Pulmonary alveolar proteinosis and glomerulonephritis in lysinuric protein intolerance: case reports and autopsy findings of four pediatric patients

Screening of Amino Acids as a Safe Energy Source for Isolated Rat Pancreatic Acini

OBJECTIVES

Amino acids play an essential role in protein synthesis, metabolism, and survival of pancreatic acini. Adequate nutritional support is important for acute pancreatitis treatment. However, high concentrations of arginine and lysine may induce acute pancreatitis. The study aimed to identify the most suitable l -amino acids as safe energy sources for pancreatic acinar cells.

MATERIALS AND METHODS

Pancreatic acini were isolated from male Wistar rats. Effects of amino acids (0.1-20 mM) on uncoupled respiration of isolated acini were studied with a Clark electrode. Cell death was evaluated with fluorescent microscopy and DNA gel electrophoresis.

RESULTS

Among the tested amino acids, glutamate, glutamine, alanine, lysine, and aspartate were able to stimulate the uncoupled respiration rate of isolated pancreatic acini, whereas arginine, histidine, and asparagine were not. Lysine, arginine, and glutamine (20 mM) caused complete loss of plasma membrane integrity of acinar cells after 24 hours of incubation. Glutamine also caused early (2-4 hours) cell swelling and blebbing. Aspartate, asparagine, and glutamate only moderately decreased the number of viable cells, whereas alanine and histidine were not toxic. DNA fragmentation assay and microscopic analysis of nuclei showed no evidence of apoptosis in cells treated with amino acids.

CONCLUSIONS

Alanine and glutamate are safe and effective energy sources for mitochondria of pancreatic acinar cells.

Delayed skeletal development and IGF-1 deficiency in a mouse model of lysinuric protein intolerance

SLC7A7 deficiency, or lysinuric protein intolerance (LPI), causes loss of function of the y+LAT1 transporter critical for efflux of arginine, lysine and ornithine in certain cells. LPI is characterized by urea cycle dysfunction, renal disease, immune dysregulation, growth failure, delayed bone age and osteoporosis. We previously reported that Slc7a7 knockout mice (C57BL/6×129/SvEv F2) recapitulate LPI phenotypes, including growth failure. Our main objective in this study was to characterize the skeletal phenotype in these mice. Compared to wild-type littermates, juvenile Slc7a7 knockout mice demonstrated 70% lower body weights, 87% lower plasma IGF-1 concentrations and delayed skeletal development. Because poor survival prevents evaluation of mature knockout mice, we generated a conditional Slc7a7 deletion in mature osteoblasts or mesenchymal cells of the osteo-chondroprogenitor lineage, but no differences in bone architecture were observed. Overall, global Slc7a7 deficiency caused growth failure with low plasma IGF-1 concentrations and delayed skeletal development, but Slc7a7 deficiency in the osteoblastic lineage was not a major contributor to these phenotypes. Future studies utilizing additional tissue-specific Slc7a7 knockout models may help dissect cell-autonomous and non-cell-autonomous mechanisms underlying phenotypes in LPI.

Plasma calprotectin is extremely high in patients with lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is a rare autosomal recessive disorder affecting the transport of cationic amino acids. Elevated plasma zinc concentrations have been described in patients with LPI. Calprotectin is a calcium- and zinc-binding protein, produced by polymorphonuclear leukocytes and monocytes. Both zinc and calprotectin have an important role in immune system. In this study, we describe plasma zinc and plasma calprotectin concentrations in Finnish LPI patients. Plasma calprotectin concentration was measured from 10 LPI patients using an enzyme-linked immunosorbent assay (ELISA) and it was remarkably high in all LPI patients (median: 622 338 μg/L) compared to that in healthy controls (608 μg/L). Plasma zinc concentration was measured by photometry and it was normal or only mildly elevated (median: 14.9 μmol/L). All the patients had decreased glomerular infiltration rate (median: 50 mL/min/1.73 m2). In conclusion, we observed extremely high plasma calprotectin concentration in patients with LPI. Mechanism of this phenomenon is unknown.

The Finnish genetic heritage in 2022 – from diagnosis to translational research

Isolated populations have been valuable for the discovery of rare monogenic diseases and their causative genetic variants. Finnish disease heritage (FDH) is an example of a group of hereditary monogenic disorders caused by single major, usually autosomal-recessive, variants enriched in the population due to several past genetic drift events. Interestingly, distinct subpopulations have remained in Finland and have maintained their unique genetic repertoire. Thus, FDH diseases have persisted, facilitating vigorous research on the underlying molecular mechanisms and development of treatment options. This Review summarizes the current status of FDH, including the most recently discovered FDH disorders, and introduces a set of other recently identified diseases that share common features with the traditional FDH diseases. The Review also discusses a new era for population-based studies, which combine various forms of big data to identify novel genotype–phenotype associations behind more complex conditions, as exemplified here by the FinnGen project. In addition to the pathogenic variants with an unequivocal causative role in the disease phenotype, several risk alleles that correlate with certain phenotypic features have been identified among the Finns, further emphasizing the broad value of studying genetically isolated populations.

Immune Dysregulation Mimicking Systemic Lupus Erythematosus in a Patient With Lysinuric Protein Intolerance: Case Report and Review of the Literature

Lysinuric protein intolerance (LPI) is an inborn error of metabolism caused by defective transport of cationic amino acids in epithelial cells of intestines, kidneys and other tissues as well as non-epithelial cells including macrophages. LPI is caused by biallelic, pathogenic variants in SLC7A7. The clinical phenotype of LPI includes failure to thrive and multi-system disease including hematologic, neurologic, pulmonary and renal manifestations. Individual presentations are extremely variable, often leading to misdiagnosis or delayed diagnosis. Here we describe a patient that clinically presented with immune dysregulation in the setting of early-onset systemic lupus erythematosus (SLE), including renal involvement, in whom an LPI diagnosis was suspected post-mortem based on exome sequencing analysis. A review of the literature was performed to provide an overview of the clinical spectrum and immune mechanisms involved in this disease. The precise mechanism by which ineffective amino acid transport triggers systemic inflammatory features is not yet understood. However, LPI should be considered in the differential diagnosis of early-onset SLE, particularly in the absence of response to immunosuppressive therapy.

