Gavrilov, Dimitar K.

Abstractα‐Methyl‐acyl‐CoA‐racemase (AMACR) deficiency (OMIM 604489) is a rare peroxisomal disorder with a variable age of onset from infancy to late adulthood. We describe a 45‐year‐old male with a history of seizures who presented with relapsing encephalopathy. Laboratory studies revealed an elevated serum pristanic acid concentration, an elevated pristanic/phytanic acid ratio, as well as the previously described homozygous mutation in the AMACR gene, c.154T>C, consistent with AMACR deficiency. This homozygous mutation is associated with a variable phenotype ranging from neonatal cholestasis to late‐onset sensorimotor neuropathy. Dietary pristanic acid restriction was attempted to improve clinical status and the patient has remained in remission for more than 16 months.

BACKGROUNDNewborn screening (NBS) for inborn errors of propionate, methionine, and cobalamin metabolism relies on finding abnormal concentrations of methionine and propionylcarnitine. These analytes are not specific for these conditions and lead to frequent false-positive results. More specific markers are total homocysteine (tHCY), methylmalonic acid (MMA), and methylcitric acid (MCA), but these markers are not detected by current NBS methods. To improve this situation, we developed a method for the detection of tHCY, MMA, and MCA in dried blood spots (DBSs) by liquid chromatography–tandem mass spectrometry (LC-MS/MS).METHODSThe analytes were extracted from a single 4.8-mm DBS punch with acetonitrile:water:formic acid (59:41:0.42) containing dithiothreitol and isotopically labeled standards (d3-MMA, d3-MCA, d8-homocystine). The extract was dried and treated with 3 N HCl in n-butanol to form butylesters. After evaporation of the butanol, the residue was reconstituted and centrifuged and the supernatant was subjected to LC-MS/MS analysis. Algorithms were developed to apply this method as an efficient and effective second-tier assay on samples with abnormal results by primary screening.RESULTSThe 99th percentiles determined from the analysis of 200 control DBSs for MMA, MCA, and HCY were 1.5, 0.5, and 9.8 μmol/L, respectively. Since 2005, prospective application of this second-tier analysis to 2.3% of all NBS samples led to the identification of 13 affected infants.CONCLUSIONSApplication of this assay reduced the false-positive rate and improved the positive predictive value of NBS for conditions associated with abnormal propionylcarnitine and methionine concentrations.