SURF1 Human

What is the role of SURF1 in human mitochondrial function?

SURF1 functions as an assembly factor for cytochrome C oxidase (COX), the fourth complex of the mitochondrial oxidative phosphorylation (OXPHOS) system. Located in the inner mitochondrial membrane, SURF1 facilitates the formation of fully assembled COX, which is essential for cellular respiration and ATP production . The loss of SURF1 reduces the assembly of functional COX complexes, leading to impaired mitochondrial energy production that particularly affects tissues with high energy demands like the brain . This bioenergetic dysfunction manifests as reduced oxygen consumption, increased reliance on glycolysis, and metabolic acidosis, contributing to the neurological pathology seen in Leigh syndrome patients .

How do SURF1 mutations contribute to Leigh syndrome pathophysiology?

SURF1 mutations represent one of the major causes of Leigh syndrome, a fatal mitochondrial necrotizing encephalopathy affecting approximately 1 in 36,000 newborns . The disease is characterized by psychomotor regression and lactic acidosis with peak mortality before age three . Recent research using patient-derived induced pluripotent stem cells (iPSCs) has revealed that SURF1 mutations disrupt neurogenesis at the neural progenitor cell (NPC) stage . Aberrant bioenergetics in SURF1-deficient NPCs impairs their neurogenic potential, leading to defects in neuronal generation and maturation . This neurogenic failure appears to be a central pathogenetic mechanism, explaining the predominant basal ganglia pathology observed in Leigh syndrome patients, as dopaminergic neurons are particularly affected .

Why have SURF1-deficient animal models failed to recapitulate human neurological phenotypes?

SURF1-deficient animal models have consistently failed to reproduce the severe neurological phenotypes seen in human Leigh syndrome patients, creating a significant obstacle in understanding disease pathogenesis . SURF1 knockout mice not only lacked neurological defects but exhibited prolonged lifespan despite mild COX deficiency and lactic acidosis . In Drosophila melanogaster, CNS-specific SURF1 knockdown increased longevity without neurological impairments, while constitutive knockdown caused embryonic lethality . SURF1 knockdown in zebrafish primarily affected the peripheral rather than central nervous system . Even SURF1 knockout pigs, which showed a severe lethal phenotype, displayed only mild neurological defects with slightly delayed CNS development, without lactic acidosis or apparent COX deficiency . These cross-species differences highlight the unique aspects of human neuronal development and metabolism, necessitating human-specific models for studying this disease .

How can researchers generate effective human models for studying SURF1 deficiency?

Researchers have developed several complementary approaches to create human models of SURF1 deficiency:

These complementary models allow researchers to study different aspects of SURF1 deficiency, from molecular mechanisms to developmental impacts .

What techniques can assess mitochondrial dysfunction in SURF1-deficient neural cells?

Researchers employ multiple techniques to characterize mitochondrial dysfunction in SURF1-deficient neural cells:

• Bioenergetic profiling:

  • Oxygen consumption rate (OCR) measurements using Seahorse XF analyzers

  • Extracellular acidification rate (ECAR) to assess glycolytic activity

  • ATP production assays to quantify energy generation capacity

• Mitochondrial morphology and network analysis:

  • Confocal microscopy with mitochondria-specific dyes or targeted fluorescent proteins

  • Analysis of mitochondrial network dynamics, fission/fusion balance, and distribution

• Complex IV (COX) activity assays:

  • Spectrophotometric measurements of cytochrome c oxidation

  • Histochemical COX staining to visualize activity in intact cells

• Molecular analysis of COX assembly:

  • Blue native polyacrylamide gel electrophoresis (BN-PAGE) to assess complex assembly

  • Western blotting for COX subunits to detect assembly defects

• Transcriptomic analysis:

  • RNA sequencing to identify dysregulated genes and pathways

  • Particular focus on genes involved in mitochondrial function, neurogenesis, and neuronal development

These comprehensive assessments have revealed that bioenergetic dysfunction occurs already at the neural progenitor cell stage, preceding and likely causing the observed neurogenic defects .

How can in utero fetal gene therapy be implemented for SURF1-related Leigh syndrome?

