BCS1L Gene

Björnstad syndrome is a rare disorder characterized by abnormal hair and hearing problems. Affected individuals have a condition known as pili torti, which means "twisted hair," so named because the strands appear twisted when viewed under a microscope. The hair is brittle and breaks easily, leading to short hair that grows slowly. In Björnstad syndrome, pili torti usually affects only the hair on the head; eyebrows, eyelashes, and hair on other parts of the body are normal. The proportion of hairs affected and the severity of brittleness and breakage can vary. This hair abnormality commonly begins before the age of 2. It may become milder with age, particularly after puberty.

People with Björnstad syndrome also have hearing problems that become evident in early childhood. The hearing loss, which is caused by changes in the inner ear (sensorineural deafness), can range from mild to severe. Mildly affected individuals may be unable to hear sounds at certain frequencies, while severely affected individuals may not be able to hear at all.

GRACILE syndrome is a severe disorder that begins before birth. GRACILE stands for the condition's characteristic features: growth retardation, aminoaciduria, cholestasis, iron overload, lactic acidosis, and early death.

In GRACILE syndrome, growth before birth is slow (intrauterine growth retardation). Affected newborns are smaller than average and have an inability to grow and gain weight at the expected rate (failure to thrive). A characteristic of GRACILE syndrome is excess iron in the liver, which likely begins before birth. Iron levels may begin to improve after birth, although they typically remain elevated. Within the first day of life, infants with GRACILE syndrome have a buildup of a chemical called lactic acid in the body (lactic acidosis). They also have kidney problems that lead to an excess of molecules called amino acids in the urine (aminoaciduria). Babies with GRACILE syndrome have cholestasis, which is a reduced ability to produce and release a digestive fluid called bile. Cholestasis leads to irreversible liver disease (cirrhosis) in the first few months of life.

Because of the severe health problems caused by GRACILE syndrome, infants with this condition do not survive for more than a few months, and about half die within a few days of birth.

Mitochondrial complex III deficiency is a genetic condition that can affect several parts of the body, including the brain, kidneys, liver, heart, and the muscles used for movement (skeletal muscles). Signs and symptoms of mitochondrial complex III deficiency usually begin in infancy but can appear later.

The severity of mitochondrial complex III deficiency varies widely among affected individuals. People who are mildly affected tend to have muscle weakness (myopathy) and extreme tiredness (fatigue), particularly during exercise (exercise intolerance). More severely affected individuals have problems with multiple body systems, such as liver disease that can lead to liver failure, kidney abnormalities (tubulopathy), and brain dysfunction (encephalopathy). Encephalopathy can cause delayed development of mental and motor skills (psychomotor delay), movement problems, weak muscle tone (hypotonia), and difficulty with communication. Some affected individuals have a form of heart disease called cardiomyopathy, which can lead to heart failure. Most people with mitochondrial complex III deficiency have a buildup of a chemical called lactic acid in the body (lactic acidosis). Some affected individuals also have buildup of molecules called ketones (ketoacidosis) or high blood sugar levels (hyperglycemia). Abnormally high levels of these chemicals in the body can be life-threatening.

Mitochondrial complex III deficiency can be fatal in childhood, although individuals with mild signs and symptoms can survive into adolescence or adulthood.

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.