Glutaric Acidemia Type 1

Clinical characteristics.

The phenotypic spectrum of untreated glutaric acidemia type 1 (GA-1) ranges from the more common form (infantile-onset disease) to the less common form (later-onset disease – i.e., after age 6 years). Of note, the GA-1 phenotype can vary widely between untreated family members with the same genotype, primarily as a function of the age at which the first acute encephalopathic crisis occurred: three months to six years in infantile-onset GA-1 and after age six years in later-onset GA-1. Characteristically these crises result in acute bilateral striatal injury and subsequent complex movement disorders. In the era of newborn screening (NBS), the prompt initiation of treatment of asymptomatic infants detected by NBS means that most individuals who would have developed manifestations of either infantile-onset or later-onset GA-1 remain asymptomatic; however, they may be at increased risk for other manifestations (e.g., renal disease) that are becoming apparent as the understanding of the natural history of treated GA-1 continues to evolve.

Diagnosis/testing.

Because the early initiation of treatment dramatically improved the outcome for persons with GA-1, an international guideline group has recommended NBS. The diagnosis of GA-1 in a proband with a positive NBS result or suggestive biochemical and/or clinical findings is confirmed by identification of biallelic pathogenic variants in GCDH or, when molecular genetic test results are uncertain, by detection of significantly reduced activity of the enzyme glutaryl-CoA dehydrogenase (GCDH) in cultured fibroblasts or leukocytes.

Management.

Prevention of primary manifestations: When GA-1 is suspected during the diagnostic evaluation of a newborn with an elevated concentration of 3-OH-GA in plasma or urine, metabolic treatment should be initiated immediately. Development and evaluation of treatment plans, training and education of affected individuals and their families, and avoidance of side effects of dietary treatment (i.e., malnutrition, growth failure) require a multidisciplinary approach by experienced subspecialists from a specialized metabolic center. The main principles of treatment are to reduce lysine oxidation and enhance physiologic detoxification of glutaryl-CoA. Combined metabolic therapy includes low-lysine diet, carnitine supplementation, and emergency treatment during episodes with the goal of averting catabolism and minimizing CNS exposure to lysine and its toxic metabolic byproducts.

Surveillance: Regular evaluations by a metabolic specialist and metabolic dietician; routine evaluation of growth parameters and head circumference, developmental progress and educational needs, clinical signs and symptoms of movement disorders, biochemical parameters, and renal function (in adolescents and adults).

Agents/circumstances to avoid: Excessive dietary protein or protein malnutrition inducing catabolic state, prolonged fasting, catabolic illness (intercurrent infection; brief febrile illness post vaccination), inadequate caloric provision during other stressors (e.g., surgery or procedure requiring fasting/anesthesia).

Evaluation of relatives at risk: Testing of all at-risk sibs of any age to allow for early diagnosis and treatment. For at-risk newborn sibs when prenatal testing was not performed: in parallel with NBS either test for the familial GCDH pathogenic variants or measure urine organic acids, plasma amino acids, and acylcarnitine profile.

Pregnancy management: It is recommended that care for a pregnant woman with GA-1 be provided by a multidisciplinary team including the treating obstetrician, a metabolic physician, and a specialist metabolic dietician.

Genetic counseling.

GA-1 is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Once the GCDH pathogenic variants in an affected family member are known, carrier testing for at-risk relatives, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible.

Suggestive Findings

Establishing the Diagnosis

The diagnosis of GA-1 in a proband with suggestive biochemical and/or clinical findings is confirmed by identification of biallelic pathogenic variants in GCDH (Table 1) or, when molecular genetic test results are uncertain, by detection of significantly reduced activity of the enzyme glutaryl-CoA dehydrogenase in cultured fibroblasts or leukocytes.

Molecular genetic testing approaches can include gene-targeted testing (single-gene testing or use of a multigene panel) and comprehensive genomic testing (typically exome sequencing) depending on the indications for testing. Gene-targeted testing requires that the clinician determine which gene(s) are likely involved, whereas genomic testing does not.

• Infants with positive newborn screening and follow-up testing (see Scenario 1) are likely to be diagnosed using gene-targeted testing.
• Symptomatic individuals with nonspecific clinical and imaging findings in whom the diagnosis of GA-1 has not been considered (see Scenario 2) are more likely to be diagnosed using comprehensive genomic testing [Marti-Masso et al 2012].

