Hypophosphatemia Cell Models for Research

Hypophosphatemia is a condition characterized by abnormally low levels of phosphate in the blood, typically defined as serum phosphate < 2.5 mg/dL. It can be acute or chronic, with causes ranging from nutritional deficiencies to genetic disorders. The global prevalence is not well-defined, but it is common in hospitalized patients, affecting up to 5% of all inpatients and up to 80% of septic patients. Chronic hypophosphatemia can lead to rickets in children and osteomalacia in adults, causing bone pain, fractures, and muscle weakness. The clinical impact is significant, with increased morbidity and mortality in severe cases. The National Cancer Institute (NCI) does not track hypophosphatemia as a cancer, but it is a common complication in cancer patients, particularly those with bone metastases or receiving certain chemotherapies. The WHO recognizes hypophosphatemia as a significant metabolic disorder, but specific incidence data are limited.

Hypophosphatemia is an ideal model for studying phosphate homeostasis, bone metabolism, and kidney function. It involves well-defined genetic causes, such as mutations in FGF23, PHEX, DMP1, and SLC34A1, which are amenable to gene editing. Public datasets, such as those from the Genotype-Tissue Expression (GTEx) project and the Human Gene Mutation Database (HGMD), provide expression and mutation data. Open questions include the molecular mechanisms of FGF23 regulation and the interplay between phosphate and vitamin D metabolism. Gene-edited cell models allow researchers to dissect these pathways in vitro, providing a controlled system to study disease mechanisms and test therapeutic interventions.

While hypophosphatemia is not a cancer, it is associated with tumor-induced osteomalacia (TIO), where tumors secrete FGF23. The major pathways involved in phosphate regulation include:

1. FGF23 signaling: FGF23 is a hormone that promotes phosphate excretion by the kidneys. It binds to FGFR1 and Klotho co-receptor, activating the MAPK/ERK pathway, which downregulates sodium-phosphate cotransporters (NaPi-IIa) in the proximal tubule.

2. PHEX regulation: PHEX is a metalloprotease that normally inactivates FGF23. Mutations in PHEX lead to increased FGF23 levels, causing X-linked hypophosphatemia (XLH).

3. DMP1 and SOST: These proteins are involved in bone mineralization and also regulate FGF23 expression. Mutations in DMP1 cause autosomal recessive hypophosphatemic rickets.

4. Vitamin D metabolism: Phosphate and vitamin D are interconnected. Low phosphate stimulates 1-alpha-hydroxylase, increasing active vitamin D, which in turn increases intestinal phosphate absorption.

The following table summarizes high-frequency genetic alterations in hypophosphatemia, based on data from ClinVar and COSMIC (for tumor-induced cases):

Data from TCGA and COSMIC for tumor-induced osteomalacia show FGF23 overexpression in certain mesenchymal tumors.

The key signaling networks deregulated in hypophosphatemia include:

• • FGF23/FGFR/Klotho axis: This pathway is central to phosphate homeostasis. Overexpression of FGF23 or mutations in Klotho can lead to hypophosphatemia.
• • MAPK/ERK pathway: Activated by FGF23 binding, leading to downregulation of NaPi-IIa.
• • Wnt/beta-catenin pathway: Involved in bone mineralization and FGF23 regulation. Mutations in Wnt1 or LRP5 can affect phosphate metabolism.
• • Vitamin D receptor (VDR) signaling: Altered vitamin D metabolism can impact phosphate absorption.

Key nodes include FGF23, PHEX, DMP1, SLC34A1, and Klotho.

Common cell lines used in hypophosphatemia research include:

Organoids derived from kidney or bone can also be used to study phosphate handling in a more physiologically relevant context.

Animal models for hypophosphatemia include:

• • Hyp mouse: A mouse model of XLH with a large deletion in the PHEX gene.
• • FGF23 knockout mice: Exhibit hyperphosphatemia, but can be used to study phosphate regulation.
• • DMP1 knockout mice: Model for autosomal recessive hypophosphatemic rickets.
• • Tumor-induced osteomalacia models: Xenograft of tumors secreting FGF23 into immunodeficient mice.

These models are valuable for studying disease mechanisms and testing therapies.

CRISPR-based gene editing allows the creation of isogenic cell lines with specific mutations in genes like FGF23, PHEX, DMP1, or SLC34A1. For example:

• • A FGF23 knockout cell line can be used to study the effects of FGF23 loss on phosphate transport.
• • A PHEX knockout cell line can model XLH and be used to test drugs that inhibit FGF23.
• • A DMP1 knockout cell line can be used to study the role of DMP1 in FGF23 regulation.
• • A SLC34A1 knock-in cell line with a known pathogenic mutation can be used to study phosphate transport defects.

These gene-edited models are commercially available from various sources and are sequence-verified, ensuring reproducibility. They accelerate research by providing a controlled system to study disease mechanisms and screen potential therapeutics.

Knockout and knock-in cell lines are essential for validating the function of genes involved in hypophosphatemia. For example:

• • Knockout of FGF23 in a kidney cell line can confirm its role in regulating phosphate transporters.
• • Knock-in of a PHEX mutation can be used to study the impact on FGF23 processing.
• • Knockout of SLC34A1 can help elucidate the mechanisms of phosphate transport.

These models allow researchers to perform gain- and loss-of-function studies in a controlled environment.

Isogenic cell line pairs (e.g., wild-type vs. PHEX knockout) can be used for high-throughput screening of compounds that modulate FGF23 expression or activity. These models are also useful for studying resistance to therapies, such as anti-FGF23 antibodies, by generating resistant cell lines through chronic exposure.

CRISPR-based synthetic lethality screens can identify genes that are essential in the context of specific mutations. For example, in cells with PHEX mutations, screening for genes that, when knocked out, cause cell death could reveal novel therapeutic targets. Additionally, gene-edited cells can be used to identify biomarkers of disease progression or drug response.

The following databases provide valuable data for hypophosphatemia research: