Primary Hyperoxaluria

The primary hyperoxalurias are autosomal recessive disorders of glyoxylate metabolism characterized by excessive production and urinary excretion of oxalate and glycolate (Primary Hyperoxaluria type II, PH2)

The glyoxylate pathway in the human hepatocyte. Alanine:glyoxylate aminotransferase (AGT). Glyoxylate reductase: hydroxypyruvate reductase (GR/HPR). Glycolate oxidase (GO). Lactate dehydrogenase (LDH). D-amino acid oxidase (DAO). Pyridoxal phosphate (PLP). Deficiency or mistargeting of AGT (PHI) results in buildup of glyoxylate and increased oxalate production. Deficiency of a protein with dual GR and HPR activities (PHII) gives rise to increased hydroxypyruvate and glyoxylate, precursors of L-glycerate and oxalate, respectively.

The term "primary hyperoxaluria" was first used by Archer and colleagues in 1957 to specifically denote a suspected metabolic origin for the marked hyperoxaluria, recurrent urolithiasis and renal and extra-renal calcium oxalate crystal deposition that characterized affected children. The urine oxalate excretion rate in affected patients is typically 3 to 6 times normal with severe clinical consequences. Kidney stones and/or calcification of the kidney occur in childhood or adolescence. Renal injury due to oxalate and consequences of the stones often leads to renal failure. Loss of renal function, if not addressed promptly by transplantation, leads to markedly increased plasma concentrations of oxalate with deposition of calcium oxalate in body tissues. Resulting organ system dysfunction including ischemic ulcers of the skin, metabolic bone disease, refractory anemia, cardiomyopathy, and cardiac conduction system abnormalities are the cause of severe morbidity and mortality. Historically, the median age at death was only 36 years.

These rare diseases (PH1 and PH2) can be caused by defects in at least 2 glyoxylate-metabolizing enzymes (see Figure 1). Since pyridoxine (vitamin B6) is a cofactor for the causative enzyme in PH1, administration of this vitamin can reduce urine oxalate levels in some PH1 patients. Recently, a third group of patients has been identified with an as-yet-unknown genetic cause of hyperoxaluria. Untreated, PH patient outcome is often poor, with death from renal failure and systemic oxalosis the norm. However, there is wide variability in outcome amongst patients, and with careful and aggressive treatment patient survival with preserved renal function to middle age (or older) is possible. The important factors that influence improved patient survival are currently poorly understood.

However, in all cases, care by a dedicated physician who is familiar with treatment of PH is essential to minimize complications, and maximize quality of life. Treatment strategies include careful dietary advice to minimize oxalate ingestion and maximize fluid intake, carefully titrated doses of pyridoxine for those patients in whom it is effective and neutral phosphate and/or citrate to reduce urinary saturation with calcium oxalate. Renal function must be monitored vigilantly and renal replacement therapy should be initiated promptly if renal clearance falls below a critical threshold, in order to prevent body-wide deposition of calcium oxalate. Kidney transplantation alone or combined kidney-liver transplantation is clearly the preferred treatment of renal failure for PH patients. Current evidence is split regarding which patients benefit the most from the riskier combined kidney-liver transplant. If transplantation is not possible, patients must be aggressively dialyzed, often 6 or 7 days per week and/or using a combination of modalities, in order to remove enough oxalate to prevent body-wide oxalosis.

Primary hyperoxaluria (PH) is the most severe of the hereditary causes of nephrolithiasis. Enzyme deficiency in the liver results in marked overproduction of oxalate that must be excreted by the kidneys. Oxalate in the urine in high concentrations is poorly insoluble when combined with calcium and thus leads to calcium oxalate crystals and stones.

Urinary tract stones cause pain, and urologic procedures are often required for their removal. Calcium oxalate crystals are also found in the parenchyma of the kidney. Damage to surrounding tissue over time results in renal failure in nearly all PH patients. As kidney function declines and excess oxalate can no longer be eliminated effectively, blood oxalate concentrations also rise, and calcium oxalate deposits in multiple organs and tissues (systemic oxalosis). Calcium oxalate in bone results in fracturing bone disease and erythropoietin resistant anemia due to replacement of marrow by oxalate. Painful ischemic ulcers of extremities occur due to involvement of blood vessels, cardiomyopathy due to deposition in myocardium, and fatal cardiac arrythmias are the consequence of deposition in the cardiac conduction system, among other problems. In the majority of patients, kidney transplantation alone is not sufficient, since the transplanted kidney is damaged by ongoing high oxalate production. Liver and kidney transplantation must be performed. Disease expression, however, varies widely. Some PH patients reach end stage renal failure within the first year of life, while others maintain satisfactory kidney function until well into middle age. This variability has not been satisfactorily explained and is only partially influenced by genotype or other factors such as degree of hyperoxaluria. Modifiers of disease expression, if identified, offer promise as potential new strategies for treatment.

