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Asymptomatic sinus bradycardia is often a sign of good physical conditioning. However, it can be pathologic, symptomatic, and potentially life-threatening. Researchers in Italy hypothesized that unexplained sinus bradycardia is associated with a mutation in the gene (HCN) that encodes the pacemaker channel, the channel responsible for sinus-node activity.
The researchers screened 52 people with bradycardia for mutations anywhere on HCN4 (the HCN subunit most highly expressed in the mammalian sinoatrial node); 1 person had a mutation in exon 7, S672R. Analysis of DNA collected from 27 family members of this person revealed an autosomal dominant pattern of inherited bradycardia. Heart-rate measurements for these individuals showed asymptomatic…