Cerebral White Matter Alterations Associated With Oligodendrocyte Vulnerability in Organic Acidurias: Insights in Glutaric Aciduria Type I.

The white matter is an important constituent of the central nervous system, containing axons, oligodendrocytes, and its progenitor cells, astrocytes, and microglial cells. Oligodendrocytes are central for myelin synthesis, the insulating envelope that protects axons and allows normal neural conduction. Both, oligodendrocytes and myelin, are highly vulnerable to toxic factors in many neurodevelopmental and neurodegenerative disorders associated with disturbances of myelination. Here we review the main alterations in oligodendrocytes and myelin observed in some organic acidurias/acidemias, which correspond to inherited neurometabolic disorders biochemically characterized by accumulation of potentially neurotoxic organic acids and their derivatives. The yet incompletely understood mechanisms underlying the high vulnerability of OLs and/or myelin in glutaric acidemia type I, the most prototypical cerebral organic aciduria, are particularly discussed.

Clinical outcomes of patients with mut-type methylmalonic acidemia identified through expanded newborn screening in China.

BACKGROUND: Isolated methylmalonic acidemia, an autosomal recessive disorder of propionate metabolism, is usually caused by mutations in the methylmalonyl-CoA mutase gene (mut-type). Because no universal consensus was made on whether mut-type methylmalonic acidemia should be included in newborn screening (NBS), we aimed to compare the outcome of this disorder detected by NBS with that detected clinically and investigate the influence of NBS on the disease course. DESIGN & METHODS: In this study, 168 patients with mut-type methylmalonic acidemia diagnosed by NBS were compared to 210 patients diagnosed after disease onset while NBS was not performed. Clinical data of these patients from 7 metabolic centers in China were analyzed retrospectively, including initial manifestations, biochemical metabolites, the responsiveness of vitamin B12 therapy, and gene variation, to explore different factors on the long-term outcome. RESULTS: By comparison of the clinically-diagnosed patients, NBS-detected patients showed younger age at diagnosis, less incidence of disease onset, better responsiveness of vitamin B12, younger age at start of treatment, lower levels of biochemical features before and after treatment, and better long-term prognosis (P < 0.01). Onset of disease, blood C3/C2 ratio and unresponsiveness of vitamin B12 were more positively associated with poor outcomes of patients whether identified by NBS. Moreover, the factors above as well as older age at start of treatment were positively associated with mortality. CONCLUSIONS: This research highly demonstrated NBS could prevent major disease-related events and allow an earlier treatment initiation. As a key prognostic factor, NBS is beneficial for improving the overall survival of infants with mut-type methylmalonic acidemia.

Generation of hiPSC lines from four glutaric aciduria type I (GA1) patients carrying pathogenic biallelic variants in GCDH.

GCDH encodes for the enzyme catalyzing the sixth step of the lysine degradation pathway. Autosomal recessive variants in GCDH are associated with glutaric aciduria type I (GA1), of which a wide genotypic spectrum of pathogenic variants have been described. In this study, hiPSC lines derived from four GA1 patients with different genotypes were generated and fully characterized. Two patients carry compound heterozygous variants in GCDH, while the other two patients carry a variant in homozygosis. These hiPSC lines can significantly contribute to better understand the molecular mechanism underlying GA1 and provide excellent models for the development of new therapeutic strategies.

Deficient brain GABA metabolism leads to widespread impairments of astrocyte and oligodendrocyte function.

