. 2025 Aug 7;20(1):408.

doi: 10.1186/s13023-025-03900-3.

Establishing a core outcome set for creatine transporter deficiency and guanidinoacetate methyltransferase deficiency

Zahra Nasseri Moghaddam^#¹, Emily K Reinhardt^#², Audrey Thurm³, Beth K Potter⁴, Maureen Smith⁵, Celeste Graham^{6

7}, Beth H Tiller⁷, Steven A Baker⁸, Deborah A Bilder⁹, Regina Bogar⁷, Jacobus Britz⁷, Rachel Cafferty⁷, Daniel P Coller^{6

7}, Ton J DeGrauw¹⁰, Vicky Hall⁷, Gerald S Lipshutz¹¹, Nicola Longo^{10

11}, Saadet Mercimek-Andrews^{10

12}, Judith S Miller¹³, Marzia Pasquali^{10

14}, Gajja S Salomons^{10

15}, Andreas Schulze^{10

16}, Celine P Wheaton⁷, Kayla F Williams⁷, Sarah P Young^{10

17}, Jasmine Li¹, Sofia Balog^{6

18}, Theresa Selucky¹⁹, Sylvia Stöckler-Ipsiroglu^#^{1

10

20}, Heidi Wallis^#^{6

7}

Affiliations

¹ Department of Pediatrics, University of British Columbia, Vancouver, BC, Canada.
² Board of Directors & Staff, Association for Creatine Deficiencies, Carlsbad, CA, USA. registry@creatineinfo.org.
³ National Institute of Mental Health, NIH, Bethesda, MD, USA.
⁴ School of Epidemiology and Public Health, Faculty of Medicine, University of Ottawa, Ottawa, ON, Canada.
⁵ Patient Partner, University of Ottawa, Ottawa, ON, Canada.
⁶ Board of Directors & Staff, Association for Creatine Deficiencies, Carlsbad, CA, USA.
⁷ Patient/Caregiver Partner, Association for Creatine Deficiencies, Carlsbad, CA, USA.
⁸ Department of Pathology, University of Utah, Salt Lake City, UT, USA.
⁹ Huntsman Mental Health Institute, University of Utah, Salt Lake City, UT, USA.
¹⁰ Scientific Medical Advisory Board, Association for Creatine Deficiencies, Carlsbad, CA, USA.
¹¹ David Geffen School of Medicine, University of California-Los Angeles, Los Angeles, CA, USA.
¹² Department of Medical Genetics, Faculty of Medicine & Dentistry, University of Alberta, Edmonton, AB, Canada.
¹³ Department of Child and Adolescent Psychiatry and Behavioral Sciences, Center for Autism Research, Children's Hospital of Philadelphia, Philadelphia, PA, USA.
¹⁴ ARUP Laboratories, Department of Pathology, University of Utah, Salt Lake City, UT, USA.
¹⁵ Laboratory Genetic Metabolic Diseases, Amsterdam Gastroenterology Endocrinology Metabolism, Amsterdam University Medical Centers, University of Amsterdam, Amsterdam, The Netherlands.
¹⁶ Hospital for Sick Children, University of Toronto, Toronto, ON, Canada.
¹⁷ Division of Genetics and Metabolism, Department of Pediatrics, Duke University Medical Center, Durham, NC, USA.
¹⁸ Ultragenyx Argentina S.R.L., Buenos Aires, Argentina.
¹⁹ Fine Point Consulting, Los Angeles, CA, USA.
²⁰ Division of Biochemical Genetics, British Columbia Children's Hospital, Vancouver, BC, Canada.

^# Contributed equally.

PMID: 40775643
PMCID: PMC12333098
DOI: 10.1186/s13023-025-03900-3

Establishing a core outcome set for creatine transporter deficiency and guanidinoacetate methyltransferase deficiency

Zahra Nasseri Moghaddam et al. Orphanet J Rare Dis. 2025.

