California's Experience with Guanidinoacetate Methyltransferase Enzyme Newborn Screening on the First 100,000 Samples
Biochemical/Metabolic and Therapeutics
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Introduction:
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. Newborn screening for GAMT was recently initiated in California using a two-tier approach for the detection of GAMT biomarkers (GUAC, creatine and creatinine) in newborn dried blood spot (DBS). Herein we review the results of the first 100,000 samples tested from July to October 2024.
Methods:
Testing on DBS for GAMT biomarkers was conducted by tier-1 screening by flow injection analysis tandem mass spectrometry (FIA-MS/MS) in multiple reaction monitoring (MRM) positive ion mode with a total run time of 1.5 minutes per sample. Samples above the tier-1 cutoff were then tested by a more specific tier-2 ultra-performance liquid chromatography (UPLC)-MS/MS method to separate GAMT biomarkers from potential isobaric interfering compounds in MRM positive ion mode with a total run time of 3.5 minutes per sample. We conducted a review of our screening algorithm, referral rates and clinical outcomes for screen-positive newborns.
Results:
In total, we tested 103,124 newborns in the first three months of GAMT screening using mass spectrometry in California. Of these newborns, 88% were in regular nursery, 10% were in neonatal intensive care unit (NICU), 0.5% from home birth, 544 (0.53%) newborns were screen indeterminate (GUAC≥3 µmol/L and Ratio= GUAC/Creatine ≥6) for GAMT by the tier-1 screening. Samples that did not have a positive result for both markers were screen negative. All 544 indeterminate newborn samples were tested by a tier-2 method; of these, 28 (5.15%) newborns were screen positive (GUAC≥5 µmol/L) and 516 were screen negative for GAMT (GUAC<5 µmol/L). Out of 28 referred newborn-the average birth weight was 1,864 grams, with 89.3 % of babies being low birth weight (< 2,500 grams), with 7.14% very low birthweight (<1000 grams). The average gestational age was 34 weeks and 82.1% of babies were born premature (< 37 weeks), with 7.14% identified as extremely premature (< 28 weeks). 92.8% of babies who were referred for evaluation were in the NICU at the time of specimen collection, and they all received total parenteral nutrition (TPN). Remaining referred cases - 1 newborn from the regular nursery and 1 newborn from the NICU were not reported to be on TPN. 3.6% of babies passed away before a genetic cause was determined. 25 (89.3%) out of 28 newborn’s GUAC tier-2 value was <7 µmol/L and 14 of these have no evidence of GAMT deficiency per clinical resolution (12 No Disorder and 2 Could not be determined), 11 are still pending resolution. The rest 3 (11%) newborn’s GUAC tier-2 was >7 µmol/L, the clinical resolution is not determined yet. No cases of GAMT deficiency were confirmed from this first 3 months of testing.
Conclusion:
Population screening using a two-tier approach is highly sensitive for the early identification of patients with GAMT disorder. Based on the review of our screening algorithm, referral rates and clinical outcomes for screen-positive cases show that for all the 14 cases with tier-2 GUAC<7 µmol/L were resolved negative for GAMT deficiency and the remaining 11 pending cases with tier-2 GUAC<7 µmol/L are highly likely to be negative for GAMT deficiency. This makes a strong case for changing tier-2 GUAC cutoff to ≥7 µmol/L, that could reduce the false positives significantly from 5.15% to under 1%.
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. Newborn screening for GAMT was recently initiated in California using a two-tier approach for the detection of GAMT biomarkers (GUAC, creatine and creatinine) in newborn dried blood spot (DBS). Herein we review the results of the first 100,000 samples tested from July to October 2024.
Methods:
Testing on DBS for GAMT biomarkers was conducted by tier-1 screening by flow injection analysis tandem mass spectrometry (FIA-MS/MS) in multiple reaction monitoring (MRM) positive ion mode with a total run time of 1.5 minutes per sample. Samples above the tier-1 cutoff were then tested by a more specific tier-2 ultra-performance liquid chromatography (UPLC)-MS/MS method to separate GAMT biomarkers from potential isobaric interfering compounds in MRM positive ion mode with a total run time of 3.5 minutes per sample. We conducted a review of our screening algorithm, referral rates and clinical outcomes for screen-positive newborns.
Results:
In total, we tested 103,124 newborns in the first three months of GAMT screening using mass spectrometry in California. Of these newborns, 88% were in regular nursery, 10% were in neonatal intensive care unit (NICU), 0.5% from home birth, 544 (0.53%) newborns were screen indeterminate (GUAC≥3 µmol/L and Ratio= GUAC/Creatine ≥6) for GAMT by the tier-1 screening. Samples that did not have a positive result for both markers were screen negative. All 544 indeterminate newborn samples were tested by a tier-2 method; of these, 28 (5.15%) newborns were screen positive (GUAC≥5 µmol/L) and 516 were screen negative for GAMT (GUAC<5 µmol/L). Out of 28 referred newborn-the average birth weight was 1,864 grams, with 89.3 % of babies being low birth weight (< 2,500 grams), with 7.14% very low birthweight (<1000 grams). The average gestational age was 34 weeks and 82.1% of babies were born premature (< 37 weeks), with 7.14% identified as extremely premature (< 28 weeks). 92.8% of babies who were referred for evaluation were in the NICU at the time of specimen collection, and they all received total parenteral nutrition (TPN). Remaining referred cases - 1 newborn from the regular nursery and 1 newborn from the NICU were not reported to be on TPN. 3.6% of babies passed away before a genetic cause was determined. 25 (89.3%) out of 28 newborn’s GUAC tier-2 value was <7 µmol/L and 14 of these have no evidence of GAMT deficiency per clinical resolution (12 No Disorder and 2 Could not be determined), 11 are still pending resolution. The rest 3 (11%) newborn’s GUAC tier-2 was >7 µmol/L, the clinical resolution is not determined yet. No cases of GAMT deficiency were confirmed from this first 3 months of testing.
Conclusion:
Population screening using a two-tier approach is highly sensitive for the early identification of patients with GAMT disorder. Based on the review of our screening algorithm, referral rates and clinical outcomes for screen-positive cases show that for all the 14 cases with tier-2 GUAC<7 µmol/L were resolved negative for GAMT deficiency and the remaining 11 pending cases with tier-2 GUAC<7 µmol/L are highly likely to be negative for GAMT deficiency. This makes a strong case for changing tier-2 GUAC cutoff to ≥7 µmol/L, that could reduce the false positives significantly from 5.15% to under 1%.