are effective due to their capacity to elicit neutralizing antibodies. Assays to determine whether antibodies are neutralizing traditionally involve infectious virus, and the assay most commonly used is the plaque-reduction neutralization test (PRNT). However, when the virus is highly pathogenic, this assay must be done under the appropriate level of containment; for tier one select agents, such as Ebola virus (EBOV), it is performed under Biological Safety Level 4 (BSL-4) conditions. Developing high-throughput neutralization assays for these viruses that can be done in standard BSL-2 laboratories should facilitate vaccine development. Our approach is to use a replication-competent hybrid virus whose genome carries the envelope gene from the pathogenic virus on the genetic backbone of a non-pathogenic virus, such as vesicular stomatitis virus (VSV). We have generated hybrid VSVs carrying the envelope genes for several species of ebolavirus. The readout for infectivity is a one-step reverse transcriptase quantitative PCR (RT-qPCR), an approach that we have used for other viruses that allows robustness and adaptability to automation. Using this method, we have shown that neutralization can be assessed within 6-16h after infection. Importantly, the titers obtained in our assay with two characterized antibodies were in agreement with titers obtained in other assays. Finally, although in this paper we describe the VSV platform to quantify neutralizing antibodies to ebolaviruses, the platform should be directly applicable to any virus whose envelope is compatible with VSV biology.

Oxidative damage targets complexes containing DNA methyltransferases, SIRT1, and polycomb members to promoter CpG Islands.

Cancer cells simultaneously harbor global losses and gains in DNA methylation. We demonstrate that inducing cellular oxidative stress by hydrogen peroxide treatment recruits DNA methyltransferase 1 (DNMT1) to damaged chromatin. DNMT1 becomes part of a complex(es) containing DNMT3B and members of the polycomb repressive complex 4. Hydrogen peroxide treatment causes relocalization of these proteins from non-GC-rich to GC-rich areas. Key components are similarly enriched at gene promoters in an in vivo colitis model. Although high-expression genes enriched for members of the complex have histone mark and nascent transcription changes, CpG island-containing low-expression genes gain promoter DNA methylation. Thus, oxidative damage induces formation and relocalization of a silencing complex that may explain cancer-specific aberrant DNA methylation and transcriptional silencing.

Differential oxidant tolerance determined by the key transcription factor Yap1 is controlled by levels of the Yap1-binding protein, Ybp1.

The Saccharomyces cerevisiae transcription factor Yap1 is a central determinant of oxidative stress tolerance. This protein is found primarily in the cytoplasm in the absence of oxidative stress but, upon exposure to oxidants, rapidly translocates to the nucleus and activates expression of target genes. Although both diamide and H(2)O(2) have been used to impose oxidative stress on cells, these different oxidants trigger Yap1 nuclear localization in distinctly different ways. Diamide appears to oxidize particular cysteine residues on Yap1, leading to inhibition of association of Yap1 with the nuclear exportin Crm1. Crm1 would normally transport Yap1 out of the nucleus. H(2)O(2) activation of Yap1 nuclear localization requires the participation of the glutathione peroxidase Gpx3 and the Yap1-binding protein Ybp1. H(2)O(2) exposure triggers formation of a dual disulfide bonded Yap1 that is catalyzed by the presence of Gpx3 and Ybp1. In the current study, we have determined that two distinct pools of Yap1 exist in the cell. These pools are designated by the level of Ybp1. Ybp1 interacts directly with Yap1 and these proteins form a stable complex in vivo. Genetic and biochemical experiments indicate that Ybp1 is rate-limiting for Yap1 oxidative folding during H(2)O(2) stress. The fungal pathogen Candida glabrata expresses a protein homologous to Ybp1 called CgYbp1. Overproduction of CgYbp1 elevated H(2)O(2) tolerance in this pathogen indicating that the determinative role of Ybp1 in setting the level of H(2)O(2) resistance has been evolutionarily conserved.

Decreased dyskerin levels as a mechanism of telomere shortening in X-linked dyskeratosis congenita.

Dyskeratosis congenita (DC) is a premature ageing syndrome characterised by short telomeres. An X-linked form of DC is caused by mutations in DKC1 which encodes dyskerin, a telomerase component that is essential for telomerase RNA stability. However, mutations in DKC1 are identifiable in only half of X-linked DC families. A four generation family with pulmonary fibrosis and features of DC was identified. Affected males showed the classic mucocutaneous features of DC and died prematurely from pulmonary fibrosis. Although there were no coding sequence or splicing variants, genome wide linkage analysis of 16 individuals across four generations identified significant linkage at the DKC1 locus, and was accompanied by reduced dyskerin protein levels in affected males. Decreased dyskerin levels were associated with compromised telomerase RNA levels and very short telomeres. These data identify decreased dyskerin levels as a novel mechanism of DC, and indicate that intact dyskerin levels, in the absence of coding mutations, are critical for telomerase RNA stability and for in vivo telomere maintenance.