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THOC2
THO complex subunit 2
THOC2 is located on the long (q) arm of chromosome X, at band Xq25. Arm ratio per GRCh38 - banding schematic.
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Overview
THOC2 encodes THO complex subunit 2, a structural component of the THO complex that plays a central role in the life cycle of messenger RNA. The THO complex assembles on newly transcribed mRNA molecules inside the nucleus and facilitates their packaging into ribonucleoprotein particles for export through nuclear pores into the cytoplasm, where translation occurs. By coordinating this export process, THOC2 ensures that genetic instructions encoded in DNA are accurately conveyed to the protein synthesis machinery. Given its fundamental role in gene expression, alterations in THOC2 can disrupt cellular function, particularly in tissues with high metabolic demands such as the developing nervous system. The gene is located on the X chromosome and encodes a protein of 1,593 amino acids organised into multiple functional domains.
What the gene does
The THOC2 protein functions as a molecular scaffold that brings together other components of the THO complex, forming a stable platform for mRNA export. It directly interacts with THOC1, THOC3, THOC5, and THOC7 subunits through distinct binding surfaces, enabling assembly of the full export machinery. THOC2 also recruits the RNA helicase DDX39B, which helps remodel RNA-protein complexes during the packaging process. Beyond mRNA export, the THO complex contributes to maintaining genomic stability by preventing the formation of aberrant DNA-RNA hybrid structures (R-loops) that can arise during transcription. These hybrids, if left unresolved, interfere with DNA replication and repair, potentially leading to chromosomal breaks. Through its dual roles in mRNA processing and genome integrity, THOC2 supports efficient gene expression while safeguarding the cell's genetic material. The protein's activity is particularly critical during early development, when rapid cell division and differentiation require precise control of gene expression programmes.
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Chromosome location
THOC2 is located on the long arm of the X chromosome at position Xq25 (Xq25 cytogenetic band). The gene spans a substantial genomic region and contains multiple exons that are spliced together to produce the mature mRNA transcript. Its position on the X chromosome means that males carry only one copy of the gene, while females carry two copies, one on each X chromosome. X-linked genes follow distinct inheritance patterns compared to autosomal genes, with males typically more severely affected by loss-of-function variants because they lack a second copy to compensate.
Protein structure
The THOC2 protein is organised into several distinct structural regions that mediate its scaffold function within the THO complex. The N-terminal anchor domain (amino acids 1-163) provides interaction surfaces for THOC5 and THOC7 subunits, establishing the foundation of the complex. Adjacent to this is the bow domain (amino acids 164-534), which binds to the dock domain of THOC1 and to THOC3, incorporating additional subunits into the assembly. A central MIF4G domain (amino acids 535-686) interacts with both THOC3 and the DDX39B helicase, coupling RNA remodelling activity to the export machinery. The stern domain (amino acids 687-1174) contains a coiled-coil region (amino acids 896-965) and a nuclear localisation signal (amino acids 923-928) that directs the protein to the nucleus where mRNA export occurs. The C-terminal charged domain (amino acids 1175-1593) is largely disordered and may provide flexibility for dynamic interactions. Two additional disordered regions (amino acids 322-341 and 1184-1593) likely contribute to the protein's adaptability during complex assembly and RNA binding.
Key variants
Pathogenic variants in THOC2 typically disrupt the protein's ability to scaffold the THO complex or impair its interactions with RNA and partner proteins. Loss-of-function changes can arise through premature termination codons, frameshift insertions or deletions, or missense substitutions that destabilise critical structural domains. Because THOC2 is located on the X chromosome, males carrying a single pathogenic variant often exhibit more pronounced effects than heterozygous females, in whom random X-inactivation may partially compensate by silencing the mutant allele in some cells.
No pathogenic or likely-pathogenic ClinVar variants recorded yet for this gene.
Associated conditions
Pathogenic variants in THOC2 have been associated with neurodevelopmental disorders characterised by intellectual disability, developmental delay, and variable additional features. The spectrum of phenotypes reflects the gene's critical role in mRNA export across all tissues, with the developing brain appearing particularly sensitive to disruptions in gene expression. Affected individuals may present with a range of neurological and systemic manifestations, though the precise clinical presentation can vary depending on the nature and location of the genetic change. Research into THOC2-related conditions continues to refine the understanding of genotype-phenotype correlations.
No disease links recorded for this gene in our reference set.
UK clinical status
THOC2 appears on several UK NHS Genomic Medicine Service gene panels, reflecting its clinical relevance for diagnostic testing. The gene is included in the DDG2P (Developmental Disorders Genotype-to-Phenotype) panel with green classification, indicating robust evidence linking variants to neurodevelopmental phenotypes. It also features on the Fetal Anomalies panel (green, version R21) and the Intellectual Disability panel (green, version R29), supporting its use in prenatal and postnatal diagnostic pathways for individuals with developmental concerns. Green classification denotes high confidence in the gene-disease association, meaning variants identified during clinical testing are more likely to be interpreted as clinically significant.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
Why is THOC2 located on the X chromosome significant?
The X-chromosomal location means males have only one copy of THOC2, so a single pathogenic variant typically causes a more pronounced effect than in females, who may partially compensate through random X-inactivation of the mutant allele. This inheritance pattern influences both the severity of clinical presentations and the recurrence risk within families.
What is the THO complex and why does it matter?
The THO complex is a multi-protein assembly that packages messenger RNA molecules and escorts them from the nucleus to the cytoplasm, ensuring efficient gene expression. THOC2 serves as a central scaffold within this complex, coordinating interactions among subunits and with RNA. Disruptions in THO complex function can impair cellular communication and development.
How are THOC2 variants identified in clinical practice?
Pathogenic variants in THOC2 are typically detected through genomic sequencing panels focused on neurodevelopmental or intellectual disability genes, or via whole-exome or whole-genome sequencing in individuals with unexplained developmental delay. Inclusion on NHS gene panels facilitates systematic testing in relevant clinical contexts.