Every now and then, conditions are ripe for a scientific breakthrough. These “conditions” include not only access to advanced technological tools that can facilitate a new discovery, but also (and, perhaps most importantly) a powerful hypothesis, talented researchers, and a certain amount of luck. All of these components converged, as Dr. Anne Comi and her co-workers recently advanced the field of child neurology by discovering the genetic basis for Sturge-Weber Syndrome.
Sturge-Weber Syndrome is a sporadic congenital neurocutaneous disorder that includes port-wine birthmark of the face, abnormal capillary venous vessels of the leptomeninges and choroid plexus, and a high incidence of glaucoma. The disorder is typically accompanied by seizures, stroke, and cognitive dysfunction.This disease has fascinated clinicians for centuries, as its puzzling involvement of skin (but only a patch of skin), brain (but usually only one hemisphere), and eyes (but usually only one eye) has left clinicians stupefied as to its etiology (see Editor’s Note below).
The notion that Sturge-Weber Syndrome may be a genetic disorder was introduced almost three decades ago by Dr. Rudolf Happle, who hypothesized that a mutation occurring early in fetal development affecting a somatic progenitor cell could lead to a set of abnormalities that are all localized to structures derived from that progenitor cell. In other words, if a mutation occurred in a fetal cell, that mutation could affect all of the structures derived from that cell. And if that cell happened to be a progenitor for structures of the eye, brain, and skin, then it could give rise to Sturge-Weber Syndrome. This hypothesis would explain the localized nature of Sturge-Weber Syndrome (because the mutation affects only those limited structures derived from that one progenitor cell) as well as the fact that the syndrome is not inherited (because the mutation occurs in a somatic cell; not the germ line).
“Discovery of the SWS gene means that a specific therapy for the disease may soon be at hand.”
This hypothesis predicts that the genetic mutation would be present within the affected tissues of a patient, but not within the unaffected tissues of that same patient. The hypothesis likewise illuminated the pathway to identify the gene: compare DNA of affected tissue with DNA of unaffected tissue from the same person with the disease; the place where they differ becomes the candidate gene.
During the immediate decades after its introduction, the Happle hypothesis remained appealing, but untested, because the technology did not yet exist to compare all DNA from two sites of an individual. By the 21st century, however, this changed, as whole-genome sequencing was introduced and as the capacity to store and compare large amounts of genomic data became available.
Enter Dr. Anne Comi, Associate Professor of Pediatrics and Neurology, and her colleagues at the Kennedy Krieger Institute and Johns Hopkins University. After establishing a multidisciplinary center for the study of Sturge-Weber Syndrome in 2003, Dr. Comi and her coworkers spent more than a decade examining patients and carefully studying clinical and scientific aspects of the disease. With the help of the Sturge-Weber Foundation and the NICHD Brain and Tissue Bank for Developmental Disorders, they acquired a large number of tissue samples.
By 2013, with the molecular techniques perfected, the computer data storage systems in place, and the bioinformatics expertise assembled, Dr. Comi and her group were ready to undertake the monumental task of DNA comparison. But whole-genome sequencing is an expensive proposition. They had the funds to conduct comparisons in only three patients. A study limited to only three patients meant that their chances of failure were high. Undaunted, they forged ahead.
The investigators painstakingly compared DNA samples from affected tissues and unaffected tissues obtained from the same individuals with Sturge-Weber Syndrome, as they sought to identify somatic single nucleotide variants. At the end of this laborious process, a single gene emerged. The gene, located on chromosome 9, was GNAQ, which codes for the protein Gαq. The research group spent the next several months validating their results and found that the mutation was indeed present in most SWS samples, absent in controls, and somatically located. The gene for Sturge-Weber Syndrome had been found, and a decades-old hypothesis had finally been validated. Dr. Comi was excited and gratified when she realized that they had found the gene that they had been seeking for so long. But she readily admits that luck played a role in their success. “I’m still amazed that it worked,” said Comi. “We had money for only three pairs of whole-genome sequencing. We were lucky that all three of the samples we chose had the mutation. In our follow-up work, we have found that most affected samples from people with SWS do have the mutation, but some affected samples don’t. Whether this is due to poor DNA quality or to a different mutation is unknown. But the point is that we could just as well have chosen a sample in which the mutation was absent. If that had happened, we would have completely missed it.” Dr. Comi and her collaborators have taken their research one important step beyond gene identification and have conducted functional studies. They have found that mutation in GNAQ hyperactivates ERK-phosphorylation, which implies that the mutation exerts its effect by overactivating the MAP kinase pathway. This is particularly exciting because drugs are already available that can inhibit ERK and other components of the MAP kinase pathway. The researchers are now devoting themselves toward understanding the mechanism, developing model systems, and investigating potential drug treatments. Thus, discovery of the SWS gene means that a specific therapy for the disease may soon be at hand.
Editor’s Note: Back in my residency days, when I first learned of Sturge-Weber Syndrome, I immediately classified it, within my mind, into that group of non-genetic syndromes. After all, like someone who’s been hit in the head with a baseball bat, people with turge- Weber Syndrome have a focal defect of one eye, one half of their brain, and the skin on one side of their face. A genetic defect couldn’t possibly lead to such a strange pattern of localized pathology. No one inherits a baseball bat injury, and no one inherits Sturge Weber Syndrome, either. Just as a baseball bat injury is obviously non-genetic, Sturge-Weber Syndrome seemed obviously non-genetic, as well. So, on the shelves of my mind, Sturge Weber Syndrome was placed neatly onto the pile of non-genetic syndromes. And that’s where it lay, undisturbed, for two decades. Now, I learn from Dr. Comi and company that I was wrong all along. Sturge Weber Syndrome is a genetic syndrome. Its pattern of pathology and lack of inheritance, which played such a crucial role in leading me to erroneously classify it as non-genetic, lay in its origins as a mutation in a somatic cell of the fetus, instead of the usual germ cell of one or both of the parents. Ah-ha! The dusty, cobweb-covered shelves of my mind have just been dumped to the floor. And suddenly, I feel like the person who’s been hit in the head with a baseball bat. DJB