To understand how this genetic mutation plays out at a cellular level, Marchetto and her colleague Alysson Muotri, an assistant professor in the department of Molecular and Cellular Medicine at the UCSD’s School of Medicine, took skin biopsies from four patients with Rett syndrome, reprogrammed them into pluripotent stem cells and experimented with a number of different conditions before they found a combination of growth factors that differentiated the stem cells into functioning human neurons.
They saw that each patient-derived stem-cell line generated a different percentage of neurons carrying the gene mutation. The defective neurons looked and acted differently from their normal counterparts, exhibiting smaller cell bodies, less dendrite connections, and impaired cell-to-cell communication.
The researchers treated neuron cultures with insulin-like growth factor (iGF1), which has been shown to reverse behavioral symptoms of Rett in mice. The drug reversed the biological symptoms of the disorder in the neurons, restoring dendrite connections and cell-to-cell signaling in defective neurons. The researchers plan to use the same process to generate neurons from more patients with both Rett syndrome and other forms of autism.
Jeffrey Neul, assistant professor of molecular and human genetics at Baylor College of Medicine, who studies Rett syndrome in mice, says animal models allow scientists to observe the behavioral effects of the disease, but this is a time- and labor-intensive process.
“The field really has been in desperate need of cellular-based assays that can be used to test therapeutic compounds,” says Neul. “And it’s really hard to push drug discovery if you don’t have something you can do in a more rapid fashion.”
The process Marchetto and Muotri have developed takes three months to generate fully functioning human neurons. While this is similar to the time frame of normal brain development, the researchers are looking for ways to speed the process up so they can rapidly generate brain cells and expose them to a variety of molecular factors and drug compounds.
The team also plans to move beyond the Petri dish once they’ve differentiated neurons from human skin cells, to see how the neurons work in a living brain. “What we can do is transplant human neurons in mouse brains and generate chimeric [hybrid human-animal] models,” says Muotri. “We can then expose these animals to different environments, and see how they will affect the human neuron.”
James Ellis, professor of molecular genetics at the University of Toronto, is doing similar work in reprogramming patients’ skin cells into brain cells. He says that Muotri and Marchetto’s findings open up a new testing ground for autism and other neurological disorders. “That’s clearly what’s going to be required of autism, where different people are going to have different mutations and mechanisms, in how they ended up with that outcome,” he says.