A global Slc7a7 knockout mouse model demonstrates characteristic phenotypes of human lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is an inborn error of cationic amino acid (arginine, lysine, ornithine) transport caused by biallelic pathogenic variants in SLC7A7, which encodes the light subunit of the y+LAT1 transporter. Treatments for the complications of LPI, including growth failure, renal disease, pulmonary alveolar proteinosis, autoimmune disorders and osteoporosis, are limited. Given the early lethality of the only published global Slc7a7 knockout mouse model, a viable animal model to investigate global SLC7A7 deficiency is needed. Hence, we generated two mouse models with global Slc7a7 deficiency (Slc7a7em1Lbu/em1Lbu; Slc7a7Lbu/Lbu and Slc7a7em1(IMPC)Bay/em1(IMPC)Bay; Slc7a7Bay/Bay) using CRISPR/Cas9 technology by introducing a deletion of exons 3 and 4. Perinatal lethality was observed in Slc7a7Lbu/Lbu and Slc7a7Bay/Bay mice on the C57BL/6 and C57BL/6NJ inbred genetic backgrounds, respectively. We noted improved survival of Slc7a7Lbu/Lbu mice on the 129 Sv/Ev × C57BL/6 F2 background, but postnatal growth failure occurred. Consistent with human LPI, these Slc7a7Lbu/Lbu mice exhibited reduced plasma and increased urinary concentrations of the cationic amino acids. Histopathological assessment revealed loss of brush border and lipid vacuolation in the renal cortex of Slc7a7Lbu/Lbu mice, which combined with aminoaciduria suggests proximal tubular dysfunction. Micro-computed tomography of L4 vertebrae and skeletal radiographs showed delayed skeletal development and suggested decreased mineralization in Slc7a7Lbu/Lbu mice, respectively. In addition to delayed skeletal development and delayed development in the kidneys, the lungs and liver were observed based on histopathological assessment. Overall, our Slc7a7Lbu/Lbu mouse model on the F2 mixed background recapitulates multiple human LPI phenotypes and may be useful for future studies of LPI pathology.

y+LAT1 and y+LAT2 contribution to arginine uptake in different human cell models: Implications in the pathophysiology of Lysinuric Protein Intolerance

y+LAT1 (encoded by SLC7A7), together with y+LAT2 (encoded by SLC7A6), is the alternative light subunits composing the heterodimeric transport system y+L for cationic and neutral amino acids. SLC7A7 mutations cause lysinuric protein intolerance (LPI), an inherited multisystem disease characterized by low plasma levels of arginine and lysine, protein-rich food intolerance, failure to thrive, hepatosplenomegaly, osteoporosis, lung involvement, kidney failure, haematologic and immunological disorders. The reason for the heterogeneity of LPI symptoms is thus far only poorly understood. Here, we aimed to quantitatively compare the expression of SLC7A7 and SLC7A6 among different human cell types and evaluate y+LAT1 and y+LAT2 contribution to arginine transport. We demonstrate that system y+L-mediated arginine transport is mainly accounted for by y+LAT1 in monocyte-derived macrophages (MDM) and y+LAT2 in fibroblasts. The kinetic analysis of arginine transport indicates that y+LAT1 and y+LAT2 share a comparable affinity for the substrate. Differences have been highlighted in the expression of SLC7A6 and SLC7A7 mRNA among different cell models: while SLC7A6 is almost equally expressed, SLC7A7 is particularly abundant in MDM, intestinal Caco-2 cells and human renal proximal tubular epithelial cells (HRPTEpC). The characterization of arginine uptake demonstrates that system y+L is operative in renal cells and in Caco-2 where, at the basolateral side, it mediates arginine efflux in exchange with leucine plus sodium. These findings explain the defective absorption/reabsorption of arginine in LPI. Moreover, y+LAT1 is the prevailing transporter in MDM sustaining a pivotal role in the pathogenesis of immunological complications associated with the disease.

Proximal renal tubular acidosis with and without Fanconi syndrome

Proximal renal tubular acidosis (RTA) is caused by a defect in bicarbonate (HCO₃⁻) reabsorption in the kidney proximal convoluted tubule. It usually manifests as normal anion-gap metabolic acidosis due to HCO₃⁻ wastage. In a normal kidney, the thick ascending limb of Henle’s loop and more distal nephron segments reclaim all of the HCO₃⁻ not absorbed by the proximal tubule. Bicarbonate wastage seen in type II RTA indicates that the proximal tubular defect is severe enough to overwhelm the capacity for HCO₃⁻ reabsorption beyond the proximal tubule. Proximal RTA can occur as an isolated syndrome or with other impairments in proximal tubular functions under the spectrum of Fanconi syndrome. Fanconi syndrome, which is characterized by a defect in proximal tubular reabsorption of glucose, amino acids, uric acid, phosphate, and HCO₃⁻, can occur due to inherited or acquired causes. Primary inherited Fanconi syndrome is caused by a mutation in the sodium-phosphate cotransporter (NaP_i-II) in the proximal tubule. Recent studies have identified new causes of Fanconi syndrome due to mutations in the EHHADH and the HNF4A genes. Fanconi syndrome can also be one of many manifestations of various inherited systemic diseases, such as cystinosis. Many of the acquired causes of Fanconi syndrome with or without proximal RTA are drug-induced, with the list of causative agents increasing as newer drugs are introduced for clinical use, mainly in the oncology field.

Overview of symptoms and treatment for lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is caused by dysfunction of the dibasic amino acid membrane transport owing to the functional abnormality of y⁺L amino acid transporter-1 (y⁺ LAT-1). LPI is associated with autosomal recessive inheritance and pathological variants in the responsible gene SLC7A7 are also observed. The pathophysiology of this disease had earlier been understood as a transport defect in polarized cells (e.g., intestinal or renal tubular epithelium); however, in recent years, transport defects in non-polarized cells such as lymphocytes and macrophages have also been recognized as important. Although the former can cause death, malnutrition, and urea cycle dysfunction (hyperammonemia), the latter can induce renal, pulmonary, and immune disorders. Furthermore, although therapeutic interventions can prevent hyperammonemic episodes to some extent, progression of pulmonary and renal complications cannot be prevented, thereby influencing prognosis. Such pathological conditions are currently being explored and further investigation would prove beneficial. In this study, we have summarized the basic pathology as revealed in recent years, along with the clinical aspects and genetic features.

