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SZT2
SZT2 subunit of KICSTOR complex
SZT2 encodes a component of the KICSTOR complex that regulates cellular nutrient sensing and growth through the mTOR pathway. The SZT2 gene provides instructions for making a protein that forms part of the KICSTOR complex, a critical regulator of cellular metabolism and growth.
SZT2 is located on the short (p) arm of chromosome 1, at band 1p34.2. Arm ratio per GRCh38 - banding schematic.
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Overview
The SZT2 gene encodes a large scaffolding protein that functions as a subunit of the KICSTOR complex, a multi-protein assembly that anchors the GATOR1 complex to lysosomal membranes. This positioning is essential for proper regulation of mTORC1, a master controller of cell growth, metabolism, and autophagy. When cellular nutrients are scarce, the KICSTOR-GATOR1 system inhibits mTORC1 activity, allowing cells to adapt their metabolic programmes accordingly.
Pathogenic variants in SZT2 disrupt this nutrient-sensing machinery, leading to dysregulated mTOR signalling in the developing brain. The resulting neurodevelopmental disorder typically presents in early childhood with seizures and developmental delays. SZT2-related conditions follow an autosomal recessive inheritance pattern, meaning that affected individuals carry pathogenic variants in both copies of the gene.
What the gene does
The SZT2 protein serves as a structural component of the KICSTOR complex, which comprises five subunits: KPTN, ITFG2, C12orf66, SZT2, and SESN2. This complex recruits and stabilises the GATOR1 complex on lysosomal membranes, positioning it to respond to amino acid availability. When amino acids are depleted, GATOR1 acts as a GTPase-activating protein for the Rag GTPases, converting them to their inactive state and thereby preventing mTORC1 recruitment to lysosomes.
The proper assembly and localisation of this nutrient-sensing machinery is essential for coordinating cellular growth with nutrient availability. In neurons, mTORC1 signalling influences protein synthesis, synaptic plasticity, and neuronal excitability. The SZT2 protein contributes a critical scaffolding function that maintains the spatial organisation of the KICSTOR complex, ensuring that nutrient signals are accurately transmitted to the mTOR pathway. Loss of functional SZT2 protein impairs the cell's ability to suppress mTORC1 during nutrient stress, leading to constitutive activation of growth signalling pathways at inappropriate times.
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Chromosome location
SZT2 is located on the short arm of chromosome 1 at position p34.2. This chromosomal region spans approximately 200 kilobases and contains the complete coding sequence for the gene. The gene comprises multiple exons that together encode a transcript producing a protein of approximately 3,432 amino acids, making it one of the larger proteins in the human proteome. The genomic architecture of SZT2 includes extensive intronic sequences that must be precisely spliced to generate the mature messenger RNA.
Protein structure
The SZT2 protein extends across approximately 3,432 amino acids and contains multiple predicted intrinsically disordered regions interspersed throughout its length. These disordered regions are identified at amino acids 699-731, 1357-1413, 1630-1686, 1816-1908, 2124-2144, 2399-2513, 2737-2757, 2862-2902, 2958-2978, and 3212-3236. Such disordered segments often serve regulatory roles or provide flexible linkers between structured domains.
A functionally characterised region spanning amino acids 1083-1189 mediates interaction with the GATOR1 complex, representing a critical interface for SZT2's role in nutrient sensing. This segment enables SZT2 to recruit GATOR1 to the lysosomal membrane as part of the KICSTOR assembly. The extensive length and multiple disordered regions suggest that SZT2 functions primarily as a scaffolding protein, providing structural organisation rather than enzymatic activity. The large size and flexible architecture allow SZT2 to simultaneously coordinate interactions with multiple protein partners within the KICSTOR and GATOR complexes.
Key variants
Pathogenic variants in SZT2 are distributed across the gene's coding sequence, with both nonsense and frameshift variants reported in individuals with neurodevelopmental conditions. These loss-of-function variants typically result in premature termination of protein synthesis or production of unstable transcripts subject to nonsense-mediated decay. The consequence is reduced or absent functional SZT2 protein, which impairs assembly of the KICSTOR complex and disrupts mTORC1 regulation.
Missense variants affecting critical functional regions, particularly those disrupting the GATOR1-interaction domain, have also been identified in affected individuals. The variant spectrum reflects the autosomal recessive nature of SZT2-related conditions, with most affected individuals carrying pathogenic variants in both copies of the gene (as compound heterozygous or homozygous variants in consanguineous families).
