Stories about: brain malformations

Science Seen: Disrupted developmental genes cause ‘split brain’

split brain syndrome
The two halves of the brain on the right, from a patient with the DCC mutation, are almost completely disconnected. The mutation — first recognized in worms — prevents axons (nerve fibers) from crossing the midline of the brain by interfering with guidance cues. Image courtesy Ellen Grant, MD, director, Fetal-Neonatal Neuroimaging and Developmental Science Center.

Tim Yu, MD, PhD, a neurologist and genomics researcher at Boston Children’s Hospital, was studying autism genes when he saw something on a list that rang a bell. It was a mutation that completely knocked out the so-called Deleted in Colorectal Carcinoma gene (DCC), originally identified in cancer patients. The mutation wasn’t in a patient with autism, but in a control group of patients with brain malformations he’d been studying in the lab of Chris Walsh, MD, PhD.

Yu’s mind went back more than 20 years. As a graduate student at University of California, San Francisco, he’d conducted research in roundworms, studying genetic mutations that made the worms, which normally move in smooth S-shaped undulations, move awkwardly and erratically.

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“Deep sequencing” finds hidden causes of brain disorders

brain malformations sequencing mosaicism
New methods can find a mutation that strikes just 1 in 10 cells.

It’s become clear that our DNA is far from identical from cell to cell and that disease-causing mutations can happen in some of our cells and not others, arising at some point after we’re conceived. These so-called somatic mutations—affecting just a percentage of cells—are subtle and easy to overlook, even with next-generation genomic sequencing. And they could be more important in neurologic and psychiatric disorders than we thought.

“There are two kinds of somatic mutations that get missed,” says Christopher Walsh, MD, PhD, chief of Genetics and Genomics at Boston Children’s Hospital. “One is mutations that are limited to specific tissues: If we do a blood test, but the mutation is only in the brain, we won’t find it. Other mutations may be in all tissues but in only a fraction of the cells—a mosaic pattern. These could be detectable through a blood test in the clinic but aren’t common enough to be easily detectable.”

That’s where deep sequencing comes in. Reporting last month in The New England Journal of Medicine, Walsh and postdoctoral fellow Saumya Jamuar, MD, used the technique in 158 patients with brain malformations of unknown genetic cause, some from Walsh’s clinic, who had symptoms such as seizures, intellectual disability and speech and language impairments.

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PVNH: Could this genetic disorder have a ‘butterfly’ effect?

butterflyThe butterfly effect is defined as “the sensitive dependence on initial conditions, where a small change at one place in a deterministic nonlinear system can result in large differences to a later state.” In medicine, the identification of a rare disease or a genetic mutation may provide insights that spread well beyond the initial discovery.

And in genetics, scientists are learning just how widespread the effects are for mutations in one gene: filaminA (FLNA).

FLNA is a common cause of periventricular nodular heterotopia (PVNH), a disorder of neuronal migration during brain development. The syndrome was first described by the late Peter Huttenlocher, MD, and the gene was identified by Christopher Walsh, MD, PhD, of Boston Children’s Hospital.

In normal brain development, neurons form in the periventricular region, located around fluid-filled ventricles near the brain’s center, then migrate outward to form six onion-like layers. In PVNH, some neurons fail to migrate to their proper position and instead form clumps of gray matter around the ventricles.

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Five people, one mutation and the evolution of human language

The Sylvian fissure (via BodyParts3D/Wikimedia Commons)
The Sylvian fissure (via BodyParts3D/Wikimedia Commons)
Five people with an unusual pattern of brain folds have afforded a glimpse into how the human brain may have evolved its language capabilities.

How the human brain develops its hills and valleys—expanding its surface area and computational capacity—has been difficult to study. Mice, the staple of scientific research, lack folds in their brains.

Christopher Walsh, MD, PhD, head of the Division of Genetics and Genomics at Boston Children’s Hospital, runs a brain development and genetics clinic and has spent 25 years studying people in whom the brain formation process goes awry. Some brains are too small (microcephaly). Some have folds, or gyri, that are too broad and thick (pachygyria). Some are smooth, lacking folds altogether (lissencephaly). And some have an abnormally large number of small, thin folds—known as polymicrogyria.

In 2005, studying people with polymicrogyria, Walsh and colleagues identified a mutation in a gene known as GPR56, a clue that this gene helps drive the formation of folds in the cortex of the human brain.

In a study published in today’s issue of Science, Walsh and his colleagues focused on five people whose brain MRIs showed polymicrogyria, but just in one location—near a large, deep furrow known as the Sylvian fissure, which includes the brain’s primary language area.

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