Test ID: AGXKM
AGXT Gene, Known Mutation

Carrier testing of individuals with a family history of primary hyperoxaluria type 1

Diagnostic confirmation of primary hyperoxaluria type 1 deficiency when familial mutations have been previously identified

Prenatal testing when 2 familial mutations have been previously identified in an affected family member

For prenatal specimens only: If amniotic fluid (non-confluent cultured cells) is received, amniotic fluid culture/genetic test will be added and charged separately. If chorionic villus specimen (non-confluent cultured cells) is received, fibroblast culture for genetic test will be added and charged separately. For any prenatal specimen that is received, maternal cell contamination studies will be added.

• Molecular Genetics-Congenital Inherited Diseases Patient Information Sheet
• Informed Consent for Genetic Testing

Polymerase Chain Reaction (PCR) Followed by DNA Sequence Analysis and Gene Dosage Analysis by Multiplex Ligation-Dependent Probe Amplification (MLPA)

(PCR is utilized pursuant to a license agreement with Roche Molecular Systems, Inc.)

This test can only be performed if a mutation has previously been identified in a family member of this individual.

Forms:

1. Molecular Genetics-Congenital Inherited Diseases Patient Information Sheet (Supply T521) in Special Instructions

2. New York Clients-Informed consent is required. Please document on the request form or electronic order that a copy is on file. An Informed Consent for Genetic Testing (Supply T576) is available in Special Instructions.

3. If not ordering electronically, submit a Molecular Genetics Request Form (Supply T245) with the specimen.

Specimen must arrive within 96 hours of collection.

Submit only 1 of the following specimens:

Specimen Type: Whole blood

Container/Tube: Lavender top (EDTA) or yellow top (ACD)

Specimen Volume: 3 mL

Collection Instructions:

1. Invert several times to mix blood. 

2. Send specimen in original tube.

Specimen Stability Information: Ambient (preferred)/Refrigerated

Due to the complexity of prenatal testing, consultation with the laboratory is required for all prenatal testing. Prenatal specimens can be sent Monday through Thursday and must be received by 5 p.m. CST on Friday in order to be processed appropriately. All prenatal specimens must be accompanied by a maternal blood specimen. Order MCC/88636 Maternal Cell Contamination, Molecular Analysis on the maternal specimen.

Specimen Type: Amniotic fluid

Container/Tube: Amniotic fluid container

Specimen Volume: 20 mL

Specimen Stability Information: Refrigerated (preferred)/Ambient

Specimen Type: Chorionic villi

Container/Tube: 15-mL tube containing 15-mL of transport media

Specimen Volume: 20 mg

Specimen Stability Information: Refrigerated

Acceptable:

Specimen Type: Confluent cultured cells

Container/Tube: T-25 flask

Specimen Volume: 2 flasks

Collection Instructions: Submit confluent cultured cells from another laboratory.

Specimen Stability Information: Ambient (preferred)/Refrigerated

No specimen should be rejected. If specimen not received at appropriate temperature or in wrong anticoagulant, include note to laboratory. If questions, contact laboratory.

Primary hyperoxaluria type 1 (PH1) is a hereditary disorder of glyoxylate metabolism caused by deficiency of alanine:glyoxylate-aminotransferase (AGT), a hepatic enzyme that converts glyoxylate to glycine. Absence of AGT activity results in conversion of glyoxylate to oxalate, which is not capable of being degraded. Therefore, excess oxalate is excreted in the urine, causing kidney stones (urolithiasis), nephrocalcinosis, and kidney failure. As kidney function declines, blood levels of oxalate increase markedly, and oxalate combines with calcium to form calcium oxalate deposits in the kidney, eyes, heart, bones, and other organs, resulting in systemic disease. Pyridoxine (vitamin B6), a cofactor of AGT, is effective in reducing urine oxalate excretion in some PH1 patients.

Presenting symptoms of PH1 include nephrolithiasis, nephrocalcinosis, or end-stage kidney disease with or without a history of urolithiasis. Age of symptom onset is variable; however, most individuals present in childhood or adolescence with symptoms related to kidney stones. In some infants with a more severe phenotype, kidney failure may be the initial presenting feature. Less frequently, affected individuals present in adulthood with recurrent kidney stones or kidney failure. End-stage kidney failure is most often seen in the third decade of life, but can occur at any age.

The exact prevalence and incidence of PH1 are not known, but prevalence rates of 1 to 3 per million population and incidences of 0.1 per million/year have been estimated from population surveys.

Biochemical testing is indicated in patients with possible primary hyperoxaluria, and can be performed prior to molecular testing. Measurement of urinary oxalate in a timed, 24-hour urine collection is strongly preferred, with correction to adult body surface area in pediatric patients (HYOX/86213 Hyperoxaluria Panel, Urine; OXU/8669 Oxalate, Urine). In very young children (incapable of performing a timed collection), random urine oxalate to creatinine ratios may be used to determine oxalate excretion. In patients with reduced kidney function, POXA/81408 Oxalate, Plasma is also recommended. Urinary excretion of oxalate of >1.0 mmol/1.73 m(2)/24 hours is strongly suggestive of, but not diagnostic for, this disorder, as there are other forms of inherited (type 2 and non-PH1/PH2) hyperoxaluria and secondary hyperoxaluria that may result in similarly elevated urine oxalate excretion rates. An elevated urine glycolate in the presence of hyperoxaluria is suggestive of PH1. Caution is warranted in interpretation of urine oxalate excretion in patients with reduced kidney function as urine oxalate concentrations may be lower due to reduced glomerular filtration rate (GFR). The plasma oxalate concentration may also be helpful in supporting the diagnosis in patients with reduced kidney function, with values >50 micromol/L highly suggestive of PH1 when GFR is <20 mL/min/1.73 m(2). Historically, the diagnosis of PH1 was confirmed by AGT enzyme analysis performed on liver biopsy; however, this has since been replaced by molecular testing, which forms the basis of confirmatory or carrier testing in most cases.   

