Abstract
Marked elevations in serum vitamin B12 (2523 pg/ml) and total serum folate (60 ng/ml) were noted in a malnourished, hypotonic eight month old hispanic male with macrocytic anemia. A positive urine nitroprusside screen was followed by measurements of homocystine(HCy)(36.5μmol/d), elevated cystathionine (40.8μmol/d) and methylmalonic acid(MMA) (457 mg/d) in 24h urine collections. This excretion of HCy and MMA suggested an aberration in vitamin B12 handling since both pathways require cobalamin(Cbl) cofactors. Analysis of pretreatment serum by TLC and bioautography showed more than half the Cbl present migrated with the cyano-form(CN-Cbl), a marked reduction in the HCy cofactor(MeCbl) and a nearly normal percent of MMA cofactor(AdoCbl). Family screening studies did not reveal comparable levels of HCy or MMA. HCy loading indicated a delayed maternal methionine clearance. A 90% reduction in 24h MMA excretion followed intramuscular hydroxocobalamin(OHCbl) but no consistent reduction in plasma or urine HCy was achieved on an unregulated diet. Anemia and macro-cytosis resolved as muscle tone, auditory responsiveness and nutritive sucking improved with alternate day OHCbl therapy. Our patient is most like Cbl-C variants previously described with severe multi-system involvement. Early macrocytic anemia, elevated serum vitamin B12, and a unique cobalamin pattern suggest a potential new variant in this metabolic pathway.
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