Characterization and chromosomal mapping of a genomic clone encoding human alanine:glyoxylate aminotransferase

Nedosiran Safety and Efficacy in PH1: Interim Analysis of PHYOX3

Introduction

Primary hyperoxaluria (PH) is a rare genetic disorder of hepatic glyoxylate metabolism. Nedosiran is an RNA interference (RNAi) therapeutic that the US Food and Drug Administration has approved for treatment of PH1. PHYOX3 is a trial evaluating monthly nedosiran in patients with PH.

Methods

In this PHYOX3 interim analysis, participants with PH1 who continued from a single-dose nedosiran trial (PHYOX1), with no previous kidney or liver transplantation, dialysis, or evidence of systemic oxalosis were eligible. The safety and efficacy of once-monthly nedosiran was assessed over 30 months.

Results

Thirteen participants completed PHYOX1 and continued into PHYOX3. At baseline, the mean (SD) and median (range) age was 24.2 (6.6) years and 23.0 (14-39) years, respectively; 53.8% were female and 61.5% were White. Mean estimated glomerular filtration rate (eGFR) remained stable (62-84.2 mL/min per 1.73 m2) to month 30. Mean 24-hour urinary oxalate (Uox) excretion showed a sustained reduction from baseline of ≥60% at every visit (months 2-30). From month 2, at least 10 of 13 (76.9%) participants achieved normal (<0.46 mmol/24h; upper limit of assay-normal [ULN]) or near-normal (≥0.46 to <0.60 mmol/24h; ≥ULN to <1.3 × ULN) 24-hour Uox excretion. All participants experienced ≥1 adverse event (AE), mostly mild or moderate in severity (primarily, injection site events). Three serious, not treatment-related AEs were reported; there were no deaths or study discontinuations due to AEs.

Conclusion

Nedosiran was well-tolerated in patients with PH1, and treatment resulted in a sustained, substantial reduction in Uox excretion for at least 30 months in this long-term study. No safety signals have been identified to date. The PHYOX3 study is ongoing.

Navigating the Evolving Landscape of Primary Hyperoxaluria: Traditional Management Defied by the Rise of Novel Molecular Drugs

Primary hyperoxalurias (PHs) are inherited metabolic disorders marked by enzymatic cascade disruption, leading to excessive oxalate production that is subsequently excreted in the urine. Calcium oxalate deposition in the renal tubules and interstitium triggers renal injury, precipitating systemic oxalate build-up and subsequent secondary organ impairment. Recent explorations of novel therapeutic strategies have challenged and necessitated the reassessment of established management frameworks. The execution of diverse clinical trials across various medication classes has provided new insights and knowledge. With the evolution of PH treatments reaching a new milestone, prompt and accurate diagnosis is increasingly critical. Developing early, effective management and treatment plans is essential to improve the long-term quality of life for PH patients.

In vivo base editing rescues primary hyperoxaluria type 1 in rats

Primary hyperoxaluria type 1 (PH1) is a childhood-onset autosomal recessive disease, characterized by nephrocalcinosis, multiple recurrent urinary calcium oxalate stones, and a high risk of progressive kidney damage. PH1 is caused by inherent genetic defects of the alanine glyoxylate aminotransferase (AGXT) gene. The in vivo repair of disease-causing genes was exceedingly inefficient before the invention of base editors which can efficiently introduce precisely targeted base alterations without double-strand DNA breaks. Adenine base editor (ABE) can precisely convert A·T to G·C with the assistance of specific guide RNA. Here, we demonstrated that systemic delivery of dual adeno-associated virus encoding a split-ABE8e could artificially repair 13% of the pathogenic allele in AgxtQ84X rats, a model of PH1, alleviating the disease phenotype. Specifically, ABE treatment partially restored the expression of alanine-glyoxylate-aminotransferase (AGT), reduced endogenous oxalate synthesis and alleviated calcium oxalate crystal deposition. Western blot and immunohistochemistry confirmed that ABE8e treatment restored AGT protein expression in hepatocytes. Moreover, the precise editing efficiency in the liver remained stable six months after treatment. Thus, our findings provided a prospect of in vivo base editing as a personalized and precise medicine for PH1 by directly correcting the mutant Agxt gene.

Genetic assessment in primary hyperoxaluria: why it matters

Accurate diagnosis of primary hyperoxaluria (PH) has important therapeutic consequences. Since biochemical assessment can be unreliable, genetic testing is a crucial diagnostic tool for patients with PH to define the disease type. Patients with PH type 1 (PH1) have a worse prognosis than those with other PH types, despite the same extent of oxalate excretion. The relation between genotype and clinical phenotype in PH1 is extremely heterogeneous with respect to age of first symptoms and development of kidney failure. Some mutations are significantly linked to pyridoxine-sensitivity in PH1, such as homozygosity for p.G170R and p.F152I combined with a common polymorphism. Although patients with these mutations display on average better outcomes, they may also present with CKD stage 5 in infancy. In vitro studies suggest pyridoxine-sensitivity for some other mutations, but confirmatory clinical data are lacking (p.G47R, p.G161R, p.I56N/major allele) or scarce (p.I244T). These studies also suggest that other vitamin B6 derivatives than pyridoxine may be more effective and should be a focus for clinical testing. PH patients displaying the same mutation, even within one family, may have completely different clinical outcomes. This discordance may be caused by environmental or genetic factors that are unrelated to the effect of the causative mutation(s). No relation between genotype and clinical or biochemical phenotypes have been found so far in PH types 2 and 3. This manuscript reviews the current knowledge on the genetic background of the three types of primary hyperoxaluria and its impact on clinical management, including prenatal diagnosis.

Safety, pharmacodynamics, and exposure-response modeling results from a first-in-human phase 1 study of nedosiran (PHYOX1) in primary hyperoxaluria

Primary hyperoxaluria (PH) is a family of ultra-rare autosomal recessive inherited disorders of hepatic glyoxylate metabolism characterized by oxalate overproduction. Nedosiran is an RNA interference agent that inhibits hepatic lactate dehydrogenase, the enzyme responsible for the common, final step of oxalate production in all three genetic subtypes of PH. Here, we assessed in a two-part, randomized, single-ascending-dose, phase 1 study (PHYOX1) the safety, pharmacokinetics, pharmacodynamics, and exposure-response of subcutaneous nedosiran in 25 healthy participants (Group A) and 18 patients with PH1 or PH2 (Group B). Group A received nedosiran (0.3, 1.5, 3.0, 6.0, then 12.0 mg/kg) or placebo, and Group B received open-label nedosiran (1.5, 3.0, or 6.0 mg/kg). No significant safety concerns were identified. Injection site reactions (four or more hours post dose) occurred in 13.3% of participants in Group A and 27.8% of participants in Group B. Mean maximum reduction in 24-hour urinary oxalate excretion from baseline to day 57 (end of study) across Group B dose cohorts was 55% (range: 22%-100%) after single-dose nedosiran, with 33% participants reaching normal 24-hour urinary oxalate excretion. Based on the available modeling and simulation data, a fixed monthly dose of nedosiran 160 mg (free acid; equivalent to 170 mg sodium salt) in adults was associated with the highest proportion of simulated individuals achieving normal or near-normal 24-hour urinary oxalate excretion and fewest fluctuations in urinary oxalate response. Thus, single-dose nedosiran demonstrated acceptable safety and evidence of a pharmacodynamic effect in both PH1 and PH2 subpopulations consistent with its mechanism of action.

