Is Allan-Herndon-Dudley Syndrome hereditary?

TL;DR: Allan-Herndon-Dudley Syndrome is a rare genetic condition inherited in an X-linked recessive pattern, meaning it is caused by mutations on the X chromosome. Because it is hereditary, it is passed from mothers who are carriers to their children, predominantly affecting males.

Is Allan-Herndon-Dudley Syndrome hereditary?

Allan-Herndon-Dudley Syndrome is strictly a genetic and hereditary condition. It is caused by mutations in the SLC16A2 gene, which provides instructions for making a protein (MCT8) essential for the transport of thyroid hormones into neurons. Because the gene is located on the X chromosome, Allan-Herndon-Dudley Syndrome follows an X-linked recessive inheritance pattern. This means that while females typically carry one mutated X chromosome and remain asymptomatic, males with only one X chromosome will manifest the full clinical features of the syndrome.

What is the risk to family members?

The risk of inheriting Allan-Herndon-Dudley Syndrome depends on the biological sex of the child and the carrier status of the mother. If a mother is a carrier of an SLC16A2 mutation, the following probabilities apply:

• Each son has a 50% chance of inheriting the mutation and being affected by Allan-Herndon-Dudley Syndrome.


• Each daughter has a 50% chance of inheriting the mutation and becoming a carrier.


• Affected males will pass the mutation to all of their daughters (who become carriers) but none of their sons.

Are de novo mutations common?

While most cases of Allan-Herndon-Dudley Syndrome are inherited from a carrier mother, de novo (spontaneous) mutations can occur. In these instances, the genetic change happens for the first time in the affected individual, even if neither parent carries the mutation. Genetic counseling is vital to determine if the mutation is familial or a de novo event, which significantly changes recurrence risks for future pregnancies.

How is genetic testing utilized?

Molecular genetic testing, specifically sequencing of the SLC16A2 gene, is the gold standard for diagnosing Allan-Herndon-Dudley Syndrome. Genetic counseling is strongly recommended for families to discuss carrier testing, prenatal diagnosis through amniocentesis or chorionic villus sampling (CVS), and preimplantation genetic testing (PGT) for those planning future pregnancies.

Next steps

• Consult a clinical geneticist to confirm a diagnosis of Allan-Herndon-Dudley Syndrome through molecular testing.


• Seek genetic counseling to map family history and understand recurrence risks.


• Connect with the 8 members on DiseaseMaps.org to share experiences and find peer support.

Medical disclaimer: This information is for educational purposes and should not replace professional medical advice, diagnosis, or treatment.

References

• NIH Genetic and Rare Diseases Information Center (GARD): Allan-Herndon-Dudley Syndrome.


• OMIM (Online Mendelian Inheritance in Man): MCT8 Deficiency; #300523.


• Orphanet: Allan-Herndon-Dudley Syndrome (ORPHA:93922).