Hartnup disease is a rare hereditary disorder that results in a skin rash and brain abnormalities because tryptophan and certain other amino acids are not well absorbed from the intestine and not well reabsorbed by the kidneys.
Hartnup disease occurs when a person inherits two copies of the abnormal gene for the disorder, one from each parent. 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. Excessive amounts of amino acids, such as tryptophan, are excreted in the urine. The body is thus left with inadequate amounts of amino acids, which are the building blocks of protein. With too little tryptophan in the blood, the body is unable to make a sufficient amount of the B-complex vitamin niacinamide, particularly under stress when more vitamins are needed.
Hartnup disease is a disorder of amino acid transport in the intestine and kidneys; otherwise, the intestine and kidneys function normally, and the effects of the disease occur mainly in the brain and skin. Symptoms may begin in infancy or early childhood, but sometimes they begin as late as early adulthood. Symptoms may be triggered by sunlight, fever, drugs, or emotional or physical stress. A period of poor nutrition nearly always precedes an attack. The attacks usually become progressively less frequent with age. Most symptoms occur sporadically and are caused by a deficiency of niacinamide. A rash develops on parts of the body exposed to the sun. Mental retardation, short stature, headaches, an unsteady gait, and collapsing or fainting are common. Psychologic problems (such as anxiety, rapid mood changes, delusions, and hallucinations) may also result.
Diagnosis and Treatment
Laboratory tests performed on urine samples reveal abnormally high excretion of amino acids and their breakdown products.
People with Hartnup disease can prevent attacks by maintaining good nutrition and supplementing their diet with niacinamide or niacin, a B-complex vitamin very similar to niacinamide. A diet that is adequate in protein can overcome the deficiency caused by poor intestinal absorption and excess excretion of amino acids into the urine.
Last full review/revision December 2006 by Peter C. Brazy, MD