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SPG7
SPG7 matrix AAA peptidase subunit, paraplegin
SPG7 is located on the long (q) arm of chromosome 16, at band 16q24.3. Arm ratio per GRCh38 - banding schematic.
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Overview
SPG7 encodes paraplegin, a mitochondrial matrix protein that forms part of the m-AAA protease complex. This complex sits within the inner mitochondrial membrane, where it performs essential quality-control functions by degrading misfolded or damaged proteins and facilitating ribosome assembly. The gene is located on chromosome 16 and produces a protein of 795 amino acids.
Pathogenic changes in SPG7 are a recognised cause of hereditary spastic paraplegia type 7, a condition marked by progressive lower-limb stiffness and weakness. The gene is also implicated in broader neurodegenerative presentations, including ataxia and optic neuropathy. Understanding SPG7 function is important for diagnosing and managing these progressive neurological conditions.
What the gene does
Paraplegin belongs to the AAA (ATPases Associated with diverse cellular Activities) protein family, a group of enzymes that regulate numerous cellular processes through ATP-dependent mechanisms. Within mitochondria, paraplegin assembles with other AAA proteins to form the m-AAA protease, a molecular machine embedded in the inner mitochondrial membrane.
The m-AAA protease performs two critical functions: it degrades non-functional or misfolded proteins within the mitochondrial matrix, and it participates in assembling mitochondrial ribosomes, the protein-synthesis machinery unique to these organelles. By removing damaged proteins, paraplegin helps maintain mitochondrial integrity and prevents the accumulation of toxic protein aggregates. This quality-control function is particularly vital in neurons, which have high energy demands and limited capacity for cellular renewal.
When paraplegin is absent or dysfunctional, mitochondrial protein homeostasis becomes disrupted. Damaged proteins accumulate, mitochondrial function declines, and energy production falters. These molecular consequences manifest particularly in long axons of spinal neurons, where energy demands are especially high and transport distances are greatest.
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Chromosome location
SPG7 is located at chromosomal band 16q24.3 on the long arm of chromosome 16. This genomic region contains the complete coding sequence for the 795-amino-acid paraplegin protein. The precise chromosomal location enables targeted genetic testing approaches for individuals presenting with hereditary spastic paraplegia or related neurodegenerative phenotypes.
Protein structure
The paraplegin protein spans 795 amino acids and contains several functionally important regions. Two disordered regions are present: one extends from amino acids 108 to 133, and another spans amino acids 751 to 795. These disordered segments typically provide structural flexibility and may facilitate protein-protein interactions.
A key functional region involves the interaction with PPIF (peptidylprolyl isomerase F), spanning amino acids 701 to 795 at the C-terminal end of the protein. This interaction region is critical for paraplegin's role within the m-AAA protease complex and its broader mitochondrial quality-control functions. The overlapping disordered region at the C-terminus likely contributes to the dynamic nature of these protein interactions.
Key variants
More than 100 pathogenic variants in SPG7 have been documented in clinical and research literature. The majority of these variants are missense changes that alter single amino acids within the paraplegin protein sequence. When these alterations occur, the protein's ability to assemble into functional m-AAA protease complexes becomes compromised, disrupting mitochondrial protein quality control. Loss-of-function variants that eliminate paraplegin production entirely also contribute to disease presentations.
No pathogenic or likely-pathogenic ClinVar variants recorded yet for this gene.
Associated conditions
Pathogenic variants in SPG7 are primarily associated with spastic paraplegia type 7, an autosomal recessive condition characterised by progressive lower-limb spasticity, muscle weakness, and gait difficulties. Age of onset and severity vary considerably among affected individuals.
Beyond classical spastic paraplegia, SPG7 variants have been linked to a broader spectrum of neurological presentations, including hereditary spastic paraplegia (all) with both autosomal dominant and autosomal recessive inheritance patterns. Some individuals present with additional features such as cerebellar ataxia, optic neuropathy, or progressive external ophthalmoplegia. The phenotypic variability reflects the widespread importance of mitochondrial function in neurological health.
- Hereditary spastic paraplegia (all)
- Spastic paraplegia type 7 Dedicated page coming soon
UK clinical status
Within the NHS Genomic Medicine Service, SPG7 holds green (high-evidence) status across multiple clinical panels. These include adult onset hereditary spastic paraplegia (R60), childhood onset hereditary spastic paraplegia (R61), hereditary ataxia, and mitochondrial disorders panels, among others. Green classification indicates strong evidence supporting the gene-disease relationship and its inclusion in diagnostic testing pathways. SPG7 testing may be considered for individuals presenting with progressive spasticity, ataxia, or unexplained neurological decline consistent with mitochondrial dysfunction.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What is the inheritance pattern of SPG7-related conditions?
Spastic paraplegia type 7 typically follows an autosomal recessive inheritance pattern, meaning two pathogenic variants (one from each parent) are usually required for disease manifestation. However, some SPG7 variants have been associated with autosomal dominant presentations or act as risk factors in combination with variants in other genes.
How does paraplegin dysfunction affect the nervous system specifically?
Long axons in the spinal cord have particularly high energy demands and rely heavily on mitochondrial function. When paraplegin is absent or non-functional, mitochondrial protein quality control fails, energy production declines, and these vulnerable neurons progressively degenerate, leading to the characteristic lower-limb spasticity and weakness.
Can SPG7 variants cause symptoms beyond spastic paraplegia?
Yes, some individuals with SPG7 variants develop additional neurological features including cerebellar ataxia (coordination difficulties), optic neuropathy (vision problems), or progressive external ophthalmoplegia (eye movement limitation). The range of presentations reflects mitochondrial dysfunction affecting multiple neurological systems.
References
- Coarelli G, Schule R, van de Warrenburg BPC. Loss of paraplegin drives spasticity rather than ataxia in a cohort of 241 patients with SPG7. Neurology. 2019. PMID: 31068484
- Patron M, Sprenger HG, Langer T. m-AAA proteases, mitochondrial calcium homeostasis and neurodegeneration. Cell research. 2018. PMID: 29451229
- SPG7 mutations are a common cause of undiagnosed ataxia. Neurology. 2015. PMID: 25941202
- Wilkinson PA, Crosby AH, Turner C. A clinical, genetic and biochemical study of SPG7 mutations in hereditary spastic paraplegia. Brain : a journal of neurology. 2004. PMID: 14985266
- Atorino L, Silvestri L, Koppen M. Loss of m-AAA protease in mitochondria causes complex I deficiency and increased sensitivity to oxidative stress in hereditary spastic paraplegia. The Journal of cell biology. 2003. PMID: 14623864
- Settasatian C, Whitmore SA, Crawford J. Genomic structure and expression analysis of the spastic paraplegia gene, SPG7. Human genetics. 1999. PMID: 10480368
- Adam MP, Bick S, Mirzaa GM. SPG7-Related Neurologic Disorder. 1993. PMID: 20301286