Andersen-Tawil syndrome is a disorder that causes episodes of muscle weakness (periodic paralysis), changes in heart rhythm (arrhythmia), and developmental abnormalities. Periodic paralysis begins early in life, and episodes last from hours to days. These episodes may occur after exercise or long periods of rest, but they often have no obvious trigger. Muscle strength usually returns to normal between episodes. However, mild muscle weakness may eventually become permanent.

In people with Andersen-Tawil syndrome, the most common changes affecting the heart are ventricular arrhythmia, which is a disruption in the rhythm of the heart's lower chambers (the ventricles), and long QT syndrome. Long QT syndrome is a heart condition that causes the heart (cardiac) muscle to take longer than usual to recharge between beats. The irregular heartbeats can lead to discomfort, such as the feeling that the heart is skipping beats (palpitations). Uncommonly, the irregular heartbeats can cause fainting (syncope), and even more rarely, sudden death.

Physical abnormalities associated with Andersen-Tawil syndrome typically affect the face, other parts of the head, and the limbs. These features often include a very small lower jaw (micrognathia), dental abnormalities (such as crowded teeth), low-set ears, widely spaced eyes, fusion (syndactyly) of the second and third toes, and unusual curving of the fingers or toes (clinodactyly). Some affected people also have short stature and an abnormal side-to-side curvature of the spine (scoliosis).

The signs and symptoms of Andersen-Tawil syndrome vary widely, and they can be different even among affected members of the same family. About 60 percent of affected individuals have all three major features (periodic paralysis, cardiac arrhythmia, and physical abnormalities).

Mutations in the KCNJ2 gene cause about 60 percent of all cases of Andersen-Tawil syndrome. When the disorder is caused by mutations in this gene, it is classified as type 1 (ATS1).

The KCNJ2 gene provides instructions for making channels that transport positively charged potassium ions across the membrane of muscle cells. The movement of potassium ions through these channels is critical for maintaining the normal function of muscles used for movement (skeletal muscles) and cardiac muscle. Mutations in the KCNJ2 gene alter the usual structure and function of these potassium channels. These changes disrupt the flow of potassium ions in skeletal and cardiac muscle, leading to the periodic paralysis and irregular heart rhythm characteristic of Andersen-Tawil syndrome.

Researchers have not determined the role of the KCNJ2 gene in bone development, and it is not known how mutations in the gene lead to the skeletal changes and other physical abnormalities often found in Andersen-Tawil syndrome.

In the 40 percent of cases not caused by KCNJ2 gene mutations, the cause of Andersen-Tawil syndrome is usually unknown. These cases are classified as type 2 (ATS2). Studies suggest that variations in at least one other potassium channel gene may underlie the disorder in some of these affected individuals.

Normal Function

The KCNJ2 gene belongs to a large family of genes that provide instructions for making potassium channels. These channels, which transport positively charged potassium ions out of cells, play key roles in a cell's ability to generate and transmit electrical signals.

The specific function of a potassium channel depends on its protein components and its location in the body. Channels made with the KCNJ2 protein are active in muscles used for movement (skeletal muscles) and in heart (cardiac) muscle. In skeletal muscle, these channels play an important role in the pattern of muscle tensing (contraction) and relaxation that allows the body to move. In the heart, the channels are involved in recharging the cardiac muscle after each heartbeat to maintain a regular rhythm. Channels formed with the KCNJ2 protein may also be involved in bone development, but their role in this process is unclear.

Researchers have determined that a molecule called PIP2 must attach (bind) to channels made with the KCNJ2 protein for the channels to function normally. PIP2 activates the ion channel and helps it stay open, which allows ions to flow across the cell membrane.

Health Conditions Related to Genetic Changes

Andersen-Tawil syndrome

More than 60 mutations in the KCNJ2 gene have been found to cause Andersen-Tawil syndrome, a disorder characterized by episodes of muscle weakness (periodic paralysis), changes in heart rhythm (arrhythmia), and physical abnormalities affecting the face, other parts of the head, and the limbs. Most of the mutations change a single protein building block (amino acid) in the KCNJ2 protein.

Mutations in the KCNJ2 gene lead to the production of a nonfunctional potassium channel. Some mutations change the shape of the channel so it cannot transport potassium ions, while other mutations prevent the channels from being inserted correctly into the cell membrane. Many KCNJ2 mutations prevent PIP2 from effectively binding to and activating potassium channels. If the KCNJ2 protein is unable to bind to PIP2, the channels remain closed and potassium ions are unable to flow across the cell membrane. Researchers believe that problems with PIP2 binding are a major cause of Andersen-Tawil syndrome.

A loss of this channel's function in skeletal and cardiac muscle cells disrupts the normal flow of potassium ions out of these cells, resulting in periodic paralysis and an irregular heart rhythm. It is not known how mutations in the KCNJ2 gene contribute to the physical abnormalities often found in Andersen-Tawil syndrome.

Short QT syndrome

Mutations in the KCNJ2 gene can also cause a heart condition called short QT syndrome, which is a type of arrhythmia. In people with this condition, the cardiac muscle takes less time than usual to recharge between beats. This change increases the risk of abnormal heart rhythm that can cause fainting or sudden death.

At least two mutations in the KCNJ2 gene have been found to cause short QT syndrome in a small number of affected families. These mutations change single amino acids in the KCNJ2 protein, which increases the activity of channels made with this protein. As a result, more potassium ions flow out of cardiac muscle cells at a critical time during the heartbeat, which can lead to an irregular heart rhythm.

Familial atrial fibrillation

MedlinePlus Genetics provides information about Familial atrial fibrillation

Other Names for This Gene

• cardiac inward rectifier potassium channel
• HHBIRK1
• HHIRK1
• HIRK1
• inward rectifier K+ channel KIR2.1
• IRK1
• IRK2_HUMAN
• KIR2.1
• LQT7
• potassium channel, inwardly rectifying subfamily J, member 2
• potassium inwardly-rectifying channel J2
• potassium inwardly-rectifying channel, subfamily J, member 2

Genomic Location

This condition is inherited in an autosomal dominant pattern, which means one copy of an altered gene in each cell is sufficient to cause the disorder. When the condition results from a mutation in the KCNJ2 gene, an affected individual may inherit the mutation from one affected parent. In other cases, the condition results from a new (de novo) mutation in the KCNJ2 gene. These cases occur in people with no history of the disorder in their family.

Andersen-Tawil syndrome is a rare genetic disorder. Its exact prevalence is unknown, although it is estimated to affect 1 in 1 million people worldwide. About 200 affected individuals have been described in the medical literature. Researchers believe that Andersen-Tawil syndrome accounts for less than 10 percent of all cases of periodic paralysis.