Our clinical and functional data enabled reclassification of p.T224M from a variant of unknown significance to pathogenic. Expression of p.T224M KCNQ1 in Chinese hamster ovary cells showed near complete loss of protein function. There was stronger clinical evidence of long QT syndrome in carriers (38.6% versus 5.5%, P=0.0006), greater history of syncope (32% versus 17%, P=0.020), and higher rate of sudden cardiac death in first degree relatives Because of the potential importance of this variant to the health of the population, additional phenotyping was performed in 88 p.T224M carriers and 54 noncarriers. The p.T224M variant, present in 1/45 Amish individuals is rare in the general population (1/248 566 in gnomAD) and was highly associated with QTc on electro-cardiogram ( P=5.53E-24, β=20.2 ms/allele). A missense variant was identified in KCNQ1 (c.671C>T, p.T224M), a gene associated with long QT syndrome type 1, which can cause syncope and sudden cardiac death.
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