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Adrenoleukodystrophy (ALD) is one of a group of genetic disorders called the leukodystrophies that cause damage to the myelin sheath, an insulating membrane that surrounds nerve cells in the brain. People with ALD accumulate high levels of saturated, very long chain fatty acids (VLCFA) in the brain and adrenal cortex because they do not produce the enzyme that breaks down these fatty acids in the normal manner. The loss of myelin and the progressive dysfunction of the adrenal gland are the primary characteristics of ALD.
ALD has two subtypes: X-linked ALD and neonatal ALD.
X-linked ALD. This is the most common form of ALD. It involves an abnormal gene located on the X-chromosome. Women have two X-chromosomes and are the carriers of the disease, but since men only have one X-chromosome and lack the protective effect of the extra X-chromosome, they are more severely affected. Onset of X-linked ALD can occur in childhood or in adulthood. The childhood form is the most severe, with onset between ages 4 and 10. The most common symptoms are usually behavioral changes such as abnormal withdrawal or aggression, poor memory, and poor school performance. Other symptoms include visual loss, learning disabilities, seizures, poorly articulated speech, difficulty swallowing, deafness, disturbances of gait and coordination, fatigue, intermittent vomiting, increased skin pigmentation, and progressive dementia. In the milder adult-onset form, which typically begins between ages 21 and 35, symptoms may include progressive stiffness, weakness or paralysis of the lower limbs, and ataxia. Although adult-onset ALD progresses more slowly than the classic childhood form, it can also result in deterioration of brain function.
Neonatal ALD. The abnormal genes that cause neonatal ALD are not located on the X-chromosome, which means that both male and female babies can be affected. Symptoms include mental retardation, facial abnormalities, seizures, retinal degeneration, weak muscle tone, enlarged liver, and adrenal dysfunction. This form usually progresses rapidly. A mild form of ALD is occasionally seen in women who are carriers of the disorder. Symptoms include progressive stiffness, weakness or paralysis of the lower limbs, ataxia, excessive muscle tone, mild peripheral neuropathy, and urinary problems.
Mutations in the ABCD1 gene cause X-linked adrenoleukodystrophy.
Mutations in the ABCD1 gene cause a shortage (deficiency) of a cellular transporter known as adrenoleukodystrophy protein. This protein is thought to play a role in the breakdown of certain fats (very long-chain fatty acids or VLCFA) inside peroxisomes. Peroxisomes are small sacs in the cell that process many types of molecules. When this transporter is lacking, the breakdown of very long-chain fatty acids is compromised and results in abnormally high levels of these fats in the body. Research suggests that the accumulation of very long-chain fatty acids is toxic to the adrenal cortex and the myelin membranes that surround many of the nerves in the brain and spinal cord.
This condition is inherited in an X-linked recessive pattern.
Adrenal function must be tested periodically in all patients with ALD. Treatment with adrenal hormones can be lifesaving. Symptomatic and supportive treatments for ALD include physical therapy, psychological support, and special education. Recent evidence suggests that a mixture of oleic acid and euric acid, known as "Lorenzo's Oil," administered to boys with X-linked ALD can reduce or delay the appearance of symptoms. Bone marrow transplants can provide long-term benefit to boys who have early evidence of X-linked ALD, but the procedure carries risk of mortality and morbidity and is not recommended for those whose symptoms are already severe or who have the adult-onset or neonatal forms. Oral administration of docosahexanoic acid (DHA) may help infants and children with neonatal ALD.