Rhizomelic chondrodysplasia punctata type 1 (RCDP1) is a peroxisomal disorder characterized by skeletal shortening, intellectual disability, seizures, cataracts, and reduced lifespans. RCDP1 is caused by biallelic loss-of-function variants in PEX7, which encodes a protein required for importing select enzymes into the peroxisome matrix, including those essential for ether lipid synthesis (e.g., plasmalogens) and the branched-chain fatty acid catabolism. Plasmalogen deficiency is a hallmark of RCDP1 and other peroxisomal disorders, including RCDP types 2-5 (RCDP2-5) and Zellweger spectrum disorders (ZSD). Here, we performed comprehensive metabolomic profiling of clinical samples from RCDP patients and Pex7-deficient mouse models. We identified profound neurometabolic disturbances in the cerebral cortex and cerebellum of Pex7-deficient mice involving multiple lipid classes, including phosphatidylethanolamines (PEs), phosphatidylcholines (PCs), acylcarnitines, and sphingomyelins. Notably, many of these neurometabolic alterations were absent in patient and Pex7-deficient mouse plasma, indicating that plasma-based profiling can underrepresent the extent of CNS lipid remodeling. Overall, these findings reveal novel insights into neurometabolic adaptations to plasmalogen deficiency and suggest the potential involvement of additional pathways that may contribute to neurological dysfunction in RCDP.

Peroxisome biogenesis disorders (PBDs) encompass a group of diseases marked by clinical and genetic heterogeneity. Phenotypes linked to PBDs include Zellweger syndrome, neonatal adrenoleukodystrophy, infantile Refsum disease (IRD), rhizomelic chondrodysplasia punctata type 1, and Heimler syndrome. PBD phenotypes manifest through hypotonia, developmental delay, facial dysmorphism, seizures, liver dysfunction, sensorineural hearing loss, and retinal dystrophy. The proband underwent comprehensive clinical evaluation, followed by whole-exome sequencing (WES) coupled with copy number analysis (CNV), aimed at identifying potential disease-causing variants aligning with the observed phenotype. Our findings detail an individual exhibiting developmental delay, hearing loss, visual impairment, hepatomegaly, and splenomegaly, attributed to a biallelic deletion of exon 4 in the PEX26 gene. The WES analysis of the index case did not uncover any pathogenic/likely pathogenic single-nucleotide variations that could account for the observed clinical findings. However, the CNV data derived from WES revealed a homozygous deletion in exon 4 of the PEX26 gene (NM_001127649.3), providing a plausible explanation for the patient's clinical features. The exon 4 region of PEX26 encodes the transmembrane domain of the protein. The transmembrane domain plays a crucial role in anchoring the protein within lipid bilayers, and its absence can disrupt proper localization and functioning. As a result, this structural alteration may impact the protein's ability to facilitate essential cellular processes related to peroxisome biogenesis and function. The index patient, which presented with hearing loss, retinal involvement and hepatic dysfunction in adolescence age, has atypical clinical course that can be considered unusual for Zellweger syndrome (ZS) and IRD phenotypes, and its rare genotypic data (in-frame single exon deletion) expands the PBD disease spectrum. This study revealed for the first time that PEX26 protein transmembrane domain loss exhibits an unusual course with clinical findings of IRD and ZS phenotypes. WES studies, incorporating CNV analyses, empower the identification of novel genetic alterations in genes seldom associated with gross deletion/duplication variations, such as those in the PEX26 gene. This not only enhances diagnostic rates in rare diseases but also contributes to broadening the spectrum of causal mutations.

Plasmalogens (Plgs) are highly abundant lipids in the retina, and their deficiency leads to severe abnormalities during eye development. The first acylation step in the synthesis of Plgs is catalyzed by the enzyme glyceronephosphate O-acyltransferase (GNPAT), which is also known as dihydroxyacetone phosphate-acyltransferase (EC 2.3.1.42). GNPAT deficiency produces rhizomelic chondrodysplasia punctata type 2, a genetic disorder associated with developmental ocular defects. Despite the relevance of retinal Plgs, our knowledge of the mechanisms that regulate their synthesis, and the role of GNPAT during eye development is limited. Using the Xenopus laevis model organism, we characterized by in situ hybridization the expression pattern of gnpat and compared it to glycerol 3-phosphate acyltransferase mitochondrial (gpam or gpat1) during eye neurogenesis, lamination, and morphogenesis. The Xenopus Gnpat was biochemically characterized in a heterologous expression system in yeast. During development, gnpat is expressed in proliferative cells of the retina and lens, and post-embryogenesis in proliferative cells of the ciliary marginal zone and lens epithelium. In contrast, gpam expression is mainly restricted to photoreceptors. Xenopus Gnpat expressed in yeast is present in both soluble and membrane fractions, but only the membrane-bound enzyme displays activity. The amino terminal of Gnpat, conserved in humans, shows lipid binding capacity that is enhanced by phosphatidic acid. Enzymes involved in the Plgs and glycerophospholipid biosynthetic pathways are differentially expressed during eye morphogenesis. The gnpat expression pattern and the molecular determinants regulating Gnpat activity advance our knowledge of this enzyme, contributing to our understanding of the retinal pathophysiology associated with GNPAT deficiency.

Peroxisome biogenesis disorders (PBDs) are a group of autosomal recessive disorders caused due to impaired peroxisome assembly affecting the formation of functional peroxisomes. PBDs are caused by a mutation in PEX gene family resulting in disease manifestation with extreme variability ranging from the onset of profound neurologic symptoms in newborns to progressive degenerative disease in adults. Disease causing variations in PEX7 is known to cause severe rhizomelic chondrodysplasia punctata type 1 and PBD 9B, an allelic disorder resulting in a milder phenotype, often indistinguishable from that of classic Refsum disease. This case report highlights the variability of PEX7 related phenotypes and suggests that other than RCDP1 and late onset phenotype similar to Refsum disease, some cases present with cataract and neurodevelopmetal abnormalities during childhood without chondrodysplasia or rhizomelia. This report also underlines the importance of considering PBD 9B in children presenting with neurodevelopmental abnormalities especially if they have congenital cataract.

Plasmalogens (Pls) are a class of membrane phospholipids which serve a number of essential biological functions. Deficiency of Pls is associated with common disorders such as Alzheimer's disease or ischemic heart disease. A complete lack of Pls due to genetically determined defective biosynthesis gives rise to rhizomelic chondrodysplasia punctata (RCDP), characterized by a number of severe disabling pathologic features and death in early childhood. Frequent cardiac manifestations of RCDP include septal defects, mitral valve prolapse, and patent ductus arteriosus. In a mouse model of RCDP, reduced nerve conduction velocity was partially rescued by dietary oral supplementation of the Pls precursor batyl alcohol (BA). Here, we examine the impact of Pls deficiency on cardiac impulse conduction in a similar mouse model (Gnpat KO). In-vivo electrocardiographic recordings showed that the duration of the QRS complex was significantly longer in Gnpat KO mice than in age- and sex-matched wild-type animals, indicative of reduced cardiac conduction velocity. Oral supplementation of BA for 2 months resulted in normalization of cardiac Pls levels and of the QRS duration in Gnpat KO mice but not in untreated animals. BA treatment had no effect on the QRS duration in age-matched wild-type mice. These data suggest that Pls deficiency is associated with increased ventricular conduction time which can be rescued by oral BA supplementation.