Abstract 11445: DPP6 Contributes to Purkinje Transient Outward Current: A Potential Molecular Mechanism for Idiopathic Ventricular Fibrillation
A genetic variant producing increased expression of DPP6, a putative Ito β subunit, was recently implicated in familial idiopathic IVF (IVF) with normal QT. Very short-coupled initiating extrasystoles appeared to originate from the apical-anterior RV Purkinje fiber (PF) network, suggesting very short local refractory periods. To investigate the role of DPP6 in PFs and IVF, we evaluated DPP6 expression in PF and LV and studied effects of DPP6 overexpression on PF and LV Ito.
Methods: mRNA was quantified by qPCR. DPP6+GFP or GFP alone (control) were overexpressed in cultured PF cells and LV-Epi cardiomyocytes by adenoviral gene transfer. Ito was recorded in GFP-expressing cells 48 hours later by whole cell patch clamp. Currents were also recorded following co-expression of DPP6 or vehicle with Kv4.3 alone, Kv4.3+KChIP2b or Kv4.3+NCS-1 in CHO cells.
Results: PF DPP6 mRNA expression was 17–49 fold higher than in LV Epi,* Endo* or midmyocardium* (*P<.05). DPP6 overexpression increased Ito density in PF (mean change +52%*) but not LV cells (−12%). PF Ito is uniquely inhibited by TEA, which leaves LV Ito unaffected. DPP6 increased TEA-inhibition of PF Ito (e.g. from 25±16% to 50±4%* at +30 mV) but did not alter TEA insensitivity of LV Ito. Besides higher DPP6 mRNA levels, PFs showed more NCS-1 (3.8-fold) and less KChIP2b (43-fold less) vs LV. Purkinje and LV Ito subunit makeup was mimicked in CHO cells: co-expression of DPP6 with “PF” Ito channels (Kv4.3/NCS-1) increased current density by ∼3.7-fold (e.g., at +30 mV, 161±36 pA/pF in Kv4.3+NCS-1+DPP6, (N=6) vs 44±8 pA/pF in Kv4.3+NCS-1,* (N=7)), whereas co-expression of DPP6 with “LV” Ito channels (Kv4.3/KChIP2b) had no effect (e.g., at +30 mV, 337±52 pA/pF in Kv4.3+KChIP2+DPP6 (N=7) vs 319±88 pA/pF in Kv4.3+KChIP2 (N=9)).
Conclusions: The Ito beta-subunit DPP6 is much more strongly expressed in PF than LV and likely conveys unique PF Ito properties like TEA-sensitivity. Overexpression of DPP6 produces Ito gain-of-function specifically in PFs, with KChIP2 appearing to block the DPP6-Kv4.3 interaction in LV. Acceleration of PF repolarization due to increased Ito as a consequence of gain-of-expression of DPP6 may produce a PF early-repolarization syndrome that causes the proarrhythmic phenotype of DPP6-related IVF.
- © 2010 by American Heart Association, Inc.