TEST CATALOG ORDERING & RESULTS SPECIMEN HANDLING CUSTOMER SERVICE EDUCATION & INSIGHTS
Test Catalog

Test ID: MHCZ    
Methylmalonic Aciduria and Homocystinuria, cblC Type, Full Gene Analysis, Varies

Useful For Suggests clinical disorders or settings where the test may be helpful

Confirmation of diagnosis of methylmalonic aciduria and homocystinuria, cblC type

 

Distinguishing between cblC, cblD, and cblF types when methylmalonic aciduria and homocystinuria are identified

 

Carrier screening in cases where there is a family history of methylmalonic aciduria and homocystinuria, but disease-causing mutations have not been identified in an affected individual

Testing Algorithm Delineates situations when tests are added to the initial order. This includes reflex and additional tests.

If skin biopsy is received, fibroblast culture for genetic test will be added and charged separately.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Multiple causes of inborn errors of cobalamin (cbl; better known as vitamin B12) metabolism have been identified. These disorders have been classified into 9 distinct complementation classes (cblA-cblH and mut, caused by mutations in the gene encoding methylmalonyl coenzyme A mutase). Complementation analysis utilizes cells from the patient to determine at what stage of the cbl metabolism pathway an error is occurring, and uses this information to differentiate between the various complementation class disorders.

 

Depending on the complementation class involved, errors in cbl metabolism can result in methylmalonic aciduria, homocystinuria, or both. The most common disorder in this group is methylmalonic aciduria and homocystinuria, cblC (cobalamin C) type, which results in both methylmalonic aciduria and homocystinuria.

 

cblC type is an autosomal recessive disorder with a variable age of onset. In the early onset form, symptoms appear in the first several years of life and include failure to thrive, developmental delay, seizures, metabolic crisis, and hydrocephalus. Patients may also have hemolytic uremic syndrome. Adults can present with confusion or other changes in mental status, cognitive decline, and megaloblastic anemia. Biochemical presentation includes methylmalonic aciduria and homocystinuria in urine organic acid or plasma amino acid analysis.

 

Other complementation class disorders, such as cblD and cblF, can result in a similar biochemical phenotype, and complementation testing or molecular testing is utilized to distinguish between these different types.

 

Mutations in the MMACHC gene are responsible for the cblC type disorder. The most common mutation (identified in approximately 40% of mutant alleles) is 271dupA. This multiethnic mutation is most frequently associated with early-onset disease, especially when present in the homozygous state. Another early-onset mutation is R111X, which is common in the Cajun and French Canadian populations. R132X is a late-onset mutation that has been identified in individuals of Indian, Pakistani, and Middle Eastern ethnicity. Although these genotype-phenotype correlations are well-established, there is often considerable variability in age of onset and expression of symptoms, even within families.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

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

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

A small percentage of individuals who are carriers or have a diagnosis of methylmalonic aciduria and homocystinuria, cblC type (MMACHC) may have a mutation that is not identified by this method (eg, large genomic deletions, promoter mutations). The absence of a mutation(s), therefore, does not eliminate the possibility of positive carrier status or the diagnosis of MMACHC. For carrier testing, it is important to first document the presence of a MMACHC gene mutation in an affected family member.

 

In some cases, DNA alterations of undetermined significance may be identified.

 

Rare polymorphisms exist that could lead to false-negative or false-positive results. If results obtained do not match the clinical and biochemical findings, additional testing should be considered.

  

Test results should be interpreted in the context of clinical findings, family history, and other laboratory data. Errors in our interpretation of results may occur if information given is inaccurate or incomplete.

Clinical Reference Recommendations for in-depth reading of a clinical nature

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. Lerner-Ellis JP, Tirone JC, Pawelek PD, et al: Identification of the gene responsible for methylmalonic aciduria and homocystinuria, cblC type. Nat Genet 2006;38:93-100

3. Morel CF, Lerner-Ellis JP, Rosenblatt DS: Combined methylmalonic aciduria and homocystinuria (cblC): Phenotype-genotype correlations and ethnic-specific observations. Mol Genet Metab 2006;88:315-321

4. Martinelli D, Deodato F, Dionisi-Vici C: Cobalamin C defect: natural history, pathophysiology, and treatment. J Inherit Metab Dis 2011;34(1):127-135

Special Instructions Library of PDFs including pertinent information and forms related to the test