Primary hyperoxalurias (PHs) are a group of rare genetic metabolic disorders that are characterized by the accumulation of a substance known as oxalate in the kidneys and other organ systems of the body. Affected individuals lack functional levels of a specific enzyme that normally prevents the accumulation of oxalate. There are three main types of PH differentiated by the specific enzyme that is deficient. In the kidneys, excess oxalate binds with calcium to form a hard compound (calcium oxalate) that is the main component of kidney and urinary stones. Common symptoms include the formation of stones throughout the urinary tract (urolithiasis) and kidneys (nephrolithiasis) and progressively increased levels of calcium in the kidneys (nephrocalcinosis), although this last finding has not been identified in individuals with PH type III as of yet. Chronic, recurrent stone formation and the accumulation of calcium oxalate in kidney tissue can cause chronic kidney disease, which can ultimately progress to kidney failure (end stage renal disease [ESRD]). Eventually, kidney function can deteriorate to the point where oxalate begins to accumulate in other organ systems. Overall, the symptoms and severity of PH may vary greatly from one person to another. Chronic kidney disease and ESRD may already be present when a diagnosis is first made. PH is a treatable disorder and complications may be minimized with early recognition and prompt treatment.
PH type I is caused by mutations in the AGXT gene. PH type II is caused by mutations in the GRHPR gene. PH type III is caused by mutations in the HOGA gene (formerly known as the DHDPSL gene). The genetic mutations that cause PH are inherited as autosomal recessive traits.
Although these disorders have been established as clear syndromes with characteristic or “core” symptoms, some aspects of these disorders are still not fully understood. Several factors including the small number of identified cases (especially with PH types II and III), the lack of large clinical studies, and the possibility of other genes influencing the disorders prevent physicians from developing a complete picture of associated symptoms and prognosis. Parents should talk to their children’s physician and medical team about their specific case, associated symptoms and overall prognosis.
The age of onset, progression, severity and specific symptoms that develop can vary greatly from one person to another, even in individuals with the same subtype and even among members of the same family. Some individuals may have mild cases that go undiagnosed well into adulthood; others may develop severe complications during infancy. In some individuals kidney involvement may progress slowly, while in others it may progress rapidly.
PRIMARY HYPEROXALURIA TYPE I
PH type I is generally broken down into different forms. The severe infantile form is associated with the failure to gain weight and grow at the expected rate for age and gender (failure to thrive), increased calcium levels in the kidneys, and/or kidney stones or stones elsewhere in the urinary tract such as the bladder or urethra. Kidney and urinary stones can cause a variety of symptoms including blood in the urine (hematuria), painful urination (dysuria), the urge to urinate often, abdominal pain (renal colic), blockage of the urinary tract, and repeated urinary tract infections. PH type I causes progressive kidney damage and eventually results in early end-stage renal failure.
When PH type I develops during childhood or adolescence, the disorder is usually characterized by recurrent stones in the kidney or elsewhere in the urinary tract such as the bladder or urethra. Younger children may experience difficulty controlling their urine and/or bedwetting (enuresis). Progressive kidney damage leading to kidney failure may develop.
Some individuals are not diagnosed with PH type I until adulthood and only experience occasional or recurrent episodes of kidney stones. In some cases, these individuals may develop kidney failure due to the obstruction of the kidneys by stones. Although usually described as a mild form of the disorder, approximately 20-50% of individuals diagnosed with PH type I in adulthood have advanced kidney disease or even end-stage renal disease. In rare cases, a diagnosis may not be made until chronic kidney disease recurs after a kidney transplant.
As kidney function declines, oxalate begins to accumulate in other organ systems of the body particularly bone, skin, retinas, the middle layer of the wall of the heart (myocardium), various blood vessels, and the central nervous system. Depending upon the organ system involved, affected individuals will develop additional symptoms including bone pain, multiple fractures, abnormal hardening and density of bone (osteosclerosis), and anemia that is difficult to treat (erythropoietin-resistant anemia); degeneration of the cranial nerve that transmit lights signals to the brain (optic atrophy) and disease of the retina (retinopathy); root resorption, pulp exposure, tooth mobility, and dental pain; damage to the nerves outside of the central nervous system (peripheral neuropathy); heart block, irregular heartbeats (arrhythmias), inflammation of the myocardium (myocarditis), and cardioembolic stroke; narrowing of a blood vessel due to spasms of the vessel (vasospasm); joint disease (arthropathy); enlargement of the liver and/or spleen (hepatosplenomegaly); and purplish mottling of the skin (livedo reticularis), tissue death (necrosis) on the hand and feet (peripheral gangrene); and a skin rash caused by calcium deposits in the skin (calcinosis cutis metastatica).
