Disorders of Intracellular Cobalamin Metabolism

[Includes: CblC, CblD, CblD Variant 1, CblD Variant 2, CblE, CblF, CblG]

Clinical Diagnosis

Vitamin B₁₂ (cobalamin) is a cofactor required by two enzymes, methionine synthase (MTR) and methylmalonyl-CoA mutase (MUT). Disorders of intracellular cobalamin metabolism may impair the function of either or both enzymes:

• Clinical syndromes involving defects in MUT function alone are called methylmalonic acidemia. Such defects can be caused by defective MUT enzyme or missing B₁₂ cofactor. Certain subtypes of methylmalonic acidemia may be further defined by complementation analysis. Complementation groups causing isolated methylmalonic acidemia include cblA and cblB (see Table 1).

• Clinical syndromes involving defective MTR function can be caused by defective MTR enzyme, missing cofactor, or defects in an enzyme that regenerates MTR: methionine synthase reductase (MTRR). Defective MTR function is associated with variable hyperhomocysteinemia and/or homocystinuria.

• Disorders that cause isolated defects in MTR function, as well as disorders that cause combined defects in both MTR and MUT function, are named by complementation group. MTR-only and combined MUT/MTR cobalamin disorders include cblC, cblD, cblD variant 1, cblD variant 2, cblE, cblF, and cblG (see Table 1).
  
  The phenotypes of the MTR-only and combined MUT/MTR cobalamin disorders are variable and can be seen in other related and unrelated conditions. Definitive diagnosis relies on specialized biochemical or molecular genetic testing (see Testing).

Testing

Note: Vitamin B₁₂-deficiency states must first be ruled out as a matter of course.

Nonspecific findings on urine organic acid analysis include the following:

• For combined MUT/MTR disorders, secondary metabolites (including 3-hydroxypropionate, methylcitrate, and tiglylglycine) may be detected using GC/MS analysis when patients are ill. When patients are well controlled, the secondary metabolites generally disappear, leaving only elevated MMA excretion.

• Methylglutaconic aciduria has been seen in at least one person with cblG [Adams et al 2006].

• Mild methylmalonic aciduria has been seen in one person with cblE when acutely ill [Tuchman et al 1988].

Plasma amino acid analysis

• Hypomethioninemia and homocystinemia and mixed disulfide can be detected using plasma amino acid analysis (see Table 3).

• Homocystine and the mixed disulfide of cysteine and homocysteine are excreted in the urine.

• In individuals with cblC, cystathioninemia/uria can be present [Mudd et al 1969].

Note: Delays in separating serum from plasma after obtaining a blood sample can artificially increase a total homocysteine result by as much as 10% an hour [Ubbink 2000, Refsum et al 2004].

Urine homocystine concentration. Measurement of urine homocystine concentration can be useful, especially in establishing the diagnosis initially. However, urine homocystine excretion is variable, making it potentially less sensitive than plasma total homocysteine concentration.

Note: Plasma homocysteine metabolism is discussed extensively in Cystathionine Beta Synthase Deficiency.

¹⁴C propionate and ¹⁴C MeTHF (¹⁴C formate) macromolecular incorporation studies are specialized biochemical laboratory investigations on patient-derived cell lines that can help establish the diagnosis of specific cobalamin disorders. Such assays rely on the incorporation of radiolabeled precursors into macromolecules. These tests are available clinically through specialized laboratories.

See Methylmalonic Acidemia for more information.

Complementation group analysis. This assay assigns a genetic group or class to the enzymatic block (i.e., cblA - cblG) using heterokaryon rescue or enzymatic cross correction [Gravel et al 1975]. In this assay the cell line from the affected individual is mixed with a panel of established cell lines of known status (e.g., cblC or cblF). Such testing is available clinically through specialized laboratories:

• CblD variants 1 and 2 display isolated adeno- and methylcobalamin synthesis, respectively.

• Complementation analysis is required to differentiate the cblC and cblF disorders.

