Familial restrictive cardiomyopathy is a genetic form of heart disease. For the heart to beat normally, the heart (cardiac) muscle must contract and relax in a coordinated way. Oxygen-rich blood from the lungs travels first through the upper chambers of the heart (the atria), and then to the lower chambers of the heart (the ventricles).

In people with familial restrictive cardiomyopathy, the heart muscle is stiff and cannot fully relax after each contraction. Impaired muscle relaxation causes blood to back up in the atria and lungs, which reduces the amount of blood in the ventricles.

Familial restrictive cardiomyopathy can appear anytime from childhood to adulthood. The first signs and symptoms of this condition in children are failure to gain weight and grow at the expected rate (failure to thrive), extreme tiredness (fatigue), and fainting. Children who are severely affected may also have abnormal swelling or puffiness (edema), increased blood pressure, an enlarged liver, an abnormal buildup of fluid in the abdominal cavity (ascites), and lung congestion. Some children with familial restrictive cardiomyopathy do not have any obvious signs or symptoms, but they may die suddenly due to heart failure. Without treatment, the majority of affected children survive only a few years after they are diagnosed.

Adults with familial restrictive cardiomyopathy typically first develop shortness of breath, fatigue, and a reduced ability to exercise. Some individuals have an irregular heart beat (arrhythmia) and may also experience a sensation of fluttering or pounding in the chest (palpitations) and dizziness. Abnormal blood clots are commonly seen in adults with this condition. Without treatment, approximately one-third of adults with familial restrictive cardiomyopathy do not survive more than five years after diagnosis.

Mutations in several genes have been found to cause familial restrictive cardiomyopathy. Mutations in the TNNI3 gene are one of the major causes of this condition. The TNNI3 gene provides instructions for making a protein called cardiac troponin I, which is found solely in the heart. Cardiac troponin I is one of three proteins that make up the troponin protein complex, which helps regulate tensing (contraction) and relaxation of the heart muscle.

TNNI3 gene mutations associated with familial restrictive cardiomyopathy result in the production of a defective cardiac troponin I protein. The altered protein disrupts the function of the troponin protein complex and does not allow the heart muscle to fully relax. As a result, not enough blood enters the ventricles, leading to a buildup in the atria and lungs. The abnormal heart relaxation and blood flow is responsible for many of the signs and symptoms of familial restrictive cardiomyopathy.

Mutations in other genes associated with familial restrictive cardiomyopathy each account for a small percentage of cases of this condition. Some people with familial restrictive cardiomyopathy do not have an identified mutation in any of the known associated genes. The cause of the disorder in these individuals is unknown.

Normal Function

The TNNI3 gene provides instructions for making a protein called cardiac troponin I, which is found solely in the heart (cardiac) muscle. Cardiac troponin I is one of three proteins that make up the troponin protein complex in cardiac muscle cells. The troponin complex is associated with a structure called the sarcomere, which is the basic unit of muscle contraction. Sarcomeres are made up of thick and thin filaments. The overlapping thick and thin filaments attach (bind) to each other and release, which allows the filaments to move relative to one another so that muscles can contract. The troponin complex, along with calcium, helps regulate tensing (contraction) of cardiac muscle.

For the heart to beat normally, cardiac muscle must contract and relax in a coordinated way. Cardiac troponin I helps to coordinate contraction of the heart. When calcium levels are low, the troponin complex binds to the thin filament. This binding blocks the interaction between the thick and thin filaments that is needed for muscle contraction. An increase in calcium levels causes structural changes in another troponin complex protein called troponin C, which then triggers the troponin complex to detach from the thin filament, allowing the heart muscle to contract.

Health Conditions Related to Genetic Changes

Familial hypertrophic cardiomyopathy

Mutations in the TNNI3 gene can cause familial hypertrophic cardiomyopathy, a condition characterized by thickening (hypertrophy) of the cardiac muscle. TNNI3 gene mutations are found in less than 5 percent of people with this condition. Although some people with hypertrophic cardiomyopathy have no obvious health effects, all affected individuals have an increased risk of heart failure and sudden death.

Most TNNI3 gene mutations in familial hypertrophic cardiomyopathy change single protein building blocks (amino acids) in the cardiac troponin I protein. The altered protein is likely incorporated into the troponin complex, but it may not function properly. It is unclear how these mutations lead to the features of familial hypertrophic cardiomyopathy.

In some people, hypertrophic cardiomyopathy develops into restrictive cardiomyopathy (described below), although it can be difficult to distinguish these two disorders.

Familial restrictive cardiomyopathy

Approximately 10 mutations in the TNNI3 gene have been found to cause familial restrictive cardiomyopathy, which is characterized by stiffening of the heart muscle. Most of these mutations change single amino acids in the cardiac troponin I protein, which impairs the protein's function. The altered protein typically cannot bind to actin. As a result, heart muscle relaxation is disrupted, leading to abnormal heart function, impaired blood flow, and the signs and symptoms of familial restrictive cardiomyopathy, such as fatigue and fainting.

Familial dilated cardiomyopathy

MedlinePlus Genetics provides information about Familial dilated cardiomyopathy

Other disorders

Mutations in the TNNI3 gene can also cause another heart conditions called dilated cardiomyopathy. This condition weakens and enlarges the heart, preventing it from pumping blood efficiently. Dilated cardiomyopathy increases the risk of heart failure and premature death.

Other Names for This Gene

• cardiac troponin I
• cTnI
• TNNI3_HUMAN
• troponin I type 3 (cardiac)
• troponin I, cardiac muscle

Genomic Location

This condition is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder.

The prevalence of familial restrictive cardiomyopathy is unknown. Although cardiomyopathy is a relatively common condition, restrictive cardiomyopathy, in which relaxation of the heart muscle is impaired, is the least common type. Some other forms of cardiomyopathy involve a weak or enlarged heart muscle with impaired contraction. In the United States and in Europe, restrictive cardiomyopathy accounts for less than five percent of all cardiomyopathies. The proportion of restrictive cardiomyopathy that runs in families is not known.