The role of the neutral amino acid transporter B0AT1 (SLC6A19) in Hartnup disorder and protein nutrition.

Hartnup disorder (OMIM 234500) is an autosomal recessive disorder, which was first described in 1956 as an aminoaciduria of neutral amino acids accompanied by a variety of symptoms, such as a photo-sensitive skin-rash and cerebellar ataxia. The disorder is caused by mutations in the neutral amino acid transporter B(0)AT1 (SLC6A19). To date 21 mutations have been identified in more than twenty families. SLC6A19 requires either collectrin or angiotensin-converting enzyme 2 for surface expression in the kidney and intestine, respectively. This ties SLC6A19 together with more complex functions such as blood-pressure control, glomerular structure, and exocytosis.

Hartnup disorder: unraveling the mystery.

Hartnup disorder is an autosomal recessive disease that can be associated with neurological, psychiatric and dermatological abnormalities or be asymptomatic. Excessive intestinal and urinary loss of neutral amino acids is an essential feature of this disorder, which had been presumed to be due to hereditary abnormalities in an apical membrane-situated amino acid transporter. As anticipated, recently, mutations in the cytoplasmic and transmembrane domains of SLC6A19, the recently cloned neutral amino acid transporter, were detected in members of families with Hartnup disorder. Presumably, deficiency in neutral amino acid absorption and consequential hypoaminoacidemia is the cause of the symptoms of the disease because SLC6A19 is not expressed in the organs affected.

[Adult-onset Hartnup disease presenting with neuropsychiatric symptoms but without skin lesions].

Hartnup disease is an inborn abnormality of renal and intestinal transport involving the neutral amino acids. Intermittent pellagra-like rash, attacks of cerebellar ataxia and psychiatric disturbance are characteristic symptoms of this disease. We described here a patient with adult-onset Hartnup disease who presented unique neuropsychiatric symptoms but no dermatologic symptoms, and reported features of amino acids transport in this patient and his family. The patient, a man aged 37 years, was referred to us because of lasting daytime bruxism. He is the second child of healthy parents who are first cousin; his elder brother who has been mentally retarded became bed-ridden and died at 32 years of age. His younger brother is completely healthy. Although the patient's development in infancy has been slightly retarded, he completed compulsory 9-year education. At 29 years of age, he experienced episodes of diplopia, ataxic gait and insomnia, and at 33 years of age, of transient stupor. There had been no history of photosensitivity or dermatitis. On neurological examination, there were trunkal ataxia, increased muscular tone and decreased mental activity besides bruxism. These symptoms remained unchanged despite of several medications including trihexyphenidyl, diazepam, halloperidol, tiapride and sulpiride. Two months later, the patient became stuporous; bruxism and hypertonicity became exaggerated. Myerson's sign, sucking reflex and grasp reflex in both hand appeared. There was no dermal lesion. A cranial computed tomography revealed a small calcification in the right frontal subcortical region and a single photon emission tomography indicated possible bifrontal hypoperfusion. Electroencephalograms demonstrated non-specific slowing. Somatosensory evoked potentials and nerve conduction velocities were normal. There were constant indicanuria and amino-aciduria.(ABSTRACT TRUNCATED AT 250 WORDS)

Apical transporters for neutral amino acids: physiology and pathophysiology.

Absorption of amino acids in kidney and intestine involves a variety of transporters for different groups of amino acids. This is illustrated by inherited disorders of amino acid absorption, such as Hartnup disorder, cystinuria, iminoglycinuria, dicarboxylic aminoaciduria, and lysinuric protein intolerance, affecting separate groups of amino acids. Recent advances in the molecular identification of apical neutral amino acid transporters has shed a light on the molecular basis of Hartnup disorder and iminoglycinuria.

Natural history of Hartnup disease.

Hartnup disease was diagnosed in 12 children and 3 of their 15 sibs in the course of routine urine screening of 6-week-old infants in New South Wales. These children were followed for up to 8 years, during which time there were only two clinical episodes which might be ascribed to Hartnup disease. The mental development of all the children was normal. 10 had height centiles less than the midparent height centiles, while 4 had centiles equal to or above the midparent centiles. The study shows that in children with Hartnup disease in Australia symptoms are very uncommon. Mental development is normal, and heights are possibly slightly below that expected. Hartnup disease has an incidence of approximately 1 in 33 000 in New South Wales.