Synthetic DNA circuits inserted into human stem cells could soon allow us to build new organs with unprecedented precision and speed. The circuits can be designed on a computer and assembled from ready-made parts ordered online. The technique could prove an efficient way of making organs for transplant without the worry of rejection, and raises the tantalising possibility that it might one day be possible to upgrade the organs we were born with. Human cells have already been used to create a tiny liver and a set of neurons.
"At the moment, the aim is to normalise cells, but in future, enhancement has to be on the menu," says Chris Mason, a professor of regenerative medicine at University College London, who wasn’t involved in the work.
Oxford Journal Nucleic Acids Research – Rapid, modular and reliable construction of complex mammalian gene circuits
To turn induced pluripotent stem (iPS) cells into a specific tissue type, they are typically placed in a soup of DNA and signalling molecules. These enter the cells and flick certain epigenetic switches. What gets turned on or off depends on the ingredients in the soup. "The problem is that there are tens of thousands of these switches that all need to be set in the right way," says Mason. Another hurdle is that all cells in the soup are influenced in the same way and grow into the same tissue type. But a piece of liver tissue, say, is not the same as a functioning liver. The issue is even more apparent with complex organs such as hearts, says Guye.
What would be more helpful is an instruction manual that each individual stem cell can follow during its development. And this is exactly what Guye’s team has provided. They started by looking at what happens in neurons and liver cells during natural embryonic development – which genes are switched on and when. They then designed and built artificial DNA control circuits to reproduce this switching in iPS cells. The circuits are slotted together using a combination of standard DNA parts – such as sequences that code for different proteins – available from online repositories and newly synthesised genetic material.
"You assemble it into one large logic circuit and put it into the cell," Guye says. "It’s interfacing with the natural system. We’re not replacing anything, we’re putting a control layer on top."
Once in the cell, the circuitry kicks into action. "The idea is that the circuit is pretty much autonomous," says Guye. It can measure activity – such as levels of gene expression in the cell – and react to it. When the circuit detects that an iPS cell has turned into a precursor cell, for example, it can initiate the next stage of development.
As yet unpublished results suggest that the technique is faster and more reliable than existing methods of creating tissues from iPS cells. In one study, his team turned iPS cells into neurons in just four days with almost 100 per cent success. "If true, it’s incredibly rapid," says Mason. "Normally it takes weeks."
ABSTRACT Rapid, modular and reliable construction of complex mammalian gene circuits –
We developed a framework for quick and reliable construction of complex gene circuits for genetically engineering mammalian cells. Our hierarchical framework is based on a novel nucleotide addressing system for defining the position of each part in an overall circuit. With this framework, we demonstrate construction of synthetic gene circuits of up to 64 kb in size comprising 11 transcription units and 33 basic parts. We show robust gene expression control of multiple transcription units by small molecule inducers in human cells with transient transfection and stable chromosomal integration of these circuits. This framework enables development of complex gene circuits for engineering mammalian cells with unprecedented speed, reliability and scalability and should have broad applicability in a variety of areas including mammalian cell fermentation, cell fate reprogramming and cell-based assays.