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Long QT Syndrome (LQTS) is an uncommon congenital heart condition in which patients affected by this syndrome are at high risks for cardiac arrest and sudden cardiac death due to mutations in cardiac ion channels (Crotti et al., 2008). There are two particular variants to the Long QT Syndrome; one is called the Jervell and Lange-Nielsen Syndrome (J-LN), which is associated with deafness, and the other has been named the Romano-Ward Syndrome (R-W), in which there is no connection with deafness (Crotti et al., 2008). The Romano-Ward Syndrome is known to be the more common type of LQTS and is autosomal dominant (Russell et al., 1996), whereas the Jervell and Lange-Nielsen Syndrome is less common and is autosomal recessive (Crotti et al., 2008).
Gene(s) responsible or implicated in the disorder
The research community has divided the Long QT Syndrome by types, depending upon the different mutations in four of the cardiac ion- channel genes, KVLQT1, HERG, SCN5A, and KCNE1 (Zareba et al., 1998). Mutations within these voltage-gated ion channels ultimately disrupt the normal nerve impulses that take place within myocardial cells. Sodium and potassium channels play key roles during action potentials as it is through these channels that their respective ions are able to enter and leave the cell in order to generate electrical excitation or inhibition throughout. Such channels are composed of subunits of proteins, and damage within even one subunit can alter the overall function of the action potential, which will alter the rate of activation, deactivation, and repolarization. Studies have shown that it is within these proteins that certain mutations occur, and they result in delayed ventricular activation and repolarization (Zareba et al., 1998), which then ultimately delay appropriate electrical impulses from being transmitted throughout the heart.
The predominant form of the Long QT Syndrome is termed LQT1 due to the presence of the mutant KVLQT1 gene at the LQT1 locus. The LQT1 locus resides in chromosome 11, and the KVLQT1 gene encodes for an alpha subunit of a potassium-channel protein, which ensures the regular level and rhythm of ion current within the potassium channel (Zareba et al., 1998). The expression of LQT1 decreases the current in the potassium channel, which ultimately affects the repolarization of the action potential. Another, but less common form of LQTS, termed LQT2, is a result of a mutation in the HERG gene at the LQT2 locus, and is present on chromosome 7. This mutation has similar effects when compared to that of LQT1 (Zareba et al., 1998). On the LQT3 locus on chromosome 3, studies have shown that there is a mutation in the SCN5A gene, and unlike LQT1 and LQT2, a LQT3 form of the Long QT...