HOW IS BIOTINIDASE DEFICIENCY INHERITED?

Biotinidase Deficiency is an autosomal recessive condition.
Biotinidase deficiency is inherited as an autosomal recessive trait. This means that the gene is on an autosome rather than a sex chromosome, and that the trait or disorder, is only expressed when an individual has two "doses" of the abnormal gene, one inherited from each parent.

People who have one gene for biotinidase deficiency and one normal gene show half normal enzyme activity levels, but are healthy because half normal levels are sufficient to prevent any symptoms. Individuals who have a single abnormal gene for biotinidase deficiency (i.e., the parents of affected children) are said to be "carriers" (also called heterozygotes) of biotinidase deficiency.

Parents of affected children must be carriers.
We are all carriers of genes for recessive genetic conditions. These genes go unnoticed unless both parents happen to be carriers of the same abnormal gene and then have a child who inherits two "doses" of the recessive gene and is affected with the condition.

Children with biotinidase deficiency inherit one copy of the gene for biotinidase deficiency from each parent. Both parents must therefore be carriers in order to have a child with biotinidase deficiency.

Carrier parents can have more than one affected child.
We all pass on one copy of each gene pair to our children. A person who carries the gene for biotinidase deficiency therefore has a 1 in 2 or 50% chance with each pregnancy of passing on the gene for biotinidase deficiency. They also have a 50% chance of passing on the normal or functional gene.

For a child to be affected, both parents must carry a gene for biotinidase deficiency, and both parents must pass on that gene. This means that for each pregnancy, carrier parents have a 1 in 4 or 25% chance of having an affected child. They also have a 25% chance of both passing on the normal gene and having an unaffected child. Finally, there is a 50% chance that a child will inherit the recessive gene from one parent and the normal gene from the other. These children will be carriers, just like the parents.

Should our other children be tested?
All of your other children should be tested for biotinidase deficiency even if they have do not have symptoms of the condition. This will identify children who are only mildly affected or have not yet developed symptoms. Biotin treatment can then be started. Testing can also identify carriers of biotinidase deficiency, and this information my be useful to your children when they grow up and are planning their own families.

What can be done in future pregnancies?
Babies that have biotinidase deficiency do not develop medical problems from the disease until after they are born. Carrier mothers apparently supply the developing baby (fetus) with enough free biotin to prevent symptoms from appearing. Therefore, although it is possible, prenatal treatment for this condition is usually not considered necessary. Some doctors recommend that women at risk for having an affected child take prenatal vitamins containing some biotin (many prenatal vitamin preparations do not contain biotin) to ensure adequate biotin for the baby.

It is possible to determine if a baby has biotinidase deficiency during a pregnancy by withdrawing a sample of amniotic fluid and measuring the activity of biotinidase in cells grown from the fluid. Amniotic fluid is the fluid or "water" inside of the womb that surrounds the baby. Amniocentesis is the procedure used to withdraw the fluid using a needle inserted into the uterus. Amniocentesis is usually performed between the 15th and 17th week of the pregnancy and is routinely offered to pregnant women for prenatal diagnosis of other genetic conditions. There is a risk of having a pregnancy complication caused by this procedure, therefore families must carefully weigh the risks and benefits of early diagnosis before deciding to have amniocentesis. These issues may be discussed with your doctor or genetic counselor.

Chorionic villus sampling (CVS) is another method of prenatal testing for various genetic conditions. Since biotinidase activity has not been measured in chorionic villus samples, it is not known if prenatal diagnosis by this method is possible.

Regardless of whether you have prenatal diagnosis, it is important for babies who are at risk of inheriting this condition to be tested shortly after birth so that affected children can be identified and treated.

Will our grandchildren have biotinidase deficiency?
Since individuals with biotinidase deficiency have two copies of the gene for biotinidase deficiency, all of their children will inherit one copy of the gene. But in order to have a child with biotinidase deficiency, their partners must also be carriers and pass on the gene for biotinidase deficiency. Only about 1 in 125 people carry the gene for biotinidase deficiency; therefore, the chance that your grandchildren will have biotinidase deficiency is about 1 in 500, but carrier testing for biotinidase deficiency can more accurately determine their risk. All of your children who have biotinidase deficiency or are carriers of the gene may wish to see a genetic counselor to discuss risks and available testing when they begin to plan their own families.

Should we discuss testing with other relatives?
Your child's aunts, uncles, cousins, and other distant relatives may also carry the gene for biotinidase deficiency. You may wish to tell other family members that biotinidase deficiency is a genetic condition. If they would like to be tested, they can discuss it with their doctors or arrange to see a genetic counselor.

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