Porokeratosis Cell Models for Research

Porokeratosis is a group of rare, chronic, progressive keratinization disorders characterized by annular plaques with a distinct histopathological feature known as the cornoid lamella. The exact global incidence is not well established, but it is estimated to affect approximately 0.1% to 0.34% of the population in certain regions, with a higher prevalence in Asian populations. The disease can manifest in various clinical forms, including disseminated superficial actinic porokeratosis (DSAP), porokeratosis of Mibelli, linear porokeratosis, and giant porokeratosis. Although porokeratosis is generally benign, there is a well-documented risk of malignant transformation, with squamous cell carcinoma (SCC) developing in up to 7.5% of cases, particularly in long-standing or widespread lesions. The risk of malignant transformation is higher in immunocompromised patients and in lesions that are large, chronic, or located on the lower extremities. The 5-year survival for porokeratosis-associated SCC is not distinctly reported, but for cutaneous SCC overall, the 5-year survival is approximately 95% for localized disease, dropping to around 50% for metastatic disease, according to NCI data. Key risk factors include ultraviolet (UV) radiation exposure, immunosuppression, and genetic predisposition, with mutations in mevalonate pathway genes being the most significant.

Porokeratosis serves as an excellent model for studying the mevalonate pathway, cholesterol biosynthesis, and their role in keratinocyte differentiation and proliferation. The disease is primarily caused by loss-of-function mutations in genes encoding enzymes of the mevalonate pathway, such as mevalonate kinase (MVK), phosphomevalonate kinase (PMVK), mevalonate diphosphate decarboxylase (MVD), and farnesyl diphosphate synthase (FDPS). These genes are also implicated in other disorders, such as mevalonate kinase deficiency (MKD), making porokeratosis a valuable model for understanding the broader implications of mevalonate pathway dysfunction. Public datasets, including those from the NCBI Gene and ClinVar, provide extensive information on pathogenic variants. Open questions remain regarding the precise molecular mechanisms linking mevalonate pathway defects to the characteristic histopathological features and the increased risk of malignant transformation. Gene-edited cell models, such as MVK knockout keratinocyte lines, are essential tools for dissecting these mechanisms and for developing targeted therapies.

The pathogenesis of porokeratosis is primarily driven by defects in the mevalonate pathway, which is essential for cholesterol biosynthesis and the synthesis of isoprenoids, such as farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP). These isoprenoids are critical for the post-translational prenylation of small GTPases, including Ras and Rho, which regulate cell proliferation, differentiation, and survival. The key steps in the pathway are:

1. Mevalonate kinase (MVK) converts mevalonic acid to 5-phosphomevalonate.

2. Phosphomevalonate kinase (PMVK) adds another phosphate to produce 5-pyrophosphomevalonate.

3. Mevalonate diphosphate decarboxylase (MVD) decarboxylates 5-pyrophosphomevalonate to isopentenyl pyrophosphate (IPP).

4. IPP is isomerized to dimethylallyl pyrophosphate (DMAPP), which then condenses to form geranyl pyrophosphate (GPP) and farnesyl pyrophosphate (FPP).

Loss-of-function mutations in any of these enzymes lead to reduced levels of FPP and GGPP, impairing protein prenylation. This results in altered signaling through the Ras/MAPK and PI3K/AKT pathways, which are frequently upregulated in porokeratosis lesions, contributing to hyperproliferation and the development of squamous cell carcinoma. Additionally, defective cholesterol biosynthesis may disrupt lipid raft formation, affecting membrane signaling and cell adhesion.

Genetic alterations in porokeratosis are predominantly germline mutations in mevalonate pathway genes, but somatic mutations can also occur in the context of malignant transformation. The following table summarizes the most frequently mutated genes, based on data from ClinVar and COSMIC:

These mutations are typically heterozygous, with a dominant-negative or haploinsufficiency effect. In malignant lesions, additional somatic mutations in TP53, RAS, and other oncogenes have been reported, similar to those found in sporadic cutaneous squamous cell carcinoma.

