A glycine-to-glutamate substitution abolishes alanine:glyoxylate aminotransferase catalytic activity in a subset of patients with primary hyperoxaluria type 1

Effect of the allelic background on the phenotype of primary hyperoxaluria type I

PURPOSE OF REVIEW

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disorder of hepatic glyoxylate metabolism leading to nephrolithiasis and kidney failure. PH1 is caused by mutations on the AGXT gene encoding alanine:glyoxylate aminotransferase (AGT). The AGXT gene has two haplotypes, the major (Ma) and the minor (mi) alleles. This review summarizes the role of the minor allele on the molecular pathogenesis and the clinical manifestations of PH1.

RECENT FINDINGS

PH1 shows high genetic variability and significant interindividual variability. Although the minor haplotype is not pathogenic on its own, it may be crucial for the pathogenicity of some mutations or amplify the effect of others, thus affecting both symptoms and responsiveness to Vitamin B6, the only pharmacological treatment effective in a selected group of PH1 patients.

SUMMARY

In the last years, new drugs based on RNA-interference are available for patients nonresponsive to Vitamin B6, but no specific biomarkers are available to predict disease course and severity. Therefore, a clinical assessment of PH1 taking into account molecular analysis of the mutations and the allelic background and the possible synergism among polymorphic and pathogenic variants should be encouraged to promote approaches of personalized medicine that improve the management of available resources.

Gly161 mutations associated with Primary Hyperoxaluria Type I induce the cytosolic aggregation and the intracellular degradation of the apo-form of alanine:glyoxylate aminotransferase

Primary Hyperoxaluria Type I (PH1) is a severe rare disorder of metabolism due to inherited mutations on liver peroxisomal alanine:glyoxylate aminotransferase (AGT), a pyridoxal 5'-phosphate (PLP)-dependent enzyme whose deficiency causes the deposition of calcium oxalate crystals in the kidneys and urinary tract. PH1 is an extremely heterogeneous disease and there are more than 150 disease-causing mutations currently known, most of which are missense mutations. Moreover, the molecular mechanisms by which missense mutations lead to AGT deficiency span from structural, functional to subcellular localization defects. Gly161 is a highly conserved residue whose mutation to Arg, Cys or Ser is associated with PH1. Here we investigated the molecular bases of the AGT deficit caused by Gly161 mutations with expression studies in a mammalian cellular system paired with biochemical analyses on the purified recombinant proteins. Our results show that the mutations of Gly161 (i) strongly reduce the expression levels and the intracellular half-life of AGT, and (ii) make the protein in the apo-form prone to an electrostatically-driven aggregation in the cell cytosol. The coenzyme PLP, by shifting the equilibrium from the apo- to the holo-form, is able to reduce the aggregation propensity of the variants, thus partly decreasing the effect of the mutations. Altogether, these results shed light on the mechanistic details underlying the pathogenicity of Gly161 variants, thus expanding our knowledge of the enzymatic phenotypes leading to AGT deficiency.

Four of the most common mutations in primary hyperoxaluria type 1 unmask the cryptic mitochondrial targeting sequence of alanine:glyoxylate aminotransferase encoded by the polymorphic minor allele

The gene encoding the liver-specific peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT, EC. 2.6.1.44) exists as two common polymorphic variants termed the "major" and "minor" alleles. The P11L amino acid replacement encoded by the minor allele creates a hidden N-terminal mitochondrial targeting sequence, the unmasking of which occurs in the hereditary calcium oxalate kidney stone disease primary hyperoxaluria type 1 (PH1). This unmasking is due to the additional presence of a common disease-specific G170R mutation, which is encoded by about one third of PH1 alleles. The P11L and G170R replacements interact synergistically to reroute AGT to the mitochondria where it cannot fulfill its metabolic role (i.e. glyoxylate detoxification) effectively. In the present study, we have reinvestigated the consequences of the interaction between P11L and G170R in stably transformed CHO cells and have studied for the first time whether a similar synergism exists between P11L and three other mutations that segregate with the minor allele (i.e. I244T, F152I, and G41R). Our investigations show that the latter three mutants are all able to unmask the cryptic P11L-generated mitochondrial targeting sequence and, as a result, all are mistargeted to the mitochondria. However, whereas the G170R, I244T, and F152I mutants are able to form dimers and are catalytically active, the G41R mutant aggregates and is inactive. These studies open up the possibility that all PH1 mutations, which segregate with the minor allele, might also lead to the peroxisome-to-mitochondrion mistargeting of AGT, a suggestion that has important implications for the development of treatment strategies for PH1.

