Is hyperammonemia helpful in detecting syndromic tubulopathies with early extrarenal manifestations? A case report of Lowe’s syndrome

Background Generally, it is not well known that Lowe’s syndrome may coexist with hyperammonemia and hipocar-nitynemia. The importance of hyperammonemia in the diagnosis of kidney diseases is not completely understood. Case presentation We present the history of a 13-year-old boy, admitted to the hospital due to proteinuria. In the past, the boy was diagnosed with binocular cataracts in infancy. Then he went through neurological diagnostic tests which diagnosed muscular hypotonia and psychomotor retardation but no inherited errors of metabolism were found. Proteinuria has been observed since the age of 2. Ultrasound imaging at the age of 5 showed the presence of a shading deposit in the kidney. At the age of 13, the boy was referred to the Pediatric Nephrology Ward. The laboratory tests revealed: a reduction of glomerular filtration rate, metabolic acidosis, proteinuria, hypercalciuria, increased activity of AST (SGOT), CK, LDH, hyperammonemia, and decreased concentration of total carnitine in blood serum. Based on the clinical presentation, Lowe’s syndrome was diagnosed. The genetic testing revealed an OCRL gene hemizygous mutation. Conclusion Lowe’s syndrome is an example of a disease in which clinical symptoms—although occurring early and in high intensity—may not raise the suspicion of tubulopathy for a long time if they are not analyzed in a complex manner. There is a necessity to educate healthcare practitioners from other fields about the extrarenal symptoms of genetically determined tubulopathies. l -carnitine deficiency may be a symptom of proximal tubulopathy, including Lowe’s syndrome. l -carnitine deficiency leads to disturbances in the efficiency of the urea cycle, which results in hyperammonemia. Hyperammone-mia is not only a symptom of inborn errors of metabolism and liver failure, but it may also lead to the diagnosis of tubulopathy. Since carnitine supplementation could have the desired beneficial effect on the patient’s general condition, it is postulated to conduct further studies on larger groups of patients with Lowe’s syndrome.

Later on, characteristic dysmorphic features and neurodevelopmental disorders with varying degrees of intellectual disability become prominent in affected children.Symptoms of (partial) Fanconi syndrome, nephrocalcinosis, and progressive chronic kidney disease are also observed.Since the gene responsible for the development of the disease was identified, the diagnosis of Lowe syndrome is based on genetic testing [1][2][3][4][5].
It is not widely known that Lowe's syndrome may coexist with hyperammonemia and hipocarnitynemia.The importance of hyperammonemia in the diagnosis of kidney diseases is not well understood.
Lowe's syndrome is an example of a disease in which clinical symptoms-although occurring early and in high intensity-may not raise the suspicion of tubulopathy for a long time if they are not analyzed in a complex manner.Early diagnosis of this rare disease may offer a chance to improve the prognosis in this group of patients.

Case presentation
A 13-year-old boy was referred to the Nephrology Department to undergo a diagnostic examination for proteinuria and erythrocyturia.In his medical history, he was born at term, with a birth weight of 3410 g and an Apgar score of 9. From early years, the boy showed visual fixation disorders, hence he was under ophthalmological care and he was diagnosed with binocular cataracts in infancy.He underwent surgical treatment and galactosemia was also ruled out then.Muscular hypotonia and psychomotor retardation were also observed in infancy and later on (sitting at around the age of 18 months, walking at around the age of 5 years, using single words at 4 years).In addition, episodes of aggression were periodically observed.Due to those symptoms, the boy underwent a neurological examination at the age of 4. MRI revealed no structural abnormalities within the brain structures.The electroencephalography (EEG) recording was unremarkable.Electromyography (EMG) revealed myopathic changes in the right deltoid muscle.The karyotype was normal (46,XY).No inherited errors of metabolism were found in tandem MS test, urine organic acid profile by GC/MS method, or transferrin isoform analysis.
In addition, since the age of 2, urine tests have shown mild proteinuria as well as intermittent erythrocyturia.Periodically, a white sediment has been observed in urine samples.Ultrasound imaging of the urinary tract performed at the age of 5 years showed the presence of a shading deposit in the right kidney, but no episodes of renal colic or urinary tract infections were noted.During this period the boy was not referred to a pediatric nephrology specialist.It is also worth noting that the boy stopped gaining weight at the age of 4. The parents, on their own initiative, undertook unconventional methods of "treating" the above-mentioned disorders; these included: a gluten-free diet (although celiac disease was not diagnosed) and herbal therapy to "detoxify" the child from heavy metals.
Finally, at the age of 13, the boy was referred to the Pediatric Nephrology Ward to undergo diagnostic tests for persistent proteinuria.On admission his general condition was good; he was identified with a mild intellectual disability, though.During physical examination the following significant abnormalities were noted: short stature (body height < 3 pcn), aphakic eyes with spectacle correction, a horizontal (thick-wave) nystagmus, microcephaly, facial malocclusion, severe tooth decay, pigeon chest, systolic murmur over the heart 2/6 on the Levin scale, reduced postural muscle tone, valgus knees, flatvalgus feet, abdominal wall muscle weakness, pelvic tilt.No hypertension was observed.A wide range of laboratory tests were performed and the selected results are presented in Table 1.
The laboratory tests revealed: a slightly reduced glomerular filtration rate (eGFR according to Schwartz formula 85 ml/min/1.73m 2 ), a mild increase in blood urea concentration, hyperchloremic metabolic acidosis with normal anion gap, persistent proteinuria with normal concentration of albumin and total protein in the blood serum.25-hydroxyvitamin D concentration was in the suboptimal range and serum PTH, calcium, and phosphorus concentration were in the normal range.Particular attention was drawn to the coexistence of l-carnitine deficiency and hyperammonemia.No specific autoantibody was detected and Complement C3 was within the normal range.Abdominal ultrasound showed the renal cortical layer poorly visible due to impaired corticomedullary differentiation and granular structure of the kidney parenchyma.Due to neurodevelopmental disorders, a neurologist was consulted, whose attention was drawn in particular to increased enzyme activities characteristic of myopathies and hyperammonemia.For this reason, he recommended repeating the entire diagnostic panel for inborn errors of metabolism, but no abnormalities were found.
Based on clinical presentations and the exclusion of inherited errors of metabolism, genetic consultation was necessary.The boy was referred to the Genetic Outpatient Clinic for genetic testing for the OCRL gene mutation.The genetic testing revealed OCRL gene hemizygous mutation (c.1489_1490delTG ex 15/24 p Try497fs), which confirmed the diagnosis.
Due to l-carnitine deficiency, its supplementation was started at a dose of 500 mg twice daily.It was well tolerated and no clinical side effects were observed.The mother noticed an increase in muscle strength and a reduction in aggressive behavior.At the laboratory check-up, a normalization of ammonemia was achieved.

