Hartnup disease is a rare hereditary disorder of amino acid metabolism that is caused by a lack of the enzymes needed to metabolize the amino acid tryptophan and certain other amino acids. Because the transport of amino acids in the intestine and kidneys is disordered, the body has inadequate amounts of these substances. Hartnup disease occurs when parents pass on to their children the defective gene that causes this disorder.
Hartnup disease is caused by a lack of the enzyme needed to metabolize the amino acid tryptophan.
Symptoms include a rash, intellectual disability, and mental health problems that are made worse by sunlight, hot weather, and nutritional deficiency.
The diagnosis is based on a urine test.
(See also Introduction to Congenital Kidney Tubular Disorders.)
Amino acids are the building blocks of proteins and have many functions in the body. The body makes some of the amino acids it needs and gets others from food.
Hartnup disease is a disorder that causes a decrease of the amino acid tryptophan and certain other amino acids. Tryptophan is an essential amino acid, which means that it cannot be made in the body but must be consumed in food. In Hartnup disease, excessive amounts of amino acids are excreted in the urine. The body is thus left with inadequate amounts of amino acids. With too little tryptophan in the blood, the body is unable to make a sufficient amount of the B-complex vitamin , particularly under stress when more vitamins are needed.
There are different types of inherited disorders. Hartnup disease occurs when a person inherits 2 copies of the abnormal gene for the disorder, 1 from each parent (see figure Non–X-Linked Recessive Disorders). Because 2 genes are needed when a recessive gene is involved, the parents are carriers of the gene but do not have the syndrome. However, siblings of children with the disorder might have it. The defective gene controls the absorption of certain amino acids from the intestine and the reabsorption of those amino acids in the kidneys. Consequently, a person with Hartnup disease cannot absorb amino acids properly from the intestine and cannot reabsorb them properly from tubules in the kidneys. (See also Overview of Hereditary Metabolic Disorders.)
Viewing the Urinary Tract
Symptoms of Hartnup Disease
The intestines and kidneys function normally, aside from the problem of transporting amino acids, and the symptoms of the disease occur mainly in the brain and skin.
Although Hartnup disease is present at birth, symptoms may begin at any time and sometimes may begin as late as early adulthood. Symptoms may be triggered by sunlight, fever, drugs or medications, or emotional or physical stress.
A period of poor nutrition nearly always precedes an attack. Most symptoms occur sporadically and are caused by a deficiency of niacin. A dietary deficiency of niacin (pellagra) causes a rash that develops on parts of the body exposed to the sun.
Intellectual disability, short stature, headaches, an unsteady gait, and collapsing or fainting are common. Mental health problems, such as anxiety, rapid mood changes, delusions, and hallucinations, may also result.
Diagnosis of Hartnup Disease
Urine tests
Laboratory tests done on urine samples reveal an abnormally high excretion of amino acids and their by-products (such as serotonin).
Genetic testing can be done to confirm the diagnosis of Hartnup disease in some people.
Treatment of Hartnup Disease
Good nutrition and supplements of niacinamide or niacin (nicotinic acid)
For attacks, nicotinamide
Avoiding sun exposure and sulfonamides
People with Hartnup disease can reduce the number and severity of attacks by maintaining good nutrition and eating enough protein.
Affected people should supplement their diet with niacinamide or (a B-complex vitamin very similar to niacinamide) and may take nicotinamide to treat attacks.
People who have Hartnup disease also should avoid sun exposure as well as antibiotics that contain sulfonamides.
Prognosis for Hartnup Disease
The prognosis for Hartnup disease is good, and the attacks usually become progressively less frequent with age.
Life expectancy is normal.
