Succinyl-CoA:3-oxoacid coenzyme A transferase (SCOT) deficiency: A rare and potentially fatal metabolic disease

Succinyl-CoA:3-oxoacid coenzyme A transferase deficiency (SCOTD) is a rare autosomal recessive disorder of ketone body utilization caused by mutations in OXCT1. We performed a systematic literature search and evaluated clinical, biochemical and genetic data on 34 previously published and 10 novel patients with SCOTD. Structural mapping and in silico analysis of protein variants is also presented. All patients presented with severe ketoacidotic episodes. Age at first symptoms ranged from 36 h to 3 years (median 7 months). About 70% of patients manifested in the first year of life, approximately one quarter already within the neonatal period. Two patients died, while the remainder (95%) were alive at the time of the report. Almost all the surviving patients (92%) showed normal psychomotor development and no neurologic abnormalities. A total of 29 missense mutations are reported. Analysis of the published crystal structure of the human SCOT enzyme, paired with both sequence-based and structure-based methods to predict variant pathogenicity, provides insight into the biochemical consequences of the reported variants. Pathogenic variants cluster in SCOT protein regions that affect certain structures of the protein. The described pathogenic variants can be viewed in an interactive map of the SCOT protein at https://michelanglo.sgc.ox.ac.uk/r/oxct. This comprehensive data analysis provides a systematic overview of all cases of SCOTD published to date. Although SCOTD is a rather benign disorder with often favourable outcome, metabolic crises can be life-threatening or even fatal. As the diagnosis can only be made by enzyme studies or mutation analyses, SCOTD may be underdiagnosed.

Severe retinal degeneration in a patient with Canavan disease

Background: Canavan disease is an autosomal recessive, neurodegenerative disorder caused by mutations in ASPA, a gene encoding the enzyme aspartoacylase. Patients present with macrocephaly, developmental delay, hypotonia, vision impairment and accumulation of N-acetylaspartic acid. Progressive white matter changes occur in the central nervous system. The disorder is often fatal in early childhood, but milder forms exist. Materials and methods: Case report. Results: We present the case of a 31-year-old male with mild/juvenile Canavan disease who had severe vision loss due to a retinal degeneration resembling retinitis pigmentosa. Prior to this case, vision loss in Canavan disease had been attributed to optic atrophy based on fundoscopic evidence of optic nerve pallor. Investigations for an alternative cause for our patient's retinal degeneration were non-revealing. Conclusion: We wonder if retinal degeneration may not have been previously recognized as a feature of Canavan disease. We highlight findings from animal models of Canavan disease to further support the association between Canavan disease and retinal degeneration.

Novel patient missense mutations in the HSD17B10 gene affect dehydrogenase and mitochondrial tRNA modification functions of the encoded protein

MRPP2 (also known as HSD10/SDR5C1) is a multifunctional protein that harbours both catalytic and non-catalytic functions. The protein belongs to the short-chain dehydrogenase/reductases (SDR) family and is involved in the catabolism of isoleucine in vivo and steroid metabolism in vitro. MRPP2 also moonlights in a complex with the MRPP1 (also known as TRMT10C) protein for N1-methylation of purines at position 9 of mitochondrial tRNA, and in a complex with MRPP1 and MRPP3 (also known as PRORP) proteins for 5'-end processing of mitochondrial precursor tRNA. Inherited mutations in the HSD17B10 gene encoding MRPP2 protein lead to a childhood disorder characterised by progressive neurodegeneration, cardiomyopathy or both. Here we report two patients with novel missense mutations in the HSD17B10 gene (c.34G>C and c.526G>A), resulting in the p.V12L and p.V176M substitutions. Val12 and Val176 are highly conserved residues located at different regions of the MRPP2 structure. Recombinant mutant proteins were expressed and characterised biochemically to investigate their effects towards the functions of MRPP2 and associated complexes in vitro. Both mutant proteins showed significant reduction in the dehydrogenase, methyltransferase and tRNA processing activities compared to wildtype, associated with reduced stability for protein with p.V12L, whereas the protein carrying p.V176M showed impaired kinetics and complex formation. This study therefore identified two distinctive molecular mechanisms to explain the biochemical defects for the novel missense patient mutations.

Clinical presentation and outcome in a series of 32 patients with 2-methylacetoacetyl-coenzyme A thiolase (MAT) deficiency

2-methylacetoacetyl-coenzyme A thiolase (MAT) deficiency, also known as beta-ketothiolase deficiency, is an inborn error of ketone body utilization and isoleucine catabolism. It is caused by mutations in the ACAT1 gene and may present with metabolic ketoacidosis. In order to obtain a more comprehensive view on this disease, we have collected clinical and biochemical data as well as information on ACAT1 mutations of 32 patients from 12 metabolic centers in five countries. Patients were between 23months and 27years old, more than half of them were offspring of a consanguineous union. 63% of the study participants presented with a metabolic decompensation while most others were identified via newborn screening or family studies. In symptomatic patients, age at manifestation ranged between 5months and 6.8years. Only 7% developed a major mental disability while the vast majority was cognitively normal. More than one third of the identified mutations in ACAT1 are intronic mutations which are expected to disturb splicing. We identified several novel mutations but, in agreement with previous reports, no clear genotype-phenotype correlation could be found. Our study underlines that the prognosis in MAT deficiency is good and MAT deficient individuals may remain asymptomatic, if diagnosed early and preventive measures are applied.