A novel mutation of human liver alanine:glyoxylate aminotransferase causes primary hyperoxaluria type 1: immunohistochemical quantification and subcellular distribution

A novel alanine:glyoxylate aminotransferase (AGT) mutation involved in primary hyperoxaluria type 1 (PH1) was studied in Japanese patients. Two mutations in exon 7, c.751T>A and c.752G>A, lead to a W251K amino acid substitution. Proband 1 (patient 1) was homozygous for the W251K mutation allele (DDBJ Accession No. AB292648), and AGT-specific activity in the patient’s liver was very low. To reveal the cause of the low enzymatic activity, the intracellular localization of AGT (W251K) was studied using immunohistochemistry and immunoelectron microscopy. The latter analysis showed that patient 2 had only one-fifth of the normal AGT expression per catalase, suggesting impairment of AGT (W251K) dependent transport into peroxisomes. Peroxisomal transport of human AGT is believed to be dependent on the presence of the type 1 peroxisomal targeting sequence. The C-terminal tripeptide of AGT, KKL is necessary for peroxisomal targeting. In cultured cells, EGFP-AGT (W251K) localized both in the peroxisome and cytosol. These results were consistent with the data obtained from liver analysis of patient 2. The subcellular distribution of AGT (W251K) and the results from a random mutagenesis study suggest that KKL is necessary for peroxisomal targeting of human AGT, but additional signal other than KKL may be necessary.

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.

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.

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.

Human wild-type alanine:glyoxylate aminotransferase and its naturally occurring G82E variant: functional properties and physiological implications

Human hepatic peroxisomal AGT (alanine:glyoxylate aminotransferase) is a PLP (pyridoxal 5'-phosphate)-dependent enzyme whose deficiency causes primary hyperoxaluria Type I, a rare autosomal recessive disorder. To acquire experimental evidence for the physiological function of AGT, the K(eq),(overall) of the reaction, the steady-state kinetic parameters of the forward and reverse reactions, and the pre-steady-state kinetics of the half-reactions of the PLP form of AGT with L-alanine or glycine and the PMP (pyridoxamine 5'-phosphate) form with pyruvate or glyoxylate have been measured. The results indicate that the enzyme is highly specific for catalysing glyoxylate to glycine processing, thereby playing a key role in glyoxylate detoxification. Analysis of the reaction course also reveals that PMP remains bound to the enzyme during the catalytic cycle and that the AGT-PMP complex displays a reactivity towards oxo acids higher than that of apoAGT in the presence of PMP. These findings are tentatively related to possible subtle rearrangements at the active site also indicated by the putative binding mode of catalytic intermediates. Additionally, the catalytic and spectroscopic features of the naturally occurring G82E variant have been analysed. Although, like the wild-type, the G82E variant is able to bind 2 mol PLP/dimer, it exhibits a significant reduced affinity for PLP and even more for PMP compared with wild-type, and an altered conformational state of the bound PLP. The striking molecular defect of the mutant, consisting in the dramatic decrease of the overall catalytic activity (approximately 0.1% of that of normal AGT), appears to be related to the inability to undergo an efficient transaldimination of the PLP form of the enzyme with amino acids as well as an efficient conversion of AGT-PMP into AGT-PLP. Overall, careful biochemical analyses have allowed elucidation of the mechanism of action of AGT and the way in which the disease causing G82E mutation affects it.

