AGAT Deficiency (GATM-related creatine synthesis defect) — Comprehensive Disease Characteristics Report

Target disease: AGAT Deficiency (autosomal recessive creatine biosynthesis disorder; a cerebral creatine deficiency disorder). (mulik2023creatinedeficiencydisorders pages 2-3, longo2011disordersofcreatine pages 2-3)

Key recent sources prioritized: - ClinGen CCDS Variant Curation Expert Panel (VCEP) specifications for GATM variant interpretation (May 2024). (goldstein2024clingenvariantcuration pages 3-4) - Review of creatine deficiency disorders (Mar 2023). (mulik2023creatinedeficiencydisorders pages 2-3) - CCDS diagnostic implementation statistics (Swiss laboratory study, Jan 2025). (kaufman2025diagnosticdelayin pages 1-2) - MRS case report demonstrating treatment response (Dec 2024). (garg2024magneticresonancespectroscopy pages 2-4)

1. Disease Information

1.1 Definition / overview

AGAT deficiency is an ultrarare inborn error of creatine biosynthesis caused by biallelic loss-of-function variants in GATM, encoding L-arginine:glycine amidinotransferase (AGAT), the first step in creatine synthesis. It causes cerebral creatine deficiency detectable by proton magnetic resonance spectroscopy (1H-MRS) and a neurodevelopmental phenotype (developmental delay/intellectual disability with prominent speech-language impairment), often with myopathy/proximal weakness. (edvardson2010larginineglycineamidinotransferase(agat) pages 1-2, stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2, longo2011disordersofcreatine pages 2-3)

1.2 Key identifiers and ontologies

• MONDO: MONDO:0012996 (AGAT deficiency) (from Open Targets disease-target association output). (ndika2012developmentalprogressand pages 1-2)
• OMIM (disease): 612718 (AGAT deficiency) (longo2011disordersofcreatine pages 2-3)
• OMIM (gene): GATM 602360 (mulik2023creatinedeficiencydisorders pages 2-3)

Not retrieved in this tool run (needs external lookup to complete): Orphanet ID, MeSH ID, ICD-10/ICD-11 codes.

1.3 Synonyms / alternative names

• L-arginine:glycine amidinotransferase deficiency (edvardson2010larginineglycineamidinotransferase(agat) pages 1-2, stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2)
• GATM-related creatine deficiency / creatine synthesis defect (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2)
• Cerebral creatine deficiency disorder/syndrome (CCDD/CCDS), enzyme-defect subgroup (kaufman2025diagnosticdelayin pages 1-2)

Note: The synonym “cerebral creatine deficiency syndrome 3” appears in a CCDS case-report context (garg2024magneticresonancespectroscopy pages 2-4) but was not consistently used across the core genetics/biochemical literature retrieved here.

1.4 Evidence sources (patient-level vs aggregated)

Most clinical knowledge is derived from individual case reports and small case series (e.g., 16 patients worldwide in a 2015 cohort) and synthesized in reviews and expert-consensus variant interpretation guidance (ClinGen VCEP). (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2, goldstein2024clingenvariantcuration pages 3-4)

2. Etiology

2.1 Disease causal factors

Primary cause: biallelic pathogenic variants in GATM leading to deficient AGAT enzyme activity (loss of function), impairing endogenous creatine synthesis. (mulik2023creatinedeficiencydisorders pages 2-3, goldstein2024clingenvariantcuration pages 3-4)

Authoritative expert consensus (ClinGen VCEP, 2024): the panel “determined loss-of-function is the disease mechanism” for GATM and applies the ACMG/AMP PVS1 framework to null variants expected to undergo NMD. (goldstein2024clingenvariantcuration pages 3-4)

2.2 Risk factors

• Genetic risk factor: inheriting two pathogenic GATM alleles (autosomal recessive). Heterozygous parents are typically asymptomatic. (mulik2023creatinedeficiencydisorders pages 2-3)
• Consanguinity: reported in at least one adult case report family structure. (verma2010arginineglycineamidinotransferasedeficiency pages 1-3)

No specific environmental/exogenous risk factors were identified in the retrieved evidence; AGAT deficiency is primarily a Mendelian enzymatic disorder.

2.3 Protective factors

No validated protective genetic variants or environmental protective factors were identified in the retrieved human evidence.

Model-organism inference: systemic AGAT deficiency in mice was associated with protection from metabolic syndrome (a separate phenotype outside the core neurodevelopmental disorder), suggesting complex systemic metabolic consequences of creatine depletion. (ndika2012developmentalprogressand pages 1-2)

2.4 Gene–environment interactions

No direct gene–environment interaction evidence specific to AGAT deficiency was identified in the retrieved literature.

3. Phenotypes

3.1 Core clinical phenotype spectrum

Across the largest human cohort retrieved (n=16), the dominant presentation was neurodevelopmental impairment with frequent myopathy: - Intellectual disability/developmental delay: 15/16 patients. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2) - Myopathy/proximal muscle weakness: 8/16 patients. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2) - Language delay / severe speech-language disorder: commonly reported. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2, pintilie2021ararebut pages 4-5) - Behavioral/psychiatric features (including autistic-like features): reported in multiple families/reviews. (pintilie2021ararebut pages 4-5, ndika2012developmentalprogressand pages 1-2) - Seizures: rare/variable; present in some reports. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2, ndika2012developmentalprogressand pages 1-2)

3.2 Age of onset, severity, and progression

• Age at diagnosis has ranged from 3 weeks to 23 years across early literature summarized in a case report review. (ndika2012developmentalprogressand pages 5-6)
• Adult presentations emphasizing insidious proximal weakness beginning in late adolescence/early adulthood have been described. (verma2010arginineglycineamidinotransferasedeficiency pages 1-3)
• Neurodevelopmental outcomes are strongly time-dependent with treatment initiation (see Treatment section). (mulik2023creatinedeficiencydisorders pages 2-3, stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2)

3.3 Laboratory/biochemical phenotypes (laboratory abnormalities)

• Low/undetectable guanidinoacetate (GAA) in urine and plasma is a hallmark. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2, mulik2023creatinedeficiencydisorders pages 2-3)
• Low (or low-normal) creatine in body fluids and absent/markedly reduced brain creatine peak by 1H-MRS. (mulik2023creatinedeficiencydisorders pages 2-3, stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 2-3)

3.4 Suggested HPO terms (non-exhaustive)

Based on retrieved clinical descriptions: - Developmental delay — HP:0001263 (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2) - Intellectual disability — HP:0001249 (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2) - Speech delay / severe speech impairment — HP:0000750 (speech delay) / HP:0002463 (aphasia may be too specific) (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2) - Hypotonia — HP:0001252 (ndika2012developmentalprogressand pages 1-2) - Proximal muscle weakness / myopathy — HP:0003701 (proximal muscle weakness) / HP:0003198 (myopathy) (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2) - Seizures — HP:0001250 (ndika2012developmentalprogressand pages 1-2) - Autistic-like behavior / behavioral abnormality — HP:0000729 (autistic behavior) / HP:0000708 (behavioral abnormality) (pintilie2021ararebut pages 4-5)

Note: HPO IDs are standard ontology suggestions; the supporting evidence for the phenotype presence is from the cited papers above.

4. Genetic/Molecular Information

4.1 Causal gene

• Gene: GATM (encodes AGAT; mitochondrial glycine amidinotransferase). (longo2011disordersofcreatine pages 2-3)

4.2 Inheritance

• Autosomal recessive inheritance with biallelic pathogenic variants. (mulik2023creatinedeficiencydisorders pages 2-3, goldstein2024clingenvariantcuration pages 3-4)

4.3 Pathogenic variant spectrum and examples

Reported variant types include truncating/null variants, missense variants, and splice variants. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 5-7)

Examples explicitly mentioned in retrieved evidence: - c.484+1G>T (splice) in a long-term supplementation case series context. (ndika2012developmentalprogressand pages 1-2) - p.W149X (nonsense) reported in the Longo review excerpt and as a shared variant in an Italian family follow-up cohort (p.Trp149). (longo2011disordersofcreatine pages 2-3, battini2017fifteenyearfollowupof pages 2-4) - 1111_1112insA (frameshift insertion) in a two-patient report. (edvardson2010larginineglycineamidinotransferase(agat) pages 1-2) - R169X* (nonsense) in an adult-onset myopathy report. (verma2010arginineglycineamidinotransferasedeficiency pages 1-3)

4.4 Variant interpretation guidance (expert consensus; 2024)

The ClinGen CCDS VCEP created gene- and disease-specific ACMG/AMP specifications for GATM, explicitly integrating disease biomarkers (GAA/creatine in body fluids and brain MRS creatine) into phenotype evidence (PP4) via a points-based system. (goldstein2024clingenvariantcuration pages 8-9, goldstein2024clingenvariantcuration pages 13-18)

Key quantitative thresholds reported for GATM by the VCEP include: - Estimated prevalence used for calculations: 1 in 3,450,000 (goldstein2024clingenvariantcuration pages 4-6) - Allele frequency thresholds: BA1 >0.0005, BS1 >0.0001, PM2_supporting <0.000055. (goldstein2024clingenvariantcuration pages 13-18)

4.5 Modifier genes / epigenetics / chromosomal abnormalities

No modifier genes, epigenetic mechanisms, or chromosomal abnormalities specific to AGAT deficiency were identified in the retrieved evidence.

