The Primary Hyperoxalurias

Primary hyperoxaluria (PH) results from endogenous (primary) overproduction of oxalic acid, as opposed to secondary hyperoxaluria, which is attributable to increased intestinal absorption (enteric) or excessive intake (dietary) of oxalate. Why are nephrologists interested in PH type 1 (PH1), the prevailing type, although it is far from being a primary renal disorder? First, the clinical treatment of patients with this “nephrologic liver disease” is quite challenging, particularly in end-stage renal failure (ESRF). Second, the study of PH1 has yielded better understanding of the pathogenesis of calcium oxalate (CaOx) crystal deposition and stone formation in general, with this disease serving as a model of nephrocalcinosis and urolithiasis. Finally, fascinating insights into the mechanisms of cellular trafficking have been obtained from the study of PH1; however, many questions remain unresolved. It has also become apparent that PH encompasses more than PH1 and PH2, and some patients who were previously classified as having secondary hyperoxaluria might actually have a type of PH. PH1 Metabolic and Genetic Basis PH1, which is an autosomal recessive disease, is caused by a defect in glyoxylate metabolism attributable to low or absent activity of the liver-specific peroxisomal enzyme alanine/glyoxylate aminotransferase (AGT) (392 amino acids, 43 kD) (Figure 1) (1,2). As a result, urinary excretion of oxalate and (in most cases) glycolate is greatly increased (2). Although massive intrarenal CaOx deposits were noted by Lepoutre in 1925 for a pediatric patient who was surgically treated for urolithiasis (abstract cited in reference 2), the disease was not named PH until 1957 (3) and the metabolic defect was localized in liver peroxisomes (therefore not within the cytosol, as previously thought) in 1986 (1). In contrast to PH1, patients with a primary peroxisomal disorder such as the Zellweger syndrome do not exhibit hyperoxaluria (2). Except for the AGT defect, peroxisomes in PH1 are normal in almost every respect; they appear only slightly smaller in electron microscopic evaluations (4). The functional deficiency of AGT in PH1 results in a failure to detoxify glyoxylate within the peroxisomes. Instead of being transaminated to glycine, glyoxylate is oxidized to oxalate and/or reduced to glycolate, resulting in greatly increased urinary excretion of oxalate and glycolate (Figure 1) (1,2,4). Glyoxylate itself is synthesized within the peroxisomes from glycine and glycolate (2).Figure 1: . Metabolic defects in primary hyperoxaluria type 1 (PH1) [alanine/glyoxylate aminotransferase (AGT)] and PH2 [glyoxylate reductase (GR)/hydroxypyruvate reductase (HPR)]. Glyoxylate reductase also exhibits hydroxypyruvate reductase activity. GO, glycolate oxidase; DAO, D-amino oxidase; LDH, lactate dehydrogenase.The gene coding for AGT, i.e., AGXT, is located on chromosome 2q37.3 and consists of 11 exons spanning approximately 10 kb (2,5). More than 30 mutations have been identified to date. There is considerable molecular heterogeneity; many patients are compound heterozygotes (frequently with only one allele identified), whereas populations with high rates of consanguinity, e.g., Israeli Arabs (6) or Northwest Africans (7), exhibit mutations in a homozygous pattern. One-half of the patients exhibit no detectable AGT catalytic activity, whereas the other half exhibit residual (2 to 48%) AGT activity (2,4). Why do individuals with residual AGT activity become ill, not differing clinically from patients with absent AGT activity? AGT is mistargeted from the peroxisomes to the mitochondria, as elegantly demonstrated by Danpure and co-workers (2,4,5). Although such patients often exhibit considerable residual enzymatic activity, only 10% of the immunoreactive AGT (cross-reacting material—positive) is localized within the peroxisomes; 90% is found within the mitochondria, where it is metabolically inactive. In his laureate speech, Gu[Combining Diaeresis]nter Blobel, the 1999 recipient of the Nobel Prize in Medicine, referred specifically to PH1, in which the altered sorting signal leads to erroneous intracellular localization of the enzyme (http://www.nobel.se). This protein-trafficking defect is unparalleled in human subjects (2). It is observed in approximately 30% of all patients with PH1, usually as the result of a 630G→A mutation (associated with a Pro11Leu polymorphism), which leads to a Gly170Arg amino acid substitution, or, less often, as the result of a 576T→A mutation (2,8). Pathophysiology Urine is a saturated solution, and its concentration can change very drastically within a short time (9). Stone formation or the development of nephrocalcinosis thus occurs when the delicate interplay between promotors (in PH1, especially oxalate) and inhibitors (e.g., citrate, magnesium, and glycosaminoglycans) of crystal formation is disturbed (9,10). In PH1, the urine is supersaturated with respect to CaOx (urinary CaOx saturation of >10 relative units) (2,8,11). This produces renal calculi, medullary nephrocalcinosis, or both. Oxalate is freely filtered in the glomerulus and is both secreted and reabsorbed in the proximal tubule (2). Oxalate transport across proximal tubular cells is complex, because this anion plays a role as a recycling substrate that functionally links the transcellular absorption of chloride to that of other anions (bicarbonate and sulfate) (12). At the basolateral