. 2025 Aug 7;20(1):408.

doi: 10.1186/s13023-025-03900-3.

Authors

Affiliations

¹ Department of Pediatrics, University of British Columbia, Vancouver, BC, Canada.
² Board of Directors & Staff, Association for Creatine Deficiencies, Carlsbad, CA, USA. registry@creatineinfo.org.
³ National Institute of Mental Health, NIH, Bethesda, MD, USA.
⁴ School of Epidemiology and Public Health, Faculty of Medicine, University of Ottawa, Ottawa, ON, Canada.
⁵ Patient Partner, University of Ottawa, Ottawa, ON, Canada.
⁶ Board of Directors & Staff, Association for Creatine Deficiencies, Carlsbad, CA, USA.
⁷ Patient/Caregiver Partner, Association for Creatine Deficiencies, Carlsbad, CA, USA.
⁸ Department of Pathology, University of Utah, Salt Lake City, UT, USA.
⁹ Huntsman Mental Health Institute, University of Utah, Salt Lake City, UT, USA.
¹⁰ Scientific Medical Advisory Board, Association for Creatine Deficiencies, Carlsbad, CA, USA.
¹¹ David Geffen School of Medicine, University of California-Los Angeles, Los Angeles, CA, USA.
¹² Department of Medical Genetics, Faculty of Medicine & Dentistry, University of Alberta, Edmonton, AB, Canada.
¹³ Department of Child and Adolescent Psychiatry and Behavioral Sciences, Center for Autism Research, Children's Hospital of Philadelphia, Philadelphia, PA, USA.
¹⁴ ARUP Laboratories, Department of Pathology, University of Utah, Salt Lake City, UT, USA.
¹⁵ Laboratory Genetic Metabolic Diseases, Amsterdam Gastroenterology Endocrinology Metabolism, Amsterdam University Medical Centers, University of Amsterdam, Amsterdam, The Netherlands.
¹⁶ Hospital for Sick Children, University of Toronto, Toronto, ON, Canada.
¹⁷ Division of Genetics and Metabolism, Department of Pediatrics, Duke University Medical Center, Durham, NC, USA.
¹⁸ Ultragenyx Argentina S.R.L., Buenos Aires, Argentina.
¹⁹ Fine Point Consulting, Los Angeles, CA, USA.
²⁰ Division of Biochemical Genetics, British Columbia Children's Hospital, Vancouver, BC, Canada.

^# Contributed equally.

PMID: 40775643
PMCID: PMC12333098
DOI: 10.1186/s13023-025-03900-3

Abstract

Background: Creatine transporter (CTD) and guanidinoacetate methyltransferase (GAMT) deficiencies are rare inborn errors of creatine metabolism, resulting in cerebral creatine deficiency. Patients with either condition commonly exhibit intellectual and developmental disabilities, often accompanied by behavior problems, delayed speech, seizures, and motor impairments. There is currently no efficacious treatment for CTD, while current management for GAMT requires lifelong treatment with a protein restricted diet and intake of high amounts of oral supplements. Efforts to conduct clinical trials on potential treatments for these disorders are made more difficult by the lack of clinical and patient-derived meaningful outcomes. A core outcome set (COS) can facilitate consistent use of outcomes in studies. The current effort included patient and caregiver perspectives into the outcome selection of a COS for CTD and GAMT.

Results: We partnered with caregivers and health professionals to establish the first COS for CTD and GAMT. The COS developed includes seven outcomes ("Adaptive Functioning", "Cognitive Functioning", "Emotional Dysregulation", "MRS Brain Creatine", "Seizure/Convulsions", "Expressive Communication", and "Fine Motor Functions") for both CTD and GAMT, and an additional outcome for GAMT ("Serum/Plasma Guanidinoacetate") that are important to stakeholders and consequently should be considered for measurement in every clinical trial. Caregivers were valued partners throughout the COS development process, which increased community engagement and facilitated caregiver empowerment.