The mevalonate pathway defects in porokeratosis lead to deregulation of several key signaling networks:

• • Ras/MAPK pathway: Reduced prenylation of Ras leads to aberrant activation of the MAPK cascade, promoting cell proliferation and survival.
• • PI3K/AKT pathway: Impaired prenylation of Rho and Rac can lead to increased PI3K/AKT signaling, enhancing cell growth and resistance to apoptosis.
• • Hedgehog signaling: Cholesterol is a critical modulator of Hedgehog (Hh) signaling; reduced cholesterol levels may alter Hh pathway activity, affecting cell differentiation.
• • Wnt/β-catenin pathway: Cholesterol and isoprenoids influence Wnt signaling; defects may lead to β-catenin stabilization and increased transcription of proliferative genes.
• • Inflammatory and immune pathways: Mevalonate pathway dysfunction can trigger the unfolded protein response (UPR) and activate NF-κB, leading to chronic inflammation and immune cell infiltration, which may contribute to malignant transformation.

Several cell lines and organoid models are used to study porokeratosis. The following table lists commonly used cell lines and their key characteristics:

Organoid models derived from patient biopsies or gene-edited keratinocytes offer a more physiologically relevant 3D architecture, allowing for the study of cell-cell interactions and drug responses. However, the lack of immune cells and stroma limits their use for studying inflammatory aspects of the disease.

Animal models for porokeratosis are limited but include:

• • Genetically engineered mouse models (GEMMs): Mice with conditional knockout of Mvk in keratinocytes (using K14-Cre) exhibit skin abnormalities resembling porokeratosis, including hyperkeratosis and altered differentiation.
• • Patient-derived xenografts (PDX): Skin grafts from porokeratosis patients onto immunodeficient mice can maintain the disease phenotype and are useful for testing topical therapies.
• • Induced models: Topical application of cholesterol synthesis inhibitors (e.g., statins) on mouse skin can induce porokeratosis-like lesions, providing a rapid and reversible model for mechanistic studies.

CRISPR-based gene editing has enabled the creation of isogenic cell lines with specific mutations in mevalonate pathway genes. These models are invaluable for studying the functional consequences of mutations in a controlled genetic background. Examples include:

• • MVK knockout HaCaT cells: Generated by CRISPR-Cas9-mediated disruption of the MVK gene, these cells exhibit reduced mevalonate kinase activity and recapitulate the metabolic defects seen in patient keratinocytes.
• • PMVK knockout NHEK cells: Primary keratinocytes with PMVK knockout can be generated using CRISPR, allowing for the study of early differentiation defects.
• • MVD knock-in cell lines: Introduction of specific patient-derived mutations (e.g., MVD c.112C>T) into wild-type cells using HDR enables the study of dominant-negative effects.

These gene-edited models are commercially available from various sources, and sequence-verified clones are provided to ensure reproducibility. They are essential for drug screening, functional genomics, and target validation studies.

Gene-edited cell lines are powerful tools for functional genomics, allowing researchers to determine the role of specific genes in porokeratosis pathogenesis. For example:

• • MVK knockout cells can be used to assess the impact of mevalonate pathway deficiency on keratinocyte proliferation, differentiation, and apoptosis.
• • PMVK knockout cells help identify downstream effectors of the pathway, such as changes in gene expression profiles (RNA-seq) and protein prenylation patterns.
• • CRISPR activation (CRISPRa) or interference (CRISPRi) can be used to modulate the expression of mevalonate pathway genes in a dose-dependent manner, revealing threshold effects.

Isogenic pairs (wild-type vs. gene-edited) are ideal for high-throughput drug screening. For porokeratosis, these models can be used to:

• • Screen for compounds that restore mevalonate pathway activity or bypass the block, such as exogenous mevalonate or farnesol.
• • Test the efficacy of statins and other cholesterol-lowering drugs in modulating disease phenotype.
• • Investigate mechanisms of resistance to therapies, such as the development of SCC in porokeratosis lesions, by exposing gene-edited cells to chemotherapeutic agents and selecting for resistant clones.

CRISPR-based synthetic lethality screens can identify genes that, when silenced, are lethal in the context of mevalonate pathway deficiency. This approach can reveal novel therapeutic targets and biomarkers. For example:

• • A genome-wide CRISPR knockout screen in MVK-deficient cells can identify genes whose loss is synthetically lethal, such as those involved in alternative isoprenoid synthesis or cholesterol uptake.
• • Secreted proteins from gene-edited cells can be profiled to identify potential serum biomarkers for disease progression or malignant transformation.

The following public databases provide valuable data for porokeratosis research: