Selective mitophagy activation and protein aggregate accumulation in MTMR5/SBF1-deficient fibroblasts

Aims: Charcot-Marie-Tooth disease type 4B3 (CMT4B3) is a rare autosomal recessive neuropathy caused by biallelic MTMR5/SBF1 variants, which encode a catalytically inactive myotubularin involved in phosphoinositide metabolism and autophagy regulation. This study investigates the impact of MTMR5/SBF1 dysfunction on autophagy and mitophagy in patient-derived fibroblasts and examines the relationship between protein aggregates and autophagic machinery. Materials and methods: Fibroblasts from a CMT4B3 patient with compound heterozygous MTMR5/SBF1 mutations were compared with a healthy control. Autophagic flux was analyzed via LC3B and SQSTM1; mitophagy was assessed through PINK1 and PRKN recruitment and by quantifying mitophagosomes and autolysosomes under mitochondrial stress. Protein aggregates were visualized using Proteostat and tested for colocalisation with autophagic structures. Key findings: CMT4B3 fibroblasts showed normal basal macroautophagy but failed to increase autophagy in response to mitochondrial stress or protein aggregates. Conversely, mitophagy was strongly activated via the PINK1-PRKN pathway. Significance: These results reveal an uncoupling between mitophagy and macroautophagy, indicating that MTMR5/SBF1 mutations modify autophagic selectivity. Our findings provide new mechanistic insights into the pathogenesis of CMT4B3 and highlight the value of patient-derived fibroblasts for studying selective autophagy defects.