PRIMARY HYPEROXALURIA TYPE II
PH type II usually presents during childhood and the disorder is more likely to have a milder presentation than PH type I. Affected individuals can develop similar symptoms to those seen in individuals with PH type I, but usually develop kidney and urinary stones less often. PH type II can eventually progress to cause end-stage renal disease, although this usually happens later than it does in PH type 1. When kidney function declines, the accumulation of oxalate in other organ systems of the body as described above may also occur.
PRIMARY OXALURIA TYPE III
Because so few cases of PH type III have been identified, it is difficult to make definite statements about disease severity and progression. The disorder is considered milder than PH types I or II. Affected individuals may have no symptoms or may only experience kidney stone formation. In some cases, the disorder may improve over time. Nephrocalcinosis and chronic kidney disease are uncommon. Advanced kidney disease has not been reported in this disorder.
PH type I is caused by mutations in the AGXT gene. PH type II is caused by mutations in the GRHPR gene. PH type III is caused by mutations in the HOGA1 gene. Genes provide instructions for creating proteins that play a critical role in many functions of the body. When a mutation of a gene occurs, the protein product may be faulty, inefficient, or absent. Depending upon the functions of the particular protein, this can affect many organ systems of the body.
Genetic diseases are determined by the combination of genes for a particular trait that are on the chromosomes received from the father and the mother. Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%. The risk is the same for males and females for genes that are not carried on either the X or Y chromosomes.
All individuals carry a few abnormal genes. Parents who are close relatives (consanguineous) have a higher chance than unrelated parents to both carry the same abnormal gene, which increases the risk to have children with a recessive genetic disorder. PH appears to be more common in countries in which consanguineous marriage is common.
Investigators have determined that the AGXT gene is located on the long arm (q) of chromosome 2 (2q37.3). The GRHPR gene is located on the short arm (p) of chromosome 9 (9p13.2). The HOGA1 gene is located on the long arm of chromosome 10 (10q24.2). Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated “p” and a long arm designated “q”. Chromosomes are further sub-divided into many bands that are numbered. For example, “chromosome 2q37.3” refers to band 37.3 on the long arm of chromosome 2. The numbered bands specify the location of the thousands of genes that are present on each chromosome.
The AGXT gene creates (encodes) the liver-specific peroxisomal enzyme alanine-glyoxylate aminotransferase. The GRHPR gene encodes the enzyme glyoxylate reductase-hydroxypyruvate reductase. Mutations in either of these genes lead to deficient levels of the corresponding enzyme. These enzymes play a role in regulating the production of oxalate. Deficient levels these enzymes ultimately lead to the overproduction of oxalate in the body. The HOGA1 gene encodes liver-specific mitochondrial enzyme 4-hydroxy-2-oxoglutarate aldolase. The exact role this enzyme plays in the production of oxalate is not fully understood. Researchers are not sure why a mutation in this gene leads to the overproduction of oxalate.
Oxalate is a chemical found in the body. It is a dicarboxylic acid that is a normal end product of metabolism. It cannot be further metabolized. Metabolism refers the various normal chemical processes that occur with a living organism. Most oxalate in the body is produced in the liver, although some may come from certain foods. Oxalate has no known role in the body and is considered a waste product of metabolism. Normally, most oxalate in the body (in the form of calcium salt) is removed from the body (excreted) through the kidneys. In individuals with PH, deficiency of the abovementioned enzymes results in the overproduction of oxalate by the liver, which causes increased oxalate excretion by the kidneys. Because of the excess amounts of oxalate, some of the chemical, in the form of calcium salt crystals, begins to accumulate in kidney tissue. This abnormal accumulation causes progressive damage to the kidneys and, if untreated, may ultimately cause end-stage renal disease.