In utero fetal gene therapy (IUFGT) represents a promising approach for treating SURF1-related Leigh syndrome before irreversible neurological damage occurs. The methodology involves:

• Vector selection: Recombinant AAV2/9 vectors are preferred due to their ability to cross the blood-brain barrier and their neuronal tropism .

• Construct design: Self-complementary AAV vectors (scAAV) expressing:

  • Initially a GFP reporter to establish optimal parameters

  • Subsequently, human SURF1 cDNA under a strong promoter like CAG

• Delivery procedure: Ultrasound-guided injection through the umbilical cord vessels at approximately E70 (for pigs; timing would differ for humans) .

• Dosing optimization: Testing different viral titers (10¹¹, 10¹², 10¹³ vg/Kg) to determine optimal concentration for transgene expression while minimizing toxicity .

• Monitoring and analysis:

  • Fetal growth monitoring via ultrasound until birth

  • Analysis of stillborn and newborn subjects by PCR and immunofluorescence

  • Assessment of GFP reporter expression in brain and peripheral tissues

  • Evaluation of correction of biochemical and histological abnormalities

This approach has shown promise in SURF1 knockout pig models, suggesting potential for preventing the severe pathology associated with SURF1 deficiency when intervention occurs early in development .

What mechanisms underlie neurogenic defects in SURF1-deficient neural cells?

Research has identified several interconnected mechanisms underlying neurogenic defects in SURF1-deficient neural cells:

• Bioenergetic dysfunction in neural progenitor cells (NPCs):

  • Reduced oxygen consumption and ATP production

  • Impaired mitochondrial respiration limiting energy available for neuronal differentiation

• Altered gene expression profiles:

  • Downregulation of genes critical for neurogenesis and nervous system development (NKX2-2, FOXA1, FOXA2, NCAM2)

  • Reduced expression of genes important for axonal outgrowth and synaptic signaling (SPON1, CNTN4)

  • Decreased expression of genes regulating synaptic function (SYP, SYT13, KCNC2)

• Metabolic reprogramming:

  • Increased expression of REST and HES1, negative regulators of glucose oxidation

  • Upregulation of glycolytic pathways as a compensatory mechanism

  • Disrupted energy homeostasis affecting cellular differentiation processes

• Impaired neuronal maturation:

  • Defects in neuronal generation from progenitor cells

  • Disrupted branching capacity of developing neurons

  • Compromised firing activity in mature neurons

These data collectively suggest that SURF1 deficiency impairs the bioenergetic capacity of neural progenitor cells, disrupting their ability to undergo proper neurogenic differentiation and leading to downstream defects in neuronal development and function .

How do transcriptomic changes in SURF1-deficient cells correlate with neurogenic defects?

Transcriptomic analysis of SURF1-deficient neural cultures has revealed significant gene expression changes that directly correlate with observed neurogenic defects:

RNA sequencing of 4-week and 8-week dopaminergic neuronal cultures showed strong correlation between datasets from different SURF1-deficient lines . Genes involved in nervous system development were consistently downregulated, while negative regulators of glucose oxidation were upregulated . These transcriptomic changes align with the observed phenotypes: reduced numbers of neurons, impaired neuronal maturation, and altered metabolic profiles in SURF1-deficient cultures . Importantly, biallelic correction of SURF1 mutations via CRISPR/Cas9 restored normal gene expression patterns, confirming the causal relationship between SURF1 deficiency and the observed transcriptomic changes .

What cellular phenotypes differentiate SURF1-deficient neurons from healthy controls?

SURF1-deficient neurons exhibit multiple distinctive cellular phenotypes compared to healthy controls:

• Morphological differences:

  • Reduced neurite length and complexity

  • Decreased branching and arborization

  • Altered soma size and shape

• Developmental abnormalities:

  • Reduced neuronal numbers in differentiated cultures

  • Impaired maturation from progenitor to neuron stage

  • Disorganized distribution of neural progenitor cells in cerebral organoids

• Functional deficits:

  • Compromised firing activity

  • Altered electrophysiological properties

  • Impaired network formation

• Metabolic alterations:

  • Reduced oxygen consumption rate

  • Increased glycolytic activity

  • Altered mitochondrial morphology and distribution

• Molecular signatures:

  • Decreased expression of neuronal markers

  • Altered distribution of synaptic proteins

  • Changed expression of axonal guidance molecules

These phenotypes are consistently observed across different patient-derived lines and are reversed upon genetic correction of the SURF1 mutation, confirming their direct relationship to SURF1 deficiency . Importantly, these cellular abnormalities align with the neurological symptoms seen in Leigh syndrome patients, providing a valuable model for understanding disease mechanisms and testing therapeutic approaches .

