Multiple loss-of-function mechanisms contribute to SCN5A-related familial sick sinus syndrome.
BACKGROUND
To identify molecular mechanisms underlying SCN5A-related sick sinus syndrome (SSS), a rare type of SSS, in parallel experiments we elucidated the electrophysiological properties and the cell surface localization of thirteen human Na(v)1.5 (hNa(v)1.5) mutant channels previously linked to this disease.


RESULTS
Mutant hNa(v)1.5 channels expressed by HEK293 cells and Xenopus oocytes were investigated by whole-cell patch clamp and two-microelectrode voltage clamp, respectively. HEK293 cell surface biotinylation experiments quantified the fraction of correctly targeted channel proteins. Our data suggested three distinct mutant channel subtypes: Group 1 mutants (L212P, P1298L, DelF1617, R1632H) gave peak current densities and cell surface targeting indistinguishable from wild-type hNa(v)1.5. Loss-of-function of these mutants resulted from altered channel kinetics, including a negative shift of steady-state inactivation and a reduced voltage dependency of open-state inactivation. Group 2 mutants (E161K, T220I, D1275N) gave significantly reduced whole-cell currents due to impaired cell surface localization (D1275N), altered channel properties at unchanged cell surface localization (T220I), or a combination of both (E161K). Group 3 mutant channels were non-functional, due to an almost complete lack of protein at the plasma membrane (T187I, W1421X, K1578fs/52, R1623X) or a probable gating/permeation defect with normal surface localisation (R878C, G1408R).


CONCLUSIONS
This study indicates that multiple molecular mechanisms, including gating abnormalities, trafficking defects, or a combination of both, are responsible for SCN5A-related familial SSS.
Cardiovascular and Genetic Medicine Research Groups, School of Biomedicine, University of Manchester, Manchester, United Kingdom.
Junhong
J
Gui
Tao
T
Wang
Richard P O
RP
Jones
Dorothy
D
Trump
Thomas
T
Zimmer
Ming
M
Lei
D000368:Aged; D001710:Biotin / Q000378:metabolism; D002460:Cell Line; D006580:Genetic Carrier Screening; D006801:Humans; D008839:Microelectrodes; D009124:Muscle Proteins / Q000235:genetics* / Q000502:physiology; D009154:Mutation; D062554:NAV1.5 Voltage-Gated Sodium Channel; D018408:Patch-Clamp Techniques; D012804:Sick Sinus Syndrome / Q000235:genetics*; D015222:Sodium Channels / Q000235:genetics* / Q000502:physiology
D009124:Muscle Proteins; D062554:NAV1.5 Voltage-Gated Sodium Channel; C568320:SCN5A protein, human; D015222:Sodium Channels; D001710:Biotin