Conclusions: Development of this COS illustrates a patient-centered approach for clinical trial readiness for CTD and GAMT that if utilized will make clinical trial results comparable, minimize bias in clinical trial outcome selection, and promote efficient use of resources.

Keywords: Core outcome set (COS); Creatine transporter deficiency; Guanidinoacetate methyltransferase deficiency; Patient focused drug development; Patient-centered research; Rare disease.

PubMed Disclaimer

Conflict of interest statement

Declarations. Ethics approval and consent to participate: All focus groups and Delphi surveys received institutional review board (IRB) approval from the North Star Review Board (protocols NB200065, NB300094; https://learningirb.org/ ). Informed consent was obtained from all focus group and Delphi survey participants prior to participation. Consent for publication: Not applicable. Competing interests: ZNM reports a stipend and travel support from ACD for this project. BKP reports a grant from INFORM RARE which receives industry matching research funds from Takeda, Biomarin, Ultragenyx, and Perkin Elmer. NL reports the following: clinical trial support for Amgen/Horizon, Amicus Therapeutics, Audentes/Astellas, BioMarin, Chiesi/Protalix, Genzyme/Sanofi, Jnana, Moderna, PTC Therapeutics, Takeda, and Ultragenyx; serves on advisory boards for Amgen/Horizon, Amicus Therapeutics, Audentes/Astellas, BioMarin, Chiesi/Protalix, Genzyme/Sanofi, Ipsen, Jaguar Gene Therapy, Jnana, Leadiant Biosciences, Moderna, Nestle Pharma, PTC Therapeutics, Reneo, and Ultragenyx Data Safety; serves on monitoring boards for Applied Therapeutics, iEcure, and Regeneron. GSL reports grant and travel support funding from ACD. JSM reports a consulting agreement with Ultragenyx and Johnson & Johnson, and has done legal consultation for a variety of cases. AS reports receiving consultation fees from Ceres Brain. EKR, AT, MS, CG, BHT, SAB, DAB, RB, JB, RC, DPC, TJDG, VH, SM-A, MP, GSS, CPW, KFW, SPY, JL, SB, TS, SS-I, and HW declare they have no conflicts of interest.

Figures

**Fig. 1**
Outcome selection process for COS development for CTD and GAMT. During the candidate outcome selection phase, the total number of candidate outcomes gathered at each stage were identified. During the project's remaining phases, the total number of remaining outcomes at the end of each stage were identified. The total number of CTD and GAMT responses from the CreatineInfo data were identified, along with the total number of patients/caregivers and health professionals from the focus groups, Delphi surveys, and consensus workshop

**Fig. 2**
Outcomes remaining after each Delphi survey round and the consensus workshop. Column headers reflect the Delphi round and each set of inclusion criteria. The inclusion criteria for each round were as follows: Delphi 1, rated ≥ 3 by ≥ 70% of any stakeholder group; Delphi 2, rated ≥ 7 by ≥ 70% of any stakeholder group; Delphi 3–1, rated ≥ 7 by ≥ 70% of any stakeholder group; Delphi 3–2, mean rating ≥ 7 for any stakeholder group; Delphi 3–3, ranked in top 10 by ≥ 15% of any stakeholder group; Consensus (C), ≥ 50% of workshop participants voted outcome as “1-Definitely In”. Stakeholders meeting the inclusion criteria for each outcome are identified by color: patients/caregivers (yellow), health professionals (blue), both patients/caregivers and health professionals (green), outcome did not meet the criteria and was excluded (gray). Some outcomes were combined and/or changed throughout the process: “Developmental Delay” and “Intellectual Disability” were combined into “Intellectual and Developmental Disability”, “Adaptive Functioning” and “Daily Living Skills” were combined into “Adaptive Functioning”, and “Expressive Language” was changed to “Expressive Communication”. Outcomes marked with ◼ came exclusively from the focus groups. Outcomes marked with ▲ were introduced by the project team during the pruning phase. Outcomes marked with ◯ were removed by the project team because they are typically already included in clinical trial designs. Outcomes marked with ◆ did not meet the consensus workshop inclusion criteria but were later included after unanimous agreement among participants. Outcomes marked with + are not included in the COS, but are worth measuring alongside the COS

