Novel PEX3 Gene Mutations Resulting in a Moderate Zellweger Spectrum Disorder

Review of childhood genetic nephrolithiasis and nephrocalcinosis

Nephrolithiasis (NL) is a common condition worldwide. The incidence of NL and nephrocalcinosis (NC) has been increasing, along with their associated morbidity and economic burden. The etiology of NL and NC is multifactorial and includes both environmental components and genetic components, with multiple studies showing high heritability. Causative gene variants have been detected in up to 32% of children with NL and NC. Children with NL and NC are genotypically heterogenous, but often phenotypically relatively homogenous, and there are subsequently little data on the predictors of genetic childhood NL and NC. Most genetic diseases associated with NL and NC are secondary to hypercalciuria, including those secondary to hypercalcemia, renal phosphate wasting, renal magnesium wasting, distal renal tubular acidosis (RTA), proximal tubulopathies, mixed or variable tubulopathies, Bartter syndrome, hyperaldosteronism and pseudohyperaldosteronism, and hyperparathyroidism and hypoparathyroidism. The remaining minority of genetic diseases associated with NL and NC are secondary to hyperoxaluria, cystinuria, hyperuricosuria, xanthinuria, other metabolic disorders, and multifactorial etiologies. Genome-wide association studies (GWAS) in adults have identified multiple polygenic traits associated with NL and NC, often involving genes that are involved in calcium, phosphorus, magnesium, and vitamin D homeostasis. Compared to adults, there is a relative paucity of studies in children with NL and NC. This review aims to focus on the genetic component of NL and NC in children.

Peroxisome disruption alters lipid metabolism and potentiates antitumor response with MAPK-targeted therapy in melanoma

Melanomas reprogram their metabolism to rapidly adapt to therapy-induced stress conditions, allowing them to persist and ultimately develop resistance. We report that a subpopulation of melanoma cells tolerate MAPK pathway inhibitors (MAPKis) through a concerted metabolic reprogramming mediated by peroxisomes and UDP-glucose ceramide glycosyltransferase (UGCG). Compromising peroxisome biogenesis, by repressing PEX3 expression, potentiated the proapoptotic effects of MAPKis via an induction of ceramides, an effect limited by UGCG-mediated ceramide metabolism. Cotargeting PEX3 and UGCG selectively eliminated a subset of metabolically active, drug-tolerant CD36+ melanoma persister cells, thereby sensitizing melanoma to MAPKis and delaying resistance. Increased levels of peroxisomal genes and UGCG were found in patient-derived MAPKi-relapsed melanomas, and simultaneously inhibiting PEX3 and UGCG restored MAPKi sensitivity in multiple models of therapy resistance. Finally, combination therapy consisting of a newly identified inhibitor of the PEX3-PEX19 interaction, a UGCG inhibitor, and MAPKis demonstrated potent antitumor activity in preclinical melanoma models, thus representing a promising approach for melanoma treatment.

Early steps in the birth of four membrane-bound organelles-Peroxisomes, lipid droplets, lipoproteins, and autophagosomes

Membrane-bound organelles allow cells to traffic cargo and separate and regulate metabolic pathways. While many organelles are generated by the growth and division of existing organelles, some can also be produced de novo, often in response to metabolic cues. This review will discuss recent advances in our understanding of the early steps in the de novo biogenesis of peroxisomes, lipid droplets, lipoproteins, and autophagosomes. These organelles play critical roles in cellular lipid metabolism and other processes, and their dysfunction causes or is linked to several human diseases. The de novo biogenesis of these organelles occurs in or near the endoplasmic reticulum membrane. This review summarizes recent progress and highlights open questions.

The Effect of a Pex3 Mutation on Hearing and Lipid Content of the Inner Ear

Peroxisome biogenesis disorders (due to PEX gene mutations) are associated with symptoms that range in severity and can lead to early childhood death, but a common feature is hearing impairment. In this study, mice carrying Pex3 mutations were found to show normal auditory development followed by an early-onset progressive increase in auditory response thresholds. The only structural defect detected in the cochlea at four weeks old was the disruption of synapses below inner hair cells. A conditional approach was used to establish that Pex3 expression is required locally within the cochlea for normal hearing, rather than hearing loss being due to systemic effects. A lipidomics analysis of the inner ear revealed a local reduction in plasmalogens in the Pex3 mouse mutants, comparable to the systemic plasmalogen reduction reported in human peroxisome biogenesis disorders. Thus, mice with Pex3 mutations may be a useful tool to understand the physiological basis of peroxisome biogenesis disorders.

Genome sequencing identifies a rare case of moderate Zellweger spectrum disorder caused by a PEX3 defect: Case report and literature review

Defects in PEX3 are associated with a severe neonatal-lethal form of Zellweger spectrum disorder. We report two moderately affected siblings whose clinical and biochemical phenotypes expand the reported spectrum of PEX3-related disease. Genome sequencing of an adolescent male with progressive movement disorder, spasticity and neurodegeneration, and previous non-diagnostic plasma very-long chain fatty acid analysis, revealed a homozygous likely pathogenic missense variant in PEX3 [c.991G > A; p.(Gly331Arg)]. A younger sibling with significant motor decline since the age of three years was also subsequently found to be homozygous for the familial PEX3 variant. A comprehensive review of the scientific literature identified three additional families with non-lethal infantile- or childhood-onset PEX3-related disease, which together with this clinical report illustrate the potential for highly variable disease severity. Our findings demonstrate the diagnostic utility of genome-wide sequencing for identifying clinically and biochemically heterogeneous inherited metabolic disorders such as the peroxisome biogenesis disorders.

Atypical PEX16 peroxisome biogenesis disorder with mild biochemical disruptions and long survival

BACKGROUND

Mutations in PEX16 cause peroxisome biogenesis disorder (PBD). Zellweger syndrome characterized by neurological dysfunction, dysmorphic features, liver disease and early death represents the severe end of this clinical spectrum. Here we discuss the diagnostic challenge of atypical PEX16 related PBD in 3 patients from highly inbred kindred and describe the role of specific metabolites analyses, fibroblasts studies, whole-exome sequencing (WES) and metabolomics profiling to establish the diagnosis.

METHODS AND PATIENTS

The proband is a 12-year-old male born to consanguineous parents. Despite normal development in the first year, regression and progressive spastic diplegia, poor coordination and dysarthria occurred thereafter. Patient 2 (3-year old female) and Patient 3 (19-month old female) shared similar clinical course with the proband. Biochemical studies on plasma and fibroblasts, WES and global metabolomics analyses were performed.

RESULTS

Very-long-chain fatty acids analysis showed subtle elevations in C26 and C26/C22. Global Metabolomics-Assisted Pathway profiling was not remarkable. Immunocytochemical investigations on fibroblasts revealed fewer catalase and PMP70-containing particles indicating aberrant peroxisomal assembly. Complementation studies were inconclusive. WES revealed a novel homozygous variant in PEX16 (c.859C>T). The biochemical profiles of Patient 2 and Patient 3 were similar to the proband and the same genotype was confirmed.

CONCLUSION

This paper highlights the diagnostic challenge of PEX16 patients due to the widely variable clinical and biochemical phenotypes. It also emphasizes the important roles of combined biochemical assays with next generation sequencing techniques in reaching diagnosis in the context of atypical clinical presentations, subtle biomarker abnormalities and consanguinity.