Updated Genetic Testing of Primary Hyperoxaluria Type 1 in a Chinese Population: Results from a Single Center Study and a Systematic Review

Primary hyperoxaluria type 1 (PH1) is a rare but devastating autosomal recessive inherited disease caused by mutations in gene AGXT. Pathogenic mutations of AGXT were mostly reported in Caucasian but infrequently in Asian, especially in Chinese. To update the genotypes of PH1 in the Chinese population, we collected and identified 7 Chinese probands with PH1 from 2013 to 2017 in our center, five of whom had delayed diagnosis and failed in kidney transplantation. Samples of peripheral blood DNA from the 7 patients and their family members were collected and sequencing analysis was performed to test the mutations of gene AGXT. Western blotting and enzyme activity analysis were conducted to evaluate the function of the mutations. Furthermore, a systematic review from 1998 to 2017 was performed to observe the genetic characteristics between Chinese and Caucasian. The results showed that a total of 12 mutations were identified in the 7 pedigrees. To the best of our knowledge, 2 novel variants of AGXT, p.Gly41Trp and p.Leu33Met, were first reported. Bioinformatics and functional analysis showed that only 7 mutations led to a reduced expression of alanine-glyoxylate amino transferase (AGT) at a protein level. The systematic review revealed significant population heterogeneity in PH1. In conclusion, new genetic subtypes and genetic characteristics of PH1 are updated in the Chinese population. Furthermore, a genotype-phenotype correlation is found in PH1.

Alanine-glyoxylate aminotransferase 1 (AGXT1) is a novel marker for hepatocellular carcinomas

Arginase-1 has been demonstrated as a marker for hepatocellular carcinoma (HCC) with higher sensitivity and specificity than HepPar-1 and glypican-3. However, its sensitivity is diminished in moderately and poorly differentiated HCCs. In the current study, we evaluated the utility of AGXT1 as a diagnostic marker. Immunostains for AGXT1 and arginase-1 were performed in tissue microarrays of 139 HCCs and 374 gastrointestinal and nongastrointestinal carcinomas. AGXT1 exhibited granular cytoplasmic immunoreactivity in contrast to the diffuse cytoplasmic staining characteristic of arginase-1 in nonneoplastic and neoplastic hepatocytes. Sensitivities of AGXT1 for all HCCs were 90.0% compared to 87.8% for arginase-1. A small number of tumors expressed only 1 of the 2 markers. Sensitivity increased to 92.1% when the presence of either marker was considered positive. Excepting 5 cases of cholangiocarcinoma, both AGXT1 and arginase-1 were negative in all non-HCC tumors with specificities of 98.7%. Our data support the consideration of AGXT1 as a novel hepatocellular marker with equally high specificity and slightly higher sensitivity as compared to arginase-1. AGXT1 may aid in diagnostic workup especially in conjunction with arginase-1 for HCCs that may otherwise defy conventional immunostaining patterns.

Folding Defects Leading to Primary Hyperoxaluria

Protein misfolding is becoming one of the main mechanisms underlying inherited enzymatic deficits. This review is focused on primary hyperoxalurias, a group of disorders of glyoxylate detoxification associated with massive calcium oxalate deposition mainly in the kidneys. The most common and severe form, primary hyperoxaluria Type I, is due to the deficit of liver peroxisomal alanine/glyoxylate aminotransferase (AGT). Various studies performed in the last decade clearly evidence that many pathogenic missense mutations prevent the AGT correct folding, leading to various downstream effects including aggregation, increased degradation or mistargeting to mitochondria. Primary hyperoxaluria Type II and primary hyperoxaluria Type III are due to the deficit of glyoxylate reductase/hydroxypyruvate reductase (GRHPR) and 4-hydroxy-2-oxoglutarate aldolase (HOGA1), respectively. Although the molecular features of pathogenic variants of GRHPR and HOGA1 have not been investigated in detail, the data available suggest that some of them display folding defects. Thus, primary hyperoxalurias can be ranked among protein misfolding disorders, because in most cases the enzymatic deficit is due to the inability of each enzyme to reach its native and functional conformation. It follows that molecules able to improve the folding yield of the enzymes involved in each disease form could represent new therapeutic strategies.

Clinical spectrum of primary hyperoxaluria type 1: Experience of a tertiary center

BACKGROUND AND AIM

Primary hyperoxalurias are rare inborn errors of metabolism resulting in increased endogenous production of oxalate that leads to excessive urinary oxalate excretion. Diagnosis of primary hyperoxaluria type 1 (PH1) is a challenging issue and depends on diverse diagnostic tools including biochemical analysis of urine, stone analysis, renal biopsy, genetic studies and in some cases liver biopsy for enzyme assay. We characterized the clinical presentation as well as renal and extrarenal phenotypes in PH1 patients.

METHODS

This descriptive cohort study included patients with presumable PH1 presenting with nephrolithiasis and/or nephrocalcinosis (NC). Precise clinical characterization of renal phenotype as well as systemic involvement is reported. AGXT mutational analysis was performed to confirm the diagnosis of PH1.

RESULTS

The study cohort included 26 patients with presumable PH1 with male to female ratio of 1.4:1. The median age at time of diagnosis was 6 years, nevertheless the median age at initial symptoms was 3 years. Thirteen patients (50%) were diagnosed before the age of 5 years. Two patients had no symptoms and were diagnosed while screening siblings of index patients. Seventeen patients (65.4%) had reached end-stage renal disease (ESRD): 6/17 (35.3%) during infancy, 4/17 (23.5%) in early childhood and 7/17 (41.29%) in late childhood. Two patients (7.7%) had clinically manifest extra renal (retina, heart, bone, soft tissue) involvement. Mutational analysis of AGXT gene confirmed the diagnosis of PH1 in 15 out of 19 patients (79%) where analysis had been performed. Fifty percent of patients with maintained renal functions had projected 10 years renal survival.

CONCLUSION

PH1 is a heterogeneous disease with wide spectrum of clinical, imaging and functional presentation. More than two-thirds of patients presented prior to the age of 5 years; half of them with the stormy course of infantile PH1. ESRD was the commonest presenting manifestation in two-thirds of our cohort.

Mutational Analysis of Agxt in Tunisian Population with Primary Hyperoxaluria Type 1

BACKGROUND

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive metabolic disorder caused by inherited mutations in the AGXT gene encoding liver peroxisomal alanine:glyoxylate aminotransferase (AGT). PH1 is a clinically and genetically heterogeneous disorder. The aim of our study was to analyze and characterize the mutational spectrum of PH1 in Tunisian patients.

MATERIALS AND METHODS

Molecular studies of 146 Tunisian patients suspected with PH were performed by PCR/Restriction fragment length polymorphism (RFLP) to detect seven mutations described as the most common. Direct sequencing for the 11 exons was performed in patients in whom any mutation was not identified.

RESULTS

The genetic diagnosis of PH1 was confirmed in 62.3% of patients. The first molecular approach based on PCR/restriction enzyme test was positive in 37.6% of patients, whereas the second molecular approach based on whole gene sequencing was successful in 24% of cases. Twelve pathogenic mutations were detected in our cohort. Two mutations were novel, and five were detected for the first time in Tunisians. The three most frequent mutations were p.Ile244Thr, p.Gly190Arg, and c.33dupC, with a frequency of 43.4%, 21.4%, and 13.1%, respectively.