5. Environmental Information

No disease-specific environmental toxins, lifestyle contributors, or infectious triggers were identified in the retrieved evidence. AGAT deficiency is primarily a genetic enzymatic deficiency. (mulik2023creatinedeficiencydisorders pages 2-3, goldstein2024clingenvariantcuration pages 3-4)

6. Mechanism / Pathophysiology

6.1 Core biochemical pathway defect

AGAT catalyzes the first and rate-limiting step of creatine biosynthesis: - Arginine + glycine → guanidinoacetate (GAA) + ornithine. (edvardson2010larginineglycineamidinotransferase(agat) pages 1-2, verma2010arginineglycineamidinotransferasedeficiency pages 1-3)

Loss of AGAT activity causes: - Marked reduction of GAA production (low/undetectable GAA in urine/plasma). (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2) - Secondary creatine deficiency, including in brain, detectable as absent/markedly reduced creatine signal by 1H-MRS. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 2-3)

6.2 Causal chain (upstream → downstream)

• Upstream trigger: biallelic loss-of-function variants in GATM → reduced/absent AGAT enzyme function. (goldstein2024clingenvariantcuration pages 3-4)
• Metabolic consequence: low GAA and deficient creatine synthesis → inadequate creatine/phosphocreatine availability, particularly in energy-demanding tissues (brain, muscle). (mulik2023creatinedeficiencydisorders pages 2-3, stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2)
• Clinical consequence: neurodevelopmental impairment with major speech/language deficits and myopathy/proximal weakness; seizures variably. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2)

6.3 Key biochemical abnormalities (diagnostic biomarkers)

• Low/undetectable GAA in plasma/urine and low creatine; absent or markedly decreased brain creatine peak on 1H-MRS. (mulik2023creatinedeficiencydisorders pages 2-3)

6.4 Suggested GO and CL terms (mechanism-oriented suggestions)

• GO:0006600 creatine biosynthetic process (general) (supported conceptually by creatine synthesis defect described in multiple sources). (mulik2023creatinedeficiencydisorders pages 2-3, edvardson2010larginineglycineamidinotransferase(agat) pages 1-2)
• GO:0003848 arginine:glycine amidinotransferase activity (enzyme function) (edvardson2010larginineglycineamidinotransferase(agat) pages 1-2)
• CL terms (likely impacted cell types):
• Neurons (e.g., cortical neurons) and skeletal muscle myocytes as high-energy demand cell types implicated by clinical phenotype and MRS/myopathy findings. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2)

7. Anatomical Structures Affected

7.1 Organ/system level

• Central nervous system: cerebral creatine deficiency on MRS, neurodevelopmental phenotype. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 2-3, garg2024magneticresonancespectroscopy pages 2-4)
• Skeletal muscle: proximal myopathy/weakness in many patients; adult myopathy presentations reported. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2, verma2010arginineglycineamidinotransferasedeficiency pages 1-3)

7.2 Suggested UBERON terms (examples)

• Brain — UBERON:0000955 (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 2-3)
• Skeletal muscle tissue — UBERON:0001134 (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2)

7.3 Subcellular localization (suggested)

AGAT is encoded by mitochondrial glycine amidinotransferase (gene-level description in review literature), suggesting mitochondrial relevance in creatine biosynthesis and cellular energy buffering. (longo2011disordersofcreatine pages 2-3)

8. Temporal Development

8.1 Onset

Common onset is in infancy/early childhood with developmental delay and speech delay, though diagnosis may occur later and adult-onset myopathy has been described. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2, verma2010arginineglycineamidinotransferasedeficiency pages 1-3, ndika2012developmentalprogressand pages 5-6)

8.2 Progression/course

• Without early treatment, persistent neurocognitive and language deficits may occur. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2, battini2017fifteenyearfollowupof pages 7-8)
• With treatment, cerebral creatine can be replenished over months (tracked by serial MRS), and muscle symptoms may improve relatively rapidly. (verma2010arginineglycineamidinotransferasedeficiency pages 1-3, battini2017fifteenyearfollowupof pages 4-7)

9. Inheritance and Population

9.1 Epidemiology (rarity)

AGAT deficiency is consistently described as ultrarare, with <20–25 individuals reported in the literature in recent summaries. (mulik2023creatinedeficiencydisorders pages 2-3, goldstein2024clingenvariantcuration pages 1-3)

9.2 Prevalence / carrier frequency estimates

• A 2023 review reports an estimated carrier frequency of 0.077% (method not detailed in excerpt). (mulik2023creatinedeficiencydisorders pages 2-3)
• ClinGen VCEP (for variant-frequency threshold setting) used an estimated prevalence of 1 in 3,450,000 for GATM-related disease. (goldstein2024clingenvariantcuration pages 4-6)

9.3 Penetrance/expressivity

ClinGen VCEP treated biallelic pathogenic variants in GATM as fully penetrant for purposes of allele-frequency calculations and variant interpretation (maximum genetic contribution and penetrance set to 100%). (goldstein2024clingenvariantcuration pages 4-6)

10. Diagnostics

10.1 Core diagnostic tests and biomarkers

Biochemical testing - Urine and plasma GAA and creatine measurements are central; low/undetectable GAA with low creatine supports AGAT deficiency. (mulik2023creatinedeficiencydisorders pages 2-3, stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2)

Neuroimaging - 1H-MRS: absent/markedly decreased brain creatine peak is a hallmark; MRI can be normal. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 2-3, garg2024magneticresonancespectroscopy pages 2-4)

Genetic confirmation - GATM sequencing (single gene, panel, WES/WGS) to confirm biallelic pathogenic variants. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 7-8)

Functional confirmation - In uncertain genetic cases, AGAT enzyme activity in fibroblasts may help resolve interpretation. (mulik2023creatinedeficiencydisorders pages 2-3, goldstein2024clingenvariantcuration pages 13-18)

10.2 Real-world diagnostic implementation and delays (statistics)

A Swiss cross-sectional/systems study (2015–2023) on cerebral creatine deficiency disorders reported: - Diagnostic/therapeutic delay 3–32 months (mean 13.8 months) in their cohort. (kaufman2025diagnosticdelayin pages 1-2) - Total 4,967 guanidinoacetate and creatine measurements performed (urine+plasma) across two Swiss centers (2015–2023). (kaufman2025diagnosticdelayin pages 4-5) - Testing volume increased from 312 analyses (2015) to 883 (2023). (kaufman2025diagnosticdelayin pages 2-4) - Urine is described as “the preferred sample for CCDD detection” and “clearly the best matrix for the initial selective screening.” (kaufman2025diagnosticdelayin pages 1-2, kaufman2025diagnosticdelayin pages 6-8)

10.3 Differential diagnosis (high-level)

AGAT deficiency should be differentiated from other cerebral creatine deficiency disorders: - GAMT deficiency (elevated GAA rather than low) and SLC6A8 creatine transporter deficiency (different urine creatine/creatinine patterns; often poor response to creatine). (mulik2023creatinedeficiencydisorders pages 1-2, kaufman2025diagnosticdelayin pages 1-2)

10.4 Screening

Newborn screening feasibility and challenges - The 2015 cohort argues AGAT deficiency is “an ideal candidate for newborn screening” because early treatment can prevent adverse outcomes, but notes it is “difficult to devise a sensitive screening algorithm based on GAA quantitation alone,” proposing multianalyte DBS algorithms or enzyme assay approaches. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 7-8)

11. Outcome / Prognosis

11.1 Prognosis with treatment (long-term data)

A 15-year follow-up cohort (Italy; 4 patients) indicates: - Long-term oral creatine is generally safe and well tolerated; renal function was preserved with monitoring, though occasional kidney stones and other side effects occurred. (battini2017fifteenyearfollowupof pages 4-7) - Early treatment can prevent adverse developmental outcomes; later-treated patients often have persistent neurocognitive deficits, but adaptive functioning can improve. (battini2017fifteenyearfollowupof pages 1-2)

11.2 Prognostic factors

Age at treatment initiation is repeatedly highlighted as the dominant prognostic factor for neurocognitive outcomes. (mulik2023creatinedeficiencydisorders pages 2-3, stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2)

12. Treatment

12.1 Standard-of-care pharmacotherapy

Creatine monohydrate supplementation is the main disease-specific therapy.

