Glutaric Acidemia Type II Panel

Glutaric acidemia (GA) type II, also known as multiple acyl-CoA dehydrogenase deficiency (MADD), is an inherited disorder of fatty acid and amino acid oxidation. GA type II is caused by defects in one of three genes (ETFA, ETFB or ETFDH). Clinical and biochemical features are not typically useful in distinguishing which gene is affected in GA type II patients. Three sub-categories of GA type II are recognized, each of which is based on severity and presentation of clinical symptoms. (Frerman and Goodman 2014).

Patients in the first group present within the first 48 hours of life with severe hypoketotic hypoglycemia, hypotonia, metabolic acidosis, possibly hepatomegaly and a “sweaty feet” odor similar to that observed in isovaleric acidemia patients, and multiple congenital anomalies, such as dysplastic kidneys, facial dysmorphism, rocker bottom feet, abdominal wall defects and abnormal external genitalia. Such patients typically die within the first week of life (Olsen et al. 2003; Schiff et al. 2006; Frerman and Goodman 2014; Xi et al. 2014).

Patients in the second group present similarly to those in the first group, but without congenital anomalies. If these patients survive beyond the first week of life, they typically succumb within the first few months, often due to cardiomyopathy (Olsen et al. 2003; Schiff et al. 2006; Frerman and Goodman 2014; Xi et al. 2014).

The patients in the third group have a more mild form of GA type II, with onset anywhere from infancy to adulthood. Clinical presentation also varies widely in this group, and may include vomiting, hypoglycemia, metabolic acidosis, hepatomegaly, progressive lipid storage proximal myopathy and exercise intolerance. Symptoms in the third group often present in an episodic manner, making biochemical analysis challenging as abnormalities may be detectable only during periods of metabolic crisis (Olsen et al. 2003; Schiff et al. 2006; Frerman and Goodman 2014; Xi et al. 2014).

In all GA type II patients, generalized aminoacidemia and aminoaciduria, hyperammonemia and metabolic acidosis may be observed. Increased levels of sarcosine in both the serum and urine are common in those with mild, later onset GA type II. Biochemically, GA type II can be distinguished from GA type I based on the presence of 2-hydroxyglutaric acid (3-hydroxyglutaric acid is present in GA type I patients) (Frerman and Goodman 2014).

To date, no effective treatments are available for patients with the severe, infantile onset forms of GA type II. Some patients with the mild, later onset form have been shown to respond well to riboflavin, glycine and L-carnitine supplementation, as well as to dietary restriction of fat and protein (Olsen et al. 2007; Wen et al. 2013; Frerman and Goodman 2014). The majority of patients reported with the late-onset, riboflavin responsive form of GA type II have had defects in the ETFDH gene (Olsen et al. 2007; Wen et al. 2010; Cornelius et al. 2012; Wen et al. 2013).

Glutaric acidemia type II is an autosomal recessive disorder caused by pathogenic variants in the ETFA, ETFB or ETFDH genes. To date, approximately 130 causative variants have been reported in the ETFDH gene, and nearly 40 causative variants have been reported between both the ETFA and ETFB genes (Human Gene Mutation Database). The majority of reported pathogenic variants are missense, although nonsense, regulatory, and splicing variants, as well as small deletions, insertions, duplications and indels have been reported, as has one gross deletion in ETFDH (Human Gene Mutation Database; Wen et al. 2010). The variants are spread throughout these genes.

The ETFA and ETFB genes encode the α- and β-subunits, respectively, of the electron transfer flavoprotein (ETF). ETFDH encodes the electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO). The ETF protein is found in the mitochondrial matrix as a heterodimer, comprised of one subunit each of α- and β-monomers. ETF accepts electrons from at least 12 other flavoprotein dehydrogenases, then transfers them to the mitochondrial respiratory chain via the electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO), which is located in the inner mitochondrial membrane. Clinical symptoms observed in ETFA, ETFB or ETFDH deficient patients are due to a disruption of electron transfer to the respiratory chain (Cornelius et al. 2012; Frerman and Goodman 2014).

Although the sensitivity of this test panel is currently unknown, most variants reported for the three genes in this panel are of the type which can be detected using direct sequencing methods and thus analytical sensitivity is expected to be high. Based on reported patient data, the detection rates for pathogenic variants in the three genes in this panel are approximately as follows: In the ETFA gene, pathogenic variants were detected in 23 of 26 studied alleles, for an overall detection rate of ~88% (Schiff et al. 2006); in the ETFB gene, pathogenic variants were detected in 16 of 18 studied alleles, for an overall detection rate of ~88% (Curcoy et al. 2003; Olsen et al. 2003; Schiff et al. 2006; Yotsumoto et al. 2008); in the ETFDH gene, pathogenic variants were detected on 64 of 70 studied alleles, for an overall detection rate of ~90% (Goodman et al. 2002; Olsen et al. 2007).

To date, only a single gross deletion has been reported in the ETFDH gene (Wen et al. 2010), and no gross deletions or duplications have been reported in the ETFA or ETFB genes. It should be noted, however, that most studies of glutaric acidemia type II patients that included ETFA, ETFB and/or ETFDH sequence analysis have not been reported to have included deletion and duplication testing. Therefore, the clinical sensitivity of this test is currently difficult to estimate.

This test is performed using Next-Gen sequencing with additional Sanger sequencing as necessary.

This panel provides 100% coverage of all coding exons of the genes plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define coverage as ≥20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).