Abnormal coagulation and enhanced fibrinolysis due to lysinuric protein intolerance associates with bleeds and renal impairment

INTRODUCTION

Lysinuric protein intolerance (LPI), a rare autosomal recessive transport disorder of cationic amino acids lysine, arginine and ornithine, affects intestines, lungs, liver and kidneys. LPI patients may display potentially life-threatening bleeding events, which are poorly understood.

AIMS

To characterize alterations in haemostatic and fibrinolytic variables associated with LPI.

METHODS

We enrolled 15 adult patients (8 female) and assessed the clinical ISTH/SSC-BAT bleeding score (BS). A variety of metabolic and coagulation assays, including fibrin generation test derivatives, clotting time (CT) and clot lysis time (CLT), thromboelastometry (ROTEM), and PFA-100 and Calibrated Automated Thrombogram (CAT), were used.

RESULTS

All patients had mild-to-moderate renal insufficiency, and moderate bleeding tendency (BS 4) without spontaneous bleeds. Mild anaemia and thrombocytopenia occurred. Traditional clotting times were normal, but in contrast, CT in fibrin generation test, and especially ROTEM FIBTEM was abnormal. The patients showed impaired primary haemostasis in PFA, irrespective of normal von Willebrand factor activity, but together with lowered fibrinogen and FXIII. Thrombin generation (TG) was reduced in vitro, according to CAT-derived endogenous thrombin potential, but in vivo TG was enhanced in the form of circulating prothrombin fragment 1 and 2 values. Very high D-dimer and plasmin-α2-antiplasmin (PAP) complex levels coincided with shortened CLT in vitro.

CONCLUSIONS

Defective primary haemostasis, coagulopathy, fibrin abnormality (FIBTEM, CT and CLT), low TG in vitro and clearly augmented fibrinolysis (PAP and D-dimer) in vivo were all detected in LPI. Altered fibrin generation and hyperfibrinolysis were associated with the metabolic and renal defect, suggesting a pathogenetic link in LPI.

Renal involvement in lysinuric protein intolerance: contribution of pathology to assessment of heterogeneity of renal lesions

Lysinuric protein intolerance (LPI) is a rare autosomal recessive disease caused by mutations in the SLC7A7 gene encoding the light subunit of a cationic amino acid transporter. Symptoms mimic primary urea cycle defects but dysimmune symptoms are also described. Renal involvement in LPI was first described in the 1980s. In 2007, it appeared that it could concern as much as 75% of LPI patients and could lead to end-stage renal disease. The most common feature is proximal tubular dysfunction and nephrocalcinosis but glomerular lesions are also reported. However, very little is known regarding histological lesions associated with LPI. We gathered every kidney biopsy of LPI-proven patients in our highly specialized pediatric and adult institution. Clinical, biological, and histological information was analyzed. Five LPI patients underwent kidney biopsy in our institution between 1986 and 2015. Clinically, 4/5 presented with proximal tubular dysfunction and 3/5 with nephrotic range proteinuria. Histology showed unspecific tubulointerstitial lesions and nephrocalcinosis in 3/5 biopsies and marked peritubular capillaritis in one child. Glomerular lesions were heterogeneous: lupus-like-full house membranoproliferative glomerulonephritis (MPGN) in one child evolved towards monotypic IgG1κ MPGN sensitive to immunomodulators. One patient presented with glomerular non-AA non-AL amyloidosis. Renal biopsy is particularly relevant in LPI presenting with glomerular symptoms for which variable histological lesions can be responsible, implying specific treatment and follow-up.

Update on Lysinuric Protein Intolerance, a Multi-faceted Disease Retrospective cohort analysis from birth to adulthood

Background

Lysinuric protein intolerance (LPI) is a rare metabolic disease resulting from recessive-inherited mutations in the SLC7A7 gene encoding the cationic amino-acids transporter subunit y⁺LAT1. The disease is characterised by protein-rich food intolerance with secondary urea cycle disorder, but symptoms are heterogeneous ranging from infiltrative lung disease, kidney failure to auto-immune complications. This retrospective study of all cases treated at Necker Hospital (Paris, France) since 1977 describes LPI in both children and adults in order to improve therapeutic management.

Results

Sixteen patients diagnosed with LPI (12 males, 4 females, from 9 families) were followed for a mean of 11.4 years (min-max: 0.4-37.0 years). Presenting signs were failure to thrive (n = 9), gastrointestinal disorders (n = 2), cytopenia (n = 6), hyperammonemia (n = 10) with acute encephalopathy (n = 4) or developmental disability (n = 3), and proteinuria (n = 1). During follow-up, 5 patients presented with acute hyperammonemia, and 8 presented with developmental disability. Kidney disease was observed in all patients: tubulopathy (11/11), proteinuria (4/16) and kidney failure (7/16), which was more common in older patients (mean age of onset 17.7 years, standard deviation 5.33 years), with heterogeneous patterns including a lupus nephritis. We noticed a case of myocardial infarction in a 34-year-old adult. Failure to thrive and signs of haemophagocytic-lymphohistiocytosis were almost constant. Recurrent acute pancreatitis occurred in 2 patients. Ten patients developed an early lung disease. Six died at the mean age of 4 years from pulmonary alveolar proteinosis. This pulmonary involvement was significantly associated with death. Age-adjusted plasma lysine concentrations at diagnosis showed a trend toward increased values in patients with a severe disease course and premature death (Wilcoxon p = 0.08; logrank, p = 0.17). Age at diagnosis was a borderline predictor of overall survival (logrank, p = 0.16).

Conclusions

As expected, early pulmonary involvement with alveolar proteinosis is frequent and severe, being associated with an increased risk of death. Kidney disease frequently occurs in older patients. Cardiovascular and pancreatic involvement has expanded the scope of complications. A borderline association between increased levels of plasma lysine and poorer outome is suggested. Greater efforts at prevention are warranted to optimise the long-term management in these patients.

