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Body + BrainBody & Brain

CRISPR and Stem Cells Could Speed Studies of Rare Diseases

ByKate TelmaNOVA NextNOVA Next

Alicia Burns is afraid to close her eyes, and more afraid for her children to sleep. Sometimes she’s scared to move, or breathe. The fear comes, she said, when “I have a big meal with family and start feeling ill and light-headed. I’m gardening and the sun is beating down on me and I feel faint.”

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“Cardiac arrest can happen at any moment.”

Burns, 38, is a mother of six and describes herself as a happily married housewife. But the internet knows her as “Brugada Girl.”

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Though she was hospitalized once in her 20s for heart attack-like symptoms, it wasn’t until her young, healthy father died in his sleep that Burns followed up. After a parade of electrocardiograms (EKG) and appointments where her faintness was dismissed by doctors, Burns was diagnosed with Brugada Syndrome, and began writing a blog.

Brugada is a rare disease, and its genetic underpinnings aren’t clearly understood. While most patients experience sudden cardiac arrest when uncoordinated electrical impulses in their heart’s ventricles disrupt blood flow, researchers are finding that each patient has a unique set of genetic mutations that cause the syndrome. And now, there might be a platform for noninvasively studying an individual’s genetic makeup.

Preparation for DNA  sequencing
Determining the mutations behind diseases can be challenging, but a new platform could help change that.

The Brugada brothers, Josep and Pedro, identified Brugada syndrome in 1992. Their younger brother, Ramon, established the genetic underpinnings in 1998. They soon realized that Brugada causes sudden unexpected nocturnal death syndrome and is endemic to regions of Southeast Asia, a phenomenon recorded by the CDC since the 1980s. Brugada also explains some cases of sudden infant death syndrome (SIDS). Currently, the NIH estimates that about five in 10,000 people are affected in the United States, and four times that in Southeast Asia. Brugada is responsible for 12% of sudden cardiac deaths.

People are clinically diagnosed with Brugada because of EKG irregularities—but EKG readings can be unreliable and cannot predict someone’s likelihood of developing Brugada later in life. With the rise of genetic sequencing, researchers identified SCN5A, a sodium channel gene, as causative. But the number of responsible genes has quickly swelled to 23, by some counts. The mutations in SCN5A alone number more than 300.

Channel proteins act as gates into the cell membrane, and allow specific ions, usually sodium, potassium, and calcium, to flow in or out. These ion tides are what create the electrical impulses that trigger muscle movement. The heart relies on channel proteins to communicate the complex electrical signals it needs to contract.

Parents with Brugada were thought to have a 50% chance of passing SCN5A to their children, and any child who received the gene would be certain of developing the syndrome. Burns had genetic tests ordered for her six kids, and three of them came back with the exact same mutation she has—but none have been diagnosed yet. While part of this is due to their age, there are an increasing number of people who have the SCN5A genotype but no physical manifestation of the Brugada syndrome phenotype and even more people who are clinically diagnosed but have a healthy SCN5A gene.

“It is a big failure in diagnostics compared to other genetic tests, where you have a good correlation between the genotype and the phenotype,” said Julien Barc, a research associate at the Institute for Medical Research in Nantes, and a co-author of several Brugada genetics studies. “We start to question if it really adds value to do diagnostics for Brugada syndrome.”

The availability of genetic testing has only complicated the understanding of the relationship between a patient’s genotype and their likelihood for developing the disease. Only about 20% of Brugada patients have SCN5A mutations. Meanwhile, the general population carries an array of harmless, one-letter changes in their DNA. When these genetic variants were found in patients from Brugada families, researchers were quick to add them to the database of Brugada-causing mutations. As a result, many of the mutations reported may not actually have anything to do with Brugada syndrome, particularly in genes other than SCN5A. “We have been polluted by variants that have been described but are not causal,” Barc said.

Barc and others have shown that the best genetic predictor of Brugada is the combination of several rare variants—DNA changes shared by fewer than 0.5% of the general population. In the past, finding these may have been difficult. But the ability to comprehensively examine a whole genome, instead of only genes that were already implicated in heart conditions, is fast expanding.

This summer, scientists at Stanford demonstrated how a single patient’s unique Brugada syndrome could be modeled in a lab dish. Researchers collected skin cells of two patients with a common SCN5A mutation, which they converted to pluripotent stem cells, cells that have the potential to become virtually any other cell in the body. They then differentiated these into cardiomyocytes, the self-contracting heart muscle cells. By measuring the electrical signals of individual cardiomyocytes, they showed that the cells behaved similarly to cells in a patient’s heart. Next, the group used the gene editing technique CRISPR-Cas9 to restore SCN5A back to its healthy version. With the gene permanently fixed, the cells began sending normal electrical signals.

This approach could be a powerful tool for determining if individual variants are responsible for Brugada syndrome. Based on a patient’s EKG and history, a clinician might not be completely sure of a diagnosis and send a sample for genome sequencing. “So you do the DNA genotype, and it comes back as some kind of variant,” said Joe Wu, professor of cardiovascular medicine and radiology, and the senior author of the study. Cardiomyocytes are made from a patient sample, and if the cells have the disease phenotype, “that will give you more confidence,” he said. If the variant is edited out and the healthy phenotype restored, that gives you additional confidence.

Physicians associate Brugada syndrome with its usual presentation: ventricular arrhythmia, the discoordination between the heart’s two larger, lower chambers. But the model reinforces that genetic mutations effect the whole organ and that Brugada mutations could also impact the upper, atrial chambers’ functioning. “Now we understand that there is a single cell predisposition that carries over to all the cells in the heart,” said Karim Sallam, a cardiovascular medicine fellow working with Wu.

Modeling individual patients—and determining if their variants are responsible for their Brugada syndrome—could aid doctors in diagnosis, assigning risk, and knowing which patients to follow up with, particularly family members. The CRISPR editing-pluripotent stem cell approach is now being used to study other rare diseases, either for those so rare that investigators can’t recruit patients or in cases where investigators want to confirm their data, Wu said. But not everyone is as optimistic about this platform’s performance.

For a model like this to be useful, it needs to accurately represent the disease. The cardiomyocytes created in the dish should have the same “Brugada phenotype” of electric signals and ion currents that show up on a patient EKG. Charles Antzelevitch, the director of research at the Lankenau Heart Institute, believes that the model fails to create the key EKG signatures of the disease and could be more specific to Brugada syndrome. “The reason for that is clear to many of us who have used induced pluripotent stem cells. These cells are very immature,” he said.

Accurately screening family of a person with Brugada syndrome might still be years away. In the meantime, Burns takes her children for frequent EKG’s, hoping that any Brugada sypmtoms will be caught before it is too late.

Image credit: NIH

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