Heterogeneous clinical spectrum of DNAJC12-deficient hyperphenylalaninemia: from attention deficit to severe dystonia and intellectual disability

BACKGROUND

Autosomal recessive mutations in DNAJC12, encoding a cochaperone of HSP70 with hitherto unknown function, were recently described to lead to hyperphenylalaninemia, central monoamine neurotransmitter (dopamine and serotonin) deficiency, dystonia and intellectual disability in six subjects affected by homozygous variants.

OBJECTIVE

Patients exhibiting hyperphenylalaninemia in whom deficiencies in hepatic phenylalanine hydroxylase and tetrahydrobiopterin cofactor metabolism had been excluded were subsequently analysed for DNAJC12 variants.

METHODS

To analyse DNAJC12, genomic DNA from peripheral blood (Sanger sequencing), as well as quantitative messenger RNA (Real Time Quantitative Polymerase Chain Reaction (RT-qPCR)) and protein expression (Western blot) from primary skin fibroblasts were performed.

RESULTS

We describe five additional patients from three unrelated families with homozygosity/compound heterozygosity in DNAJC12 with three novel variants: c.85delC/p.Gln29Lysfs*38, c.596G>T/p.*199Leuext*42 and c.214C>T/p.(Arg72*). In contrast to previously reported DNAJC12-deficient patients, all five cases showed a very mild neurological phenotype. In two subjects, cerebrospinal fluid and primary skin fibroblasts were analysed showing similarly low 5-hydroxyindolacetic acid and homovanillic acid concentrations but more reduced expressions of mRNA and DNAJC12 compared with previously described patients. All patients responded to tetrahydrobiopterin challenge by lowering blood phenylalanine levels.

CONCLUSIONS

DNAJC12 deficiency appears to result in a more heterogeneous neurological phenotype than originally described. While early identification and institution of treatment with tetrahydrobiopterin and neurotransmitter precursors is crucial to ensure optimal neurological outcome in DNAJC12-deficient patients with a severe phenotype, optimal treatment for patients with a milder phenotype remains to be defined.

3-Hydroxy-3-methylglutaryl-coenzyme A lyase deficiency: Clinical presentation and outcome in a series of 37 patients

3-Hydroxy-3-methylglutaryl-coenzyme A lyase deficiency (HMGCLD) is a rare inborn error of ketone body synthesis and leucine degradation, caused by mutations in the HMGCL gene. In order to obtain a comprehensive view on this disease, we have collected clinical and biochemical data as well as information on HMGCL mutations of 37 patients (35 families) from metabolic centers in Belgium, Germany, The Netherlands, Switzerland, and Turkey. All patients were symptomatic at some stage with 94% presenting with an acute metabolic decompensation. In 50% of the patients, the disorder manifested neonatally, mostly within the first days of life. Only 8% of patients presented after one year of age. Six patients died prior to data collection. Long-term neurological complications were common. Half of the patients had a normal cognitive development while the remainder showed psychomotor deficits. We identified seven novel HMGCL mutations. In agreement with previous reports, no clear genotype-phenotype correlation could be found. This is the largest cohort of HMGCLD patients reported so far, demonstrating that HMGCLD is a potentially life-threatening disease with variable clinical outcome. Our findings suggest that the clinical course of HMGCLD cannot be predicted accurately from HMGCL genotype. The overall outcome in HMGCLD appears limited, thus rendering early diagnosis and strict avoidance of metabolic crises important.

Biallelic Mutations in DNAJC12 Cause Hyperphenylalaninemia, Dystonia, and Intellectual Disability

Phenylketonuria (PKU, phenylalanine hydroxylase deficiency), an inborn error of metabolism, can be detected through newborn screening for hyperphenylalaninemia (HPA). Most individuals with HPA harbor mutations in the gene encoding phenylalanine hydroxylase (PAH), and a small proportion (2%) exhibit tetrahydrobiopterin (BH4) deficiency with additional neurotransmitter (dopamine and serotonin) deficiency. Here we report six individuals from four unrelated families with HPA who exhibited progressive neurodevelopmental delay, dystonia, and a unique profile of neurotransmitter deficiencies without mutations in PAH or BH4 metabolism disorder-related genes. In these six affected individuals, whole-exome sequencing (WES) identified biallelic mutations in DNAJC12, which encodes a heat shock co-chaperone family member that interacts with phenylalanine, tyrosine, and tryptophan hydroxylases catalyzing the BH4-activated conversion of phenylalanine into tyrosine, tyrosine into L-dopa (the precursor of dopamine), and tryptophan into 5-hydroxytryptophan (the precursor of serotonin), respectively. DNAJC12 was undetectable in fibroblasts from the individuals with null mutations. PAH enzyme activity was reduced in the presence of DNAJC12 mutations. Early treatment with BH4 and/or neurotransmitter precursors had dramatic beneficial effects and resulted in the prevention of neurodevelopmental delay in the one individual treated before symptom onset. Thus, DNAJC12 deficiency is a preventable and treatable cause of intellectual disability that should be considered in the early differential diagnosis when screening results are positive for HPA. Sequencing of DNAJC12 may resolve any uncertainty and should be considered in all children with unresolved HPA.

