Thrombocytopenia 2 (THC2) Cell Models for Research

Thrombocytopenia 2 (THC2) is a rare autosomal dominant disorder characterized by low platelet counts (thrombocytopenia) and a predisposition to myeloid malignancies. The exact prevalence is unknown, but it is estimated to affect fewer than 1 in 1,000,000 individuals. THC2 is caused by mutations in the ANKRD26 gene, which encodes a protein involved in the regulation of thrombopoietin (TPO) signaling. Patients typically present with mild to moderate thrombocytopenia, bleeding tendencies, and an increased risk of developing acute myeloid leukemia (AML) and other hematological cancers. The clinical impact is significant due to the risk of malignant transformation, which requires lifelong monitoring and management.

THC2 serves as an excellent model for studying the molecular mechanisms of thrombopoiesis and leukemogenesis. The disease is monogenic, making it amenable to gene editing and functional studies. Public datasets, such as those from the International Working Group for Myeloproliferative Neoplasms Research and Treatment, provide clinical and genetic data. Open questions include the precise role of ANKRD26 in platelet production and how mutations lead to leukemia. Gene-edited cell models are invaluable for dissecting these pathways and testing therapeutic interventions.

• • The primary pathway affected in THC2 is the thrombopoietin (TPO) signaling pathway. ANKRD26 acts as a negative regulator of TPO receptor (MPL) expression. Mutations in the 5' untranslated region (UTR) of ANKRD26 lead to increased expression of ANKRD26, which in turn downregulates MPL, impairing megakaryocyte differentiation and platelet production. Additionally, aberrant ANKRD26 expression may activate the MAPK/ERK pathway, contributing to leukemic transformation. The steps are:
• • TPO binds to MPL receptor.
• • Normal ANKRD26 expression limits MPL expression.
• • Mutant ANKRD26 overexpression reduces MPL levels.
• • Impaired MPL signaling leads to defective megakaryopoiesis.
• • Over time, additional mutations may activate oncogenic pathways, leading to AML.

Data from TCGA and COSMIC databases.

• • Key signaling networks deregulated in THC2 include:
• • TPO/MPL/JAK2/STAT pathway: Impaired signaling leads to thrombocytopenia.
• • MAPK/ERK pathway: Overactivated in leukemic cells, promoting proliferation.
• • PI3K/AKT pathway: Contributes to cell survival and resistance to apoptosis.
• • Wnt/β-catenin pathway: May be involved in leukemic stem cell maintenance.

Key nodes: MPL, JAK2, STAT5, ERK, AKT, β-catenin.

Organoids derived from patient iPSCs can recapitulate megakaryopoiesis and are useful for studying THC2.

• • Patient-derived xenografts (PDX): Immunodeficient mice engrafted with THC2 patient cells to study leukemia progression.
• • Genetically engineered mouse models (GEMM): Mice with Ankrd26 mutations mimicking human THC2.
• • Induced models: CRISPR-mediated knockout of Ankrd26 in mouse hematopoietic stem cells.

CRISPR-based gene editing allows the creation of isogenic cell lines with specific ANKRD26 mutations. For example, a knockout line with complete loss of ANKRD26 function can be used to study the effects of loss-of-function, while a knock-in line with a patient-specific 5' UTR mutation can model the overexpression phenotype. These models are sequence-verified and commercially available, accelerating research without the need for primary patient samples. They are essential for drug screening and functional validation.

Gene-edited cells enable the validation of ANKRD26 mutations in thrombopoiesis. For example, knocking out ANKRD26 in K562 cells can reveal its role in megakaryocytic differentiation. Knock-in of a mutant allele can confirm the gain-of-function effect on MPL downregulation. These models are used to identify downstream targets and pathways.

Isogenic pairs (wild-type vs. mutant) are used to screen compounds that restore MPL expression or inhibit MAPK signaling. Resistance mechanisms can be studied by exposing mutant cells to drugs and selecting for resistant clones. This helps in developing targeted therapies for THC2-associated leukemias.

CRISPR synthetic lethality screens can identify genes that are essential in ANKRD26-mutant cells but not in wild-type cells. This can lead to the discovery of novel biomarkers and therapeutic targets. For example, inhibiting a synthetic lethal partner may selectively kill mutant cells.