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Long QT Syndrome

The long QT syndrome (LQTS) is a heart disease in which there is an abnormally long delay between the electrical excitation (or depolarization) and relaxation (repolarization) of the ventricles of the heart. It is associated with syncope (loss of consciousness) and with sudden death due to ventricular arrhythmias. Arrhythmias in individuals with LQTS are often associated with exercise or excitement. The cause of sudden cardiac death in individuals with LQTS is ventricular fibrillation. more...

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Individuals with LQTS have a prolongation of the QT interval on the ECG. The Q point on the ECG corresponds to the beginning of ventricular depolarization while the T point corresponds to the beginning of ventricular repolarization. The QT interval is measured from the Q point to the end of the T wave. While many individuals with LQTS have persistent prolongation of the QT interval, some individuals do not always show the QT prolongation; in these individuals, the QT interval may prolong with the administration of certain medications.


The two most common types of LQTS are genetic and drug-induced. Genetic LQTS can arise from mutation to one of several genes. These mutations tend to prolong the duration of the ventricular action potential (APD), thus lengthening the QT interval. LQTS can be inherited in an autosomal dominant or an autosomal recessive fashion. The autosomal recessive forms of LQTS tend to have a more severe phenotype, with some variants having associated syndactyly or congenital neural deafness. A number of specific genes loci have been identified that are associated with LQTS. Following is a list of the most common mutations:

  • LQT1 - mutations to the alpha subunit of the slow delayed rectifier potassium channel (KvLQT1 or KCNQ1). The current through the heteromeric channel (KvLQT1+minK) is known as IKs. This mutation is thought to cause LQT by reducing the amount of repolarizing action potential current that prolongs action potential duration (APD). These mutations tend to be the most common yet least severe.
  • LQT2 - mutations to the alpha subunit of the fast delayed rectifier potassium channel (HERG + miRP). Current through this channel is known as IKr. This phenotype is also probably caused by a reduction in repolarizing current.
  • LQT3 - mutations to the alpha subunit of the sodium channel (SCN5A). Current through is 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 - mutations in an anchor protein Ankyrin B which anchors the ion channels in the cell. Very rare.
  • LQT5 - mutations in the beta subunit MinK which coassembles with KvLQT1.
  • LQT6 - mutations in the beta subunit MiRP1 which coassembles with HERG.
  • LQT7 - mutations in the potassium channel KCNJ2 which leads to Andersen-Tawil syndrome.
  • LQT8 - mutations in the calcium channel Cav1.2 encoded by the gene CACNA1c leading to Timothy's syndrome

Other mutations affect the beta subunits ion channels. For example LQT6 affects minK (aka KCNE1) which is the beta subunit that coassembles with KCNQ1 to form IKs channels.


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Secrets underlie lethal heart condition - long QT syndrome
From Science News, 7/26/97 by Nathan Seppa

Genetic mutations that can cause the heart to stop suddenly in otherwise healthy people are proving to be more varied and insidious than researchers had suspected. The condition, called long QT syndrome, may also be more prevalent than the currently estimated rate of 1 in 9,000.

Long QT syndrome is an inherited disease that causes a rapid heartbeat, fainting, and sometimes death--most often, apparently, during stress. It could account for half of the approximately 8,000 sudden, unexplained deaths among children and adults each year, says G. Michael Vincent, a cardiologist at the University of Utah School of Medicine in Salt Lake City.

Researchers have identified mutations in several genes, any one of which can produce the heart problems. Three of these genes had been detected by 1996 (SN: 1/13/96, p. 31), two were discovered in the last year, and another is suspected. Carriers of one of these mutated genes have a 50 percent chance of passing it on to their children. In rare cases, the mutation can cause deafness, too.

The condition gets its name from the unusually long interval between two points, labeled Q and T, on an electrocardiogram (EKG), which records a person's heart rhythm. The excessive interval signals that cells in the heart aren't recharging fully between beats. Sudden jolts of fear, excitement, or physical exertion may increase this already long gap. The person suffers arrhythmia, a fluttery heartbeat that pumps insufficient blood to the brain, and faints. The worst episodes conclude with the heart's stopping, causing death, Vincent says.

Recent findings on the syndrome continue to turn up surprises.

For example, the QT interval can vary greatly in people who have the syndrome, even among those who are related. Moreover, genetic screening made possible by the new findings turns up many potential cases that would have gone undetected in the past, says Katherine Timothy, research coordinator of the Utah team. For 10 to 15 percent of those who have one of the characteristic mutations, an EKG shows no sign of the ailment.

Another 30 percent of carriers have borderline long QT intervals, Vincent reported last week at an American Heart Association seminar in Portland, Ore. "That further complicates the diagnosis." He believes that these slightly longer than normal intervals could be present in as many as 1 in 5,000 people.

Long QT syndrome is treatable. Early diagnosis is especially important because the disorder causes many deaths during puberty. Beta blockers, which inhibit the nervous system's influence on the heart, are effective against long QT syndrome in about 90 percent of cases. "Beta blockers slow the heart rate and make the heart resistant to irregular rhythms," says Charles R. Bridges Jr., a heart surgeon at the University of Pennsylvania in Philadelphia. The recent QT findings are "fascinating stuff," he adds.

Scientists have recently come to suspect that a low-protein diet or drug use can also trigger the heart problem in people with long QT syndrome. Several people with the condition have died while smoking marijuana, and one died while using cocaine, Vincent says. Anecdotal evidence suggests that some prescription medications may also exacerbate the condition.

One rare form of long QT syndrome is accompanied by congenital deafness, but only when both parents pass along a copy of a defective gene. Studies in mice, published earlier this year, indicate that this gene variant may be recessive, unlike the other long QT mutations.

Women are diagnosed with long QT syndrome more often than men, Vincent says. Women normally have slightly longer QT intervals and thus may be more susceptible to the syndrome's effects, he suggests. In addition, many boys who die of unidentified causes may in fact have succumbed to long QT syndrome, leaving fewer men than women with the syndrome. "That skews the subsequent evaluation," he says.

Because long QT syndrome so often goes undiagnosed, the Utah researchers have amassed data by surveying relatives of people suspected of having had it. After Reader's Digest published a story on Vincent's work in 1996, 2,500 people called his office to relate an unexplained sudden death, usually of a young person, in the family. Vincent is now seeking funding to investigate those cases.

COPYRIGHT 1997 Science Service, Inc.
COPYRIGHT 2004 Gale Group

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