Hartnup disease

Hartnup disease (also known as Hartnup disorder or pellagra-like dermatosis) is a rare inherited metabolic condition caused by defective transport of neutral (monoamino-monocarboxylic) amino acids in the small intestine and kidneys. It is caused by mutations in the SLC6A19 gene, which encodes the sodium-dependent neutral amino acid transporter B0AT1. This transporter defect leads to impaired intestinal absorption and excessive urinary loss of neutral amino acids, particularly tryptophan. Since tryptophan is a precursor to niacin (vitamin B3), its deficiency can produce symptoms resembling pellagra. The disease primarily affects the skin, nervous system, and gastrointestinal tract. Classic clinical features include a photosensitive, pellagra-like skin rash on sun-exposed areas, intermittent cerebellar ataxia, psychiatric symptoms (such as emotional instability, anxiety, and psychosis), and occasionally diarrhea. Symptoms are often episodic and may be triggered by sunlight exposure, poor nutrition, febrile illness, or physiological stress. Many individuals identified through newborn screening remain asymptomatic throughout life, and the clinical expression of the disease is highly variable. Treatment of Hartnup disease is generally straightforward and effective. Oral supplementation with nicotinamide (niacinamide) is the mainstay of therapy, which can prevent and treat the pellagra-like manifestations. A high-protein diet is also recommended to compensate for amino acid malabsorption. Sun protection is advised to reduce photosensitive skin reactions. With appropriate management, the long-term prognosis is generally excellent, and many patients experience a decrease in symptom frequency with age.

Common questions about Hartnup disease