Hartnup disease

Hartnup disease, or Hartnup's disease, or Hartnup disorder, is an autosomal recessive genetic metabolic disorder in the absorption of the amino acid tryptophan that can be, in turn, converted into Serotonin, Melatonin and Niacin. Niacin is a precursor to nicotinamide, a necessary component of NAD+.

The causative gene is located on chromosome 5.

Symptoms
Hartnup disease manifest during infancy with variable clinical presentation: failure to thrive, photosensitivity, intermittent ataxia, nystagmus and tremor. Nicotinamide is necessary for neutral amino acid transporter production in the proximal renal tubules found in the kidney, and intestinal mucosal cells found in the small intestine. Therefore, a symptom stemming from this disorder results in increased amounts of amino acids in the urine.

Pellagra is also caused by low nicotinamide; this disorder results in dermatitis, diarrhea and dementia.