Dosing ranges reported across studies: - 100–800 mg/kg/day across cohorts and reviews. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2, mulik2023creatinedeficiencydisorders pages 2-3) - A 2023 review states: “While all 3 disorders are currently treated with creatine supplementation,” and specifies for AGAT deficiency that oral creatine has been used and benefit depends on early initiation. (mulik2023creatinedeficiencydisorders pages 1-2)

Evidence for time-dependent neurocognitive benefit (review): - Cognitive restoration reported when treatment started <2 years, but not when started after age 10 in the 2023 review summary. (mulik2023creatinedeficiencydisorders pages 2-3)

Long-term treatment strategies / tapering (Italian follow-up cohort): - Symptomatic patients began at ~400 mg/kg/day, later reduced to 200–300 mg/kg/day, then 100 mg/kg/day guided by MRS and biochemical monitoring. (battini2017fifteenyearfollowupof pages 1-2)

12.2 Treatment outcomes

• Creatine supplementation often increases cerebral creatine on MRS and improves myopathy; however, complete normalization of cerebral creatine is not universal even with very high doses. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 7-8)
• In an MRS-focused clinical case report (44-month-old), pretreatment MRS had no definable creatine peak at 3.0 ppm and follow-up MRS at 5 months showed reappearance of the creatine peak after supplementation. (garg2024magneticresonancespectroscopy pages 2-4)

12.3 Adverse effects and monitoring

Reported adverse events/side effects with chronic creatine therapy include: - Weight gain, polyuria/polydipsia, transient diarrhea with dose increases, urinary creatine crystals, and kidney stones (at least one asymptomatic). (battini2017fifteenyearfollowupof pages 4-7, stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 7-8)

Monitoring strategies include serial urine/plasma creatine and GAA and periodic MRS (1H/31P) to track brain replenishment and guide dosing. (battini2017fifteenyearfollowupof pages 4-7)

12.4 Pregnancy management (real-world implementation)

A pregnancy case report (2020) describes a woman with AGAT deficiency requiring close monitoring due to increased creatine demand: - Abstract quote: “Biochemical monitoring of Cr in biological fluids of the mother revealed a decline of the Cr concentrations… requiring prompt correction of the Cr dose.” (Sep 2020). (alessandri2020increasedcreatinedemand pages 1-2) - The mother’s creatine dose was increased (e.g., to 3 g/day mid-pregnancy), and the infant had normal brain creatine and typical developmental milestones at one year. (alessandri2020increasedcreatinedemand pages 2-4, alessandri2020increasedcreatinedemand pages 1-2)

12.5 Suggested MAXO terms (treatment action ontology; suggestions)

• Creatine supplementation therapy (oral) (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2)
• Magnetic resonance spectroscopy monitoring (treatment-response monitoring) (battini2017fifteenyearfollowupof pages 4-7)
• Genetic counseling (autosomal recessive disorder; prenatal testing possible when familial variants known) (mulik2023creatinedeficiencydisorders pages 2-3)

13. Prevention

13.1 Primary/secondary prevention

No primary prevention exists for a Mendelian enzymatic deficiency, but secondary prevention via early detection and early creatine supplementation is repeatedly emphasized.

Early treatment prevention concept: - A 2023 review notes the disorders are treatable with creatine, and early treatment in AGAT deficiency can prevent adverse outcomes. (mulik2023creatinedeficiencydisorders pages 2-3) - A 2015 cohort states: “Early treatment seems to prevent adverse developmental outcomes.” (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 7-8)

13.2 Newborn screening implementation

• Early Check expanded newborn screening program includes AGAT deficiency on its screening panel and operationalizes confirmatory testing and genetic counseling after positive screens (observational cohort; estimated enrollment 30,000 newborns). (NCT03655223 chunk 2)

14. Other Species / Natural Disease

No naturally occurring veterinary cases were retrievable in the accessible full texts for this run (a relevant 2026 dog paper was listed as unobtainable by the search tool). Therefore, no curated cross-species natural disease entry can be provided from the current evidence set.

15. Model Organisms

15.1 Mouse model evidence

An AGAT-deficient mouse model has been used to study systemic creatine depletion and metabolic consequences; one study reports that AGAT deficiency protects from metabolic syndrome (model-organism phenotype not directly equivalent to the human neurodevelopmental disorder but informative for systemic pathway biology). (ndika2012developmentalprogressand pages 1-2)

Key abstract quotes (supporting major claims)

1. Creatine deficiency disorders review (Mar 2023):

2. “Biallelic pathogenic variants in GATM result in l-arginine:glycine amidinotransferase deficiency.” (mulik2023creatinedeficiencydisorders pages 2-3)

3. Pregnancy in AGAT deficiency (Sep 2020):

4. “Biochemical monitoring of Cr in biological fluids of the mother revealed a decline of the Cr concentrations… requiring prompt correction of the Cr dose.” (alessandri2020increasedcreatinedemand pages 1-2)

5. Long-term Italian follow-up (Feb 2017):

6. “Consecutive MRS examinations have confirmed that Cr depletion in AGAT-d patients is reversible under Cr supplementation.” (ndika2012developmentalprogressand pages 1-2)

Summary artifact

The following table provides a compact disease knowledge-base-ready summary (identifiers, biomarkers, phenotypes, diagnostics, dosing, outcomes, screening):

Table: This table condenses the most actionable disease-level facts for GATM-related AGAT deficiency, including identifiers, biochemical hallmarks, clinical presentation, diagnostics, and treatment evidence with dosing ranges. It is useful as a quick-reference artifact for a disease knowledge base entry.

Clinical trials / real-world studies relevant to AGAT deficiency

1. Early Check: Expanded Screening in Newborns (ClinicalTrials.gov NCT03655223, posted 2018; observational; estimated enrollment 30,000)

2. Explicitly lists Agat Deficiency on its screening panel and provides confirmatory testing and genetic counseling workflow. URL: https://clinicaltrials.gov/study/NCT03655223 (NCT03655223 chunk 2)

3. Biomarker for Creatine Deficiency Syndromes (BioCDS) (ClinicalTrials.gov NCT02934854, posted 2018; observational; withdrawn, enrollment 0)

4. Proposed DBS LC/MRM-MS biomarker discovery/validation with GATM sequencing; withdrawn due to “Transition into BioMetabol.” URL: https://clinicaltrials.gov/study/NCT02934854 (NCT02934854 chunk 1)

Important limitations of this report (data not retrieved in-tool)

• Orphanet/MeSH/ICD codes were not retrieved via the available full texts and would require targeted database queries.
• Variant-level allele frequencies in gnomAD and comprehensive ClinVar variant lists are not included beyond the ClinGen thresholds and exemplar variants available in the retrieved excerpts.
• Many phenotype frequencies beyond IDD (15/16) and myopathy (8/16) were not extractable from the provided excerpts and would require full-table extraction from primary cohorts.

References

1. (mulik2023creatinedeficiencydisorders pages 2-3): Crystal Mulik and Saadet Mercimek-Andrews. Creatine deficiency disorders: phenotypes, genotypes, diagnosis, and treatment outcomes. Turkish Archives of Pediatrics, 58:129-135, Mar 2023. URL: https://doi.org/10.5152/turkarchpediatr.2023.23022, doi:10.5152/turkarchpediatr.2023.23022. This article has 13 citations.

2. (longo2011disordersofcreatine pages 2-3): Nicola Longo, Orly Ardon, Rena Vanzo, Elizabeth Schwartz, and Marzia Pasquali. Disorders of creatine transport and metabolism. American Journal of Medical Genetics Part C: Seminars in Medical Genetics, 157:72-78, Feb 2011. URL: https://doi.org/10.1002/ajmg.c.30292, doi:10.1002/ajmg.c.30292. This article has 135 citations.

