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GM2-gangliosidosis, AB variant is a rare inherited disorder that causes progressive destruction of nerve cells in the brain and spinal cord (the central nervous system).
Signs and symptoms of the AB variant begin in infancy. Infants with this disorder typically appear normal until the age of 3 to 6 months, when development slows and muscles used for movement weaken. Affected infants lose motor skills such as turning over, sitting, and crawling. They also develop an exaggerated startle reaction to loud noises. As the disease progresses, children with the AB variant develop seizures, vision and hearing loss, mental retardation, and paralysis. An eye abnormality called a cherry-red spot, which can be identified with an eye examination, is characteristic of this disorder. Children with the AB variant usually live only into early childhood.
The AB variant is extremely rare; only a few cases have been reported worldwide.
Mutations in the GM2A gene cause GM2-gangliosidosis, AB variant.
The GM2A gene provides instructions for making a protein called the GM2 ganglioside activator. This protein is required for the normal function of beta-hexosaminidase A, a critical enzyme in the central nervous system. The GM2 ganglioside activator and beta-hexosaminidase A work together in cellular structures called lysosomes, which act as the cell's recycling centers. Within lysosomes, beta-hexosaminidase A and the activator protein form a complex that breaks down a fatty substance called GM2 ganglioside.
If mutations disrupt the activity of the GM2 ganglioside activator, beta-hexosaminidase A cannot perform its normal function. As a result, GM2 ganglioside can accumulate to toxic levels in the brain and spinal cord. Progressive damage caused by the buildup of this substance leads to the destruction of nerve cells, which causes the severe medical problems characteristic of the AB variant.
Because the AB variant impairs the function of a lysosomal enzyme and involves the buildup of GM2 ganglioside, this condition is sometimes referred to as a lysosomal storage disorder.
This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. Most often, the parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but do not show signs and symptoms of the condition.