3-Methylcrotonyl-CoA Carboxylase (3-MCC) deficiency is an autosomal recessive inborn error of leucine metabolism . 3-MCC is a biotin-dependent enzyme in the L-leucine degradation pathway. Newborn screening which includes testing for 3-MCC by tandem mass spectrometry, may reveal increased levels of 3-hydroxyisovalerylcarnitine (C5-OH).The clinical course has been shown to vary considerably, ranging from entirely asymptomatic to death in infancy . Severe and mild phenotypes are not clearly defined, but the vast majority of individuals have mild phenotypes which may be asymptomatic, while a subgroup shows mild unspecific symptoms like fatigue and weakness during catabolic episodes or mild developmental delay.
Isolated 3-MCC deficiency, which is not responsive to treatment with biotin, can be distinguished from the biotin-responsive multiple-carboxylase deficiencies, which are due to disorders of biotin metabolism (biotinidase deficiency and holocarboxylase synthetase deficiency) and affect all four of the biotin-dependent carboxylases. Infants with elevated C5-OH may also be due to maternal 3-MCC deficiency.
The 3-MCC enzyme consists of two subunits encoded by the MCCC1 gene (or MCCA) on 3q26 and the MCCC2 gene (or MCCB) on 5q13. Sequencing analysis is available to test for mutations in the MCCC1 and MCCC2 genes, associated with 3-MCC deficiency.
1. Sweetman and Williams. Branched Chain Organic Acidurias, in: C.R. Scriver, A.L. Beaudet, W. Sly, D. Valle (Eds.), The Metabolic and Molecular Bases of Inherited Disease, McGraw-Hill, New York, 2001, pp. 2135-2137.
2. Gibson et al. 3-Methylcrotonyl-coenzyme A carboxylase deficiency in Amish/Mennonite adults identified by detection of increased acylcarnitines in blood spots of their children. J Pediatr 1998, 132:519.
3. Stadler et al. Newborn Screening for 3-Methylcrotonyl-CoA Carboxylase Deficiency: Population Heterogeneity of MCCA and MCCB Mutations and Impact on Risk Assessment. Hum Mutat 2006, 27(8):748-759.
4. Baumgartner et al. The molecular basis of human 3-methylcrotonyl-CoA carboxylase deficiency. J Clin Invest 2001, 107:495-504.
5. Dantas et al. 3-Methylcrotonyl-CoA carboxylase deficiency: mutation analysis in 28 probands, 9 symptomatic and 19 detected by newborn screening. Hum Mutat 2005, 26:164-175.
6. Holzinger et al. Cloning of the human MCCA and MCCB genes and mutations therein reveal the molecular cause of 3-methylcrotonyl-CoA: carboxylase deficiency. Hum Mol Genet 2001, 10(12):1299-1306.
7. Baumgartner MR. Molecular mechanism of dominant expression in 3-methylcrotonyl-CoA carboxylase deficiency. J Inherit Metab Dis 2005, 28(3):301-9.
This test is indicated for:
- Individuals with clinical and biochemical findings consistent with 3-MCC deficiency.
- Carrier testing in individuals with a family history of 3-MCC deficiency.
DNA isolated from peripheral blood is hybridized to a CGH array to detect deletions and duplications. The targeted CGH array has overlapping probes which cover the entire genomic region.
Detection is limited to duplications and deletions. The CGH array will not detect point or intronic mutations. 3-MCC deficiency is rare with incidence estimates of 1:84,700 live births .
Results of molecular analysis must be interpreted in the context of the patient's clinical and/or biochemical phenotype.
Isolation using the Perkin Elmer™Chemagen™ Chemagen™ Automated Extraction method or Qiagen™ Puregene kit for DNA extraction is recommended.
Infants and Young Children (<2 years of age): 2-3 ml
Children > 2 years of age to 10 years old: 3-5 ml
Older Children & Adults: 5-10 ml
Autopsy: 2-3 ml unclotted cord or cardiac blood
- Organic Acids (OA) - Urine and Acylcarnitine Profile (AR) - Plasma are used in the diagnosis of a patient with 3-MCC deficiency.
- Known Mutation Analysis (KM) is available to family members if mutations are identified by sequencing.
Prenatal Custom Diagnostics is available to couples who are confirmed carriers of mutations. Please contact the laboratory genetic counselor to discuss appropriate testing prior to collecting a prenatal specimen.