PH1 is inherited as an autosomal recessive disorder caused by mutations in the AGXT gene, which encodes the enzyme AGT. Several common AGXT mutations have been identified including c.33dupC, p.Gly170Arg (c.508G->A), and p.Ile244Thr (c.731T->C). These mutations account for at least 1 of the 2 affected alleles in approximately 70% of individuals with PH1. Direct sequencing of the AGXT gene is predicted to identify 99% of alleles in individuals who are known by enzyme analysis to be affected with PH1.

While age of onset and severity of disease is variable and not necessarily predictable by genotype, a correlation between pyridoxine responsiveness and homozygosity for the p.Gly170Arg mutation has been observed. (Note: molecular testing is available as AGXMS/89915 AGXT Gene, Full Gene Analysis and, for the p.Gly170Arg mutation only, as AGXT/83643 Alanine:Glyoxylate Aminotransferase [AGXT] Mutation Analysis [G170R], Blood). Pyridoxine (vitamin B6) is a known cofactor of AGT and is effective in reducing urine oxalate excretion in some PH1 patients treated with pharmacologic doses. Individuals with 2 copies of the p.Gly170Arg mutation have been shown to normalize their urine oxalate when treated with pharmacologic doses of pyridoxine and those with a single copy of the mutation show reduction in urine oxalate. This is valuable because not all patients have been shown to be responsive to pyridoxine, and strategies that help to identify the individuals most likely to benefit from such targeted therapies are desirable.

Site-specific (known mutation) testing for mutations that have already been identified in an affected patient is useful for confirming a suspected diagnosis in a family member. It is also useful for determining whether at-risk individuals are carriers of the disease and, subsequently, at risk for having a child with PH1 deficiency.

An interpretive report will be provided.

An interpretative report will indicate if results are diagnostic for primary hyperoxaluria type 1 (2 mutations identified), if the patient is a carrier for primary hyperoxaluria type 1 (1 mutation identified), or if no mutations are identified.

The identification of a disease-causing mutation in an affected family member is necessary before predictive testing for other family members can be offered. If a familial mutation has not been previously identified, order AGXMS/89915 AGXT Gene, Full Gene Analysis.

Analysis is performed for the familial mutation(s) provided only. This assay does not rule out the presence of other mutations within this gene or within other genes that may be associated with PH1.  

We strongly recommend that patients undergoing predictive testing receive genetic counseling both prior to testing and after results are available.  

Predictive testing of an asymptomatic child is not recommended.  

Test results should be interpreted in the context of clinical findings, family history, and other laboratory data. Any error in the diagnosis or in the pedigree provided to us, including false-paternity, could lead to erroneous interpretation of results.

A previous bone marrow transplant from an allogenic donor will interfere with testing. Call Mayo Medical Laboratories for instructions for testing patients who have received a bone marrow transplant.

1. Milliner DS: The primary hyperoxalurias: an algorithm for diagnosis. Am J Nephrol 2005;25(2):154-160

2. Monico CG, Rossetti S, Olson JB, Milliner DS: Pyridoxine effect in type I primary hyperoxaluria is associated with the most common mutant allele. Kidney Int 2005;67(5):1704-1709

3. Monico CG, Rossetti S, Schwanz HA, et al: Comprehensive mutation screening in 55 probands with type 1 primary hyperoxaluria shows feasibility of a gene-based diagnosis. J Am Soc Nephrol 2007;18:1905-1914

4. Rumsby G, Williams E, Coulter-Mackie M: Evaluation of mutation screening as a first line test for the diagnosis of the primary hyperoxalurias. Kidney Int 2004;66(3):959-963

5. Williams EL, Acquaviva C, Amoroso, A, et al: Primary hyperoxaluria type I: update and additional mutation analysis of the AGXT gene. Hum Mutat 2009;30:910-917

6. Williams E, Rumsby G: Selected exonic sequencing of the AGXT gene provides a genetic diagnosis in 50% of patients with primary hyperoxaluria type 1. Clin Chem 2007;53(7):1216-1221

7. Communique April 2007: Laboratory and Molecular Diagnosis of Primary Hyperoxaluria and Oxalosis

DNA sequencing or gene dosage analysis is used to test for the presence of a specific mutation(s) in the AGXT gene that was previously identified in an affected family member. (Unpublished Mayo method)

Monday; 10 a.m.

81479- Unlisted molecular pathology procedure

81265 - Comparative analysis using Short Tandem Repeat {STRI markers; patient and comparative specimen (eg. Pretransplant recipient and donor germline testing, posttransplant non-hematopoietic recipient germline [eg, buccal swab or other germline tissue sample! and donor testing. twin zygosity testing, or maternal cell contamination of fetal cells) (if appropriate)

Amniotic Fluid Culture for Genetic Testing

88235-Tissue culture for amniotic fluid (if appropriate)

88240-Cryopreservation (if appropriate)

Fibroblast Culture for Genetic Testing

88233-Tissue culture, skin or solid tissue biopsy (if appropriate)

88240-Cryopreservation (if appropriate)