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.

A comparative analysis of the Arabidopsis mutant amp1-1 and a novel weak amp1 allele reveals new functions of the AMP1 protein

Ethylene and gibberellins have a synergistic stimulatory effect on hypocotyl elongation of light-grown Arabidopsis thaliana (L.) Heynh. seedlings. A screen for mutants with decreased response to these hormones led to the isolation of a novel allele (ampl-7) of the ALTERED MERISTEM PROGRAM (AMP) 1 locus. The amp1-7 allele contains a missense mutation causing a phenotype, which is weaker than that of the amp1-1 mutant that carries a nonsense mutation. The mutant phenotype prompted the hypothesis that AMP1 is involved in ethylene and GA signalling pathways or in a parallel pathway-controlling cell and hypocotyl elongation and cellular organization. Amp1 mutants contain higher zeatin concentrations causing enlargement of the apical meristem, which was confirmed by cytokinin application to wild type seedlings. Light grown amp1 seedlings have shorter hypocotyls than wild type; however, application of cytokinins promotes hypocotyl elongation of both Col-0 and amp1. We suggest that in amp1 mutants either zeatin overproduction or its action is strictly localized.

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.

Primary hyperoxaluria type 1 in Japan

BACKGROUND/AIMS

Current status of primary hyperoxaluria (PH) has not been surveyed in Japan.

METHODS

Japanese patients with PH were reviewed in the published literature.

RESULTS

Fifty-nine patients were diagnosed as PH from 1962 to 2003. The median ages both at diagnosis and at the onset of initial symptoms were 17 (range: 0.02-63) and 13 (range: 0-58) years, respectively. Twenty-nine (49%) patients were older than 20 years at diagnosis, among whom 26 (90%) already presented end-stage renal failure (ESRF) or soon evolved into ESRF. Among 30 (51%) diagnosed as PH under 20 years old, only 13 (43%) were already in a terminal stage of renal insufficiency. Ten patients were diagnosed as PH1 by liver biopsy. We identified two types of enzymatic phenotypes in 3 of those patients examined. In 1 case, immunoreactive SPT/AGT protein level was very low due to accelerated proteolysis, while in other 2 cases, the immunoreactivity was detected on mitochondria due to mistargeting. Of 9 cases having been subjected to kidney transplantation at a median age of 20 years (range 7.3-40.0), it was only 2 cases that were reported to be successful, while the median survival time of the kidney grafts being 1.4 years (range 0-7). Of 4 patients having undergone combined liver/kidney transplantations (at the ages of 1.3, 1.4, 9 and 41 years, respectively), the surgery was successful in 3 cases; in the remaining one case, however, rejection required removal of the transplanted kidney was observed. The overall survival ratio of all the 59 PH cases accounted for 77, 71 and 55% at 5, 10 and 20 years, respectively.

CONCLUSION

Assuming that the majority of the 59 patients with PH reported was classified as PH1, it is postulated that morbidity of violent infantile PH1 in Japan might be less than those in the USA and Europe, and symptoms of elderly Japanese PH1 patients seem to be milder than those of Western patients. Establishment of an early detection system of PH1 and more popular application of combined liver/kidney transplantation deserve further study.

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.

Overexpression of human alanine:glyoxylate aminotransferase in Escherichia coli: renaturation from guanidine–HCl and affinity for pyridoxal phosphate co-factor