• Complementation analysis is used to differentiate cblE from cblG. CblE cells show sensitivity to reductant concentration in vitro [Rosenblatt et al 1984].

Cobalamin distribution. Cells are incubated in radio-labeled cyanocobalamin, following which various cobalamin derivatives are measured using HPLC to determine the ability of the cultured cells to carry out cobalamin-derivative interconversion.

Expanded newborn screening. In the past several years, the implementation of tandem mass spectrometry (MS/MS) in newborn screening by many states in the US and countries worldwide has identified newborns with methylmalonic acidemia through detection of elevated propionylcarnitine (C3), a metabolite increased in the blood of individuals with methylmalonic acidemia and the related disorder, propionic acidemia [Chace et al 2001].

Elevated C3 is usually observed during MS/MS testing of blood spots from individuals with MUT-only disorders.

MS/MS screening can detect persons with cblC and theoretically cblD variant 2 and cblF.

Although total homocysteine (tHcy) can be detected by MS/MS in newborn blood spots, it has not been used in newborn screening to date.

Molecular Genetic Testing

GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information.—ED.

Genes. The following genes (complementation groups) are the only ones known to be associated with MTR-only and combined MTR/MUT cobalamin disorders:

• MMACHC (cblC)

• MTRR (cblE)

• MTR (cblG)

• MMACHC (cblD)

• LMBRD1 (cblF)

Clinical testing

• Sequence analysis of MMADHC (formerly referred to as C2orf25), MTR, MTRR, and MMACHC is available on a clinical basis.

• Targeted mutation analysis that detects the c.271dupA mutation in MMACHC is available on a clinical basis.

Table 4 summarizes molecular genetic testing for these disorders.

Table 4. Molecular Genetic Testing Used in Disorders of Intracellular Cobalamin Metabolism

Gene Symbol	Complementation Group	Test Method	Mutations Detected	Mutation Detection Frequency by Gene and Test Method	Test Availability
MMACHC	cblC	Sequence analysis	Sequence variants including c271dupA	Unknown	Clinical
		Targeted mutation analysis	c.271dupA
MTRR	cblE	Sequence analysis	Sequence variants		Clinical
MTR	cblG		Sequence variants		Clinical
MMADHC (C2orf25)	cblD		Sequence variants		Clinical
LMBRD1	cblF	Direct DNA 1	Sequence variants		Research 2

1. Direct DNA methods may include mutation analysis, mutation scanning, sequence analysis, or other means of molecular genetic testing to detect a genetic alteration in LMBRD1.

2. No laboratories offering clinical molecular genetic testing for this disease are listed in the GeneTests Laboratory Directory. However, clinical confirmation of mutations identified in research laboratories may be available for families in which the disease-causing mutations have been identified. For laboratories offering such testing, see .

Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.

Testing Strategy

Establishing the diagnosis of an MTR-only or combined MUT-MTR disorder in a proband

• Metabolic screening tests such as urine organic acid analysis and plasma amino acid analysis help to categorize the clinical syndrome.

  • Urine organic acid analysis. If elevated MMA excretion is detected, then the following intracellular cobalamin-related inborn errors should be considered: cblA, cblB, cblC, cblD, cblD variant 1, and cblF.

  • Plasma amino acid analysis

  • Other metabolites
    Plasma concentrations of MMA, methylcitrate, free and total carnitine, and fractionated acylcarnitines (to document propionylcarnitine) should be obtained.
    
    Plasma concentration of homocysteine should be measured as total homocysteine (not free homocysteine).
    
    Note: Urine homocystine measurements can provide supportive data but are not as sensitive as plasma total homocysteine concentration.

  • Serum vitamin B₁₂ analysis should be included to rule out vitamin B₁₂ deficiency as a cause of elevated blood or urine MMA levels.

• Molecular genetic testing. The role of molecular genetic testing in diagnosis is evolving. When molecular genetic testing identifies specific disease-causing mutations in a family, it may be faster and less expensive than complementation analysis; however, given the risk of identifying missense mutations of unknown significance, complementation analysis will be more definitive in some situations.

