Glutaric Acidemia Type II via the ETFB Gene

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, more than 10 causative variants have been reported in the ETFB gene (Human Gene Mutation Database). The majority of reported pathogenic variants are missense variants and small deletions, although nonsense and splice variants have also been reported. The variants are spread throughout the gene.

The ETFB gene is located on chromosome 19 and contains 6 exons. ETFB encodes the β-subunit of the electron transfer flavoprotein (ETF). 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), encoded by the ETFDH gene. Clinical symptoms observed in ETFB deficient patients are due to a disruption of electron transfer to the respiratory chain (Cornelius et al. 2012; Frerman and Goodman 2014).

Glutaric acidemia II patients are split between those with causative variants in the ETFA, ETFB and ETFDH genes, and it is difficult to estimate the fraction with variants in each gene. However, to date all reported causative variants in the ETFB gene are detectable via direct sequencing, and the majority of ETFB patients reported in the literature have been found to have two causative ETFB variants. For example, collective date from four studies shows that approximately 16 causative ETFB variants were found in a total of 18 studied ETFB alleles (Curcoy et al. 2003; Olsen et al. 2003; Schiff et al. 2006; Yotsumoto et al. 2008).

To date, no large deletions or duplications have been described in the ETFB gene, although most studies of glutaric acidemia type II patients that included ETFB analysis have not been reported to have included deletion and duplication testing.

This test provides full coverage of all coding exons of the ETFB gene 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 full 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).