Functional analysis of thirty-four suspected pathogenic missense variants in ALDH5A1 gene associated with succinic semialdehyde dehydrogenase deficiency.

Deficiency of succinate semialdehyde dehydrogenase (SSADH; aldehyde dehydrogenase 5a1 (ALDH5A1), OMIM 271980, 610045), the second enzyme of GABA degradation, represents a rare autosomal-recessively inherited disorder which manifests metabolically as gamma-hydroxybutyric aciduria. The neurological phenotype includes intellectual disability, autism spectrum, epilepsy and sleep and behavior disturbances. Approximately 70 variants have been reported in the ALDH5A1 gene, half of them being missense variants. In this study, 34 missense variants, of which 22 novel, were evaluated by in silico analyses using PolyPhen2 and SIFT prediction tools. Subsequently, the effect of these variants on SSADH activity was studied by transient overexpression in HEK293 cells. These studies showed severe enzymatic activity impairment for 27 out of 34 alleles, normal activity for one allele and a broad range of residual activities (25 to 74%) for six alleles. To better evaluate the alleles that showed residual activity above 25%, we generated an SSADH-deficient HEK293-Flp-In cell line using CRISPR-Cas9, in which these alleles were stably expressed. This model proved essential in the classification as deficient for one out of the seven studied alleles. For 8 out of 34 addressed alleles, there were discrepant results among the used prediction tools, and/or in correlating the results of the prediction tools with the functional data. In case of diagnostic urgency of missense alleles, we propose the use of the transient transfection model for confirmation of their effect on the SSADH catalytic function, since this model resulted in fast and robust functional characterization for the majority of the tested variants. In selected cases, stable transfections can be considered and may prove valuable.

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.

Deficiency in SLC25A1, encoding the mitochondrial citrate carrier, causes combined D-2- and L-2-hydroxyglutaric aciduria.

The Krebs cycle is of fundamental importance for the generation of the energetic and molecular needs of both prokaryotic and eukaryotic cells. Both enantiomers of metabolite 2-hydroxyglutarate are directly linked to this pivotal biochemical pathway and are found elevated not only in several cancers, but also in different variants of the neurometabolic disease 2-hydroxyglutaric aciduria. Recently we showed that cancer-associated IDH2 germline mutations cause one variant of 2-hydroxyglutaric aciduria. Complementary to these findings, we now report recessive mutations in SLC25A1, the mitochondrial citrate carrier, in 12 out of 12 individuals with combined D-2- and L-2-hydroxyglutaric aciduria. Impaired mitochondrial citrate efflux, demonstrated by stable isotope labeling experiments and the absence of SLC25A1 in fibroblasts harboring certain mutations, suggest that SLC25A1 deficiency is pathogenic. Our results identify defects in SLC25A1 as a cause of combined D-2- and L-2-hydroxyglutaric aciduria.

RNA sequencing of creatine transporter (SLC6A8) deficient fibroblasts reveals impairment of the extracellular matrix.

Creatine transporter (SLC6A8) deficiency is the most common cause of cerebral creatine syndromes, and is characterized by depletion of creatine in the brain. Manifestations of this X-linked disorder include intellectual disability, speech/language impairment, behavior abnormalities, and seizures. At the moment, no effective treatment is available. In order to investigate the molecular pathophysiology of this disorder, we performed RNA sequencing on fibroblasts derived from patients. The transcriptomes of fibroblast cells from eight unrelated individuals with SLC6A8 deficiency and three wild-type controls were sequenced. SLC6A8 mutations with different effects on the protein product resulted in different gene expression profiles. Differential gene expression analysis followed by gene ontology term enrichment analysis revealed that especially the expression of genes encoding components of the extracellular matrix and cytoskeleton are altered in SLC6A8 deficiency, such as collagens, keratins, integrins, and cadherins. This suggests an important novel role for creatine in the structural development and maintenance of cells. It is likely that the (extracellular) structure of brain cells is also impaired in SLC6A8-deficient patients, and future studies are necessary to confirm this and to reveal the true functions of creatine in the brain.

