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The team found that the disease is caused by a mutation in the gene known as Calsequestrin 2 (CASQ2), which plays a vital role in the contraction and relaxation of the heart. The mutation impairs the ability of the CASQ2 protein to attract and release calcium ions upon demand.
Published in the American Journal of Human Genetics, the study was performed by doctoral student Hadas Lahat, her advisors, Prof. Michael Eldar, Chief of the Heart Institute at the Sheba Medical Center and Dr. Elon Pras of the Danek Gartner Institute of Human Genetics, Sheba Medical Center, and Dr. Avidan, Dr. Tsviya Olender, Dr. Edna Ben-Asher, and Prof. Doron Lancet of the Weizmann Institute's Department of Molecular Genetics.
The study began when an eight-year-old girl in the tribe fainted following physical exertion, and was admitted to the Rambam Medical Center. Her two younger brothers suffered from the same symptoms, and two other siblings had previously died of the disease. While the hospital physicians believed that the children's symptoms were due to tachycardia, they were unable to identify the precise cause. The family then turned to Prof. Michael Eldar, Chief of the Heart Institute at the Sheba Medical Center, who diagnosed the children as suffering from PVT. During his meeting with them, Eldar learned that several other families in the village had a similar medical history.
The girl's family belongs to a Bedouin community in northern Israel, thought to be the descendants of three brothers who had settled there over 200 years ago. The nature of the disease and the common Bedouin custom of familial intermarriage alerted researchers to possible genetic involvement. Hypothesizing that one of these brothers had harbored a genetic mutation, Lahat, of the Danek Gartner Institute of Human Genetics, visited the community and tested the families. It appeared she was on the right track. In seven families alone, thirteen children were identified as having PVT and were given appropriate medication. Nine untreated children in these families had earlier died from this condition.
But the researchers had yet to identify the exact gene responsible for PVT from a possible 80 candidate genes. At the time the Human Genome Project was far from complete and information was available for only half of the 80 suspect genes. Lahat and her supervisor, Dr. Elon Pras, decided to approach Weizmann Institute scientists for help.
At the Institute Drs. Olender and Avidan initially had little success, yet as the Human Genome Project generated new information at an increasing speed, the researchers fortuitously came upon a newly mapped gene, Calsequestrin 2 (CASQ2), just as they were losing hope. The CASQ2 protein appeared as a good candidate because it serves as a calcium ion reservoir in heart muscle cells. By binding, holding, and releasing calcium ions, the CASQ2 protein could thus play a key role in the contraction and relaxation of heart muscles. A second clue came from an unexpected source. Only four months earlier a different research team had found that a mutation in another gene, known as RYR2, also causes a form of PVT. Furthermore, RYR2 was found to belong to the same cellular pathway as CASQ2.
The researchers soon found that the children suffering from PVT had a mutation in their CASQ2 gene, and were able to pinpoint how things had earlier gone wrong in those that had died. They discovered that the mutation was surprisingly small -- characterized by only a single base change, from G to C, in one of the DNA's nucleotides. Nevertheless this change causes the body to produce the amino acid histidine instead of aspartic acid, which impairs the CASQ2 protein's ability to attract and release calcium ions.
'The protein carries a very strong negative charge, thus binding a large number of positively charged calcium ions and releasing them when necessary,' says Avidan. 'Unfortunately, in the mutated CASQ2 protein, the overall negative charge is smaller, since the single base change replaces the aspartic acid which is negatively charged -- with the positively charged histidine. This most likely damages the protein's ability to attract calcium ions, leading to heart failure.'
Other scientists collaborating in this study are Etgar Levy-Nissenbaum and Dr. Boleslaw Goldman of the Danek Gartner Institute of Human Genetics and Dr. Asad Khoury and Dr. Avraham Lorber of the Rambam Medical Center.
Donor support: The Crown Human Genome Center and the Israel Science Foundation Grant
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