COVID-19 and Hartnup disease: an affair of intestinal amino acid malabsorption.

Since the outbreak of COVID-19, clinicians have tried every effort to fight the disease, and multiple drugs have been proposed. However, no proven effective therapies currently exist, and different clinical phenotypes complicate the situation. In clinical practice, many severe or critically ill COVID-19 patients developed gastrointestinal (GI) disturbances, including vomiting, diarrhoea, or abdominal pain, even in the absence of cough and dyspnea. Understanding the mechanism of GI disturbances is warranted for exploring better clinical care for COVID-19 patients. With evidence collected from clinical studies on COVID-19 and basic research on a rare genetic disease (i.e., Hartnup disorder), we put forward a novel hypothesis to elaborate an effective nutritional therapy. We hypothesize that SARS-CoV-2 spike protein, binding to intestinal angiotensin-converting enzyme 2, negatively regulates the absorption of neutral amino acids, and this could explain not only the GI, but also systemic disturbances in COVID-19. Amino acid supplements could be recommended.Level of evidence No level of evidence: Hypothesis article.

Biochemical phenotyping of multiple myeloma patients at diagnosis reveals a disorder of mitochondrial complexes I and II and a Hartnup-like disturbance as underlying conditions, also influencing different stages of the disease.

The aim of this study was to identify novel plasma metabolic signatures with possible relevance during multiple myeloma (MM) development and progression. A biochemical quantitative phenotyping platform based on targeted electrospray ionization tandem mass spectrometry technology was used to aid in the identification of any eventual perturbed biochemical pathway in peripheral blood plasma from 36 MM patients and 73 healthy controls. Our results showed that MM cases present an increase in short and medium/long-chain species of acylcarnitines resembling Multiple AcylCoA Dehydrogenase Deficiency (MADD), particularly, associated with MM advanced International Staging System (ISS). Lipids profile showed lower concentrations of phosphatidylcholine (PC), lysophosphatidylcholine (LPC) and sphingomyelins (SM) in the MM patients and its respective ISS groups. MM cases were accompanied by a drop in the concentration of essential amino acids, especially tryptophan, with a significant inverse correlation between the progressive drop in tryptophan with the elevation of ß2-microglobulin, with the increase in systemic methylation levels (Symmetric Arginine Dimethylation, SDMA) and with the accumulation of esterified carnitines in relation to free carnitine (AcylC/C0). Serotonin was significantly elevated in cases of MM, without a clear association with ISS. Kynurenine/tryptophan ratio demonstrates that the activity of dioxigenases is even higher in the cases classified as ISS 3. In conclusion, our study showed that MM patients at diagnosis showed metabolic disorders resembling both mitochondrial complexes I and II and Hartnup-like disturbances as underlying conditions, also influencing different stages of the disease.

Collectrin and ACE2 in renal and intestinal amino acid transport.

Neutral amino acid transporters of the SLC6 family are expressed at the apical membrane of kidney and/or small intestine, where they (re-)absorb amino acids into the body. In this review we present the results concerning the dependence of their apical expression with their association to partner proteins. We will in particular focus on the situation of B0AT1 and B0AT3, that associate with members of the renin-angiotensin system (RAS), namely Tmem27 and angiotensin-converting enzyme 2 (ACE2), in a tissue specific manner. The role of this association in relation to the formation of a functional unit related to Na+ or amino acid transport will be assessed. We will conclude with some remarks concerning the relevance of this association to Hartnup disorder, where some mutations have been shown to differentially interact with the partner proteins.

Impaired nutrient signaling and body weight control in a Na+ neutral amino acid cotransporter (Slc6a19)-deficient mouse.

Amino acid uptake in the intestine and kidney is mediated by a variety of amino acid transporters. To understand the role of epithelial neutral amino acid uptake in whole body homeostasis, we analyzed mice lacking the apical broad-spectrum neutral (0) amino acid transporter B(0)AT1 (Slc6a19). A general neutral aminoaciduria was observed similar to human Hartnup disorder which is caused by mutations in SLC6A19. Na(+)-dependent uptake of neutral amino acids into the intestine and renal brush-border membrane vesicles was abolished. No compensatory increase of peptide transport or other neutral amino acid transporters was detected. Mice lacking B(0)AT1 showed a reduced body weight. When adapted to a standard 20% protein diet, B(0)AT1-deficient mice lost body weight rapidly on diets containing 6 or 40% protein. Secretion of insulin in response to food ingestion after fasting was blunted. In the intestine, amino acid signaling to the mammalian target of rapamycin (mTOR) pathway was reduced, whereas the GCN2/ATF4 stress response pathway was activated, indicating amino acid deprivation in epithelial cells. The results demonstrate that epithelial amino acid uptake is essential for optimal growth and body weight regulation.

