Thymoma: curation-oriented mechanistic summary

Modeling decisions applied from issue #1198

The dismech curation unit is the disease-level mechanism graph, not every oncology ontology subclass. Thymoma therefore remains one disease entry rather than separate pages for WHO types A, AB, B1, B2, B3, or invasion-status variants.
disease_term stays MONDO-first as MONDO:0006456 (thymoma).
The entry should also carry a disease-level NCIT mapping to NCIT:C3411 (Thymoma), because NCIT is the more oncology-native coding system for downstream cancer interoperability.
Subtypes are best represented as flat facet axes. For thymoma, the highest-yield axes are:
histological_pattern: Type A (NCIT:C6454), Type AB (NCIT:C6885), Type B1 (NCIT:C6887), Type B2 (NCIT:C6888), Type B3 (NCIT:C7997)
invasion_status: Encapsulated thymoma (NCIT:C7386), invasive thymoma (NCIT:C7904)
The NCIT-grounded subtype terms are ontology grounding only. They should not imply separate dismech pages or a “Not Yet Curated” renderer state.

Disease overview

Thymoma is a thymic epithelial tumor of the anterior mediastinum whose biology is unusual among solid tumors because the neoplasm also distorts thymic T-cell selection. The two dominant disease-level themes are therefore:

Tumor-cell intrinsic oncogenesis, especially the GTF2I-enriched, low-mutation-burden program of indolent type A/AB thymoma.
Tumor-microenvironment immune dysfunction, in which abnormal thymic epithelial architecture impairs central tolerance and promotes paraneoplastic autoimmunity.

Subtype biology

Type A / Type AB thymoma: indolent histotypes with the strongest GTF2I enrichment. PMID:24974848 reports that GTF2I mutation is detected in 82% of type A and 74% of type AB thymomas.
Type B1 / Type B2 thymoma: more cortical, lymphocyte-rich histotypes with increasing invasiveness and myasthenia gravis association.
Type B3 thymoma: epithelial-predominant thymoma with substantially greater local invasion and worse disease-free survival. PMID:15063231 reports a monotonic increase in neighboring-organ invasion across A → AB → B1 → B2 → B3.
Encapsulated vs invasive thymoma: a separate invasion-status axis is useful because clinical behavior spans indolent, organ-confined disease to locally invasive or metastatic disease even within the same broad disease entry (PMID:20207296).

Core mechanisms

1. GTF2I-driven thymic epithelial transformation

PMID:24974848: “We analyzed 28 thymic epithelial tumors (TETs) using next-generation sequencing and identified a missense mutation ... in GTF2I at high frequency in type A thymomas, a relatively indolent subtype.”
PMID:32034314: “Our findings identify GTF2I mutation as a new oncogenic driver that is responsible for transformation of thymic epithelial cells.”

Interpretation for YAML: this supports a disease node centered on thymic epithelial transformation, with subtype restriction to type A/AB thymoma and downstream linkage to proliferative and metabolic programs.

2. Metabolic stress adaptation downstream of mutant GTF2I

PMID:32034314: “Gtf2i L424H knockin cells exhibited cell transformation, aneuploidy, and increase tumor growth and survival under glucose deprivation or DNA damage.”
PMID:32034314: “Gtf2i mutation also increased the expression of several glycolytic enzymes, cyclooxygenase-2, and caused modifications of lipid metabolism.”

Interpretation for YAML: metabolic rewiring is worth its own atomic node rather than being bundled into the transformation node.

3. Reduced MHC class II antigen presentation

PMID:18567864: “expression levels of class II major histocompatibility complex (MHCII) genes are variably decreased in thymomas, most prominently in histological WHO types A and AB”

Interpretation for YAML: this directly supports an atomic node for reduced antigen presentation via MHC class II, especially in type A/AB thymoma.

4. AIRE loss in thymic epithelial cells

PMID:18567864: “expression of the autoimmune regulator (AIRE) gene is absent from approximately 95% of thymomas.”

Interpretation for YAML: AIRE loss should remain separate from reduced MHC II expression because both are central-tolerance defects but represent distinct molecular lesions.

5. Central tolerance failure with autoreactive T-cell escape

PMID:41098393: “Since thymomas are mainly composed of the cortex, with few medullae, MG may be caused by immature thymoma-derived T cells that fail to undergo negative selection and have not yet acquired sufficient self-tolerance.”
PMID:18567864: “Generation of autoreactive CD4(+) effector T cells and defective production of regulatory CD4(+) T cells inside thymomas contribute to the development of myasthenia gravis (MG) in >90% of MG(+) thymomas.”

Interpretation for YAML: negative-selection failure, autoreactive effector-T-cell generation, and defective Treg production belong in a linked but still atomic disease mechanism chain.

6. Neuromuscular autoantigen niche in MG-associated thymoma

PMID:41098393: “we identified neuromuscular medullary thymic epithelial cells (nmTECs) as neuromuscular antigen-expressing cell populations.”
PMID:41098393: “We observed spatial nmTEC colocalization and an immune niche, inferring an interaction and suggesting a pathological role of nmTECs in MG.”

Interpretation for YAML: this is a more specific downstream mechanism that helps explain why myasthenia gravis is the dominant autoimmune phenotype.

Hallmark phenotype profile

Anterior mediastinal mass: characteristic anatomic presentation (PMID:37761349).
Myasthenia gravis: dominant autoimmune phenotype (PMID:20207296).
Pure red cell aplasia: recurrent hematologic autoimmune complication (PMID:40983285).
Good syndrome and recurrent infections: thymoma-associated immunodeficiency phenotype with hypogammaglobulinemia and infection susceptibility (PMID:39180607).

Treatment implications

Complete resection / thymectomy remains the central intervention for localized disease (PMID:20207296; PMID:40983285).
Postoperative radiotherapy is mainly relevant for invasive disease or aggressive histology (PMID:20207296).
Platinum-anthracycline chemotherapy is the standard systemic approach for inoperable or metastatic thymoma (PMID:20207296).
Systemic immunosuppression remains necessary when autoimmune complications persist or newly arise after thymectomy (PMID:40983285).

Ontology grounding used in the YAML

Disease anchor: MONDO:0006456 thymoma
Disease-level NCIT mapping: NCIT:C3411 Thymoma
Histologic subtype grounding:
NCIT:C6454 Thymoma Type A
NCIT:C6885 Thymoma Type AB
NCIT:C6887 Thymoma Type B1
NCIT:C6888 Thymoma Type B2
NCIT:C7997 Thymoma Type B3
Invasion-status subtype grounding:
NCIT:C7386 Encapsulated Thymoma
NCIT:C7904 Invasive Malignant Thymoma

NCIT subtype identifiers were checked against the EVS neoplasm hierarchy page for thymoma subclasses while keeping the disease anchor MONDO-first in the dismech entry.