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SPG21
SPG21 abhydrolase domain containing, maspardin
SPG21 is located on the long (q) arm of chromosome 15, at band 15q22.31. Arm ratio per GRCh38 - banding schematic.
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Overview
SPG21 resides on chromosome 15 and encodes maspardin, a member of the alpha-beta hydrolase superfamily. Although the precise enzymatic activity of maspardin remains under investigation, research suggests the protein plays a role in vesicle trafficking and lysosomal biology within cells. Alterations in SPG21 have been associated with certain forms of hereditary spastic paraplegia, a group of genetic disorders affecting motor neurone function. The gene's contribution to cellular housekeeping processes positions it as a factor in maintaining neuronal health, particularly in the long axons of the corticospinal tract.
What the gene does
Maspardin is thought to participate in intracellular membrane trafficking, particularly pathways involving endosomes and lysosomes. Evidence suggests the protein may assist in the sorting and transport of cargo within the cell, ensuring that proteins and lipids reach their correct destinations. This trafficking function is essential for cellular homeostasis, waste clearance, and organelle integrity. Disruption of maspardin activity may compromise lysosomal degradation pathways and autophagy, processes critical for removing damaged cellular components. The protein's structural classification within the alpha-beta hydrolase fold superfamily hints at potential catalytic or binding functions, though specific substrates have not been conclusively identified. Dysfunction in these trafficking mechanisms can lead to accumulation of cellular debris, oxidative stress, and ultimately neuronal degeneration.
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Chromosome location
SPG21 is located on the long arm of chromosome 15 at cytogenetic band 15q22.31. This chromosomal region contains a cluster of genes involved in diverse cellular processes. The precise number of exons and the full genomic span of SPG21 vary across transcript isoforms, reflecting alternative splicing patterns that may influence protein function in different tissues. The gene's position within the genome does not coincide with known structural rearrangement hotspots, and pathogenic variants typically arise from point mutations or small insertions and deletions rather than large chromosomal aberrations.
Protein structure
Domain architecture has not been experimentally characterised in detail for this protein. Maspardin belongs to the alpha-beta hydrolase fold superfamily based on sequence homology, suggesting it may adopt a core structure featuring a central beta-sheet flanked by alpha-helices. However, specific functional domains, catalytic residues, and binding pockets remain to be mapped through crystallographic or detailed biochemical studies. The absence of annotated domains in current protein databases reflects the early stage of structural investigation for maspardin.
Key variants
Pathogenic variants in SPG21 are predominantly loss-of-function alterations, including nonsense mutations, frameshift insertions or deletions, and splice-site changes that disrupt normal transcript processing. Missense variants affecting conserved residues within the predicted hydrolase fold have also been reported. The variant spectrum reflects the gene's relatively recent identification in clinical genetics, with ongoing efforts to catalogue pathogenic alleles and establish genotype-phenotype correlations. Carrier frequency varies across populations, and most pathogenic variants are rare or private to individual families.
Sample of pathogenic variants
9 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.118C>T | p.Arg40Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Hereditary spastic paraplegia |
c.119dup | p.Ser41fs | Pathogenic/Likely pathogenic | ★★☆☆ | Mast syndrome |
c.601dup | p.Thr201fs | Pathogenic | ★★☆☆ | SPG21-related disorder |
c.736C>T | p.Arg246Ter | Pathogenic/Likely pathogenic | ★★☆☆ | SPG21-related disorder |
c.137_138del | p.Leu46fs | Pathogenic | ★☆☆☆ | Mast syndrome |
c.345_346del | p.Gln116fs | Pathogenic | ★☆☆☆ | Mast syndrome |
c.118del | p.Arg40fs | Pathogenic | - | Mast syndrome |
c.322G>C | p.Ala108Pro | Pathogenic | - | Mast syndrome |
c.487del | p.Lys162_Ile163insTer | Pathogenic | - | Mast syndrome |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Variants in SPG21 have been linked to hereditary spastic paraplegia, a clinically and genetically heterogeneous group of disorders characterised by progressive lower-limb spasticity and weakness. The condition typically presents with gait disturbance, muscle stiffness, and hyperreflexia, though additional neurological features may occur depending on the specific genetic subtype. Inheritance patterns associated with SPG21-related disease can vary, and clinical severity ranges from mild ambulatory impairment to significant disability. Diagnosis relies on genetic testing alongside clinical evaluation and neuroimaging to exclude other causes of spasticity.
No disease links recorded for this gene in our reference set.
UK clinical status
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What is the role of maspardin in cells?
Maspardin is thought to participate in intracellular trafficking, particularly the movement of materials between endosomes and lysosomes. This function supports cellular waste clearance and organelle maintenance, processes essential for long-term neuronal health.
How do SPG21 variants cause neurological symptoms?
Pathogenic variants typically reduce or eliminate maspardin function, disrupting cellular trafficking and lysosomal pathways. This can lead to accumulation of cellular debris and oxidative damage, particularly affecting the long axons of motor neurones responsible for lower-limb movement.
Can SPG21 variants affect other organs besides the nervous system?
Current evidence suggests SPG21-related conditions primarily affect the nervous system, with progressive spasticity and weakness as the main features. Additional systemic involvement has not been consistently reported, though research into the full clinical spectrum continues.