Alanine:glyoxylate aminotransferase-1 (AGT) is a human liver peroxisomal enzyme whose deficiency results in, primary hyperoxaluria type 1 (PH1), a fatal metabolic disease. AGT requires a pyridoxal phosphate (PLP) co-factor in its active site. The AGT gene usually exists in one of two polymorphic forms, the major and minor alleles. We describe here an overexpression system for normal and mutant variants of human AGT in Escherichia coli BL21 (DE3) pLysS. We have extracted functional AGT from inclusion bodies using guanidine-HCl. Denaturation and re-folding of the overexpressed AGT after guanidine-HCl treatment produces high yields of biologically active protein and provides a strategy for generating an apoenzyme to investigate PLP-binding. K(M)s for PLP were determined by reconstitution of the apoenzyme. Successful folding was independent of the presence of PLP. The K(M) for PLP for minor allele AGT was significantly higher than that for major allele AGT. This decreased affinity could be attributed to I340M, a polymorphism associated with the minor allele. G170R, located on the minor allele and the most common PH1 mutation, had no effect on the affinity for PLP. PH1 mutations, G41V and G41R, showed enhanced activity after re-folding. We suggest that the renaturation/re-folding and reconstitution strategies provide an approach for studying the maturation of AGT under optimal conditions and in isolation from cellular quality control and chaperoning processes. Furthermore, our data show that mutations with serious consequences in vivo may not be inherently catalytically inactive and may be rescuable.

Implications of genotype and enzyme phenotype in pyridoxine response of patients with type I primary hyperoxaluria

BACKGROUND

Marked hyperoxaluria due to liver-specific deficiency of alanine:glyoxylate aminotransferase activity (AGT) characterizes type I primary hyperoxaluria (PHI). Approximately half of PHI patients experience improvement in the degree of hyperoxaluria following pyridoxine (VB6) treatment. Recently, we showed an association between VB6 response and the commonest PHI mutation G170R, with patients possessing one or two copies showing 50% reduction or complete to near complete normalization of oxaluria, respectively. Two patients showed responses varying from this pattern. To further clarify the molecular basis of VB6 response in PHI, we performed additional genotyping.

METHODS

23 PHI patients diagnosed via hepatic enzyme analysis, hyperoxaluria and hyperglycolic aciduria or homozygosity for a known mutation, availability of pre- and post-VB6 24-hour urine oxalate and GFR >40 ml/min/1.73 m2 were included. Data was retrieved retrospectively, oxalate measured by oxalate oxidase, and genotyping performed by PCR-based methods.

RESULTS

VB6 response was associated with the G170R and F152I mutations. Eight new sequence changes were detected.

CONCLUSIONS

In PHI, two mutations resulting in AGT mistargeting are associated with VB6 response. Whether this favorable effect is specific to the peroxisomal-to-mitochondrial mistargeting caused by these changes or due to another mechanism remains to be determined.

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.

Crystal structure of alanine:glyoxylate aminotransferase and the relationship between genotype and enzymatic phenotype in primary hyperoxaluria type 1

A deficiency of the liver-specific enzyme alanine:glyoxylate aminotransferase (AGT) is responsible for the potentially lethal hereditary kidney stone disease primary hyperoxaluria type 1 (PH1). Many of the mutations in the gene encoding AGT are associated with specific enzymatic phenotypes such as accelerated proteolysis (Ser205Pro), intra-peroxisomal aggregation (Gly41Arg), inhibition of pyridoxal phosphate binding and loss of catalytic activity (Gly82Glu), and peroxisome-to-mitochondrion mistargeting (Gly170Arg). Several mutations, including that responsible for AGT mistargeting, co-segregate and interact synergistically with a Pro11Leu polymorphism found at high frequency in the normal population. In order to gain further insights into the mechanistic link between genotype and enzymatic phenotype in PH1, we have determined the crystal structure of normal human AGT complexed to the competitive inhibitor amino-oxyacetic acid to 2.5A. Analysis of this structure allows the effects of these mutations and polymorphism to be rationalised in terms of AGT tertiary and quaternary conformation, and in particular it provides a possible explanation for the Pro11Leu-Gly170Arg synergism that leads to AGT mistargeting.