**Fig. 3**
Distribution of locations for the participants (n = 150) who completed the Delphi surveys. Represented countries were categorized into three groups: 1–5 participants (light orange, n = 22), 6–10 participants (orange, n = 4), and 11 or more participants (dark orange, n = 1)

See this image and copyright information in PMC

Update of

Establishing a Core Outcome Set for Creatine Transporter Deficiency and Guanidinoacetate Methyltransferase Deficiency.
Nasseri Moghaddam Z, Reinhardt EK, Thurm A, Potter BK, Smith M, Graham C, Tiller BH, Baker SA, Bilder DA, Bogar R, Britz J, Cafferty R, Coller DP, DeGrauw TJ, Hall V, Lipshutz GS, Longo N, Mercimek-Andrews S, Miller JS, Pasquali M, Salomons GS, Schulze A, Wheaton CP, Williams KF, Young SP, Li J, Balog S, Selucky T, Stockler-Ipsiroglu S, Wallis H. Nasseri Moghaddam Z, et al. medRxiv [Preprint]. 2024 Sep 10:2024.09.06.24313213. doi: 10.1101/2024.09.06.24313213. medRxiv. 2024. Update in: Orphanet J Rare Dis. 2025 Aug 7;20(1):408. doi: 10.1186/s13023-025-03900-3. PMID: 39371127 Free PMC article. Updated. Preprint.

References

1. Sun W, Wang Y, Wu M, Wu H, Peng X, Shi Y, et al. Fourteen cases of cerebral creatine deficiency syndrome in children: a cohort study in China. Transl Pediatr. 2023;12(5):927–37. 10.21037/tp-23-164. - PMC - PubMed
1. Tise CG, Palma MJ, Cusmano-Ozog KP, Matalon DR. Creatine transporter deficiency presenting as failure to thrive: a case report of a novel SLC6A8 variant causing a treatable but likely underdiagnosed genetic disorder. J Investig Med High Impact Case Rep. 2023. 10.1177/23247096231154438. - PMC - PubMed
1. Fernandes-Pires G, Braissant O. Current and potential new treatment strategies for creatine deficiency syndromes. Mol Genet Metab. 2022;135(1):15–26. 10.1016/j.ymgme.2021.12.005. - PubMed
1. Salomons GS, van Dooren SJ, Verhoeven NM, Cecil KM, Ball WS, Degrauw TJ, et al. X-linked creatine-transporter gene (SLC6A8) defect: a new creatine-deficiency syndrome. Am J Hum Genet. 2001;68(6):1497–500. 10.1086/320595. - PMC - PubMed
1. Dunbar M, Jaggumantri S, Sargent M, Stöckler-Ipsiroglu S, van Karnebeek CDM. Treatment of X-linked creatine transporter (SLC6A8) deficiency: systematic review of the literature and three new cases. Mol Genet Metab. 2014;112(4):259–74. 10.1016/j.ymgme.2014.05.011. - PubMed

MeSH terms

Actions
Actions
Actions
Actions
Actions
Actions
Actions
Actions
Actions
Actions
Actions
Actions
Actions
Actions

Substances

Actions
Actions
Actions
Actions
Actions

Save citation to file

Email citation

Add to Collections

Add to My Bibliography

Your saved search

Create a file for external citation management software

Your RSS Feed

Establishing a core outcome set for creatine transporter deficiency and guanidinoacetate methyltransferase deficiency

Affiliations

Establishing a core outcome set for creatine transporter deficiency and guanidinoacetate methyltransferase deficiency

Authors

Affiliations

Abstract

Conflict of interest statement

Figures

Update of

References

MeSH terms

Substances

Supplementary concepts

Grants and funding

LinkOut - more resources

Medical