A novel missense mutation in the SLC6A19 gene in a Chinese family with Hartnup disorder.

BACKGROUND: Hartnup disease is a rare autosomal-recessive abnormality of renal and gastrointestinal neutral amino acid transport associated with neurologic, psychiatric, and dermatologic symptoms. Mutations in the SLC6A19 gene have been proposed to be responsible for the underlying changes in this disorder. AIM: To investigate a pedigree with Hartnup disorder and to search for the mutation in the SLC6A19 gene in this pedigree. METHODS: The encoding exons of the SLC6A19 gene were amplified and sequenced from genomic DNA samples. Amino acids were determined in urine samples from the proband and her family members. RESULTS: The proband and her brother had a homozygous mutation of c.850G > A in the SLC6A19 gene, causing G284R in the transmembrane domain of the SLC6A19 transporter, inherited from their parents who were heterozygous carriers. Their urine samples showed increased values of eight neutral amino acids. CONCLUSION: We found a novel homozygous mutation of G284R in the transmembrane domain of the SLC6A19 transporter in the proband, with typical dermatologic and neurologic manifestations and increased levels of urinary neutral amino acids.

Tissue-specific amino acid transporter partners ACE2 and collectrin differentially interact with hartnup mutations.

BACKGROUND & AIMS: Hartnup amino acid transporter B(0)AT1 (SLC6A19) is the major luminal sodium-dependent neutral amino acid transporter of small intestine and kidney proximal tubule. The expression of B(0)AT1 in kidney was recently shown to depend on its association with collectrin (Tmem27), a protein homologous to the membrane-anchoring domain of angiotensin-converting enzyme (ACE) 2. METHODS: Because collectrin is almost absent from small intestine, we tested the hypothesis that it is ACE2 that interacts with B(0)AT1 in enterocytes. Furthermore, because B(0)AT1 expression depends on an associated protein, we tested the hypothesis that Hartnup-causing B(0)AT1 mutations differentially impact on B(0)AT1 interaction with intestinal and kidney accessory proteins. RESULTS: Immunofluorescence, coimmunoprecipitation, and functional experiments using wild-type and ace2-null mice showed that expression of B(0)AT1 in small intestine critically depends on ACE2. Coexpressing new and previously identified Hartnup disorder-causing missense mutations of B(0)AT1 with either collectrin or ACE2 in Xenopus laevis oocytes showed that the high-frequency D173N and the newly identified P265L mutant B(0)AT1 transporters can still be activated by ACE2 but not collectrin coexpression. In contrast, the human A69T and R240Q B(0)AT1 mutants cannot be activated by either of the associated proteins, although they function as wild-type B(0)AT1 when expressed alone. CONCLUSIONS: We thus show that ACE2 is necessary for the expression of the Hartnup transporter in intestine and suggest that the differential functional association of mutant B(0)AT1 transporters with ACE2 and collectrin in intestine and kidney, respectively, participates in the phenotypic heterogeneity of human Hartnup disorder.

A protein complex in the brush-border membrane explains a Hartnup disorder allele.

Protein absorption in the intestine is mediated by proteases and brush-border peptidases together with peptide and amino acid transporters. Neutral amino acids are generated by a variety of aminopeptidases and carboxypeptidases and are subsequently taken up by the amino acid transporter B(0)AT1 (SLC6A19), which is mutated in Hartnup disorder. Coexpression of B(0)AT1 together with the brush-border carboxypeptidase angiotensin-converting enzyme 2 (ACE2) in Xenopus laevis oocytes led to a dramatic increase of transporter expression at the oocyte surface. Other members of the SLC6 family were not stimulated by coexpression with ACE2. Addition of a peptide containing a carboxyterminal leucine residue to ACE2- and B(0)AT1-coexpressing oocytes caused inward currents due to Na(+)-leucine cotransport, demonstrating the formation of a metabolic complex. Coexpression of the Hartnup disorder causing mutation B(0)AT1(R240Q) showed reduced interaction with ACE2 and its renal paralogue collectrin. This would result in reduced surface expression in both kidney and intestine, thereby explaining the onset of the disorder in individuals carrying this mutation.

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.

