Impaired proteasome activity and neurodegeneration with brain iron accumulation in FBXO7 defect

Abstract

FBXO7 is implicated in the ubiquitin-proteasome system and parkin-mediated mitophagy. FBXO7defects cause a levodopa-responsive parkinsonian-pyramidal syndrome(PPS).

Methods: We investigated the disease molecular bases in a child with PPS and brain iron accumulation.

Results: A novel homozygous c.368C>G (p.S123*) FBXO7 mutation was identified in a child with spastic paraplegia, epilepsy, cerebellar degeneration, levodopa nonresponsive parkinsonism, and brain iron deposition. Patient's fibroblasts assays demonstrated an absence of FBXO7 RNA expression leading to impaired proteasome degradation and accumulation of poly-ubiquitinated proteins.

Conclusion: This novel FBXO7 phenotype associated with impaired proteasome activity overlaps with neurodegeneration with brain iron accumulation disorders.

Figure 1

MRI proband’s studies: T1WI sagittal images (A) at 10 years old (B), 12 years old (C), 15 years old and (D) 17 years old show a progression from slight vermian atrophy to cerebellar atrophy of vermis and hemispheres, cerebral volume loss and brainstem atrophy, enlarged ventricles, and cerebral sulci at the age of 17. Axial T2WI of the pons at (E) 12 years old, (F) 15 years old and (G) 17 years old. T2 hyperintensity delineating a cross through the pons known as the “hot cross bun” sign and representing selective degeneration of pontocerebellar tracts appear at the age of 17. Gradient recall echo [GRE] sequence basal ganglia at (H) 12 years old (I), 15 years old and (J) and 17 years. Bilateral hypointensities in pallidum suggest that iron deposition appeared at 15 years.

Figure 2

Pedigree and analysis of the FBXO7 p.S123* mutation: (A) FBXO7 c.368C>G (p.S123*) mutation family’s pedigree. The index case (II:1) is homozygous for the mutation (black circle with an arrow). Her healthy relatives are heterozygous carriers (half‐shaded symbols). (B) Distribution of pathogenic mutations reported in FBXO7 according to protein domains; p.S123* (in bold) is located in an inter‐domain between the UBL and the CDK6 domains. (C) Absence of FBXO7 protein western blot in patient’s fibroblasts (II:1) compared to two controls fibroblasts’ lysates (C1 and C2) immunodetected with an antibody against FBXO7. (D) mRNA expression was evaluated by qPCR in fibroblasts from the patient (II:1) and one control with and without emetine treatment, a translation inhibitor. FBXO7 mRNA levels in patient’s fibroblasts (II:1) are rescued when treated with emetine. Data were obtained from one biological sample and three independent technical replicates. Error bars represent SEM. Mann–Whitney–Wilcoxon test: *P < 0.05; n.s: nonsignificant. (E) UPS activity assay performed in fibroblasts from the patient (II:1) and one control (C3) shows a reduction of 20% UPS activity in patient. Data were obtained from one biological sample and six independent technical replicates. Error bars represent SEM. Mann–Whitney–Wilcoxon test: *P < 0.05. (F) Detection of a trend to increase in poly‐UB proteins in patient (II:1) fibroblasts by Western blot and SDS‐PAGE. Actin was used to validate equal protein loading. Data were obtained from one biological sample and three independent technical replicates. Error bars represent SEM. Mann–Whitney–Wilcoxon test. n.s: nonsignificant