Primary hyperoxaluria type 1

Other Names: HP1; Oxalosis 1; Glycolic aciduria; Alanine-glyoxylate aminotransferase deficiency; Peroxisomal alanine glyoxylate aminotransferase deficiency; Hepatic AGT deficiency; Serine pyruvate aminotransferase deficiency

Primary hyperoxaluria type 1 (PH1) is a rare disorder that mainly affects the kidneys. It results from buildup of a substance called oxalate, which normally is filtered through the kidneys and excreted in the urine. In people with PH1, the accumulated oxalate is deposited in the kidneys and urinary tract. It combines with calcium, forming the main component of kidney and bladder stones (calcium oxalate).

Cause

Primary hyperoxaluria type 1 is caused by mutations in a gene called AGXT. This gene gives the body instructions for making an enzyme called alanine-glyoxylate aminotransferase. This enzyme is found in cell structures called peroxisomes in liver cells. It converts a compound called glyoxylate to the amino acid glycine. Mutations in the AGXT gene lead to a decrease in the amount or function of the enzyme, preventing the breakdown of glyoxylate. This is turn causes glyoxylate to accumulate, and it is converted to oxalate instead of glycine. Excess oxalate that is not excreted from the body then combines with calcium to form calcium oxalate, which damages the kidneys and other organs.

Inheritance

Primary hyperoxaluria type 1 is inherited in an autosomal recessive manner.This means that to be affected, a person must have a mutation in both copies of the responsible gene in each cell. Affected people inherit one mutated copy of the gene from each parent, who is referred to as a carrier. Carriers of an autosomal recessive condition typically do not have any signs or symptoms (they are unaffected). When 2 carriers of an autosomal recessive condition have children, each child has a:

• 25% chance to be affected
• 50% chance to be an unaffected carrier like each parent
• 25% chance to be unaffected and not a carrier

Signs and symptoms

The symptoms and severity of primary hyperoxaluria type 1 (PH1) can vary. The age that symptoms begin ranges from birth to the sixth decade of life (although there are exceptions). About 19% of people with PH1 have a severe, very early-onset form that becomes apparent within a few months after birth. At the milder end of the spectrum, some people with PH1 go without any symptoms for over 40 or 50 years. The median age of onset is about 5-6 years.

Features of renal involvement can range from nephrocalcinosis and renal failure in infancy, to only occasional stones diagnosed in adulthood. Kidney stones are commonly the first sign of hyperoxaluria. Symptoms of kidney stones can include sudden abdominal or flank pain; blood in the urine; frequent urge to urinate; pain while urinating; or fever and chills. Untreated PH1 leads to kidney failure, which is life-threatening. Symptoms of kidney failure can include decreased or no urine output; feeling ill or tired; loss of appetite; nausea and vomiting; and pale skin due to anemia.

For most diseases, symptoms will vary from person to person. People with the same disease may not have all the symptoms listed. 80%-99% of people have these symptoms

• Abnormality of circulating enzyme level
• Anemia(Low number of red blood cells or hemoglobin)
• Calcinosis(Calcium buildup in soft tissues of body)
• Hyperoxaluria(High urine oxalate levels)
• Metabolic acidosis
• Nephrocalcinosis(Too much calcium deposited in kidneys)
• Nephrolithiasis(Kidney stones)

30%-79% of people have these symptoms

• Decreased glomerular filtration rate
• Dysuria(Painful or difficult urination)
• Failure to thrive(Faltering weight)
• Hematuria(Blood in urine)

5%-29% of people have these symptoms

• Abnormality of the skeletal system(Skeletal abnormalities)
• Enuresis
• Recurrent urinary tract infections(Frequent urinary tract infections)
• Stage 5 chronic kidney disease

1%-4% of people have these symptoms

• Abnormality of the dentition(Abnormal dentition)
• Atherosclerosis(Narrowing and hardening of arteries)
• Stroke

Diagnosis

The diagnosis of PH1 is established in a proband with hyperoxaluria or hyperoxalemia by identification biallelic pathogenic variants in AGXT on molecular genetic testing. Failure to identify at least one AGXT pathogenic variant should prompt examination for other types of primary hyperoxaluria and in occasional circumstances may require consideration of liver biopsy to assay the activity of the enzyme alanine: glyoxylate-aminotransferase (AGT). However, given the wide availability of genetic testing, liver biopsy to obtain tissue for enzymatic activity is now rarely employed.

Treatment

The goal of treatment for primary hyperoxaluria type 1 (PH1) is to minimize calcium oxalate deposition and maintain renal function. Early diagnosis and prompt therapy is critical to preserve the function of the kidneys for as long as possible. General therapies for preventing kidney stones benefit all people with PH1. Recommendations for this include:

• drinking large amounts of fluid
• oral potassium citrate to inhibit calcium oxalate crystallization
• drugs such as thiazides to decrease calcium in the urine
• avoiding significant intake of vitamin C or D (they promote stone formation)
• supplementation of dietary calcium

Treatment for kidney stones may involve shock wave lithotripsy, percutaneous nephrolithotomy, and/or ureteroscopy.

• Reducing the body's production of oxalate involves treatment with pyridoxine. While only about 10%-30% of people with PH1 respond to treatment with pyridoxine, it has been recommended that all people with PH1 receive a minimum 3-month trial at the time of initial diagnosis.
• Dialysis to remove oxalate in people with PH1 has limitations, but may be indicated in specific circumstances in some people with PH1.
• Lastly, organ transplantation is an option for therapy. There has been much discussion among experts regarding the best transplantation strategy for people with PH1. Depending on each person's response to other therapies and the disease severity, options may include combined liver-kidney transplant; sequential liver-kidney transplant; an isolated kidney transplant, or an isolated liver transplant.
• Other therapies for PH1 are under investigation and may become options for people with PH1 in the future.
• People with questions about the treatment of PH1 for themselves or family members should speak with their doctor for treatment options and advice.

Prognosis

The progression and severity of primary hyperoxaluria type 1 (PH1) varies. Specific mutations in the responsible gene (AGXT) may correspond with particular symptoms, disease progression, and response to treatment. For example, some people with PH1 respond to treatment with vitamin B6 (pyridoxine), while others do not. Some research suggests that specific mutations in the AGXT gene are associated with later onset end stage renal failure. The outlook is very poor if PH1 is left untreated. An early and accurate diagnosis leading to aggressive supportive treatment is a major factor in short- and long-term outcomes. In the future, the prognosis may be improved by new therapies.

NIH genetic and rare disease info

• NIH info on Primary hyperoxaluria type 1

Primary hyperoxaluria type 1 is a rare disease.

Primary hyperoxaluria type 1 Resources
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