Alanine:glyoxylate aminotransferase peroxisome-to-mitochondrion mistargeting in human hereditary kidney stone disease

The pyridoxal-phosphate (PLP)-dependent enzyme alanine:glyoxylate aminotransferase (AGT) is mistargeted from peroxisomes to mitochondria in patients with the hereditary kidney stone disease primary hyperoxaluria type 1 (PH1) due to the synergistic interaction between a common Pro(11)Leu polymorphism and a PH1-specific Gly(170)Arg mutation. The kinetic partitioning of newly synthesised AGT between peroxisomes and mitochondria is determined by the combined effects of (1) the generation of cryptic mitochondrial targeting information, and (2) the inhibition of AGT dimerization. The crystal structure of AGT has recently been solved, allowing the effects of the various polymorphisms and mutations to be rationalised in terms of AGT's three-dimensional conformation. Procedures that increase dimer stability and/or increase the rate of dimer formation have potential in the formulation of novel strategies to treat this otherwise intractable life-threatening disease.

The AGT gene in Africa: a distinctive minor allele haplotype, a polymorphism (V326I), and a novel PH1 mutation (A112D) in Black Africans

We describe a novel missense mutation (A112D) and polymorphism (V326I) in the human AGT gene in two black African patients with primary hyperoxaluria type 1, an autosomal recessive disease resulting from a deficiency of the liver peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT; EC 2.6.1.44). V326I was found in DNA from normal control Blacks with an allele frequency of 3%. Expression studies confirmed that A112D reduced AGT enzyme activity by 95% while V326I had no effect. Both A112D and V326I were homozygous in both patients and lie on a variant of the minor allele of the AGT gene. This variant haplotype, Mi(A), includes an intron 1 duplication and intron 4 VNTR (38 repeat) but lacks the P11L and I340M normally associated with the minor allele in Caucasians. Among the South African Blacks tested, the Mi(A) haplotype had an allele frequency of 12% compared to 3 % for the Caucasian-type minor allele haplotype.

Three novel deletions in the alanine:glyoxylate aminotransferase gene of three patients with type 1 hyperoxaluria

We describe three novel deletions in the human AGT gene in three patients with primary hyperoxaluria type 1, an autosomal recessive disease resulting from a deficiency of the liver peroxisomal enzyme, alanine glyoxylate aminotransferase (AGT; EC 2.6.1.44). A deletion of 4 nucleotides in the exon 6/intron 6 splice junction (679-IVS6+2delAAgt) is expected to cause missplicing. It would also code for a K227E missense alteration in any mRNA successfully spliced. A 2-bp deletion in exon 11 (1125-1126del CG, cDNA) results in a frameshift. A deletion of at least 5-6 kb, EX1 EX5del, spanned exons 1-5 and contiguous upstream sequence. All three deletions are heterozygous with previously documented missense mutations; the intron 6 deletion with F152I, the exon 11 deletion with G82E, and EX1 EX5del with the common mistargeting mutation, G170R.

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.

The optimal approach to the patient with oxalosis

There are many challenges in oxalosis, including prompt, clinical recognition of this inborn error of metabolism, management of its many medical problems, provision of adequate care at end-stage kidney disease, and optimizing both the timing and results of liver and kidney allografts. This review provides a framework for the interested clinician to understand the many problems, and to begin to assimilate knowledge about an increasingly recognized, metabolic disorder. It ends with potential, innovative therapies that are not yet at the patient's bedside.

Functional synergism between the most common polymorphism in human alanine:glyoxylate aminotransferase and four of the most common disease-causing mutations