Sample of pathogenic variants
10 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.203G>A | p.Trp68Ter | Pathogenic | ★★☆☆ | Developmental and epileptic encephalopathy, 18 |
c.2363C>G | p.Ser788Ter | Pathogenic/Likely pathogenic | ★★☆☆ | not provided |
c.2476C>T | p.Arg826Ter | Pathogenic/Likely pathogenic | ★★☆☆ | not provided |
c.3640C>T | p.Arg1214Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Developmental and epileptic encephalopathy, 18 |
c.3787C>T | p.Arg1263Ter | Pathogenic | ★★☆☆ | not provided |
c.4141C>T | p.Arg1381Ter | Pathogenic/Likely pathogenic | ★★☆☆ | not provided |
c.4396C>T | p.Arg1466Ter | Pathogenic | ★★☆☆ | not provided |
c.558dup | p.Tyr187fs | Pathogenic/Likely pathogenic | ★★☆☆ | SZT2-related disorder |
c.5872C>T | p.Arg1958Ter | Pathogenic/Likely pathogenic | ★★☆☆ | not provided |
c.5905C>T | p.Arg1969Ter | Pathogenic/Likely pathogenic | ★★☆☆ | not provided |
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
Pathogenic variants in both copies of the SZT2 gene cause a neurodevelopmental disorder characterised by early-onset epilepsy, intellectual disability, and macrocephaly. Seizures typically begin within the first few years of life and may be difficult to control with standard anti-epileptic medications. Affected individuals generally experience global developmental delays, with particular impact on cognitive development and motor skills.
The neurological features arise from dysregulated mTOR signalling during critical periods of brain development. Excessive mTORC1 activity in the absence of functional SZT2 disrupts normal neuronal differentiation, migration, and circuit formation. Some individuals may also present with additional features including hypotonia, behavioural difficulties, and distinctive facial characteristics.
No disease links recorded for this gene in our reference set.
Inheritance pattern
Conditions caused by pathogenic SZT2 variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
SZT2 is included on multiple NHS England PanelApp gene panels with green (high evidence) classification, reflecting strong evidence for its role in neurodevelopmental and epilepsy conditions. The gene appears on the DDG2P (Developmental Disorders Genotype-to-Phenotype) panel, the Early Onset or Syndromic Epilepsy panel (R59), and the Intellectual Disability panel (R29).
These green-rated panel memberships indicate that SZT2 testing is appropriate within NHS genomic medicine pathways for individuals presenting with early-onset epilepsy, intellectual disability, or unexplained developmental delay. Clinical geneticists and paediatric neurologists may request SZT2 analysis as part of targeted gene panels when the clinical presentation suggests disrupted mTOR signalling or when broad epilepsy gene panels are indicated.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Diet & lifestyle considerations
Research into mTOR pathway disorders suggests that dietary interventions modulating mTOR activity may influence seizure control in some affected individuals, though evidence specific to SZT2-related conditions remains limited. Some studies have explored ketogenic diets and other metabolic therapies in disorders of mTOR dysregulation, as these dietary approaches can influence cellular energy metabolism and mTOR signalling.
Families should work closely with paediatric neurologists and specialist paediatric dietitians before implementing any dietary modifications, as seizure management requires careful clinical oversight. General principles of good health, including adequate sleep, stress management, and avoidance of known seizure triggers, may support overall wellbeing in individuals with epilepsy, though these considerations apply broadly rather than specifically to SZT2-related conditions.
Supplement considerations
There is no conclusive evidence that any specific dietary supplement prevents or ameliorates the neurological features of SZT2-related conditions. Some research has investigated rapamycin and related mTOR inhibitors in disorders of mTOR hyperactivation, though these are prescription medications rather than supplements and require specialist medical supervision.
Families should discuss any proposed supplement use with their child's neurologist or clinical geneticist before starting, as interactions with anti-epileptic medications or effects on seizure threshold must be considered. Nutritional adequacy is important for neurodevelopment, and a paediatric dietitian can provide guidance on ensuring appropriate macro- and micronutrient intake within any therapeutic dietary framework.
Frequently asked questions
What inheritance pattern does SZT2 follow?
SZT2-related conditions follow an autosomal recessive inheritance pattern. This means an individual must inherit pathogenic variants in both copies of the gene, one from each parent, to be affected. Parents who each carry one pathogenic variant are typically unaffected carriers.
How does SZT2 relate to mTOR signalling?
SZT2 encodes a scaffolding protein within the KICSTOR complex, which anchors the GATOR1 complex to lysosomal membranes. GATOR1 inhibits mTORC1 when nutrients are scarce. Loss of functional SZT2 disrupts this inhibitory mechanism, leading to inappropriate mTORC1 activation even when cellular resources are limited.
At what age do SZT2-related seizures typically begin?
Seizures in individuals with pathogenic variants in both copies of the SZT2 gene typically begin in infancy or early childhood, often within the first few years of life. The epilepsy is often characterised as early-onset and may prove challenging to manage with conventional anti-epileptic therapies.
Can SZT2 variants be detected through carrier screening?
Yes, SZT2 can be included in expanded carrier screening panels, particularly those focused on autosomal recessive neurogenetic conditions. Carrier screening identifies individuals who carry one pathogenic variant, which is relevant for reproductive planning, especially if both partners are identified as carriers.
Why is SZT2 included on NHS epilepsy gene panels?
SZT2 has been validated with strong evidence (green rating) for causing early-onset epilepsy and intellectual disability through disrupted mTOR regulation. Its inclusion on NHS PanelApp panels reflects robust clinical and research data linking pathogenic variants in both copies of the gene to neurodevelopmental phenotypes, making genetic testing clinically appropriate for individuals with compatible presentations.