Hartnup Disease: Neutral Amino Acid Transport Defect
| Primary Defect: | SLC6A19 Transporter (Autosomal Recessive) |
| Affected Sites: | Intestinal Mucosa & Renal Proximal Tubules |
| Primary Missing Precursor: | Tryptophan (Leads to Niacin / Vitamin B3 deficiency) |
| Clinical Phenotype: | Pellagra-like Triad (Dermatitis, Diarrhea, Dementia) |
High-Yield Core Realities:
- The Transporter Block: Mutations in the SLC6A19 gene destroy the function of the sodium-dependent neutral amino acid transporter B(0)AT1. This transporter is responsible for reabsorbing neutral amino acids from the lumen of the small intestine and the proximal convoluted tubules of the kidney.
- The Tryptophan-to-Niacin Shunt Failure: While many neutral amino acids are lost in the urine, the clinical presentation is driven entirely by the wasting of Tryptophan. Under normal physiological conditions, tryptophan is metabolized into quinolinic acid to produce about 50% of the body’s de novo NAD+ and NADP+ (Niacin/Vitamin B3) requirements. Losing tryptophan shuts down this pathway, leading to intracellular NAD+ depletion.
- The Pellagra Clinical Mirror: Patients present during infancy or early childhood with symptoms mimicking dietary pellagra. This includes a symmetric, scaly, hyperpigmented photosensitive rash localized to sun-exposed areas (classic Casal necklace appearance), intermittent cerebellar ataxia, and neuropsychiatric fluctuations ranging from emotional lability to delirium.
- The Indican Urine Diagnostic Clue: Unabsorbed tryptophan remaining in the intestinal lumen undergoes bacterial degradation by gut microflora into indole derivatives. These indoles are absorbed into the portal circulation, conjugated in the liver to form indican, and excreted in massive amounts into the urine. When urine from these patients is exposed to air, the indican can oxidize, occasionally turning the urine a distinctive blue hue.
- Therapeutic Rescue Strategy: Symptoms are completely reversed or prevented by administering high-dose oral Nicotinic Acid or Nicotinamide (Vitamin B3), which bypasses the missing tryptophan synthesis loop to restore systemic NAD+ pools. Additionally, a high-protein diet allows adequate absorption of essential neutral amino acids in the form of dipeptides and tripeptides via the unaffected PEPT1 transporter.