Glutaric Acidemia Type I · GA-1

Description

Glutaric acidemia type I, GA-1, is an autosomal recessive disorder of lysine, hydroxylysine, and tryptophan catabolism. The enzyme glutaryl-CoA dehydrogenase, encoded by GCDH on chromosome 19, catalyzes the conversion of glutaryl-CoA to crotonyl-CoA. Pathogenic variants in GCDH reduce or abolish enzyme activity. Glutaric acid, 3-hydroxyglutaric acid, and glutarylcarnitine accumulate, and glutaryl-CoA derivatives are toxic to the basal ganglia, the striatum in particular.

Macrocephaly is present at or shortly after birth in a substantial fraction of affected children and is often the first finding. Without screening, the typical clinical course is silent for the first months of life, then an acute encephalopathic crisis triggered by febrile illness, infection, or surgery between ages six months and three years. The crisis produces irreversible striatal injury and a movement disorder, most often dystonia, sometimes severe and lifelong. A smaller group of children develops striatal injury insidiously without an identifiable acute crisis. Outcomes correlate with how early the diagnosis is made and how aggressively crises are prevented.

Detection is by newborn screening on the dried blood spot, using tandem mass spectrometry to identify elevated glutarylcarnitine, C5DC. Confirmation uses urine organic acids showing elevated glutaric acid and 3-hydroxyglutaric acid, plasma acylcarnitines, and GCDH sequencing. Two biochemical phenotypes are recognized: high excretors with markedly elevated urinary glutaric acid and low excretors whose urine organic acids may be normal between episodes; the C5DC marker on screening detects both. Reported live-birth incidence in unselected populations is roughly 1 in 100,000. Founder populations with markedly higher rates include the Old Order Amish of Pennsylvania, the Lumbee of North Carolina, the Oji-Cree of Canada, and several populations in Saudi Arabia and the United Kingdom Irish Traveller community, with population-specific rates ranging from 1 in 250 to 1 in 5,000.

Treatments to date

There is no FDA-approved disease-specific drug therapy for GA-1. Standard of care has two components: a lysine-restricted diet with a lysine-free, tryptophan-reduced medical formula combined with measured natural protein, and an aggressive emergency protocol during febrile illness or other catabolic stress. The emergency protocol provides intravenous dextrose at high rates, additional carnitine, and protein elimination for twenty-four to forty-eight hours, started promptly when illness begins. L-carnitine supplementation is continuous and lifelong.

Outcomes change dramatically when both components are in place from the neonatal period. Cohort studies from Germany, the United States, and Canada report rates of dystonic movement disorder below 10 percent in children identified through newborn screening and treated by experienced metabolic centers, compared with 80 to 90 percent in unscreened or late-diagnosed cohorts. Once striatal injury occurs, treatment is supportive, with physical therapy, antispasticity and antidystonic medications, and assistive technology. Liver transplantation has been reported in selected cases and reduces metabolite production but does not reverse established neurological injury. Gene therapy programs are in preclinical development.