Adrenoleukodystrophy

Inheritance

How is X-linked adrenoleukodystrophy inherited?

X-linked adrenoleukodystrophy is inherited in an X-linked manner, because the ABCD1 gene is located on the X chromosome. Women who are carriers for X-linked adrenoleukodystrophy have a 50% chance of passing the changed ABCD1 gene to each of their children. If a daughter inherits the change, she too will be a carrier, or mildly affected. If a son inherits the change he will likely exhibit full symptoms of X-linked adrenoleukodystrophy. Men who have X-linked adrenoleukodystrophy will not pass their genetic change on to any of their sons, but all of their daughters will be carriers.

References
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What does it mean to have inherited a “variant” in the gene for X-linked adrenoleukodystrophy?

How often is X-linked adrenoleukodystrophy inherited from a parent?

What does it mean to be a carrier for X-linked adrenoleukodystrophy?

If a woman is a carrier for X-linked adrenoleukodystrophy what is the risk that her children will have X-linked adrenoleukodystrophy?

What are the risks to the children of a man with X-linked adrenoleukodystrophy?

If a woman knows she is a carrier for X-linked adrenoleukodystrophy what are her options for having children?

What is preimplantation genetic testing in relation to adrenoleukodystrophy?

What is preimplantation genetic testing?

What does it mean to have inherited a “variant” in the gene for X-linked adrenoleukodystrophy?

If a person inherits a "variant of uncertain significance" in the gene associated with adrenoleukodystrophy, ABCD1, this means that we see a change in the gene, but we are unsure if the change will actually have an effect on the person. Further research will be needed to see if a genetic change is disease-causing or if it is benign. If a person learns they have a variant of uncertain significance, their diagnosis and medical management would be based on symptoms and blood testing for very long chain fatty acids in the blood.

References
How often is X-linked adrenoleukodystrophy inherited from a parent?

X-linked adrenoleukodystrophy is inherited from a parent approximately 95% of the time. In the remaining 4-5% of cases, there is a new mutation, or genetic change, in the ABCD1 gene of the affected patient. A new genetic change is also called a de novo change or mutation.

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What does it mean to be a carrier for X-linked adrenoleukodystrophy?

Only women can be carriers of adrenoleukodystrophy. Carriers of X-linked adrenoleukodystrophy have a change in one of their ABCD1 genes but also have another copy that is functioning properly. Females can be carriers because they have two copies of the X chromosome and therefore two copies of ABCD1. Most women who are carriers do not show signs and symptoms of the condition, but an estimated 20% of carrier females will show moderate paraparesis (difficulty using the lower limbs), usually in middle age or later life.

Males cannot be carriers because they only have one X chromosome. Therefore, if a male has a change to ABCD1, he has no normally functioning copy of that gene, and will express adrenoleukodystrophy in some way.

References
If a woman is a carrier for X-linked adrenoleukodystrophy what is the risk that her children will have X-linked adrenoleukodystrophy?

Children of a woman who is a carrier of adrenoleukodystrophy each have a 50% (1/2) chance of inheriting the same change. If the child is a male, he would be expected to show symptoms. The specific sub-type of adrenoleukodystrophy he fits into may vary. If the child is a female, we would expect that she would be a carrier for the condition (like her mother) and show no symptoms or much less severe symptoms.

References
What are the risks to the children of a man with X-linked adrenoleukodystrophy?

Males have one X and one Y chromosome. All the daughters of a male with X-linked adrenoleukodystrophy will inherit the ABCD1 gene that isn't working properly because that is the only X chromosome a male has to pass on. That daughter would most likely inherit a normally functioning ABCD1 gene from her mother, and therefore she would be a carrier. Most carriers do not show symptoms of the condition. If they do show symptoms, these symptoms are less severe than those of a male. Sons of a male with X-linked adrenoleukodystrophy will have no risk of inheriting the condition, because they do not receive their father's X chromosome, only his Y chromosome.

References
If a woman knows she is a carrier for X-linked adrenoleukodystrophy what are her options for having children?

There are multiple options for a woman who is a carrier for X-linked adrenoleukodystrophy to expand her family. These options are deeply personal and there is no one "right" way to go about building a family. Options include using preimplantation genetic diagnosis and in vitro fertilization to ensure the child will not have X-linked adrenoleukodystrophy, having a natural pregnancy and doing prenatal testing, having a natural pregnancy and testing for X-linked adrenoleukodystrophy at birth, using a donor egg to conceive, and/or choosing to adopt additional children. Some of these options, such as preimplantation genetic diagnosis and some forms of prenatal testing, require that the specific genetic change in ABCD1 be identified. The best time to discuss your options for family-building is prior to pregnancy. A genetic counselor can help explain each of these options in more detail and help you evaluate which option is best for you. The genetic counselor will not tell you which option you should pursue. Genetic counselors can be found on the National Society of Genetic Counselors website.

References
What is preimplantation genetic testing in relation to adrenoleukodystrophy?

Preimplantation genetic testing (PGT) is a way that embryos can be tested for a familial genetic condition before they are implanted. It can be done for adrenoleukodystrophy if the specific genetic change in the family has been identified. After eggs are fertilized using in vitro fertilization, one or more cells are taken from the embryo and genetic testing is performed to look for the change in ABCD1. Then embryos that do not have the change are specifically chosen to be implanted for the pregnancy.

What is preimplantation genetic testing?

Preimplantation genetic testing (PGD) is a tool that can be used in combination with IVF to help couples who are at risk for having a child with a genetic condition have a healthy child. During the embryo culture step of IVF, the healthiest embryos are selected for PGD. Once the embryos have at least 8 cells, one or more cells is removed from the embryo and sent to the laboratory for genetic testing of the particular condition the couple is at risk for, such as cystic fibrosis or thalassemia. Removing the cells at this early stage of the embryos growth does not affect the embryo at all. After testing, only the embryos that do not have the genetic condition are selected and implanted in the woman’s uterus. Prenatal diagnostic testing such as amniocentesis or chorionic villus sampling (CVS) is typically recommended in the rare case that the cell tested was not the same as the other cells of the embryo (a situation called mosaicism).

References
  • http://www.pennmedicine.org/fertility/patient/clinical-services/pgd-preimplantation-genetic-diagnosis/

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