A second phase of PH occurs when the glomerular filtration rate (GFR) of the kidneys drops far enough. The GFR is the flow rate of filtered fluid through the kidneys. When the GFR drops low enough, the kidneys are no longer able to handle the excess amounts of oxalate and the chemical begins to accumulate in other tissues of the body causing a wide variety of symptoms.
The progression and severity of PH is highly variable. This may be due, in part, to specific mutations in genes corresponding with specific symptoms or disease progression. The association of a specific mutation in a gene to specific symptoms is known as genotype-phenotype correlation. For example, some individuals with PH type 1 respond to treatment with vitamin B6 (pyridoxine) while others do not; this may be due to having a specific mutation of the AGXT gene. There have been more than 170 different mutations in the AGXT gene identified that cause PH type 1. No specific genotype-phenotype correlation has been established yet in the PHs, although some research suggests that specific mutations of the AGXT gene are associated with later onset end stage renal failure.
It is likely that other factors contribute to disease variability in PH. Such factors may include environmental factors and additional genetic factors (i.e., modifier genes). However, no specific environmental or additional genetic factors have been identified.
PH affects males and females in equal numbers. The exact incidence and prevalence of these disorders is unknown. Because some cases go undiagnosed or misdiagnosed, determining these disorders’ true frequency in the general population is difficult. PH type I is the most common form. One estimate places the prevalence of PH type I at 1-3 cases per 1,000,000 people in the general population and the incidence at 1 case per 120,000 live births per year in Europe.
A diagnosis of PH is based upon identification of characteristic symptoms (e.g. chronic stone formation), a detailed patient history, a thorough clinical evaluation and a variety of specialized tests. Children with high levels of calcium in the kidneys or kidney stones should be screened for PH. PH may be suspected in individuals with a history of recurrent kidney stones and/or nephrocalcinosis. Because of the disorders’ rarity, there may a delay from symptom onset to diagnosis.
Clinical Testing and Workup
Chemical analysis of urine samples may reveal elevated levels of oxalate, although this is a fluctuating and variable finding. Glycolate (glycolic acid) in PH type I and L-glycerate in PH type II may also be elevated in urine samples, but are nonspecific (i.e., they may be elevated for reasons other than PH). In some cases of PH type III, urinary calcium levels are abnormally high. Some individuals with PH type III have elevated levels of 4-hydroxyglutamate, which can be incorporated into multiple-analyte panel for newborn screening for inborn errors of metabolism. Blood tests can reveal high plasma oxalate concentration in individuals with PH who have advanced chronic kidney disease. Otherwise, plasma oxalate levels are usually normal.
X-ray examinations can reveal the presence of kidney stones or calcium oxalate deposits in tissue. Computed tomography (CT) scanning, a specialized imaging technique, uses a computer and x-rays to create a film showing cross-sectional images of certain tissue structures such as kidney tissue.
A biopsy of affected kidney tissue can also reveal the abnormal accumulation of oxalate. A biopsy involves the surgical removal and microscopic examination of a piece of affected tissue. A liver biopsy may be used to obtain a tissue sample to conduct an enzyme assay. An enzyme assay is a test that measures the activity of a specific enzyme. An assay can demonstrate low levels of the specific enzymes that are associated with the specific forms of PH.
Examination of kidney stones can differentiate PH from other disorders associated with kidney stone formation or from idiopathic kidney stone formation. The stones associated with PH tend to consist of more than 95% calcium oxalate monohydrate (whewellite), are usually pale-colored and may come varying sizes, shapes and appearances (non-homogeneous).
Molecular genetic testing can confirm a diagnosis of PH in some cases. Molecular genetic testing can detect mutations in specific genes known to cause PH, but is available only on a clinical basis.
In families where disease-causing mutations have been previously identified, diagnosis before birth (prenatal diagnosis) may be possible through amniocentesis or chorionic villus sampling (CVS). During amniocentesis, a sample of fluid that surrounds the developing fetus is removed and analyzed for the disease-causing mutation known to run in the family. During CVS, a tissue sample is removed from the placenta.
Such families may also consider pre-implantation genetic diagnosis (PGD). PGD is performed on embryos created through in vitro fertilization. PGD involves testing an embryo to determine whether the embryo has the same genetic abnormality as the parent.
