3-Methylcrotonylglycinuria: MCCC1/MCCC2 Sequencing

Clinical Information

3-Methylcrotonylglycinuria (3MCC) is an autosomal recessive inborn error of leucine metabolism caused by the deficiency of 3-methylcrotonyl-CoA carboxylase. Patients are often identified through newborn screening. Mildly affected or asymptomatic mothers with 3MCC deficiency have also been identified through positive newborn screening in their children. The presentation of this disorder is highly variable with severe cases experiencing significant neurological abnormalities, psychomotor retardation, seizures, cardio-respiratory failure and coma. Ketoacidosis, hypoglycemia and hyperammonemia are often seen. Mild cases may be asymptomatic or display fatigue, muscle weakness and/or mild developmental delay. 3MCC can be caused by a variant in either the gene coding the alpha subunit (MCCC1) or the beta subunit (MCCC2) of the enzyme.

Technical Information

• This test is performed by Sanger sequencing.
• Sequencing of the MCCC1 and MCCC2 genes will detect variants in 99% of individuals with 3MCC.
• All 19 coding exons of MCCC1 and 17 coding exons of MCCC2 are subjected to polymerase chain reaction amplification. Each amplified product is analyzed by a standard fluorescent sequencing protocol in both the forward and reverse direction.

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 5µg 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 sent 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 MCCC1 and MCCC2 genes 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.