Inhaled Sargramostim Induces Resolution of Pulmonary Alveolar Proteinosis in Lysinuric Protein Intolerance

Pulmonary alveolar proteinosis (PAP) is a potentially fatal complication of lysinuric protein intolerance (LPI), an inherited disorder of cationic amino acid transport. The patients often present with mild respiratory symptoms, which may rapidly progress to acute respiratory failure responding poorly to conventional treatment with steroids and bronchoalveolar lavations (BALs). The pathogenesis of PAP in LPI is still largely unclear. In previous studies, we have shown disturbances in the function and activity of alveolar macrophages of these patients, suggesting that increasing the activity and the number of macrophages by recombinant human GM-CSF (rhuGM-CSF) might be beneficial in this patient group.Two LPI patients with complicated PAP were treated with experimental inhaled rhuGM-CSF (sargramostim) after poor response to maximal conventional therapy. BAL fluid and cell samples from one patient were studied with light microscopy and transmission electron microscopy.Excellent response to therapy was observed in patient 1 with no compliance problems or side effects. Macrophages with myelin figure-like structures were seen in her BAL sample. Slight improvement of the pulmonary function was evident also in patient 2, but the role of sargramostim could not be properly evaluated due to the complicated clinical situation.In conclusion, inhaled rhuGM-CSF might be of benefit in patients with LPI-associated PAP.

Imbalance of plasma amino acids, metabolites and lipids in patients with lysinuric protein intolerance (LPI)

BACKGROUND

Lysinuric protein intolerance (LPI [MIM 222700]) is an aminoaciduria with defective transport of cationic amino acids in epithelial cells in the small intestine and proximal kidney tubules due to mutations in the SLC7A7 gene. LPI is characterized by protein malnutrition, failure to thrive and hyperammonemia. Many patients also suffer from combined hyperlipidemia and chronic kidney disease (CKD) with an unknown etiology.

METHODS

Here, we studied the plasma metabolomes of the Finnish LPI patients (n=26) and healthy control individuals (n=19) using a targeted platform for analysis of amino acids as well as two analytical platforms with comprehensive coverage of molecular lipids and polar metabolites.

RESULTS

Our results demonstrated that LPI patients have a dichotomy of amino acid profiles, with both decreased essential and increased non-essential amino acids. Altered levels of metabolites participating in pathways such as sugar, energy, amino acid and lipid metabolism were observed. Furthermore, of these metabolites, myo-inositol, threonic acid, 2,5-furandicarboxylic acid, galactaric acid, 4-hydroxyphenylacetic acid, indole-3-acetic acid and beta-aminoisobutyric acid associated significantly (P<0.001) with the CKD status. Lipid analysis showed reduced levels of phosphatidylcholines and elevated levels of triacylglycerols, of which long-chain triacylglycerols associated (P<0.01) with CKD.

CONCLUSIONS

This study revealed an amino acid imbalance affecting the basic cellular metabolism, disturbances in plasma lipid composition suggesting hepatic steatosis and fibrosis and novel metabolites correlating with CKD in LPI. In addition, the CKD-associated metabolite profile along with increased nitrite plasma levels suggests that LPI may be characterized by increased oxidative stress and apoptosis, altered microbial metabolism in the intestine and uremic toxicity.

Renal Involvement in a French Paediatric Cohort of Patients with Lysinuric Protein Intolerance

Lysinuric protein intolerance (LPI) is a rare autosomal recessive metabolic disorder, caused by defective transport of cationic amino acids at the basolateral membrane of epithelial cells, typically in intestines and kidneys. The SLC7A7 gene, mutated in LPI patients, encodes the light subunit (y+LAT1) of a member of the heterodimeric amino acid transporter family.The diagnosis of LPI is difficult due to unspecific clinical features: protein intolerance, failure to thrive and vomiting after weaning. Later on, patients may present delayed growth osteoporosis, hepatosplenomegaly, muscle hypotonia and life-threatening complications such as alveolar proteinosis, haemophagocytic lymphohistiocytosis and macrophage activation syndrome. Renal involvement is also a serious complication with tubular and more rarely, glomerular lesions that may lead to end-stage kidney disease (ESKD). We report six cases of LPI followed in three different French paediatric centres who presented LPI-related nephropathy during childhood. Four of them developed chronic kidney disease during follow-up, including one with ESKD. Five developed chronic tubulopathies and one a chronic glomerulonephritis. A histological pattern of membranoproliferative glomerulonephritis was first associated with a polyclonal immunoglobulin deposition, treated by immunosuppressive therapy. He then required a second kidney biopsy after a relapse of the nephrotic syndrome; the immunoglobulin deposition was then monoclonal (IgG1 kappa). This is the first observation of an evolution from a polyclonal to a monotypic immune glomerulonephritis. Immune dysfunction potentially attributable to nitric oxide overproduction secondary to arginine intracellular trapping is a debated complication in LPI. Our results suggest all LPI patients should be monitored for renal disease regularly.

Dysfunction in macrophage toll-like receptor signaling caused by an inborn error of cationic amino acid transport

Amino acids, especially arginine, are vital for the well-being and activity of immune cells, and disruption of amino acid balance may weaken immunity and predispose to infectious and autoimmune diseases. We present here a model of an inborn aminoaciduria, lysinuric protein intolerance (LPI), in which a single mutation in y(+)LAT1 cationic amino acid transporter gene SLC7A7 leads to a multisystem disease characterized by immunological complications, life-threatening pulmonary alveolar proteinosis and nephropathy. Macrophages are suggested to play a central role in LPI in the development of these severe secondary symptoms. We thus studied the effect of the Finnish y(+)LAT1 mutation on monocyte-derived macrophages where toll-like receptors (TLRs) act as the key molecules in innate immune response against external pathogens. The function of LPI patient and control macrophage TLR signaling was examined by stimulating the TLR2/1, TLR4 and TLR9 pathways with their associated pathogen-associated molecular patterns. Downregulation in expression of TLR9, IRF7, IRF3 and IFNB1 and in secretion of IFN-α was detected, suggesting an impaired response to TLR9 stimulation. In addition, secretion of TNF-α, IL-12 and IL-1RA by TLR2/1 stimulation and IL-12 and IL-1RA by TLR4 stimulation was increased in the LPI patients. LPI macrophages secreted significantly less nitric oxide than control macrophages, whereas plasma concentrations of inflammatory chemokines CXCL8, CXCL9 and CXCL10 were elevated in the LPI patients. In conclusion, our results strengthen the relevance of macrophages in the pathogenesis of LPI and, furthermore, suggest that cationic amino acid transport plays an important role in the regulation of innate immune responses.

Urine Beta2-Microglobulin Is an Early Marker of Renal Involvement in LPI

OBJECTIVE

Lysinuric protein intolerance (LPI) is a rare autosomal recessive disorder affecting the transport of cationic amino acids. It has previously been shown that approximately one third of the Finnish LPI patients have impaired renal function. The aim of this study was to analyse in detail urine beta2-microglobulin values, renal dysfunction, oral L-citrulline doses and plasma citrulline concentrations in Finnish LPI patients.

