NEW YORK – Through a molecular autopsy, researchers have identified a novel pathogenic duplication in the genomes of Amish individuals who experienced exercise-associated sudden death.
Each year, thousands of otherwise healthy individuals die suddenly, and some of these cases can be traced to pathogenic variants in key cardiac-related genes. Researchers from the Mayo Clinic have now examined an Amish family in which four siblings died suddenly following exertion. By sequencing the exomes and analyzing copy number variants in the genomes of those siblings and other family members, the researchers uncovered a duplication shared by all the affected family members, including other distant relatives, as they reported on Wednesday in JAMA Cardiology. Additionally, they found the same duplication in another affected Amish family, suggesting that it might be a founder mutation within the population.
"Alarmingly, people with this condition often have an unremarkable clinical course and routine cardiology clinical testing, such as an electrocardiogram or even stress testing, that do not reveal an individual at risk for a sudden cardiac event," Michael Ackerman, a genetic cardiologist at the Mayo Clinic and senior author on the paper, said in a statement. He added that doctors may now be able to identify others in the community who may be at risk and to offer them counseling.
While exome sequencing of the four siblings who died following exercise did not uncover any suspicious homozygous or compound heterozygous mutations, copy-number variant analysis did. The four siblings shared a homozygous tandem duplication 344,085 base pairs in length, composed of 26,000 base pairs of intergenic sequence, the 5'UTR and promoter region of RYR2, and the first four exons of RYR2.
Their parents and unaffected siblings were heterozygous for this duplication.
But a handful of distant relatives — who experienced sudden cardiac arrest or death while playing or swimming — also harbored this duplication, as did additional people from an unrelated Amish family with a history of sudden unexplained death or sudden cardiac arrest. In all, 23 individuals within these two families were identified as having this duplication, and 18 of them died young.
Mutations in RYR2 have previously been linked to ventricular arrhythmias and sudden cardiac death through two mechanisms, the researchers noted. A gain-of-function variant, for instance, leads to excessive calcium leakage after depolarization, while a loss-of-function variant leads to a gradual calcium overload.
But individuals with this newly uncovered RYR2 duplication only displayed intermittent prolonged QT intervals or prominent U-waves and sometimes even had normal exercise stress tests, the researchers noted. This suggested that the duplication might lead to a partial loss of function of RYR2 and that cardiological tests might not spot family members at risk.
Genetic testing might instead be a means of identifying at-risk people within the Amish community. The researchers said that this duplication — as it is shared by two unrelated Amish families — might be a founder mutation.
"A simple genetic test can be performed to identify family members at risk for their own sudden cardiac arrest," Ackerman said. "In fact, identification of unaffected heterozygous carriers may provide potentially lifesaving premarital counseling and reproductive planning for the Amish community."
The researchers also plan to develop a "disease-in-the-dish" model of the disease by transforming blood samples into heart cells to further tease out the molecular and cellular mechanisms of the disease, which they tentatively named calcium release channel deficiency syndrome.