CONCLUSION

The two novel mutations detected in our study extend the spectrum of known AGXT gene mutations. The screen for the mutations identified in this study can provide a useful, cost-effective, and first-line investigation in Tunisian PH1 patients.

Two novel AGXT mutations identified in primary hyperoxaluria type-1 and distinct morphological and structural difference in kidney stones

Primary hyperoxaluria type 1 (PH1) is a rare genetic disease characterized by excessive oxalate accumulation in plasma and urine, resulting in various phenotypes because of allelic and clinical heterogeneity. This study aimed to detect disease-associated genetic mutations in three PH1 patients in a Chinese family. All AGXT exons and 3 common polymorphisms which might synergistically interact with mutations, including P11L, I340 M and IVSI+74 bp were analyzed by direct sequencing in all family members. It demonstrated that in each of three patients, a previously reported nonsense mutation p.R333^* was in cis with a novel missense mutation p.M49L in the minor allele characterized by the polymorphism of 74-bp duplication in intron 1, while the other novel missense mutation p.N72I was in trans with both p.R333^* and P.M49L in the major allele. Kidney stones from two sibling patients were also observed though stereomicroscopic examination and scanning electron microscopy. Distinct morphological and inner-structure differences in calculi were noticed, suggesting clinical heterozygosity of PH1 to a certain extent. In brief, two novel missense mutations were identified probably in association with PH1, a finding which should provide an accurate tool for prenatal diagnosis, genetic counseling and screening for potential presymptomatic individuals.

Recurrent truncating mutations in alanine-glyoxylate aminotransferase gene in two South Indian families with primary hyperoxaluria type 1 causing later onset end-stage kidney disease

Primary hyperoxaluria type 1 is an autosomal recessive inborn error of metabolism due to liver-specific peroxisomal enzyme alanine-glyoxylate transaminase deficiency. Here, we describe two unrelated patients who were diagnosed to have primary hyperoxaluria. Homozygous c.445_452delGTGCTGCT (p.L151Nfs*14) (Transcript ID: ENST00000307503; human genome assembly GRCh38.p2) (HGMD ID CD073567) mutation was detected in both the patients and the parents were found to be heterozygous carriers. Our patients developed end-stage renal disease at 23 years and 35 years of age. However, in the largest series published from OxalEurope cohort, the median age of end-stage renal disease for null mutations carriers was 9.9 years, which is much earlier than our cases. Our patients had slower progressions as compared to three unrelated patients from North India and Pakistan, who had homozygous c.302T>C (p.L101P) (HGMD ID CM093792) mutation in exon 2. Further, patients need to be studied to find out if c.445_452delGTGCTGCT mutation represents a founder mutation in Southern India.

Primary hyperoxalurias: diagnosis and treatment

Primary hyperoxalurias (PH) comprise a group of three distinct metabolic diseases caused by derangement of glyoxylate metabolism in the liver. Recent years have seen advances in several aspects of PH research. This paper reviews current knowledge of the genetic and biochemical basis of PH, the specific epidemiology and clinical presentation of each type, and therapeutic approaches in different disease stages. Potential future specific therapies are discussed.

Surgical management of stone disease in patients with primary hyperoxaluria

OBJECTIVE

To present our experience with surgical management of nephrolithiasis in patients with primary hyperoxaluria (PH).

METHODS

A retrospective chart review from 1994 to 2012 was performed to identify patients with diagnosis of PH.

RESULTS

A total of 14 patients with PH were identified with a median follow-up of 18.6 years (range, 0.9-51 years). Median ages at initial symptom and subsequent diagnosis were 6.7 years (range, 1.1-35.5 years) and 0.42 years (range, 0-33.25 years), respectively. Patients underwent a total of 54 procedures at our institution, including ureteroscopy (27 [50%]), percutaneous nephrolithotomy (15 [28%]), shock wave lithotripsy (8 [15%]), and combined procedures (4 [7%]). Overall nonintraparenchymal stone-free rate after the first, second, and third procedures were 59%, 76%, and 78%, respectively. On average, 1.6 procedures (range, 1-4) were required to rid patients of symptomatic stones, which subsequently afforded them a mean of 3.62 years (range, 0.25-21.5 years) without the need for additional intervention. There were 6 Clavien grade ≥III complications in 4 patients, including immediate postoperative end-stage renal disease in 3 patients.

CONCLUSION

Despite optimal medical and surgical management, patients experience recurrent acute stone events requiring multiple urologic interventions. Significant complications such as end-stage renal disease can occur secondary to surgical intervention.

Phenotype-Genotype Correlations and Estimated Carrier Frequencies of Primary Hyperoxaluria

Primary hyperoxaluria (PH) is a rare autosomal recessive disease characterized by oxalate accumulation in the kidneys and other organs. Three loci have been identified: AGXT (PH1), GRHPR (PH2), and HOGA1 (PH3). Here, we compared genotype to phenotype in 355 patients in the Rare Kidney Stone Consortium PH registry and calculated prevalence using publicly available whole-exome data. PH1 (68.4% of families) was the most severe PH type, whereas PH3 (11.0% of families) showed the slowest decline in renal function but the earliest symptoms. A group of patients with disease progression similar to that of PH3, but for whom no mutation was detected (11.3% of families), suggested further genetic heterogeneity. We confirmed that the AGXT p.G170R mistargeting allele resulted in a milder PH1 phenotype; however, other potential AGXT mistargeting alleles caused more severe (fully penetrant) disease. We identified the first PH3 patient with ESRD; a homozygote for two linked, novel missense mutations. Population analysis suggested that PH is an order of magnitude more common than determined from clinical cohorts (prevalence, approximately 1:58,000; carrier frequency, approximately 1:70). We estimated PH to be approximately three times less prevalent among African Americans than among European Americans because of a limited number of common European origin alleles. PH3 was predicted to be as prevalent as PH1 and twice as common as PH2, indicating that PH3 (and PH2) cases are underdiagnosed and/or incompletely penetrant. These results highlight a role for molecular analyses in PH diagnostics and prognostics and suggest that wider analysis of the idiopathic stone-forming population may be beneficial.

Vitamin B6 in primary hyperoxaluria I: first prospective trial after 40 years of practice

BACKGROUND AND OBJECTIVES

Primary hyperoxaluria type I (PH I) is caused by deficiency of the liver-specific enzyme alanine-glyoxylate:aminotransferase (AGT). Many mutations are known to perturb AGT protein folding. Vitamin B6 (B6) is the only specific drug available for treatment. Although B6 has been used for >40 years, controlled data on B6 efficacy are lacking. Therefore, this study investigated the absolute and relative change of urinary oxalate (Uox) excretion under increasing dosages of B6, the first prospective trial to do so.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

B6 response was studied in 12 patients (7 male patients) with genetically confirmed PH I (3 Gly170Arg homozygous, 5 compound Gly170Arg and/or Phe152Ile heterozygous, and 4 negative for Gly170Arg and/or Phe152Ile mutations) and noncompromised renal function. Efficacy was defined as a 30% relative reduction in Uox excretion. B6 was administered orally starting at 5 mg/kg body weight per day and given in increments of 5 mg/kg every 6 weeks, up to a final dosage of 20 mg/kg per day at week 24. Uox and serum B6 levels were measured every 6 weeks.

