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Test ID PHEGP Phenylalanine Disorders Gene Panel, Varies


Ordering Guidance


The recommended first-tier test for disorders of phenylalanine metabolism is quantitative plasma amino acids (AAQP / Amino Acids, Quantitative, Plasma), as well as neurotransmitters in cerebrospinal fluid and pterin metabolite analysis in blood and urine.



Shipping Instructions


Specimen preferred to arrive within 96 hours of collection.



Specimen Required


Patient Preparation: A previous bone marrow transplant from an allogenic donor will interfere with testing. Call 800-533-1710 for instructions for testing patients who have received a bone marrow transplant.

Specimen Type: Whole blood

Container/Tube:

Preferred: Lavender top (EDTA) or yellow top (ACD)

Acceptable: Any anticoagulant

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


Forms

1. New York Clients-Informed consent is required. Document on the request form or electronic order that a copy is on file. The following documents are available in Special Instructions:

-Informed Consent for Genetic Testing (T576)

-Informed Consent for Genetic Testing (Spanish) (T826)

2. Molecular Genetics: Biochemical Disorders Patient Information (T527) in Special Instructions

Useful For

Follow up for abnormal biochemical results suggestive of a phenylalanine disorder

 

Establishing a molecular diagnosis for patients with phenylalanine disorders

 

Identifying variants within genes known to be associated with phenylalanine disorders, allowing for predictive testing of at-risk family members

Method Name

Custom Sequence Capture and Targeted Next-Generation Sequencing followed by Polymerase Chain Reaction (PCR) and Sanger Sequencing

Reporting Name

Phenylalanine Disorders Gene Panel

Specimen Type

Varies

Specimen Minimum Volume

See Specimen Required

Specimen Stability Information

Specimen Type Temperature Time Special Container
Varies Varies

Clinical Information

Hyperphenylalaninemia is a heterogeneous disorder of phenylalanine catabolism caused by a deficiency of any one of 6 enzymes involved in the conversion of phenylalanine to tyrosine.

 

Phenylketonuria (PKU) is the most frequent inherited disorder of amino acid metabolism (about 1:10,000-1:15,000) and was the first successfully treated inborn error of metabolism and included in newborn screening programs worldwide. It is inherited in an autosomal recessive manner and is caused by a defect in the enzyme phenylalanine hydroxylase (PAH), which converts the essential amino acid phenylalanine to tyrosine. Deficiency of PAH results in decreased levels of tyrosine and an accumulation of phenylalanine in blood and tissues. Untreated, PKU leads to severe brain damage with intellectual impairment, behavior abnormalities, seizures, and spasticity. The level of enzyme activity differentiates classic PKU (PAH activity <1%) from other milder forms; however, all are characterized by increased levels of phenylalanine (hyperphenylalaninemia). Treatment includes the early introduction of a diet low in phenylalanine.

 

Approximately 2% of patients with hyperphenylalaninemia have a deficiency of tetrahydrobiopterin (BH4), which causes a secondary deficit of the neurotransmitters dopamine and serotonin. There are 4 autosomal recessive disorders associated with BH4 deficiency plus hyperphenylalaninemia; guanosine triphosphate cyclohydrolase deficiency (GCH1), 6-pyruvoyl tetrahydropterine synthase deficiency (PTS), dihydropteridine reductase deficiency (QDPR), and pterin-4 alpha carbinolamine dehydratase (PCD) deficiency (PCBD1). This group of disorders, with the exception of PCD, is characterized by progressive dystonia, truncal hypotonia, extremity hypertonia, seizures, and mental retardation though milder presentations exist. PCD has no symptoms other than transient alterations in tone. Treatment may include administration of BH4, L-dopa (and carbidopa) 5-hydroxytryptophan supplements, and a low phenylalanine diet.

 

Recently, variants in DNAJC12, which encodes a heat-shock protein that interacts with the phenylalanine, tyrosine, and tryptophan hydroxylases to help catalyze the conversion of the substrates to their respective products, has been shown to cause hyperphenylalaninemia, progressive neurodegeneration, and dystonia. Treatment may include early administration of BH4 and/or neurotransmitter precursors.

 

Related additional disorders of neurotransmitter metabolism include:

-Aromatic l-amino acid decarboxylase (AADC) deficiency, caused by variants in DDC, is an autosomal recessive inborn error in neurotransmitter metabolism that leads to combined serotonin and catecholamine deficiency.

-Patients with dopa-responsive dystonia due to variants in SPR causing sepiapterin reductase deficiency have progressive psychomotor retardation and dystonia.

-Variants in tyrosine hydroxylase (TH) prevent the conversion of L-tyrosine to L-dopa resulting in Segawa syndrome.

-Variants in SLC18A2, a vesicular transporter of dopamine, cause infantile parkinsonism-dystonia-2 (PKDYS2)

Reference Values

An interpretive report will be provided.

Interpretation

All detected alterations are evaluated according to American College of Medical Genetics and Genomics (ACMG) recommendations.(1) Variants are classified based on known, predicted, or possible pathogenicity and reported with interpretive comments detailing their potential or known significance.

Clinical Reference

1. Richards S, Aziz N, Bale S, et al: Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology. Genet Med. 2015 May;17(5):405-424

2. Burgard P, Luo X, Levy HL, Hoffmann GF: Phenylketonuria. In: Sarafoglou K, Hoffmann GF, Roth KS, eds. Pediatric Endocrinology and Inborn Errors of Metabolism. 2nd ed. McGraw-Hill Education; 2017:251-258

3. Blau N, Thony B: Hyperphenylalanemias: Disorders of tetrahydrobiopterin metabolism. In: Sarafoglou K, Hoffmann GF, Roth KS, eds. Pediatric Endocrinology and Inborn Errors of Metabolism. 2nd ed. McGraw-Hill Education;  2017:259-266

4. Anikster Y, Haack TB, Vilboux T, et al: Biallelic mutations in DNAJC12 cause hyperphenylalaninemia, dystonia, and intellectual disability. Am J Hum Genet. 2017;100(2):257-266. doi: 10.1016/j.ajhg.2017.01.002

5. OMIM. Johns Hopkins University; Accessed January 2, 2020. Available at https://omim.org/

Test Classification

This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

CPT Code Information

81405

81406 x 2

81479

LOINC Code Information

Test ID Test Order Name Order LOINC Value
PHEGP Phenylalanine Disorders Gene Panel In Process

 

Result ID Test Result Name Result LOINC Value
608788 Test Description 62364-5
608789 Specimen 31208-2
608790 Source 31208-2
608791 Result Summary 50397-9
608792 Result 82939-0
608793 Interpretation 69047-9
608794 Resources In Process
608795 Additional Information 48767-8
608796 Method 85069-3
608797 Genes Analyzed 48018-6
608798 Disclaimer 62364-5
608799 Released By 18771-6

Day(s) Performed

Varies

Report Available

3 to 4 weeks