Molecular and clinical heterogeneity in primary hyperoxaluria type 1

The autosomal recessive disease primary hyperoxaluria type 1 (PH1) is caused by a functional deficiency of the liver-specific peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT). An analysis of liver samples from 59 PH1 patients showed considerable heterogeneity at the enzymic level. Approximately two thirds of patients had zero AGT catalytic activity, whereas the remaining one third had activities that ranged from 3% to 48% of the mean normal level. Two thirds of patients with zero AGT activity also had zero immunoreactive AGT protein, while the other one third, together with all the patients with detectable AGT catalytic activity, had levels of immunoreactive AGT protein that varied from normal to only a few percent of normal. All patients with AGT catalytic activity had their enzyme in the wrong intracellular compartment (ie, mitochondria). On the other hand, in all but one of the patients with immunoreactive AGT protein, but zero catalytic activity, the inactive AGT was correctly located within the peroxisomes. This enzymic heterogeneity was matched by considerable heterogeneity at the clinical level (eg, age at onset, rate of progression, age at renal failure, etc). No simple relationship was found between the level of hepatic AGT and the severity of the disease. It is suggested that a lack of AGT might be responsible for a broader pathological phenotype than classically associated with PH1. The possibility is advanced that some patients with idiopathic oxalate stone disease might owe their predisposition to stone formation to a functional deficiency of AGT.     

Primary hyperoxaluria: genotype-phenotype correlation

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disorder caused by a deficiency of alanine-glyoxylate aminotransferase (AGT), which is encoded by a single copy gene (AGXT). Molecular diagnosis was used in conjunction with clinical, biochemical and enzymological data to evaluate genotype-phenotype correlation. Patients can present a severe form of PH1, an adult form and a mild to moderate decrease in renal function. Biochemical diagnosis is made by plasma, urine and dialyzate oxalate and glycolate assays, and by liver AGT activity and pyridoxine responsitivity. Molecular genetic diagnosis can be made using different techniques, for example, the single strand conformation polymorphism technique (SSCP), followed by the sequencing of the 11 AGXT exons. The disease is clinically and genetically classified as highly heterogeneous. Mutant alleles can be recognised in 80- 90% of chromosomes, depending on the techniques used. Mutations in exons 1, 2, 4 and 10 are more frequent in Italian patients. Normalized AGT activity seems to be lower in the severe form than in the adult form. Double heterozygous patients present a lower age at disease onset and they were more frequent in the more severe than in mild severe disease. The 444T>C mutation was more frequent in the severe form, while the opposite was observed for 630G>A. 630G>A mutation homozygotes had a higher AGT residual activity. The presence of allelic heterogeneity of the AGXT could be responsible, to some extent, for the phenotypic heterogeneity in PH1. Homozygous genotypes were more frequent than expected and were associated with a less severe form of the disease.     

AGXT gene mutations and their influence on clinical heterogeneity of type 1 primary hyperoxaluria

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disorder that is caused by a deficiency of alanine: glyoxylate aminotransferase (AGT), which is encoded by a single copy gene (AGXT). Molecular diagnosis was used in conjunction with clinical, biochemical, and enzymological data to evaluate genotype-phenotype correlation. Twenty-three unrelated, Italian PH1 patients were studied, 20 of which were grouped according to severe form of PH1 (group A), adult form (group B), and mild to moderate decrease in renal function (group C). All 23 patients were analyzed by using the single-strand conformation polymorphism technique followed by the sequencing of the 11 AGXT exons. Relevant chemistries, including plasma, urine and dialyzate oxalate and glycolate assays, liver AGT activity, and pyridoxine responsiveness, were performed. Both mutant alleles were found in 21 out of 23 patients, and 13 different mutations were recognized in exons 1, 2, 4, and 10. Normalized AGT activity was lower in the severe form than in the adult form (P < 0.05). Double heterozygous patients presented a lower age at the onset of the disease (P = 0.025), and they were more frequent in group A (75%) than in the group B (14%; P = 0.0406). The T444C mutation was more frequent in the severe form (P < 0.05), and the opposite was observed for G630A (P < 0.05). G630A mutation homozygotes had a higher AGT residual activity (P = 0.00001). This study confirms the allelic heterogeneity of the AGXT, which could to some extent be responsible for the phenotypic heterogeneity in PH1.     