What gene therapy strategies show promise for treating SURF1-deficient Leigh syndrome?

Several gene therapy approaches are being investigated for SURF1-deficient Leigh syndrome:

• Gene Augmentation Therapy (GAT):

  • Involves inserting a healthy copy of the SURF1 gene rather than removing the mutated gene

  • Uses adeno-associated virus (AAV) vectors, particularly AAV2/9 for CNS delivery

  • This approach is already being tested for other neurological diseases

  • Avoids the complexity of precise gene editing while providing functional SURF1 protein

• In Utero Fetal Gene Therapy (IUFGT):

  • Targets the disease before birth, potentially preventing developmental defects

  • Involves ultrasound-guided injection of viral vectors expressing SURF1 into fetal circulation

  • Promising results in SURF1 knockout pig models suggest this approach may prevent severe phenotypes

  • Preliminary experiments use GFP-expressing vectors to establish optimal viral concentration and tissue distribution

• CRISPR/Cas9-based approaches:

  • Research in cellular models demonstrates successful biallelic correction of SURF1 mutations

  • Corrected cells show restored COX assembly and mitochondrial function

  • Reversal of neurogenic defects confirms the causal relationship between SURF1 mutations and neuronal pathology

Research indicates that restoring SURF1 function at the neural progenitor cell stage may be sufficient to rescue neurogenic defects, suggesting intervention at early developmental stages could be most effective for preventing neurological damage .

Which pharmacological interventions may improve mitochondrial function in SURF1 deficiency?

Research has identified several pharmacological agents that could potentially improve mitochondrial function in SURF1 deficiency:

• Bezafibrate:

  • Currently being investigated as a potential treatment

  • Stimulates lipid metabolism and activates PGC-1 alpha, a regulator of cell metabolism

  • Preliminary research suggests it may improve energy output in progenitor cells

  • Already approved for dietary support in adults, facilitating potential clinical translation

• Metabolic modulators:

  • Compounds targeting specific aspects of energy metabolism

  • May help compensate for reduced COX activity

  • Could potentially improve ATP production through alternative pathways

• Mitochondrial biogenesis activators:

  • Increase mitochondrial mass to partially compensate for dysfunction

  • May enhance residual COX activity

  • Target upstream regulators of mitochondrial biogenesis

The identification of bioenergetic dysfunction in neural progenitor cells as a key pathogenic mechanism provides a rationale for early pharmacological intervention . Given the developmental nature of the neurogenic defects, treatments may need to be initiated as early as possible, potentially even prenatally, to prevent irreversible neurological damage .

How can organoid models advance therapeutic development for SURF1-related disorders?

Cerebral organoids provide valuable advantages for therapeutic development in SURF1-related disorders:

• Three-dimensional modeling of complex brain development:

  • Organoids better recapitulate human brain development compared to standard cell cultures

  • Allow visualization of ventricular zones and cortical layers affected by SURF1 deficiency

  • Enable assessment of cell-cell interactions and tissue organization

• Phenotypic readouts for therapeutic assessment:

  • SURF1-deficient organoids are smaller and show disorganized neural progenitor zones

  • They exhibit defective generation of neurons with abnormal distribution

  • These observable phenotypes provide clear endpoints for therapeutic evaluation

• Experimental advantages for intervention testing:

  • Allow longitudinal monitoring of development over months

  • Enable testing of interventions at different developmental stages

  • Provide system for dose-finding and optimization studies

• Translational relevance:

  • Derived from patient cells, maintaining disease-relevant genetic background

  • More accurately reflect human neural development compared to animal models

  • Can be genetically corrected to provide isogenic controls for direct comparison

In SURF1-deficient organoids, researchers have observed that genetic correction restores neurogenic progenitor zones and enables proper neuron generation . This demonstrates the utility of organoids for validating potential therapeutic approaches before advancing to more complex models or clinical studies .

