Mutation in SCN5A (Na+ channel, INa)
Long QT Syndrome Type 3 (LQT3)
- Congenitally inherited (Genetic) or acquired (Drug therapy)
- Cardiovascular disorder – prolonged ventricular repolarization.
- Prevalence level – 8%
- Clinical phenotype – Prolonged QT wave causing extended
- Statistics – 1 in 10,000 individuals are affected with LQT
syndromes 8% of these individuals diagnosed with LQT3.
Approximately 3,000 to 4,000 individuals in the U.S die as a
consequence of sudden death.
- Fainting (Syncope)
- Sudden death
- Cardiac arrest
- Diagnose through the use of the electrocardiograph (EKG). Abnormal T wave (phenotype) readings can result in determining the form of LQT syndrome the patient has. Any QT interval above 440 milliseconds (ms) is considered prolonged.
- Autopsy of LQT3 syndrome can only be diagnosed through observing for abnormal chemical analysis of the SCN5A gene.
Treatments & Their Effectiveness
- Pacemakers and Automatic Defibrillators
- Medication – Mexiletine and Lidocaine
Risk Factors 
- History of syncope
- Duration of episode and QT interval
- Congenital deafness
- Males: More susceptible during childhood.
- Females: During pregnancy. Cardiac events are common
Preventive Measures 
- Individuals who present the following should consider being tested for LQT3 syndrome: sudden and unexpected lose of consciousness during childhood and teenage years, sudden or unexplained cardiac arrest, unexplained deaths in family or epilepsy in children.
Functional Role of Transmembrane Proteins (SCN5A)
- Caused by a disorder in the Sodium (Na+) ion channel in the
SCN5A gene. There are approximately 29 known mutations that cause the long QT syndrome where most are the consequence of a single nucleotide substitution. SCN5A intercedes in the permeability of excitable membranes as a function of the alpha () channel . Mutation of the SCN5A protein has been associated with the III-IV linker region responsible for the inactivation of the sodium gates. It is believed that the linker region acts a blocking particle that goes into the channel to block the flow of sodium ions. (Linked to chromosome 13)
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Figure 1: ECG reading provided by
Late Updated: May 3, 2003.