Vision on gyrate atrophy: why treat the eye?

In the April issue of this Journal, Boffa and coworkers put forward a new therapeutic approach for Gyrate Atrophy of the Choroid and Retina (GACR; OMIM 258870) (Boffa et al, 2023). The authors propose to apply gene therapy to the liver for GACR, a metabolic disease primarily affecting eyesight due to retinal degeneration. Their vision is enthusiastically supported by a News and Views comment in the same issue (Seker Yilmaz and Gissen, 2023). However, based on disease pathology, patient's needs, ethical considerations, therapeutic developmental time lines, and current state of the art of gene therapy for liver and eye, we have a different view on this issue: We argue below that local treatment of the eye is the preferred option for GACR.

Untargeted metabolomics and infrared ion spectroscopy identify biomarkers for pyridoxine-dependent epilepsy.

BackgroundPyridoxine-dependent epilepsy (PDE-ALDH7A1) is an inborn error of lysine catabolism that presents with refractory epilepsy in newborns. Biallelic ALDH7A1 variants lead to deficiency of α-aminoadipic semialdehyde dehydrogenase/antiquitin, resulting in accumulation of piperideine-6-carboxylate (P6C), and secondary deficiency of the important cofactor pyridoxal-5'-phosphate (PLP, active vitamin B6) through its complexation with P6C. Vitamin B6 supplementation resolves epilepsy in patients, but intellectual disability may still develop. Early diagnosis and treatment, preferably based on newborn screening, could optimize long-term clinical outcome. However, no suitable PDE-ALDH7A1 newborn screening biomarkers are currently available.MethodsWe combined the innovative analytical methods untargeted metabolomics and infrared ion spectroscopy to discover and identify biomarkers in plasma that would allow for PDE-ALDH7A1 diagnosis in newborn screening.ResultsWe identified 2S,6S-/2S,6R-oxopropylpiperidine-2-carboxylic acid (2-OPP) as a PDE-ALDH7A1 biomarker, and confirmed 6-oxopiperidine-2-carboxylic acid (6-oxoPIP) as a biomarker. The suitability of 2-OPP as a potential PDE-ALDH7A1 newborn screening biomarker in dried bloodspots was shown. Additionally, we found that 2-OPP accumulates in brain tissue of patients and Aldh7a1-knockout mice, and induced epilepsy-like behavior in a zebrafish model system.ConclusionThis study has opened the way to newborn screening for PDE-ALDH7A1. We speculate that 2-OPP may contribute to ongoing neurotoxicity, also in treated PDE-ALDH7A1 patients. As 2-OPP formation appears to increase upon ketosis, we emphasize the importance of avoiding catabolism in PDE-ALDH7A1 patients.FundingSociety for Inborn Errors of Metabolism for Netherlands and Belgium (ESN), United for Metabolic Diseases (UMD), Stofwisselkracht, Radboud University, Canadian Institutes of Health Research, Dutch Research Council (NWO), and the European Research Council (ERC).

Sialic acid catabolism by N-acetylneuraminate pyruvate lyase is essential for muscle function.

Sialic acids are important components of glycoproteins and glycolipids essential for cellular communication, infection, and metastasis. The importance of sialic acid biosynthesis in human physiology is well illustrated by the severe metabolic disorders in this pathway. However, the biological role of sialic acid catabolism in humans remains unclear. Here, we present evidence that sialic acid catabolism is important for heart and skeletal muscle function and development in humans and zebrafish. In two siblings, presenting with sialuria, exercise intolerance/muscle wasting, and cardiac symptoms in the brother, compound heterozygous mutations [chr1:182775324C>T (c.187C>T; p.Arg63Cys) and chr1:182772897A>G (c.133A>G; p.Asn45Asp)] were found in the N-acetylneuraminate pyruvate lyase gene (NPL). In vitro, NPL activity and sialic acid catabolism were affected, with a cell-type-specific reduction of N-acetyl mannosamine (ManNAc). A knockdown of NPL in zebrafish resulted in severe skeletal myopathy and cardiac edema, mimicking the human phenotype. The phenotype was rescued by expression of wild-type human NPL but not by the p.Arg63Cys or p.Asn45Asp mutants. Importantly, the myopathy phenotype in zebrafish embryos was rescued by treatment with the catabolic products of NPL: N-acetyl glucosamine (GlcNAc) and ManNAc; the latter also rescuing the cardiac phenotype. In conclusion, we provide the first report to our knowledge of a human defect in sialic acid catabolism, which implicates an important role of the sialic acid catabolic pathway in mammalian muscle physiology, and suggests opportunities for monosaccharide replacement therapy in human patients.