Exome sequencing reveals mutated SLC19A3 in patients with an early-infantile, lethal encephalopathy.

To accomplish a diagnosis in patients with a rare unclassified disorder is difficult. In this study, we used magnetic resonance imaging pattern recognition analysis to identify patients with the same novel heritable disorder. Whole-exome sequencing was performed to discover the mutated gene. We identified seven patients sharing a previously undescribed magnetic resonance imaging pattern, characterized by initial swelling with T2 hyperintensity of the basal nuclei, thalami, cerebral white matter and cortex, pons and midbrain, followed by rarefaction or cystic degeneration of the white matter and, eventually, by progressive cerebral, cerebellar and brainstem atrophy. All patients developed a severe encephalopathy with rapid deterioration of neurological functions a few weeks after birth, followed by respiratory failure and death. Lactate was elevated in body fluids and on magnetic resonance spectroscopy in most patients. Whole-exome sequencing in a single patient revealed two predicted pathogenic, heterozygous missense mutations in the SLC19A3 gene, encoding the second thiamine transporter. Additional predicted pathogenic mutations and deletions were detected by Sanger sequencing in all six other patients. Pathology of brain tissue of two patients demonstrated severe cerebral atrophy and microscopic brain lesions similar to Leigh's syndrome. Although the localization of SLC19A3 expression in brain was similar in the two investigated patients compared to age-matched control subjects, the intensity of the immunoreactivity was increased. Previously published patients with SLC19A3 mutations have a milder clinical phenotype, no laboratory evidence of mitochondrial dysfunction and more limited lesions on magnetic resonance imaging. In some, cerebral atrophy has been reported. The identification of this new, severe, lethal phenotype characterized by subtotal brain degeneration broadens the phenotypic spectrum of SLC19A3 mutations. Recognition of the associated magnetic resonance imaging pattern allows a fast diagnosis in affected infants.

Non-invasive prenatal diagnosis of beta-thalassemia and sickle-cell disease using pyrophosphorolysis-activated polymerization and melting curve analysis.

OBJECTIVE: The aim of this study was to develop a pyrophosphorolysis-activated polymerization (PAP) assay for non-invasive prenatal diagnosis (NIPD) of ß-thalassemia major and sickle-cell disease (SCD). PAP is able to detect mutations in free fetal DNA in a highly contaminating environment of maternal plasma DNA. METHODS: Pyrophosphorolysis-activated polymerization primers were designed for 12 informative SNPs, genotyped by melting curve analysis (MCA) in both parents. The PAP assay was tested in a series of 13 plasma DNA samples collected from pregnant women. A retrospective NIPD was performed in a couple at risk for SCD. RESULTS: All PAP reactions were optimized and able to detect <3% target gDNA in a background of >97% wildtype gDNA. In all 13 cases, the paternal allele was detected by PAP in maternal plasma at 10 to 18 weeks of gestation. For the couple at risk, PAP showed presence of the normal paternal SNP allele in maternal plasma, which was confirmed by results of the chorionic villus sampling analysis. CONCLUSIONS: In contrast to other methods used for NIPD, the combined PAP and MCA analysis detecting the normal paternal allele is also applicable for couples at risk carrying the same mutation, provided that a previously born child is available for testing to determine the linkage to the paternal SNPs.

IDH2 mutations in patients with D-2-hydroxyglutaric aciduria.

Heterozygous somatic mutations in the genes encoding isocitrate dehydrogenase-1 and -2 (IDH1 and IDH2) were recently discovered in human neoplastic disorders. These mutations disable the enzymes' normal ability to convert isocitrate to 2-ketoglutarate (2-KG) and confer on the enzymes a new function: the ability to convert 2-KG to d-2-hydroxyglutarate (D-2-HG). We have detected heterozygous germline mutations in IDH2 that alter enzyme residue Arg(140) in 15 unrelated patients with d-2-hydroxyglutaric aciduria (D-2-HGA), a rare neurometabolic disorder characterized by supraphysiological levels of D-2-HG. These findings provide additional impetus for investigating the role of D-2-HG in the pathophysiology of metabolic disease and cancer.