Article

Partial expression defect for the SCN5A missense mutation G1406R depends on splice variant background Q1077 and rescue by mexiletine

Department of Medicine, University of Wisconsin–Madison, Madison, Wisconsin, United States
AJP Heart and Circulatory Physiology (Impact Factor: 4.01). 11/2006; 291(4):H1822-8. DOI: 10.1152/ajpheart.00101.2006
Source: PubMed

ABSTRACT Mutations in the cardiac Na(+) channel gene SCN5A cause loss of function and underlie arrhythmia syndromes. SCN5A in humans has two splice variants, one lacking a glutamine at position 1077 (Q1077del) and one containing Q1077. We investigated the effect of splice variant background on loss of function and rescue for G1406R, a mutation reported to cause Brugada syndrome. Mutant and wild-type (WT) channels in both backgrounds were transfected into HEK-293 cells and incubated for up to 72 h with and without mexiletine. At 8 h, neither current nor cell surface expression was observed for the mutant in either background, but both were present in WT channels. At 24 h, small (<10% compared with WT) currents were noted and accompanied by cell surface expression. At 48 h, current density was approximately 40% of WT channels for the mutant in the Q1077del variant background but remained at <10% of WT channels in Q1077. Current levels were stable by 72 h. Coexpression with beta(1)- or beta(3)-subunits or insertion of the polymorphism H558R in the background did not significantly affect current expression. Mexiletine restored current density of the mutant channel in both backgrounds to nearly WT levels. The mutant channels also showed a negative shift in inactivation, slower recovery, and enhanced slow inactivation, consistent with a loss of function phenotype. These data show that a trafficking defect may be partial and time dependent and may differ with the splice variant background. Also, expression defects and gating abnormalities may contribute to loss of function for the same mutation.

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    • "in reduced or ab - sent inward sodium currents in heterologous expression systems such as HEK 293 cells . The channel activity of these mutants has been rescued by incubation of transfected cells at reduced temperatures , treatment with channel blockers , or over - expression of ␤ - subunits ( Valdivia et al . , 2004 ; Poelz - ing et al . , 2006 ; Tan et al . , 2006 ; Pfahnl et al . , 2007 ; Rusconi et al . , 2007 ) . Lowering the incubation temperature is thought to slow the kinetics of protein folding , allow - ing more time for the mutant proteins to acquire a functional conformation before sorting to the cell membrane ( Ulloa - Aguirre et al . , 2004 ) . In the case of the Na v 1 . 6 - S21P mut"

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