Congenital disorders of creatine synthesis and transport

INTRODUCTION — Creatine is a nitrogenous organic acid that is produced primarily in the kidney and liver and is stored in tissues with high energy demands, such as skeletal muscle and the brain. Its phosphorylated form (creatine-phosphate or phosphocreatine) is involved in the formation of adenosine triphosphate (ATP), which is used as an energy source for a number of intracellular metabolic processes.

There are three identified congenital metabolic disorders that lead to creatine deficiency [1-3]. Two are autosomal recessive disorders that affect the biosynthesis of creatine. They are arginine:glycine amidinotransferase (AGAT; also called glycine amidinotransferase [GATM]) deficiency and guanidinoacetate methyltransferase (GAMT) deficiency (figure 1). The third disorder, X-linked creatine transporter (CRTR) deficiency, is caused by a defect in the transport of creatine into the brain and muscle.

The pathogenesis, clinical features, diagnosis, and management of these disorders are reviewed here. Other inborn errors of metabolism are reviewed separately. (See "Inborn errors of metabolism: Classification" and "Inborn errors of metabolism: Epidemiology, pathogenesis, and clinical features" and "Inborn errors of metabolism: Identifying the specific disorder".)

OVERVIEW — This section briefly reviews creatine metabolism and transport. It also reviews the common clinical features and the general diagnostic and treatment approaches for these disorders. The clinical features and management approaches are discussed in the separate sections on each disorder.

Creatine metabolism — Creatine synthesis involves two enzymatic steps and occurs primarily in the liver, kidney, and pancreas (figure 1). The first step involves L-arginine:glycine amidinotransferase (AGAT), which catalyzes the formation of guanidinoacetate (GAA) from arginine and glycine. The second involves guanidinoacetate methyltransferase (GAMT), which catalyzes the formation of creatinine from GAA and S-adenosylmethionine. Creatine is taken up by the tissues, mainly brain and muscle, by the creatine transporter (CRTR, solute carrier family 6 member 8 [SLC6A8]). It is nonenzymatically converted to creatinine and excreted into the urine.

Clinical manifestations — One of the primary features of congenital disorders of creatine metabolism and transport is intellectual disability, which can range from mild to severe (table 1) [4]. Clinical manifestations also may include behavioral problems, autism, speech delay, epilepsy, and movement disorders [2]. Additional features seen in patients with AGAT and CRTR deficiencies include myopathy and muscular hypotrophy, respectively. In general, GAMT deficiency had a more severe phenotype than AGAT or CRTR deficiency.

Another disorder affecting AGAT, but without (cerebral) creatine deficiency, is caused by heterozygous autosomal-dominant missense variants in GATM [5]. These particular variants result in intramitochondrial aggregates that cause Fanconi syndrome and kidney failure.

Diagnosis — Patients with intellectual disability associated with autistic behaviors should be screened for congenital disorders of creatine synthesis and transport. Initial screening for these disorders includes measurement of the creatine signal in the brain by proton magnetic resonance spectroscopy (MRS) and measurement of GAA, creatine, and creatinine in the urine, plasma, and/or cerebrospinal fluid (CSF) (table 1) [6-9]. The laboratory studies are easier to obtain, less costly, and do not require invasive sedation and, therefore, are normally performed first. Measurement of levels in the urine only is generally sufficient. However, MRS is more sensitive and specific and, therefore, is usually performed as a confirmatory test.

There is almost complete depletion of the cerebral creatine pool on brain MRS in all patients with AGAT and GAMT deficiencies and in males with CRTR deficiency. Cerebral creatine levels are partially depleted to normal in females with CRTR deficiency [10]. The diagnosis is confirmed by identification of the genetic defect [1]. Functional testing by assaying enzymatic activity in cultured skin fibroblasts (GAMT) and Epstein-Barr virus (EBV) transformed lymphoblasts (GAMT and AGAT) is performed if genetic testing is inconclusive; for example, if a new variant of unknown pathogenicity is identified [11-13].

Carrier testing for at-risk relatives and prenatal testing are possible in families where the disease-causing variant has been identified [1]. GAMT deficiency can also be identified by measurement of GAA in blood spot cards. DNA is extracted from the blood spot card for GAMT molecular analysis if the GAA value is above the cutoff. Algorithms have been developed for neonatal screening of GAMT deficiency [14-16], and pilot newborn screening projects are underway, although no cases of the rare disorder have been identified yet. Methods suitable for newborn screening for AGAT deficiency or CRTR deficiency have not been developed, since testing using the available biomarkers is not sufficiently reliable.

