Cerebrotendinous xanthomatosis is a disorder characterized by abnormal storage of fats (lipids) in many areas of the body. People with this disorder cannot break down certain lipids effectively, specifically different forms of cholesterol, so these fats accumulate in the body in the form of fatty yellow nodules called xanthomas. These xanthomas are most commonly found in the brain and in connective tissue called tendons that attach muscle to bone, which is reflected in the condition name (cerebro- meaning brain and -tendinous referring to tendons).

People with cerebrotendinous xanthomatosis often develop neurological problems in early adulthood that are thought to be caused by an abnormal accumulation of fats and an increasing number of xanthomas in the brain. These neurological problems include recurrent seizures (epilepsy), movement disorders, impaired speech (dysarthria), loss of sensation in the arms and legs (peripheral neuropathy), decline in intellectual function (dementia), hallucinations, and depression. Xanthomas can accumulate in the fatty substance that insulates and protects nerves (myelin), causing the destruction of myelin and disrupting nerve signaling in the brain. Degeneration (atrophy) of brain tissue caused by excess lipid deposits also contributes to the neurological problems.

Xanthomas in the tendons begin to form in early adulthood. The most common areas for xanthomas to develop are tendons in the hands, elbows, knees, neck, and in the Achilles tendon, which connects the heel of the foot to the calf muscles in the leg. Tendon xanthomas may cause discomfort and interfere with tendon flexibility. While many affected people develop tendon xanthomas, these nodules may not be easily visible underneath the skin.

Other features of cerebrotendinous xanthomatosis include clouding of the lenses of the eyes (cataracts) and chronic diarrhea in childhood; a reduced ability to produce and release a digestive fluid called bile (cholestasis), which can lead to a yellowing of the skin or whites of the eyes (jaundice); and progressively brittle bones that are prone to fracture (osteoporosis). People with cerebrotendinous xanthomatosis are also at an increased risk of developing cardiovascular disease or respiratory failure because of lipid accumulation in the heart or lungs, respectively. If untreated, the signs and symptoms related to cerebrotendinous xanthomatosis worsen over time; however, this condition varies greatly among those who are affected.

Mutations in the CYP27A1 gene cause cerebrotendinous xanthomatosis. The CYP27A1 gene provides instructions for producing an enzyme called sterol 27-hydroxylase. This enzyme works in the pathway that breaks down cholesterol to form acids used in the digestion of fats (bile acids), specifically a bile acid called chenodeoxycholic acid.

Mutations in the CYP27A1 gene lead to the production of a nonfunction or abnormal sterol 27-hydroxylase that cannot help form chenodeoxycholic acid. As a result, other molecules are formed by an alternative pathway. A molecule called cholestanol, which is similar to cholesterol, is produced as well as substances called bile alcohols. Cholestanol and bile alcohols are increased in the blood, while blood cholesterol levels are typically normal. In various tissues in the body, including the brain and heart, cholesterol and cholestanol levels are increased. These lipids make up much of the fats found in xanthomas. The accumulation of cholesterol and cholestanol throughout the body's tissues causes the signs and symptoms of cerebrotendinous xanthomatosis.

Normal Function

The CYP27A1 gene is a member of the cytochrome P450 gene family. Enzymes produced from the cytochrome P450 genes are involved in the formation and breakdown of various molecules and chemicals within cells. The CYP27A1 gene provides instructions for producing an enzyme called sterol 27-hydroxylase. This enzyme is located in the energy-producing centers of cells (mitochondria), where it is involved in the pathway that breaks down cholesterol to form acids used to digest fats (bile acids). Specifically, sterol 27-hydroxylase breaks down cholesterol to form a bile acid called chenodeoxycholic acid. The formation of bile acids from cholesterol is the body's main pathway for cholesterol removal. Sterol 27-hydroxylase plays a key role in maintaining normal cholesterol levels in the body.

Health Conditions Related to Genetic Changes

Cerebrotendinous xanthomatosis

At least 90 mutations that cause cerebrotendinous xanthomatosis have been identified in the CYP27A1 gene. Cerebrotendinous xanthomatosis is a disorder characterized by abnormal storage of fats (lipids) in many areas of the body. Most CYP27A1 gene mutations change one protein building block (amino acid) in the sterol 27-hydroxylase enzyme. The most common mutation changes the amino acid arginine to the amino acid cysteine at position 362 in the protein (written as Arg362Cys or R362C). Changes in amino acids typically disrupt the normal function of the protein and impair its ability to help form chenodeoxycholic acid. Other mutations cause no functional enzyme to be made. As a result, other molecules are formed by an alternative pathway. A molecule called cholestanol, which is similar to cholesterol, is produced and accumulates in blood and tissues. Cholesterol also accumulates in tissues, but levels in blood are typically normal. The accumulation of cholesterol and cholestanol throughout the body's tissues causes the signs and symptoms of cerebrotendinous xanthomatosis.

Other Names for This Gene

• 5-beta-cholestane-3-alpha, 7-alpha, 12-alpha-triol 27-hydroxylase
• CP27
• CP27A_HUMAN
• CTX
• CYP27
• cytochrome P-450C27/25
• cytochrome P450, family 27, subfamily A, polypeptide 1
• cytochrome P450, subfamily XXVIIA (steroid 27-hydroxylase, cerebrotendinous xanthomatosis), polypeptide 1
• sterol 27-hydroxylase
• vitamin D(3) 25-hydroxylase

Genomic Location

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

The symptoms seen in cerebrotendinous xanthomatosis are listed below. We have also included the typical age when each symptom appears.

• Chronic diarrhea (infancy)
• Cataracts (early childhood)
• Mental impairment (infancy or at puberty)
• Xanthomas (adolescents to early adulthood)
• Dementia with slow deterioration in intellectual abilities (early adulthood)
• Spasticity (early adulthood)
• Cerebellar signs such as intention tremor, difficulty with fast hand movements, nystagmus, truncal ataxia, and rhomberg's sign) (early adulthood)
• Behavioral changes (early adulthood)
• Hallucinations (early adulthood)
• Agitation (early adulthood)
• Aggression (early adulthood)
• Depression (early adulthood)
• Suicide attempt (early adulthood)

Other symptoms may include dystonia, atypical parkinsonism, seizures, and peripheral neuropathy.

The incidence of cerebrotendinous xanthomatosis is estimated to be 1 per million individuals worldwide. This condition is more common in the Moroccan Jewish population with an incidence of 1 in 108 individuals.