• Skin biopsy to construct a primary fibroblast cell line should be obtained as soon as the affected individual is stable, or, in the event of poor suspected outcome, prior to death. Studies on the cell line are needed for definitive diagnosis through cellular biochemical studies.
  
  Note: If the in vivo response to intramuscular vitamin B₁₂ in the form of hydroxycobalamin (OH-Cbl) is questionable or borderline, OH-Cbl administration should be continued until the results of the in vitro studies are available.

Carrier testing for at-risk relatives requires prior identification of the disease-causing mutations in the family.

Note: Carriers are heterozygotes for an autosomal recessive disorder and are not at risk of developing the disorder.

Prenatal diagnosis for at-risk pregnancies requires prior identification of the disease-causing mutations in the family.

Genetically Related (Allelic) Disorders

No other phenotypes are known to be associated with mutations in MMADHC (C2orf25), MTR, MTRR, or MMACHC.

Natural History

The clinical manifestations of the disorders of intracellular cobalamin metabolism (cblC, cblD, cblF, cblE, and cblG) can be highly variable even within a single complementation class.

Genotype-Phenotype Correlations

Preliminary reports have discussed some association between disease manifestation and mutations.

Morel et al [2006] compiled a review of phenotype/genotype correlations among 37 previously reported individuals with cblC:

• Of the 25 persons with early-onset disease, 17 were homozygous for c.271dupA or c.331C>T, or were compound heterozygotes for the two mutations.

• Of the 12 persons with late-onset disease, nine had a primarily neurodegenerative disease onset. Of those nine, four were c.394C>T homozygotes, two were c.271dupA/c.394C>T compound heterozygotes, and three were c.271dupA/other missense mutation heterozygotes.

Precise delineation of these relationships requires additional study.

Coelho et al [2008] identified the gene responsible for cblD. They described nine mutations in seven persons affected with cblD (as defined by complementation analysis). Each of the mutations was tested with cell culture assays designed to test methylcobalamin and adenosylcobalamin synthesis. Among the nine mutations, there were three each associated with MUT-only, MTR-only and combined MUT/MTR metabolic patterns. Analysis of the mutations associated with each pattern suggested a genotype/phenotype correlation. All of mutations that caused the MUT-only pattern were nonsense and frame-shifting mutations in exons 3 and 4. In contrast, the MTR-only mutations were all missense mutations in exons 6 – 8. Three frame-shifting mutations in exon 5, exon 8 and intron 7 caused a combined MUT/MTR phenotype. These data suggested to the authors that the MUT- and MTR- related MMADHC activities were localized to 3’ and 5’ regions of the gene respectively, and that translational re-initiation occurred in the open reading frame.

Nomenclature

The nomenclature for inherited disorders of intracellular cobalamin transport and processing is based on cellular complementation analysis that defines cobalamin groups A-G (cblA - cblG).

In general, cobalamin disorders are classified into those that affect MUT only (causing methylmalonic aciduria), MTR only (causing homocystinuria/hypomethioninemia), or both MUT and MTR.

Prevalence

The true prevalence of the cobalamin disorders is unknown. Hundreds of cases have been described for cblC, tens of cases for cblE and cblG, and fewer than ten cases each for cblD and cblF.

Disorders Causing Primarily Elevations in Methylmalonic Acid Excretion

Methylmalonic acidemia. As discussed in detail in Methylmalonic Acidemia, mutations in MUT, encoding methylmalonyl mutase (along with the cblA and cblB), cause a MUT-only phenotype.

"Benign" methylmalonic acidemia. Newborn screening in the province of Quebec identified infants with mild to moderate urinary MMA excretion. Follow-up revealed resolution in more than 50% of children, as well as an apparently benign, persistent, low-moderate hypermethylmalonic acidemia in some [Sniderman et al 1999]. Additional individuals with a relatively benign type of methylmalonic acidemia have been reported [Martens et al 2002].

