Letters in 2011

Anterior foregut endoderm is the precursor of various tissues that might be amenable to cell replacement therapy, including thymus, thyroid, lung and trachea. Green et al. present a method for generating this endoderm subtype from human embryonic stem cells and induced pluripotent stem cells.

Profiles of biomarker accumlation may sometimes offer more clinically useful information than assays taken at discrete time points. Ling et al. use implantable sensors to show that cumulative measurements of three clinically relevant cardiac biomarkers correlate with infarct size in a mouse model of heart attack.

The inadvertent cutting of nerves is a common adverse event during surgery. To ease visual identification of nerve fibers, Whitney et al. use phage display to develop a peptide that specifically stains peripheral nerves in living mice and in human tissue samples.

TALEs (transcription activator–like effectors) contain a large number of nearly identical repeats, which makes it difficult to synthesize new variants. Feng et al. describe a facile method for assembling TALEs and show TALEs' utility for activating expression of endogenous human genes.

Synthetic biology requires methods for modular, scalable control of gene expression. Liu et al. show that unnatural amino acids can be used to regulate transcription and use the approach to create NOR and OR gates.

Ye et al. mimic a natural pathway for IgG transfer to deliver a vaccine across mucosal barriers. Intranasal immunization of mice with a fusion of a herpes simplex virus type-2 (HSV-2) antigen to an Fc fragment induces long-lasting protection after intravaginal challenge with HSV-2.

Messenger RNA has received little attention as a potential therapeutic agent. Kormann et al. show that intramuscular injection of chemically modified erythropoietin mRNA substantially increases the hematocrit in mice and demonstrate the curative potential of pulmonary mRNA delivery in a mouse model of congenital surfactant protein B deficiency.

Sequencing a human genome using next-generation methods does not distinguish between the two copies of each chromosome. Kitzman et al. determine a haplotype-resolved genome sequence by efficiently constructing and sequencing long-insert clones that cover the diploid genome with a low likelihood of overlap.

Cui et al. generate transgenic rats and mice bearing targeted genomic integrations by enhancing the rate of homologous recombination in single-cell embryos with zinc-finger nucleases. The approach avoids the time-consuming backcrossing involved in generating mutant mice with ES cells and should be applicable to species for which ES cells have not been isolated.

The use of shRNA in vivo to investigate genes involved in proliferation and survival is confounded by the competitive advantage of cells with insufficient shRNA expression. Zuber et al. address this issue with a system that combines Tet-regulated shRNA expression, two fluorescent reporters and robust transactivator production.

Song et al. present the first method for global analysis of 5-hydroxymethylcytosine, a recently identified epigenetic modification in mammalian cells. They use a bacteriophage-derived enzyme to tag the hydroxymethyl group with an azide-modified glucose residue that can be used for affinity purification and sequencing of modified genomic DNA fragments.

If transgenes are to be introduced into the genome for cell therapies, the integration events should permit high transgene expression without altering the expression of endogenous genes. Papapetrou et al. propose five criteria to define such 'safe harbors' in the human genome and demonstrate high A-globin expression from a safe-harbor site in erythroid-lineage cells derived from induced pluripotent stem (iPS) cells.