RESULTS

Mean relative Uox reduction was 25.5%. Uox declined from 2.09±0.55 (mean±SD) at baseline to 1.52±0.60 mmol/1.73 m(2) per day (P=0.01) at week 24. Serum B6 levels increased from 22.5±8.7 to 1217±776 ng/ml (P<0.001). Six patients showed a ≥30% relative reduction of Uox at week 24.

CONCLUSION

This first prospective trial confirmed B6 efficacy in 50% of patients (three of three homozygous, one of five heterozygous, and two of four patients negative for the Gly170Arg and/or Phe152Ile mutations). Interestingly, no complete biochemical remission was observed, even in the homozygous Gly170Arg study participants. Future trials are necessary to learn more about genotype-related B6 response and B6 metabolism.

Protein homeostasis defects of alanine-glyoxylate aminotransferase: new therapeutic strategies in primary hyperoxaluria type I

Alanine-glyoxylate aminotransferase catalyzes the transamination between L-alanine and glyoxylate to produce pyruvate and glycine using pyridoxal 5′-phosphate (PLP) as cofactor. Human alanine-glyoxylate aminotransferase is a peroxisomal enzyme expressed in the hepatocytes, the main site of glyoxylate detoxification. Its deficit causes primary hyperoxaluria type I, a rare but severe inborn error of metabolism. Single amino acid changes are the main type of mutation causing this disease, and considerable effort has been dedicated to the understanding of the molecular consequences of such missense mutations. In this review, we summarize the role of protein homeostasis in the basic mechanisms of primary hyperoxaluria. Intrinsic physicochemical properties of polypeptide chains such as thermodynamic stability, folding, unfolding, and misfolding rates as well as the interaction of different folding states with protein homeostasis networks are essential to understand this disease. The view presented has important implications for the development of new therapeutic strategies based on targeting specific elements of alanine-glyoxylate aminotransferase homeostasis.

Primary hyperoxalurias: disorders of glyoxylate detoxification

Glyoxylate detoxification is an important function of human peroxisomes. Glyoxylate is a highly reactive molecule, generated in the intermediary metabolism of glycine, hydroxyproline and glycolate mainly. Glyoxylate accumulation in the cytosol is readily transformed by lactate dehydrogenase into oxalate, a dicarboxylic acid that cannot be metabolized by mammals and forms tissue-damaging calcium oxalate crystals. Alanine-glyoxylate aminotransferase, a peroxisomal enzyme in humans, converts glyoxylate into glycine, playing a central role in glyoxylate detoxification. Cytosolic and mitochondrial glyoxylate reductase also contributes to limit oxalate production from glyoxylate. Mitochondrial hydroxyoxoglutarate aldolase is an important enzyme of hydroxyproline metabolism. Genetic defect of any of these enzymes of glyoxylate metabolism results in primary hyperoxalurias, severe human diseases in which toxic levels of oxalate are produced by the liver, resulting in progressive renal damage. Significant advances in the pathophysiology of primary hyperoxalurias have led to better diagnosis and treatment of these patients, but current treatment relies mainly on organ transplantation. It is reasonable to expect that recent advances in the understanding of the molecular mechanisms of disease will result into better targeted therapeutic options in the future.

Biochemical analyses are instrumental in identifying the impact of mutations on holo and/or apo-forms and on the region(s) of alanine:glyoxylate aminotransferase variants associated with primary hyperoxaluria type I

Primary Hyperoxaluria Type I (PH1) is a disorder of glyoxylate metabolism caused by mutations in the human AGXT gene encoding liver peroxisomal alanine:glyoxylate aminotransferase (AGT), a pyridoxal 5'-phosphate (PLP) dependent enzyme. Previous investigations highlighted that, although PH1 is characterized by a significant variability in terms of enzymatic phenotype, the majority of the pathogenic variants are believed to share both structural and functional defects, as mainly revealed by data on AGT activity and expression level in crude cellular extracts. However, the knowledge of the defects of the AGT variants at a protein level is still poor. We therefore performed a side-by-side comparison between normal AGT and nine purified recombinant pathogenic variants in terms of catalytic activity, coenzyme binding mode and affinity, spectroscopic features, oligomerization, and thermal stability of both the holo- and apo-forms. Notably, we chose four variants in which the mutated residues are located in the large domain of AGT either within the active site and interacting with the coenzyme or in its proximity, and five variants in which the mutated residues are distant from the active site either in the large or in the small domain. Overall, this integrated analysis of enzymatic activity, spectroscopic and stability information is used to (i) reassess previous data obtained with crude cellular extracts, (ii) establish which form(s) (i.e. holoenzyme and/or apoenzyme) and region(s) (i.e. active site microenvironment, large and/or small domain) of the protein are affected by each mutation, and (iii) suggest the possible therapeutic approach for patients bearing the examined mutations.

Nephrocalcinosis and urolithiasis in children

The incidence of adult urolithiasis has increased significantly in industrialized countries over the past decades. Sound incidence rates are not available for children, nor are they known for nephrocalcinosis, which can appear as a single entity or together with urolithiasis. In contrast to the adult kidney stone patient, where environmental factors are the main cause, genetic and/or metabolic disorders are the main reason for childhood nephrocalcinosis and urolithiasis. While hypercalciuria is considered to be the most frequent risk factor, several other metabolic disorders such as hypocitraturia or hyperoxaluria, as well as a variety of renal tubular diseases, e.g., Dent's disease or renal tubular acidosis, have to be ruled out by urine and/or blood analysis. Associated symptoms such as growth retardation, intestinal absorption, or bone demineralization should be evaluated for diagnostic and therapeutic purposes. Preterm infants are a special risk population with a high incidence of nephrocalcinosis arising from immature kidney, medication, and hypocitraturia. In children, concise evaluation will reveal an underlying pathomechanism in >75% of patients. Early treatment reducing urinary saturation of the soluble by increasing fluid intake and by providing crystallization inhibitors, as well as disease-specific medication, are mandatory to prevent recurrent kidney stones and/or progressive nephrocalcinosis, and consequently deterioration of renal function.

Primary hyperoxaluria

Primary hyperoxalurias (PH) are inborn errors in the metabolism of glyoxylate and oxalate. PH type 1, the most common form, is an autosomal recessive disorder caused by a deficiency of the liver-specific enzyme alanine, glyoxylate aminotransferase (AGT) resulting in overproduction and excessive urinary excretion of oxalate. Recurrent urolithiasis and nephrocalcinosis are the hallmarks of the disease. As glomerular filtration rate decreases due to progressive renal damage, oxalate accumulates leading to systemic oxalosis. Diagnosis is often delayed and is based on clinical and sonographic findings, urinary oxalate assessment, DNA analysis, and, if necessary, direct AGT activity measurement in liver biopsy tissue. Early initiation of conservative treatment, including high fluid intake, inhibitors of calcium oxalate crystallization, and pyridoxine in responsive cases, can help to maintain renal function in compliant subjects. In end-stage renal disease patients, the best outcomes have been achieved with combined liver-kidney transplantation which corrects the enzyme defect.

Selected AGXT gene mutations analysis provides a genetic diagnosis in 28% of Tunisian patients with primary hyperoxaluria

Background

Primary hyperoxaluria type I (PH1) is a rare genetic disorder characterized by allelic and clinical heterogeneity. Four mutations (G170R, 33_34insC, I244T and F152I) account for more than 50% of PH1 alleles and form the basis for diagnostic genetic screening for PH1. We aimed to analyze the prevalence of these specific mutations causing PH1, and to provide an accurate tool for diagnosis of presymptomatic patients as well as for prenatal diagnosis in the affected families.