The neurometabolic disorder succinic semialdehyde dehydrogenase (SSADH) deficiency leads to great neurochemical imbalances and severe neurological manifestations. The cause of the disease is loss of function of the enzyme SSADH, leading to impaired metabolism of the principal inhibitory neurotransmitter GABA. Despite the known identity of the enzymatic deficit, the underlying pathology of SSADH deficiency remains unclear. To uncover new mechanisms of the disease, we performed an untargeted integrative analysis of cerebral protein expression, functional metabolism, and lipid composition in a genetic mouse model of SSADH deficiency (ALDH5A1 knockout mice). Our proteomic analysis revealed a clear regional vulnerability, as protein alterations primarily manifested in the hippocampus and cerebral cortex of the ALDH5A1 knockout mice. These regions displayed aberrant expression of proteins linked to amino acid homeostasis, mitochondria, glial function, and myelination. Stable isotope tracing in acutely isolated brain slices demonstrated an overall maintained oxidative metabolism of glucose, but a selective decrease in astrocyte metabolic activity in the cerebral cortex of ALDH5A1 knockout mice. In contrast, an elevated capacity of oxidative glutamine metabolism was observed in the ALDH5A1 knockout brain, which may serve as a neuronal compensation of impaired astrocyte glutamine provision. In addition to reduced expression of critical oligodendrocyte proteins, a severe depletion of myelin-enriched sphingolipids was found in the brains of ALDH5A1 knockout mice, suggesting degeneration of myelin. Altogether, our study highlights that impaired astrocyte and oligodendrocyte function is intimately linked to SSADH deficiency pathology, suggesting that selective targeting of glial cells may hold therapeutic potential in this disease.

Functional Characterization of a Spectrum of Genetic Variants in a Family with Succinic Semialdehyde Dehydrogenase Deficiency.

Succinic semialdehyde dehydrogenase (SSADH) is a mitochondrial enzyme involved in the catabolism of the neurotransmitter γ-amino butyric acid. Pathogenic variants in the gene encoding this enzyme cause SSADH deficiency, a developmental disease that manifests as hypotonia, autism, and epilepsy. SSADH deficiency patients usually have family-specific gene variants. Here, we describe a family exhibiting four different SSADH variants: Val90Ala, Cys93Phe, and His180Tyr/Asn255Asp (a double variant). We provide a structural and functional characterization of these variants and show that Cys93Phe and Asn255Asp are pathogenic variants that affect the stability of the SSADH protein. Due to the impairment of the cofactor NAD+ binding, these variants show a highly reduced enzyme activity. However, Val90Ala and His180Tyr exhibit normal activity and expression. The His180Tyr/Asn255Asp variant exhibits a highly reduced activity as a recombinant species, is inactive, and shows a very low expression in eukaryotic cells. A treatment with substances that support protein folding by either increasing chaperone protein expression or by chemical means did not increase the expression of the pathogenic variants of the SSADH deficiency patient. However, stabilization of the folding of pathogenic SSADH variants by other substances may provide a treatment option for this disease.

Generation and characterization of six human induced pluripotent stem cell lines (hiPSCs) from three individuals with SSADH Deficiency and CRISPR-corrected isogenic controls.

Succinic Semialdehyde Dehydrogenase Deficiency (SSADHD) is an ultra-rare autosomal recessive neurometabolic disorder caused by ALDH5A1 mutations presenting with autism and epilepsy. Here, we report the generation and characterization of human induced pluripotent stem cells (hiPSCs) derived from fibroblasts of three unrelated SSADHD patients - one female and two males with the CRISPR-corrected isogenic controls. These individuals are clinically diagnosed and are being followed in a longitudinal clinical study.

Phenotypic correlates of structural and functional protein impairments resultant from ALDH5A1 variants.