Neutral amino acid transport in epithelial cells and its malfunction in Hartnup disorder.

Hartnup disorder is an autosomal recessive abnormality of renal and gastrointestinal neutral amino acid transport. A corresponding transport activity has been characterized in kidney and intestinal cells and named system B(0). The failure to resorb amino acids in this disorder is thought to be compensated by a protein-rich diet. However, in combination with a poor diet and other factors, more severe symptoms can develop in Hartnup patients, including a photosensitive pellagra-like skin rash, cerebellar ataxia and other neurological symptoms. Homozygosity mapping in a Japanese family and linkage analysis on six Australian pedigrees placed the Hartnup disorder gene at a locus on chromosome 5p15. This fine mapping facilitated a candidate gene approach within the interval, which resulted in the cloning and characterization of a novel member of the sodium-dependent neurotransmitter transporter family (B(0)AT1, SLC6A19) from mouse and human kidney, which shows all properties of system B(0). Flux experiments and electrophysiological recording showed that the transporter is Na(+) dependent and Cl(-) independent, electrogenic and actively transports most neutral amino acids. In situ hybridization showed strong expression in intestinal villi and in the proximal tubule of the kidney. Expression of B(0)AT1 was restricted to kidney, intestine and skin. A total of ten mutations have been identified in SLC6A19 that co-segregate with disease in the predicted recessive manner, with the majority of affected individuals being compound heterozygotes. These mutations lead to altered neutral amino acid transport function compared to the wild-type allele in vitro. One of the mutations occurs in members of the original Hartnup family described in 1956, thereby defining SLC6A19 as the 'Hartnup'-gene.

Molecular cloning of mouse amino acid transport system B0, a neutral amino acid transporter related to Hartnup disorder.

Resorption of amino acids in kidney and intestine is mediated by transporters, which prefer groups of amino acids with similar physico-chemical properties. It is generally assumed that most neutral amino acids are transported across the apical membrane of epithelial cells by system B(0). Here we have characterized a novel member of the Na(+)-dependent neurotransmitter transporter family (B(0)AT1) isolated from mouse kidney, which shows all properties of system B(0). Flux experiments showed that the transporter is Na(+)-dependent, electrogenic, and actively transports most neutral amino acids but not anionic or cationic amino acids. Superfusion of mB(0)AT1-expressing oocytes with neutral amino acids generated inward currents, which were proportional to the fluxes observed with labeled amino acids. In situ hybridization showed strong expression in intestinal microvilli and in the proximal tubule of the kidney. Expression of mouse B(0)AT1 was restricted to kidney, intestine, and skin. It is generally assumed that mutations of the system B(0) transporter underlie autosomal recessive Hartnup disorder. In support of this notion mB(0)AT1 is located on mouse chromosome 13 in a region syntenic to human chromosome 5p15, the locus of Hartnup disorder. Thus, the human homologue of this transporter is an excellent functional and positional candidate for Hartnup disorder.

Homozygosity mapping to chromosome 5p15 of a gene responsible for Hartnup disorder.

Hartnup disorder is an autosomal recessive phenotype involving a transporter for monoamino-monocarboxylic acids. Genetic analysis of the mouse model mapped its locus to human chromosome 11q13 (8). We report here the results of linkage analysis in two Japanese first cousin-marriage families. In the first family, the proband had Hartnup disorder and his deceased older brother was reported to have had typical Hartnup symptoms. The younger brother of the proband was shown to have decreased tryptophan absorption by oral loading test. In the second family, a 6-year-old girl, the proband, had specific hyperaminoaciduria. DNA was isolated from either blood samples or umbilical cord stumps. Genome-wide screening by homozygosity mapping was conducted. Taking into account that the older brother was affected and the younger brother was a carrier in the first family, homozygosity mapping (LOD score = 3.55) and GENEHUNTER (LOD score = 3.28) locates the locus of the Hartnup disorder on 5p15.

A candidate mouse model for Hartnup disorder deficient in neutral amino acid transport.