3. (goldstein2024clingenvariantcuration pages 3-4): Jennifer Goldstein, Amanda Thomas-Wilson, Emily Groopman, Vimla Aggarwal, Simona Bianconi, Raquel Fernandez, Kim Hart, Nicola Longo, Nicole Liang, Daniel Reich, Heidi Wallis, Meredith Weaver, Sarah Young, and Saadet Mercimek-Andrews. Clingen variant curation expert panel recommendations for classification of variants in gamt, gatm and slc6a8 for cerebral creatine deficiency syndromes. Molecular Genetics and Metabolism, 142:108362, May 2024. URL: https://doi.org/10.1016/j.ymgme.2024.108362, doi:10.1016/j.ymgme.2024.108362. This article has 12 citations and is from a peer-reviewed journal.

4. (kaufman2025diagnosticdelayin pages 1-2): Christina Kaufman, Anaïs D’Andrea, Annette Hackenberg, Martin Poms, Olivier Braissant, and Johannes Häberle. Diagnostic delay in cerebral creatine deficiency disorders: lessons learned from a cross-sectional single center study, and guanidinoacetate and creatine measurements in switzerland between 2015 and 2023. Molecular and Cellular Pediatrics, Jan 2025. URL: https://doi.org/10.1186/s40348-024-00188-4, doi:10.1186/s40348-024-00188-4. This article has 2 citations.

5. (garg2024magneticresonancespectroscopy pages 2-4): Ankita Garg, Rajiv Gupta, Jayesh Ashok Kumar Modi, and Debolina Kabiraj. Magnetic resonance spectroscopy as a diagnostic tool in cerebral creatine deficiency syndrome 3. Case Reports in Clinical Radiology, 0:1-5, Dec 2024. URL: https://doi.org/10.25259/crcr_92_2024, doi:10.25259/crcr_92_2024. This article has 0 citations.

6. (edvardson2010larginineglycineamidinotransferase(agat) pages 1-2): Simon Edvardson, Stanley H. Korman, Amir Livne, Avraham Shaag, Ann Saada, Ruppen Nalbandian, Hyla Allouche-Arnon, J. Moshe Gomori, and Rachel Katz-Brull. L-arginine:glycine amidinotransferase (agat) deficiency: clinical presentation and response to treatment in two patients with a novel mutation. Molecular Genetics and Metabolism, 101:228-232, Oct 2010. URL: https://doi.org/10.1016/j.ymgme.2010.06.021, doi:10.1016/j.ymgme.2010.06.021. This article has 72 citations and is from a peer-reviewed journal.

7. (stockleripsiroglu2015arginineglycineamidinotransferase(agat) pages 1-2): Sylvia Stockler-Ipsiroglu, Delia Apatean, Roberta Battini, Suzanne DeBrosse, Kimberley Dessoffy, Simon Edvardson, Florian Eichler, Katherine Johnston, David M. Koeller, Sonia Nouioua, Meriem Tazir, Ashok Verma, Monica D. Dowling, Klaas J. Wierenga, Andrea M. Wierenga, Victor Zhang, and Lee-Jun C. Wong. Arginine:glycine amidinotransferase (agat) deficiency: clinical features and long term outcomes in 16 patients diagnosed worldwide. Molecular Genetics and Metabolism, 116:252-259, Dec 2015. URL: https://doi.org/10.1016/j.ymgme.2015.10.003, doi:10.1016/j.ymgme.2015.10.003. This article has 82 citations and is from a peer-reviewed journal.

8. (ndika2012developmentalprogressand pages 1-2): Joseph D.T. Ndika, Kathreen Johnston, James A. Barkovich, Michael D. Wirt, Patricia O'Neill, Ofir T. Betsalel, Cornelis Jakobs, and Gajja S. Salomons. Developmental progress and creatine restoration upon long-term creatine supplementation of a patient with arginine:glycine amidinotransferase deficiency. Molecular Genetics and Metabolism, 106:48-54, May 2012. URL: https://doi.org/10.1016/j.ymgme.2012.01.017, doi:10.1016/j.ymgme.2012.01.017. This article has 48 citations and is from a peer-reviewed journal.

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19. (battini2017fifteenyearfollowupof pages 4-7): Roberta Battini, M. Grazia Alessandrì, Claudia Casalini, Manuela Casarano, Michela Tosetti, and Giovanni Cioni. Fifteen-year follow-up of italian families affected by arginine glycine amidinotransferase deficiency. Orphanet Journal of Rare Diseases, Feb 2017. URL: https://doi.org/10.1186/s13023-017-0577-5, doi:10.1186/s13023-017-0577-5. This article has 22 citations and is from a peer-reviewed journal.

20. (goldstein2024clingenvariantcuration pages 1-3): Jennifer Goldstein, Amanda Thomas-Wilson, Emily Groopman, Vimla Aggarwal, Simona Bianconi, Raquel Fernandez, Kim Hart, Nicola Longo, Nicole Liang, Daniel Reich, Heidi Wallis, Meredith Weaver, Sarah Young, and Saadet Mercimek-Andrews. Clingen variant curation expert panel recommendations for classification of variants in gamt, gatm and slc6a8 for cerebral creatine deficiency syndromes. Molecular Genetics and Metabolism, 142:108362, May 2024. URL: https://doi.org/10.1016/j.ymgme.2024.108362, doi:10.1016/j.ymgme.2024.108362. This article has 12 citations and is from a peer-reviewed journal.

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22. (kaufman2025diagnosticdelayin pages 4-5): Christina Kaufman, Anaïs D’Andrea, Annette Hackenberg, Martin Poms, Olivier Braissant, and Johannes Häberle. Diagnostic delay in cerebral creatine deficiency disorders: lessons learned from a cross-sectional single center study, and guanidinoacetate and creatine measurements in switzerland between 2015 and 2023. Molecular and Cellular Pediatrics, Jan 2025. URL: https://doi.org/10.1186/s40348-024-00188-4, doi:10.1186/s40348-024-00188-4. This article has 2 citations.

23. (kaufman2025diagnosticdelayin pages 2-4): Christina Kaufman, Anaïs D’Andrea, Annette Hackenberg, Martin Poms, Olivier Braissant, and Johannes Häberle. Diagnostic delay in cerebral creatine deficiency disorders: lessons learned from a cross-sectional single center study, and guanidinoacetate and creatine measurements in switzerland between 2015 and 2023. Molecular and Cellular Pediatrics, Jan 2025. URL: https://doi.org/10.1186/s40348-024-00188-4, doi:10.1186/s40348-024-00188-4. This article has 2 citations.

24. (kaufman2025diagnosticdelayin pages 6-8): Christina Kaufman, Anaïs D’Andrea, Annette Hackenberg, Martin Poms, Olivier Braissant, and Johannes Häberle. Diagnostic delay in cerebral creatine deficiency disorders: lessons learned from a cross-sectional single center study, and guanidinoacetate and creatine measurements in switzerland between 2015 and 2023. Molecular and Cellular Pediatrics, Jan 2025. URL: https://doi.org/10.1186/s40348-024-00188-4, doi:10.1186/s40348-024-00188-4. This article has 2 citations.

25. (mulik2023creatinedeficiencydisorders pages 1-2): Crystal Mulik and Saadet Mercimek-Andrews. Creatine deficiency disorders: phenotypes, genotypes, diagnosis, and treatment outcomes. Turkish Archives of Pediatrics, 58:129-135, Mar 2023. URL: https://doi.org/10.5152/turkarchpediatr.2023.23022, doi:10.5152/turkarchpediatr.2023.23022. This article has 13 citations.

26. (battini2017fifteenyearfollowupof pages 1-2): Roberta Battini, M. Grazia Alessandrì, Claudia Casalini, Manuela Casarano, Michela Tosetti, and Giovanni Cioni. Fifteen-year follow-up of italian families affected by arginine glycine amidinotransferase deficiency. Orphanet Journal of Rare Diseases, Feb 2017. URL: https://doi.org/10.1186/s13023-017-0577-5, doi:10.1186/s13023-017-0577-5. This article has 22 citations and is from a peer-reviewed journal.

27. (alessandri2020increasedcreatinedemand pages 1-2): Maria Grazia Alessandrì, Francesca Strigini, Giovanni Cioni, and Roberta Battini. Increased creatine demand during pregnancy in arginine: glycine amidino-transferase deficiency: a case report. BMC Pregnancy and Childbirth, Sep 2020. URL: https://doi.org/10.1186/s12884-020-03192-4, doi:10.1186/s12884-020-03192-4. This article has 15 citations and is from a peer-reviewed journal.

28. (alessandri2020increasedcreatinedemand pages 2-4): Maria Grazia Alessandrì, Francesca Strigini, Giovanni Cioni, and Roberta Battini. Increased creatine demand during pregnancy in arginine: glycine amidino-transferase deficiency: a case report. BMC Pregnancy and Childbirth, Sep 2020. URL: https://doi.org/10.1186/s12884-020-03192-4, doi:10.1186/s12884-020-03192-4. This article has 15 citations and is from a peer-reviewed journal.

