Clinical characteristics: Disorders of intracellular cobalamin metabolism have a variable phenotype and age of onset that are influenced by the severity and location within the pathway of the defect. The prototype and best understood phenotype is cblC; it is also the most common of these disorders. The age of initial presentation of cblC spans a wide range:
In utero with fetal presentation of nonimmune hydrops, cardiomyopathy, and intrauterine growth restriction
Newborns, who can have microcephaly, poor feeding, and encephalopathy
Infants, who can have poor feeding and slow growth, neurologic abnormality, and, rarely, hemolytic uremic syndrome (HUS)
Toddlers, who can have poor growth, progressive microcephaly, cytopenias (including megaloblastic anemia), global developmental delay, encephalopathy, and neurologic signs such as hypotonia and seizures
Adolescents and adults, who can have neuropsychiatric symptoms, progressive cognitive decline, thromboembolic complications, and/or subacute combined degeneration of the spinal cord
Diagnosis/testing: The diagnosis of a disorder of intracellular cobalamin metabolism in a symptomatic individual is based on clinical, biochemical, and molecular genetic data. Evaluation of the methylmalonic acid (MMA) level in urine and blood and plasma total homocysteine (tHcy) level are the mainstays of biochemical testing. Diagnosis is confirmed by identification of biallelic pathogenic variants in one of the following genes (associated complementation groups indicated in parentheses): MMACHC (cblC), MMADHC (cblD-combined and cblD-homocystinuria), MTRR (cblE), LMBRD1 (cblF), MTR (cblG), ABCD4 (cblJ), THAP11(cblX-like), ZNF143(cblX-like), or a hemizygous variant in HCFC1 (cblX, which can show a cblC complementation class).
Management: Treatment of manifestations: Critically ill individuals must be stabilized, preferably in consultation with a metabolic specialist, by treating acidosis, reversing catabolism, and initiating parenteral hydroxocobalamin. Treatment of thromboembolic complications (e.g., HUS and thrombotic microangiopathy) includes initiation of hydroxocobalamin (OHCbl) and betaine or an increase in their doses. Long-term management focuses on improving the metabolic derangement by lowering plasma tHcy and MMA concentrations and maintaining plasma methionine concentrations within the normal range. Gastrostomy tube placement for feeding may be required; infantile spasms, seizures, congenital heart malformations, and hydrocephalus are treated using standard protocols.
Prevention of primary manifestations: Early institution of injectable hydroxocobalamin improves survival and may reduce but not completely prevent primary manifestations. To prevent metabolic decompensations, patients are advised to avoid situations that result in catabolism, such as prolonged fasting and dehydration, and always remain on a weight-appropriate dose of hydroxocobalamin.
Surveillance: During the first year of life, infants may need to be evaluated once or twice a month by a metabolic specialist to assess growth, nutritional status, feeding ability, and developmental and neurocognitive progress. Toddlers and school-age children should be evaluated at least twice a year to adjust medication dosing (hydroxocobalamin, betaine) during growth and evaluate nutritional status. Teens and adults may be seen on a yearly basis. Routine ophthalmologic, neurologic, and cardiac evaluations may also be appropriate.
Agents/circumstances to avoid: Prolonged fasting (longer than overnight without dextrose-containing intravenous fluids); dietary protein intake below the recommended dietary allowance for age or more than that prescribed by a metabolic specialist; methionine restriction including use of medical foods that do not contain methionine; and the anesthetic nitrous oxide.
Evaluation of relatives at risk: If the pathogenic variants in the family are known, at-risk sibs may be tested prenatally to allow initiation of treatment in utero or as soon as possible after birth.
If the newborn sib of an affected individual has not undergone prenatal testing, molecular genetic testing can be performed in the first week of life if the pathogenic variants in the family are known. Otherwise, evaluation of urine organic acids and plasma amino acids, measurement of total plasma homocysteine, serum methylmalonic acid analysis, and acylcarnitine profile analysis can be used for the purpose of early diagnosis and treatment.
Genetic counseling: The majority of disorders of intracellular cobalamin metabolism are inherited in an autosomal recessive manner. 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. The disorder of intracellular cobalamin metabolism caused by pathogenic variants in HCFC1 is inherited in an X-linked manner. The risk to sibs depends on the genetic status of the mother. If the mother of the proband has an HCFC1 pathogenic variant, the chance of transmitting it in each pregnancy is 50%. Males who inherit the pathogenic variant will be affected. Females who inherit the pathogenic variant will be heterozygous and will usually not be affected (no affected females have been described to date).
Once the pathogenic variant(s) have been identified in an affected family member, carrier testing for at-risk relatives, molecular genetic prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible.
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