Affected children with this disorder frequently show symptoms of autism spectrum disorders combined with a constellation of physical problems. Abnormalities included changes in the composition of cells in the cortex, the largest brain structure in humans, and of neurons that secrete two key chemical messengers. Neurons that make long-distance connections between the brain’s hemispheres tended to be in short supply.

Most patients with Timothy syndrome meet diagnostic criteria for an autism spectrum disorder. However, unlike most cases of autism, Timothy syndrome is known to be caused by a single genetic mutation. “Studying the consequences of a single mutation, compared to multiple genes with small effects, vastly simplifies the task of pinpointing causal mechanisms,” explained Ricardo Dolmetsch, PhD, from Stanford University (Stanford, CA, USA), a US National Institute of Mental Health (NIMH; Bethesda, MD, USA) grantee who led the study.

Dr. Dolmetsch, and colleagues, reported on their findings November 27, 2011, in the journal Nature Medicine. “Unlike animal research, the cutting-edge technology employed in this study makes it possible to pinpoint molecular defects in a patient’s own brain cells,” said NIMH director Thomas R. Insel, MD. “It also offers a way to screen more rapidly for medications that act on the disordered process.”

Before this research, researchers knew that Timothy syndrome is caused by a tiny malfunction in the gene which codes for a calcium channel protein in cell membranes. The mutation results in too much calcium entering cells, causing a characteristic set of abnormalities throughout the body. Accurate functioning of the calcium channel is known to be particularly critical for proper heart rhythm--many patients die in childhood of arrhythmias--but its role in brain cells was less well understood.

To understand more, Dr. Dolmetsch and colleagues used a new technology called induced pluripotent stem cells (iPSCs). They first transformed skin cells from Timothy syndrome patients into stem cells and then coaxed these to differentiate into neurons.

“Remarkable reproducibility” observed across multiple iPSC lines and individuals validated that the technique can reveal defects in neuronal differentiation--such as whether cells assume the correct identity as the brain gets wired-up in early development, said the researchers. Compared to those from controls, fewer neurons from Timothy syndrome patients became neurons of the lower layers of the cortex and more became upper layer neurons. The lower layer cells that remained were more likely to be the sort that project to areas below the cortex. In comparison, there were fewer-than-normal neurons equipped to form a structure, called the corpus callosum, which makes possible communications between the left and right hemispheres.

Many of these defects were also seen in equivalent studies of mice with the same genetic mutation found in Timothy syndrome patients. This supports the link between the mutation and the developmental abnormalities.

Several genes earlier implicated in autism were among hundreds found to be expressed abnormally in Timothy syndrome neurons. Excess cellular calcium levels also caused an overproduction of neurons that make key chemical messengers. Timothy syndrome neurons secreted 3.5 times more norepinephrine and 2.3 times more dopamine than control neurons. Addition of a drug that blocks the calcium channel reversed the abnormalities in cultured neurons, reducing the proportion of catecholamine-secreting cells by 68%.

The findings in Timothy syndrome patient iPSCs follow those in Rett syndrome, another single gene disorder that frequently includes autism-like symptoms. About one year ago, Alysson Muotri, PhD, and colleagues at University of California, San Diego (USA), reported deficits in the protrusions of neurons, called spines, which help form connections, or synapses. The scientists’ discovery of earlier (neuronal fate) and later (altered connectivity) defects suggest that disorders on the autism spectrum affect multiple stages in early brain development.

“Most of these abnormalities are consistent with other emerging evidence that ASDs arise from defects in connectivity between cortex areas and show decreased size of the corpus callosum,” concluded Dr. Dolmetsch. “Our study reveals how these might be traceable to specific mechanisms set in motion by poor regulation of cellular calcium. It also demonstrates that neurons derived from iPSCs can be used to identify the cellular basis of a neurodevelopmental disorder.”

The processes identified in this study may become potential targets for developing new therapies for Timothy Syndrome and may also provide insights into the neural basis of deficits in other forms of autism, according to Dr. Dolmetsch.

Related Links:
Stanford University
National Institute of Mental Health