Tarsal-carpal coalition syndrome is a rare, inherited bone disorder that affects primarily the hands and feet. Several individual bones make up each wrist (carpal bones) and ankle (tarsal bones). In tarsal-carpal coalition syndrome, the carpal bones fuse together, as do the tarsal bones, which causes stiffness and immobility of the hands and feet. Symptoms of the condition can become apparent in infancy, and they worsen with age. The severity of the symptoms can vary, even among members of the same family.

In this condition, fusion at the joints between the bones that make up each finger and toe (symphalangism) can also occur. Consequently, the fingers and toes become stiff and difficult to bend. Stiffness of the pinky fingers and toes (fifth digits) is usually noticeable first. The joints at the base of the pinky fingers and toes fuse first, and slowly, the other joints along the length of these digits may also be affected. Progressively, the bones in the fourth, third, and second digits (the ring finger, middle finger, and forefinger, and the corresponding toes) become fused. The thumb and big toe are usually not involved. Affected individuals have increasing trouble forming a fist, and walking often becomes painful and difficult. Occasionally, there is also fusion of bones in the upper and lower arm at the elbow joint (humeroradial fusion). Less common features of tarsal-carpal coalition syndrome include short stature or the development of hearing loss.

Tarsal-carpal coalition syndrome is caused by mutations in the NOG gene, which provides instructions for making a protein called noggin. This protein plays an important role in proper bone and joint development by blocking (inhibiting) signals that stimulate bone formation. The noggin protein attaches (binds) to proteins called bone morphogenetic proteins (BMPs), which keeps the BMPs from triggering signals for the development of bone.

NOG gene mutations that cause tarsal-carpal coalition syndrome reduce the amount of functional noggin protein. With decreased noggin function, BMPs abnormally stimulate bone formation in joint areas, where there should be no bone, causing the bone fusions seen in people with tarsal-carpal coalition syndrome.

Mutations in the NOG gene are involved in several disorders with overlapping signs and symptoms. Because of a shared genetic cause and overlapping features, researchers have suggested that these conditions, including tarsal-carpal coalition syndrome, represent a spectrum of related conditions referred to as NOG-related-symphalangism spectrum disorder (NOG-SSD).

Normal Function

The NOG gene provides instructions for making a protein called noggin. This protein is involved in the development of many body tissues, including nerve tissue, muscles, and bones. Noggin's role in bone development makes it important for proper joint formation.

Noggin interacts with members of a group of proteins called bone morphogenetic proteins (BMPs). These proteins help control the development of bone and other tissues. In order to begin these developmental processes, BMPs attach (bind) to other proteins called receptors, and this binding stimulates specific cellular processes. The noggin protein regulates the activity of certain BMPs by attaching to them and blocking them from binding to the receptor, which leads to a decrease in BMP signaling.

Health Conditions Related to Genetic Changes

Tarsal-carpal coalition syndrome

Several mutations in the NOG gene have been identified in people with a condition called tarsal-carpal coalition syndrome. This condition is characterized by fusion of the individual bones in the wrists (the carpal bones) and in the ankles (the tarsal bones) as well as fusion at the joints between the bones that make up each finger and toe (symphalangism). Symphalangism makes the fingers and toes stiff and difficult to bend.

This condition is caused by mutations in the NOG gene that change single protein building blocks (amino acids) in the noggin protein. These mutations alter the structure or stability of noggin, impair the transport of noggin out of the cell, or reduce the protein's ability to bind to BMPs, resulting in a reduction of functional noggin protein. With decreased noggin function, BMPs abnormally stimulate bone formation in joint areas, where there should be no bone, causing the bone fusions seen in people with tarsal-carpal coalition syndrome.

Other disorders

Several other bone-related disorders are caused by mutations in the NOG gene. These gene mutations change single protein building blocks (amino acids) in the noggin protein.

Proximal symphalangism is characterized by fusion at the joints between the bones in the fingers and toes, particularly at the joint at the base of the digit. Other signs and symptoms include abnormally short middle fingers, webbed toes, and hearing loss that is due to fusion of the bones in the ears (stapes fixation).

Multiple synostoses syndrome 1 is characterized by symphalangism and characteristic facial features, such as a broad nose and thin lips. In addition, affected individuals can have fusion of the bones in the hands, feet, hips, and upper part of the spine (cervical vertebrae). People with this condition can have hearing loss due to stapes fixation.

People with stapes ankylosis with broad thumb and toes (also known as Teunissen-Cremers syndrome) have hearing loss due to stapes fixation, farsightedness, and broad thumbs and big toes. Some affected individuals may have fusion of the cervical vertebrae and characteristic facial features like those seen in multiple synostoses syndrome 1.

Brachydactyly type B2 is characterized by short fingers and toes, which occurs because the middle bone of the digit or the bone that forms the tip of the digit is abnormally small or absent. In addition, people with this condition can have symphalangism, fusion of the carpal bones, and connection of the skin between two or more fingers or toes (syndactyly).

As in tarsal-carpal coalition syndrome, the NOG gene mutations that cause these conditions reduce the amount of functional noggin protein. For reasons that are unknown, the same mutations can cause different disorders in different people. Because of a shared genetic cause and overlapping features, researchers have suggested that these conditions, including tarsal-carpal coalition syndrome, represent a spectrum of related conditions referred to as NOG-related-symphalangism spectrum disorder (NOG-SSD).

Other Names for This Gene

• NOGG_HUMAN
• noggin precursor
• SYM1
• symphalangism 1 (proximal)
• SYNS1

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.

This condition is very rare; however, the exact prevalence is unknown.