Long QT Syndrome classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]

Classification

The two most common types of LQTS are genetic and drug-induced. Genetic LQTS can arise from mutation to one of several genes. Following is a list of the most common mutations:

Type	OMIM	Mutation	Notes
LQT1	192500	alpha subunit of the slow delayed rectifier potassium channel (KvLQT1 or KCNQ1)	The current through the heteromeric channel (KvLQT1 + minK) is known as IKs. These mutations often cause LQT by reducing the amount of repolarizing current that is required to terminate the action potential, leading to an increase in the action potential duration (APD). These mutations tend to be the most common yet least severe.
LQT2	152427	alpha subunit of the rapid delayed rectifier potassium channel (HERG + MiRP1)	Current through this channel is known as IKr. This phenotype is also probably caused by a reduction in repolarizing current.
LQT3	603830	alpha subunit of the sodium channel (SCN5A)	Current through this channel is commonly referred to as INa. Depolarizing current through the channel late in the action potential is thought to prolong APD. The late current is due to failure of the channel to remain inactivated and hence enter a bursting mode in which significant current can enter when it should not. These mutations are more lethal but less common.
LQT4	600919	anchor protein Ankyrin B	LQT4 is very rare. Ankyrin B anchors the ion channels in the cell.
LQT5	176261	beta subunit MinK (or KCNE1) which coassembles withKvLQT1	-
LQT6	603796	beta subunit MiRP1 (or KCNE2) which coassembles with HERG	-
LQT7	170390	potassium channel KCNJ2 (or Kir2.1)	The current through this channel and KCNJ12 (Kir2.2) is called IK1. LQT7 leads to Andersen-Tawil syndrome.
LQT8	601005	alpha subunit of the calcium channel Cav1.2 encoded by the gene CACNA1c.	Leads to Timothy's syndrome.
LQT9		Caveolin 3
LQT10		SCN4B

Drug induced LQT is usually a result of treatment by anti-arrhythmic drugs such as amiodarone or a number of other drugs that have been reported to cause this problem (e.g. cisapride). Someanti-psychotic drugs, such as Haloperidol and Ziprasidone, have a prolonged QT interval as a rare side effect. Genetic mutations may make one more susceptible to drug induced LQT.