METHODS AND RESULTS

Of the 41 Finnish LPI patients, 56% had proteinuria and 53% hematuria. Mean plasma creatinine concentration was elevated in 48%, serum cystatin C in 62%, and urine beta2-microglobulin in 90% of the patients. Seventeen per cent of the patients developed ESRD, and five of them received a kidney transplant. L-citrulline doses and fasting plasma citrulline concentrations were similar in adult LPI patients with decreased and normal GFR (mean ± SD 79.5 ± 29.2 vs. 82.4 ± 21.9 mg/kg/day, P = 0.619, and 80.3 ± 20.1 vs. 64.8 ± 23.0 μmol/l, P = 0.362, respectively).

CONCLUSIONS

Urine beta2-microglobulin is a sensitive early marker of renal involvement, and it should be monitored regularly in LPI patients. Weight-based oral L-citrulline doses and plasma citrulline concentrations were not associated with renal function. LPI patients with ESRD were successfully treated with dialysis and kidney transplantation.

Potential association between membranous nephropathy and sargramostim therapy for pulmonary alveolar proteinosis

We present the case of a 43-year-old woman with a diagnosis of pulmonary alveolar proteinosis, on chronic treatment with sargramostim, a recombinant granulocyte-macrophage colony-stimulating factor, who presented with the nephrotic syndrome secondary to biopsy-proven membranous nephropathy. We discuss potential underlying mechanisms, including speculated effects of sargramostim on mesangial cells and the kidney resident macrophages, and review the existing literature on the potential association between these two disorders.

Diffuse lung disease of infancy: a pattern-based, algorithmic approach to histological diagnosis

Diffuse lung disease (DLD) of infancy has multiple aetiologies and the spectrum of disease is substantially different from that seen in older children and adults. In many cases, a specific diagnosis renders a dire prognosis for the infant, with profound management implications. Two recently published series of DLD of infancy, collated from the archives of specialist centres, indicate that the majority of their cases were referred, implying that the majority of biopsies taken for DLD of infancy are first received by less experienced pathologists. The current literature describing DLD of infancy takes a predominantly aetiological approach to classification. We present an algorithmic, histological, pattern-based approach to diagnosis of DLD of infancy, which, with the aid of appropriate multidisciplinary input, including clinical and radiological expertise and ancillary diagnostic studies, may lead to an accurate and useful interim report, with timely exclusion of inappropriate diagnoses. Subsequent referral to a specialist centre for confirmatory diagnosis will be dependent on the individual case and the decision of the multidisciplinary team.

Fanconi syndrome with lysinuric protein intolerance

We present the case of a 9-year-old child with lysinuric protein intolerance and Fanconi syndrome. She was referred to our hospital with a persistent metabolic acidosis and polyuria. Renal investigations revealed all laboratory signs of Fanconi syndrome, with glucosuria, generalized aminoaciduria, phosphaturia and severe hypercalciuria. The diagnosis of Fanconi syndrome was confirmed by a renal biopsy that showed extensive lesions of proximal tubular epithelial cells with vacuolation of these cells and a sloughing of the brush border.

An official American Thoracic Society clinical practice guideline: classification, evaluation, and management of childhood interstitial lung disease in infancy

BACKGROUND

There is growing recognition and understanding of the entities that cause interstitial lung disease (ILD) in infants. These entities are distinct from those that cause ILD in older children and adults.

METHODS

A multidisciplinary panel was convened to develop evidence-based guidelines on the classification, diagnosis, and management of ILD in children, focusing on neonates and infants under 2 years of age. Recommendations were formulated using a systematic approach. Outcomes considered important included the accuracy of the diagnostic evaluation, complications of delayed or incorrect diagnosis, psychosocial complications affecting the patient's or family's quality of life, and death.

RESULTS

No controlled clinical trials were identified. Therefore, observational evidence and clinical experience informed judgments. These guidelines: (1) describe the clinical characteristics of neonates and infants (<2 yr of age) with diffuse lung disease (DLD); (2) list the common causes of DLD that should be eliminated during the evaluation of neonates and infants with DLD; (3) recommend methods for further clinical investigation of the remaining infants, who are regarded as having "childhood ILD syndrome"; (4) describe a new pathologic classification scheme of DLD in infants; (5) outline supportive and continuing care; and (6) suggest areas for future research.

CONCLUSIONS

After common causes of DLD are excluded, neonates and infants with childhood ILD syndrome should be evaluated by a knowledgeable subspecialist. The evaluation may include echocardiography, controlled ventilation high-resolution computed tomography, infant pulmonary function testing, bronchoscopy with bronchoalveolar lavage, genetic testing, and/or lung biopsy. Preventive care, family education, and support are essential.

Lysine triggers apoptosis through a NADPH oxidase-dependent mechanism in human renal tubular cells

Progressive chronic kidney disease (CKD) is common in lysinuric protein intolerance (LPI), a primary inherited aminoaciduria characterized by massive Lysine excretion in urine. However, by which mechanisms Lysine may cause kidney damage to tubule cells is still not understood. This study determined whether Lysine overloading of human proximal tubular cells (HK-2) in culture enhances apoptotic cell loss and its associated mechanisms. Overloading HK-2 with Lysine levels reproducing those observed in urine of patients affected by LPI (10 mM) increased apoptosis (+30%; p < 0.01 vs.C), as well as Bax and Apaf-1 expressions (+30-50% p < 0.05), while downregulated Bcl-2 (-40% p < 0.05). Apoptosis induced by high Lysine was no longer observed after addition of caspase-9 and caspase-3 inhibitors while caspase-8 inhibitor had no protective effect. High Lysine induced elevations in ROS generation and NADPH oxidase subunits mRNAs (p22 (phox) +106 ± 23%, p67 (phox) +108 ± 22% and gp91 (phox) +75 ± 4% p < 0.05-0.01). In addition, the NADPH oxidase inhibitor DPI prevented both ROS production and apoptosis. Treating HK-2 with antioxidants, such as Cysteine and its analog, N-acetyl-L-cysteine (NAC), rescued the HK-2 from apoptosis induced by Lysine. In summary, our data show that high Lysine in vitro increases the permissiveness of proximal tubule kidney cells to apoptosis by triggering a pathway involving NADPH oxidase signaling. This event may represent a key cellular effect in the increasing the susceptibility of human tubular cells to apoptosis when the tubules cope with a high Lysine load. This effect is instrumental to renal damage and disease progression in patients with LPI.