Some of these individuals had a combined biochemical genetic phenotype of malonic and methylmalonic acidemia and therefore may not have a defect in methylmalonyl-CoA mutase activity or cobalamin metabolism [Sniderman et al 1999].

The long-term outcome and clinical phenotype of these individuals awaits further description.

SUCLA2 deficiency. Mild methylmalonic aciduria with Leigh syndrome is associated with deficiency in succinyl-CoA ligase [Ostergaard et al 2007, Carrozzo et al 2007].

Reye-like syndrome. A Reye-like syndrome of hepatomegaly and obtundation during a mild intercurrent infection is an unrecognized presentation of a number of inborn errors of metabolism, including methylmalonic acidemia [Chang et al 2000].

Other entities

• Benign methylmalonic acidemia with distal renal tubular acidosis (one sibship) [Dudley et al 1998]

• Malonyl-CoA decarboxylase deficiency, usually associated with significant MMA elevations and malonic acid increase relative to methylmalonic acidemia [Brown et al 1984]

• Elevated MMA and malonic acid with MMA predominating [Gregg et al 1998]

• Isolated methylmalonic acidemia, possibly caused by methylmalonic semialdehyde dehydrogenase deficiency [Roe et al 1998]

• Isolated methylmalonic aciduria and normal plasma concentrations of methylmalonic acidemia (one family) [Martens et al 2002]

• Cobalamin-unresponsive methylmalonic acidemia and progressive neurodegenerative disease with microcephaly and cataracts (two siblings) [Stromme et al 1995, Mayatepek et al 1996]

• A mitochondrial deletion syndrome with a combined propionic and methylmalonic acidemia syndrome [Yano et al 2003]

Disorders Causing Both Elevated Methylmalonic Acid Excretion and Increased Homocysteine

Vitamin B₁₂ deficiency. Individuals with vitamin B₁₂ deficiency can have methylmalonic acidemia and homocystinuria, as can the newborns of mothers who have vitamin B₁₂ deficiency. To establish the diagnosis of vitamin B₁₂ deficiency, it is necessary to measure vitamin B₁₂ serum concentration in both affected newborns and their mothers.

B₁₂ deficiency can occur in a breast-fed infant of a vegan mother and in an infant born to a mother with subclinical pernicious anemia. The mother may not necessarily have a very low serum concentration of vitamin B₁₂. Maternal vitamin B₁₂ deficiency can produce a methylmalonic acidemia syndrome in an infant that ranges from severe encephalopathy [Higginbottom et al 1978] to elevated propionylcarnitine detected by newborn screening [Chace et al 2001].

Intramuscular replacement therapy to normalize vitamin B₁₂ serum concentration reverses the metabolic abnormality.

Imerslund Gräsbeck syndrome. Features of this autosomal recessive disorder may include poor cobalamin absorption, abnormal renal tubular protein reabsorption, and urinary tract malformations. Two genes encoding intrinsic factor receptor components, CUBN encoding cubulin and AMN encoding amnionless, have been implicated [Grasbeck 2006].

Transcobalamin II deficiency. Transcobalamin II (distinguished from transcobalamin I AKA haptocorrin) is required for the movement of cobalamin from intestinal enterocytes into cells throughout the body. Transcobalamin II deficiency, a rare condition, is characterized by the infantile onset of megaloblastic anemia, failure to thrive, neurologic disease, and immunologic disease. Serum cobalamin concentrations are generally normal with a reduced (untreated) unsaturated B₁₂ binding capacity (UBBS) and a reduced level of transcobalamin II (the latter detected by an immunoassay) [Kaikov et al 1991, Rosenblatt & Fenton 2001]

Disorders Causing Primarily Hyperhomocysteinemia

Cystathionine beta synthase deficiency (CBS, classic homocystinuria) is a disorder of homocysteine catabolism with a Marfan syndrome-like phenotype, soft skin, lens dislocation, developmental delays/cognitive impairment, and thromboembolism. CBS is progressive with onset typically in childhood. Biochemically it is characterized by elevated serum concentration of methionine and decreased serum concentration of cysteine.

