On this page
TCIRG1
T cell immune regulator 1, ATPase H+ transporting V0 subunit a3
TCIRG1 is located on the long (q) arm of chromosome 11, at band 11q13.2. Arm ratio per GRCh38 - banding schematic.
Explore chromosome 11 in the library →Available at Jeen Health
Clinical tests that include this
Overview
TCIRG1 (T cell immune regulator 1, ATPase H+ transporting V0 subunit a3) is located on chromosome 11q13.2 and encodes a 830-amino-acid protein that forms part of the vacuolar H+-ATPase complex. This molecular machine pumps protons across cellular membranes to create the acidic environments required for numerous biological processes. The a3 subunit plays an essential role in osteoclasts, the cells responsible for breaking down bone tissue during skeletal remodelling.
Pathogenic variants in TCIRG1 follow an autosomal recessive inheritance pattern and are responsible for approximately half of all cases of severe infantile osteopetrosis. This condition results from the failure of osteoclasts to adequately dissolve bone mineral, leading to abnormally dense bones that are paradoxically brittle and prone to fracture. TCIRG1 is included on UK carrier screening panels due to the severity of associated disease manifestations.
What the gene does
The TCIRG1 protein serves as the a3 subunit within vacuolar H+-ATPase (V-ATPase) complexes, which are multi-subunit molecular pumps that transport protons against concentration gradients. By moving positively charged hydrogen atoms across membranes, V-ATPases regulate the pH of cellular compartments and their surrounding microenvironments. This pH control is essential for numerous biochemical reactions to proceed correctly.
Within osteoclasts, V-ATPases containing the a3 subunit are embedded in a highly folded membrane structure called the ruffled border, which faces the bone surface. These pumps actively transport protons into the sealed compartment formed between the osteoclast and the bone matrix, creating an extremely acidic environment. This low pH is necessary to dissolve the mineral component of bone, allowing osteoclasts to carry out bone resorption as part of the continuous remodelling process that maintains skeletal strength and mineral homeostasis. Without functional TCIRG1, osteoclasts cannot acidify this resorption compartment effectively, preventing normal bone turnover.
Video: Genetics 101
Chromosome location
TCIRG1 is located on the long arm of chromosome 11 at position 13.2 (11q13.2). The gene spans genomic DNA and produces transcript variants that encode the 830-amino-acid a3 subunit protein. This chromosomal region contains multiple genes involved in diverse cellular functions, though TCIRG1 mutations specifically affect bone remodelling due to the critical role of the a3 subunit in osteoclast function.
Protein structure
The TCIRG1 protein comprises 830 amino acids and contains two disordered regions: one spanning amino acids 139-158 and another from amino acids 681-701. These disordered segments lack stable secondary structure and may provide flexibility for protein-protein interactions or conformational changes during the V-ATPase catalytic cycle. The a3 subunit integrates into the V0 domain of the complete V-ATPase complex, which anchors the pump in cellular membranes and forms the proton channel through which hydrogen ions are transported. The specific structural features that distinguish the a3 isoform from other V-ATPase a subunits determine its specialised function in osteoclasts and other cell types requiring robust acidification capacity.
Key variants
More than 60 pathogenic variants in TCIRG1 have been identified in individuals with autosomal recessive osteopetrosis. These genetic changes include missense variants that alter single amino acids within the a3 subunit, nonsense variants that introduce premature stop codons, and splicing variants that disrupt normal transcript processing. Many variants impair the assembly or stability of the V-ATPase complex, whilst others reduce the pump's ability to transport protons effectively. The severity of osteopetrosis generally correlates with the degree of residual V-ATPase function.