Differential diagnosis — In addition to the three known congenital disorders of creatine synthesis and transport, there are disorders that may have associated secondary deficiency of cerebral creatine, including argininosuccinic aciduria (due to argininosuccinate lyase deficiency), citrullinemia type 1 (due to argininosuccinate synthetase enzyme deficiency), and gyrate atrophy of the choroid and retina (due to ornithine aminotransferase enzyme deficiency) [17,18]. These patients have partial cerebral creatine deficiency identified by MRS, but a normal urine creatine-to-creatinine ratio, and unremarkable or nonspecific changes of GAA concentrations in body fluids.

Treatment — The goal of timely diagnosis and treatment of congenital disorders of creatine biosynthesis and transport is to prevent or at least minimize brain damage and development of the associated clinical manifestations. An understanding that correction of cerebral creatine depletion will improve clinical outcomes has driven the development of treatment strategies. These strategies include oral supplementation of high-dose creatine-monohydrate for all three congenital creatine deficiency disorders (table 1). GAA-reducing strategies (high-dose ornithine, arginine-restricted diet) are additional essential treatment measures for GAMT deficiency [1,3]. Supplementation of substrates for intracerebral creatine synthesis (eg, arginine, glycine) can be tried to treat CRTR deficiency [1,3].

Treatment is associated with a significant increase of cerebral creatine levels and reduction of urinary, plasma, and CSF GAA levels as well as improvement or stabilization of clinical symptoms in all symptomatic cases of GAMT deficiency [19]. Early diagnosis and treatment can result in normal development [19]. Similar findings are reported for treatment of AGAT deficiency [11,20,21], but treatment of CRTR deficiency has been less successful [3,22].

ARGININE:GLYCINE AMIDINOTRANSFERASE (AGAT) DEFICIENCY — AGAT deficiency (MIM #612718) is a rare inborn error of metabolism caused by homozygous or compound heterozygous mutations in the glycine amidinotransferase (GATM) gene located at 15q15 [11]. This defect results in reduced guanidinoacetate (GAA) levels in the urine and plasma (table 1 and figure 1). Less than 20 patients have been identified globally [23].

Clinical features — The first reported cases were two female siblings and their male cousin [6,11]. Their clinical features included mild-to-moderate intellectual disability, autistic behavior, speech and language delay, and occasional seizures. Depletion of brain creatine was reversible with creatine supplementation and was associated with clinical improvement.

Myopathy is a manifestation that appears later in the disease course [23-26]. This was first noted in two siblings, ages 18 and 12 years, from a Yemenite Jewish family, who had a childhood history of poor weight gain, developmental delay, and fatigability [24]. They both went on to have proximal muscle weakness, with moderately elevated creatine kinase (CK) levels (500 to 600 units/L) and a myopathic electromyography. Findings on muscle biopsy included tubular aggregates and decreased respiratory chain enzyme activity. Strength and stamina improved after treatment with supplemental creatine.

Treatment — The treatment of AGAT deficiency consists of 300 to 400 mg/kg/day of oral creatine supplementation (administered as creatine monohydrate) [3], although longitudinal brain magnetic resonance spectroscopy (MRS) studies in affected patients suggest that oral supplementation of 100 mg/kg of creatine monohydrate is sufficient to restore and maintain cerebral creatine levels [27]. This therapy is effective in replenishing the cerebral creatine pool and improving developmental outcomes [6,21,27,28]. It is also effective in patients with myopathy [24-26]. Early treatment may prevent the development of intellectual disability and other disease manifestations [20,21]. Measurement of cerebral creatine levels by MRS is used to monitor the therapeutic response [3,27] and clinical outcomes including muscle weakness and cognitive and behavioral function.

Long-term outcome — Treatment results in significant improvement of myopathy. Improvement and normalization of developmental delay have been reported in patients who received early treatment [23,27]. As an example, the younger sibling of two sisters with AGAT deficiency was diagnosed prenatally and started on creatine supplementation at four months of age. This boy demonstrated normal development at 18 months of age, in contrast to his two older siblings, who had developmental delay at that age [11,20]. A cousin of these children was diagnosed and started on treatment at two years of age. He had a borderline normal intelligence quotient (IQ) at eight years of age, whereas two other affected relatives who were not started on treatment until five and seven years of age had moderate intellectual disability at the ages 11 and 13 years, respectively [23,27].