Metabolism of pyridoxine in mild metabolic hyperoxaluria and primary hyperoxaluria (type 1).

Plasma pyridoxine metabolites in plasma and 4-pyridoxic acid excretions in urine were measured in normal subjects, in 7 patients with type-1 hyperoxaluria and in 8 patients with mild metabolic hyperoxaluria, while receiving various doses of pyridoxine. Compliance with ingestion of pyridoxine was verified by measuring urinary 4-pyridoxic acid. In the normal subjects the maximum level of pyridoxal phosphate was obtained after only 10 mg/day of pyridoxine. The patients were divided into nonresponders, good responders and poor responders to pyridoxine according to the fall in urinary oxalate and glycollate excretions. In patients taking pyridoxine, the plasma pyridoxal phosphate levels were as for normal subjects in primary hyperoxaluria, lower than for normal subjects in mild metabolic hyperoxaluria (p less than 0.01), and in the latter group lower in partial responders than in good responders (p = 0.04). Hence in mild metabolic hyperoxaluria there may be difficulty in converting pyridoxine to pyridoxal phosphate.     

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.     

Skeletal features of primary hyperoxaluria type 1, revisited

Purpose  The purpose of this study was to describe the skeletal manifestations of primary hyperoxaluria type 1 (PH1), the most common of the primary hyperoxalurias. Methods  We clinically and radiographically reviewed 12 consecutive patients diagnosed with PH1, aged between 2 and 17 years. All patients had evidence of some type of renal involvement, 4 of whom were at end-stage renal disease (ESRD) and were under dialysis. Results  The main symptom was skeletal pain and was present only in the 4 severely involved patients and appeared during the second year of dialysis. The 2 most severely involved patients had evidence of pathological fractures. Radiological signs were present in patients with or without symptoms. These radiological signs were of two distinct types: those almost specific of oxalosis, such as dense and radiolucent metaphyseal bands and vertebral osteocondensations, which are found mainly in the severely involved individuals, and those less specific, such as signs of renal osteodystrophy, which are also found in less severely involved patients. Interestingly, our study revealed the presence of spondylolysis in 25% of cases. This latter finding is unique and has not previously been reported in the literature. Conclusions  The skeletal manifestations of PH1 include specific and less specific radiological signs, with some patients being asymptomatic, and others presenting with bone pain and pathological fractures, as well as spondylolysis.     

Genetic analysis: a diagnostic tool for primary hyperoxaluria type 1

Pediatric Nephrology -     

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.     

中国大陆原发性高草酸尿症1型临床特点及诊治情况总结

目的探讨原发性高草酸尿症1型(PH1)的临床表现、治疗及预后。方法检索维普网、中国知网、万方数据库、PubMed、Web of Science、Embase和Cochrane数据库中的相关文献,收集57例PH1患者的临床资料,对其临床表现、诊疗经过及预后情况进行分析。结果共纳入符合标准的文献35篇,共57例PH1患者,其中男39例,女18例,年龄0.2~57.0岁,发病年龄为出生后至42岁。57例PH1患者的临床症状表现特异性较低,肾结石41例,肾钙化和(或)肾钙质沉积21例,泌尿系统外草酸沉积12例,腰背腹痛12例,输尿管结石8例,此外尿量减少、代谢性酸中毒、水电解质紊乱、贫血、肉眼血尿等症状均有报道,有33例在诊断时已进入终末期肾病(ESRD)阶段。26例患者接受移植治疗,肾移植17例(2例因结石复发、恢复透析再次接受肝肾联合移植,1例恢复透析并再次接受肝移植),肝肾联合移植7例,肝移植2例,肝肾序贯移植3例。31例患者未接受移植治疗。接受移植治疗的患者与未接受移植治疗的患者存活率差异有统计学意义(85%比58%,P<0.05)。结论PH1的临床表现多样且缺乏特异性,发现时多已进入ESRD阶段,接受移植治疗较未接受移植治疗的患者预后更佳,应优先考虑预先肝移植或肝肾联合移植。     