Lysinuric protein intolerance (LPI): a multi organ disease by far more complex than a classic urea cycle disorder

Lysinuric protein intolerance (LPI) is an inherited defect of cationic amino acid (lysine, arginine and ornithine) transport at the basolateral membrane of intestinal and renal tubular cells caused by mutations in SLC7A7 encoding the y(+)LAT1 protein. LPI has long been considered a relatively benign urea cycle disease, when appropriately treated with low-protein diet and l-citrulline supplementation. However, the severe clinical course of this disorder suggests that LPI should be regarded as a severe multisystem disease with uncertain outcome. Specifically, immune dysfunction potentially attributable to nitric oxide (NO) overproduction secondary to arginine intracellular trapping (due to defective efflux from the cell) might be a crucial pathophysiological route explaining many of LPI complications. The latter comprise severe lung disease with pulmonary alveolar proteinosis, renal disease, hemophagocytic lymphohistiocytosis with subsequent activation of macrophages, various auto-immune disorders and an incompletely characterized immune deficiency. These results have several therapeutic implications, among which lowering the l-citrulline dosage may be crucial, as excessive citrulline may worsen intracellular arginine accumulation.

Exploring the transcriptomic variation caused by the Finnish founder mutation of lysinuric protein intolerance (LPI)

Lysinuric protein intolerance (LPI) is an autosomal recessive disorder caused by mutations in cationic amino acid transporter gene SLC7A7. Although all Finnish patients share the same homozygous mutation, their clinical manifestations vary greatly. The symptoms range from failure to thrive, protein aversion, anemia and hyperammonaemia, to immunological abnormalities, nephropathy and pulmonary alveolar proteinosis. To unravel the molecular mechanisms behind those symptoms not explained directly by the primary mutation, gene expression profiles of LPI patients were studied using genome-wide microarray technology. As a result, we discovered 926 differentially-expressed genes, including cationic and neutral amino acid transporters. The functional annotation analysis revealed a significant accumulation of such biological processes as inflammatory response, immune system processes and apoptosis. We conclude that changes in the expression of genes other than SLC7A7 may be linked to the various symptoms of LPI, indicating a complex interplay between amino acid transporters and various cellular processes.

Arginine transport in human monocytic leukemia THP-1 cells during macrophage differentiation

L-arginine metabolism in myeloid cells plays a central role in the processes of macrophage activation and in the regulation of immune responses. In this study, we investigated arginine transport activity and the expression of the related transporter genes during the differentiation of monocytes to macrophages. We show here that the induction of THP-1 monocyte differentiation by PMA markedly increases the expression of SLC7A7 mRNA and of y(+)LAT1 protein and consequently, the activity of system y(+)L-mediated arginine transport. Conversely, the activity of system y(+) decreases during macrophage differentiation as a result of a reduction in CAT1 protein expression. The PMA-induced, macrophage-differentiated phenotype and the increased activity of system y(+)L through the induction of SLC7A7 gene are mediated by the specific activation of PKCβ. SLC7A7 gene silencing causes a significant reduction of system y(+)L activity and a subsequent, marked increase of arginine and lysine cell content, thus suggesting that in macrophagic cells, system y(+)L activity is mainly directed outwardly. Differentiating agents other than PMA, i.e., VD3 and ATRA, are equally effective in the stimulation of system y(+)L transport activity through the increased expression of SLC7A7 mRNA and y(+)LAT1 protein. Moreover, we found that also during differentiation of human monocytes from peripheral blood SLC7A7 mRNA and system y(+)L activity are increased. These findings point to SLC7A7 gene as a marker of macrophage differentiation.

Pulmonary alveolar proteinosis: an overview for internists and hospital physicians

Pulmonary alveolar proteinosis (PAP) is a rare diffuse lung disease characterized by abnormal accumulation of surfactant-associated phospholipoproteinaceous material in the pulmonary alveoli. The clinical findings of slow-onset dyspnea or dyspnea on exertion and persistent dry cough are nonspecific; radiographic findings of "bat-wing configuration" and "crazy paving" appearance in high-resolution computed tomography are suggestive, but not diagnostic of PAP. The current gold standard of PAP diagnosis involves histopathological examination of alveolar specimens obtained from bronchoalveolar lavage and transbronchial lung biopsy. The characteristic histopathological features are intraalveolar periodic acid Schiff (PAS)-positive eosinophilic homogeneous material with well-preserved architecture ofalveolar septa. The current standard medical treatment of PAP involves the physical removal of the surfactant-associated phospholipoproteinaceous alveolar deposit by whole lung lavage, which causes clinical and radiological improvement in a majority of patients. Some patients have been successfully treated with recombinant granulocyte-macrophage colony-stimulating factor (GM-CSF).

SP-D counteracts GM-CSF-mediated increase of granuloma formation by alveolar macrophages in lysinuric protein intolerance

BACKGROUND

Pulmonary alveolar proteinosis (PAP) is a syndrome with multiple etiologies and is often deadly in lysinuric protein intolerance (LPI). At present, PAP is treated by whole lung lavage or with granulocyte/monocyte colony stimulating factor (GM-CSF); however, the effectiveness of GM-CSF in treating LPI associated PAP is uncertain. We hypothesized that GM-CSF and surfactant protein D (SP-D) would enhance the clearance of proteins and dying cells that are typically present in the airways of PAP lungs.

METHODS

Cells and cell-free supernatant of therapeutic bronchoalveolar lavage fluid (BALF) of a two-year-old patient with LPI were isolated on multiple occasions. Diagnostic BALF samples from an age-matched patient with bronchitis or adult PAP patients were used as controls. SP-D and total protein content of the supernatants were determined by BCA assays and Western blots, respectively. Cholesterol content was determined by a calorimetic assay or Oil Red O staining of cytospin preparations. The cells and surfactant lipids were also analyzed by transmission electron microscopy. Uptake of Alexa-647 conjugated BSA and DiI-labelled apoptotic Jurkat T-cells by BAL cells were studied separately in the presence or absence of SP-D (1 microg/ml) and/or GM-CSF (10 ng/ml), ex vivo. Specimens were analyzed by light and fluorescence microscopy.