29. (NCT03655223 chunk 2): Early Check: Expanded Screening in Newborns. RTI International. 2018. ClinicalTrials.gov Identifier: NCT03655223

30. (garg2024magneticresonancespectroscopy pages 4-5): Ankita Garg, Rajiv Gupta, Jayesh Ashok Kumar Modi, and Debolina Kabiraj. Magnetic resonance spectroscopy as a diagnostic tool in cerebral creatine deficiency syndrome 3. Case Reports in Clinical Radiology, 0:1-5, Dec 2024. URL: https://doi.org/10.25259/crcr_92_2024, doi:10.25259/crcr_92_2024. This article has 0 citations.

31. (garg2024magneticresonancespectroscopy pages 1-2): Ankita Garg, Rajiv Gupta, Jayesh Ashok Kumar Modi, and Debolina Kabiraj. Magnetic resonance spectroscopy as a diagnostic tool in cerebral creatine deficiency syndrome 3. Case Reports in Clinical Radiology, 0:1-5, Dec 2024. URL: https://doi.org/10.25259/crcr_92_2024, doi:10.25259/crcr_92_2024. This article has 0 citations.

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Key Findings

Finding 1: AGAT Deficiency is a Rare Autosomal Recessive Creatine Biosynthesis Disorder Caused by Biallelic GATM Mutations

AGAT deficiency is the rarest of the three cerebral creatine deficiency syndromes (CCDS). The GATM gene (HGNC:4175, NCBI Gene 2628, Ensembl ENSG00000171766) on chromosome 15q21.1 encodes the mitochondrial enzyme L-arginine:glycine amidinotransferase. In the largest published cohort, 16 patients from 8 families of 8 different ethnic backgrounds were characterized (PMID: 26490222). The ClinGen Creatine Deficiency Syndromes Variant Curation Expert Panel (CCDS VCEP) has curated 45 variants in GATM to date (PMID: 38452609). Key identifiers include OMIM 612718, Orphanet 35704, and MONDO:0012999. As stated by Stockler-Ipsiroglu et al.: "Arginine:glycine aminotransferase (AGAT) (GATM) deficiency is an autosomal recessive inborn error of creative synthesis" (PMID: 26490222).

Finding 2: Clinical Phenotype — Intellectual Disability, Language Impairment, Myopathy, Behavioral Disturbances, and Epilepsy

In the cohort of 16 patients, 15/16 (94%) had intellectual disability or developmental delay, and 8/16 (50%) had myopathy or proximal muscle weakness (PMID: 26490222). Common features include severe language impairment and behavioral disorders. Recently, the phenotype has been expanded to include epilepsy: "This study presents the first reported epilepsy cases in AGAT deficiency" (PMID: 40674085). Detailed characterization revealed that "Two individuals had focal epilepsy with sensory seizures characterized by a prominent 'tingling' sensation. Three experienced febrile seizures plus and marked temperature sensitivity. Corpus callosum dysmorphisms were observed in three cases" (PMID: 40323733). Cortical thickness was significantly reduced across multiple brain regions despite creatine supplementation.

Finding 3: Creatine Supplementation is Effective, Especially When Started Early

Oral creatine monohydrate at 100–800 mg/kg/day results in almost complete restoration of brain creatine levels (PMID: 26490222). Two patients treated since ages 4 and 16 months had normal cognitive and behavioral development at ages 10–11 years. In one patient treated from 16 months, "8 years post initiation of oral creatine supplementation, patient demonstrates superior nonverbal and academic abilities, with average verbal skills" (PMID: 22386973). Late-treated patients showed limited cognitive improvement but significant myopathy improvement. The 15-year follow-up confirmed that "Cr treatment is considered safe and well tolerated but side effects, including weight gain and kidney stones, have been reported. Early treatment prevents adverse developmental outcome" (PMID: 28148286).

Finding 4: Distinctive Biochemical Signature Enables Differential Diagnosis

Low/undetectable GAA and low creatine in body fluids, combined with absent brain creatine on MRS, form the pathognomonic biochemical profile. As reported: "Common biochemical denominators were low/undetectable guanidinoacetate (GAA) concentrations in urine and plasma, and low/undetectable cerebral creatine levels" (PMID: 26490222). This distinguishes AGAT deficiency from GAMT deficiency (elevated GAA, low creatine) and creatine transporter deficiency (elevated urine creatine/creatinine ratio): "low guanidinoacetate and low creatine levels in body fluids in l-arginine:glycine amidinotransferase deficiency, and elevated creatine-to-creatinine ratio in urine in creatine transporter deficiency" (PMID: 36856349).

Finding 5: Mouse Model Reveals Cardiac and Muscle Vulnerability

AGAT-knockout mice exhibit reduced cardiac contractility with significantly lower L-type calcium channel current amplitude, slower inactivation, and slower calcium transient decay, rescued by creatine supplementation (PMID: 33275525). Additionally, "Simvastatin-induced motor impairment was exacerbated in AGAT-deficient mice compared with AGAT-overexpressing GAMT" deficient mice (PMID: 31853708). The GATM gene was associated with statin-induced myopathy in two human populations, suggesting translational relevance.

1. Disease Information

Overview

AGAT deficiency (also known as GATM deficiency, arginine:glycine amidinotransferase deficiency, or cerebral creatine deficiency syndrome 3) is the rarest of the three cerebral creatine deficiency syndromes (CCDS). It results from complete or near-complete loss of AGAT enzymatic activity, preventing the first step in endogenous creatine biosynthesis. The disease was first described in the early 2000s, following earlier discoveries of GAMT deficiency (1994) and creatine transporter deficiency (2001).

Key Identifiers

Synonyms and Alternative Names

• Arginine:glycine amidinotransferase deficiency (AGAT-d)
• GATM deficiency
• Cerebral creatine deficiency syndrome 3 (CCDS3)
• L-arginine:glycine amidinotransferase deficiency
• Creatine deficiency syndrome due to AGAT deficiency
• Glycine amidinotransferase deficiency

Information Source

This report is derived from aggregated disease-level resources (OMIM, Orphanet, PubMed literature, ClinVar, ClinGen) and individual patient-level case reports/case series. The largest single cohort study characterized 16 patients from 8 families (PMID: 26490222).

2. Etiology

Disease Causal Factors

AGAT deficiency is exclusively genetic in origin. It is caused by biallelic (homozygous or compound heterozygous) pathogenic variants in the GATM gene. As confirmed: "biallelic pathogenic variants in GATM result in l-arginine:glycine amidinotransferase deficiency" (PMID: 36856349). There are no known environmental, infectious, or lifestyle causes. The disease follows strict Mendelian autosomal recessive inheritance.

Risk Factors

Genetic risk factors: - Carrier status for pathogenic GATM variants in both parents (obligate heterozygotes) - Consanguinity significantly increases risk; multiple reported families have consanguineous parents (PMID: 23770102) - No known susceptibility loci or modifier genes beyond GATM itself

Environmental risk factors: - Dietary creatine intake may modify phenotypic severity; vegetarian diets provide no dietary creatine, and individuals depend entirely on endogenous synthesis, which is absent in AGAT deficiency (PMID: 21387089) - Statin medications may exacerbate myopathy in carriers or affected individuals; the GATM gene has been associated with statin-induced myopathy in two human populations (PMID: 31853708)

Protective Factors

Genetic protective factors: - No specific protective alleles identified - Residual AGAT activity from hypomorphic variants may theoretically ameliorate phenotype, though no clear genotype-phenotype correlation exists (PMID: 27233232)

Environmental protective factors: - Dietary creatine from meat and fish provides approximately half of daily needs in omnivores - Early initiation of creatine supplementation is the most powerful protective intervention

Gene-Environment Interactions

• Statin exposure interacts with GATM genotype: simvastatin-induced motor impairment is exacerbated in AGAT-deficient mice (PMID: 31853708)
• Pregnancy increases creatine demand; an AGAT-deficient woman required dose escalation during pregnancy (PMID: 32883247)

3. Phenotypes

Core Clinical Features

As reported: "15 patients diagnosed between 16 months and 25 years of life had intellectual disability/developmental delay (IDD). 8 patients also had myopathy/proximal muscle weakness" (PMID: 26490222).

Laboratory Abnormalities

Quality of Life Impact

AGAT deficiency profoundly affects quality of life: intellectual disability ranges from mild to severe requiring special educational support; severe language impairment limits social interaction and independence; proximal weakness (Gowers sign positive) limits physical activities; and behavioral disturbances complicate caregiving and social integration. Most untreated or late-treated patients require lifelong supervision and support.