To investigate the genotype-to-protein-to-phenotype correlations of succinic semialdehyde dehydrogenase deficiency (SSADHD), an inherited metabolic disorder of γ-aminobutyric acid catabolism. Bioinformatics and in silico mutagenesis analyses of ALDH5A1 variants were performed to evaluate their impact on protein stability, active site and co-factor binding domains, splicing, and homotetramer formation. Protein abnormalities were then correlated with a validated disease-specific clinical severity score and neurological, neuropsychological, biochemical, neuroimaging, and neurophysiological metrics. A total of 58 individuals (1:1 male/female ratio) were affected by 32 ALDH5A1 pathogenic variants, eight of which were novel. Compared to individuals with single homotetrameric or multiple homo and heterotetrameric proteins, those predicted not to synthesize any functional enzyme protein had significantly lower expression of ALDH5A1 (p = 0.001), worse overall clinical outcomes (p = 0.008) and specifically more severe cognitive deficits (p = 0.01), epilepsy (p = 0.04) and psychiatric morbidity (p = 0.04). Compared to individuals with predictions of having no protein or a protein impaired in catalytic functions, subjects whose proteins were predicted to be impaired in stability, folding, or oligomerization had a better overall clinical outcome (p = 0.02) and adaptive skills (p = 0.04). The quantity and type of enzyme proteins (no protein, single homotetramers, or multiple homo and heterotetramers), as well as their structural and functional impairments (catalytic or stability, folding, or oligomerization), contribute to phenotype severity in SSADHD. These findings are valuable for assessment of disease prognosis and management, including patient selection for gene replacement therapy. Furthermore, they provide a roadmap to determine genotype-to-protein-to-phenotype relationships in other autosomal recessive disorders.

Human aromatic amino acid decarboxylase is an asymmetric and flexible enzyme: Implication in aromatic amino acid decarboxylase deficiency.

Human aromatic amino acid decarboxylase (AADC) is a pyridoxal 5'-phosphate-dependent enzyme responsible for the biosynthesis of dopamine and serotonin, essential neurotransmitters involved in motor and cognitive abilities. Mutations in its gene lead to AADC deficiency, a monogenic rare neurometabolic childhood parkinsonism characterized by severe motor and neurodevelopmental symptoms. Here, for the first time, we solved the crystal structure of human holoAADC in the internal aldimine (1.9 Å) and in the external aldimine (2.4 Å) of the substrate analog L-Dopa methylester. In this intermediate, the highly flexible AADC catalytic loop (CL) is captured in a closed state contacting all protein domains. In addition, each active site, composed by residues of both subunits, is connected to the other through weak interactions and a central cavity. By combining crystallographic analyses with all-atom and coarse-grained molecular dynamics simulations, SAXS investigations and limited proteolysis experiments, we realized that the functionally obligate homodimeric AADC enzyme in solution is an elongated, asymmetric molecule, where the fluctuations of the CL are coupled to flexibility at the edge between the N-terminal and C-terminal domains. The structural integrity of this peripheral protein region is essential to catalysis, as assessed by both artificial and 37 AADC deficiency pathogenic variants leading to the interpretation that structural dynamics in protein regions far from the active site is essential for CL flexibility and the acquirement of a correct catalytically competent structure. This could represent the molecular basis for pathogenicity prediction in AADC deficiency.

Identifying and predicting the pathogenic effects of a novel variant inducing severe early onset MMA: a bioinformatics approach.

BACKGROUND: Methylmalonic acidemia (MMA) is a rare metabolic disorder resulting from functional defects in methylmalonyl-CoA mutase. Mutations in the MMAB gene are responsible for the cblB type of vitamin B12-responsive MMA. RESULTS: This study used Whole-exome sequencing (WES), Sanger sequencing, linkage analysis, and in-silico evaluation of the variants' effect on protein structure and function to confirm their pathogenicity in a 2-day-old neonate presenting an early-onset metabolic crisis and death. WES revealed a homozygous missense variant on chromosome 12, the NM_052845.4 (MMAB):c.557G > A, p.Arg186Gln, in exon 7, a highly conserved and hot spot region for pathogenic variants. After being confirmed by Sanger sequencing, the wild-type and mutant proteins' structure and function were modeled and examined using in-silico bioinformatics tools and compared to the variant NM_052845.4 (MMAB):c.556C > T, p.Arg186Trp, a known pathogenic variant at the same position. Comprehensive bioinformatics analysis showed a significant reduction in the stability of variants and changes in protein-protein and ligand-protein interactions. Interestingly, the variant c.557G > A, p.Arg186Gln depicted more variations in the secondary structure and less binding to the ATP and B12 ligands compared to the c.556C > T, p.Arg186Trp, the known pathogenic variant. CONCLUSION: This study succeeded in expanding the variant spectra of the MMAB, forasmuch as the variant c.557G > A, p.Arg186Gln is suggested as a pathogenic variant and the cause of severe MMA and neonatal death. These results benefit the prenatal diagnosis of MMA in the subsequent pregnancies and carrier screening of the family members. Furthermore, as an auxiliary technique, homology modeling and protein structure and function evaluations could provide geneticists with a more accurate interpretation of variants' pathogenicity.