Clinical Description

The phenotypic spectrum of untreated glutaric acidemia type 1 (GA-1) ranges from the more common form (infantile-onset disease) to the less common form (later-onset disease after age 6 years). Of note, the GA-1 phenotype can vary widely among untreated family members with the same genotype, primarily as a function of the age at which the first acute encephalopathic crisis occurred: three months to three years in infantile-onset GA-1 and after age six years in later-onset GA-1 [López-Laso et al 2007, Wang et al 2014]. Characteristically these crises result in acute bilateral striatal injury and subsequent complex movement disorders. Patients may also develop insidious-onset basal ganglia injury in the absence of an identified acute encephalopathic crisis.

In the era of newborn screening (NBS), the prompt initiation of treatment of asymptomatic infants detected by NBS means that most individuals who would have developed manifestations of either infantile-onset or later-onset GA-1 remain asymptomatic.

Infantile-onset GA-1. If untreated, 80%-90% of children with infantile-onset GA-1 will experience an acute encephalopathic crisis, 95% of which occur in the first 24 months of life. These crises can be precipitated by intercurrent febrile illness, febrile reaction to vaccinations, or fasting and catabolic stressors associated with anesthesia and surgical procedures [Kölker et al 2006, Boy et al 2017b]. Characteristically these crises result in acute bilateral striatal injury and are followed (typically between ages 3 months and 3 years; in rare cases, between ages 3 and 6 years) by progressive complex neurologic movement disorders. Disability and mortality are high after acute crises [Kyllerman et al 2004, Kölker et al 2006].

Dietary treatment and intense emergency treatment during intercurrent illness (see Management) have reduced the frequency of acute encephalopathic crises and movement disorders to 10%-20%.

Subdural hemorrhages, a rare manifestation of GA-1, may develop even in individuals diagnosed on NBS, managed appropriately, and without macrocephaly [Zielonka et al 2015, Ishige et al 2017]. Subdural hemorrhages may appear spontaneously or following mild head trauma in GA-1; they can also resolve spontaneously. Isolated subdural hemorrhage without other features of GA-1 on brain MRI is extremely uncommon [Vester et al 2015, Vester et al 2016].

Seizures are reported in 7% of individuals with GA-1 [Kölker et al 2015a]. While self-limited seizures may accompany the acute encephalopathic crisis, in other instances they may be the presenting manifestation [McClelland et al 2009]. Infantile spasms have been reported in some [Young-Lin et al 2013, Liu et al 2015].

When GA-1 is diagnosed after the onset of neurologic manifestations, outcome is poor and the therapeutic effect of the usual interventions is more limited [Hoffmann et al 1996, Bjugstad et al 2000, Busquets et al 2000a, Kyllerman et al 2004, Kölker et al 2006, Kamate et al 2012, Wang et al 2014]. Nonetheless, therapeutic intervention may prevent additional progressive neurologic deterioration in some [Hoffmann et al 1996, Bjugstad et al 2000, Kölker et al 2006, Badve et al 2015, Fraidakis et al 2015].

With early diagnosis and adherence to treatment, 80%-90% of individuals with GA-1 remain largely asymptomatic [Strauss et al 2011, Viau et al 2012, Couce et al 2013, Lee et al 2013, Boy et al 2018].

Insidious onset of manifestations was previously seen in an estimated 10%-20% of symptomatic individuals [Kölker et al 2006]; it now appears to be more common because early diagnosis and treatment of GA-1 have reduced the incidence of acute encephalopathic crises [Boy et al 2018].

Individuals who adhere to maintenance and emergency treatments rarely develop dystonia; those who do not are at high risk of developing a movement disorder [Kölker et al 2007, Heringer et al 2010, Strauss et al 2011, Kölker et al 2012, Boy et al 2018]. Those who have insidious onset generally have less severe movement disorders and less extensive lesions on brain MRI than those with acute encephalopathic crisis [Boy et al 2019]. The insidious phenotype may correlate with lack of adherence to chronic dietary treatment [Boy et al 2018].