Requests for prenatal testing for conditions that do not affect intellect and may have some treatment available are not common. Differences in perspective exist among medical professionals as well as within families regarding the use of such procedures. Families interested in prenatal testing or PGD should seek the counsel of a certified genetics professional.
The treatment of PH is directed toward the specific symptoms that are apparent in each individual. Treatment may require the coordinated efforts of a team of specialists. Pediatricians, surgeons, specialists who assess and treat problems of the kidneys (nephrologists), specialists who assess and treat problems of the liver (hepatologists), specialists who assess and treat problems of the urinary tract (urologists), dieticians, and other healthcare professionals may need to systematically and comprehensively plan an affect child’s treatment. Genetic counseling may be of benefit for affected individuals and their families. Psychosocial support for the entire family is essential as well.
Specific therapeutic procedures and interventions may vary, depending upon numerous factors, such as disease stage; specific subtype; responsiveness to pyridoxine; the presence or absence of certain symptoms; an individual’s age and general health; and/or other elements. Decisions concerning the use of particular drug regimens and/or other treatments should be made by physicians and other members of the health care team in careful consultation with the patient based upon the specifics of his or her case; a thorough discussion of the potential benefits and risks, including possible side effects and long-term effects; patient preference; and other appropriate factors.
Prompt diagnosis and early therapy are essential to slowing progression of the disorder and preserving kidney function as long as possible. Early conservative methods can reduce or prevent kidney stone formation. Adequate fluid intake can help to prevent the formation of kidney stones. Drinking large amounts of water prevents the accumulation of oxalate and flushes out the kidneys. Some infants and small children may require a procedure known as gastrostomy to ensure proper fluid intake and dilution of the urine. With this procedure, a thin tube is placed into the stomach via a small incision in the abdomen, allowing for the direct intake of food, fluids, and/or medicine.
Certain medications may be used to treat individuals with PH including potassium citrate, thiazides, or orthophosphates. These medications can prevent the crystallization of oxalate and calcium. Certain drugs that promote the production of urine, known as loop diuretics, should be avoided by individuals with PH. These drugs are specific diuretics that work on a structure within the kidney known as the loop of Henle and can cause calcium to build up in the urine, contributing to the formation of calcium oxalate stones.
Some individuals with PH type I (but not with other forms of PH) respond to dietary supplementation with pyridoxine, also known as vitamin B6. In such cases, pyridoxine supplementation leads to a reduction in oxalate levels. Not all individuals with PH type I respond to pyridoxine therapy. Pyridoxine is a metabolic precursor to a co-factor necessary for function of AGT, the protein affected in PH type I.
For individuals who experience repeated stone formation, sometimes referred to as a high stone burden, a procedure that uses shock waves (lithotripsy) to break up stones in the urinary tract and kidneys may be recommended. The most common type, extracorporeal shock wave lithotripsy, is not recommended for individuals with PH because it may damage surrounding tissue and is often ineffective in treating the recurrent stones associated with PH. Instead, minimally invasive methods such as ureteroscopic laser lithotripsy may be recommended. During this procedure, a small tube or scope is inserted into the bladder and the ureter (the tube through which urine passes from the kidneys to the bladder). If a stone is small, the scope can be used to remove the stone whole. If the stone is too large, the stone will be broken apart with a laser. The stone fragments are then removed.
Some physicians recommend dietary restriction of foods high in oxalate as a precautionary measure, although intestinal oxalate has a limited effect on disease progression. Foods high in oxalate include chocolate, rhubarb, and starfruit. Vitamin D and vitamin C should be avoided in large doses. Affected individuals should also avoid becoming dehydrated as maintaining dilute urine is extremely important in preventing stone formation.
If kidney function continues to decline, or in cases where an individual is first diagnosed with PH after the development of advanced kidney disease or end stage renal disease, additional more aggressive treatment will be required including dialysis, a liver transplant, a combined liver-kidney transplant or a kidney transplant. The specific therapy used will depend upon an individual’s specific case and requirements.
