TBX6-associated congenital scoliosis (TACS) is a congenital axial-skeleton malformation in which vertebral mis-segmentation — characteristically hemivertebrae — produces congenital scoliosis. It is the most common defined genetic cause of congenital scoliosis and follows a distinctive compound inheritance model: a rare loss-of-function (null) TBX6 allele — frequently a recurrent 16p11.2 deletion encompassing TBX6 — in trans with a common hypomorphic TBX6 risk haplotype, so that TBX6 dosage falls below the threshold required for normal somite formation while a heterozygous null alone is insufficient. TBX6 is a T-box transcription factor of the paraxial mesoderm / determination wavefront that is required to form somites, the serially repeated precursors of the vertebrae. TACS is filed in OMIM/MONDO under spondylocostal dysostosis 5 (OMIM:122600) but is clinically a milder, often more localized congenital scoliosis than classic recessive spondylocostal dysostosis. It conforms to the axial-skeleton serial-homology module across its wavefront, somite-formation, and vertebral-malsegmentation nodes.
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name: TBX6-Associated Congenital Scoliosis
creation_date: "2026-06-29T00:00:00Z"
category: Mendelian
description: >-
TBX6-associated congenital scoliosis (TACS) is a congenital axial-skeleton
malformation in which vertebral mis-segmentation — characteristically
hemivertebrae — produces congenital scoliosis. It is the most common defined
genetic cause of congenital scoliosis and follows a distinctive compound
inheritance model: a rare loss-of-function (null) TBX6 allele — frequently a
recurrent 16p11.2 deletion encompassing TBX6 — in trans with a common
hypomorphic TBX6 risk haplotype, so that TBX6 dosage falls below the threshold
required for normal somite formation while a heterozygous null alone is
insufficient. TBX6 is a T-box transcription factor of the paraxial mesoderm /
determination wavefront that is required to form somites, the serially
repeated precursors of the vertebrae. TACS is filed in OMIM/MONDO under
spondylocostal dysostosis 5 (OMIM:122600) but is clinically a milder, often
more localized congenital scoliosis than classic recessive spondylocostal
dysostosis. It conforms to the axial-skeleton serial-homology module across
its wavefront, somite-formation, and vertebral-malsegmentation nodes.
disease_term:
preferred_term: TBX6-associated congenital scoliosis
term:
id: MONDO:0007389
label: spondylocostal dysostosis 5
parents:
- Vertebral Malformation Disorders
inheritance:
- name: Compound Inheritance (Null Allele plus Hypomorphic Haplotype)
description: >-
A rare TBX6 null allele (often a 16p11.2 deletion) in trans with a common
hypomorphic TBX6 risk haplotype; a heterozygous null alone is insufficient,
so the phenotype reflects compound TBX6 dosage insufficiency.
evidence:
- reference: PMID:25564734
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: Compound inheritance of a rare null mutation and a hypomorphic allele of TBX6 accounted for up to 11% of congenital scoliosis cases in the
explanation: >-
Establishes the compound (null-plus-hypomorphic) TBX6 inheritance model
and its substantial contribution to congenital scoliosis.
prevalence:
- population: Sporadic congenital scoliosis (Han Chinese series)
percentage: ~11%
notes: >-
TBX6 null variants plus the common hypomorphic haplotype accounted for up to
11% of sporadic congenital scoliosis cases in the original Han Chinese
series; TACS is the most common defined monogenic-dosage cause of congenital
scoliosis identified to date.
evidence:
- reference: PMID:25564734
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: we identified a total of 17 heterozygous TBX6 null mutations in the 161
explanation: >-
Reports 17 TBX6 null mutations among 161 persons with sporadic congenital
scoliosis (~11%).
pathophysiology:
- name: TBX6 Dosage Insufficiency at the Determination Wavefront
conforms_to: "axial_segmentation_serial_homology#Segmentation Clock and Wavefront Dysfunction"
description: >-
TBX6 is a T-box transcription factor of the paraxial mesoderm and
determination wavefront required for somite formation. Compound inheritance
of a rare null allele and a common hypomorphic haplotype lowers functional
TBX6 below the threshold needed to drive normal segmentation; a heterozygous
null allele alone is insufficient, indicating a dosage-dependent wavefront
defect.
role: trigger
genes:
- preferred_term: TBX6
term:
id: hgnc:11605
label: TBX6
cell_types:
- preferred_term: Presomitic (Paraxial) Mesoderm Cell
term:
id: CL:0011007
label: paraxial cell
biological_processes:
- preferred_term: Somitogenesis
term:
id: GO:0001756
label: somitogenesis
modifier: ABNORMAL
evidence:
- reference: PMID:25564734
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: heterozygous TBX6 null mutation is insufficient to cause congenital scoliosis
explanation: >-
Shows a single null allele is insufficient, establishing the
dosage-dependent TBX6 wavefront mechanism.