The mutant mouse strain HPH2 (hyperphenylalaninemia) was isolated after N-ethyl-N-nitrosourea (ENU) mutagenesis on the basis of delayed plasma clearance of an injected load of phenylalanine. Animals homozygous for the recessive hph2 mutation excrete elevated concentrations of many of the neutral amino acids in the urine, while plasma concentrations of these amino acids are normal. In contrast, mutant homozygotes excrete normal levels of glucose and phosphorus. These data suggest an amino acid transport defect in the mutant, confirmed in a small reduction in normalized values of 14C-labeled glutamine uptake by kidney cortex brush border membrane vesicles (BBMV). The hyperaminoaciduria pattern is very similar to that of Hartnup Disorder cases also show niacin deficiency symptoms, of Hartnup Disorder cases also show niacin deficiency symptoms, which are thought to be multifactorially determined. Similarly, the HPH2 mouse exhibits a niacin-reversible syndrome that is modified by diet and by genetic background. Thus, HPH2 provides a candidate mouse model for the study of Hartnup Disorder, an amino acid transport deficiency and a multifactorial disease in the human.

Hartnup syndrome, progressive encephalopathy and allo-albuminaemia. A clinico-pathological case study.

Clinical, biochemical, neuropathological and neurochemical findings in a case of Hartnup syndrome are reported. After initially normal development, the affected girl suffered progressive neuropsychiatric decline with statomotor and mental retardation and intractable seizures and died at the age of 2 years. Postmortem neuropathological and neurochemical investigations showed a combination of extensive neuronal degeneration and cerebral dysmyelination. Pathogenetic hypotheses and the relationship between neuropsychiatric disease and Hartnup syndrome are discussed. Additionally, a fast type bisalbuminaemia present in the girl and her mother is described.

[Inherited amino acid transport disorders].

Disorders due to inherited amino acids transport defect are reviewed. The disorders were categorized into three types of transport defects, namely, brush-border membrane of epithelial cells of small intestine and kidney tubules (Hartnup disease, blue diaper syndrome, cystinuria, iminoglycinuria and lysine malabsorption syndrome), basolateral membrane (lysinuric protein intolerance) and membrane of intracellular organelles (cystinosis and hyperornitinemia-hyperammonemia-homocitrullinuria syndrome). Pathogenesis, clinical feature, laboratory findings, diagnosis, genetics and treatment of these disorders are described, briefly. There is not much data for the transport systems themselves, so that further investigation in molecular and gene levels for transport systems is necessary to clarify the characteristics of the transport and heterogeneity of phenotypes in inherited amino acids transport disorders.

[Clinical picture of Hartnup disease. Without urine amino acids or any other identified metabolic disorder (a new entity)]. / Quadro clínico de doença de Hartnup. Sem aminoaciduria ou outra alteração metabólica identificada (entidade Nova).

Harthnup disease clinical picture without aminoaciduria or other identified metabolic disturb (New entity?). The authors present a patient with clinical picture superposed to the Hartnup disease's, a rare, autosomic and recessive metabolic disturbance, characterized by typical aminoaciduria consequent to tryptophan and other neutral aminoacids defective transport by jejunal mucous membrane and renal tubules, clinically expressed by photosensitive pellagra-like dermatitis, mental retardation and intermittent cerebellar ataxia. The laboratorial results did not confirm Hartnup aminoaciduria nor other identified metabolic change that justify his clinical manifestations.

Circumvention of defective neutral amino acid transport in Hartnup disease using tryptophan ethyl ester.

Tryptophan ethyl ester, a lipid-soluble tryptophan derivative, was used to bypass defective gastrointestinal neutral amino acid transport in a child with Hartnup disease. The child's baseline tryptophan concentrations in serum (20 +/- 6 microM) and cerebrospinal fluid (1.0 +/- 0.2 microM) were persistently less than 50% of normal values. Cerebrospinal fluid 5-hydroxyindoleacetic acid (5-HIAA), a serotonin metabolite, was also less than 50% of normal (21 +/- 2 ng/ml). Serum tryptophan concentrations increased only modestly and briefly after an oral challenge with 200 mg/kg of oral L-tryptophan, reflecting the absorptive defect. An oral challenge with 200 mg/kg of tryptophan ethyl ester resulted in a prompt increase in serum tryptophan to a peak of 555 microM. Sustained treatment with 20 mg/kg q6h resulted in normalization of serum (66 +/- 15 microM) and cerebrospinal fluid tryptophan concentrations (mean = 2.3 microM). Cerebrospinal fluid 5-HIAA increased to more normal concentrations (mean = 33 ng/ml). No toxicity was observed over an 8-mo period of treatment, chronic diarrhea resolved, and body weight, which had remained unchanged for 7 mo before ester therapy, increased by approximately 26%. We concluded that tryptophan ethyl ester is effective at circumventing defective gastrointestinal neutral amino acid transport and may be useful in the treatment of Hartnup disease.

[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)