Methylenetetrahydrofolate reductase (MTHFR) deficiency is a defect in folate-dependent methylation pathways that results in diminished conversion of homocysteine to methionine. The biochemical hallmarks are moderate homocystinuria/homocysteinemia with low to normal plasma methionine levels. In contrast to methionine biosynthetic defects like cblE and cblG, megaloblastic anemia does not occur. The clinical course is characterized by varying severity, cognitive impairment, and white-matter disease [Fenton et al 2001].

Mild hyperhomocysteinemia can result from folate deficiency, vitamin B₁₂ deficiency, or mild mutations in enzymes in remethylation enzymes. It is thought to be a risk for premature vascular via thrombophilia.

Evaluations Following Initial Diagnosis

To establish the extent of disease in an individual diagnosed with an MTR-only cobalamin syndrome, the following evaluations are recommended:

• A serum chemistry panel (concentrations of Na+, K+, CI-, glucose, urea, creatinine, AST, ALT, alkaline phosphatase, bilirubin [T/U], triglycerides, and cholesterol)

• A complete blood count with differential

• Quantitative plasma amino acid analysis

• Plasma MMA concentration

• Urine organic acid analysis by GC/MS

• Plasma total homocysteine concentration

• Measurement of serum concentrations of vitamin B₁₂ and RBC folate to determine if a nutritional deficiency is present and determine if folate trapping (as MeTHF) in the face of normal plasma B₁₂ concentrations is occurring

• For affected children with elevated MMA excretion, arterial (or venous) blood gas, plasma ammonium concentration, serum lactate concentration, and urine ketone measurements

Note: These investigations can be used to assess/follow disease in the presence of a known diagnosis.

Treatment of Manifestations

Critically ill individuals must be stabilized, preferably in consultation with a metabolic specialist.

The severe acidotic/ketotic crises seen in isolated methylmalonic acidemia are less common in the combined MUT/MTR disorders. However, acute neurologic deterioration can punctuate a course marked by the insidious neurologic and developmental abnormalities seen in most patients. Aggressive therapy to both treat acidosis and reverse catabolism is warranted during acute deterioration:

• Base deficits may be corrected with volume replacement and intravenous bicarbonate solutions if indicated.

• Calories are initially supplied using dextrose-containing intravenous fluid given at high rates. During acute illness, adequate calories are important to arrest/prevent decompensation.

• Dietary protein is initially restricted, then liberalized as tolerated. Most patients can tolerate more natural protein than patients with isolated mutase lesions.

Exacerbations can be minimized using a combination of dietary management (with a low-protein, high-calorie diet low in propiogenic amino acid precursors) and cofactor therapy. Dietary manipulation has not been systematically evaluated for MTR-only cobalamin disorders.

Cobalamin supplementation is the mainstay of therapy and must be provided after plasma vitamin B₁₂ concentrations are measured and vitamin B₁₂ responsiveness is determined. Hydroxycobalamin* (1-mg intramuscular injections daily) is a reasonable starting point for therapy. Dosing practices vary; some providers increase the interval between injections based on clinical assessment and MMA excretion.

* Note: Hydroxycobalamin (not the cyano form) is the preferred preparation for treatment of cobalamin disorders.

Other manifestations such as infantile spasms and seizures need specialized management.

Gastrostomy tube placement for feeding is often required when feeding problems and growth failure are present.

Prevention of Primary Manifestations

Dietary managment. Patients with combined MUT/MTR disorders may be able to tolerate a normal diet. The optimal management of such patients is under debate [Bartholomew et al 1988].

Patients with MTR-only cobalamin disorders generally do not need protein restriction.

Hydroxycobalamin injections. Intramuscular injections of 1 mg hydroxycobalamin approximately once every one to three days are usually required in responsive individuals. However, some individuals can be managed with less frequent injections.