AUTOSOMAL DOMINANT AGAT AGGREGATION SYNDROME — This condition is caused by mitochondrial aggregation of fully penetrant heterozygous GATM missense variants in renal tubular cells [5]. Patients develop renal Fanconi syndrome with glucosuria, hyperphosphaturia, generalized aminoaciduria, low-molecular-weight proteinuria, and metabolic acidosis. Debilitating rickets or bone deformities have not been described in these patients, nor were neurologic manifestations or cerebral creatine deficiency reported.

GUANIDINOACETATE METHYLTRANSFERASE (GAMT) DEFICIENCY — GAMT deficiency (MIM #612736) is a rare inborn error of metabolism caused by homozygous or compound heterozygous mutations in the guanidinoacetate N-methyltransferase (GAMT) gene located at 19p13.3 [29]. This defect results in elevated guanidinoacetate (GAA) levels in the urine, plasma, and cerebrospinal fluid (CSF) (table 1 and figure 1). GAMT deficiency has been identified in approximately 100 patients worldwide [19,29-33].

Clinical features — The initial manifestations of GAMT deficiency present within the first year of life [19]. Developmental delay and intellectual disability range from severe to mild. Expressive speech is impaired and is often resistant to treatment interventions. Most individuals with GAMT deficiency have behavioral disturbances that can include autistic features, aggression, hyperactivity, and self-mutilation. A significant proportion of patients have neurologic manifestations, including pyramidal and extrapyramidal movement disorders and epilepsy.

Specific neurologic findings in young children include hypotonia, ataxia, and hyperkinetic extrapyramidal movements [19,29-31]. Intermittent ataxia during febrile illnesses was reported in one patient [19]. Dystonia occurs later in the disease course. Chorea, including new onset of choreatic storm, and hemiballism have been described in other patients. Additionally, there are reports of presentations masquerading as Leigh syndrome and mitochondrial encephalopathy [34].

Seizures can be absence or tonic-clonic, focal, or generalized. Specific seizure types reported include atonic seizures with head drop/nodding, myoclonic seizures, myoclonic astatic seizures (myoclonic followed by atonic), and infantile spasms. Electroencephalography (EEG) changes are usually nonspecific, but several patients have demonstrated high-amplitude theta-delta background activities with multifocal spikes [19,31]. Basal ganglia changes occur mostly on the globus pallidus, with bilateral hypointensities in T1-weighted images and hyperintensities in T2-weighted images seen on magnetic resonance imaging (MRI).

Treatment — Therapy for GAMT deficiency has a dual aim: restoration of cerebral creatine levels and reduction of GAA levels [3,19]. Oral supplementation of creatine is used to restore cerebral creatine levels. Higher doses of creatine monohydrate (400 to 800 mg/kg/day, orally or enterally) are often required to treat GAMT deficiency compared with arginine:glycine amidinotransferase (AGAT) deficiency [3,27]. There are several strategies to reduce GAA levels, including competitive inhibition of AGAT activity via high-dose L-ornithine supplementation (400 to 800 mg/kg/day orally or enterally) [16,19,35] and substrate deprivation via an arginine-restricted diet.

An arginine-restricted diet consists of consuming 0.3 to 0.4 g/kg/day of natural protein that contains approximately 250 mg/kg/day of L-arginine together with an arginine-free essential amino acid supplement to achieve the age-related daily required intake (DRI) for protein [19]. In patients whose plasma GAA levels are well controlled upon L-ornithine supplementation, the effect of additional dietary L-arginine restriction has shown variable efficacy on further reducing plasma GAA levels [16,19]. It is unclear if a less arduous low-protein diet, consisting of 0.8 to 1.5 g/kg/day of natural protein, with or without supplementation of an arginine-free formula, reduces GAA accumulation [19].

Sodium benzoate as an additional treatment to reduce the production of GAA via conjugation with glycine to form hippuric acid is controversial. One case study reported that sodium benzoate had no effect on further reduction of plasma GAA levels [36], whereas another case series demonstrated a beneficial effect of sodium benzoate [35].

