SUCLG1 Gene

Succinate-CoA ligase deficiency is an inherited disorder that affects the early development of the brain and other body systems. One of the earliest signs of the disorder is very weak muscle tone (severe hypotonia), which appears in the first few months of life. Severe hypotonia delays the development of motor skills such as holding up the head and rolling over. Many affected children also have muscle weakness and reduced muscle mass, which prevents them from standing and walking independently.

Additional features of succinate-CoA ligase deficiency can include progressive abnormal curvature of the spine (scoliosis or kyphosis), uncontrolled movements (dystonia), severe hearing loss, and seizures beginning in childhood. In most affected children, a substance called methylmalonic acid builds up abnormally in the body and is excreted in urine (methylmalonic aciduria). Most children with succinate-CoA ligase deficiency also experience a failure to thrive, which means that they gain weight and grow more slowly than expected.

Succinate-CoA ligase deficiency causes breathing difficulties that often lead to recurrent infections of the respiratory tract. These infections can be life-threatening, and most people with succinate-CoA ligase deficiency live only into childhood or adolescence.

A few individuals with succinate-CoA ligase deficiency have had an even more severe form of the disorder known as fatal infantile lactic acidosis. Affected infants develop a toxic buildup of lactic acid in the body (lactic acidosis) in the first day of life, which leads to muscle weakness and breathing difficulties. Children with fatal infantile lactic acidosis usually live only a few days after birth.

Nonsyndromic hearing loss is a partial or total loss of hearing that is not associated with other signs and symptoms. In contrast, syndromic hearing loss occurs with signs and symptoms affecting other parts of the body.

Nonsyndromic hearing loss can be classified in several different ways. One common way is by the condition's pattern of inheritance: autosomal dominant (DFNA), autosomal recessive (DFNB), X-linked (DFNX), or mitochondrial (which does not have a special designation). Each of these types of hearing loss includes multiple subtypes. DFNA, DFNB, and DFNX subtypes are numbered in the order in which they were first described. For example, DFNA1 was the first type of autosomal dominant nonsyndromic hearing loss to be identified.

The characteristics of nonsyndromic hearing loss vary among the different types. Hearing loss can affect one ear (unilateral) or both ears (bilateral). Degrees of hearing loss range from mild (difficulty understanding soft speech) to profound (inability to hear even very loud noises). The term "deafness" is often used to describe severe-to-profound hearing loss. Hearing loss can be stable, or it may be progressive, becoming more severe as a person gets older. Particular types of nonsyndromic hearing loss show distinctive patterns of hearing loss. For example, the loss may be more pronounced at high, middle, or low tones.

Most forms of nonsyndromic hearing loss are described as sensorineural, which means they are associated with a permanent loss of hearing caused by damage to structures in the inner ear. The inner ear processes sound and sends the information to the brain in the form of electrical nerve impulses. Less commonly, nonsyndromic hearing loss is described as conductive, meaning it results from changes in the middle ear. The middle ear contains three tiny bones that help transfer sound from the eardrum to the inner ear. Some forms of nonsyndromic hearing loss, particularly a type called DFNX2, involve changes in both the inner ear and the middle ear. This combination is called mixed hearing loss.

Depending on the type, nonsyndromic hearing loss can become apparent at any time from infancy to old age. Hearing loss that is present before a child learns to speak is classified as prelingual or congenital. Hearing loss that occurs after the development of speech is classified as postlingual.