What experimental approaches might better elucidate the tissue-specific effects of SURF1 deficiency?

Several experimental approaches could advance our understanding of tissue-specific SURF1 deficiency effects:

• Multi-lineage organoid models:

  • Generate region-specific brain organoids (forebrain, midbrain, hindbrain)

  • Compare vulnerability across different neural tissues

  • Create multi-tissue organoids to study tissue-specific energy requirements

• Single-cell omics approaches:

  • Apply single-cell RNA sequencing to identify vulnerable cell populations

  • Use spatial transcriptomics to map expression patterns within 3D organoids

  • Combine with metabolomics to correlate gene expression with metabolic profiles

• Advanced imaging techniques:

  • Implement live-cell imaging to track mitochondrial dynamics in different cell types

  • Use super-resolution microscopy to visualize COX assembly in situ

  • Apply functional imaging to measure ATP production and oxygen consumption at single-cell resolution

• Comparative studies across species:

  • Systematic comparison of human, mouse, and pig SURF1-deficient models

  • Identify molecular factors explaining differential vulnerability across species

  • Explore compensatory mechanisms present in resistant species

These approaches would help explain why certain tissues and cell types are more affected by SURF1 deficiency, potentially revealing new therapeutic targets and intervention strategies for patients with Leigh syndrome .

What challenges remain in translating SURF1 therapeutic approaches to clinical applications?

Despite promising advances, several challenges remain in translating SURF1 therapies to clinical applications:

• Timing of intervention:

  • Neurogenic defects occur early in development, potentially requiring prenatal intervention

  • Ethical and technical challenges of in utero treatments

  • Need to establish therapeutic windows through developmental studies

• Delivery challenges:

  • Efficient targeting of CNS tissues with therapeutic agents

  • Crossing the blood-brain barrier with sufficient concentration

  • Achieving widespread distribution in affected brain regions

• Patient heterogeneity:

  • Different SURF1 mutations may respond differently to treatments

  • Variable disease progression and severity among patients

  • Need for personalized approaches based on genetic profile

• Safety considerations:

  • Long-term safety of gene therapy approaches

  • Potential off-target effects of CRISPR/Cas9-based interventions

  • Risk-benefit assessment for a progressive, fatal disease

• Clinical trial design:

  • Rare disease with limited patient populations

  • Lack of established biomarkers for treatment response

  • Need for sensitive outcome measures to detect therapeutic effects

Addressing these challenges will require coordinated efforts between basic scientists, clinicians, and regulatory agencies to advance promising therapeutic approaches from laboratory models to clinical applications for this devastating pediatric disease .

How might advances in SURF1 research impact our understanding of other mitochondrial disorders?

Advances in SURF1 research have broader implications for understanding and treating other mitochondrial disorders:

• Methodological advances:

  • iPSC-derived models established for SURF1 provide templates for studying other mitochondrial diseases

  • CRISPR/Cas9 approaches for mutation correction can be applied to other genetic disorders

  • Organoid-based drug screening platforms can accelerate therapeutic development across conditions

• Mechanistic insights:

  • Recognition of neurogenesis defects as a pathogenic mechanism may apply to other mitochondrial encephalopathies

  • Understanding neural progenitor vulnerability could explain selective CNS involvement in diverse mitochondrial disorders

  • Identification of metabolic-genetic regulatory networks may reveal common therapeutic targets

• Therapeutic strategies:

  • Gene augmentation approaches developed for SURF1 may work for other mitochondrial assembly factor deficiencies

  • In utero intervention concepts could be extended to other congenital mitochondrial disorders

  • Pharmacological compounds identified may have broader applications across mitochondrial diseases

• Developmental perspective:

  • Recognition that many "degenerative" mitochondrial disorders may have developmental origins

  • Shift toward earlier intervention strategies across mitochondrial diseases

  • Integration of developmental biology with mitochondrial medicine

The pioneering work on SURF1-related Leigh syndrome provides a valuable framework for understanding the complex interplay between mitochondrial dysfunction, cellular energy metabolism, and neurodevelopmental processes across the spectrum of mitochondrial disorders .