4. Genetic/Molecular Information

Causal Gene

The gene encodes L-arginine:glycine amidinotransferase, a mitochondrial enzyme catalyzing: L-arginine + glycine → L-ornithine + guanidinoacetate (GAA). As described: "There are two enzyme deficiencies, guanidinoacetate methyltransferase (GAMT), encoded by GAMT and arginine-glycine amidinotransferase (AGAT), encoded by GATM, which are involved in the synthesis of creatine" (PMID: 38452609).

Pathogenic Variants

The ClinGen CCDS VCEP has curated 45 variants in GATM (PMID: 38452609).

Selected reported pathogenic variants:

Variant characteristics: - Classification: Pathogenic and likely pathogenic per ACMG/AMP guidelines, curated by the CCDS VCEP - Variant types: Missense, nonsense, frameshift, splice-site variants have all been reported - Allele frequency: Extremely rare in population databases (gnomAD); most variants are private or near-private - Origin: All reported variants are germline - Functional consequence: Loss of function; seven missense variants showed 0% residual wild-type AGAT activity (PMID: 27233232)

Genotype-phenotype correlation: "Two patients with mild phenotype had a nonsense missense variant. Severe phenotype was present in patients with missense as well as truncating variants. There seems to be no phenotype and genotype correlation" (PMID: 27233232).

Modifier Genes

No modifier genes have been identified. Variation in phenotypic severity appears primarily influenced by age at treatment initiation.

Epigenetic Information

Transcriptomic analysis in AGAT-knockout mouse brains revealed homoarginine- and creatine-dependent gene regulation changes (PMID: 32182846). Creatine biosynthesis consumes approximately 40% of all S-adenosylmethionine (SAM)-derived methyl groups, suggesting that AGAT deficiency could indirectly affect the methylation landscape (PMID: 21387089).

Chromosomal Abnormalities

No large-scale chromosomal abnormalities are associated. A distinct GATM gain-of-function mechanism has been associated with autosomal dominant renal Fanconi syndrome (PMID: 29654216), but this is unrelated to AGAT deficiency.

5. Environmental Information

Environmental Factors

AGAT deficiency is purely genetic. No environmental toxins, radiation, or occupational exposures are implicated.

Lifestyle Factors

• Diet: Dietary creatine (primarily from meat and fish) provides an external source; vegetarian or vegan diets would theoretically exacerbate the phenotype
• Exercise: Physical activity increases creatine demand and may unmask or worsen myopathy
• Pregnancy: Substantially increases creatine demand; documented case required dose escalation (PMID: 32883247)

Infectious Agents

Not applicable.

6. Mechanism / Pathophysiology

Molecular Pathways

AGAT catalyzes the first and rate-limiting step of endogenous creatine biosynthesis:

L-Arginine + Glycine  ──AGAT──>  L-Ornithine + Guanidinoacetate (GAA)
                                    │
                              GAA + SAM  ──GAMT──>  Creatine + SAH
                                                        │
                                              Creatine + ATP  ──CK──>  Phosphocreatine + ADP

Key pathway identifiers: - KEGG: hsa00260 (Glycine, serine and threonine metabolism); hsa00330 (Arginine and proline metabolism) - Reactome: R-HSA-71288 (Creatine metabolism) - GO:0006601 (creatine biosynthetic process)

Cellular Processes and Functions of Creatine

Creatine and the creatine kinase/phosphocreatine (CK/PCr) system serve as: 1. Temporal energy buffer: PCr rapidly regenerates ATP in tissues with high and fluctuating energy demands 2. Spatial energy shuttle: Transports high-energy phosphate groups from mitochondria to sites of ATP consumption 3. Neuromodulator: "Creatine modulates GABAergic and glutamatergic cerebral pathways, presynaptic CRTR (SLC6A8) ensuring re-uptake of synaptic creatine" (PMID: 26542286) 4. Antioxidant: Direct antioxidant properties 5. Osmolyte: Contributes to cellular water retention in muscle

Causal Chain: From Genetic Defect to Clinical Manifestation

GATM biallelic mutations
 │
 ▼
Loss of AGAT enzyme activity (mitochondrial)
 │
 ├──> No GAA production ──> No substrate for GAMT ──> No endogenous creatine
 │
 ├──> Depletion of homoarginine (hArg) ──> Impaired NO signaling? ──> Cardiovascular effects
 │
 ▼
Systemic creatine depletion (dependent on dietary creatine only)
 │
 ├──> Brain: Cerebral creatine deficiency
 │         ├──> Impaired neuronal energy metabolism ──> Intellectual disability
 │         ├──> Disrupted GABAergic/glutamatergic signaling ──> Behavioral disturbances, epilepsy
 │         └──> Impaired brain development ──> Speech delay, corpus callosum dysmorphisms
 │
 ├──> Skeletal muscle: Muscle creatine depletion
 │         └──> Impaired energy metabolism ──> Proximal myopathy, hypotonia
 │
 └──> Heart: Cardiac creatine depletion
   └──> Altered calcium handling ──> Reduced contractility

Protein Dysfunction

AGAT (UniProt: P50440) is a mitochondrial matrix enzyme. Pathogenic variants result in protein truncation, misfolding, or catalytic site disruption. All characterized pathogenic missense variants show 0% residual activity (PMID: 27233232).

Metabolic Changes

• Creatine/phosphocreatine depletion: Primary metabolic abnormality; brain creatine undetectable on MRS
• GAA depletion: Absence of the creatine precursor (diagnostic hallmark)
• Homoarginine depletion: AGAT also synthesizes L-homoarginine; its deficiency may contribute to cardiovascular and cerebrovascular risk (PMID: 30370846)
• Reduced SAM consumption: GAMT-mediated creatine synthesis normally consumes ~40% of total SAM flux; in AGAT deficiency this demand is eliminated (PMID: 21387089)

Relevant CHEBI terms: CHEBI:16919 (creatine), CHEBI:17437 (guanidinoacetate), CHEBI:15354 (phosphocreatine), CHEBI:59560 (L-homoarginine), CHEBI:67079 (S-adenosylmethionine)

Cardiac Involvement (Mouse Model)

"Creatine-deficient mice, which lack arginine-glycine amidinotransferase (AGAT) to synthesize creatine and homoarginine, exhibit reduced cardiac contractility" (PMID: 33275525). The cardiac calcium handling defects are rescued by creatine supplementation.

Biochemical Abnormalities

The specific enzyme deficiency is in AGAT (EC 2.1.4.1, glycine amidinotransferase): - Substrate: L-arginine + glycine - Product: L-ornithine + guanidinoacetate - Localization: Mitochondrial matrix - Additional observations: Decreased respiratory chain complex activity in muscle tissue has been reported (PMID: 20682460), and tubular aggregates on electron microscopy

Molecular Profiling

Transcriptomics: Transcriptome analysis of AGAT-knockout mouse brains revealed gene regulation changes dependent on homoarginine and creatine status (PMID: 32182846). No transcriptomic, proteomic, metabolomic, or single-cell studies have been performed on human AGAT-deficiency patient samples due to extreme rarity.

7. Anatomical Structures Affected

Organ Level

Tissue and Cell Level

Subcellular Level

Localization

• Cerebral cortex (UBERON:0000956): Reduced cortical thickness, especially parieto-occipital
• Corpus callosum (UBERON:0002336): Dysmorphisms in 3/4 affected family members
• Proximal muscles of lower limbs (UBERON:0004518): Primary myopathy site
• Kidney cortex (UBERON:0001225): Primary GAA synthesis organ
• Bilateral involvement typical for both brain and muscle manifestations

8. Temporal Development

Onset

• Typical age of onset: Infancy to early childhood (symptoms noticed between 6 months and 3 years)
• Age at diagnosis: Ranged from 16 months to 25 years in the largest cohort (PMID: 26490222)
• Onset pattern: Insidious; developmental milestones progressively delayed rather than lost
• Epilepsy onset: 4–6 years in reported cases (PMID: 40674085)

Progression

• Disease course: Chronic, progressive without treatment; stable to improving with creatine
• Duration: Lifelong condition

Critical Periods

The first months to years of life represent the critical therapeutic window. Two patients treated from ages 4 and 16 months achieved normal development by ages 10–11, while patients treated later showed limited cognitive improvement (PMID: 26490222). One patient treated from 16 months demonstrated "superior nonverbal and academic abilities, with average verbal skills" at 8-year follow-up (PMID: 22386973). This suggests irreversible damage from brain creatine depletion during neurodevelopmental critical periods.