All the pathological sequelae of the primary hyperoxalurias are related to the increased synthesis and excretion of oxalate. Marked hyperoxaluria is present from birth on, with 2 to 8 times the upper limit of normal urine oxalate being characteristic. Blood in the urine or pain related to stones, stone passage, or urinary tract infection are the most common symptoms of the disease. Over time, frequent stone recurrences and the need for multiple stone removal procedures are typical. The majority of patients are symptomatic before 10 years of age. In some cases, however, the disease may go unrecognized either due to the absence of symptoms or to incorrect diagnosis, until patients reach 30 to 50 years of age. Oxalate at high concentrations with calcium in the urine forms crystals that form in the urinary tract leading to stones, and also deposit in kidney tissue causing nephrocalcinosis. Calcium oxalate crystals are also directly injurious to renal cells and incite a granulomatous reaction in the renal interstitium. Over time the effects of such injury, often combined with intermittent obstruction or infection related to stones, lead to kidney failure.

Earlier literature showed that about 50% of patients developed kidney failure by 15 years of age, and about80% developed kidney failure by age 30. With improved diagnosis and management, more recent information shows the median age at renal failure to be 33 years. However, some patients present with kidney failure as the first manifestation of the disease, as early as 4 months of age. Patients with type 2 disease appear to have a milder course overall than those with type 1, including better preservation of renal function. The reasons for such variation in clinical expression are poorly understood, and if elucidated may provide valuable insights as to potentially remediable factors that can be exploited for therapeutic benefit. Once renal function declines to less than 30-40 ml/min/1.73 m2, plasma oxalate concentration rises, exceeding the supersaturation threshold for calcium oxalate. Systemic oxalosis with associated morbidity and death result. Transplantation is required for satisfactory long term outcomes. Early diagnosis of primary hyperoxaluria is of vital importance so that treatment can be initiated as soon as possible. Yet, due to lack of familiarity with the disease, delays of many years from onset of symptoms to diagnosis are common. Among patients in the IPHR, the mean time from symptom onset to diagnosis was 5.6 years (0.0, 6.5 25th, 75th%ile). Nineteen percent were first recognized to have PH only after developing irreversible renal failure.

The only definitive treatment for type 1 PH is liver transplantation to replace the missing AGT enzyme. However, doing so requires complete removal of the patient’s otherwise normal liver, exposing the patient to the risks of the transplant procedure, as well as the risks of life-long immune suppression. In 30-50% of PH1 patients, reduction in urine oxalate excretion can be achieved by treatment with pharmacologic doses of pyridoxine, suggesting enhancement of AGT enzyme activity. As shown by work from our IPHR, this effect is specific to certain mutations. The pyridoxine effect is partial when there is heterozygosity for these mutations, but complete normalization of urine oxalate occurs with homozygosity. AGT requires pyridoxal phosphate as a co-factor. Though the mechanism of its effect in PH1 patients remains to be proven, in vitro studies demonstrate molecular stabilization of mutant enzyme by chaperones such as pyridoxal phosphate and betaine. Other treatment measures, directed to reduction in crystal and stone formation, include high fluid intake to reduce urine oxalate concentration, and oral medications that inhibit calcium oxalate crystal formation, specifically citrate and phosphate. All of these modalities have been in use for more than 20 years. Though long term outcomes have improved with earlier diagnosis and currently available treatment, renal failure nonetheless still develops in 70% of patients by 60 years of age. More effective treatments are urgently needed. Promising new directions using molecular chaperones, oxalate degrading bacteria, and exploitation of oxalate transport physiology are in various stages of investigation.

