Biallelic mutations in SUPV3L1 cause a variable leukodystrophy due to impaired mitochondrial degradosome function

Technical Summary: Biallelic Variants in SUPV3L1 Cause a Variable Leukodystrophy Due to Impaired Mitochondrial Degradosome Function

Problem Statement
Inherited white matter disorders (IWMDs) encompass a heterogeneous group of conditions, approximately 10% of which are linked to mitochondrial genes. While the mitochondrial degradosome—a complex comprising the helicase SUV3 (encoded by SUPV3L1) and the PNPase PNPT1 (encoded by PNPT1)—is known to be critical for mitochondrial integrity through the surveillance and degradation of double-stranded RNA (dsRNA), the clinical spectrum of SUPV3L1 dysfunction remained poorly defined. Previous reports of PNPT1 mutations established a link to early-onset encephalopathy and activation of type I interferon signaling due to cytosolic leakage of mitochondrial dsRNA. However, only a handful of patients with biallelic SUPV3L1 variants had been described, with inconsistent phenotypes ranging from optic atrophy to neurodegeneration, leaving the genetic, clinical, and radiological boundaries of the disorder unclear.

Methodology
The study employed an international collaborative approach to characterize the SUPV3L1-associated phenotype and elucidate its pathophysiology:

• Patient Cohort: Twenty-one individuals from fourteen unrelated families were ascertained via GeneMatcher. All patients harbored biallelic SUPV3L1 variants identified through next-generation sequencing. Clinical and radiological data were retrospectively collected and evaluated.
• In Silico and In Vitro Functional Assays: Variants were assessed for pathogenicity using conservation analysis and in silico prediction tools (CADD, PolyPhen2, SIFT, SpliceAI). A cell-based complementation assay was performed in U2OS cells using siRNA knockdown of endogenous SUPV3L1 followed by re-expression of wild-type or variant constructs. Pathogenicity was determined by measuring intracellular dsRNA accumulation via immunofluorescence.
• In Vivo Zebrafish Modeling: A supv3l1 knock-out (crispant) model was generated in zebrafish using CRISPR/Cas9. The study utilized specific transgenic reporter lines: Tg(mpeg1:mCherryCAAX) to label macrophages/microglia, Tg(AnnexinV:mVenus) to label dying cells, and a novel Tg(mpeg1.1:mts-mNeonGreen) line to visualize mitochondrial morphology in the microglial lineage.
• Immunological and Molecular Analysis: Interferon signaling was assessed in patient blood via quantitative PCR (qPCR) for interferon-stimulated genes (ISGs). In zebrafish larvae, qPCR was used to quantify antiviral immune pathway genes, and high-resolution confocal imaging was employed to analyze microglial morphology, apoptosis (via TUNEL and AnnexinV staining), and systemic immune activation.

Key Contributions and Results

• Clinical and Radiological Spectrum: The study defines a broad clinical spectrum for SUPV3L1-associated disease. All 21 patients presented in childhood, with onset typically under 12 months. The phenotype is characterized by significant neurodevelopmental involvement, including moderate to severe motor delay and intellectual disability (15/21). Common features include microcephaly (16/21), spasticity (19/21), and seizures (6/21). A distinct feature observed in 9/21 patients was skin hypopigmentation, suggestive of an autoimmune component.
• Radiological Findings: Neuroimaging (MRI/CT) in 15 patients revealed variable white matter abnormalities, ranging from mild T2 hyperintensity to confluent bilateral changes. Specific findings included subcortical cysts in the temporal poles (5 patients), cerebellar atrophy, and basal ganglia calcification (3/4 patients with CT). One patient (P11) presented with severe antenatal lissencephaly and early lethality.
• Functional Validation: All 15 identified variants were confirmed to impair SUV3 function. In the cell-based assay, re-expression of variant proteins failed to rescue the accumulation of intracellular dsRNA seen in SUPV3L1-depleted cells, confirming loss-of-function.
• Mechanistic Insights (Zebrafish Model): Loss of supv3l1 in zebrafish resulted in reduced survival, liver necrosis, and defective mitochondrial morphology (increased fusion/fission) specifically within the microglial lineage.
• Immune Activation: The supv3l1 crispant zebrafish exhibited an activated microglial phenotype (rounder cell bodies, membrane ruffling) and a significant increase in apoptotic cells in the optic tectum. Crucially, qPCR revealed a >400-fold upregulation of antiviral immune genes (including ifnF1, isg15, and mxa), confirming systemic activation of type I interferon signaling. This was corroborated in human patients, where 3 out of 4 tested individuals showed an elevated interferon signature in peripheral blood.

Significance
This study establishes SUPV3L1 as a critical gene for neurodevelopment and defines the clinical and radiological spectrum of its associated disorder, which presents as a variable leukodystrophy. The authors propose that the underlying pathophysiology involves the failure of the mitochondrial degradosome to degrade mitochondrial dsRNA, leading to its leakage and the subsequent activation of the type I interferon innate immune pathway. The observation of skin hypopigmentation and interferon signature upregulation suggests an autoimmune or inflammatory component driven by dysplastic microglia. These findings position SUPV3L1-associated disease within the broader category of interferonopathies and suggest that immunomodulatory therapies could be a viable therapeutic strategy, particularly for patients experiencing episodic regression.