Unravelling 5-oxoprolinuria (pyroglutamic aciduria) due to bi-allelic OPLAH mutations: 20 new mutations in 14 families

Primary 5-oxoprolinuria (pyroglutamic aciduria) is caused by a genetic defect in the γ-glutamyl cycle, affecting either glutathione synthetase or 5-oxoprolinase. While several dozens of patients with glutathione synthetase deficiency have been reported, with hemolytic anemia representing the clinical key feature, 5-oxoprolinase deficiency due to OPLAH mutations is less frequent and so far has not attracted much attention. This has prompted us to investigate the clinical phenotype as well as the underlying genotype in patients from 14 families of various ethnic backgrounds who underwent diagnostic mutation analysis following the detection of 5-oxoprolinuria. In all patients with 5-oxoprolinuria studied, bi-allelic mutations in OPLAH were indicated. An autosomal recessive mode of inheritance for 5-oxoprolinase deficiency is further supported by the identification of a single mutation in all 9/14 parent sample sets investigated (except for the father of one patient whose result suggests homozygosity), and the absence of 5-oxoprolinuria in all tested heterozygotes. It is remarkable, that all 20 mutations identified were novel and private to the respective families. Clinical features were highly variable and in several sib pairs, did not segregate with 5-oxoprolinuria. Although a pathogenic role of 5-oxoprolinase deficiency remains possible, this is not supported by our findings. Additional patient ascertainment and long-term follow-up is needed to establish the benign nature of this inborn error of metabolism. It is important that all symptomatic patients with persistently elevated levels of 5-oxoproline and no obvious explanation are investigated for the genetic etiology.

Expanding the phenotype in aminoacylase 1 (ACY1) deficiency: characterization of the molecular defect in a 63-year-old woman with generalized dystonia

Aminoacylase 1 (ACY1) deficiency is an organic aciduria due to mutations in the ACY1 gene. It is considered much underdiagnosed. Most individuals known to be affected by ACY1 deficiency have presented with neurologic symptoms. We report here a cognitively normal 63-year-old woman who around the age of 12 years had developed dystonic symptoms that gradually evolved into generalized dystonia. Extensive investigations, including metabolic diagnostics and diagnostic exome sequencing, were performed to elucidate the cause of dystonia. Findings were only compatible with a diagnosis of ACY1 deficiency: the urinary metabolite pattern with N-acetylated amino acids was characteristic, there was decreased ACY1 activity in immortalized lymphocytes, and two compound heterozygous ACY1 mutations were detected, one well-characterized c.1057C>T (p.Arg353Cys) and the other novel c.325A>G (p.Arg109Gly). Expression analysis in HEK293 cells revealed high residual activity of the enzyme with the latter mutation. However, following co-transfection of cells with stable expression of the c.1057C>T variant with either wild-type ACY1 or the c.325A>G mutant, only the wild-type enhanced ACY1 activity and ACY1 presence in the Western blot, suggesting an inhibiting interference between the two variants. Our report extends the clinical spectrum of ACY1 deficiency to include dystonia and indicates that screening for organic acidurias deserves consideration in patients with unexplained generalized dystonia.

A novel common large genomic deletion and two new missense mutations identified in the Romanian phenylketonuria population

The mutation spectrum for the phenylalanine hydroxylase (PAH) gene was investigated in a cohort of 84 hyperphenylalaninemia (HPA) patients from Romania identified through newborn screening or neurometabolic investigations. Differential diagnosis identified 81 patients with classic PAH deficiency while 3 had tetrahydropterin-cofactor deficiency and/or remained uncertain due to insufficient specimen. PAH-genetic analysis included a combination of Sanger sequencing of exons and exon–intron boundaries, MLPA and NGS with genomic DNA, and cDNA analysis from immortalized lymphoblasts. A diagnostic efficiency of 99.4% was achieved, as for one allele (out of a total of 162 alleles) no mutation could be identified. The most prevalent mutation was p.Arg408Trp which was found in ~ 38% of all PKU alleles. Three novel mutations were identified, including the two missense mutations p.Gln226Lys and p.Tyr268Cys that were both disease causing by prediction algorithms, and the large genomic deletion EX6del7831 (c.509 + 4140_706 + 510del7831) that resulted in skipping of exon 6 based on PAH-cDNA analysis in immortalized lymphocytes. The genomic deletion was present in a heterozygous state in 12 patients, i.e. in ~ 8% of all the analyzed PKU alleles, and might have originated from a Romanian founder.