Familial hemophagocytic lymphohistiocytosis syndrome due to lysinuric protein intolerance: a patient with a novel compound heterozygous pathogenic variant in SLC7A7.

Lysinuric protein intolerance (LPI) (MIM#222700) is a rare autosomal recessive defect in bibasic amino acid transport caused by pathogenic variants in solute carrier family 7 member 7 gene ( SLC7A7). The symptoms begin after weaning from breast milk and include refusal of feeding, vomiting, and consequent failure to thrive. Some metabolic disorders, including LPI, are complicated by hemophagocytic lymphohistiocytosis (HLH); however, the frequency of HLH caused by inborn errors of metabolism is very rare in the HLH cohort. SLC7A7 consists of 11 exons, and has 66 known pathogenic variants. SLC7A7 is associated with HLH. Here, we report the case of a 32-year-old woman who presented with LPI and HLH. Genetic analysis revealed a novel compound heterozygosity in SLC7A7 with two pathogenic variants, c.713C>T (p. Sre238Phe) and c.625+1G>A (splicing acceptor site) inherited from her father and mother, respectively.

Anesthetic Management of a Patient With S-Adenosylhomocysteine Hydrolase Deficiency: A Case Report.

S-adenosylhomocysteine hydrolase (AHCY) deficiency is a rare congenital disorder in methionine metabolism with minimal guidelines regarding anesthetic management. This case report describes a 19-year-old man presenting for a liver biopsy in interventional radiology due to a history of elevated aminotransferases and creatine kinase. He received dextrose-containing fluids and a total intravenous anesthetic to avoid rhabdomyolysis and hyperkalemia. Anesthetic goals for patients with AHCY deficiency should focus on avoiding rhabdomyolysis, minimizing postoperative ventilatory compromise, monitoring for potential coagulopathy, and providing anxiolysis.

Mutations in Hcfc1 and Ronin result in an inborn error of cobalamin metabolism and ribosomopathy.

Combined methylmalonic acidemia and homocystinuria (cblC) is the most common inborn error of intracellular cobalamin metabolism and due to mutations in Methylmalonic Aciduria type C and Homocystinuria (MMACHC). Recently, mutations in the transcriptional regulators HCFC1 and RONIN (THAP11) were shown to result in cellular phenocopies of cblC. Since HCFC1/RONIN jointly regulate MMACHC, patients with mutations in these factors suffer from reduced MMACHC expression and exhibit a cblC-like disease. However, additional de-regulated genes and the resulting pathophysiology is unknown. Therefore, we have generated mouse models of this disease. In addition to exhibiting loss of Mmachc, metabolic perturbations, and developmental defects previously observed in cblC, we uncovered reduced expression of target genes that encode ribosome protein subunits. We also identified specific phenotypes that we ascribe to deregulation of ribosome biogenesis impacting normal translation during development. These findings identify HCFC1/RONIN as transcriptional regulators of ribosome biogenesis during development and their mutation results in complex syndromes exhibiting aspects of both cblC and ribosomopathies.

Spectrum and characterization of bi-allelic variants in MMAB causing cblB-type methylmalonic aciduria.

