Primary hyperoxaluria type 2 is a rare condition characterized by the overproduction of a substance called oxalate (also called oxalic acid). In the kidneys, the excess oxalate combines with calcium to form calcium oxalate, a hard compound that is the main component of kidney stones. Deposits of calcium oxalate can lead to kidney damage, kidney failure, and injury to other organs. Primary hyperoxaluria type 2 is caused by the shortage (deficiency) of an enzyme called glyoxylate reductase/hydroxypyruvate reductase (GRHPR) that normally prevents the buildup of oxalate. This enzyme shortage is caused by mutations in the GRHPR gene. Primary hyperoxaluria type 2 is inherited in an autosomal recessive pattern.

GRHPR mutations that cause this condition. These mutations either introduce signals that disrupt production of the glyoxylate reductase/hydroxypyruvate reductase enzyme or alter its structure. As a result, enzyme activity is absent or dramatically reduced. Glyoxylate builds up because of the enzyme shortage, and is converted to a compound called oxalate instead of glycolate. Oxalate, in turn, combines with calcium to form calcium oxalate, which the body cannot readily eliminate. Deposits of calcium oxalate can lead to the characteristic features of primary hyperoxaluria type 2.

Normal Function

The GRHPR gene provides instructions for making an enzyme called glyoxylate and hydroxypyruvate reductase. This enzyme plays a role in preventing the buildup of a potentially harmful substance called glyoxylate by converting it to a substance called glycolate, which is easily excreted from the body. Additionally, this enzyme can convert a compound called hydroxypyruvate to D-glycerate, which is eventually converted to the simple sugar glucose (by other enzymes) and used for energy.

Health Conditions Related to Genetic Changes

Primary hyperoxaluria

More than 25 mutations in the GRHPR gene have been found to cause primary hyperoxaluria type 2. This condition is caused by the overproduction of a substance called oxalate. Excess amounts of this substance lead to kidney and bladder stones, which begin in childhood and often result in kidney disease by early adulthood. Deposition of oxalate in multiple other tissues throughout the body (systemic oxalosis) can cause additional health problems.

GRHPR gene mutations either disrupt production of the glyoxylate and hydroxypyruvate reductase enzyme or alter its structure. As a result, enzyme activity is absent or severely reduced and the conversion of glyoxylate to glycolate is impaired. Glyoxylate builds up and is converted to a compound called oxalate. The oxalate is filtered through the kidneys and is either excreted in urine as a waste product or combines with calcium to form calcium oxalate, a hard compound that is the main component of kidney and bladder stones. Increased oxalate levels in the blood can lead to systemic oxalosis, particularly affecting bones and the walls of blood vessels in people with primary hyperoxaluria type 2.

Other Names for This Gene

• D-glycerate dehydrogenase
• GLXR
• glyoxylate reductase/hydroxypyruvate reductase
• GRHPR_HUMAN
• PH2

Genomic Location

The GRHPR gene is found on chromosome 9.

Primary hyperoxaluria type 2 is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Primary hyperoxaluria type 2 is characterized by recurrent nephrolithiasis (deposition of calcium oxalate in the kidney and urinary tract), nephrocalcinosis (deposition of calcium oxalate in the kidney tissue), and end-stage renal disease (ESRD). After ESRD, oxalosis (widespread tissue deposition of calcium oxalate) usually develops. Presenting symptoms are typically those associated with the presence of kidney stones, including hematuria, renal colic (a type of abdominal pain caused by kidney stones), or obstruction of the urinary tract. The symptoms of primary hyperoxaluria type 2 are typically less severe than primary hyperoxaluria type 1 and may be limited to kidney stone formation. Symptom onset may occur in childhood or adolescence. End stage renal disease is rarely observed in childhood.

The current management strategy includes high fluid intake, treatment with inhibitors of calcium oxalate crystallization, and temporary intensive dialysis for end-stage renal disease (ESRD) followed by kidney transplantation. Varying success has been reported following transplantation, with recurrence being a real possibility since hyperoxaluria and elevated L-glycerate levels persist. Careful management in the postoperative period, with attention to brisk urine output and use of calcium oxalate urinary inhibitors may help prevent complications.To date, liver-kidney transplantation has not been used in primary hyperoxaluria type 2. This strategy may be considered, however, as there is more enzyme in the liver than in other tissues. More studies are needed before liver transplantation can be recommended. Other treatment modalities needing further investigation include liver cell transplantation and recombinant gene therapy to replace the missing enzyme.

While development of end-stage renal disease (ESRD) may be less common (and later occurring) in people with primary hyperoxaluria type 2 than in those with type 1, chronic and terminal renal insufficiency have been described.Prognosis depends on early treatment and management of hyperoxaluria and associated renal deterioration. Patients who develop ESRD and undergo kidney transplantation seem to carry a high risk of disease recurrence.