Genetics and Epilepsy: What We Know Today
Holmes, Gregory L., The Exceptional Parent
The importance of genetics in epilepsy has long been recognized. Dispelling the notion that epilepsy was a sacred disease, Hippocrates, in the fourth century B.C., argued that epilepsy was inherited. During the last decade there has been an explosion of new information about the genetics of epilepsy. With mapping of the entire human genome complete, it is inevitable that our understanding of the role of genetics in epilepsy will continue to increase rapidly.
A large number of children with various forms of epilepsy have a genetic basis for the disorder. It is important to make the distinction between forms of epilepsy that can be inherited and other inherited disorders that can cause epilepsy, such as tuberous sclerosis. In some genetic conditions, such as juvenile myoclonic epilepsy, children inherit a gene or genes that result only in seizures. Juvenile myoclonic epilepsy is characterized by brief jerks of the extremities (termed myoclonus), generalized tonic clonic seizures (often upon awakening in the morning), and, occasionally, absence seizures. In all other regards the child is normal. In most of the forms of "genetic" epilepsies, most children and adults are otherwise clinically normal.
In other inherited conditions, epilepsy is one symptom of a complex of abnormalities. Children with tuberous sclerosis, for example, may have severe seizures. However the genes responsible for tuberous sclerosis may also lead to facial lesions (adenoma sebaceum), white spots (hypopigmented macules) on the skin, lesions of the brain (cortical tubers and subependymal nodules), kidney and eye disease, and mental retardation. While epilepsy is a major component of the syndrome, it occurs because the tuberous sclerosis gene leads to abnormalities in the brain which, in turn, causes seizures. As the effects of the tuberous sclerosis gene in the brain are variable, not all children with tuberous sclerosis have seizures.
In addition to genes resulting directly in the inheritance of epilepsy, genetic factors are known to play a role in whether a child develops a seizure disorder after a brain injury. For example, two children may have identical head injuries, yet one child develops seizures and the other does not. One of the factors determining whether the child develops epilepsy are genes that may make him or her more susceptible to seizures. Likewise, children may inherit the gene or genes responsible for febrile seizures but will only have a seizure in the context of a febrile illness.
GENETIC PRINCIPLES AND DEFINITIONS
The basic starting point of any discussion of genetics is with deoxyribonucleic acid (DNA). DNA is made of two chains consisting of base pairs (nucleotides) consisting of purines (adenine or guanine) or pyrimidines (cytosine or thymine). The two strands are paired together; adenine is paired with thymine; guanine is paired with cytosine. A gene can be defined as a region of DNA comprising the nucleotides necessary to produce a functional protein. A gene may have multiple alleles, whereby different nucleotide sequences make up the sequence of the gene but result in the same gene product.
DNA directs the synthesis of ribonucleic acid (RNA). While DNA stores genetic information in a monotonous double-stranded form, RNA is more versatile, having multiple functions and structures. Most RNA is single stranded and the nucleotide uracil replaces thymine. DNA produces RNA--a process called transcription--which, in turn, forms proteins--a process called translation. Proteins are the active working components of the cellular machinery. Whereas DNA stores the information for protein synthesis and RNA carries out the instructions encoded in DNA, most biological activities are carried out by proteins. The synthesis of proteins is critical to the function of all cells in the body. The loss or over-production of a critical protein can result in epilepsy.
Chromosomes consist of many genes linked together. …