Pathogenic variants in MMAB cause cblB-type methylmalonic aciduria, an autosomal-recessive disorder of propionate metabolism. MMAB encodes ATP:cobalamin adenosyltransferase, using ATP and cob(I)alamin to create 5'-deoxyadenosylcobalamin (AdoCbl), the cofactor of methylmalonyl-CoA mutase (MMUT). We identified bi-allelic disease-causing variants in MMAB in 97 individuals with cblB-type methylmalonic aciduria, including 33 different and 16 novel variants. Missense changes accounted for the most frequent pathogenic alleles (p.(Arg186Trp), N = 57; p.(Arg191Trp), N = 19); while c.700C > T (p.(Arg234*)) was the most frequently identified truncating variant (N = 14). In fibroblasts from 76 affected individuals, the ratio of propionate incorporation in the presence and absence of hydroxocobalamin (PI ratio) was associated to clinical cobalamin responsiveness and later disease onset. We found p.(Arg234*) to be associated with cobalamin responsiveness in vitro, and clinically with later onset; p.(Arg186Trp) and p.(Arg191Trp) showed no clear cobalamin responsiveness and early onset. Mapping these and novel variants onto the MMAB structure revealed their potential to affect ATP and AdoCbl binding. Follow-up biochemical characterization of recombinant MMAB identified its three active sites to be equivalent for ATP binding, determined by fluorescence spectroscopy (Kd = 21 µM) and isothermal calorimetry (Kd = 14 µM), but function as two non-equivalent AdoCbl binding sites (Kd1 = 0.55 µM; Kd2 = 8.4 µM). Ejection of AdoCbl was activated by ATP (Ka = 24 µM), which was sensitized by the presence of MMUT (Ka = 13 µM). This study expands the landscape of pathogenic MMAB variants, provides association of in vitro and clinical responsiveness, and facilitates insight into MMAB function, enabling better disease understanding.

Assessing Prevalence and Carrier Frequency of Succinic Semialdehyde Dehydrogenase Deficiency.

Pathogenic variants in ALDH5A1 cause succinic semialdehyde dehydrogenase (SSADH) deficiency, with >180 cases reported worldwide. However, a nonspecific neurologic presentation and inconsistent variant nomenclature have limited diagnoses. In this study, pathogenic variants in ALDH5A1 were curated and variant prevalence assessed in the Genome Aggregation Database (gnomAD) to determine a minimum carrier frequency and to estimate disease prevalence. Stringent population variant analysis, including 98 reported disease-associated ALDH5A1 variants, indicates a pan-ethnic carrier frequency of ∼1/340, supporting a prevalence of SSADH deficiency of ∼1/460 000 worldwide, with highest carrier frequencies observed in East Asian and South Asian populations. Because heterozygous loss of function alleles are rare in gnomAD and >60% of reported disease-causing variants were missense changes that were not present in gnomAD, the pan-ethnic carrier frequency for SSADH deficiency is likely not fully represented in this study. Additional analyses to investigate the potential impact of more common ALDH5A1 variants with reduced but not deficient enzyme activity, including analysis in diverse populations, are needed to fully assess the prevalence of this ultra-rare disease.

Mitochondrial damage in renal epithelial cells is potentiated by protein exposure in propionic aciduria.

Propionic aciduria (PA) is caused by deficiency of the mitochondrial enzyme propionyl-CoA carboxylase (PCC). Due to inefficient propionate catabolism patients are endangered by life-threatening ketoacidotic crisis. Protein and amino acid restriction are major therapeutic pillars. However, long-term complications like neurological deterioration and cardiac abnormalities cannot be prevented. Chronic kidney disease (CKD), which is a well-known characteristic of methylmalonic aciduria two enzymatic steps downstream from PCC, has been recognized as a novel late-onset complication in PA. The pathophysiology of CKD in PA is unclear. We investigated mitochondrial structure and metabolism in human renal tubular cells of healthy controls and PA patients. The cells were exposed to either standard cell culture conditions (NT), high protein (HP) or high concentrations of isoleucine and valine (I/V). Mitochondrial morphology changed to condensed, fractured morphology in PA cells irrespective of the cell culture medium. HP and I/V exposure, however, potentiated oxidative stress in PA cells. Mitochondrial mass was enriched in PA cells, and further increased by HP and I/V exposure suggesting a need for compensation. Alterations in the tricarboxylic acid cycle intermediates and accumulation of medium- and long-chain acylcarnitines pointed to altered mitochondrial energy metabolism. Mitophagy was silenced while autophagy as cellular defense mechanisms was highly active in PA cells. The data demonstrate that PA is associated with renal mitochondrial damage which is aggravated by protein and I/V load. Preservation of mitochondrial energy homeostasis in renal cells may be a potential future therapeutic target.

Postmortem whole-genome sequencing on a dried blood spot identifies a novel homozygous SUOX variant causing isolated sulfite oxidase deficiency.

Rapid whole-genome sequencing (rWGS) has shown that genetic diseases are a common cause of infant mortality in neonatal intensive care units. Dried blood spots collected for newborn screening allow investigation of causes of infant mortality that were not diagnosed during life. Here, we present a neonate who developed seizures and encephalopathy on the third day of life that was refractory to antiepileptic medications. The patient died on day of life 16 after progressive respiratory failure and sepsis. The parents had lost two prior children after similar presentations, neither of whom had a definitive diagnosis. Postmortem rWGS of a dried blood spot identified a pathogenic homozygous frameshift variant in the SUOX gene associated with isolated sulfite oxidase deficiency (c.1390_1391del, p.Leu464GlyfsTer10). This case highlights that early, accurate molecular diagnosis has the potential to influence prenatal counseling and guide management in rare, genetic disorders and has added importance in cases of a strong family history and risk factors such as consanguinity.

Epimutation of MMACHC compound to a genetic mutation in cblC cases.

BACKGROUND: Methylmalonic aciduria (MMA) combined with homocystinuria, cobalamin(cbl)C deficiency type (OMIM 277400), is the most common autosomal recessive inherited disorder of intracellular cobalamin metabolism caused by mutations in the MMACHC gene (OMIM 609831), of which more than 100 mutations have been identified to date. In this study, we only identified a coding mutation in one allele at the MMACHC gene locus, and no large fragments deletion or duplication were found. Up to now, only three epimutation cblC cases were reported. We hypothesized whether the MMACHC was hypermethylated. METHODS: To address this hypothesis, the entire coding region and adjacent splice sites of the panel genes involved in metabolic diseases were sequenced using the Illumina HiSeq X platform, followed by confirmation via Sanger sequencing in their parents and brothers. Methylation analysis of the MMACHC was performed using an EpiTect Bisulfite Kit and methylation-specific PCR (MSP) to investigate the role of epimutations in cblC disease. RESULTS: We identified a clearly pathogenic single heterozygous c.658_660del, p. (K220del) mutation, which was also identified in the mother. Analysis of the MMACHC indicated a heterozygous epimutation consisting of 34 hypermethylated CpG sites in a CpG island encompassing the promoter and first exon of the MMACHC, which was also identified in the father. Furthermore, we identified a single heterozygous c.*2C>T mutation in the sixth exon of the PRDX1 (OMIM 176763) in patients and their fathers, which was the only sequence variation that segregated with the MMACHC methylation. Neither c.658_660del and epimutation in MMACHC nor c.*2C>T in PRDX1 was discovered in her brother. CONCLUSION: We report compound heterozygotes in MMACHC for a genetic mutation and an epimutation in cblC cases. To our best knowledge, this is the first report of two cblC cases from China caused by compound heterozygous mutations with a coding mutation in one allele and an epimutation in the other at the MMACHC locus.

The first knock-in rat model for glutaric aciduria type I allows further insights into pathophysiology in brain and periphery.

