Hartnup Disease
Overview
Plain-Language Overview
Hartnup Disease is a rare inherited condition that affects how the body absorbs certain amino acids, especially tryptophan, from the intestines and kidneys. This problem mainly impacts the digestive system and the nervous system. Because the body cannot properly absorb these amino acids, it can lead to symptoms like a skin rash, similar to sunburn, and neurological issues such as confusion or ataxia (difficulty coordinating movements). The disease is caused by a genetic defect that affects the transport of amino acids. People with this condition may experience episodes of symptoms triggered by stress or illness. The main health effects come from the lack of important nutrients needed for normal body functions.
Clinical Definition
Hartnup Disease is an autosomal recessive disorder caused by mutations in the SLC6A19 gene, which encodes a sodium-dependent neutral amino acid transporter primarily expressed in the renal proximal tubules and intestinal epithelium. This defect leads to impaired absorption and reabsorption of neutral amino acids, especially tryptophan, resulting in increased urinary excretion and decreased systemic availability. The deficiency of tryptophan reduces synthesis of niacin (vitamin B3), causing pellagra-like symptoms including photosensitive dermatitis, cerebellar ataxia, and neuropsychiatric manifestations. The disease typically presents in childhood or adolescence with intermittent episodes triggered by stressors such as infection or poor nutrition. Despite the aminoaciduria, patients often have normal growth and development between episodes. The clinical significance lies in the potential for reversible neurological and dermatological symptoms if diagnosed and managed appropriately.
Inciting Event
Initial clinical symptoms often appear after periods of stress or illness that increase metabolic demand for niacin.
Exposure to intense sunlight can trigger or worsen the characteristic photosensitive rash.
Dietary niacin deficiency or low tryptophan intake can precipitate symptom onset.
Latency Period
Symptoms typically develop during childhood or adolescence after a latent period of asymptomatic aminoaciduria.
Clinical manifestations may appear weeks to months after dietary or environmental triggers.
Variable latency depending on severity of transporter defect and nutritional status.
Diagnostic Delay
Symptoms often mimic pellagra or other dermatologic and neurologic disorders, leading to misdiagnosis.
Lack of awareness of aminoaciduria and its significance delays biochemical testing.
Mild or intermittent symptoms can cause clinicians to overlook Hartnup disease in differential diagnosis.
Clinical Presentation
Signs & Symptoms
Photosensitive dermatitis with rash on face, neck, and hands
Cerebellar ataxia and unsteady gait in some patients
Psychiatric symptoms including anxiety, depression, and hallucinations
Diarrhea and other gastrointestinal symptoms due to niacin deficiency
Glossitis and stomatitis reflecting mucosal involvement
History of Present Illness
Patients present with photosensitive rash on sun-exposed areas, often with scaling and erythema.
Neurologic symptoms include ataxia, tremor, and psychiatric disturbances such as anxiety or depression.
Gastrointestinal complaints like diarrhea and malabsorption may be reported.
Symptoms often worsen with sun exposure and improve with niacin supplementation.
Past Medical History
History of recurrent photosensitive dermatitis or unexplained neurologic symptoms.
Previous episodes of pellagra-like symptoms or malnutrition.
No significant history of renal or hepatic disease unless secondary complications occur.
Family History
Positive family history of similar dermatologic or neurologic symptoms suggests autosomal recessive inheritance.
Consanguinity in parents increases risk of affected offspring.
Siblings may have asymptomatic aminoaciduria or mild symptoms.
Physical Exam Findings
Photosensitive rash resembling pellagra, often erythematous and scaly on sun-exposed areas
Ataxia and other cerebellar signs in severe cases
Nystagmus and other ocular abnormalities
Hyperpigmentation in sun-exposed skin regions
Glossitis and stomatitis may be present
Diagnostic Workup
Diagnostic Criteria
Diagnosis is established by detecting increased urinary excretion of neutral amino acids, particularly tryptophan, on amino acid chromatography. Clinical features such as photosensitive rash and neurological symptoms support the diagnosis. Genetic testing confirming mutations in the SLC6A19 gene provides definitive confirmation. Differential diagnosis includes other causes of pellagra and aminoacidurias, which must be excluded. The combination of characteristic clinical presentation and biochemical findings is essential for diagnosis.
Pathophysiology
Key Mechanisms
Defective neutral amino acid transport in the renal proximal tubules and intestinal epithelium due to mutations in the SLC6A19 gene causes impaired absorption and reabsorption of amino acids like tryptophan.
Reduced tryptophan availability leads to decreased synthesis of niacin (vitamin B3), resulting in symptoms resembling pellagra.
Excessive urinary excretion of neutral amino acids causes aminoaciduria, a hallmark biochemical finding.