membrane, oxalate enters the cell in exchange for sulfate or bicarbonate, via Sat-1 (12). At the luminal brush border membrane, oxalate is transported out of the cell in exchange for chloride and is transported back into the cell in exchange for sulfate. The overall result is a net secretion rate of 10 to 30%, corresponding to fractional oxalate excretion (Cox/Cin) of 1.09 to 1.28 (2). With physiologic concentrations of oxalate, occasional crystals either are passed as crystalluria particles or are endocytosed by renal epithelial cells (13). Endocytosed crystals are eliminated or are exocytosed to the basolateral side of the cells. From there, the crystals migrate to the interstitium, where they may eventually be destroyed by local inflammatory reactions involving macrophages (14). In PH1, far more oxalate is filtered in the glomeruli than under normal conditions, leading to extremely high oxalate concentrations within the proximal tubular cells. Until recently, these high oxalate concentrations were not considered to be harmful themselves, apart from the risk of CaOx deposit formation in the renal interstitium, followed by foreign-body reactions. However, it is now well established that high oxalate levels have direct toxic effects on renal tubular cells (15). Oxalate reduces both the growth rate and life span of LLC-PK1 cells (which resemble proximal tubular cells), in a concentration-dependent manner (15,16). Whereas oxalate acts as a mitogen at low concentrations, it is a toxic agent at high concentrations (16). These negative effects at the cellular level resemble those observed in other tissues after oxidative stress (15). Indeed, oxalate seems to promote the production of free radicals, which may explain its cellular toxicity (15,17). Because such effects are clearly concentration-dependent, this mechanism may directly contribute to rapid deterioration of renal function in PH1, which is much greater than that observed with nephrocalcinosis of other origins. In addition, the high plasma oxalate levels of patients with PH1 and renal insufficiency may exert toxic effects on other organs and tissues (18). Both plasma oxalate levels and plasma CaOx saturation values are significantly higher for patients with PH1 than for normal control subjects, even with normal renal function (19,20,21). The two parameters increase concomitantly (19,20,21) and are inversely correlated with the GFR (19). Plasma CaOx supersaturation (plasma CaOx saturation of >1), with plasma oxalate levels of >30 μM, is observed with a GFR of <45 ml/min per 1.73 m2, in contrast to non-PH1 patients, for whom supersaturation occurs only with a GFR of <8 ml/min per 1.73m2 (18,19,21). CaOx crystal deposition therefore begins early in PH1, and patients are at risk of systemic CaOx deposition before the stage of chronic renal failure (19). Diagnosis Unfortunately, the diagnosis of PH1 is often overlooked or delayed; therefore, different diagnostic procedures are briefly discussed here. Analysis of Urine. Hyperoxaluria is the hallmark of PH1. Urinary oxalate excretion is usually greatly elevated among patients with PH1 and recurrent urolithiasis and/or nephrocalcinosis, exceeding 2 mmol/24 h per 1.73 m2 and sometimes even 4 mmol/24 h per 1.73 m2 (normal, <0.5 mmol/24 h per 1.73 m2 or <45 mg/24 h per 1.73 m2) (2,8). However, family studies have demonstrated that some untreated patients with PH1 may exhibit only slightly elevated (0.5 to 1 mmol/24 h per 1.73 m2) or even normal urinary oxalate excretion (22). Frequent sources of error are (1) incorrect urine collection (acidification to pH <2 is required to prevent deposition of insoluble CaOx), (2) failure to convert 24-h oxalate excretion values for pediatric patients to the adult surface area (1.73 m2) or to consult age-related tables for oxalate/creatinine ratios (8,10), and (3) renal insufficiency associated with oxalate retention and reduced urinary excretion. In such cases, plasma oxalate determinations may be helpful. Glycolate levels are elevated in only approximately two-thirds of patients with PH1; therefore, findings of normal values (which are in the same range as those for oxalate, in milligrams or millimoles) do not exclude this diagnosis (2,8). Plasma Oxalate Measurement. In patients with PH1, plasma oxalate levels, which are already elevated (>6.3 μM) with normal renal function, are significantly higher (>80 to 100 μM) in ESRF, compared with those for patients without PH1 (40 to 60 μM) (18,19,21). Liver Biopsy Assessment. A definitive diagnosis (which is essential if liver transplantation is being considered) requires assessments of AGT activity and immunoreactivity in hepatic tissue (minimum of 2 mg), unless the diagnosis has been established at the molecular level (2,8). Testing for both PH1 and PH2 can be performed with the same needle-biopsy specimen of the liver (23). DNA Analysis. Patients with PH1 are frequently compound heterozygotes, and the mutation on the second allele often remains unidentified, which renders DNA analysis impractical as a diagnostic procedure (2,8). However, selective screening among populations with high incidences of specific homozygous mutations (e.g., Ile244Thr in North African patients) would be feasible (7). Prenatal diagnosis can be performed by linkage analysis using chorionic villous biopsy samples, if the family has been demonstrated to be informative for the linkage marker (24). Genetic counseling must take into account, however, the fact