Cerebrotendinous Xanthomatosis (CTX)

  Cerebrotendinous Xanthomatosis (CTX)

Summary

Cerebrotendinous xanthomatosis (CTX) is a rare autosomal recessive genetic disorder caused by an abnormality in the CYP27A1 gene, resulting in a deficiency of the mitochondrial enzyme sterol 27-hydroxylase. The lack of this enzyme prevents cholesterol from being converted into a bile acid called chenodeoxycholic acid. Deposits of cholesterol and a related compound called cholestanol accumulate in the nerve cells and membranes potentially causing damage to the brain, spinal cord, tendons, lens of the eye and arteries. Affected individuals can experience diarrhea and cataracts in childhood and may develop benign, fatty tumors (xanthomas) of the tendons during adolescence. If untreated, CTX can lead to progressive neurologic problems such as seizures, cognitive impairment, and difficulties with coordination and balance (ataxia). Coronary heart disease is common. Some individuals with the later-onset symptoms of CTX experienced cholestatic jaundice during infancy. The specific symptoms and progression of this disorder can vary greatly from one individual to another, even for twins with the same abnormality in the CYP27A1 gene. Long-term therapy with chenodeoxycholic acid has been effective in treating affected individuals.

Introduction

CTX was first described in the medical literature in 1937. CTX is classified as a bile acid synthesis disorder (due to the underlying genetic mutation that causes deficiency in an important enzyme in the bile acid synthesis pathway; sterol 27-hydroxylase). Bile acids (chenodeoxycholic and cholic acid) are mostly synthesized in the liver. They are an important component of bile and help the intestine to absorb fats. The disorder can also be classified as a lipid storage disorder (due to fat deposition in various tissues of the body) or a leukodystrophy (due to the involvement of central nervous system white matter).

Signs & Symptoms

The presentation of CTX is highly variable and is associated with a wide range of potential abnormalities. Originally, the disorder was believed only to be a neurological disorder of abnormal fat (lipid) storage not associated with liver disease. It is now known that CTX can occasionally present in childhood with cholestatic liver disease that can be severe or can be mild and resolve on its own in individuals who may later develop other complications of the disorder such as neurological disease. Cholestatic liver disease refers to the interruption or suppression of the flow of bile from the liver (cholestasis). Features of cholestasis include yellowing of the skin, mucous membranes and whites of the eyes (jaundice), failure to thrive, and growth deficiency. Enlargement of the liver (hepatomegaly) and/or spleen (splenomegaly) may also occur.

Generally, systemic symptoms develop earlier than neurologic symptoms. The first symptom may be chronic diarrhea in infancy. Diarrhea is often resistant to treatment (intractable). Infantile spasms have also been reported as a possible symptom. Juvenile cataracts in the first decades of life are often an initial symptom of CTX. Tendinous xanthomas (fatty tumors) may appear in the second or third decade and can be located on the Achilles tendon, extensor tendons of the elbows and hands, and the knees.

Most affected individuals experience a decline in mental function beginning at puberty, but some show impairment beginning in childhood. Cognitive impairment can be mild to severe. CTX is often diagnosed through neurological symptoms of the disease that will continue to get worse without treatment. Seizures and epilepsy have been reported as symptoms. Psychiatric abnormalities including behavioral changes, hallucinations, agitation, aggression, depression, and suicidal tendencies can also occur, although specific expression varies greatly. Increased muscle tone and stiffness (spasticity) can occur.

In some patients, additional neurologic findings may occur including impaired coordination of voluntary movements due to underdevelopment (hypoplasia) of the brain’s cerebellum (cerebellar ataxia); symptoms that resemble Parkinson disease (atypical parkinsonism); and dystonia, which is a general term for a large group of movement disorders that vary in their symptoms, causes, progression, and treatments. Dystonia is generally characterized by involuntary muscle contractions that force the body into abnormal, sometimes painful, movements and positions (postures).

Cardiovascular disease has been reported in individuals with CTX, although the exact prevalence of this finding is unknown. Hardening of the arteries (atherosclerosis) and coronary heart disease may occur. Additional symptoms that have been reported include underactivity of the thyroid (hypothyroidism) and skeletal abnormalities such as porous, brittle bones (osteoporosis) and a higher incidence of bone fractures.

Causes

CTX is caused by a disease-causing (pathogenic) variant in the CYP27A1 gene. Genes provide instructions for creating proteins that play a critical role in many functions of the body. When a pathogenic variant of a gene occurs, the protein product or enzyme may be faulty, inefficient, or absent. Depending upon the functions of the particular protein, this can affect many organ systems of the body, including the brain. In CTX, the gene variant is inherited in an autosomal recessive manner.