Other therapies that are usually given but have not been systematically evaluated include the following:

• Carnitine. For the combined MUT/MTR cobalamin disorders, carnitine can be given at a dose of 50-100 mg/kg/day. As a dietary supplement, carnitine may increase intracellular CoA pools and enhance the excretion of propionylcarnitine. However, the contribution of propionylcarnitine excretion to the total propionate load is small. The relief of intracellular CoA accretion may be the mechanism by which carnitine benefits some individuals.

• Betaine. Betaine augments the nonmethionine synthase-dependent conversion of homocysteine to methionine. Some reports have questioned the efficacy of this therapy in persons with disorders of intracellular cobalamin metabolism. Further studies are required to examine dosing and useful monitoring.
  
  Betaine is provided at the following doses:

  • Under age three years. 100 mg/kg/day, by mouth, divided into two doses

  • Over age three years. Up to 250 mg/kg/day, by mouth, divided into two doses [Bodamer & Lee 2006]

• Pyridoxine. Vitamin B₆ is a cofactor for cystathionine beta synthase and therefore has been proposed as a means of maximizing the removal of homocysteine. Persons with disorders of intracellular cobalamin metabolism generally do not respond to pyridoxine unless a dietary deficiency is present.

• Folate and folinic acid. Folic acid and folinic acid can potentially augment remethylation. The adult dose of folate is 1 mg by mouth per day, titratable down to 0.5 mg for maintenance. Doses for children and infants are available in the Harriet Lane Handbook and other common reference texts.

• Methionine supplementation. Hypomethioninemia associated with MUT/MTR and MTR-only disorders have been treated with methionine supplementation. The efficacy of this therapy is uncertain [Robb et al 1984, Smith et al 2006].

Surveillance

During the first year of life, infants may need to be evaluated once or twice a month. No guidelines recommend how often certain laboratory tests should be performed.

Routine medical care:

• Ophthalmologic evaluation for retinal and optic nerve changes

• Neurologic evaluation for early signs of psychomotor retardation, behavioral disturbances, seizures, and myelopathy

• Special attention to growth and development

Agents/Circumstances to Avoid

Potentially exacerbating circumstances:

• Prolonged fasting (longer than overnight without dextrose-containing intravenous fluids)

• Excessive dietary protein intake (greater than RDA for age or more than prescribed by a metabolic specialist)

Testing of Relatives at Risk

It is appropriate to measure the concentration of MMA, total plasma homocysteine, and/or related metabolites in urine and/or plasma of the sibs of a proband as soon as possible in order to institute treatment promptly in those who are affected.

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.

Other

MedicAlert^® bracelets and emergency treatment protocols outlining fluid, electrolyte, and cofactor therapy should be available for all affected individuals.

The use of cobalamin preparations other than hydroxycobalamin, including oral cyanocobalamin and intramuscular methylcobalamin injections (the latter for MTR-only types), has not been systematically evaluated.

Intramuscular cobalamin supplementation has been provided to pregnant women when the fetus is known to be affected by an intracellular cobalamin disorder. Several reports have documented the progression of disease-related complications in patients treated in utero [Patton et al 2000, Bodamer et al 2005].

Genetics clinics, staffed by genetics professionals, provide information for individuals and families regarding the natural history, treatment, mode of inheritance, and genetic risks to other family members as well as information about available consumer-oriented resources. See the GeneTests Clinic Directory.

See Consumer Resources for disease-specific and/or umbrella support organizations for this disorder. These organizations have been established for individuals and families to provide information, support, and contact with other affected individuals.

Mode of Inheritance

All of the disorders of intracellular cobalamin metabolism are inherited in an autosomal recessive manner.

Risk to Family Members

Parents of a proband

• The parents of an affected child are obligate heterozygotes (i.e., carriers of one mutant allele).

• Heterozygotes (carriers) are asymptomatic.

Sibs of a proband

• At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.

• Once an at-risk sib is known to be unaffected, the risk of his/her being a carrier is 2/3.

• Heterozygotes (carriers) are asymptomatic.

Offspring of a proband. The offspring of an individual with a disorder of intracellular cobalamin metabolism are obligate heterozygotes (carriers) for a disease-causing mutation.