that family members carrying identical mutations may exhibit grossly discordant clinical features (22). Stone Analysis. The calculi of patients with PH1 consist almost exclusively of pure CaOx monohydrate (whewellite); this finding may yield a diagnostic clue (25,26). Microscopic analysis reveals a characteristic subtype (subtype 1c) that is virtually pathognomonic for PH1 (25). Bone biopsies for patients with renal failure, to demonstrate birefringent CaOx crystals, were previously used for diagnosis. Eyeground examinations are diagnostically important (Figure 2).Figure 2: . Clinical findings of calcium oxalate (CaOx) deposits in PH1. (A) X-ray (plain film of the abdomen), demonstrating multiple kidney stones (some with a staghorn appearance) in a 7-yr-old patient with PH1 and renal failure. A double-J catheter was placed in the right ureter. Initial oxalate osteopathy, with cystic lesions in both femoral necks and pelvic bones, can also be observed. (B) Ultrasonogram (right kidney), demonstrating a “white” kidney, attributable to diffuse nephrocalcinosis, in a 3-mo-old patient with renal failure. (C) Eyeground examination image (6-yr-old patient), demonstrating numerous pitch-black foci (consisting of multiple layers of hyperplastic and hypertrophic retinal pigment epithelium) and multiple CaOx crystals in the inner retina.Clinical Manifestations Recurrent urolithiasis and nephrocalcinosis are the main symptoms, and the combination of the two conditions, leading to progressive loss of renal function, is characteristic for PH1 (Figure 2). One-half of the patients exhibit their first symptoms by the age of 5 yr (range, 1st month of life to 6th decade) (27,28). Diagnosis is usually delayed by ≥5 yr, except among infants (28). Although PH1 is a monogenic disease, the clinical severity is not correlated with the mutation or the degree of residual functional AGT activity (2,4,22). The clinical, biochemical, and genetic heterogeneity is very large, with some patients presenting in infancy with renal failure and others experiencing only occasional passage of stones in adult life, with maintained renal function (2,4,22,27,28). Family screening has demonstrated that some patients are completely without symptoms (with neither nephrocalcinosis nor stones) (22,28). Even members of the same family, with identical mutations, may exhibit completely different clinical phenotypes, e.g., severe infantile oxalosis, compared with absent clinical findings (22). In a malignant variant (infantile PH1), the first symptoms occur very early (median 4 and infants at a age of with the of a failure to severe metabolic and all of which are secondary to renal failure This infantile is by rapid to and severe systemic Why these infants diffuse nephrocalcinosis (Figure not urolithiasis is not Oxalate is when the saturation for plasma oxalate (plasma oxalate levels of >30 μM) is i.e., early in renal insufficiency (19). occurs in every and tissue except the liver and leads to which be by all The are the most of CaOx The oxalate in is higher to than that among patients without PH1 (2 to The lesions are characteristic both in a and diffuse with a and in assessments of CaOx and Clinical of oxalate are and CaOx which are to may be one of the first of systemic (Figure important of CaOx deposits are the of the (with and the the the and the and Because the diagnosis is often delayed or overlooked the of PH1 is from the and that 1 in to has PH1 The disease is far more in such as where PH1 is the of for of pediatric patients, compared with those treated for in North and Clinical on PH1 are of patients were experiencing by the age of yr This finding with from performed in and which demonstrated that in of patients by the age of yr and in of patients by the age of yr (27,28). However, infantile has a particularly with of the patients experiencing at the time of diagnosis and by the age of yr The of for patients is very if not Indeed, almost no renal disease is more in this Patients with PH1 with GFR of ml/min may remain in for many may and residual function of urinary or the for patients with PH1 to be if the disease is than in the The of are to oxalate production and to increase the urinary of The of and of to of be is an essential for such as AGT, and of from 5 to per to urinary oxalate are to significantly at hyperoxaluria in of patients The patients most to are those with residual AGT activity of values of for urinary oxalate excretion are A of not less than is of plasma oxalate levels is in renal insufficiency although the effects are within 1 to 2 in most infants may exhibit different Indeed, a (with a 630G→A who to to high of in 2 a within 1 yr high are and may to even low of may is at the molecular level (2). to the glyoxylate or glycolate have not been of of the of CaOx is The most important and is to a intake surface area per to urinary oxalate concentrations exceeding at most untreated patients with PH1 often exhibit values well 1 this may via a or In addition, that increase the urinary are of to 60 per has been demonstrated to stone formation and CaOx deposition and the same is for not only calcium also is an of CaOx crystal and growth (9). or a of and to per in or Although is and clinical for the effects of this treatment it is to its because of the sometimes of PH1. is of CaOx because it of to oxalate. a combination of and might the Oxalate absorption studies using a that intestinal absorption is in subjects However, the of