Amino acids are building blocks for protein. There are three types of amino acids based on the polarity of the side chain. It includes acidic, basic, and neutral amino acids. Hartnup disease is an autosomal recessive inherited nutritional disorder due to decreased absorption of neutral amino acids from the gut and kidney. It has a wide range of clinical spectrum including neutral aminoaciduria, indicanuria, photosensitive pellagra like skin rash, cerebellar ataxia, anxiety, depression, mild intellectual disability. The clinical symptoms occur intermittently and are influenced by environmental factors, season, stress, and malnutrition. Some patients remain asymptomatic and are diagnosed through routine screening.
Hartnup disease has a genetic etiology. SLC6A19 is a gene located on the short arm of chromosome 5 that encodes transport protein B0AT1 present at the apical cell surface of the small intestine and kidney. Immunohistochemical studies showed maximum expression of B0AT1 in early proximal convoluted tubule in kidney and jejunum in the small intestine.
Mutation in the SLC6A19 gene is responsible for Hartnup disease. The mutations noted to cause the disease are missense, splice site, frameshift, and nonsense. The activation and trafficking of B0AT1 to the cell surface occurs via specific type 1 membrane protein. These are collectrin at kidneys and angiotensin-converting enzyme 2 at intestines. They activate it by catabolic reactions and transport B0AT1 to the apical cell surface. Human ACE 2 gene encodes angiotensin-converting enzyme 2, and the CLTRN gene encodes collectrin protein. Both genes are located at X chromosome band Xp22.2. Mutations in these genes result in sequestration and reduced surface expression of B0AT1 transporter, which presents with clinical symptoms of Hartnup disease.
Hartnup disease has a wide range of clinical symptoms as it can result from a mutation in the transport protein B0AT1, or co-receptors collectrin and ACE2.
The incidence of Hartnup disease is 1 in 15,000 live births. Hartnup disease was first described by Baron et al. in 1956 in a family in England in which four siblings were affected. The condition was first described in a boy in Hartnup family who present with photosensitive rash, ataxia, hand tremors, and gait disturbances. On investigation of further family members, his other three siblings were also found to have the same syndrome with varying clinical symptoms. Hartnup disease has an early age of onset, with most patients presenting in childhood. As it is inherited in an autosomal recessive pattern and does not have a sexual predilection.
Protein digestion begins in the stomach by the action of pepsinogen. The proteins are then broken down into smaller peptides by the action of various enzymes secreted by the pancreas and enzymes present at the brush border of the small intestine. These enzymes break down proteins into tri-peptides, di-peptides, and amino acids. The majority of amino acids are absorbed in jejunum through special transport protein located at the apical surface of brush borders. B0AT1 is a sodium-dependent co-transport protein for the absorption of neutral amino acids present at the apical surface of the small intestine and renal tubular cells. The absorption of neutral amino acids is independent of chloride concentration. The absorbed neutral amino acids are transported via the blood to other organs. In the kidney, it is filtered through glomerulus, where it is reabsorbed through B0AT1 membrane protein located in proximal tubular cells.
In Hartnup disease, there is a defect in B0AT1 protein, resulting in excessive excretion of neutral amino acids in urine and feces. A small amount of neutral amino acids is absorbed in the gut directly via paracellular transport and as oligopeptides from proton counter transport proteins.
Excess tryptophan present in the gut is converted to indolic compounds by the action of colonic bacteria. These indolic compounds are conjugated in the liver and are excreted in urine as indican. Tryptophan is a neutral amino acid that is a precursor for niacin. Wasting of tryptophan results in niacin deficiency manifesting as pellagra like skin eruptions and neurological symptoms.
The patients present with intermittent and reversible episodes of cutaneous and neurological clinical symptoms aggravated by sunlight, hot weather, and nutritional deficiency. The skin eruptions that resemble pellagra are erythematous, scaly, and present at sun-exposed areas of skin. There have been reports of improvement of skin rash with nicotinamide supplementation. Neurological symptoms range from tremors, ataxia, mood disorders, depression, convulsions, and psychosis.
Developmental milestones are normal in children, but below normal academic performance and short stature have been noted. The clinical symptoms improve with age when protein demand decreases. Patients may remain asymptomatic throughout life if their niacin intake is adequate.
The diagnosis of Hartnup disease is by urine analysis showing neutral aminoaciduria except for proline. The neutral amino acids (valine, serine, phenylalanine, histidine, glutamine, leucine, asparagine, citrulline, isoleucine, threonine, alanine, tyrosine, tryptophan) undergo analysis by paper chromatography of urine. Indican is also present in the urine, produced as a by-product of degradation of tryptophan by colonic bacteria.
The main aim of treatment is to prevent the occurrence of clinical episodes. Hartnup disease is treatable by a high protein diet. Patients should have a well-balanced diet as episodes are more frequent in individuals who are malnourished and have maize as their staple diet. Nicotinamide supplements ameliorate skin eruptions and neurological symptoms in patients with niacin deficiency. Patients with neurological symptoms require complete neurological workup. Appropriate psychiatric and neurological treatments may be needed to resolve the neuropsychiatric symptoms.
Hartnup disease requires differentiation from other conditions that result in a photosensitive skin rash. The common causes include:
Nutritional pellagra has normal urine analysis. Lupus has specific antinuclear (ANA), anti-Smith, and anti-double-stranded (anti-ds Ab) antibodies in plasma. Carcinoid syndrome is associated with flushing, diarrhea, and elevated levels of 5-hydroxy indoleacetic acid(5-HIAA) levels in urine. Seborrheic eczema has yellow crusting on skin lesions, and there is a history of allergies.
Patients should take preventive measures to prevent episodes of Hartnup disease. Preventive measures include avoiding direct sunlight, wearing protective clothing, and applying sunblock in outdoors as hot weather and sunlight exacerbate skin lesions.
Drugs like sulfonamides, nonsteroidal anti-inflammatory medicines, tetracyclines, oral contraceptives that increase sensitivity to light should also be avoided.
A multidisciplinary team involving a primary care physician, nutritionist, and neurologist should manage the patient. Routine newborn screening for Hartnup disease is not standard at present. Patients presenting with skin rash similar to pellagra, tremors, or ataxia at a young age should be evaluated for Hartnup disease as it is easily treatable. Interprofessional collaboration is important to improve patient outcomes in Hartnup disease.
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