Clinical Trials

1. Correlation of Disease Expression with Specific Genetic Mutations in Primary Hyperoxaluria

   This protocol will demonstrate that certain mutations predispose individuals with hyperoxaluria to more severe disease.

   Oxalate excretion is in part genetically determined. To date, mutations in two separate loci are known to cause hyperoxaluria. Primary hyperoxaluria types 1 (OMIM 259900) and 2 (OMIM 260000), result from mutations in AGXT (2q36-q37) and GR/HPR (9q13-q21), respectively. An influence from other genetic loci is suspected, however, based on a wide phenotypic spectrum of disease expression. To date, over 100 mutations have been catalogued in AGXT and GR-HPR. Different mutations appear to have widely differing effects on AGT and GR-HPR protein quantity and activity. Attempts to correlate specific genetic mutations with disease phenotype have been disappointing, however. There are also many polymorphisms in both genes, some of which are functional, further complicating correlations between genotype and phenotype.. Because many mutations are private or family-specific, segregation analysis is often needed, requiring genotyping in family members.

2. Efficacy of Betaine for Reduction of Urine Oxalate in Patients with Type I Primary Hyperoxaluria

   In this proposal, we will specifically assess the clinical efficacy of betaine for the treatment of partially VB6 responsive and VB6 refractory PHI patients. Through its chemical chaperone role in enhancing AGT enzymatic activity, betaine will effectively reduce urine oxalate excretion in patients harboring the I244T mutation or other missense sequence changes predicted or shown to result in protein aggregation, degradation or mis-trafficking. If effective, betaine could represent a new and safe treatment option for a subset of PHI patients, particularly those with either partially VB6 responsive or VB6 refractory hyperoxaluria, or those with adverse effects such as peripheral neuropathy from large doses of VB6. The primary end-point will be changes in urinary oxalate excretion.

3. Evaluating Oxalobacter formigenes in Primary Hyperoxaluria (PH)

   A research trial to determine the effectiveness and safety of Oxalobacter formigenes treatment in patients with PH Type I and II has started. Patient enrollment has started in the Netherlands, UK and Germany and will be initiated in France and US after Ethics/IRB approvals have been obtained. This trial is being conducted by OxThera, a clinical stage biotechnology company focused on development of treatments for Primary and Secondary hyperoxaluria. Oxalobacter formigenes is a unique oxalate degrading bacteria normally present in the human gut. This bacterium can use only "oxalate" as its food and therefore it efficiently degrades all the oxalate it can find to obtain energy for its survival. When Oxalobacter formigenes is taken the oxalate degrading bacteria are released into the intestine where they continuously degrade oxalate. This excessive break down of oxalate in the intestine may help to pull out the extra oxalate being made by the liver of the PH patients. If the bacterium can do this, less oxalate will be going through the kidneys into the urine.

   Participants in this study will have an equal chance of receiving Oxalobacter formigenes or a placebo. The duration of the study is approximately 6 months and the study medication is a capsule given orally twice a day every day. Tests of your eligibility will be conducted at the study center. You should always consult your doctor before participating in a clinical trial.

4. Investigations into the Genotype and Phenotype of Unclassified Hyperoxaluria: Enteric Oxalate Absorption Study

   This study will measure urinary oxalate excretion using 13C-labelled oral oxalate loads. Flavored gelatin (children) or a capsule (adults or children) containing 13C-labelled (a stable isotope) oxalic acid will be ingested by each subject. Urine specimens will be analyzed for 13C oxalate by gas chromatography/mass spectroscopy (GC/MS). The utility of using urine oxalate as an indicator of oxalate absorption is based on the finding that > 90% of an absorbed dose of radioisotope labeled oxalate can be recovered from the urine in 24-36 hours. Most of the oxalate excretion after administration of 13C-labelled oxalate occurs within the first 6 hours after ingestions. We anticipate that the cause of oxalate excess in the patients is due to either intestinal factors or secondary to metabolic overproduction. The degree of hyperoxaluria that has generally been observed in patients with malabsorption states is in the range of < 1 mmol/1.73 m2/24 hrs (oxalate excretion rate in normal adults is between 0.11 and 0.4 mmol/1.73m2/24 hrs via the oxalate oxidase method). Based on this information, and the marked hyperoxaluria of greater than 1 mmol/1.73m2/24 hrs, we anticipate enteric oxalate absorption to be normal in patients with unclassified primary hyperoxaluria.