Other family members of a proband. Each sib of the proband's parents is at a 50% risk of being a carrier.

Carrier Detection

Molecular genetic testing. Carrier testing using molecular genetic techniques is possible for some cobalamin disorders. Such testing requires that the disease-causing mutations in a family are known, and that clinical sequencing of the affected gene is available.

Biochemical genetic testing is not reliable enough for carrier testing.

Related Genetic Counseling Issues

See Management, Testing of Relatives at Risk for information on testing at-risk relatives for the purpose of early diagnosis and treatment.

Family planning

• The optimal time for determination of genetic risk and discussion of the availability of prenatal testing is before pregnancy.

• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected or at risk of being carriers.

DNA banking. Because it is likely that testing methodology and our understanding of genes, mutations, and diseases will improve in the future, consideration should be given to banking DNA (typically extracted from white blood cells) of affected individuals for possible future use. DNA banking is particularly relevant when available testing is inconclusive or molecular genetic testing is available on a research basis only. See for a list of laboratories offering DNA banking.

Prenatal Testing

Molecular genetic testing. Prenatal diagnosis for pregnancies at increased risk for cblC, cblD, cblE, or cblG is possible by analysis of DNA extracted from fetal cells obtained by amniocentesis usually performed at approximately 15 to 18 weeks' gestation or chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation. Both disease-causing alleles of an affected family member must be identified before prenatal testing can be performed.

Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.

Biochemical testing. Prenatal testing for pregnancies at 25% risk for MTR-only and MUT/MTR-combined cobalamin syndromes is possible by:

• Enzyme analysis of cultured fetal cells obtained by amniocentesis usually performed at approximately 15-18 weeks' gestation or chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation. Biochemical confirmation of the diagnosis in an affected family member must be obtained before prenatal testing can be performed.

• Amniotic fluid analysis of metabolites. The absolute positive predictive and negative predictive values of metabolite analysis only have yet to be determined but appear to aid the diagnosis in most cases [Morel et al 2005].

Prenatal diagnosis for both combined MUT/MTR and MTR-only cobalamin syndrome types has been demonstrated using complementation analysis of cultured amniocytes [Morel et al 2005]. Although clinical testing is available through specialized laboratories, examples of positive screens (thus confirming the efficacy of the method) have not been reported for all complementation classes.

Note: The most prudent recommendation is to perform both metabolite analysis AND enzymatic assays of cultured CVS or amniocytes.

No laboratories offering molecular genetic testing for prenatal diagnosis of cblF are listed in the GeneTests Laboratory Directory. However, prenatal testing may be available for families in which the disease-causing mutations have been identified. For laboratories offering custom prenatal testing, see .

Preimplantation genetic diagnosis (PGD) may be available for families in which the disease-causing mutations have been identified. For laboratories offering PGD, see .

Molecular Genetic Pathogenesis

Figure 1. Pathways utilizing cobalamin derivatives. (more...)

Figure 1. Pathways utilizing cobalamin derivatives. Cbl indicates cobalt atom and possible valence state (indicated by superscripted Roman numerals).
Modified from Chandler & Venditti [2005]

Figure1

Literature Cited

Published Statements and Policies Regarding Genetic Testing

No specific guidelines regarding genetic testing for this disorder have been developed.

Author Notes

Dr. Adams is a pediatrician and biochemical geneticist. He is an attending physician at the National Human Genome Research Institute at the National Institutes of Health.

Dr. Venditti is a pediatrician and biochemical geneticist. He is the director of the Organic Acid Disorder Research Unit at the National Human Genome Research Institute and an attending physician at the National Institutes of Health Clinical Center.

Revision History

• 11 August 2009 (cd) Revision: targeted mutation analysis for c.271dupA mutation in cblC and prenatal diagnosis for cblD and variants available clinically.

• 2 June 2009 (cd) Revision: sequence analysis available clinically for MMADHC mutations causing cblD, cblD variant 1, and cblD variant 2

• 25 February 2008 (me) Review posted to live Web site

• 22 December 2006 (cpv) Original submission