RESULTS

Here we show that large amounts of cholesterol, and large numbers of cholesterol crystals, dying cells, and lipid-laden foamy alveolar macrophages were present in the airways of the LPI patient. Although SP-D is present, its bioavailability is low in the airways. SP-D was partially degraded and entrapped in the unusual surfactant lipid tubules with circular lattice, in vivo. We also show that supplementing SP-D and GM-CSF increases the uptake of protein and dying cells by healthy LPI alveolar macrophages, ex vivo. Serendipitously, we found that these cells spontaneously generated granulomas, ex vivo, and GM-CSF treatment drastically increased the number of granulomas whereas SP-D treatment counteracted the adverse effect of GM-CSF.

CONCLUSIONS

We propose that increased GM-CSF and decreased bioavailability of SP-D may promote granuloma formation in LPI, and GM-CSF may not be suitable for treating PAP in LPI. To improve the lung condition of LPI patients with PAP, it would be useful to explore alternative therapies for increasing dead cell clearance while decreasing cholesterol content in the airways.

Nutrient intake in lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is a rare autosomal recessive disorder characterized by defective transport of cationic amino acids. Poor intestinal absorption and increased renal loss of arginine, ornithine and lysine lead to low plasma concentrations of these amino acids and, subsequently, to impaired urea cycle function. The patients therefore have decreased nitrogen tolerance, which may lead to hyperammonaemia after ingestion of normal amounts of dietary protein. As a protective mechanism, most patients develop strong aversion to protein-rich foods early in life. Oral supplementation with citrulline, which is absorbed normally and metabolized to arginine and ornithine, improves protein tolerance to some extent, as do sodium benzoate and sodium phenylbutyrate also used by some patients. Despite effective prevention of hyperammonaemia, the patients still consume a very restricted diet, which may be deficient in energy, essential amino acids and some vitamins and minerals. To investigate the potential nutritional problems of patients with lysinuric protein intolerance, 77 three- to four-day food records of 28 Finnish LPI patients aged 1.5-61 years were analysed. The data suggest that the patients are clearly at risk for many nutritional deficiencies, which may contribute to their symptoms. Their diet is highly deficient in calcium, vitamin D, iron and zinc. Individualized nutritional supplementation accompanied by regular monitoring of dietary intake is therefore an essential part of the treatment of LPI.

Nephropathy advancing to end-stage renal disease: a novel complication of lysinuric protein intolerance

OBJECTIVE

To analyze systemically the prevalence of renal involvement in a cohort of Finnish patients with lysinuric protein intolerance (LPI) and to describe the course and outcome of end-stage renal disease in 4 patients.

STUDY DESIGN

The clinical information in a cohort of 39 Finnish patients with LPI was analyzed retrospectively.

RESULTS

Proteinuria was observed in 74% of the patients and hematuria was observed in 38% of the patients during follow-up. Elevated blood pressure was diagnosed in 36% of the patients. Mean serum creatinine concentration increased in 38% of the patients, and cystatin C concentration increased in 59% of the patients. Four patients required dialysis, and severe anemia with poor response to erythropoietin and iron supplementation also developed in these patients.

CONCLUSIONS

Our findings suggest that renal function of patients with LPI needs to be carefully monitored, and hypertension and hyperlipidemia should be treated effectively. Special attention also should be paid to the prevention of osteoporosis and carnitine deficiency in the patients with end-stage renal disease associated with LPI. The primary disease does not prohibit treatment by dialysis and renal transplantation.

Successful whole lung lavage in pulmonary alveolar proteinosis secondary to lysinuric protein intolerance: a case report

BACKGROUND

Pulmonary alveolar proteinosis (PAP) is a rare disease characterised by accumulation of lipoproteinaceous material within alveoli, occurring in three clinically distinct forms: congenital, acquired and secondary. Among the latter, lysinuric protein intolerance (LPI) is a rare genetic disorder caused by defective transport of cationic amino acids. Whole Lung Lavage (WLL) is currently the gold standard therapy for severe cases of PAP.

CASE PRESENTATION

We describe the case of an Italian boy affected by LPI who, by the age of 10, developed digital clubbing and, by the age of 16, a mild restrictive functional impairment associated with a high-resolution computed tomography (HRCT) pattern consistent with pulmonary alveolar proteinosis. After careful assessment, he underwent WLL.

CONCLUSION

Two years after WLL, the patient has no clinical, radiological or functional evidence of pulmonary disease recurrence, thus suggesting that WLL may be helpful in the treatment of PAP secondary to LPI.

Long-term oral lysine supplementation in lysinuric protein intolerance

In lysinuric protein intolerance (LPI), defective transport of cationic amino acids at the basolateral membrane of the polar epithelial cells in the intestine and renal tubules leads to decreased intestinal absorption and excessive renal loss of lysine, arginine, and ornithine. Citrulline supplementation partially restores the function of the urea cycle that is impaired by deficiency of arginine and ornithine, but does not correct the chronic lysine deficiency. Previous attempts to supplement lysine orally have been hindered by profuse diarrhea, probably caused by excess lysine remaining unabsorbed in the gut. However, individually adjusted minute doses of L-lysine hydrochloride at mealtimes are tolerated well, but the long-term benefits of this therapy remain unknown. The aim of the study was to investigate the long-term benefits and possible adverse effects of oral lysine supplementation in patients with LPI. Supplementation of meals with low doses of oral lysine improved fasting plasma lysine concentrations in 27 Finnish patients with LPI without causing hyperammonemia or other recognizable side effects during 12 months of follow-up. In conclusion, low-dose oral lysine supplementation is potentially beneficial to patients with LPI and can be started safely at an early age.

