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Diseases

Genetic and Rare Diseases Information Center (GARD)

Down syndrome

*

* Not a rare disease
Información en español

Other Names for this Disease
  • Down's syndrome
  • Trisomy 21
See Disclaimer regarding information on this site. Some links on this page may take you to organizations outside of the National Institutes of Health.

Your Question

My daughter is 9 months old and has Down syndrome. She also has a strong, fish-like odor coming from the mouth. Is it possible that she could also have trimethylaminuria? How might a person be tested for this condition?

Our Answer

We have identified the following information that we hope you find helpful. If you still have questions, please contact us.

Is it possible for a person to have both Down syndrome and trimethylaminuria?

The genetic changes that cause trimethylaminuria (on chromosome 1) are very different from those that cause Down syndrome (and involve chromosome 21) and the two conditions are completely unrelated. Although it is uncommon, it is possible for a person to have Down syndrome, as well as an autosomal recessive condition such as trimethylaminuria.

Trimethylaminuria is usually caused by changes (mutations) in the FMO3 gene and is inherited in an autosomal recessive manner. This means that to be affected, a person must have a mutation in both copies of the responsible gene in each cell. The parents of an affected person usually each carry one mutated copy of the gene and are referred to as carriers. Carriers typically do not show signs or symptoms of the condition, although carriers of trimethylaminuria may experience mild symptoms or have temporary episodes of fish-like odor.[1] When two carriers of an autosomal recessive condition have children, each child has a 25% (1 in 4) risk to have the condition, a 50% (1 in 2) risk to be a carrier like each of the parents, and a 25% chance to not have the condition and not be a carrier.

Most cases of Down syndrome result from trisomy 21, which means each cell in the body has three copies of chromosome 21 instead of the usual two copies. The extra genetic material disrupts the normal course of development, causing the characteristic features of Down syndrome. A small percentage of people with Down syndrome have an extra copy of chromosome 21 in only some of the body's cells (mosaic Down syndrome), and others may have it because part of chromosome 21 becomes attached (translocated) to another chromosome during the formation of reproductive cells (eggs and sperm) or very early in fetal development (translocation Down syndrome).[2]
Last updated: 3/26/2015

How is trimethylaminuria diagnosed?

A diagnosis of trimethylaminuria is often suspected based on the presence of characteristic signs and symptoms. A urine test and/or genetic testing for a change (mutation) in the FMO3 gene can then be ordered to confirm the diagnosis. The urine test measures the level of trimethylamine in the urine when the person is on a non-restricted diet or following a choline challenge (a person is given a dose of choline by mouth prior to the test). People with trimethylaminuria have elevated levels of trimethylamine in the urine.[3][4]

GeneReviews offers more specific information about the diagnosis of trimethylaminuria. To access this resource, please click here.
Last updated: 3/26/2015

Is genetic testing available for trimethylaminuria?

Yes, genetic testing is available for FMO3, the gene known to cause trimethylaminuria.[3] Carrier testing for at-risk relatives and prenatal testing are possible if the disease-causing mutations in the family are known.

The Genetic Testing Registry (GTR) is a centralized online resource for information about genetic tests. The intended audience for the GTR is health care providers and researchers. Patients and consumers with specific questions about a genetic test should contact a health care provider or a genetics professional.
Last updated: 3/26/2015

References
Other Names for this Disease
  • Down's syndrome
  • Trisomy 21
See Disclaimer regarding information on this site. Some links on this page may take you to organizations outside of the National Institutes of Health.