9. Inheritance and Population

Epidemiology

• Prevalence: Ultra-rare; fewer than 50 patients identified worldwide; estimated <1 per 1,000,000
• Incidence: Unknown; too rare for reliable estimation
• Orphanet classification: Ultra-rare disease

Genetic Inheritance

• Inheritance pattern: Autosomal recessive (HP:0000007)
• Penetrance: Complete for biochemical phenotype; clinical severity variable
• Expressivity: Variable; influenced primarily by age at treatment initiation
• Genetic anticipation: Not applicable
• Germline mosaicism: Not reported but cannot be excluded
• Consanguinity role: Significant; multiple families consanguineous (PMID: 23770102)
• Carrier frequency: Unknown; estimated extremely low; functional characterization of rare GATM variants was performed to estimate frequency (PMID: 27233232)
• Founder effects: The c.446G>A, p.(Trp149Ter) variant appears recurrent in Italian families (PMID: 40674085)

Population Demographics

• Affected populations: 16 patients from 8 families of 8 different ethnic backgrounds, indicating no ethnic predilection (PMID: 26490222)
• Geographic distribution: Cases from Europe, Middle East, North Africa, North and South America; no clustering
• Sex ratio: Approximately equal (autosomal inheritance)
• Age distribution: Diagnosis ranges from infancy to adulthood; most in childhood

10. Diagnostics

Clinical Tests

Biochemical testing:

Methods include LC-MS/MS: "LC-MS/MS measurements of guanidinoacetic acid (GAA) and creatine in urine and plasma are an important screening test to identify the deficit" (PMID: 30858092).

Imaging: - Brain MRI: Corpus callosum dysmorphisms, reduced cortical thickness (PMID: 40323733) - Brain ¹H-MRS: Absent creatine peak at 3.0 ppm (key diagnostic finding) - Muscle electron microscopy: Tubular aggregates (PMID: 20682460)

Electrophysiology: - EMG: Myopathic pattern (PMID: 23770102) - EEG: Epileptiform activity in patients with seizures

Genetic Testing

• Recommended approach: Biochemical screening followed by GATM sequencing
• Gene panels: CCDS panels including GATM, GAMT, and SLC6A8 (PMID: 28055022)
• WES/WGS: Useful for unexplained ID workup
• ACMG technical standard: Guidelines standardize diagnostic procedures for all CCDS (PMID: 28055022)
• ClinGen VCEP guidelines: Disease-specific variant classification for GATM (PMID: 38452609)
• Enzyme activity assay: AGAT activity in lymphoblasts for functional confirmation (PMID: 22386973)

Differential Diagnosis

Screening

• Newborn screening: GATM deficiency is "a good candidate for newborn screening" given normal neurodevelopment in presymptomatically treated individuals (PMID: 27233232). Detection requires identifying LOW GAA, which is technically more challenging than detecting elevated GAA (used for GAMT-d screening). GAMT deficiency has been added to newborn screening in some US states (PMID: 34389248; PMID: 35120844).
• Cascade screening: Siblings and family members should be tested immediately when a proband is identified.
• Metabolic screening in autism/ID: Has identified CCDS cases; "An inborn error of metabolism was found in 13 (12.4%) patients. Five patients (4.8%) had cerebral creatine deficiency syndrome" (PMID: 36604934).

11. Outcome/Prognosis

Survival and Mortality

• Life expectancy: Likely near-normal with treatment; no deaths directly attributable to AGAT deficiency reported
• Mortality rate: Not established due to rarity
• Disease-specific mortality: None reported

Morbidity and Function

Complications

• Untreated: Progressive ID, severe language impairment, increasing disability
• Treatment-related: Weight gain, kidney stones (PMID: 28148286)
• Epilepsy: May develop even with supplementation (PMID: 40674085)
• Brain atrophy: May persist despite treatment (PMID: 40323733)

Prognostic Factors

The single most important prognostic factor is age at treatment initiation. As confirmed: "Early treatment prevents adverse developmental outcome, while patients diagnosed and treated at an older age showed partial but signif[icant improvement]" (PMID: 28148286).

12. Treatment

Pharmacotherapy: Creatine Monohydrate (Primary Treatment)

• MAXO term: MAXO:0001298 (dietary supplement therapy)
• Dose: 100–800 mg/kg/day orally, divided into multiple daily doses
• Mechanism: Replaces endogenous creatine; restores intracellular creatine and phosphocreatine pools

Treatment outcomes: "Treatment with creatine monohydrate (100-800 mg/kg/day) resulted in almost complete restoration of brain creatine levels and significant improvement of myopathy. The 2 patients treated since age 4 and 16 months had normal cognitive and behavioral development at age 10 and 11 years. Late treated patients had limited improvement of cognitive functions" (PMID: 26490222).

Long-term efficacy: "8 years post initiation of oral creatine supplementation, patient demonstrates superior nonverbal and academic abilities, with average verbal skills" (PMID: 22386973).

Side effects: Weight gain, kidney stones; generally safe and well tolerated (PMID: 28148286).

Potential Alternative: GAA Supplementation (Investigational)

GAA has been proposed as an alternative with potentially better brain bioavailability. However: "AGAT patients might benefit from oral GAA due to upgraded bioavailability and convenient utilization of the compound, while possible drawbacks (e.g. brain methylation issues, neurotoxicity, and hyperhomocysteinemia) should be accounted as well" (PMID: 28971744). This remains experimental.

Anticonvulsant Medications

For patients with epilepsy, carbamazepine and valproate/lacosamide combinations have been used (PMID: 40674085).

Supportive and Rehabilitative Care

• Speech therapy (MAXO:0000930): For language delay
• Physical therapy (MAXO:0000502): For myopathy and motor development
• Special education (MAXO:0000016): For intellectual disability
• Behavioral therapy: For behavioral disturbances
• Antiepileptic drugs (MAXO:0000759): If seizures present

Pregnancy Management

Pregnant women with AGAT deficiency require increased creatine doses with close monitoring (PMID: 32883247).

Advanced Therapeutics and Patient Advocacy

No gene therapy trials specific to AGAT deficiency are registered. The Association for Creatine Deficiencies (ACD) is actively advancing the field, supporting "advancements in disease diagnosis, investments in various therapeutic modalities, creation of a collaborative research community" (PMID: 40078706).

Treatment Algorithm

1. Confirm diagnosis biochemically and genetically
2. Initiate creatine monohydrate immediately (starting 200–400 mg/kg/day)
3. Monitor brain creatine by MRS at regular intervals
4. Adjust dose upward (to 800 mg/kg/day if needed) based on response
5. Monitor for side effects (weight, renal function, kidney stones)
6. Provide supportive therapies (speech, PT, OT) as needed
7. Screen siblings and treat presymptomatically if affected
8. Lifelong supplementation and monitoring

13. Prevention

Primary Prevention

• Genetic counseling (MAXO:0000079): For families with known GATM variants; 25% recurrence risk per pregnancy
• Preimplantation genetic diagnosis (PGD): Available for families with known mutations
• Prenatal testing: Molecular testing in at-risk pregnancies
• Consanguinity counseling: Important in populations with high consanguinity rates

Secondary Prevention (Early Detection)

• Newborn screening: AGAT deficiency is a strong NBS candidate due to treatability, but detection of LOW GAA presents technical challenges distinct from GAMT-d screening (elevated GAA)
• Cascade family screening: Critical; siblings of diagnosed patients should be immediately tested
• Metabolic screening in developmental delay cohorts: Children with unexplained DD, especially with myopathy, should be screened for CCDS

Tertiary Prevention

• Continuous lifelong creatine supplementation
• Regular monitoring: brain MRS, developmental assessments, renal function
• Dose adjustment during physiological stress (pregnancy, illness, growth spurts)
• Seizure surveillance and management

14. Other Species / Natural Disease

Taxonomy and Orthologous Genes

Natural Disease

No naturally occurring AGAT deficiency has been described in non-human species. The disorder has only been studied through engineered knockout mouse models. The creatine biosynthesis pathway (AGAT → GAMT) is evolutionarily conserved across vertebrates, indicating its fundamental importance in energy metabolism.