Outcome — Early treatment results in the most beneficial intellectual outcomes [19]. Various case series have shown that treatment is effective in reducing seizure activity, improving movement disorder, and reversing basal ganglia changes. There is also a moderate effect on improving behavioral disturbances and stabilizing intellectual disability [19,31,33].

CREATINE TRANSPORTER (CRTR) DEFICIENCY — CRTR deficiency (MIM #300352) is the most common of the congenital disorders of CRTR and metabolism. It is caused by a hemizygous variant in the solute carrier family 6 (neurotransmitter transporter, creatine), member 8 (SLC6A8) gene, located at Xq28 [37]. Approximately one-third of patients have a de novo pathogenic variant [38]. CRTR loss of function is mostly caused by missense variants and small deletions that are concentrated in the transmembrane domains 7 and 8 of the protein [38]. The prevalence of this disorder in at-risk populations, such as males with intellectual disability, epilepsy, or autism spectrum disorder, is approximately 1 to 3.5 percent [39-41].

Clinical manifestations — CRTR deficiency typically presents in the first six years of life [38]. The most common findings include intellectual disability that progresses with age, severe expressive speech delay, epilepsy, and autistic disorders. Seizures are usually intermittent and treatment responsive. They can occur with or without fever. Reported behavioral problems include attention deficit hyperactivity disorder (ADHD), social anxiety, impulsivity, and aggression.

Additional clinical manifestations include dysmorphic facial features (broad forehead, mid-face hypoplasia, ptosis, short nose) and slender, poorly developed muscles with hypotonia. Hyperextensible joints and ataxic and dystonic movement disorders have also been reported. The clinical spectrum of CRTR deficiency spans from mild to moderate and severe phenotypes depending upon the severity and combination of clinical findings [42].

Single-case reports include gastrointestinal (chronic constipation/ileus), urogenital, ophthalmologic, and hearing abnormalities as well as cardiomyopathy, delayed myelination, thin corpus callosum, and progressive cerebral/cerebellar atrophy [2,22].

An association was identified between variants with residual activity and relatively milder phenotypes [38]. In comparison, large deletions of the SLC6A8 gene, including complex rearrangements, present with more severe clinical findings.

Female heterozygotes (carriers) often have learning disabilities or mild intellectual disability, although more severe presentations have also been observed and are most likely due to skewed X-inactivation in favor of the mutated allele [10]. The most severe phenotype reported is in a girl with mild/moderate intellectual disability, behavioral problems, and intractable epilepsy [43].

Diagnosis — The urinary creatine-to-creatinine ratio is used as a screening marker [41,44]. Impaired renal tubular reuptake of creatine is presumed to result in an elevated urinary creatine excretion [8], whereas urinary creatinine excretion is low due to decreased creatine concentration in muscle and brain tissue. As a result, the urinary creatine-to-creatinine ratio is high in hemizygous males [38]. However, this marker is noninformative in heterozygous clinically affected or unaffected females because the ratio is often normal to only mildly elevated [10]. A high rate of false-positive results also limits the validity of this screening test. (See 'Diagnosis' above.)

The diagnosis of CRTR deficiency is confirmed by molecular analysis of SLC6A8. Creatine uptake studies in cultured fibroblasts are performed when the diagnosis is strongly suspected in a male patient (eg, elevated urinary creatine-to-creatinine ratio or creatine deficiency found on brain magnetic resonance spectroscopy [MRS]) who had no detected pathogenic variant or a novel variant of uncertain pathogenicity [45]. Creatine uptake <10 percent of normal control fibroblasts is diagnostic. This assay can also be used to confirm the diagnosis in a symptomatic heterozygous female with a novel variant of uncertain pathogenicity.

