Genetics and Epilepsy
What is genetics?
Genetics is the study of genes, which are the basic units of heredity. Human beings have many thousands of genes. Each of these genes influences certain traits such as hair color, eye color, blood type, and many other characteristics. People are different with regard to these traits because their genes are different. Children look similar to one or both parents, for example, or have traits similar to their grandparents and other relatives, because of certain genes which have been passed or inherited from one generation to the next.
I thought we inherited different traits because of our chromosomes. What is
the difference between genes and chromosomes?
An easy way of thinking about a chromosome is as a "package" filled with many genes.
The human body is made up of millions of cells. There are many different types of cells such as brain cells, muscle cells, and skin cells. Most cells in the body contain chromosomes. Each of these cells holds 23 pairs of chromosomes (46 total). One member of each pair is inherited from the mother and the other from the father. Genes are located on chromosomes and, thus, are passed from both parents to the child.
Genes, in turn, are made up of a substance called DNA (deoxyribonucleic acid). Genes give the cell instructions to make proteins, which are necessary for the body to do all the things that it does.
That sounds so complicated. What if something goes wrong?
It is complicated. Genes can be altered in many ways. Changes in the DNA that cause a protein to not work normally or stop working altogether are called "mutations." Environmental factors can also affect the way cells work.
Some mutations may result in disease and some of these conditions can be passed on to future generations.
Is epilepsy inherited?
Some types of epilepsy are. Epilepsy is not a single disorder, but a collection of many disorders that all have in common the tendency to cause a person to have seizures. When individual characteristics are caused by single genes, they are called "simple" genetic traits. Only a few rare types of epilepsy are caused by alterations in single genes. Most seem to be caused by a complex interaction among multiple genes and environmental influences.
I'm pregnant and I have epilepsy. Is my baby likely to have seizures, too?
Certain types of epilepsy do seem to run in families. Some studies have shown that the risk of epilepsy in brothers, sisters and children of people with seizure disorders ranges from 4-8% (that is, between about 1 in 25 and about 1 in 12). The risk in the general population is about 1-2% (between 1 in 100 and 1 in 50). However, it's important to remember that although the risk is higher than in the general population, most people with epilepsy do not have any relatives with seizures, and the great majority of parents with epilepsy do not have children with epilepsy.
Which types of epilepsy are most likely to be inherited?
People who have a generalized epilepsy (one where the EEG pattern shows both sides of the brain involved at the beginning of a seizure) seem somewhat more likely to have other family members with seizures than those with a localization-related epilepsy (also called partial or focal, where the EEG pattern shows seizures beginning in a single area of the brain). Parents with a history of generalized absence (childhood petit mal) seizures are more likely to have children with the same condition than those with other generalized seizures or focal seizures.
What are some of the other factors that seem to be involved in inheritance?
1) Age when epilepsy begins -- Children of people whose seizures started early in life (for example, before 20 years of age) have a greater risk of developing epilepsy than children of people whose seizures started later in life.
2) Mothers and fathers with epilepsy -- Studies have shown that the risk of epilepsy is about twice as high in children of women with epilepsy than in children of men with epilepsy. Research has shown that this is not related to pregnancy or birth complications, maternal seizures during pregnancy or maternal antiepileptic drugs during pregnancy. More research is needed to explain this difference.
3) Cause of epilepsy -- The risk for developing epilepsy does not seem to be increased, compared with the general population, in relatives of people who have epilepsy caused by serious brain injury that occurs after birth due to conditions such as strokes, brain tumors, severe head trauma or brain infections.
Can epilepsy skip a generation?
Yes, it can. As discussed earlier, most types of epilepsy are caused by multiple genes and environmental influences. Not everyone who carries genes that make him or her more likely to develop epilepsy will, in fact, do so. Therefore, even if the genes are passed on, not every generation in a family will have seizures.
How can I find out what the risk is for my baby to inherit my type of epilepsy?
Ask your physician to refer you for genetic counseling. Specially trained physicians or nurses, genetic counselors, and other health care professionals can help you study your medical history, find out facts about your family history and, if possible, calculate the risk for you and your baby. They may recommend certain laboratory tests to get more information.
It's important to remember that although there is a lot of evidence that genes play an important role in causing epilepsy, exactly which genes are involved has not been identified for most people who have seizures. Studying the families of people who have epilepsy will help increase our knowledge and perhaps, in the future, will lead to new treatments for epilepsy or even measures to prevent epilepsy.