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.1191del | p.Phe398fs | Pathogenic | ★★☆☆ | Autosomal recessive osteopetrosis 1 |
c.1427G>A | p.Trp476Ter | Pathogenic/Likely pathogenic | ★★☆☆ | TCIRG1-related disorder |
c.1480C>T | p.Gln494Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Autosomal recessive osteopetrosis 1 |
c.1682delinsTT | p.Gly561fs | Pathogenic/Likely pathogenic | ★★☆☆ | Autosomal recessive osteopetrosis 1 |
c.1787G>A | p.Trp596Ter | Pathogenic | ★★☆☆ | Autosomal recessive osteopetrosis 1 |
c.2001del | p.Ala668fs | Pathogenic | ★★☆☆ | Autosomal recessive osteopetrosis 1 |
c.2177del | p.Gly726fs | Pathogenic/Likely pathogenic | ★★☆☆ | Autosomal recessive osteopetrosis 1 |
c.631-1G>A | - | Pathogenic/Likely pathogenic | ★★☆☆ | Autosomal recessive osteopetrosis 1 |
c.745C>T | p.Gln249Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Autosomal recessive osteopetrosis 1 |
c.927C>A | p.Cys309Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Autosomal recessive osteopetrosis 1 |
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 TCIRG1 cause osteopetrosis (infantile, malignant), also known as autosomal recessive osteopetrosis or ARO. This severe form of the condition typically presents in infancy with failure to thrive, fractures despite increased bone density, and compression of cranial nerves due to narrowed foramina. Affected individuals may develop anaemia and thrombocytopenia as dense bone encroaches on the marrow cavity, alongside hepatosplenomegaly from extramedullary haematopoiesis. Without treatment, infantile osteopetrosis is often fatal in early childhood. TCIRG1-related osteopetrosis accounts for the majority of severe autosomal recessive cases, making it the most common genetic cause of this life-threatening skeletal disorder.
Inheritance pattern
Conditions caused by pathogenic TCIRG1 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
TCIRG1 is included on multiple NHS Genomic Medicine Service panels with green classification, indicating strong evidence for clinical validity. The gene appears on the Osteopetrosis panel (R104.4), the broader Skeletal dysplasia panel (R104), and the Fetal anomalies panel (green, R21) for prenatal diagnosis. It is also listed on the Hydrocephalus panel (R86) due to the cranial manifestations of severe osteopetrosis. Additionally, TCIRG1 features on the COVID-19 research panel, reflecting investigations into immune and inflammatory pathways.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What is the role of TCIRG1 in bone health?
TCIRG1 encodes a subunit of the proton pump that osteoclasts use to create the acidic environment needed to dissolve bone mineral during normal bone remodelling. Without functional TCIRG1, bones become abnormally dense because old bone cannot be broken down and replaced.
How is TCIRG1-related osteopetrosis inherited?
TCIRG1-related osteopetrosis follows an autosomal recessive pattern, meaning an affected individual inherits one pathogenic variant from each parent. Carriers with a single pathogenic variant typically have no symptoms but can pass the variant to their children.
Why is TCIRG1 included on carrier screening panels?
TCIRG1 is included on carrier screening panels because pathogenic variants cause severe infantile osteopetrosis, a life-threatening condition that presents in early infancy. Identifying carriers before or during pregnancy allows informed reproductive planning and early diagnosis if a child is affected.
References
- Susani L, Pangrazio A, Sobacchi C. TCIRG1-dependent recessive osteopetrosis: mutation analysis, functional identification of the splicing defects, and in vitro rescue by U1 snRNA. Human mutation. 2004. PMID: 15300850
- Scimeca JC, Quincey D, Parrinello H. Novel mutations in the TCIRG1 gene encoding the a3 subunit of the vacuolar proton pump in patients affected by infantile malignant osteopetrosis. Human mutation. 2003. PMID: 12552563
- Nishi T, Forgac M. The vacuolar (H+)-ATPases--nature's most versatile proton pumps. Nature reviews. Molecular cell biology. 2002. PMID: 11836511
- Sobacchi C, Frattini A, Orchard P. The mutational spectrum of human malignant autosomal recessive osteopetrosis. Human molecular genetics. 2001. PMID: 11532986
- Frattini A, Orchard PJ, Sobacchi C. Defects in TCIRG1 subunit of the vacuolar proton pump are responsible for a subset of human autosomal recessive osteopetrosis. Nature genetics. 2000. PMID: 10888887
- Kornak U, Schulz A, Friedrich W. Mutations in the a3 subunit of the vacuolar H(+)-ATPase cause infantile malignant osteopetrosis. Human molecular genetics. 2000. PMID: 10942435