Discussion
The multidirectional symptoms of Lowe's syndrome may occur with varying degrees of intensity and often do not raise any direct suspicion of tubulopathy.Therefore, despite progress in diagnostics, delayed diagnoses may still happen, as in the presented case.Additionally, data found in the literature do not highlight any rare symptoms of the syndrome, such as hyperammonaemia, observed in our patient.What is the explanation of the mechanism of hyperammonaemia in Lowe's syndrome?When Lowe's syndrome was first described in 1952, it was defined as 'organic aciduria, decreased renal ammonia production, hydrophthalmos, and mental retardation' .Bockenhauer D. et al. confirmed the selective dysfunction of proximal tubules in patients with Lowe's syndrome in their study [6,7].
Ammonia is the primary toxic metabolite, synthesized as a byproduct of protein catabolism in the intestines, kidneys, and muscles.Ammonia is primarily eliminated in the urea cycle, which has a prerequisite step: the formation of carbamoyl phosphate is catalyzed by carbamoyl phosphate synthetase 1 (CPS1).This step controls the rate of urea production, making the regulation of CPS1 a critical factor in ammonia elimination.This enzyme is regulated by N-acetylglutamine (NAG), which is synthesized with the participation of acetyl-CoA and glutamine.Furthermore, ammonia is generated through metabolic transformations of glutamine in the cells of virtually the entire renal tubular system.Approximately 60-70% of the total renal ammonia production occurs in the proximal tubules.The enzyme that regulates glutamine metabolism is phosphate-dependent glutaminase (PDG), which increases its activity in metabolic acidosis.During such conditions, proximal tubular cells can produce up to 80% of the total renal ammonia output.
Under normal conditions, metabolic acidosis increases the excretion of ammonia in the urine and stimulates the metabolism of ammonia in the kidneys.Renal ammonia metabolism is the primary mechanism therefore the kidneys maintain acid-base homeostasis.However, the molecular mechanisms governing ammonia production in the kidneys remain unclear [8].
Why does hyperammonemia occur in patients with Lowe's syndrome?Dysfunction of the proximal tubule is characterized by different combinations of excessive urinary excretion of glucose, phosphate, bicarbonate, amino acids, and other solutes excreted (or not properly absorbed) by this segment of the nephron, including l-carnitine.Under normal metabolic conditions, acetylcarnitine is the primary representative of the acyl group and participates in both anabolic and catabolic pathways of cellular metabolism.Maintaining the mitochondrial acyl-CoA/CoA ratio was indicated as a crucial role for carnitine because many enzymes involved in the citric acid cycle, gluconeogenesis, the urea cycle, and fatty acid oxidation are regulated by the ratio of the above-mentioned particles [9].
In our patient, in addition to hyperammonemia, elevated activity of transaminases, lactate dehydrogenase (LDH), creatine kinase (CK), and decreased carnitine levels were observed.In both primary and secondary carnitine deficiencies, hyperammonemia was observed, which may provide a basis for explaining the phenomenon in our patient.In the course of proximal tubule disorders, there is a decreased concentration of free carnitine.Although Lowe's syndrome has been described from the beginning as a condition of reduced renal ammonia production.Patients with Lowe's syndrome exhibit hyperammonemia despite normal liver function.The secondary nature of hyperammonemia can be attributed to carnitine deficiency [10].
Maria Adeva-Andany et al. explain the possible impact of carnitine deficiency on hyperammonemia.The occurrence of hyperammonemia suggests a connection between the metabolism of fatty acids, glucose, and amino acids, but the mechanism underlying the increase in serum ammonia levels is unclear in these conditions.In the mitochondrial matrix of hepatocytes, acetyl-CoA is essential for urea synthesis.The enzyme N-acetylglutamine synthase generates N-acetylglutamine from acetyl-CoA and glutamine.N-acetylglutamine activates carbamoyl-phosphate synthetase-1, which combines bicarbonate and ammonia to produce carbamoyl phosphate, initiating the urea cycle.Impaired mitochondrial fatty acid oxidation due to a deficiency of l-carnitine can reduce acetyl-CoA production thereby inhibiting N-acetylglutamine synthesis.A deficiency of N-acetylglutamine may induce hyperammonemia by inhibiting the urea cycle [11,12].
