A-320 Determination of Guanidinoacetate Methyltransferase Enzyme Biomarkers in Newborn Dried Blood Spot by Tandem Mass Spectrometry-A Two Tier Approach
Authors: Dhillon K , Zhou J , Aduviso J , Neogi P , Sharma R 
Published: October 2024

Abstract

Background
Guanidinoacetate methyltransferase enzyme (GAMT) deficiency is a rare, inherited metabolic disorder that affects the synthesis of creatine. Creatine is an essential metabolite for regenerating energy, particularly in the brain and muscles. Creatine is synthesized from guanidinoacetate (GUAC) by the GAMT enzyme. Mutations in the GAMT gene result in a shortage of the GAMT enzyme that causes a deficiency of creatine and accumulation of GUAC in the body, which is a neurotoxin. Following a recommendation by the Recommended Uniform Screening Panel and in line with California State statute, all California newborns will be screened for GAMT deficiency beginning July 2024. Here, we developed and validated a quantitative two-tier tandem mass spectrometry (MS/MS) assay for the detection of GAMT biomarkers (GUAC, creatine and creatinine) in newborn dried blood spot (DBS).

Methods
This method punches a 3.2 mm disc of controls and newborn patient DBS into a 96-well plate. 100 μL of extraction solution, containing internal standards of GAMT biomarkers, is added to each well and incubated at 40°C for 45 minutes. The extract is transferred, evaporated, and derivatized into a 3N-butanol-HCl solution at 60°C for 30 minutes. The excess HCl is evaporated to dryness. The conversion of GUAC and creatine to their butyl esters has improved their detection sensitivity by MS/MS. The butyl esters are reconstituted into the mobile phase and shaken for 10 minutes at 27°C. The final 10 μL extract was analyzed by flow injection analysis (FIA)-MS/MS in multiple reaction monitoring (MRM) positive ion mode with a total run time of 1.5 minutes per sample. The presumptive positive samples from initial analysis are prepared following the same method, and a 5 μL extract is resolved through an ultra-performance liquid chromatography (UPLC) column and analyzed by UPLC-MS/MS in MRM positive ion mode with a total run time of 3.5 minutes per sample. The validation was performed by assessing the linearity, accuracy, imprecision, carryover, limit of detection (LOD) and lower limit of quantitation (LLOQ), matrix effect, analytical measurement range (AMR), percent total allowable error (TEa), and reference range.

Results
This method is linear for GUAC, creatine and creatinine over a measured range of 1.17-40.00, 121.20-857.36 and 41.34-387.46 µmol/L respectively. The R2 values=1.000, 0.997, 0.9998; slope=0.999, 0.9945, 0.9982; intercept=0.0398, 4.881, 1.7027; mean recoveries=99-104%, 100-104%, 100-104%; intra-assay precision (n=20) =4.05-5.95%, 4.62-5.31%, 3.27-5.19%, inter-assay precision (n=40)=3.7-12.6%, 4.3-7.6%, 3.8-7.4%; TEa=6-21%, 7-17%, 6-17%; LOD=0.12, 0.54, 1.11 µmol/L; AMR=0.33-1300, 1.11-2700, 3.50-4500 µmol/L; median patient results observed from 2293 normal patient=1.48, 494.02, 89.97 μmol/L for GUAC, creatine, and creatinine respectively. The tier-1 and tier-2 assay screening cutoff for GUAC at the population 95th percentile is 2.53 and 4.55 μmol/L respectively.

Conclusions
The tier-1 and tier-2 methods has been developed and validated to determine GAMT biomarkers in newborn DBS, making it suitable for routine screening of ∼2000 newborn specimens per day by tier-1 and followed by more specific tier-2 to separate GAMT biomarkers from potential isobaric interfering compounds. This two-tier approach greatly improved the test performance and significantly reduced false positive rate.