ANP is a circulating hormone that, through stimulation of the intracellular second messenger cGMP, plays a primary physiological role in the regulation of intravascular blood volume and vascular tone through natriuresis, diuresis, and vaso-dilatation.12
Through cGMP signaling, ANP also modulates currents of sodium, calcium, and potassium channels in cardiac myocytes.13–15
Moreover, in atria of intact human hearts, ANP has been shown to shorten atrial conduction time and the effective refractory period, which provides a potential electrophysiological substrate for arrhythmia.16
ANP has also been shown to cause dose-dependent, autonomically mediated shortening of the atrial MAP duration and the effective refractory period in dogs.17
Thus, the suggestion that a mutation in NPPA
could be responsible for the development of atrial fibrillation is consistent with some of the known aspects of ANP physiology.
To elucidate the mechanism by which a mutation in NPPA
could lead to familial atrial fibrillation, we first showed that the mutation we identified results in the production of a mutant protein product. The concentration of circulating mANP was several times as high as that of wild-type ANP. One possible explanation for this difference in concentration is that mANP may have a prolonged half-life. Indeed, natriuretic peptides with a longer carboxyl terminus have increased resistance to degradation by neutral endopeptidase 24.11.18
In particular, Dendroaspis augusticeps
natriuretic peptide (DNP), a unique natriuretic peptide isolated from snake venom, has a carboxyl-terminal extension of 15 amino acids and increased cGMP-stimulating potency.18,19
The phenotype that we observed in family members could thus be explained by high levels of circulating mANP with ANP-like activity. Such a mechanism would be consistent with previous experimental studies showing electrophysiological derangements on exposure of atrial myocytes to pathophysiological doses of ANP.14–17
Moreover, the mild structural remodeling, despite effective ventricular rate control, in several affected members of the family we studied is consistent with a proapoptotic effect on myocytes observed with excessive ANP–cGMP signaling.20
However, regardless of potential dose-related and structural remodeling effects, our isolated (denervated) heart model showed a direct effect of mANP, but not wild-type ANP, on atrial electrophysiology. Although an additional novel function of the mANP fusion protein cannot be ruled out, the 12-residue carboxyl terminus has no strong sequence homology with known proteins. Atrial fibrillation developed in affected subjects over a period of several decades, as observed in patients with primary defects in ion channels and gap junctions,7–10
which suggests insidious but progressive electrical remodeling that conferred susceptibility to atrial arrhythmia.
The linkage analysis we performed has some limitations. Several subjects in the family who were classified as “unaffected” had not yet reached the mean age at which atrial fibrillation was diagnosed in other family members. If the analysis is based only on the family members who were known to be affected, the lod score was 2.66, which is below the threshold of 3.0 commonly accepted to confirm linkage. However, the fact that the mutation in the gene encoding ANP segregates with known disease and the demonstration that mANP has electrophysiological effects that could confer a predisposition to atrial fibrillation strongly suggest that we have correctly identified the causative mutation.
In families with atrial fibrillation, investigators have identified mutations in ion channels that are predicted to either shorten or lengthen the duration of cardiac action potentials.21
Our findings uncover a novel molecular genetic basis for abnormal repolarization and electrical instability in the cardiac atria and suggest the ANP–cGMP signaling pathway as a potentials therapeutic target.22,23