The autosomal recessive disorder primary hyperoxaluria type 1 (PH1) is caused by a deficiency of the liver-specific pyridoxal-phosphate-dependent enzyme alanine:glyoxylate aminotransferase (AGT). Numerous mutations and polymorphisms in the gene encoding AGT have been identified, but in only a few cases has the causal relationship between genotype and phenotype actually been demonstrated. In this study, we have determined the effects of the most common naturally occurring amino acid substitutions (both normal polymorphisms and disease-causing mutations) on the properties, especially specific catalytic activity, of purified recombinant AGT. The results presented in this paper show the following: 1) normal human His-tagged AGT can be expressed at high levels in Escherichia coli and purified in a correctly folded, dimerized and catalytically active state; 2) presence of the common P11L polymorphism decreases the specific activity of purified recombinant AGT by a factor of three; 3) AGTs containing four of the most common PH1-specific mutations (G41R, F152I, G170R, and I244T) are all soluble and catalytically active in the absence of the P11L polymorphism, but in its presence all lead to protein destabilization and aggregation into inclusion bodies; 4) naturally occurring and artificial amino acid substitutions that lead to peroxisome-to-mitochondrion AGT mistargeting in mammalian cells also lead to destabilization and aggregation in E. coli; and 5) the PH1-specific G82E mutation abolishes AGT catalytic activity by interfering with cofactor binding, as does the artificial K209R mutation at the putative site of cofactor Shiff base formation. These results are discussed in the light of the high allelic frequency ( approximately 20%) of the P11L polymorphism and its importance in determining the phenotypic manifestations of mutations in PH1.

Primary hyperoxaluria type 1: diagnostic relevance of mutations and polymorphisms in the alanine:glyoxylate aminotransferase gene (AGXT)

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disorder of glyoxylate metabolism caused by deficiency of the hepatic peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT). The disease shows considerable phenotypic, enzymatic and genetic heterogeneity. To date, 7 polymorphisms and 11 point mutations have been described in the gene encoding AGT. We report on the prevalence of these polymorphisms and mutations in 79 patients with PH1 with the aim of assessing their diagnostic relevance. A strong association of the C154T, intron 1 insertion and C386T polymorphisms is confirmed and this linkage extends to include the type 1 variant of a polymorphic tandem repeat in intron 4. Only 64 of 158 (40%) PH1 alleles have one of the defined mutations, with the G630A mutation accounting for 39 of these and T853C for 14. Overall only 20 (25%) of the patients studied had the genetic basis of their disease fully explained: 7 were homozygous for the G630A mutation, 5 were homozygous for the T853C mutation, 1 was homozygous for the C819T mutation, and 7 had two different mutations identified and were presumed to be compound heterozygotes. Only the two more frequent G630A and T853C mutations are of general diagnostic relevance for mutation screening. It seems likely that there are a significant number of other mutations, perhaps family-specific, still to be described. There was no apparent difference in the types of mutations in patients presenting in the first year of life (36%), suggesting that other factors, such as periods of dehydration or urinary tract infections, might contribute more to the clinical manifestation than genotype.

Primary hyperoxaluria type 1: a cluster of new mutations in exon 7 of the AGXT gene

Primary hyperoxaluria type 1 (PH1) is a severe autosomal recessive inborn error of glyoxylate metabolism caused by deficiency of the hepatic peroxisomal enzyme alanine:glyoxylate aminotransferase. This enzyme is encoded by the AGXT gene on chromosome 2q37.3. DNA samples from 79 PH1 patients were studied using single strand conformation polymorphism analysis to detect sequence variants, which were then characterised by direct sequencing and confirmed by restriction enzyme digestion. Four novel mutations were identified in exon 7 of AGXT: a point mutation T853C, which leads to a predicted Ile244Thr amino acid substitution, occurred in nine patients. Two other mutations in adjacent nucleotides, C819T and G820A, mutated the same codon at residue 233 from arginine to cysteine and histidine, respectively. The fourth mutation, G860A, introduced a stop codon at amino acid residue 246. Enzyme studies in these patients showed that AGT catalytic activity was either very low or absent and that little or no immunoreactive protein was present. Together with a new polymorphism in exon 11 (C1342A) these findings underline the genetic heterogeneity of the AGXT gene. The novel mutation T853C is the second most common mutation found to date with an allelic frequency of 9% and will therefore be of clinical importance for the diagnosis of PH1.