Methods

Polymerase chain reaction/Restriction Fragment Length Polymorphism, were used to detect the four mutations in the AGXT gene in DNA samples from 57 patients belonging to 40 families.

Results

Two mutations causing PH1 were detected in 24 patients (42.1%), with a predominance of the I244T mutation (68% of patients) and 33_34insC (in the remaining 32%). In 92% of cases, mutated alleles were in homozygous state.

The presented clinical features were similar for the two mutations. The age of onset was heterogeneous with a higher frequency of the pediatric age. In 58.3% of cases, the presentation corresponded to advanced renal disease which occurred early (< 5 years) in the two mutations. In adolescents, only the I244T mutation was detected (41.1%). I244T and 33_34insC mutations were observed in adult patients, with 17.6% and 12.5% respectively.

Conclusion

Limited mutation analysis can provide a useful first line investigation for PH1. I244T and 33_34insC presented 28.2% of identified mutations causing disease in our cohort. This identification could provide an accurate tool for prenatal diagnosis in the affected families, for genetic counselling and for detection of presymptomatic individuals.

Primary hyperoxaluria in a compound heterozygote infant

BACKGROUND

Primary hyperoxaluria type 1 is a rare disorder caused by a defect in the hepatic metabolism of glyoxylate. Cases presenting in infancy are very uncommon and often have a severe course leading to early end-stage renal failure.

METHODS

We treated a case of early presentation of primary hyperoxaluria type 1 and reviewed the relevant literature.

RESULTS

A 4-month-old female infant was admitted to our hospital because of acute renal failure and nephrocalcinosis. Mutational analysis of alanine-glyoxylate aminotransferase gene revealed compound heterozygosity in the infant, confirming the development of primary hyperoxaluria type 1. A few weeks later, the condition of the infant worsened during an interdialytic period and died.

CONCLUSIONS

Interest of this case is based on the coexistence of two mutations of alanine-glyoxylate aminotransferase gene recently reported, and it confirms the severe course of the disease when it presents in infancy. It also highlights the importance of the association of nephrocalcinosis and urolithiasis as key diagnostic manifestations of primary hyperoxaluria type 1.

Human liver peroxisomal alanine:glyoxylate aminotransferase: characterization of the two allelic forms and their pathogenic variants

The hepatic peroxisomal alanine:glyoxylate aminotransferase (AGT) is a pyridoxal 5'-phosphate (PLP)-enzyme whose deficiency is responsible for Primary Hyperoxaluria Type 1 (PH1), an autosomal recessive disorder. In the last few years the knowledge of the characteristics of AGT and the transfer of this information into some pathogenic variants have significantly contributed to the improvement of the understanding at the molecular level of the PH1 pathogenesis. In this review, the spectroscopic features, the coenzyme's binding affinity, the steady-state kinetic parameters as well as the sensitivity to thermal and chemical stress of the two allelic forms of AGT, the major (AGT-Ma) and the minor (AGT-Mi) allele, have been described. Moreover, we summarize the characterization obtained by means of biochemical and bioinformatic analyses of the following PH1-causing variants in the recombinant purified forms: G82E associated with the major allele, F152I encoded on the background of the minor allele, and the G41 mutants which co-segregate either with the major allele (G41R-Ma and G41V-Ma) or with the minor allele (G41R-Mi). The data have been correlated with previous clinical and cell biology results, which allow us to (i) highlight the functional differences between AGT-Ma and AGT-Mi, (ii) identify the structural and functional molecular defects of the pathogenic variants, (iii) improve the correlation between the genotype and the enzymatic phenotype, (iv) foresee or understand the molecular basis of the responsiveness to pyridoxine treatment of patients bearing these mutations, and (v) pave the way for new treatment strategies. This article is part of a Special Issue entitled: Pyridoxal Phospate Enzymology.

Primary hyperoxaluria type 1: update and additional mutation analysis of the AGXT gene

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive, inherited disorder of glyoxylate metabolism arising from a deficiency of the alanine:glyoxylate aminotransferase (AGT) enzyme, encoded by the AGXT gene. The disease is manifested by excessive endogenous oxalate production, which leads to impaired renal function and associated morbidity. At least 146 mutations have now been described, 50 of which are newly reported here. The mutations, which occur along the length of the AGXT gene, are predominantly single-nucleotide substitutions (75%), 73 are missense, 19 nonsense, and 18 splice mutations; but 36 major and minor deletions and insertions are also included. There is little association of mutation with ethnicity, the most obvious exception being the p.Ile244Thr mutation, which appears to have North African/Spanish origins. A common, polymorphic variant encoding leucine at codon 11, the so-called minor allele, has significantly lower catalytic activity in vitro, and has a higher frequency in PH1 compared to the rest of the population. This polymorphism influences enzyme targeting in the presence of the most common Gly170Arg mutation and potentiates the effect of several other pathological sequence variants. This review discusses the spectrum of AGXT mutations and polymorphisms, their clinical significance, and their diagnostic relevance.

Effect of conservative treatment on the renal outcome of children with primary hyperoxaluria type 1

Primary hyperoxaluria type 1 results from alanine:glyoxylate aminotransferase deficiency. Due to genotype/phenotype heterogeneity in this autosomal recessive disorder, the renal outcome is difficult to predict in these patients and the long-term impact of conservative management in children is unknown. We report here a multicenter retrospective study on the renal outcome in 27 affected children whose biological diagnosis was based on either decreased enzyme activity or identification of mutations in the patient or his siblings. The median age at first symptoms was 2.4 years while that at initiation of conservative treatment was 4.1 years; 6 children were diagnosed upon family screening. The median follow-up was 8.7 years. At diagnosis, 15 patients had an estimated glomerular filtration rate (eGFR) below 90, and 7 children already had stage 2-3 chronic kidney disease. The median baseline eGFR was 74, which rose to 114 with management in the 22 patients who did not require renal replacement therapy. Overall, 20 patients had a stable eGFR, however, 7 exhibited a decline in eGFR of over 20 during the study period. In a Cox regression model, the only variable significantly associated with deterioration of renal function was therapeutic delay with a relative risk of 1.7 per year. Our study strongly suggests that early and aggressive conservative management may preserve renal function of compliant children with this disorder, thereby avoiding dialysis and postponing transplantation.

Flecked-retina syndromes

A number of retinal disorders may present with fleck-like lesions in the retina. We describe the case of a 13-year-old girl who presented with a complaint of decreased vision and prior diagnosis of "fleck-retina." Further examination revealed that the patient had an autosomal recessive disorder associated with systemic manifestations. In the current article, the authors describe the case report and briefly review the various autosomal-recessive disorders that may present with "retinal flecks."

Partial trypsin digestion as an indicator of mis-folding of mutant alanine:glyoxylate aminotransferase and chaperone effects of specific ligands. Study of a spectrum of missense mutants

Alanine:glyoxylate aminotransferase (AGT) is a liver peroxisomal enzyme whose deficiency results in primary hyperoxaluria type 1 (PH1). More than 75 PH1 mutations are now documented in the AGT gene (AGXT), of which about 50% are missense. We have previously demonstrated that many such mutants expressed by transcription/translation are subject to generalized degradation by the proteasome and a specific limited trimming by an endogenous ATP-independent protease activity. Here, we report the results of partial digestion using trypsin as a mimic for the endogenous non-proteasomal protease and the use of N-terminal protein sequencing to determine the sensitive site. Partial trypsin digestion also provided an indicator of proper folding of the mutant enzyme. For selected mutations the sensitivity to trypsin could be ameliorated by addition of pyridoxal phosphate or aminooxy acetic acid as specific pharmacological chaperones.