Genetic diseases are determined by the combination of genes for a particular trait that are on the chromosomes received from the father and the mother. Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%. The risk is the same for males and females.

Disease-causing variants in the CYP27A1 gene result in deficiency of the mitochondrial enzyme sterol 27-hydroxylase. The lack of this enzyme prevents cholesterol from being converted into the bile acid chenodeoxycholic acid. The block in synthesis of this bile acid causes accumulation of bile acid pathway intermediates and cholestanol in blood and tissues of affected individuals. Cholestanol deposits can accumulate in nerve cells and membranes and cause damage to the brain, spinal cord, tendons, lens of the eye and arteries.

Diagnosis

CTX is diagnosed based on a thorough clinical evaluation, a detailed patient and family history, identification of characteristic clinical findings, and specialized tests including genetic testing and biochemical tests on blood and urine.

Genetic testing can confirm a diagnosis of CTX by detecting disease-causing (pathogenic) variants in the CYP27A1 gene known to cause the disorder. This type of testing can confirm the presence of gene variants that have already been described in the literature to cause CTX. Sometimes novel (unknown) variants are uncovered by genetic testing and in these cases biochemical testing is helpful to confirm the biochemical defect is present.

Certain specialized laboratories can conduct analysis to detect biochemical features that are indicative of CTX. Due to the nature of the biochemical defect, the cholestanol concentration in plasma or serum (derived from blood) is high, while the plasma cholesterol concentration is normal to low. In addition, the concentration of bile acid pathway intermediates is elevated, and the concentration of bile alcohols in plasma, bile and urine is increased. Bile alcohols are formed in an alternative pathway present in CTX that generates some cholic acid. Due to the nature of the biochemical defect in CTX there is little or no formation of chenodeoxycholic acid.

CTX has been nominated as a candidate disorder to add the recommended uniform screening panel of disorders (the RUSP) to screen for in newborns. Researchers have developed testing for CTX to identify dried bloodspots from newborns with the disorder and large prospective pilot studies are underway to confirm that the testing is able to identify newborns with CTX.

In some cases, specialized imaging techniques may include computerized tomography (CT) scanning of the head and magnetic resonance imaging (MRI) of the brain may assist in assessing disease progression in individuals suspected of CTX. During CT scanning, a computer and x-rays are used to create a film showing cross-sectional images of certain tissue structures. An MRI uses a magnetic field and radio waves to produce cross-sectional images of particular organs and bodily tissues. These tests may show cerebellar lesions and white matter damage in individuals with CTX.

Treatment

Because oral bile acid replacement therapy can halt disease progression or prevent symptoms from occurring in asymptomatic individuals, early diagnosis and treatment of CTX is extremely important to prevent disease complications. Researchers have recently shown that CTX patients who started treatment later (after the age of 25 years) had a worse outcome and were significantly more limited in ambulation and more cognitively impaired than those that started treatment earlier (before the age of 25 years).

Successful long-term treatment of a number of affected individuals who were asymptomatic in childhood has been reported in the literature.

Treatment with chenodeoxycholic acid normalizes the production of cholestanol. The efficacy of treatment with chenodeoxycholic acid can be monitored using biochemical testing to confirm a decrease in blood cholestanol. Treatment can prevent symptoms in asymptomatic individuals and stop the progression of disease symptoms in affected individuals. After significant disease progression, treatment does not readily reverse the neurological deficits that have already occurred.

It may be effective to give a drug that inhibits HMG-CoA reductase (an enzyme that plays a role in the creation of cholesterol in the liver) in conjunction with chenodeoxycholic acid. There are concerns that treatment with HMG-CoA reductase inhibitors (better known as statins) could boost the activity of receptors for low-density lipoprotein (LDL) cholesterol, thereby increasing cholesterol uptake and potentially worsening CTX. HMG-CoA reductase inhibitors can also cause muscle damage.

In 2009, the U.S. Food and Drug Administration (FDA) re-approved an artificially made (synthetic) form of chenodeoxycholic acid known as Chenodal as a treatment for gallstones. This drug is also used as a first-line therapy to treat individuals with CTX. Chenodal received an orphan drug designation from the FDA for the treatment of CTX in the U.S.

Cholic acid, another bile acid, has also been used to treat young children with CTX. Although chenodeoxycholic acid is considered as a first-line therapy to treat CTX, cholic acid lacks the potential toxic effects on the liver (hepatotoxicity) sometimes associated with chenodeoxycholic acid.

Additional treatment is symptomatic and supportive. For example, cataract surgery may be necessary before 50 years of age.

Genetic counseling is recommended for affected families and individuals

TYPE OF DOCTOER AND DEPARTMENT: NEUROLOGIST SPEIALIST CAN DIAGNOSE THIS DISEASE.