D-2-hydroxyglutaric aciduria Type I: Functional analysis of D2HGDH missense variants.

D-2-hydroxyglutaric aciduria Type I (D-2-HGA Type I), a neurometabolic disorder with a broad clinical spectrum, is caused by recessive variants in the D2HGDH gene encoding D-2-hydroxyglutarate dehydrogenase (D-2-HGDH). We and others detected 42 potentially pathogenic variants in D2HGDH of which 31 were missense. We developed functional studies to investigate the effect of missense variants on D-2-HGDH catalytic activity. Site-directed mutagenesis was used to introduce 31 missense variants in the pCMV5-D2HGDH expression vector. The wild type and missense variants were overexpressed in HEK293 cells. D-2-HGDH enzyme activity was evaluated based on the conversion of [2 H4 ]D-2-HG to [2 H4 ]2-ketoglutarate, which was subsequently converted into [2 H4 ]L-glutamate and the latter quantified by LC-MS/MS. Eighteen variants resulted in almost complete ablation of D-2-HGDH activity and thus, should be considered pathogenic. The remaining 13 variants manifested residual activities ranging between 17% and 94% of control enzymatic activity. Our functional assay evaluating the effect of novel D2HGDH variants will be beneficial for the classification of missense variants and determination of pathogenicity.

An overview of combined D-2- and L-2-hydroxyglutaric aciduria: functional analysis of CIC variants.

Combined D-2- and L-2-hydroxyglutaric aciduria (D/L-2-HGA) is a devastating neurometabolic disorder, usually lethal in the first years of life. Autosomal recessive mutations in the SLC25A1 gene, which encodes the mitochondrial citrate carrier (CIC), were previously detected in patients affected with combined D/L-2-HGA. We showed that transfection of deficient fibroblasts with wild-type SLC25A1 restored citrate efflux and decreased intracellular 2-hydroxyglutarate levels, confirming that deficient CIC is the cause of D/L-2-HGA. We developed and implemented a functional assay and applied it to all 17 missense variants detected in a total of 26 CIC-deficient patients, including eight novel cases, showing reduced activities of varying degrees. In addition, we analyzed the importance of residues affected by these missense variants using our existing scoring system. This allowed not only a clinical and biochemical overview of the D/L-2-HGA patients but also phenotype-genotype correlation studies.

Clinically Distinct Phenotypes of Canavan Disease Correlate with Residual Aspartoacylase Enzyme Activity.

We describe 14 patients with 12 novel missense mutations in ASPA, the gene causing Canavan disease (CD). We developed a method to study the effect of these 12 variants on the function of aspartoacylase-the hydrolysis of N-acetyl-l-aspartic acid (NAA) to aspartate and acetate. The wild-type ASPA open reading frame (ORF) and the ORFs containing each of the variants were transfected into HEK293 cells. Enzyme activity was determined by incubating cell lysates with NAA and measuring the released aspartic acid by LC-MS/MS. Clinical data were obtained for 11 patients by means of questionnaires. Four patients presented with a non-typical clinical picture or with the milder form of CD, whereas seven presented with severe CD. The mutations found in the mild patients corresponded to the variants with the highest residual enzyme activities, suggesting that this assay can help evaluate unknown variants found in patients with atypical presentation. We have detected a correlation between clinical presentation, enzyme activity, and genotype for CD.