Mutation-based diagnostic testing for primary hyperoxaluria type 1: survey of results

OBJECTIVES

To test for specific mutations in the alanine:glyoxylate aminotransferase (AGT) gene, in order to diagnose primary hyperoxaluria type 1 (PH1).

DESIGN AND METHODS

Samples of liver and/or DNA from 81 patients were submitted to our laboratory for diagnostic testing for PH1. Using a panel of selected mutations, DNA was examined in 64 cases, of which 36 had the diagnosis of PH1 confirmed by liver AGT assay. DNA sequencing was employed if mutation testing revealed only one mutation.

RESULTS

Identification of 100% of the mutations in the AGT-confirmed samples led to the development of a focused testing panel currently involving 4 common mutations, 7 mutations recurring at lower frequency and 5 with apparent ethnic associations.

CONCLUSIONS

This mutation panel alone would have identified the two causative mutations in 64% of the PH1 samples.

Nephrolithiasis in identical twins: the impact of nature vs nurture

OBJECTIVE

To assess possible underlying metabolic abnormalities in three sets of monozygotic twins, to evaluate the interplay among the factors of kidney stone formation, a complex multifactorial process influenced by environmental, genetic and anatomical factors.

PATIENTS AND METHODS

Three sets of identical twins with either cystine or calcium oxalate stones were identified. Demographic data, medical histories and the results of 24-h urine testing, with samples collected on self-selected diets, were reviewed and analysed.

RESULTS

The cystinuric twins had very similar cystine excretion rates, while stone activity was significantly more pronounced in one. Metabolic abnormalities were concordant in one set of twins with calcium oxalate stones, both being hypercalciuric and hyperuricosuric. However, metabolic abnormalities were discordant in the other pair, one twin with hypercalciuria and the other with hypocitraturia. Two of the three pairs had low urinary volume.

CONCLUSIONS

These results support previous observations that environmental, genetic and potentially, anatomical factors play roles in kidney-stone formation. Additional controlled studies of monozygotic stone-forming twins might help to define the interplay between environmental and genetic factors, and allow the identification of susceptibility genes involved in stone generation.

Selected exonic sequencing of the AGXT gene provides a genetic diagnosis in 50% of patients with primary hyperoxaluria type 1

BACKGROUND

Definitive diagnosis of primary hyperoxaluria type 1 (PH1) requires analysis of alanine:glyoxylate aminotransferase (AGT) activity in the liver. We have previously shown that targeted screening for the 3 most common mutations in the AGXT gene (c.33_34insC, c.508G>A, and c.731T>C) can provide a molecular diagnosis in 34.5% of PH1 patients, eliminating the need for a liver biopsy. Having reviewed the distribution of all AGXT mutations, we have evaluated a diagnostic strategy that uses selected exon sequencing for the molecular diagnosis of PH1.

METHODS

We sequenced exons 1, 4, and 7 for 300 biopsy-confirmed PH1 patients and expressed the identified missense mutations in vitro.

RESULTS

Our identification of at least 1 mutation in 224 patients (75%) and 2 mutations in 149 patients increased the diagnostic sensitivity to 50%. We detected 29 kinds of sequence changes, 15 of which were novel. Four of these mutations were in exon 1 (c.2_3delinsAT, c.30_32delCC, c.122G>A, c.126delG), 7 were in exon 4 (c.447_454delGCTGCTGT, c.449T>C, c.473C>T, c.481G>A, c.481G>T, c.497T>C, c.424-2A>G), and 4 were in exon 7 (c.725insT, c.737G>A, c.757T>C, c.776 + 1G>A). The missense changes were associated with severely decreased AGT catalytic activity and negative immunoreactivity when expressed in vitro. Missense mutation c.26C>A, previously described as a pathological mutation, had activity similar to that of the wild-type enzyme.

CONCLUSIONS

Selective exon sequencing can allow a definitive diagnosis in 50% of PH1 patients. The test offers a rapid turnaround time (15 days) with minimal risk to the patient. Demonstration of the expression of missense changes is essential to demonstrate pathogenicity.

Comprehensive mutation screening in 55 probands with type 1 primary hyperoxaluria shows feasibility of a gene-based diagnosis

Mutations in AGXT, a locus mapped to 2q37.3, cause deficiency of liver-specific alanine:glyoxylate aminotransferase (AGT), the metabolic error in type 1 primary hyperoxaluria (PH1). Genetic analysis of 55 unrelated probands with PH1 from the Mayo Clinic Hyperoxaluria Center, to date the largest with availability of complete sequencing across the entire AGXT coding region and documented hepatic AGT deficiency, suggests that a molecular diagnosis (identification of two disease alleles) is feasible in 96% of patients. Unique to this PH1 population was the higher frequency of G170R, the most common AGXT mutation, accounting for 37% of alleles, and detection of a new 3' end deletion (Ex 11_3'UTR del). A described frameshift mutation (c.33_34insC) occurred with the next highest frequency (11%), followed by F152I and G156R (frequencies of 6.3 and 4.5%, respectively), both surpassing the frequency (2.7%) of I244T, the previously reported third most common pathogenic change. These sequencing data indicate that AGXT is even more variable than formerly believed, with 28 new variants (21 mutations and seven polymorphisms) detected, with highest frequencies on exons 1, 4, and 7. When limited to these three exons, molecular analysis sensitivity was 77%, compared with 98% for whole-gene sequencing. These are the first data in support of comprehensive AGXT analysis for the diagnosis of PH1, obviating a liver biopsy in most well-characterized patients. Also reported here is previously unavailable evidence for the pathogenic basis of all AGXT missense variants, including evolutionary conservation data in a multisequence alignment and use of a normal control population.

Consequences of missense mutations for dimerization and turnover of alanine:glyoxylate aminotransferase: study of a spectrum of mutations

Alanine:glyoxylate aminotransferase (AGT) is a liver peroxisomal enzyme, deficiency of which results in primary hyperoxaluria type 1 (PH1). More than 65 PH1-related mutations are now documented in the AGT gene (AGXT), of which about 50% are missense. We have generated a spectrum of 15 missense changes including the most common PH1 mutation, G170R, and expressed them on the appropriate background of the major or minor allele, in an Escherichia coli overexpression system and in a rabbit reticulocyte transcription/translation system. We have investigated their effects on enzyme activity, dimerization, aggregation, and turnover. The effect of pyridoxal phosphate (PLP) on dimerization and stability was also investigated. Although all 15 mutant AGTs were expressed as intact proteins in E. coli, only three: G41R and G41V on the major allele, and the common mutation G170R, resulted in significant amounts of enzymatic activity. Dimerization failure was a frequent observation (13/15) except for G41V and D183N. Dimerization was poor with S187F but was substantially improved with PLP. Proteasome-mediated protein degradation was observed for all the mutations except G41R on the major allele, G41V, D183N, G170R, and S218L. Increases in the stability of the mutant enzymes in the presence of PLP were small; however, G41R on the minor allele showed a direct relationship between its half life and the concentration of PLP. The minor allele AGT product and many of the mutants were subject to a limited non-proteasomal proteolytic cleavage when ATP was depleted.

