Hyperphosphatemia Cell Models for Research

Hyperphosphatemia, defined as serum phosphate levels above 4.5 mg/dL, is a common electrolyte disorder, particularly in patients with chronic kidney disease (CKD). According to the World Health Organization (WHO), CKD affects approximately 10% of the global population, and hyperphosphatemia is a major complication in advanced stages. The National Cancer Institute (NCI) does not track hyperphosphatemia directly, but it is a significant comorbidity in cancer patients, especially those with bone metastases or tumor lysis syndrome. Hyperphosphatemia is associated with increased cardiovascular mortality, vascular calcification, and renal osteodystrophy. In CKD patients, the prevalence of hyperphosphatemia increases as glomerular filtration rate declines, affecting up to 70% of patients on dialysis. The condition is often asymptomatic but can lead to severe complications if untreated.

Hyperphosphatemia is an ideal model for studying phosphate homeostasis and related signaling pathways. The disease involves multiple organs, including the kidney, bone, and parathyroid glands, and is regulated by hormones such as FGF23, PTH, and vitamin D. Research models are essential to understand the molecular mechanisms of phosphate transport, hormonal regulation, and the pathological consequences of elevated phosphate levels. Public datasets, such as those from the Genotype-Tissue Expression (GTEx) project and the Gene Expression Omnibus (GEO), provide valuable resources for studying gene expression changes in hyperphosphatemia. Open questions include the precise role of phosphate in vascular calcification and the development of targeted therapies.

Hyperphosphatemia is not a cancer itself but is often associated with cancer and can promote tumor progression. The major pathways involved in phosphate homeostasis include:

• • FGF23-Klotho pathway: FGF23, secreted by osteocytes, binds to Klotho-FGFR complexes in the kidney, promoting phosphate excretion and suppressing 1,25-dihydroxyvitamin D synthesis.
• • PTH pathway: Parathyroid hormone (PTH) increases renal phosphate excretion by inhibiting sodium-phosphate cotransporters in the proximal tubule.
• • Vitamin D pathway: 1,25-dihydroxyvitamin D increases intestinal phosphate absorption and regulates FGF23 expression.
• • Sodium-phosphate cotransporters: The SLC34 family (NaPi-IIa, NaPi-IIb, NaPi-IIc) and SLC20 family (PiT-1, PiT-2) mediate phosphate uptake in the intestine and kidney.

In cancer, hyperphosphatemia can activate signaling pathways such as the PI3K/AKT and MAPK pathways, promoting cell proliferation and survival. Elevated phosphate levels can also induce epithelial-mesenchymal transition (EMT) and enhance metastatic potential.

While hyperphosphatemia is primarily an acquired condition, genetic mutations in phosphate-regulating genes can cause inherited forms of hyperphosphatemia. Key genes include:

Data from ClinVar and COSMIC indicate that these mutations are rare but have significant clinical impact. In cancer, somatic mutations in phosphate transporters or regulators may contribute to altered phosphate metabolism, but large-scale sequencing studies (TCGA) have not identified recurrent mutations in these genes. However, epigenetic alterations and dysregulated expression are more common.

Hyperphosphatemia disrupts several signaling networks:

• • FGF23 signaling: In hyperphosphatemia, FGF23 levels are often elevated as a compensatory mechanism, but in CKD, Klotho deficiency leads to FGF23 resistance, resulting in persistent hyperphosphatemia.
• • PTH signaling: Secondary hyperparathyroidism in CKD increases PTH levels, which attempt to increase phosphate excretion but are insufficient.
• • Vitamin D metabolism: Reduced renal 1α-hydroxylase activity leads to decreased active vitamin D, which can increase PTH and affect phosphate balance.
• • Wnt/β-catenin pathway: Phosphate can activate Wnt signaling, promoting vascular calcification.
• • PI3K/AKT pathway: Phosphate can activate this pathway in cancer cells, promoting survival and proliferation.

Key nodes include FGF23, Klotho, PTH, VDR, SLC34A1, SLC34A2, and SLC20A1.

Several cell lines are used to study phosphate metabolism and hyperphosphatemia:

Organoids derived from kidney or intestinal tissues can recapitulate phosphate transport and hormonal responses, providing a more physiologically relevant model.

Animal models for hyperphosphatemia include:

• • Diet-induced models: High-phosphate diet in rodents induces hyperphosphatemia and vascular calcification.
• • 5/6 nephrectomy model: Surgical removal of kidney mass induces CKD and hyperphosphatemia.
• • Genetically engineered mouse models (GEMM): Knockout of FGF23, Klotho, or SLC34A1 in mice leads to hyperphosphatemia.
• • Patient-derived xenografts (PDX): Used in cancer research to study the effects of hyperphosphatemia on tumor growth.

CRISPR gene editing enables the creation of isogenic cell lines with precise mutations in phosphate-regulating genes. These models are essential for studying gene function and drug response. Examples include:

• • FGF23 knockout cell lines: Generated in HK-2 or HEK293 cells to study the role of FGF23 in phosphate transport.
• • SLC34A1 knock-in cell lines: Introducing disease-associated mutations to study their effect on phosphate transport.
• • Klotho knockout cell lines: To model FGF23 resistance.

These gene-edited cell lines are commercially available and sequence-verified, ensuring reproducibility. They are used for drug screening, target validation, and mechanistic studies.

Gene-edited cell lines are used to validate the function of genes involved in phosphate homeostasis. For example, knocking out FGF23 in kidney cells can confirm its role in regulating phosphate transporter expression. Similarly, introducing specific mutations in SLC34A1 can help determine the impact on phosphate uptake. These models are essential for understanding the molecular basis of hyperphosphatemia and identifying potential therapeutic targets.

Isogenic cell line pairs (wild-type vs. knockout) are used in high-throughput drug screening to identify compounds that selectively target cells with specific genetic alterations. For instance, a FGF23 knockout cell line can be used to screen for drugs that bypass FGF23 signaling. Additionally, gene-edited cells can be used to study drug resistance mechanisms, such as the upregulation of alternative phosphate transporters.

CRISPR-based synthetic lethality screens can identify genes that are essential for survival in the context of hyperphosphatemia. By knocking out genes in a high-phosphate environment, researchers can discover novel biomarkers or therapeutic targets. For example, a screen in kidney cells might reveal that certain phosphate transporters are critical for cell survival under phosphate overload, making them potential drug targets.