Lymphangioleiomyomatosis (LAM) Cell Models for Research

Lymphangioleiomyomatosis (LAM) is a rare, progressive, systemic disease that predominantly affects women, typically of childbearing age. The global incidence is estimated at 3-5 per million women, with a prevalence of about 6-10 per million. The disease is characterized by cystic lung destruction, lymphatic abnormalities, and renal angiomyolipomas. According to the WHO, LAM is classified as a low-grade malignant neoplasm with metastatic potential. The 10-year survival rate is approximately 80-90% with current therapies, but respiratory failure remains the leading cause of death. The disease can occur sporadically (S-LAM) or in association with tuberous sclerosis complex (TSC-LAM), with the latter accounting for about 30-40% of cases. Key risk factors include female gender, age, and genetic mutations in TSC1 or TSC2.

LAM serves as an excellent model for studying tumor suppressor genes, mTOR signaling, and hormonal influences on tumorigenesis. The disease is driven by loss-of-function mutations in TSC1 or TSC2, leading to constitutive activation of the mTOR pathway. This provides a clear genetic basis for mechanistic studies. Public datasets, such as those from the TCGA and GEO, offer transcriptomic and genomic data for LAM patient samples and cell lines. Open questions include the role of estrogen in disease progression, the metastatic mechanisms of LAM cells, and the development of targeted therapies beyond mTOR inhibitors. Gene-edited cell models are crucial for dissecting these pathways and validating therapeutic targets.

The primary pathway dysregulated in LAM is the PI3K/AKT/mTOR signaling cascade, which is constitutively activated due to loss of TSC1/TSC2 function. The steps involved are:

1. Loss of TSC1 or TSC2 function (via mutation or loss of heterozygosity).

2. Hyperactivation of Rheb (Ras homolog enriched in brain).

3. Activation of mTORC1 (mammalian target of rapamycin complex 1).

4. Increased phosphorylation of S6K1 and 4E-BP1, leading to enhanced protein synthesis and cell growth.

5. Upregulation of genes involved in cell cycle progression, angiogenesis, and metastasis.

Additionally, the Wnt/β-catenin pathway is implicated, with TSC2 loss leading to β-catenin stabilization and transcriptional activation of target genes. The MAPK/ERK pathway may also be activated via cross-talk with mTOR signaling.

Data from TCGA and COSMIC databases indicate that TSC2 mutations are the most frequent, with a small percentage of cases showing TSC1 mutations. Other alterations are rare and may contribute to disease progression.

Key signaling networks deregulated in LAM include:

• • PI3K/AKT/mTOR pathway: Central to LAM pathogenesis, with TSC1/TSC2 loss leading to constitutive activation.
• • Wnt/β-catenin pathway: TSC2 loss stabilizes β-catenin, promoting cell proliferation and epithelial-mesenchymal transition.
• • MAPK/ERK pathway: Cross-talk with mTOR signaling can lead to enhanced cell survival and proliferation.
• • Estrogen receptor signaling: Estrogen promotes LAM cell growth, possibly via activation of mTOR and other pathways.
• • Autophagy: TSC2 loss inhibits autophagy, contributing to cell survival and metabolic reprogramming.

Key nodes include mTORC1, S6K1, 4E-BP1, β-catenin, and ERK1/2.

Organoids derived from LAM patient samples are emerging as more physiologically relevant models, recapitulating the cystic structure and cellular heterogeneity. They can be used for drug screening and studying cell-cell interactions.

Animal models for LAM include:

• • Genetically engineered mouse models (GEMMs): Tsc2 heterozygous or conditional knockout mice develop renal and lung lesions, but not full LAM phenotype.
• • Patient-derived xenografts (PDX): LAM cells injected into immunodeficient mice can form tumors, but lung cysts are not reproduced.
• • Induced models: Administration of estrogen to Tsc2+/- mice enhances tumor growth, mimicking hormonal influence.
• • Rat models: Tsc2 mutant rats (Eker rat) develop renal tumors and are used for studying TSC-related diseases.

These models are useful for studying tumorigenesis and testing therapies, but they have limitations in recapitulating the full LAM phenotype.

CRISPR-based gene editing enables the creation of isogenic cell lines with precise mutations in TSC1 or TSC2. These models are invaluable for studying the molecular consequences of specific mutations and for drug discovery. For example:

• • TSC2 knockout cell lines: Generated by introducing frameshift mutations in TSC2, leading to loss of tuberin expression. These cells exhibit constitutive mTORC1 activation and can be used to screen for mTOR inhibitors.
• • TSC2 point mutation knock-in lines: Introduction of specific patient-derived mutations (e.g., R1459X) allows for studying genotype-phenotype correlations.
• • TSC1 knockout lines: Similar to TSC2, but less common.

These gene-edited models are commercially available from various sources and are sequence-verified, ensuring reproducibility. They accelerate research by providing consistent, isogenic backgrounds for functional studies.

Gene-edited cells are used to validate the function of genes implicated in LAM. For example:

• • TSC2 knockout lines are used to confirm the role of tuberin in regulating mTORC1 activity.
• • Knock-in of specific TSC2 mutations allows for assessing the impact on protein stability and downstream signaling.
• • CRISPR screens using libraries targeting genes in the mTOR pathway can identify novel modifiers of LAM cell growth.

These models provide a controlled system to study gene function and identify potential therapeutic targets.

Isogenic pairs (wild-type vs. TSC2 knockout) are ideal for high-throughput drug screening. They enable identification of compounds that selectively inhibit mutant cells while sparing normal cells. For example:

• • Screening for mTOR inhibitors (e.g., rapamycin analogs) using TSC2-null cells.
• • Testing combination therapies that target both mTOR and other pathways (e.g., autophagy inhibitors).
• • Modeling drug resistance by exposing TSC2-null cells to increasing concentrations of inhibitors and selecting resistant clones.

These approaches facilitate the development of more effective therapies for LAM.

CRISPR-based synthetic lethality screens can identify genes that are essential for survival in TSC2-null cells but not in wild-type cells. These genes may serve as novel therapeutic targets or biomarkers. For example:

• • Screening for genes that, when knocked out, selectively kill TSC2-null cells.
• • Identifying secreted proteins or metabolites that are differentially produced by TSC2-null cells, which could serve as biomarkers for disease progression.

Gene-edited cell models provide a powerful platform for discovering biomarkers and therapeutic targets.