Das, B. B. | bioRxiv Science

bioRxiv2026

Homozygous c.820G>A variant in MGME1 contributes to multi-systemic mitochondrial dysfunction in an Indian patient cohort

Mitochondrial DNA (mtDNA) maintenance disorders arise from defects in mtDNA replication or repair, frequently resulting in extensive deletions or depletion of mtDNA. Mitochondrial genome maintenance exonuclease 1 (MGME1) is a nuclear-encoded nuclease essential for mtDNA replication and genome stability, and biallelic pathogenic variants in MGME1 cause mitochondrial DNA depletion syndrome 11. Here, we report a novel homozygous MGME1 missense variant c.820G>A (p. Ala274Thr) in five affected individuals from unrelated South Indian families presenting with proximal myopathy, chronic progressive external ophthalmoplegia, and cardiac and renal involvement. Patient-derived cells exhibited a significant reduction in mtDNA copy number, consistent with impaired mtDNA maintenance. Mechanistic analyses combining imaging-based and biochemical approaches demonstrated that the MGME1 variant disrupts both mtDNA replication and repair. Functional characterization further revealed defective oxidative phosphorylation and reduced mitochondrial membrane potential, confirming mitochondrial dysfunction. Collectively, our findings establish the pathogenicity of this novel MGME1 variant and expand the clinical and molecular spectrum of MGME1-associated mitochondrial disease, linking impaired mtDNA replication to multisystemic mitochondrial dysfunction. Graphical abstract summarySchematic overview of mitochondrial DNA (mtDNA) replication in wild-type and MGME1 mutant conditions. In wild-type cells, the coordinated activity of the mitochondrial replication machinery (TWNK, POLG, LIG3, TFAM, and MGME1) maintains mtDNA integrity and supports normal oxidative phosphorylation and ATP production. In contrast, the MGME1 mutation disrupts mtDNA replication and processing, leading to replication defects, mtDNA depletion, and mitochondrial dysfunction. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=116 SRC="FIGDIR/small/725852v1_ufig1.gif" ALT="Figure 1"> View larger version (42K): org.highwire.dtl.DTLVardef@406ce7org.highwire.dtl.DTLVardef@1adc863org.highwire.dtl.DTLVardef@3ef92aorg.highwire.dtl.DTLVardef@585559_HPS_FORMAT_FIGEXP M_FIG C_FIG

genetics↗

bioRxiv2019

Proteasomal Inhibition Triggers Viral Oncoprotein Degradation via Autophagy-Lysosomal Pathway

Epstein-Barr virus (EBV) nuclear oncoprotein EBNA3C is essential for B-cell transformation and development of several B-cell lymphomas particularly those are generated in an immuno-compromised background. EBNA3C recruits ubiquitin-proteasome machinery for deregulating multiple cellular oncoproteins and tumor suppressor proteins. Although EBNA3C is found to be ubiquitinated at its N-terminal region and interacts with 20S proteasome, the viral protein is surprisingly stable in growing B-lymphocytes. EBNA3C can also circumvent autophagy-lysosomal mediated protein degradation and subsequent antigen presentation for T-cell recognition. Recently, we have shown that EBNA3C enhances autophagy, which serve as a prerequisite for B-cell survival particularly under growth deprivation conditions. We now demonstrate that proteasomal inhibition by MG132 induces EBNA3C degradation both in EBV transformed B-lymphocytes and ectopic-expression systems. Interestingly, MG132 treatment promotes degradation of two EBNA3 family oncoproteins - EBNA3A and EBNA3C, but not the viral tumor suppressor protein EBNA3B. EBNA3C degradation induced by proteasomal inhibition is partially blocked when autophagy-lysosomal pathway is inhibited. In response to proteasomal inhibition, EBNA3C is predominantly K63-linked polyubiquitinated, colocalized with the autophagy-lsyosomal fraction in the cytoplasm and participated within p62-LC3B complex, which facilitates autophagy-mediated degradation. We further show that the degradation signal is present at the first 50 residues of the N-terminal region of EBNA3C. Proteasomal inhibition reduces the colony formation ability of this important viral oncoprotein, increases transcriptional activation of both latent and lytic gene expression and induces viral reactivation from EBV transformed B-lymphocytes. Altogether, this study offers rationale to use proteasome inhibitors as potential therapeutic strategy against multiple EBV associated B-cell lymphomas, where EBNA3C is expressed.\n\nAuthor SummaryEpstein-Barr virus (EBV) establishes latent infection in B-lymphocytes and is associated with a number of human malignancies, both of epithelial and lymphoid origin. EBV encoded EBNA3 family of nuclear latent antigens comprising of EBNA3A, EBNA3B, and EBNA3C are unique to immunoblastic lymphomas. While EBNA3A and EBNA3C are involved in blocking many important tumor suppressive mechanisms, EBNA3B exhibits tumor suppressive functions. Although EBNA3 proteins, in particular EBNA3C, interact with and employ different protein degradation machineries to induce B-cell lymphomagenesis, these viral proteins are extremely stable in growing B-lymphocytes. To this end, we now demonstrate that proteasomal inhibition leads to specifically degradation of oncogenic EBNA3A and EBNA3C proteins, whereas EBNA3B remains unaffected. Upon proteasomal inhibition, EBNA3C degradation occurs via autophagy-lysosomal pathway, through labeling with K63-linked polyubiquitination and participating in p62-LC3B complex involved in ubiquitin-mediated autophagy substrate selection and degradation through autolysosomal process. We also demonstrate that the N-terminal domain is responsible for autophgy-lysosomal mediated degradation, while the C-terminal domain plays a crucial role in cytoplasmic localization. Fascinatingly, while proteasomal inhibition reduces EBNA3Cs oncogenic property, it induces both latent and lytic gene expressions and promotes viral reactivation from EBV transformed B-lymphocytes. This is the first report which demonstrates a viral oncoprotein degrades through autophagy-lysosomal pathway upon proteasomal inhibition. In sum, the results promise development of novel strategies specifically targeting proteolytic pathway for the treatment of EBV associated B-cell lymphomas, particularly those are generated in immunocompromised individuals.

cancer biology↗