Granulocyte-macrophage colony-stimulating factor increases L-arginine transport through the induction of CAT2 in bone marrow-derived macrophages

L-arginine transport is crucial for macrophage activation because it supplies substrate for the key enzymes nitric oxide synthase 2 and arginase I. These enzymes participate in classic and alternative activation of macrophages, respectively. Classic activation of macrophages is induced by type I cytokines, and alternative activation is induced by type II cytokines. The granulocyte macrophage colony-stimulating factor (GM-CSF), in addition to inducing proliferation and differentiation of macrophages, activates arginase I, but its action on L-arginine transport is unknown. We studied the L-arginine transporters that are active in mouse primary bone marrow-derived macrophages (BMM) and examined the effect of GM-CSF treatment on transport activities. Under basal conditions, L-arginine entered mainly through system y(+)L (>75%). The remaining transport was explained by system y(+) (<10%) and a diffusion component (10-15%). In response to GM-CSF treatment, transport activity increased mostly through system y(+) (>10-fold), accounting for about 40% of the total L-arginine transport. The increase in y(+) activity correlated with a rise in cationic amino acid transporter (CAT)-2 mRNA and protein. Furthermore, GM-CSF induced an increase in arginase activity and in the conversion of L-arginine to ornithine, citrulline, glutamate, proline, and polyamines. BMM obtained from CAT2-knockout mice responded to GM-CSF by increasing arginase activity and the expression of CAT1 mRNA, which also encodes system y(+) activity. Nonetheless, the increase in CAT1 activity only partially compensated the lack of CAT2 and L-arginine metabolism was hardly stimulated. We conclude that BMM present mainly y(+)L activity and that, in response to GM-CSF, l-arginine transport augments through CAT2, thereby increasing the availability of this amino acid to the cell.

B and T cell immunity in patients with lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is characterized by defective cellular transport of the dibasic amino acids, secondary dysfunction of the urea cycle, aversion to dietary protein, failure to thrive, hepatosplenomegaly and osteoporosis. Because several patients have suffered from recurrent respiratory infections and/or severe generalized varicella, and a few have developed systemic lupus, vasculitis or other autoimmune diseases, we have now evaluated the function of patients' immune systems. Serum concentrations of one to three IgG subclasses were decreased in 10 of the 12 patients studied. Antibody titres against diphtheria, tetanus and Haemophilus influenzae (Hib) were below the detection limit of the assay in four, three and eight of the 11 patients examined, respectively. (Re)vaccination of these 11 patients led to satisfactory responses against tetanus, but two patients still failed to develop measurable antibodies against diphtheria, two against Hib and six against one or more of the three serotypes of 23-valent pneumococcus vaccine. The proportions of T cells of all lymphocytes and the proliferative responses of the peripheral blood mononuclear cells were normal. In conclusion, humoral immune responses in some patients with LPI are defective and these patients may benefit from intravenous immunoglobulin therapy.

The value of classifying interstitial pneumonitis in childhood according to defined histological patterns

AIMS

Interstitial pneumonitis in children is very rare and most cases have been classified according to their counterparts in adults, although the term 'chronic pneumonitis of infancy' has recently been proposed for a particular pattern of interstitial lung disease in infants. We reviewed our paediatric cases of interstitial pneumonitis, first, to look at the spectrum of histological patterns found in this age group and, second, to determine whether the classification of such cases in childhood is both appropriate and worthwhile.

METHODS AND RESULTS

Twenty-five of 38 open lung biopsies showed an overlapping spectrum of interstitial pneumonitis, including three cases that fulfilled the histological criteria for chronic pneumonitis of infancy. There were 11 cases of reactive pulmonary lymphoid hyperplasia (either lymphoid interstitial pneumonitis or follicular bronchiolitis), five of which were associated with abnormalities of the immune system. Four cases were classified as desquamative interstitial pneumonitis and the remaining seven cases were classified as nonspecific interstitial pneumonitis. There were no cases with the histological features of usual interstitial pneumonitis. Most patients responded to steroids but tended to have a residual deficit in lung function. Mortality appeared to be associated with presentation at a young age.

CONCLUSION

Classification of interstitial pneumonitis according to their adult counterparts is appropriate for this younger age group and can provide valuable information for the clinician. The term 'chronic pneumonitis of infancy' refers to a specific histological pattern, but whether it represents a separate disease or a reflection of pulmonary immaturity remains to be proven.

Lysinuric protein intolerance (LPI) gene maps to the long arm of chromosome 14

Lysinuric protein intolerance (LPI) is an autosomal recessive disease characterized by defective transport of cationic amino acids and by hyperammonemia. Linkage analysis in 20 Finnish LPI families assigned the LPI gene locus to the proximal long arm of chromosome 14. Recombinations placed the locus between framework markers D14S72 and MYH7, a 10-cM interval in which the markers D14S742, D14S50, D14S283, and TCRA showed no recombinations with the phenotype. The phenotype was in highly significant linkage disequilibrium with markers D14S50, D14S283, and TCRA. The strongest allelic association obtained with marker TCRA, resulting in a P(excess) value of .98, suggests that the LPI gene locus lies in close proximity to this marker, probably within a distance of < 100 kb.

Necropsy findings in lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is a rare autosomal recessive inborn error of metabolism, characterised by defective transport of the cationic amino acids lysine, arginine and ornithine. To date there are few reported necropsy cases. This report describes the necropsy findings in a 21 year old female patient originally diagnosed as having LPI in 1973. Liver function tests deteriorated and immediately before death jaundice, hyperammonaemia, coma, metabolic acidosis, and a severe bleeding diathesis developed. At necropsy, there was micronodular cirrhosis of the liver with extensive fatty change in hepatocytes. The lungs showed pulmonary alveolar proteinosis. Immunofluorescence and electron microscopy revealed the presence of a glomerulonephritis with predominant IgA deposition. These necropsy findings reflect the spectrum of lesions reported in LPI, providing further evidence of an association between this condition and pulmonary alveolar proteinosis, cirrhosis and glomerulonephritis.

Lysinuric protein intolerance characterized by bone marrow abnormalities and severe clinical course

STUDY OBJECTIVE

To evaluate phenotypic variability of lysinuric protein intolerance in a cohort of nine Italian patients.

DESIGN

Retrospective analysis of patient records.

SUBJECTS

Nine Italian patients (seven independent families), all originating from southern Italy, observed during the last 14 years.

RESULTS

Some of the patients had unique clinical features, including bone marrow abnormalities featuring erythroblastophagocytosis (five patients) and clinical course and the outcome of the disease, have also been observed: respiratory involvement was present in five cases, with a lethal picture of "alveolar proteinosis" in one. Severe kidney involvement, with both glomerular and tubular damage and rapidly progressing to chronic renal failure, has been observed in one case.

CONCLUSION

Lysinuric protein intolerance may cause severe multisystem involvement, which requires early and careful monitoring. Some peculiar clinical findings observed in Italian patients point to a genetic heterogeneity of lysinuric protein intolerance.