Glutaric aciduria type I (GA-I, OMIM # 231670) is an inborn error of metabolism caused by a deficiency of glutaryl-CoA dehydrogenase (GCDH). Patients develop acute encephalopathic crises (AEC) with striatal injury most often triggered by catabolic stress. The pathophysiology of GA-I, particularly in brain, is still not fully understood. We generated the first knock-in rat model for GA-I by introduction of the mutation p.R411W, the rat sequence homologue of the most common Caucasian mutation p.R402W, into the Gcdh gene of Sprague Dawley rats by CRISPR/CAS9 technology. Homozygous Gcdhki/ki rats revealed a high excretor phenotype, but did not present any signs of AEC under normal diet (ND). Exposure to a high lysine diet (HLD, 4.7%) after weaning resulted in clinical and biochemical signs of AEC. A significant increase of plasmatic ammonium concentrations was found in Gcdhki/ki rats under HLD, accompanied by a decrease of urea concentrations and a concomitant increase of arginine excretion. This might indicate an inhibition of the urea cycle. Gcdhki/ki rats exposed to HLD showed highly diminished food intake resulting in severely decreased weight gain and moderate reduction of body mass index (BMI). This constellation suggests a loss of appetite. Under HLD, pipecolic acid increased significantly in cerebral and extra-cerebral liquids and tissues of Gcdhki/ki rats, but not in WT rats. It seems that Gcdhki/ki rats under HLD activate the pipecolate pathway for lysine degradation. Gcdhki/ki rat brains revealed depletion of free carnitine, microglial activation, astroglyosis, astrocytic death by apoptosis, increased vacuole numbers, impaired OXPHOS activities and neuronal damage. Under HLD, Gcdhki/ki rats showed imbalance of intra- and extracellular creatine concentrations and indirect signs of an intracerebral ammonium accumulation. We successfully created the first rat model for GA-I. Characterization of this Gcdhki/ki strain confirmed that it is a suitable model not only for the study of pathophysiological processes, but also for the development of new therapeutic interventions. We further brought up interesting new insights into the pathophysiology of GA-I in brain and periphery.

Identification of 2,2-Dimethylbutanoic Acid (HST5040), a Clinical Development Candidate for the Treatment of Propionic Acidemia and Methylmalonic Acidemia.

Propionic acidemia (PA) and methylmalonic acidemia (MMA) are rare autosomal recessive disorders of propionyl-CoA (P-CoA) catabolism, caused by a deficiency in the enzymes P-CoA carboxylase and methylmalonyl-CoA (M-CoA) mutase, respectively. PA and MMA are classified as intoxication-type inborn errors of metabolism because the intramitochondrial accumulation of P-CoA, M-CoA, and other metabolites results in secondary inhibition of multiple pathways of intermediary metabolism, leading to organ dysfunction and failure. Herein, we describe the structure-activity relationships of a series of short-chain carboxylic acids which reduce disease-related metabolites in PA and MMA primary hepatocyte disease models. These studies culminated in the identification of 2,2-dimethylbutanoic acid (10, HST5040) as a clinical candidate for the treatment of PA and MMA. Additionally, we describe the in vitro and in vivo absorption, distribution, metabolism, and excretion profile of HST5040, data from preclinical studies, and the synthesis of the sodium salt of HST5040 for clinical trials.

Cobalamin J disease detected on newborn screening: Novel variant and normal neurodevelopmental course.

Cobalamin J disease (CblJ) is an ultra-rare autosomal recessive disorder of intracellular cobalamin metabolism associated with combined methylmalonic acidemia and homocystinuria. It is caused by pathogenic variants in ABCD4, which encodes an ATP-binding cassette (ABC) transporter that affects the lysosomal release of cobalamin (Cbl) into the cytoplasm. Only six cases of CblJ have been reported in the literature. Described clinical features include feeding difficulties, failure to thrive, hypotonia, seizures, developmental delay, and hematological abnormalities. Information on clinical outcomes is extremely limited, and no cases of presymptomatic diagnosis have been reported. We describe a now 17-month-old male with CblJ detected by newborn screening and confirmed by biochemical, molecular, and complementation studies. With early detection and initiation of treatment, this patient has remained asymptomatic with normal growth parameters and neurodevelopmental function. To the best of our knowledge, this report represents the first asymptomatic and neurotypical patient with CblJ.