Galactosemia: GALT Sequencing

Clinical Information

Galactosemia is an autosomal recessive inborn error of carbohydrate metabolism caused by the deficiency of galactose-1-phosphate uridyl transferase (GALT), which performs the second enzymatic step in the conversion of galactose to glucose-1-phosphate. As a result of the enzyme deficiency, excess galactose is excreted in the urine and the substrate for GALT, galactose-1-phosphate, accumulates throughout the body.

Manifestations of the disease appear within days of the initiation of milk feedings, and include vomiting, jaundice and failure to thrive. If left untreated for a prolonged period of time, patients will develop hepatomegaly, cataracts and intellectual disability. Speech delay and premature ovarian failure are also observed in many galactosemia patients, even in those who are diagnosed early and put on treatment. Galactosemia is typically detected very early because of newborn screening programs, which measures GALT enzyme activity in dried blood spots. Mutations in the GALT gene are responsible for the classic form of galactosemia (G/G). This sequencing analysis can help identify the disease causing mutations in patients.

Technical Information

• This test is performed by Sanger sequencing.
• Genomic DNA is used to amplify one coding exon of the GALT gene by standard polymerase chain reaction. Each amplified product is analyzed by a standard fluorescent sequencing protocol in both the forward and reverse orientations.

Specimen Requirements

• The preferred sample type is 3-4 ml of peripheral blood collected in an EDTA (purple top) tube. Dried blood spots, extracted DNA, and saliva are also accepted for this test. Blood and saliva kits are available by request.
• Approximately 75 µL of blood should be applied to each of the five circles on a filter paper dried blood spot card. Allow blood to dry for at least 4 hours before shipping.
• Send approximately 5 µg of extracted DNA at a requested concentration of 90-130 ng/µl.
• Saliva samples must be submitted in an approved saliva kit.

Transport Instructions

• The blood specimen should be kept at room temperature and delivered via overnight shipping. If shipment is delayed by one or two days, the specimen should be refrigerated and shipped at room temperature. Do not freeze the specimen. Samples collected on Friday can be safely designated for Monday delivery.
• DBS cards should be shipped at room temperature.
• Extracted DNA should be sent at room temperature via overnight delivery.
• Saliva is stable at room temperature and can be delivered via overnight or ground shipping.

Prenatal Testing Information

Prenatal diagnosis may be available for familial pathogenic or likely pathogenic variants; full sequencing of the GALT gene is not available. For variants identified at external laboratories, GGC will need to review the variant classification prior to sample receipt. Prenatal diagnosis is not available for variants of uncertain clinical significance. Additional fees for cell culture and maternal cell contamination may apply. Maternal cell contamination studies are required for all prenatal molecular tests. Contact the laboratory prior to sending a prenatal specimen to confirm that targeted testing can be accepted.

Prenatal Specimen Requirements

• Amniotic Fluid: Direct amniotic fluid will be accepted for analysis given there is sufficient volume for back-up culture to be established. A back-up culture at a reference lab OR Greenwood lab is required. Additional charges may apply if cell culture is required after receipt of the sample. If sending direct fluid for molecular analysis only, 10-20 ml of amniotic fluid is requested. Chromosome studies will require an additional 10-15 ml of fluid.
• Chorionic Villi Sample (CVS): 10-50 mg of chorionic villi is requested in a sterile tube with CVS media. Direct testing on CVS is not available; an additional 1-3 weeks may be needed for cultures to grow.
• Cultured amniocytes/CVS: 2x T25 confluent flasks
• A maternal sample is required, and accepted sample types include blood, saliva, and extracted DNA. Please refer to the specimen requirements above for more information.

Prenatal Transport Instructions

• Amniotic fluid should be kept at room temperature; do not freeze or refrigerate. Specimen should be sent by courier or overnight mail to arrive at the laboratory the next day. FedEx is preferred.
• CVS and cultured amniocytes should be kept at room temperature; do not freeze or refrigerate. Specimen should be sent by courier or overnight mail to arrive at the laboratory the next day. FedEx is preferred.
• Please refer to the transport instructions above for sending the maternal sample.