Dialysis may be used to treat individuals with PH. Dialysis is a procedure in which a machine is used to perform the kidney’s basic functions of fluid and waste removal. Dialysis can help clear oxalate from the body. However, dialysis (both conventional hemodialysis and peritoneal dialysis) fail to adequately remove enough oxalate to prevent oxalate accumulation. Dialysis may be used in specific situations or until a kidney transplant can be performed.
For some individuals, a liver transplantation may be recommended. Since the liver is the only organ that produces the enzyme that is deficient in PH type I, a new liver will restore production of the missing enzyme. A liver transplantation may be considered in individuals without advanced kidney disease (preemptive liver transplantation), but its use as an isolated procedure early in the course of the renal decline is controversial because the benefits must be weighed against a relatively high immediate post-operative mortality.
In some cases of PH type I, affected individuals may be recommended for a combined liver-kidney transplant if the kidneys are already too damaged (i.e. stage 4 chronic kidney disease). In other cases (stage 5 chronic kidney disease), sequential transplantation, the liver followed by the kidney, may be recommended.
An isolated kidney transplant may also be performed in individuals with PH type I, but has generally been replaced by preemptive liver transplantation or combined liver-kidney transplantation. Since the underlying defect in PH type I is in the liver, an isolated kidney transplant has a high level of recurrence of kidney disease.
PH type II may be treated by isolated kidney transplantation, which has been varyingly successful. Unlike PH type I, the enzyme that is deficient in PH type II is widespread throughout tissues of the body. Consequently, a combined liver-kidney transplant has not been used in individuals with PH type II. However, such an approach may have merit because more of the GRHPR enzyme is found in the liver than any other tissues of the body.
At this time, PH type III has not been associated with advanced kidney disease or end stage renal failure.
Gene therapy is also being studied as another approach to therapy for individuals with PH. In gene therapy, the defective gene present in a patient is replaced with a normal gene to enable the production of the active enzyme and prevent the development and progression of the disease in question. Given the permanent transfer of the normal gene, which is able to produce active enzyme at all sites of disease, this form of therapy is theoretically most likely to lead to a “cure.” However, at this time, there remain some technical difficulties to resolve before gene therapy can be advocated as a viable alternative approach.
Similar to gene therapy is cell therapy, in which liver cells (hepatocytes) are repopulated into the liver of an individual with PH type I. If successful, these normal or genetically-modified liver cells would restore the proper activity of the enzyme alanine-glyoxylate aminotransferase. However, as with gene therapy, significant technical difficulties remain to be resolved.
Researchers have speculated that the use of pharmacological chaperones may have a role in treating some individuals with PH. The AGT enzyme associated with PH type I may, in some cases, be capable of at least partial enzymatic activity but is misshapen (misfolded) becoming trapped within the cell or is broken down by cellular quality control processes. Researchers are studying drugs that may be able to stabilize and guide (chaperone) the defective AGT enzyme to the proper place. Researchers believe that these pharmacological chaperones can bind with receptor proteins preserving enough of the natural shape and function of the proteins that they do not become trapped within the cells or targeted by the quality control processes, and can travel to their proper destination and perform their intended function. More research is necessary to determine the long-term safety and effectiveness of these potential treatments for individuals with PH.
Probiotics, which are foods or dietary supplements that contain live bacteria that replace or complement beneficial bacteria normally found in the gastrointestinal tract, may have role in spurring intestinal oxalate excretion. Certain probiotics such as Oxalobacter formigenes break down oxalate, increase the excretion of oxalate and reduce the amount for intestinal absorption. Initial studies into such probiotics, however, have produced disappointing results.
A registry for hereditary calcium stone disorders has been set up at the Mayo Clinic. A registry is a special database that contains information about individuals with a specific disorder or group of conditions. The collection of data about rare disorders may enable researchers to increase the understanding of such disorders, expand the search for treatments, and accelerate clinical trials into specific treatment options. For more information, contact:
International Registry for Hereditary Calcium Stone Diseases
200 First Street SW
Rochester, MN 55905
Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. government funding, and some supported by private industry, are posted on this government web site.
For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:
Toll-free: (800) 411-1222
TTY: (866) 411-1010
For information about clinical trials sponsored by private sources, in the main, contact:
For more information about clinical trials conducted in Europe, contact: https://www.clinicaltrialsregister.eu/
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