The role of preemptive liver transplantation in primary hyperoxaluria type 1

In primary hyperoxaluria the deficiency or mistargeting of hepatic alanine-glyoxylate aminotransferase (AGT) leads to the overproduction of oxalate resulting in hyperoxaluria and renal damage due to urolithiasis and/or nephrocalcinosis. Presently, the cure of the metabolic defect can be achieved only by liver transplantation. While for patients with end-stage renal disease combined hepatorenal transplantation is recommended, the concept of preemptive liver transplantation (PLTX), i.e. cure of the metabolic defect before renal damage occurs, has received considerable attention. Due to the heterogenous clinical course in PH1, optimal timing of PLTX is a matter of debate. Advocators of PLTX would consider a patient with a slowly declining GFR, reaching levels of 40–60 ml/min/1.73 m², as an ideal candidate, while others would continue medical treatment in these patients and opt for rapid combined liver-kidney transplantation if GFR reaches even lower levels. This review will discuss the background and rationale of PLTX and gives an update on 11 patients with PLTX who have been reported in the literature to date.Dedicated to Professor Dirk E. Müller-Wiefel on occasion of his 60th birthday.     

Liver-kidney-transplantation in type 1 primary hyperoxaluria: description and comments on a case

BACKGROUND: Primary hyperoxaluria leads to oxalosis, a systemic illness with fatal prognosis in uremic youngsters because of systemic complications. Case report: A 14-year old boy with primary type 1 hyperoxaluria who had a long-lasting history of nephrolithiasis and passed from normal renal function to end-stage renal disease within 7 months. MEASUREMENT of alanine: glyoxylate aminotransferase (AGT) catalytic activity in the liver biopsy disclosed very low activity which was not. responsive to pyridoxin., thus the patient entered onto a priority national waiting list for liver-kidney transplantation and a week later received a combined transplant. In order to increase body clearance of oxalate, the patient underwent medical treatment to increase urine oxalate solubility (sodium and potassium citrate oral therapy, magnesium supplementation and increase of diuresis) and intensive dialysis both before and after transplantation. COMMENT: The medical approach to the treatment of this rare illness is discussed. Since the major risk for the grafted kidney is related to the oxalate burden, i.e. oxalate deposition from the body deposits to the kidney that becomes irreversibly damaged, treatment consists of increasing the body clearance of oxalate both by increasing oxalate solubility in the urine and with intensive dialysis performed both before and after combined transplantation. To the same extent (by limiting body oxalate deposits), a relatively early (native GFR 20-25 ml/minute) transplantation is advisable.     

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.     

Genetic basis of primary hyperoxaluria type II

Primary hyperoxaluria Type II (PH2) is a rare monogenic disease characterized by excessive urinary oxalate and L-glycerate excretion. The severity of clinical complications in PH2 patients can range from none to end-stage renal failure secondary to massive deposits of calcium oxalate crystals in the kidney. The disease is a result of the absence of an enzyme with glyoxylate reductase and hydroxypyruvate reductase activities (GRHPR). Recent breakthroughs have occurred in our understanding of the molecular basis of PH2. In this article, we briefly review the literature concerning the clinical and biochemical characteristics of the disease and the enzyme associated with it. We describe the identification of the cDNA for the GRHPR enzyme using the expressed sequence tag database, the characterization of the human GRHPR gene, and the identification of mutations in patients with PH2. Insights gained from the molecular biology underlying this disease as they relate to relevant clinical issues such as potential therapeutic strategies are discussed.     