Secondary niacin deficiency causes dermatitis, diarrhea, and dementia, the classic triad of clinical manifestations.
| Involvement | Details |
|---|---|
| Organs | Small intestine is the primary site of defective neutral amino acid absorption in Hartnup disease. |
Kidneys contribute to aminoaciduria due to impaired reabsorption in the proximal tubules. | |
Skin is affected by photosensitive dermatitis resulting from niacin deficiency. | |
Brain may be involved causing neurological symptoms such as ataxia and psychiatric manifestations due to niacin deficiency. | |
| Tissues | Intestinal mucosa is involved due to impaired absorption of neutral amino acids. |
Renal proximal tubule epithelium is affected by defective reabsorption of amino acids leading to aminoaciduria. | |
| Cells | Enterocytes in the small intestine are defective in neutral amino acid transport due to mutations in the SLC6A19 gene, leading to impaired absorption. |
Renal tubular cells show defective reabsorption of neutral amino acids causing aminoaciduria. | |
| Chemical Mediators | Tryptophan is a key neutral amino acid whose defective absorption leads to niacin deficiency and symptoms of Hartnup disease. |
Niacin deficiency results in decreased synthesis of NAD and NADP, causing pellagra-like manifestations. |
Treatments
Pharmacological Treatments
Niacin (Vitamin B3) supplementation
- Mechanism:
Repletes niacin levels to prevent and treat pellagra-like symptoms caused by defective tryptophan absorption.
- Side effects:
Flushing
Gastrointestinal upset
Hepatotoxicity
- Clinical role:
First-line
Non-pharmacological Treatments
High-protein diet to increase intake of tryptophan and other neutral amino acids.
Sunlight avoidance and use of sunscreen to prevent photosensitive dermatitis.
Supportive care for neurological symptoms including physical therapy.
Prevention
Pharmacological Prevention
Oral niacin (nicotinic acid) supplementation to prevent pellagra symptoms
High-dose nicotinamide to replenish niacin stores and reduce neurological symptoms
Tryptophan supplementation may be used adjunctively to improve amino acid levels
Non-pharmacological Prevention
Sun protection measures including sunscreen and protective clothing to prevent rash
Dietary optimization with high-protein foods rich in niacin and tryptophan
Regular monitoring for early detection of neurological or dermatological symptoms
Outcome & Complications
Complications
Pellagra-like symptoms including dermatitis, diarrhea, and dementia
Neurological impairment such as ataxia and peripheral neuropathy
Psychiatric manifestations including psychosis and depression
Secondary infections due to skin barrier disruption
| Short-term Sequelae | Long-term Sequelae |
|---|---|
|
|
Differential Diagnoses
Hartnup Disease versus Pellagra (Niacin Deficiency)
Hartnup Disease | Pellagra (Niacin Deficiency) |
|---|---|
Genetic defect in neutral amino acid transport causing impaired tryptophan absorption | Dietary deficiency or malabsorption leading to low niacin intake |
Normal niacin levels but increased neutral amino acids in urine | Low serum niacin and its metabolites |
Improvement requires niacin supplementation plus high-protein diet | Improvement with niacin supplementation alone |
Hartnup Disease versus Carcinoid Syndrome
Hartnup Disease | Carcinoid Syndrome |
|---|---|
Normal 5-HIAA levels with increased neutral aminoaciduria | Elevated 5-hydroxyindoleacetic acid (5-HIAA) in urine |
Intermittent photosensitive dermatitis and ataxia without flushing | Chronic symptoms with flushing, diarrhea, and bronchospasm |
No tumor on imaging; diagnosis confirmed by amino acid transport defect | Imaging shows carcinoid tumor or metastases |
Hartnup Disease versus Vitamin B6 (Pyridoxine) Deficiency
Hartnup Disease | Vitamin B6 (Pyridoxine) Deficiency |
|---|---|
Normal pyridoxine levels with defective amino acid transport | Low serum pyridoxal phosphate levels |
Photosensitive pellagra-like rash and cerebellar ataxia | Peripheral neuropathy and sideroblastic anemia |
Symptoms improve with niacin and protein supplementation | Symptoms improve with pyridoxine supplementation |
Hartnup Disease versus Cystinuria
Hartnup Disease | Cystinuria |
|---|---|
Increased urinary excretion of neutral amino acids like tryptophan | Increased urinary excretion of cystine and dibasic amino acids |
Neurologic symptoms and photosensitive rash without kidney stones | Recurrent nephrolithiasis due to cystine stones |
Autosomal recessive defect in intestinal and renal neutral amino acid transport | Autosomal recessive inheritance affecting renal tubular transport |
Hartnup Disease versus Biotinidase Deficiency
Hartnup Disease | Biotinidase Deficiency |
|---|---|
Photosensitive dermatitis and ataxia presenting in childhood | Seizures, alopecia, and metabolic acidosis in infancy |
Normal biotinidase activity with defective amino acid transport | Low biotinidase enzyme activity |
Improvement with niacin and protein supplementation | Rapid improvement with biotin supplementation |