Test ID: GALTK
Galactosemia Gene Analysis, Known Mutation

Diagnostic confirmation of galactosemia when familial mutations have been previously identified

Carrier screening of at-risk individuals when a mutation in the galactose-1-phosphate uridyltransferase (GALT) gene has been identified in an affected family member

Documentation of the specific familial mutation(s) must be provided with the specimen in order to perform this test.

For prenatal specimens only: If amniotic fluid (nonconfluent cultured cells) is received, amniotic fluid culture/genetic test will be added and charged separately. If chorionic villus specimen (nonconfluent 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-Biochemical Disorders Patient Information Sheet
• Informed Consent for Genetic Testing

Polymerase chain reaction (PCR)-based assay is utilized to test for the presence of a specific mutation previously identified in an affected family member.

(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-Biochemical Disorders Patient Information Sheet (Supply T527) 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           

Preferred: Lavender top (EDTA)

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

Acceptable:

Specimen Type: Blood spots

Container/Tube: Whatman Protein Saver 903 Paper

Specimen Volume: 5 blood spots

Collection Instructions:

1. Let blood dry on the filter paper at ambient temperature in a horizontal position for 3 hours.

2. Do not expose specimen to heat or direct sunlight.

3. Do not stack wet specimens.

4. Keep specimen dry.

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

Classic galactosemia is an autosomal recessive disorder of galactose metabolism caused by mutations in the galactose-1-phosphate uridyltransferase (GALT) gene. The complete or near complete deficiency of the GALT enzyme is life threatening. If left untreated, complications include liver failure, sepsis, mental retardation, and death. Galactosemia is treated by a galactose-free diet, which allows for rapid recovery from the acute symptoms and a generally good prognosis. Despite adequate treatment from an early age, children with galactosemia remain at increased risk for developmental delays, speech problems, and abnormalities of motor function. Females with galactosemia are at increased risk for premature ovarian failure. The prevalence of classic galactosemia is approximately 1 in 30,000.

Duarte variant galactosemia (compound heterozygosity for the Duarte variant, N314D, and a classic mutation) is generally associated with higher levels of GALT activity (5%-20%) than classic galactosemia (<5%); however, this may be indistinguishable by newborn screening assays. Typically, individuals with Duarte variant galactosemia have a milder phenotype, but are often treated with a low galactose diet during infancy. The LA variant, consisting of N314D and a second change, L218L, is associated with higher levels of GALT activity than the Duarte variant alone.

Newborn screening, which identifies potentially affected individuals by measuring total galactose (galactose and galactose-1-phosphate) and/or determining the activity of the GALT enzyme, varies from state to state. The diagnosis of galactosemia is established by follow-up quantitative measurement of GALT activity. If enzyme activity levels are indicative of carrier or affected status, molecular testing for common GALT mutations may be performed. If 1 or both disease-causing mutations are not detected by targeted mutation analysis and biochemical testing has confirmed the diagnosis of galactosemia, sequencing of the GALT gene is available to identify private mutation(s).

The GALT gene maps to 9p13. More than 180 mutations have been identified in the GALT gene. Several disease-causing mutations are common in patients with classic galactosemia (G/G genotype). The most frequently observed is the Q188R mutation. This mutation accounts for 60% to 70% of classic galactosemia alleles. The S135L mutation is the most frequently observed mutation in African Americans and accounts for approximately 50% of the mutant alleles in this population. The K285N mutation is common in those of eastern European descent and accounts for 25% to 40% of the alleles in this population. The L195P mutation is observed in 5% to 7% of classic galactosemia. The Duarte variant (N314D) is found in 5% of the general United States population.

The above mutations, plus the LA variant, are included in GCT/84360 Galactosemia Reflex, Blood, which is the preferred test for the diagnosis of galactosemia or for follow-up to positive newborn screening results. These mutations are also included in GAL6/84366 Galactosemia Gene Analysis (6-Mutation Panel). Full sequencing of the GALT gene can be useful for the identification of mutations when 1 or no mutations are found with these tests in an individual with demonstrated GALT enzyme deficiency. Full sequencing of the GALT gene identifies over 95% of the sequence variants in the coding region and splice junctions.

An interpretive report will be provided.

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

Results should be interpreted in the context of biochemical results.

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 GALTM/88877 GALT 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 galactosemia.

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. Elsas LJ 2nd, Lai K: The molecular biology of galactosemia. Genet Med 1998 Nov-Dec;1(1):40-48

2. Novelli G, Reichardt JK: Molecular basis of disorders of human galactose metabolism: past, present, and future. Mol Genet Metab 2000 Sept-Oct;71(1-2):62-65

3. Bosch AM, Ijlst L, Oostheim W, et al: Identification of novel mutations in classical galactosemia. Hum Mutat 2005 May:25(5):502

4. Richards CS, Bale S, Bellissimo DB, et al: ACMG recommendations for standards for interpretation and reporting of sequence variations: Revisions 2007. Genet Med 2008;10(4):294-300

A PCR-based assay utilizing fluorescence resonance energy transfer (FRET) probes, melt-curve analysis, and LightCycler technology is used to examine the GALT gene for the presence of the known familial mutation(s).(Wittwer CT, Herrmann MG, Gundry CN, Elenitoba-Johnson KS: Real-time multiplex PCR assays. Methods 2001 Dec;25[4]:430-442)

Friday; 10 a.m.

81403-Known familial variant not otherwise specified, for gene listed in Tier 1 or Tier 2, DNA sequence analysis, each variant exon