Guanidinoacetate methyltransferase deficiency, a treatable cause of intellectual disability in late childhood

INTRODUCTION

Identifying a clear cause for intellectual disability (ID) in children remains challenging, particularly in resource-limited settings. Many cases remain unexplained, leaving families without answers or options for targeted treatment. Yet, some underlying causes, such as creatine deficiency disorders (CDDs), are treatable. Early diagnosis can make a significant difference, helping prevent or minimize long-term neurodevelopmental impairment.^[1]

CDDs are rare inherited metabolic disorders characterized by impaired synthesis or transport of creatine, a crucial molecule involved in energy metabolism in the brain and muscles. Among these, guanidinoacetate methyltransferase (GAMT) deficiency is one such entity that presents with developmental delays, seizures, behavioral challenges, and autistic features. This case highlights the importance of considering treatable metabolic conditions like GAMT deficiency when evaluating children with unexplained developmental delays.

CASE REPORT

A 15-year-old girl, born to second-degree consanguineous parents, presented with global developmental delay, mainly affecting language and social domains. Her mother noted absent meaningful speech by age two, prompting early interventions such as speech and occupational therapy. Despite these efforts, the child showed only limited progress. At age five, she began having drop attacks, which were managed at a local hospital. Brain imaging of magnetic resonance imaging (MRI) and electroencephalography was reported at that time to be normal, and she was started on multiple antiepileptic drugs. Despite polytherapy, she continued to have intermittent myoclonic-atonic seizures. There was no significant prenatal, perinatal, or postnatal history suggestive of hypoxic or infectious insults. Her neonatal metabolic screening did not include CDDs, and no family history of similar conditions was reported.

Her current developmental assessment revealed her gross motor skills matched those of 4 years old, while her fine motor, language, and social skills were those of an 18 months old. Anthropometric measurements, including head circumference, were normal. No obvious phenotype specific to any syndrome type was observed. Neurological examination showed truncal ataxia and head nodding, but no other abnormalities. Screening with the Childhood Autism Rating Scale indicated mild-to-moderate autism spectrum disorder.

Routine metabolic tests, including serum lactate, ammonia, and liver enzymes, were normal. With inconclusive biochemical tests and persistent seizures despite multiple medications, further testing was pursued. Magnetic resonance spectroscopy (MRS) of the brain demonstrated reduced creatine peaks (Figure 1 illustrates the MRS findings, highlighting the markedly reduced creatine peaks), and hence, genetic testing was planned, suspecting CDDs. Biochemical confirmation showed low serum creatine levels with elevated guanidinoacetate levels in urine and plasma.

Close

Figure 1:

The magnetic resonance spectroscopic findings with the red arrow highlighting the markedly reduced creatine peak. The X-axis denoting the chemical shift (parts per million-ppm) and the Y-axis denoting signal intensity (arbitrary units-a.u.).

Export to PPT

Whole exome sequencing revealed a pathogenic homozygous missense mutation (c.59G>C; p.Trp20Ser) in exon 1 of the GAMT gene, confirming the diagnosis of GAMT deficiency.

Core differentials for the disorders such as Angelman syndrome and Rett syndrome were elaborated in Table 1.

Following the diagnosis, the family received extensive counseling by a multidisciplinary team including pediatric neurologist, child development unit, and metabolic specialist, regarding the disorder, its treatment, prognosis, and the need for lifelong supplementation. Carrier testing for the parents was also advised.

The child was started on oral creatine monohydrate at 800 mg/kg/day in two divided doses and L-ornithine at 60 mg/kg/day to reduce guanidinoacetate accumulation. She continued levetiracetam and ethosuximide, with lamotrigine added later due to ongoing drop attacks.

After 6 months of follow-up, the child showed mild but noticeable improvement in social interaction, cognition, and reduced hyperactivity. At present, the financial challenges due to the cost of long-term supplementation are being addressed with philanthropic support.

DISCUSSION

Creatine is synthesized from arginine and glycine through the action of arginine glycine amidinotransferase (AGAT) and GAMT, using S-adenosylmethionine as a methyl donor. Creatine is then transported into the brain and muscles by the creatine transporter (CRTR). Inside cells, creatine plays an essential role in regenerating adenosine triphosphate, the cell’s primary energy molecule.

Mutations in AGAT, GAMT, or CRTR genes cause creatine deficiency syndromes, which present primarily with neurological symptoms. GAMT and AGAT deficiencies are inherited in an autosomal recessive manner, while CRTR deficiency is X-linked, affecting males more severely.

GAMT deficiency leads to elevated guanidinoacetate due to impaired conversion, while AGAT deficiency shows low guanidinoacetate from reduced synthesis. Both present with low urine creatine. CRTR deficiency features normal synthesis but impaired transport, causing high urine creatine. Our patient had low serum creatine and elevated guanidinoacetate, confirming GAMT deficiency. Biochemical findings for all three types are summarized in Figure 2.

Close

Figure 2:

All the biochemical and radiological findings for all three subtypes of creatine deficiency. GAMT: Guanidinoacetate methyltransferase, AGAT: Arginine glycine amidinotransferase, CRTR: Creatine transporter .

Export to PPT

Symptoms often emerge in infancy or early childhood and include developmental delays, ID, seizures, behavioral issues, and autistic features. MRI brain scans may show hyperintensities in the globus pallidus, and MRS typically reveals absent or reduced creatine peaks, which is a key diagnostic clue.^[1]

The estimated prevalence of CDDs among children with unexplained ID is around 2.7% and fewer than 7/1000 children with autism spectrum disorder.^[2] GAMT deficiency is rare globally, with an estimated incidence ranging from 1 in 550,000 to 1 in 2.6 million live births, though data from India are scarce.^[3]

Management involves oral creatine monohydrate supplementation (typically 400–800 mg/kg/day), which helps replenish brain creatine stores. L-ornithine supplementation can lower guanidinoacetate levels, which may be neurotoxic at high concentrations. Some protocols also suggest dietary arginine restriction. Supportive therapies such as speech, occupational, and behavioral therapy are crucial for maximizing functional outcomes. Early diagnosis and prompt treatment can prevent or greatly reduce cognitive impairment and seizures.

In our patient, despite the late diagnosis at age 15, targeted treatment resulted in observable improvements in social and behavioral functioning, underscoring the benefit of treatment even when initiated later in life.

CONCLUSION

In children with non-syndromic intellectual disability, especially when associated with seizures or autistic features, treatable metabolic conditions like GAMT deficiency should be considered. Timely diagnosis and targeted therapy can significantly improve outcomes. Increasing awareness and access to genetic and metabolic testing are vital to ensure that treatable causes of intellectual disability are not missed.