Basic Research

Ongoing Primary Hyperoxaluria research includes multidisciplinary collaboration with investigators in Nephrology, Urology, Radiology, Laboratory Medicine and the basic sciences:

• Determination of specific genetic mutations in primary hyperoxaluria patients, and correlation with disease outcomes
• Development of newer radiology techniques in order to measure renal calcium content and stones
• Development of better rat models of hyperoxaluria
• Study of the mechanism(s) by which oxalate changes cell function
• Development of a Tissue Bank for urine, plasma, whole blood and liver samples collected from patients to facilitate investigation and collaborative research.

REGISTRY MISSION STATEMENT

The International Registry for Calcium Stone Diseases is a vehicle through which the international scientific community can pool information regarding these rare diseases, in order to collectively advance knowledge, and ultimately improve the quality of life of affected individuals and their families.

The Registry Database Study design is that of an observational cohort, following patients forward in time from the date of enrollment in the Registry. Limited data will also be collected regarding patient status at time of diagnosis, which may be several years earlier. It is expected that patients will be evaluated approximately annually, though this cannot be mandated. As enrollment in the Registry is voluntary, it will not be possible to estimate disease prevalence or incidence. However, it is hoped that sufficient numbers of representative patients will be enrolled to allow us to characterize the disease(s) and describe its natural history.

Registry date can be collected and entered either by physician participation in the Registry or by patient cooperation with Registry staff. Physicians engaged in the management of patients with PH complete a web-based on-line enrollment form with information on their name, specialty, type of practice, and contact information. In order to enter patients the participating physician will be responsible for securing Institutional Review Board (IRB) or Independent Ethics Committee (IEC) approval for the research, and a patient authorization form if required by the local IRB. To help in this, a sample Patient Authorization request form will be submitted to the physician upon receipt of physician enrollment. A copy of each signed research authorization, if required by the local IRB, should be retained at the enrolling institution. After the approvals are obtained, physicians may enroll patients into the Registry by completing secure web based patient enrollment forms. After a patient is enrolled, the referring physician will be sent an invitation letter for the patient to participate in other activities of the Registry. If the patient consents, kits will be sent for blood and urine samples, as appropriate. A patient who does not have a physician willing to complete the necessary steps to enrollment can participate by providing his/her medical records to the Mayo Clinic staff, and signing a consent for Registry participation. Mayo staff will then enter the medical information in the Registry. Patient enrollment forms are completed by an enrolled physician or his/her designated staff following patient authorization and IRB approval, as required by the physician’s site. These have been converted to a Web-based format. A short initial form is completed for each patient to confirm eligibility. If the patient meets eligibility requirements, a unique study number is assigned and a registration form is completed to include the patient’s authorization of participation and demographic information. Thereafter, the patient is identified only by his/her unique alphanumeric code. An initial enrollment form for each patient establishes specific findings at diagnosis, and key features of the clinical course to that date. A current status form is then completed. The current status form will be updated annually. A confirmation of patient enrollment is sent on satisfactory receipt of patient enrollment form and will include a unique Registry identification number, coded patient initials, gender and date of birth. For each subject, a unique identification number is provided by the Registry. Following assignment of the unique patient identifier (letters and number), in order to best protect patient confidentiality, only this identifier will be used in subsequent communications with the physician or study center.

Your participation is very important for building a database of information to assist in developing therapeutic strategies for Primary Hyperoxaluria. It will help to outline more about disease expression, and also about diagnosis, therapy, transplantation procedures and outcomes. A study coordinator is available to answer questions or help in any way with data entry. To contact the study coordinator please call (800) 270-4637 OR E-mail to hyperoxaluriacenter@mayo.edu

• Mayo Clinic Hyperoxaluria Center is a resource for patients and their families and physicians. The center facilitates collaborative research to provide better understanding of this disorder. Write hyperoxaluriacenter@mayo.edu or call 800-270-4637
• Oxalosis and Hyperoxaluria Foundation (OHF)
  This is the only organization in the world dedicated to improving the care and treatment and finding a cure for Oxalosis, PH and related stone diseases
• International Registry for Hereditary Calcium Stone Diseases
  The purpose of this registry is to identify worldwide as many affected individuals as possible, and to collect as much clinical information about these patients as is feasible.