D-2-Hydroxyglutaric Aciduria Type II via the IDH2 Gene - Targeted Variants Analysis

D-2-Hydroglutaric Aciduria Type II (D2HGA-II) is a rare inborn error of metabolism caused by a defect in the processing of isocitrate, ultimately resulting in hyperproduction of the metabolite D-2-hydroxyglutaric acid (D2HG). D2HGA-II occurs as a result of specific sequence variants that alter the active site of the mitochondrial isocitrate dehydrogenase-2 (IDH2) enzyme. Affected individuals have presented in infancy or early childhood, usually by 2 years of age, with developmental delay, hypotonia, seizures and dysmorphic features. Approximately half of the reported patients have also presented with cardiomyopathy. Biochemically, D2HGA-II patients are found to have markedly increased levels of D2HG in body fluids (urine, plasma and cerebral spinal fluid), typically higher than the levels observed in D2HGA-I patients (Kranendijk et al. 2010; Nota et al. 2013; Struys et al. 2016). Approximately half of the patients reported with elevated D2HG have been found to have D2HGA-I and the other half have D2HGA-II (Kranendijk et al. 2010).

Pathogenic variants in other genes may also lead to increased levels of D2HG in body fluids. These genes include D2HGDH, IDH1, SLC25A1, and possibly ALDH5A1 and ETFA, ETFB and ETFDH (Struys et al. 2006; Struys et al. 2016). Additional metabolic investigations may be able to distinguish the various causes of elevated D2HG levels.

It should be noted that pathogenic variants in the IDH2 gene have also been reported in patients with a variety of cancers (Kranendijk et al. 2010; Nota et al. 2013; Hamadou et al. 2016). It is not currently clear if D2HGA-II patients are at risk for developing malignancies in the future (Nota et al. 2013).

D2HGA-II shows an autosomal dominant mode of inheritance, and is caused by specific pathogenic variants in the IDH2 gene. To date, the only variants that have been shown to be causative for this disorder are p.Arg140Gln (c.419G>A, the most common cause) and p.Arg140Gly (c.418C>G)(Kranendijk et al. 2010; Kranendijk et al. 2011). These pathogenic variants have apparently arisen de novo in the majority of patients, though at least one patient inherited the p.Arg140Gln pathogenic variant from her unaffected mother. Her mother showed somatic mosaicism for the p.Arg140Gln pathogenic variant in her blood, and additional evidence strongly suggested the mother was also germline mosaic for the variant (Kranendijk et al. 2010; Nota et al. 2013).

The IDH2 gene encodes the mitochondrial isocitrate dehydrogenase-2 enzyme, which normally converts the metabolite isocitrate to 2-ketoglutarate (2-KG). The p.Arg140 residue sits within the active site of the IDH2 enzyme. The p.Arg140Gln or p.Arg140Gly missense substitutions alter the function of the enzyme, conferring on it the ability to convert 2-KG to D2HG. This results in hyperproduction of D2HG, which is thought to overwhelm the metabolic capacity of the D-2-hydroxyglutarate dehydrogenase (D2HGDH) enzyme and ultimately result in accumulation of vast amounts of D2HG (Kranendijk et al. 2011; Struys et al. 2016).

In a study of 17 unrelated idiopathic D-2-Hydroglutaric Aciduria patients (D2HGA-II) (those with no identified D2HGDH pathogenic variants and normal D-2-HGDH enzyme activity levels), 15 patients were found to be heterozygous for either the p.Arg140Gln (14/15) or p.Arg140Gly (1/15) substitutions, suggesting a clinical sensitivity of ~88% in idiopathic D2HGA-II patients (Kranendijk et al. 2010).

This test is specifically designed for the detection of specific heritable germline mutations and is not appropriate for the detection of somatic mutations in tumor tissue.

This test entails bidirectional Sanger sequencing of the IDH2 c.418 and c.419 nucleotides, plus 20 bp of flanking DNA on each side. At this time, sequence IDH2 variants not located at nucleotides c.418 and c.419 will not be reported. We will also sequence this region in family members of patients known or suspected to carry these variants, or to confirm research results that suggest the presence of one of the two targeted variants.