Miller-Dieker lissencephaly syndrome

Inheritance

How is Miller-Dieker lissencephaly syndrome inherited?

About 80% of cases of Miller-Dieker lissencephaly syndrome are not inherited. These are called “de novo” cases. In these cases, there is no family history and the chromosome 17p13.3 deletion occurs randomly during the formation of the sperm cells or egg cells, or in early development in the womb. There is nothing parents can do to cause or prevent a chromosome 17p13.3 deletion in a child if it happens randomly. This does not commonly happen twice within a family.

About 20% of the time, deletions of chromosome 17p13.3 are due to an unbalanced chromosome rearrangement. An unbalanced chromosome rearrangement is the medical term for pieces of chromosomes which are attached to a different chromosome; this can cause missing (deletion) or extra (duplication) genetic material. Sometimes, a parent has a balanced chromosome rearrangement that can cause a chromosome 17p13.3 deletion in their child. When a parent has a balanced chromosome rearrangement, they have all of the chromosomal material, but it is just rearranged. The parent does not show signs or symptoms because all of the chromosomal material is present. However, when a parent goes to have a child, this parent is at an increased risk to have a child with an unbalanced amount of chromosomal material [too much (duplication) and/or too little (deletion)].

When this happens with chromosome 17p13.3, the child would be at increased chance to have a deletion of this region, and thus have Miller-Dieker syndrome.

The best person to explain this concept is a genetic counselor. If your child does not have a genetic counselor, ask your main doctor or use the Find a Genetic Counselor tool on the National Society of Genetic Counselors website.

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Are all chromosome 17p13.3 deletions the same and cause Miller-Dieker syndrome?

If a child if found to have Miller-Dieker lissencephaly syndrome and parents’ genetic testing is normal, what is the chance for parents to have another child with Miller-Dieker lissencephaly syndrome?

Are all chromosome 17p13.3 deletions the same and cause Miller-Dieker syndrome?

Not all chromosome 17p13.3 deletions are the same. Chromosome 17p13.3 deletions can be different sizes. Some deletions of this region are smaller than typically seen in Miller-Dieker syndrome and thus may not involve the critical genes to give the signs and symptoms of MDS. Also some deletions can be much larger than the typical region for MDS, and therefore, may contain other important genes necessary for proper development.

Sometimes, a child can have another chromosome difference in addition to a chromosome 17p13.3 deletion.

If you are unsure the size of your child's deletion or what genes are specifically involved, speaking with a genetic counselor may be best. If your child does not have a genetic counselor, ask your main doctor who they would recommend or use the Find a Genetic counselor tool on the National Society of Genetic Counselor's website.

If a child if found to have Miller-Dieker lissencephaly syndrome and parents’ genetic testing is normal, what is the chance for parents to have another child with Miller-Dieker lissencephaly syndrome?

If a child is found to have Miller-Dieker lissencephaly syndrome and parental genetic testing is normal, the chance to have another child with Miller-Dieker lissencephaly syndrome is very low, likely less than 1%. There is the rare chance of gonadal mosaicism where some of the mother’s egg cells or some of the father’s sperm cells have the chromosome difference. This usually cannot be detected through a blood test. Even if this is the case, the recurrence risk for future pregnancies is likely very low.

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