Strategies for the prenatal diagnosis of primary hyperoxaluria type 1

Primary hyperoxaluria type 1 (PH1) is a potentially lethal autosomal recessive disorder of glyoxylate metabolism caused by a deficiency of the liver-specific peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT). Over the past 13 years, various strategies have been adopted for its prenatal diagnosis, including (1) glyoxylate metabolite analysis of amniotic fluid in the second trimester; (2) AGT enzyme assay, immunoassay, and immuno-electron microscopy of fetal liver biopsies also in the second trimester; and (3) linkage and mutation analysis of DNA isolated from chorionic villus samples in the first trimester. These methods have evolved in parallel with our increased understanding of the molecular aetiology and pathogenesis of the disease. Although the usefulness of metabolite analysis remains unproven, all the other methods have been successfully applied to the prenatal diagnosis of PH1. In this review, examples of the use of the available methodologies are provided, and their pros and cons are discussed with reference to specific cases.

DNA diagnosis of X-linked adrenoleukodystrophy

The X-linked adrenoleukodystrophy (ALD) gene was identified recently and is predicted to encode a 745-amino-acid peroxisomal membrane protein. Strategies have been designed for the search for mutations in the ALD gene in patients. Several mutations have now been found and it seems that many different mutations are responsible for ALD. There is no straightforward correlation between genotype and phenotype since the same mutation can cause different ALD phenotypes in the same family. However, once a mutation has been found in a family, it can be traced in all at-risk individuals of that family, both post- and prenatally, without the need for very long-chain fatty acid (VLCFA) analysis. Segregation analysis with extragenic and intragenic polymorphisms may remain useful in families where mutation analysis is not possible for practical reasons; VLCFA analysis and measurement of the peroxisomal beta-oxidation with C26:0 fatty acid as a substrate will remain the alternative. We also briefly discuss the possibilities of DNA diagnosis for other peroxisomal disorders.

Molecular characterization and clinical use of a polymorphic tandem repeat in an intron of the human alanine:glyoxylate aminotransferase gene

The autosomal recessive disease primary hyperoxaluria type 1 (PH1) is caused by a deficiency of the liver-specific peroxisomal enzyme alanine:glyoxylate amino-transferase (AGT). This paper concerns the identification, characterization and clinical use of an unusual discretely polymorphic tandem repeat sequence in the fourth intron of the human AGT gene (gene locus designation AGXT). In a random Caucasian population, three alleles could be clearly recognized that consisted of either 12 (type III), 17 (type II) or approximately 38 (type I) tandemly repeated copies of a highly conserved 29/32-bp sequence with frequencies of 33%, 7% and 60%, respectively. In a random Japanese population, the allelic frequencies were markedly different (i.e. 31%, 45% and 19%, respectively). In addition, a fourth allele was identified, consisting of approximately 32 repeats (type IV), with an allelic frequency of approximately 5% in Japanese. The repetitive sequence was similar to previously identified mammalian sequences with homology to the Epstein-Barr virus IR3 repetitive element involving a 12/15-bp region GCA(GGN)GGAGGAGGG within the repeat unit. This IR3-like sequence was interspersed with a 17-bp sequence with no similarity to any currently known repetitive element. The type I and type III alleles were judged to be equivalent to a previously identified TaqI polymorphism. Two polymorphisms previously shown to be associated with the peroxisome-to-mitochondrion mistargeting of AGT in PH1 (a C154-->T point substitution in exon 1 and a 74-bp duplication in intron 1) were found to segregate exclusively with the type I intron 4 polymorphism in Caucasians, but not in Japanese. The polymorphic nature of the intron 4 tandem repeats makes them of potential use in the prenatal diagnosis of PH1, especially when coupled with the exon 1 C154-->T substitution or intron 1 duplication polymorphisms. A PH1 family, in which a fetus had been predicted previously to be either normal or a carrier by AGT enzymic analysis of a fetal liver biopsy, but who had been shown to be only partially informative with respect to the C154-->T/intron 1 polymorphisms, was analysed retrospectively. The family was completely informative for the intron 4 tandem repeat polymorphism and the carrier status of the fetus was confirmed.