Treatment — Treatment of CRTR deficiency consists of high-dose oral creatine supplementation (400 mg/kg/day of creatine-monohydrate) with the intent to maximize creatine transport into the brain via residual function of the CRTR or alternative lower-affinity pathways. Supraphysiologic oral doses of the creatine precursors, L-arginine (400 mg/kg/day) and/or glycine (150 mg/kg/day), have been tried in several patients to enhance cerebral creatine synthesis [3,7]. Improvements seen with these therapies are more limited than what is seen in the disorders of creatine metabolism, mainly because the interventions do not result in a significant increase in cerebral creatine levels [43,46-48]. In a systematic review of the literature, 10 of 28 patients (36 percent) had a demonstrable response to treatment, with improved clinical parameters (including cognitive ability, psychiatric and behavioral disturbances, and epilepsy) and/or a demonstrated moderate increase in cerebral creatine [22]. The majority of patients with clinical improvement were milder cases that had detectable cerebral creatine prior to treatment, most likely due to residual CRTR function. Combined creatine, arginine, and glycine therapy was shown to stop disease progression in males and improve phenotype in females in a case series including data from 14 males and 3 females with CRTR deficiency [42]. S-adenosylmethionine given to one patient as an additional substrate for cerebral creatine synthesis did not result in an increase of cerebral creatine levels [49].

There are several apparent barriers to correction of the clinical and biochemical features of CRTR deficiency. CRTR is not expressed in astrocytes in rodents, although this finding has not been confirmed in humans [50]. In addition, coexpression of arginine:glycine amidinotransferase (AGAT) and guanidinoacetate methyltransferase (GAMT) in brain cells is uncommon [51]. Thus, uptake of GAA by the CRTR is required to synthesize sufficient creatine in the brain. Furthermore, failure of neurons to reuptake endogenously synthesized creatine leads to increased turnover in the cerebrospinal fluid (CSF) [52].

Alternative treatments with creatine analogs (cyclocreatine) and nanoparticle encapsulated creatine are in development.

SUMMARY

●There are two autosomal-recessive disorders that affect the biosynthesis of creatine: arginine:glycine amidinotransferase (AGAT) deficiency and guanidinoacetate methyltransferase (GAMT) deficiency (figure 1). A third congenital metabolic disorder that is X linked, creatine transporter (CRTR) deficiency, is caused by a defect in the transport of creatine into the brain and muscle. (See 'Introduction' above and 'Creatine metabolism' above.)

●One of the primary features of congenital disorders of creatine synthesis and transport is intellectual disability, which can range from mild to severe (table 1). Clinical manifestations also may include behavioral problems, autism, speech delay, epilepsy, and movement disorders. However, patients with autosomal-dominant renal AGAT aggregation syndrome do not have neurologic or neurodevelopmental manifestations. Additional features seen in patients with AGAT and CRTR deficiencies include myopathy and muscular hypotrophy. CRTR deficiency also has several unique features. In general, GAMT deficiency had a more severe phenotype than AGAT or CRTR deficiency. (See 'Clinical manifestations' above and 'Arginine:glycine amidinotransferase (AGAT) deficiency' above and 'Autosomal dominant AGAT aggregation syndrome' above and 'Guanidinoacetate methyltransferase (GAMT) deficiency' above and 'Creatine transporter (CRTR) deficiency' above.)

●Patients with intellectual disability should be screened for congenital disorders of creatine synthesis and transport. A marked reduction of the creatine signal in magnetic resonance spectroscopy (MRS) of the brain is an indicator of disorders of creatine biosynthesis and transport. Determination of urinary guanidinoacetate (GAA) and the urinary creatine-to-creatinine ratio is fundamental to the diagnosis and ruling out other disorders. The diagnosis is confirmed by identification of the genetic defect. An assay of enzymatic activity is performed if genetic testing is inconclusive. (See 'Diagnosis' above and 'Differential diagnosis' above and 'Creatine transporter (CRTR) deficiency' above.)

●Treatment strategies include oral supplementation of high-dose creatine-monohydrate for all three congenital disorders of creatine biosynthesis and transport. GAA-reducing strategies (high-dose ornithine, arginine-restricted diet) are additional treatment strategies for GAMT deficiency. Supplementation of substrates for intracerebral creatine synthesis (arginine, glycine) is often also used to treat CRTR deficiency. Treatment is associated with significant biochemical and clinical improvement or stabilization in patients with AGAT and GAMT deficiencies. Early treatment and diagnosis of these two disorders can result in normal development. Therapy has been less successful for CRTR deficiency. (See 'Treatment' above and 'Arginine:glycine amidinotransferase (AGAT) deficiency' above and 'Guanidinoacetate methyltransferase (GAMT) deficiency' above and 'Creatine transporter (CRTR) deficiency' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Sylvia Stockler-Ipsiroglu, MD, PhD, MBA, FRCPC, who contributed to earlier versions of this topic review.