- reference: PMID:25564734
supports: SUPPORT
evidence_source: IN_VITRO
snippet: In vitro functional assays suggested that the risk haplotype is a hypomorphic allele.
explanation: >-
Confirms the common risk haplotype is hypomorphic, the second hit lowering
TBX6 dosage. Evidence source is IN_VITRO (functional assays).
downstream:
- target: Disrupted Paraxial Mesoderm Somite Formation
description: Reduced TBX6 dosage impairs somite formation in the paraxial mesoderm.
causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
intermediate_mechanisms:
- sub-threshold TBX6 transcriptional output in the presomitic mesoderm
- name: Disrupted Paraxial Mesoderm Somite Formation
conforms_to: "axial_segmentation_serial_homology#Disrupted Somite Boundary Formation"
description: >-
Sub-threshold TBX6 activity corrupts the formation and boundary positioning
of somites in the presomitic mesoderm, mis-specifying the metameric template
from which the vertebrae derive.
role: central_effector
cell_types:
- preferred_term: Presomitic (Paraxial) Mesoderm Cell
term:
id: CL:0011007
label: paraxial cell
biological_processes:
- preferred_term: Segmentation
term:
id: GO:0035282
label: segmentation
modifier: ABNORMAL
evidence:
- reference: PMID:17965051
supports: SUPPORT
evidence_source: MODEL_ORGANISM
snippet: The FGF pathway establishes a posterior-to-anterior signaling gradient in the presomitic mesoderm (PSM), which controls cell maturation and is involved in the positioning of segmental boundaries.
explanation: >-
Describes the PSM determination wavefront positioning segmental
boundaries, the somite-formation step disrupted by reduced TBX6 dosage.
Evidence source is MODEL_ORGANISM (mouse).
downstream:
- target: Hemivertebrae and Congenital Scoliosis
description: Mis-formed somites yield hemivertebrae and a curved spine.
causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
intermediate_mechanisms:
- asymmetric/incomplete vertebral body formation from mis-specified somites
- name: Hemivertebrae and Congenital Scoliosis
conforms_to: "axial_segmentation_serial_homology#Vertebral and Costal Malsegmentation"
description: >-
The somite-formation defect manifests as vertebral malsegmentation —
characteristically hemivertebrae — that produces a congenital lateral
curvature of the spine. Because the vertebrae are serially homologous somite
derivatives, one or more segments are malformed.
role: consequence
cell_types:
- preferred_term: Chondrocyte
term:
id: CL:0000138
label: chondrocyte
- preferred_term: Osteoblast
term:
id: CL:0000062
label: osteoblast
biological_processes:
- preferred_term: Embryonic Skeletal System Morphogenesis
term:
id: GO:0048704
label: embryonic skeletal system morphogenesis
modifier: ABNORMAL
evidence:
- reference: PMID:25564734
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: Hemivertebrae are characteristic of TBX6-associated congenital scoliosis.
explanation: >-
Identifies hemivertebrae as the characteristic vertebral malsegmentation
of TACS, the module consequence.
- reference: PMID:25564734
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: Congenital scoliosis is a common type of vertebral malformation.
explanation: >-
Frames congenital scoliosis as a vertebral malformation, the clinical
endpoint of the malsegmentation.
phenotypes:
- name: Hemivertebrae
description: >-
Hemivertebrae (incomplete, wedge-shaped vertebral bodies) are the
characteristic vertebral malsegmentation of TACS.
phenotype_term:
preferred_term: Hemivertebrae
term:
id: HP:0002937
label: Hemivertebrae
evidence:
- reference: PMID:25564734
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: Hemivertebrae are characteristic of TBX6-associated congenital scoliosis.
explanation: Hemivertebrae are the characteristic vertebral defect of TACS.
- name: Congenital Scoliosis
description: >-
Congenital lateral spinal curvature resulting from the vertebral
malsegmentation.
phenotype_term:
preferred_term: Scoliosis
term:
id: HP:0002650
label: Scoliosis
evidence:
- reference: PMID:25564734
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: Congenital scoliosis is a common type of vertebral malformation.
explanation: TACS presents as congenital scoliosis, a vertebral malformation.
treatments:
- name: Scoliosis Surgical Management
description: >-
Management of congenital scoliosis ranges from observation and bracing to
growth-friendly instrumentation or corrective/fusion surgery for progressive
curves; hemivertebra resection may be indicated for focal deformity.
treatment_term:
preferred_term: orthopedic surgical procedure
term:
id: NCIT:C16186
label: Orthopedic Surgical Procedure