Adrenoleukodystrophy and other peroxisomal diseases

The gene predisposing for X-linked adrenoleukodystrophy (ALD), the most common peroxisomal disorder, has been identified recently by positional cloning. The ALD protein is a 75 kDa peroxisomal membrane protein belonging to the family of ATP-binding cassette transporter proteins. With the combination of genetic complementation and candidate gene approaches, two genes responsible for Zellweger syndrome, a group of genetically heterogeneous disorders affecting peroxisome biogenesis, have also been identified.

Primary hyperoxaluria type 1: genotypic and phenotypic heterogeneity

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disease caused by a deficiency of the liver-specific peroxisomal enzyme alanine: glyoxylate aminotransferase (AGT). The disease is notable for its extensive heterogeneity at the clinical, biochemical, enzymic and molecular genetic levels. A study of 116 PH1 patients over the past 8 years has revealed four main enzymic phenotypes: (1) absence of both AGT catalytic activity and immunoreactive AGT protein (approximately 40% of patients); (2) absence of AGT catalytic activity but presence of immunoreactive protein (approximately 16% of patients); (3) presence of both AGT catalytic activity and immunoreactive protein (approximately 41% of patients), in most of which cases the AGT is mistargeted to the mitochondria instead of the peroxisomes; and (4) a variation of the mistargeting phenotype in which AGT is equally distributed between peroxisomes and mitochondria, but in which that in the peroxisomes is aggregated into matrical core-like structures (approximately 3% of patients). Various point mutations, all occurring at conserved positions in the coding regions of the AGT gene, have been identified in these patients. The five mutations discussed in the present study, which have been found in individuals manifesting all of the four major enzymic phenotypes, account for the expressed alleles in about half of all Caucasian PH1 patients. The most common mutation found so far leads to a Gly170-->Arg amino acid substitution. This mutation, in combination with a normally occurring Pro11-->Leu polymorphism, appears to be responsible for the unprecedented peroxisome-to-mitochondrion mistargeting phenotype.

ATP-dependent degradation of a mutant serine: pyruvate/alanine:glyoxylate aminotransferase in a primary hyperoxaluria type 1 case

Primary hyperoxaluria type 1 (PH 1), an inborn error of glyoxylate metabolism characterized by excessive synthesis of oxalate and glycolate, is caused by a defect in serine:pyruvate/alanine:glyoxylate aminotransferase (SPT/AGT). This enzyme is peroxisomal in human liver. Recently, we cloned SPT/AGT-cDNA from a PH 1 case, and demonstrated a point mutation of T to C in the coding region of the SPT/AGT gene encoding a Ser to Pro substitution at residue 205 (Nishiyama, K., T. Funai, R. Katafuchi, F. Hattori, K. Onoyama, and A. Ichiyama. 1991. Biochem. Biophys. Res. Commun. 176:1093-1099). In the liver of this patient, SPT/AGT was very low with respect to not only activity but also protein detectable on Western blot and immunoprecipitation analyses. Immunocytochemically detectable SPT/AGT labeling was also low, although it was detected predominantly in peroxisomes. On the other hand, the level of translatable SPT/AGT-mRNA was higher than normal, indicating that SPT/AGT had been synthesized in the patient's liver at least as effectively as in normal liver. Rapid degradation of the mutant SPT/AGT was then demonstrated in transfected COS cells and transformed Escherichia coli, accounting for the low level of immunodetectable mutant SPT/AGT in the patient's liver. The mutant SPT/AGT was also degraded much faster than normal in an in vitro system with a rabbit reticulocyte extract, and the degradation in vitro was ATP dependent. These results indicate that a single amino acid substitution in SPT/AGT found in the PH1 case leads to a reduced half-life of this protein. It appears that the mutant SPT/AGT is recognized in cells as an abnormal protein to be eliminated by degradation.