15. Model Organisms

AGAT-Knockout Mouse (Primary Model)

Key phenotypic findings:

1. Cardiac: "Creatine-deficient mice, which lack arginine-glycine amidinotransferase (AGAT) to synthesize creatine and homoarginine, exhibit reduced cardiac contractility" — reduced L-type calcium channel current, slower calcium transient decay; rescued by creatine (PMID: 33275525)

2. Muscular: "Simvastatin-induced motor impairment was exacerbated in AGAT-deficient mice compared with AGAT-overexpressing GAMT" — GATM associated with statin myopathy in humans (PMID: 31853708)

3. Cerebral: Homoarginine- and creatine-dependent gene regulation changes in brain (PMID: 32182846)

Model limitations: - Behavioral phenotyping shows only mild and limited alterations compared to the significant human cognitive impairment - Brain development timelines differ between species - Speech and language assessment is not possible in mice

GAMT-Knockout Mouse (Related Model)

Shares creatine depletion phenotype but additionally accumulates GAA (potentially neurotoxic), providing a complementary model for studying creatine deficiency vs. GAA toxicity effects (PMID: 34440375).

Mechanistic Model / Interpretation

The pathophysiology of AGAT deficiency can be understood as a biosynthetic energy deficiency disorder with tissue-specific vulnerability:

       ┌─────────────────────────┐
       │   GATM Gene Mutations    │
       │   (Biallelic, LOF)       │
       └─────────┬───────────────┘
                 │
       ┌─────────▼───────────────┐
       │ AGAT Enzyme Absent       │
       │ (Mitochondrial Matrix)   │
       └─────────┬───────────────┘
                 │
      ┌──────────────────┼──────────────────┐
      │                  │                   │
    ┌─────────▼──────┐  ┌───────▼───────┐  ┌───────▼────────┐
    │ No GAA Produced │  │ No Homoarginine│  │ Reduced SAM    │
    │                 │  │ (hArg)         │  │ Consumption    │
    └─────────┬──────┘  └───────┬───────┘  └───────┬────────┘
      │                 │                   │
    ┌─────────▼──────┐  ┌───────▼───────┐  ┌───────▼────────┐
    │ No Endogenous  │  │ Cardiovascular │  │ Methylation    │
    │ Creatine       │  │ Risk?          │  │ Balance Shift  │
    └─────────┬──────┘  └───────────────┘  └────────────────┘
      │
    ┌─────────▼───────────────────────────────────┐
    │ SYSTEMIC CREATINE/PHOSPHOCREATINE DEPLETION │
    └──────┬──────────────┬──────────────┬────────┘
   │              │              │
  ┌────────▼─────┐ ┌─────▼─────┐ ┌──────▼──────┐
  │   BRAIN      │ │  MUSCLE   │ │   HEART     │
  │ - ID         │ │ - Myopathy│ │ - Altered   │
  │ - Speech ↓   │ │ - Hypotonia│ │   Ca²⁺     │
  │ - Behavior   │ │ - Weakness│ │   handling  │
  │ - Epilepsy   │ │           │ │             │
  │ - Atrophy    │ │           │ │             │
  └──────────────┘ └───────────┘ └─────────────┘

The brain is most severely affected because: (1) it has the highest energy demand per unit mass; (2) the blood-brain barrier has limited permeability for peripheral creatine ("SLC6A8 is expressed by microcapillary endothelial cells at the blood-brain barrier, but is absent from surrounding astrocytes" — PMID: 26861125); and (3) intracerebral creatine synthesis is disrupted. The critical dependence on the neurodevelopmental time window explains why early treatment prevents damage while late treatment has limited cognitive benefit.

Evidence Base

Key Literature

Ontology Term Summary

Disease Ontology

• MONDO: MONDO:0012999
• OMIM: 612718
• Orphanet: ORPHA:35704

HPO (Human Phenotype Ontology)

• HP:0001249 (Intellectual disability) | HP:0001263 (Global developmental delay)
• HP:0000750 (Delayed speech and language development) | HP:0002474 (Expressive language delay)
• HP:0003198 (Myopathy) | HP:0003701 (Proximal muscle weakness)
• HP:0001252 (Muscular hypotonia) | HP:0000729 (Autistic behavior)
• HP:0000708 (Atypical behavior) | HP:0001250 (Seizures) | HP:0007359 (Focal-onset seizure)
• HP:0001508 (Failure to thrive) | HP:0002120 (Cerebral cortical atrophy)
• HP:0001273 (Abnormal corpus callosum morphology) | HP:0000007 (Autosomal recessive inheritance)

GO (Gene Ontology)

• GO:0006601 (creatine biosynthetic process) | GO:0006600 (creatine metabolic process)
• GO:0015068 (glycine amidinotransferase activity) | GO:0016740 (transferase activity)
• GO:0005759 (mitochondrial matrix) | GO:0005739 (mitochondrion) | GO:0005737 (cytoplasm)

CL (Cell Ontology)

• CL:0000540 (neuron) | CL:0000127 (astrocyte) | CL:0000128 (oligodendrocyte)
• CL:0000187 (muscle cell) | CL:0000746 (cardiac muscle cell) | CL:0002306 (renal tubular epithelial cell)

UBERON (Anatomical Ontology)

• UBERON:0000955 (brain) | UBERON:0000956 (cerebral cortex) | UBERON:0002336 (corpus callosum)
• UBERON:0001134 (skeletal muscle tissue) | UBERON:0000948 (heart) | UBERON:0002113 (kidney)

CHEBI (Chemical Entities)

• CHEBI:16919 (creatine) | CHEBI:17437 (guanidinoacetate) | CHEBI:15354 (phosphocreatine)
• CHEBI:59560 (L-homoarginine) | CHEBI:67079 (S-adenosylmethionine) | CHEBI:16737 (creatinine)

MAXO (Medical Action Ontology)

• MAXO:0001298 (dietary supplement therapy) | MAXO:0000079 (genetic counseling)
• MAXO:0000930 (speech-language therapy) | MAXO:0000502 (physical therapy)
• MAXO:0000016 (education) | MAXO:0000759 (antiepileptic drug therapy)

Limitations and Knowledge Gaps

1. Extreme rarity: Fewer than 50 patients identified worldwide; all data from small case series and case reports. Robust epidemiological data, natural history studies, and clinical trials are not feasible.

2. Ascertainment bias: The nonspecific early phenotype (developmental delay, speech delay) means AGAT deficiency is almost certainly underdiagnosed. Many patients may carry diagnoses of "idiopathic intellectual disability."

3. No genotype-phenotype correlation: Despite functional characterization of multiple variants, outcome appears determined primarily by age at treatment rather than mutation type.

4. Limited long-term data: Longest follow-up is 15 years. Lifelong trajectory including potential late-onset complications (cardiovascular, renal) remains unknown.

5. Cardiac phenotype uncharacterized in humans: The mouse model clearly shows cardiac dysfunction, but systematic cardiac evaluation in human patients has not been reported.

6. Persistent brain changes: Reduced cortical thickness and corpus callosum dysmorphisms persist despite supplementation, suggesting some structural changes are irreversible or independent of creatine status.

7. Homoarginine deficiency: Clinical significance of concurrent homoarginine depletion in AGAT-deficient patients is poorly understood.

8. Newborn screening not implemented: Unlike GAMT deficiency (added to US RUSP), AGAT deficiency detection requires identifying LOW GAA, presenting distinct technical challenges.

9. No molecular profiling in humans: No transcriptomic, proteomic, or metabolomic studies on patient samples exist.

Proposed Follow-up Studies and Actions

1. Newborn screening for AGAT deficiency: Develop and validate dried blood spot assays optimized to detect low GAA, in parallel with existing GAMT-d programs.

2. International patient registry: Establish a centralized, prospective registry for systematic natural history data and outcome collection.

3. Cardiac evaluation protocol: Conduct echocardiographic and cardiac MRI evaluation of all known AGAT-deficient patients, based on compelling mouse model evidence.

4. Homoarginine supplementation study: Investigate whether L-homoarginine supplementation provides additional cardiovascular or neurological benefit.

5. Longitudinal brain imaging: Perform volumetric MRI and diffusion tensor imaging to characterize structural changes and their relationship to treatment timing.

6. Multi-omics profiling: Conduct metabolomics and transcriptomics on patient-derived samples to identify biomarkers and secondary metabolic disturbances.

7. Statin pharmacogenomics: Consider GATM genotyping in patients experiencing statin adverse effects, given the human genetic association and mouse model data.

8. Gene therapy feasibility: Explore AAV-mediated GATM gene replacement in the mouse model, given the monogenic nature and clear biochemical endpoints.

9. Metabolic screening in DD/ID/ASD cohorts: Implement routine creatine metabolism screening in all children with unexplained developmental delay, especially when consanguinity is present.

10. GAA supplementation pilot: Design a carefully monitored study of oral GAA as creatine adjunct, with close monitoring of methylation status and homocysteine.

Report compiled from systematic review of 38 publications and comprehensive database searches. All citations verified against PubMed abstracts. Last updated: May 2026.