Late-onset GA-1. Late-onset GA-1 is defined as onset of manifestations after age six years. Some individuals with late-onset GA-1 (e.g., mothers diagnosed due to the birth of a child with an abnormal NBS result) are entirely asymptomatic. Others have a variety of neurologic findings. Among eight symptomatic individuals ages eight to 71 years, the following were observed: chronic headaches (4), macrocephaly (4), epilepsy (2), tremor (2), and dementia (2). All had MRI evidence of frontotemporal hypoplasia and abnormal signal of the white matter; five had subependymal nodules. All showed the high excreting phenotype [Boy et al 2017a]. Others have reported clinical and neuroimaging findings [Külkens et al 2005, Pierson et al 2015, Zhang & Luo 2017].

Other reported manifestations of late-onset GA-1 include the following:

• Peripheral neuropathy (1 adult) [Herskovitz et al 2013]
• Brain neoplasms (in several adults and children) [Korman et al 2007, Burlina et al 2012, Herskovitz et al 2013, Pierson et al 2015, Serrano Russi et al 2018]. This finding has led to an as-yet unsubstantiated speculation about possible increased susceptibility to brain neoplasms in adults.

Non-neurologic disease manifestations observed in individuals in GA-1 regardless of age of onset. Chronic kidney disease may occur in those with GA-1, even with adherence to treatment, and may be an extracerebral manifestation in adults with GA-1 [Kölker et al 2015b].

Note: Infants with biochemical findings consistent with GA-1 on NBS, but normal blood levels of GA and 3-OH-GA and only one identifiable GCDH pathogenic variant, may warrant close clinical follow up. However, given the high sensitivity of GCDH molecular genetic testing, the chances that an infant with these findings is affected and at risk of developing acute striatal necrosis are low.

Genotype-Phenotype Correlations

Most GCDH variants reported to date are missense variants [Schmiesing et al 2017].

GA-1 biochemical (excreter) subtypes. GA-1 was originally divided into two arbitrarily defined biochemical subtypes: high excreters of urinary glutaric acid (GA) and low excreters of urinary GA [Baric et al 1999]. High excreters and low excreters are at the same risk for striatal injury [Christensen et al 2004, Kölker et al 2006]. While excreter status has no clear correlation with the clinical phenotype in childhood, evidence suggests that high excreters have higher concentrations of GA and 3-OH-GA in the CNS and have increased prevalence of progressive white matter lesions on MRI [Boy et al 2017a].

• High excreters have no or very low glutaryl-CoA dehydrogenase activity (0%-3%) [Goodman et al 1998, Baric et al 1999, Busquets et al 2000b]. NBS sensitivity for the high excreter biochemical phenotype is 100% [Boy et al 2018].
• Low excreters have up to 30% residual glutaryl-CoA dehydrogenase activity [Goodman et al 1998, Busquets et al 2000b]. They have biallelic GCDH pathogenic variants, at least one of which is a hypomorphic missense variant. NBS sensitivity for the low excreter biochemical phenotype is 84% [Boy et al 2018]. See Table 10 for details on variants associated with low excreter status.

Prevalence

Well over 500 individuals with GA-1 have been reported to date [Boy et al 2017b]. Prevalence estimates for GA-1 vary between 1:30,000 and 1:100,000-110,000 [Kyllerman & Steen 1980, Lindner et al 2004, Tsai et al 2017].

Details on founder variants reported in Ojibway-Cree First Nation Canadians of Manitoba and Ontario, South African Xhosa peoples, Pennsylvania Amish, Lumbee Native Americans of North Carolina, and Irish Traveler communities in the Republic of Ireland are included in Table 10.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual following diagnosis of GA-1, the evaluations summarized in Table 3 (if not performed as part of the evaluation that led to diagnosis) are recommended.

Treatment of Manifestations

All children with GA-1 and feeding difficulties require supervision of a specialist metabolic dietitian with experience in managing diet in GA-1. German (D)-Austrian (A)-Swiss (CH) (DACH) recommendations have been used in several clinical trials and have resulted in positive outcomes [Kölker et al 2007, Heringer et al 2010, Kölker et al 2012, Boy et al 2013].

The main principles of treatment are to reduce lysine oxidation and enhance physiologic detoxification of glutaryl-CoA. Combined metabolic therapy includes the following [Boy et al 2013]:

• Low-lysine diet
• Carnitine supplementation
• Emergency treatment during episodes with the goal of averting catabolism and minimizing CNS exposure to lysine and its toxic metabolic byproducts