Despite the explosion of interest in induced pluripotent stem cells, researchers continue to be frustrated by two key riddles: Why do so few differentiated cells reprogram to a pluripotent state, and why does it take them so long to do it?

In an effort to make the contents of the reprogramming black box a bit less opaque, researchers led by Alexander Meissner at the Broad Institute in Cambridge, Massachusetts, conducted a genomic analysis of reprogrammed, partially reprogrammed and differentiated cells to show what genes are induced and what they might be doing1. The team uncovered several reasons why cells might fail to reprogram. For example, some cells become trapped in differentiated states even if pluripotency genes are active. The researchers could boost reprogramming rates for such cells by adding both a small molecule (a DNA methyltransferase inhibitor known as AZA) that helps erase epigenetic markings and small RNAs to knock down lineage-specifying transcription factors.

Additionally, Linzhao Cheng and his colleagues at Johns Hopkins University School of Medicine, in Baltimore, Maryland, just presented an improved tool to prod cells to pluripotency: a technique that nearly halves the reprogramming time and dramatically increases reprogramming efficiency2. In addition, Cheng derived an induced pluripotent stem (iPS) cell line from a commercially available cell line with mutations for sickle cell anaemia, marking the first publication of a human iPS cell line derived for a specific genetic disease.

Cheng's group used an established four-gene cocktail, created by Shinya Yamanaka, of Kyoto University in Japan, that is known to induce pluripotency, and added an oncogene known as SV40 large T antigen (T). With T in the mix, cells were induced to pluripotency in 12–14 days compared to the 3–4 weeks required without it. Moreover, 23 times more cells successfully reprogrammed when T was added than when the four factors were used alone. Adding T to the four factors in a slightly different recipe created by James Thomson, of the University of Wisconsin–Madison, resulted in a 70-fold increase.

Previous work by George Daley's group at Harvard University, in Cambridge, Massachusetts, had shown that extra factors, including T, assist in reprogramming human cells as part of a six-factor mix3. Cheng was able to produce dramatic success with smaller combinations — even T, Sox2 and Oct4 without the other reprogramming factors In contrast to Daley's results, Cheng's group showed that the lentivirus expressing T did integrate into the iPS genome.

More work is needed to prove that the rapidly reprogramming cells are truly pluripotent like embryonic stem (ES) cells. Most, but not all, of the ES-like colonies showed markers of ES cells, and some, but not all, could form the three basic embryonic germ layers in teratoma in mice. Cheng has also found that the cells are able to differentiate into trophectoderm cell lineage — something mouse ES cells cannot do but human ones can.

Faster reprogramming could hold the clues for getting around one of the key problems with iPS cells today — the use of viral vectors to introduce the necessary genes. The vectors create a safety concern because they integrate into the cell's DNA. If the reprogramming can be completed in less time, researchers might be able to figure out how to do it with means other than viral vectors, says Cheng. Because T's molecular interactions are well studied, researchers also have a potential new target for trying to trigger reprogramming.

Cell lines derived from Cheng's line could prove valuable as a cellular model for disease as they contain mutations found in sickle cell anaemia. Previous work from Rudolf Jaenisch has shown that iPS cells can be created from a mouse with sickle cell anaemia and then genetically modified to eliminate the disease in mice4. Although there is still a long way to go before similar cells can be thought safe and effective for humans, Cheng's sickle cell anaemia iPS cells will likely be a useful system for studying the disease, developing therapies and screening drugs. Cheng is already working to establish an iPS line for a sickle cell anaemia patient at his university's hospital.