Leigh Syndrome Health Article

Definition

Leigh syndrome is a rare inherited neurometabolic disorder characterized by degeneration of the central nervous system (brain, spinal cord, and optic nerve), meaning that it gradually loses its ability to function properly.

Description

First described in 1951, Leigh syndrome usually occurs between the ages of three months and two years. The disorder worsens rapidly; the first signs may be loss of head control, poor sucking ability and loss of previously acquired motor skills, meaning the control of particular groups of muscles. Loss of appetite, vomiting, seizures, irritability, and/or continuous crying may accompany these symptoms. As the disorder becomes worse, other symptoms such as heart problems, lack of muscle tone (hypotonia), and generalized weakness may develop, as well as lactic acidosis, a condition by which the body produces too much lactic acid. In rare cases, Leigh syndrome may begin late in adolescence or early adulthood, and in these cases, the progression of the disease is slower than the classical form.

The disorder usually occurs in three stages, the first between eight and 12 months involving vomiting and failure to thrive, the second in infancy, characterized by loss of motor ability, eye problems and respiratory irregularity. The third stage occurs between two and 10 years of age and is characterized by hypotonia and feeding difficulties.

In most cases, Leigh syndrome is inherited as an autosomal recessive genetic trait. However, X-linked recessive, autosomal dominant, and mitochondrial inheritance can also occur. Several different types of genetic enzyme defects are thought to cause Leigh syndrome, meaning that the disorder may be caused by defective enzymes, the proteins made by the body to speed up the biochemical reactions required to sustain life.

Commonly known as Leigh's disease, Leigh syndrome is also known as Leigh necrotizing encephalopathy, necrotizing encephalomyelopathy of Leigh's and subacute necrotizing encephalopathy (SNE). When it occurs in adolescence and adulthood, it may be called adult-onset subacute necrotizing encephalomyelopathy.

Genetic profile

Several different types of genetic metabolic defects are thought to lead to Leigh syndrome. A deficiency of one or a number of different enzymes may be the cause.

Classic Leigh syndrome

The usual form of Leigh syndrome is inherited as an autosomal recessive genetic trait. It has been linked to a genetic defect in one of two genes known as E2 and E3, which cause either a deficiency of the enzyme pyruvate dehydrogenase, or an abnormality in other enzymes that make pyruvate dehydrogenase work. Other cases of autosomal recessive Leigh syndrome are associated with other genetic enzyme deficiencies (i.e., NADH-CoQ and Cytochrome C oxidase), although the gene or genes responsible for these deficiencies are not known. All of these different genetic defects seem to have a common effect on the central nervous system.

In autosomal recessive inheritance, a single abnormal gene on one of the autosomal chromosomes (one of the first 22 "non-sex" chromosomes) from both parents can cause the disease. Both of the parents must be carriers in order for the child to inherit the disease and neither of the parents has the disease (since it is recessive).

A child whose parents are carriers of the disease has a 25% chance of having the disease; a 50% chance of being a carrier of the disease, meaning that he is not affected by the disease, and a 25% chance of receiving both normal genes, one from each parent, and being genetically normal for that particular trait.

X-linked Leigh syndrome

Evidence also exists for an X-linked recessive form of Leigh syndrome, which has been linked to a specific defect in a gene called E1-alpha, a part of the enzyme pyruvate dehydrogenase.

X-linked recessive disorders are conditions that are coded on the X chromosome. All humans have two chromosomes that determine their gender: females have XX, males have XY. X-linked recessive, also called sexlinked, inheritance affects the genes located on the X chromosome. It occurs when an unaffected mother carries a disease-causing gene on at least one of her X chromosomes. Because females have two X chromosomes, they are usually unaffected carriers. The X chromosome that does not have the disease-causing gene compensates for the X chromosome that does. Generally for a woman to have symptoms of the disorder, both X chromosomes would have the disease-causing gene. That is why women are less likely to show such symptoms than males.

If a mother has a female child, the child has a 50% chance of inheriting the disease gene and being a carrier who can pass the disease gene on to her sons. On the other hand, if a mother has a male child, he has a 50% chance of inheriting the disease-causing gene because he has only one X chromosome. If a male inherits an X-linked recessive disorder, he is affected. All of his daughters will also be carriers.