Primary hyperoxaluria type 1: AGT mistargeting highlights the fundamental differences between the peroxisomal and mitochondrial protein import pathways

Primary hyperoxaluria type 1 (PH1) is an atypical peroxisomal disorder, as befits a deficiency of alanine:glyoxylate aminotransferase (AGT), which is itself an atypical peroxisomal enzyme. PH1 is characterized by excessive synthesis and excretion of the metabolic end-product oxalate and the progressive accumulation of insoluble calcium oxalate in the kidney and urinary tract. Disease in many patients is caused by a unique protein trafficking defect in which AGT is mistargeted from peroxisomes to mitochondria, where it is metabolically ineffectual, despite remaining catalytically active. Although the peroxisomal import of human AGT is dependent upon the PTS1 import receptor PEX5p, its PTS1 is exquisitely specific for mammalian AGT, suggesting the presence of additional peroxisomal targeting information elsewhere in the AGT molecule. This and many other functional peculiarities of AGT are probably a consequence of its rather chequered evolutionary history, during which much of its time has been spent being a mitochondrial, rather than a peroxisomal, enzyme. Analysis of the molecular basis of AGT mistargeting in PH1 has thrown into sharp relief some of the fundamental differences between the requirements of the peroxisomal and mitochondrial protein import pathways, particularly the properties of peroxisomal and mitochondrial matrix targeting sequences and the different conformational limitations placed upon importable cargos.

Molecular aetiology of primary hyperoxaluria type 1

Primary hyperoxaluria type 1 (PH1) is a rare autosomal-recessive disorder, caused by a deficiency of the liver-specific intermediary-metabolic enzyme alanine:glyoxylate aminotransferase (AGT). AGT deficiency results in increased synthesis and excretion of the metabolic end-product oxalate and the deposition of insoluble calcium oxalate in the kidney and urinary tract. Numerous mutations and polymorphisms have been identified in the gene (AGXT) that encodes AGT, some of which interact synergistically to cause a variety of complex enzyme phenotypes, including AGT intraperoxisomal aggregation, accelerated degradation, and peroxisome-to-mitochondrion mistargeting. The latter is the single most common cause of PH1 and results from the functional interaction between a common Pro11Leu polymorphism and a disease-specific Gly170Arg mutation. The recent solution of the crystal structure of AGT has enabled the effects of several mutations and polymorphisms to be rationalised in terms of their likely effects on AGT conformation. Increased understanding of the molecular aetiology of PH1 has led to significant improvements in all aspects of the clinical management of the disorder, including diagnosis (by enzyme assay of percutaneous needle liver biopsies), prenatal diagnosis (by DNA analysis of chorionic villus samples) and treatment (by liver transplantation as a form of enzyme replacement therapy).

Molecular etiology of primary hyperoxaluria type 1: new directions for treatment

Primary hyperoxaluria type 1 (PH1) is a rare autosomal-recessive disorder caused by a deficiency of the liver-specific enzyme alanine:glyoxylate aminotransferase (AGT). AGT deficiency results in increased synthesis and excretion of the metabolic end-product oxalate and deposition of insoluble calcium oxalate in the kidney and urinary tract. Classic treatments for PH1 have tended to address the more distal aspects of the disease process (i.e. the symptoms rather than the causes). However, advances in the understanding of the molecular etiology of PH1 over the past decade have shifted attention towards the more proximal aspects of the disease process (i.e. the causes rather than the symptoms). The determination of the crystal structure of AGT has enabled the effects of some of the most important missense mutations in the AGXT gene to be rationalised in terms of AGT folding, dimerization and stability. This has opened up new possibilities for the design pharmacological agents that might counteract the destabilizing effects of these mutations and which might be of use for the treatment of a potentially life-threatening and difficult-to-treat disease.

Preliminary evidence for ethnic differences in primary hyperoxaluria type 1 genotype

BACKGROUND

Primary hyperoxaluria type 1 (PH1) is caused by a deficiency of peroxisomal alanine:glyoxylate aminotransferase (AGT). In about one third of patients, enzymatically active AGT is synthesized but is mistargeted to mitochondria. There are more than 50 mutations identified in the gene for AGT. Four mutations, G170R, 33_34insC, F152I and I244T account for more than 50% of PH1 alleles. The question arose whether there are ethnic differences in PH1 genotype.

METHODS

The published data on mutations in the AGT gene were examined with respect to recurrences and geographic or ethnic association. The mutations that have been found in at least 2 unrelated individuals were considered.

RESULTS

Two common mutations, G170R and 33_34insC showed no obvious ethnic associations and have been found in a variety of populations. A third common PH1 mutation, I244T, has a strong association with people from a Spanish or North African background. A particularly high frequency among Canary Islands PH1 patients suggests a probable founder effect. Between these two extremes are a number of mutations that recur at low frequency within certain ethnic groups.

CONCLUSIONS

Ethnic associations of PH1 genotypes span a spectrum ranging from limited recurrences confined to a population group, to a probable founder effect.

Intra-familial clinical heterogeneity: absence of genotype-phenotype correlation in primary hyperoxaluria type 1 in Israel

BACKGROUND/AIMS

Primary hyperoxaluria type 1 (PH1) is caused by the deficiency of the liver enzyme alanine:glyoxylate aminotransferase which results in increased synthesis and excretion of oxalate. The clinical manifestations of PH1 are heterogeneous with respect to the age of onset and rate of progression. The aim of this study was to investigate possible relationships between a given genotype, the biochemical profile and the clinical phenotype.

METHODS

We conducted a study of 56 patients from 22 families with PH1 from Israel. The clinical and biochemical data were compiled and the genotype was determined for each family.

RESULTS

The prevalent phenotype was of early onset with progression to end-stage renal disease during the first decade of life. Fifteen PH1-causing mutations were detected in 21 families: 10 were first described in this patient population. Marked intra-familial clinical heterogeneity was noted, meaning that there was no correlation between a given genotype and the phenotype.

CONCLUSIONS

The clinical course of patients with PH1 is not dictated primarily by its genotype. Other genetic and/or environmental factors play a role in determining the ultimate phenotype.

A comparative analysis of the evolutionary relationship between diet and enzyme targeting in bats, marsupials and other mammals

The subcellular distribution of the enzyme alanine:glyoxylate aminotransferase (AGT) in the livers of different mammals appears to be related to their natural diets. Thus, AGT tends to be mitochondrial in carnivores, peroxisomal in herbivores, and both mitochondrial and peroxisomal in omnivores. To what extent this relationship is an incidental consequence of phylogenetic structure or an evolutionarily meaningful adaptive response to changes in dietary selection pressure is unknown. In order to distinguish between these two possibilities, we have determined the subcellular distribution of AGT in the livers of 22 new mammalian species, including members of three orders not studied before. In addition, we have analysed the statistical relationship between AGT distribution and diet in all 77 mammalian species, from 12 different orders, for which the distribution is currently known. Our analysis shows that there is a highly significant correlation between AGT distribution and diet, independent of phylogeny. This finding is compatible with the suggestion that the variable intracellular targeting of AGT is an adaptive response to episodic changes in dietary selection pressure. To our knowledge, this is the first example of such a response being manifested at the molecular and cellular levels across the breadth of Mammalia.