Novel mutations of the AGXT gene causing primary hyperoxaluria type 1

BACKGROUND: Primary hyperoxaluria type 1 (PH1), an inherited cause of nephrolithiasis, is due to a functional defect of the liver-specific peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT). A definitive PH1 diagnosis can be established by analyzing AGT activity in liver tissue or mutation analysis of the AGXT gene. METHODS: The molecular basis of PH1 in three Chinese patients, two with adult-onset and one with childhood-onset recurrent nephrolithiasis, was established by analyzing the entire AGXT gene. RESULTS: Three novel mutations (c2T>C, c817insAG and c844C>T) and two previously reported mutations (c33insC and 679-IVS6+2delAAgt) were identified. c2T>C converts the initiation codon from ATG to ACG, which predicts significant reduction, if not complete abolition, of protein translation. c817insAG leads to a frameshift and changes the amino acid sequence after codon 274. c844C>T changes glutamine at codon 282 to a termination codon, resulting in protein truncation. CONCLUSIONS: This is the first report describing AGXT gene mutations in Chinese patients with PH1. AGXT genotypes cannot fully explain the clinical heterogeneity of PH1, and other factors involved in disease pathogenesis remain to be identified. Our experience emphasizes the importance of excluding PH1 in patients with recurrent nephrolithiasis to avoid delay or inappropriate management.     

Combined liver-kidney transplantation for primary hyperoxaluria type 1 in young children.

BACKGROUND: Primary hyperoxaluria type 1 (PH1) is a rare condition in which deficiency of the liver enzyme alanine:glyoxylate aminotransferase leads to renal failure and systemic oxalosis. Combined liver-kidney transplantation (LKT) is recommended for end-stage renal failure (ESRF) in adults, but management of infants and young children is controversial. We retrospectively reviewed six children who underwent LKT for PH1. METHODS: The median age at diagnosis was 1.8 years (range 3 weeks to 7 years). Two children presented with severe infantile oxalosis at 3 and 9 weeks, five patients had ESRF with nephrocalcinosis and systemic oxalosis, (median duration of dialysis 1.3 years), and one had progressive chronic renal failure. Four children underwent combined LKT, one child staged liver then kidney, and one infant had an isolated liver transplant. The median age at transplantation was 8.9 years (range 1.7-15 years). RESULTS: Overall patient survival was four out of six. The two infants with PH1 and severe systemic oxalosis died (2 and 3 weeks post-transplant) due to cardiovascular oxalosis and sepsis. The other four children are well at median follow-up of 10 months (range 6 months to 7.4 years). No child developed hepatic rejection and all have normal liver function. Renal rejection occurred in three patients. Despite maximal medical management, oxalate deposits recurred in all renal grafts, contributing to graft loss in one (one of the infants who died), and significant dysfunction requiring haemodialysis post-transplant for 6 months. CONCLUSIONS: LKT is effective therapy for primary oxalosis with ESRF but has a high morbidity and mortality rate in children who present in infancy with nephrocalcinosis and systemic oxalosis. We feel that earlier LKT, or pre-emptive liver transplantation, may be a better therapeutic strategy to improve the outlook for these patients.     

Oxalobacter formigenes: a potential tool for the treatment of primary hyperoxaluria type 1

Primary hyperoxaluria is characterized by severe urolithiasis, nephrocalcinosis, and early renal failure. As treatment options are scarce, we aimed for a new therapeutic tool using colonic degradation of endogenous oxalate by Oxalobactor formigenes. Oxalobacter was orally administered for 4 weeks as frozen paste (IxOC-2) or as enteric-coated capsules (IxOC-3). Nine patients (five with normal renal function, one after liver-kidney transplantation, and three with renal failure) completed the IxOC-2 study. Seven patients (six with normal renal function and one after liver-kidney transplantation) completed the IxOC-3 study. Urinary oxalate or plasma oxalate in renal failure was determined at baseline, weekly during treatment and for a 2-week follow-up. The patients who showed >20% reduction both at the end of weeks 3 and 4 were considered as responders. Under IxOC-2, three out of five patients with normal renal function showed a 22-48% reduction of urinary oxalate. In addition, two renal failure patients experienced a significant reduction in plasma oxalate and amelioration of clinical symptoms. Under IxOC-3 treatment, four out of six patients with normal renal function responded with a reduction of urinary oxalate ranging from 38.5 to 92%. Although all subjects under IxOC-2 and 4 patients under IxOC-3 showed detectable levels of O. formigenes in stool during treatment, fecal recovery dropped directly at follow up, indicating only transient gastrointestinal-tract colonization. The preliminary data indicate that O. formigenes is safe, leads to a significant reduction of either urinary or plasma oxalate, and is a potential new treatment option for primary hyperoxaluria.