De novo dominant variants affecting the motor domain of KIF1A are a cause of PEHO syndrome.

PEHO syndrome (OMIM no. 260565) is characterized by myoclonic jerking and infantile spasms, profound psychomotor retardation with the absence of motor milestones and speech, absence or early loss of visual fixation with atrophy of optic discs by 2 years of age and progressive brain atrophy on neuroimaging. We describe the results of a genomic study of a girl with PEHO syndrome and review the literature on cases with a disease-causing variant in the same gene. Exome sequencing of the index and unaffected parents followed by Sanger confirmation identified nine candidate genes harboring nonsynonymous rare variants identified by trio whole-exome sequencing. The de novo variant, a missense variant (c.296C>T, p.(T99M)), affecting the motor domain of KIF1A was considered the pathogenic mutation. The literature review revealed 24 cases with disease-causing variants in the motor domain of KIF1A, of which three met all the criteria for PEHO syndrome and an additional patient with incomplete clinical data met four of the five criteria. If the criteria were modified to include cases with any convulsive disorder and less profound intellectual disability, a total of six patients met all five of the criteria, three patients met four of the criteria and six met three of the criteria. Our results indicate that the molecular basis for PEHO syndrome, in at least a subset of patients, is a dominant KIF1A variant affecting the motor domain of the protein. Variable expressivity is seen with recurrent variants causing the full phenotype of PEHO syndrome in some patients and in other patients, a partial or milder PEHO phenotype.

RMND1 deficiency associated with neonatal lactic acidosis, infantile onset renal failure, deafness, and multiorgan involvement.

RMND1 is an integral inner membrane mitochondrial protein that assembles into a large 240 kDa complex to support translation of the 13 polypeptides encoded on mtDNA, all of which are essential subunits of the oxidative phosphorylation (OXPHOS) complexes. Variants in RMND1 produce global defects in mitochondrial translation and were first reported in patients with severe neurological phenotypes leading to mortality in the first months of life. Using whole-exome sequencing, we identified compound heterozygous RMND1 variants in a 4-year-old patient with congenital lactic acidosis, severe myopathy, hearing loss, renal failure, and dysautonomia. The levels of mitochondrial ribosome proteins were reduced in patient fibroblasts, causing a translation defect, which was rescued by expression of the wild-type cDNA. RMND1 was almost undetectable by immunoblot analysis in patient muscle and fibroblasts. BN-PAGE analysis showed a severe combined OXPHOS assembly defect that was more prominent in patient muscle than in fibroblasts. Immunofluorescence experiments showed that RMND1 localizes to discrete foci in the mitochondrial network, juxtaposed to RNA granules where the primary mitochondrial transcripts are processed. RMND1 foci were not detected in patient fibroblasts. We hypothesize that RMND1 acts to anchor or stabilize the mitochondrial ribosome near the sites where the mRNAs are matured, spatially coupling post-transcriptional handling mRNAs with their translation, and that loss of function variants in RMND1 are associated with a unique constellation of clinical phenotypes that vary with the severity of the mitochondrial translation defect.

A novel recurrent mutation in ATP1A3 causes CAPOS syndrome.

BACKGROUND: We undertook genetic analysis of three affected families to identify the cause of dominantly-inherited CAPOS (cerebellar ataxia, areflexia, pes cavus, optic atrophy and sensorineural hearing loss) syndrome. METHODS: We used whole-exome sequencing to analyze two families affected with CAPOS syndrome, including the original family reported in 1996, and Sanger sequencing to assess familial segregation of rare variants identified in the probands and in a third, apparently unrelated family with CAPOS syndrome. RESULTS: We found an identical heterozygous missense mutation, c.2452G > A (p.(Glu818Lys)), in the Na⁺/K⁺ ATPase α3(ATP1A3) gene in the proband and his affected sister and mother, but not in either unaffected maternal grandparent, in the first family. The same mutation was also identified in the proband and three other affected members of the second family and in all three affected members of the third family. This mutation was not found in more than 3600 chromosomes from unaffected individuals. CONCLUSION: Other mutations in ATP1A3 have previously been demonstrated to cause rapid-onset dystonia-parkinsonism (also called dystonia-12) or alternating hemiplegia of childhood. This study shows that an allelic mutation in ATP1A3 produces CAPOS syndrome.