Clinical implications of mutation analysis in primary hyperoxaluria type 1

BACKGROUND

Primary hyperoxaluria type 1 (PH1) is an inborn error of glyoxylate metabolism with an extensive clinical and genetic heterogeneity. Although over 50 disease-causing mutations have been identified, the relationship between genotype and clinical outcome remains unclear. The aim of this study was to determine this association in order to find clues for improvement of patient care.

METHODS

AGXT mutation analysis and assessment of biochemical characteristics and clinical outcome were performed on patients from a Dutch PH1 cohort.

RESULTS

Thirty-three of a cohort of 57 PH1 patients, identified in The Netherlands over a period of 30 years, were analyzed. Ten different mutations were found. The most common mutations were the Gly170Arg, Phe152Ile, and the 33insC mutations, with an allele frequency of 43%, 19%, and 15%, respectively. Homozygous Gly170Arg and Phe152Ile mutations were associated with pyridoxine responsiveness and a preserved renal function over time when treatment was timely initiated. All patients homozygous for the 33insC mutation had end-stage renal disease (ESRD) before the first year of age. In two unrelated patients, a new Val336Asp mutation was found coupled with the Gly170Arg mutation on the minor allele. We also found 3 patients homozygous for a novel Gly82Arg mutation with adverse outcome in 2 of them.

CONCLUSION

Early detection of Gly170Arg and Phe152Ile mutations in PH1 has important clinical implications because of their association with pyridoxine responsiveness and clinical outcome. The association of a homozygous 33insC mutation with severe infantile ESRD, resulting in early deaths in 2 out of 3 cases, warrants a choice for prenatal diagnostics in affected families.

Metabolic and molecular basis of peroxisomal disorders: a review

The group of peroxisomal disorders now includes 17 different disorders with Zellweger syndrome as prototype. Thanks to the explosion of new information about the functions and biogenesis of peroxisomes, the metabolic and molecular basis of most of the peroxisomal disorders has been resolved. A review of peroxisomal disorders is provided in this paper.

Evaluation of mutation screening as a first line test for the diagnosis of the primary hyperoxalurias11The nomenclature used in this paper is based on that recommended by Antonarakis SE, and the Nomenclature Working Group (1998): Recommendations for a nomenclature system for human gene mutations. Hum Mutat 11:1–3, 1998, where “c” denotes cDNA sequence and nucleotide numbering uses the “A” of the ATG translation initiation start site as +1

BACKGROUND

A definitive diagnosis of primary hyperoxaluria type 1 (PH1) and primary hyperoxaluria type 2 (PH2) requires the measurement of alanine:glyoxylate aminotransferase (AGT) and glyoxylate reductase (GR) activities, respectively, in a liver biopsy. We have evaluated a molecular genetic approach for the diagnosis of these autosomal-recessive diseases.

METHODS

Polymerase chain reaction (PCR) was used to detect three common mutations in the AGXT gene (c.33_34insC, c.508G>A, and c.731T>C) and one, c.103delG, in the GRHPR gene in DNA samples from 365 unrelated individuals referred for diagnosis of PH1 and/or PH2 by liver enzyme analysis.

RESULTS

One or more of these mutations was found in 183 (68.8%) biopsy proven cases of PH1 and PH2 with a test negative predictive value of 62% and 2%, respectively. 102 (34.1%) patients were homozygous or compound heterozygous, making a molecular diagnosis possible. Age of onset and presenting features were similar in patients homozygous for any of the four mutations. Of the AGXT homozygotes, only the c.508G>A mutant was associated with significant AGT catalytic activity and in two of these activity was in the low normal range, possibly reflecting variation in mitochondrial content of the biopsy as this particular mutation is associated with mitochondrial mistargeting.

CONCLUSION

Limited mutation analysis can provide a useful first line test for PH1 and PH2 in patients in whom primary hyperoxaluria is suspected and in whom secondary causes have been excluded. Those patients in whom a single mutation, or no mutation, is found can then be selectively targeted for liver biopsy.

Genetic heterogeneity in primary hyperoxaluria type 1: impact on diagnosis

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disease characterized by progressive kidney failure due to renal deposition of calcium oxalate. The disease is caused by a deficiency of alanine:glyoxylate aminotransferase (AGT) which catalyzes the conversion of glyoxylate to glycine. When AGT is absent, glyoxylate is converted to oxalate which forms insoluble calcium salts that accumulate in the kidney and other organs. In the most common phenotype there is a unique phenomenon wherein AGT is mis-targeted to the mitochondria instead of the peroxisomes. The diagnosis of PH1 is complicated by heterogeneity of clinical presentation, course of the disease, biochemical markers, AGT enzymatic activity and genotype. More than 50 mutations and polymorphisms have been reported in the AGT gene; three common mutations accounting for almost 50% of PH1 alleles. The mutations are of all types, with missense making up the largest fraction. There are some mutations with apparent ethnic associations and at least one that appears to be pan-ethnic. Although correlations can in some cases be made between biochemical phenotype and genotype, correlation with clinical phenotype is complicated by the involvement of other genetic and non-genetic factors that affect disease severity. A number of polymorphisms have been described in the AGT gene some of which cause missense changes and, in some cases, alter enzyme activity. As DNA testing becomes more commonly used for diagnosis it is important to correlate observed sequence changes with previously documented changes as an aid to assessing their potential significance.

Primary hyperoxaluria type 1 in the Canary Islands: a conformational disease due to I244T mutation in the P11L-containing alanine:glyoxylate aminotransferase

Primary hyperoxaluria type 1 (PH1) is an inborn error of metabolism resulting from a deficiency of alanine:glyoxylate aminotransferase (AGXT; EC 2.6.1.44). Most of the PH1 alleles detected in the Canary Islands carry the Ile-244 --> Thr (I244T) mutation in the AGXT gene, with 14 of 16 patients homozygous for this mutation. Four polymorphisms within AGXT and regional microsatellites also were shared in their haplotypes (AGXT*LTM), consistent with a founder effect. The consequences of these amino acid changes were investigated. Although I244T alone did not affect AGXT activity or subcellular localization, when present in the same protein molecule as Leu-11 --> Pro (L11P), it resulted in loss of enzymatic activity in soluble cell extracts. Like its normal counterpart, the AGXT*LTM protein was present in the peroxisomes but it was insoluble in detergent-free buffers. The polymorphism L11P behaved as an intragenic modifier of the I244T mutation, with the resulting protein undergoing stable interaction with molecular chaperones and aggregation. This aggregation was temperature-sensitive. AGXT*LTM expressed in Escherichia coli, as a GST-fusion protein, and in insect cells could be purified and retained enzymatic activity. Among various chemical chaperones tested in cell culture, betaine substantially improved the solubility of the mutant protein and the enzymatic activity in cell lysates. In summary, I244T, the second most common mutation responsible for PH1, is a protein conformational disease that may benefit from new therapies with pharmacological chaperones or small molecules to minimize protein aggregation.

Urologic manifestations of nonurologic disease urolithiasis

Patients with nephrolithiasis may have coexistent diseases that play a causative role in stone formation. A stone event may be the initial manifestation of the disorder and the urologist may play a major diagnostic role. Regulation or correction of the disorder may eradicate or dramatically attenuate stone activity. In some patients, metabolic abnormalities persist, however, and the therapeutic strategies that were reviewed in this manuscript may need to be implemented. Stone disease may also be induced by drugs that are used to treat a number of disease processes. When this occurs, the drug will usually need to be discontinued and alternative therapy should be instituted.