oxalate to the oxalate in the urine is to in individuals and exhibits to who the intestinal exhibit greater oxalate absorption However, for patients with oxalate of to hyperoxaluria, because of their extremely high levels of endogenous oxalate It is therefore to with very high oxalate and is not because less intestinal calcium would be to oxalate and to insoluble CaOx in the has no because glycine, the of glyoxylate (Figure is not an essential amino is not although acid of oxalate) to be used with and only after because it may the of patients with PH1 and nephrocalcinosis in nor is to with the endogenous production rate of oxalate, much less the oxalate In the oxalate of renal to per 1.73 m2 surface only the endogenous oxalate production of 2 to CaOx and CaOx crystals are in other organs in to the Although the oxalate is greater with than with oxalate values are for the two renal even the combination of the two or the of or is to prevent oxalate retention plasma oxalate levels and CaOx saturation values remain extremely elevated or increase even (18). with or may be until transplantation is Except among the patients who well to recurrent nephrocalcinosis and stone formation are to risk are or and of The results of kidney performed in in the were very with rates of only for and for patients This treatment has thus been there, in of In kidney transplantation with is in the The results were with rates of for patients and for however, the rate was only The first liver for a patient with PH1 was performed in before the enzymatic defect been identified Because the metabolic defect is in the it is to a although the liver is normal in every other respect (2). liver transplantation is not an because it would not oxalate overproduction by the liver have been performed in to the rates in patients) were for patients and for liver renal function has with rates of to 60 ml/min per 1.73 m2 after 5 risk are age and of the renal begins to function, is no to or of of and treatment with the first are because of the of the of CaOx In both plasma oxalate levels and plasma CaOx saturation as well as urinary oxalate remain elevated for or even after transplantation In increased levels among patients without PH are within after transplantation Liver The for liver of until seems Indeed, if liver transplantation were a procedure and were no of transplantation would be an treatment for many patients with PH1. However, the of loss or even the of a patient who might have without is the for with that may residual renal function and the of renal more than a patients with PH1 who were not experiencing renal failure have been treated in this with results However, some of the patients might have maintained their renal function without The the of because the of the disease is to liver transplantation can be only if renal failure is not far the GFR is or even to with plasma oxalate levels of the GFR is ml/min per 1.73 m2, is a high risk of rapid secondary kidney transplantation the overall results of liver transplantation be better with better residual renal However, with better of renal function, the and become with respect to the of such a in among and in for infants with the malignant variant of PH1 Because of their such patients for which is leading to progressive systemic oxalosis, until transplantation exhibit a high of PH1 the and for transplantation Because is not a treatment for PH1, except for very in both is an that questions for via liver transplantation might be considered an of gene must be before gene in the to liver transplantation (which not because oxalate is in gene would of far more than or 30% of liver which is to with the level of AGT per cell by using AGT this would not yield higher overall the of cells gene even with such would be to for the endogenous oxalate production by cells is a of The of often such gene would to be is also unresolved. PH2 PH2 is a with and is to be Clinical are not as severe as in PH1 and consist of urolithiasis. is in of and systemic is The age at is occur has not been observed in the pediatric age The of PH2 are high levels of urinary excretion of oxalate and acid however, normal acid excretion was observed in one PH2 results from the deficiency of a enzyme with glyoxylate hydroxypyruvate and (Figure 1) The tissue concentration of glyoxylate reductase is high in the liver low in the and (23). of PH2 can be by of the glyoxylate reductase activity in liver biopsy (23). The was to the of chromosome and spanning kb mutations have been As for PH1, treatment of high of and of is no for the of transplantation to because hyperoxaluria and elevated excretion More are required before liver transplantation can be PH As has been the for other metabolic the of enzyme has clearly demonstrated that PH more than PH1 and There have been of pediatric patients, for whom both PH1 and PH2 were and who not of secondary hyperoxaluria, who early 2 of urolithiasis attributable to hyperoxaluria, sometimes with this be There is considerable that this is and (e.g., and has been in and the molecular genetic and enzyme diagnostic have been established for PH1 and However, considerable is required to explain the between and in PH1 and to the metabolic defects in PH. diagnosis and treatment are the for patients with PH1 and renal The fact that no of is to prevent progressive systemic in is The and results have been with which the metabolic in PH also the of patients with secondary a also by the and Hyperoxaluria