The first clinical symptom in children with Lowe's syndrome is severe neonatal hypotonia, often observed in the absence of deep tendon reflexes.The hypotonia is primarily of central origin.However, muscle biopsy in some cases revealed selective type-1 fiber atrophy, resembling congenital fiber type disproportion myopathy.Additionally, creatine kinase and/or lactate dehydrogenase activity are typically elevated in Lowe's syndrome.One should also remember skeletal development disorders, polyuria, a tendency to episodes of dehydration, or metabolic crises associated with acidosis.These symptoms, although not observed or perhaps not noticed in our patient, may, if properly interpreted, significantly accelerate the diagnosis of tubulopathy.
However, currently genetic is crucial for confirming the diagnosis of Lowe's syndrome.
In a group of 11 children with either cystinosis or Lowe's syndrome, reduced content of plasma and muscle carnitine was observed due to renal loss.Following treatment with oral l-carnitine at a dose of 100 mg/kg per day divided every 6 h, plasma carnitine concentrations normalized in all subjects within 2 days [13].The vast majority (> 99%) of body carnitine is situated in the intracellular compartment.Circulating carnitine represents only about 0.5% of body carnitine.Normal carnitine levels are maintained by a balance between dietary intake, endogenous synthesis, and renal reabsorption.In the kidneys the majority of carnitine (90-99% of filtered load) is reabsorbed, until saturation is reached.The renal threshold for carnitine excretion is around 50 µmol/L.The kidneys are very efficient in maintaining normal levels of plasma carnitine by modulating urinary carnitine excretion according to the intake from the diet.
Previous studies have demonstrated that treatment with l-carnitine may have beneficial effects in managing conditions such as diabetes, insulin resistance, elevated blood pressure, and dyslipidemia.However, in contrast, other studies have suggested that l-carnitine may increase fasting triglyceride levels in patients with type 2 diabetes [14].
In 2012, the systematic evaluation of the efficacy and safety of l-carnitine in inborn metabolic disorders appeared in the Cochrane database [15].The authors stated that credible publications or randomized trials assessing the effectiveness of administering carnitine in metabolic disorders are missing.This has resulted in a lack of clear guidelines regarding the appropriate dosage, safety, and frequency of l-carnitine administration.The absence of such data does not imply that administering l-carnitine is ineffective or should not be used in metabolic disorders.According to Nasser et al., clinicians should base their decisions regarding l-carnitine therapy on clinical experience and the patient's clinical presentation [15].
Carnitine is well tolerated with few side effects only.Diarrhea and abdominal discomfort could be observed with high doses.Bacterial metabolism in the intestine can produce trimethylamine, which has a fishy odor.This side effect may respond to a reduce carnitine dose; otherwise, a course of oral metronidazole and/or probiotics may be indicated [14,15].In the short-term observation of our patient, starting carnitine supplementation resulted in ammonia level normalization.

Conclusion
Lowe's syndrome is an example of a disease in which clinical symptoms-although occurring early and in high intensity-may not raise the suspicion of tubulopathy for a long time if they are not analyzed in a complex manner.Early diagnosis of this rare disease may offer a chance to improve the prognosis in this group of patients.There is a necessity to educate healthcare practitioners from other fields about the extrarenal symptoms of genetically determined tubulopathies.
Since carnitine supplementation could have the desired beneficial effect on the patient's general condition, it is postulated to conduct further studies on larger groups of patients with Lowe's syndrome.
The screening for proteinuria in children with extrarenal manifestation may help in the early detection of rare tubulopathies.

Table 1
Selected laboratory tests supporting the diagnosis CK creatine kinase, LDH lactate dehydrogenase, AST serum aspartate aminotransferase, 25-OH-D 25-hydroxyvitamin D, eGFR estimated glomerular filtration rate