Genetic disorders of calcium, phosphorus, and bone homeostasis

Genetic Disorders of Calcium and Phosphorus Metabolism

In this review, we describe genetic mutations affecting metabolic pathways of calcium and phosphorus homeostasis. Calcium and phosphorus homeostasis has tight hormonal regulation by three major hormones: vitamin D, parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23). We describe the physiology and pathophysiology of disorders, their biochemical profile, clinical characteristics, diagnostics, and treatments.

Hiding in plain sight: Gene panel and genetic markers reveal 26-year undiagnosed tumor-induced osteomalacia of the rib concurrently misdiagnosed as X-linked hypophosphatemia

Tumor-induced osteomalacia (TIO), caused by phosphaturic mesenchymal tumors (PMTs), is a rare paraneoplastic syndrome characterized by frequent bone fractures, bone pain, muscle weakness, and affected gait. These tumors typically secrete high levels of Fibroblastic Growth Factor 23 (FGF23), a hormone which acts on the kidney to cause hypophosphatemia, ultimately impairing bone mineralization. In this case report, we present a 41-year-old female with FGF23-mediated hypophosphatemia with a 26-year delay in TIO diagnosis and a concurrent misdiagnosis of X-linked hypophosphatemic rickets (XLH). Given an absence of family history of hypophosphatemia, a 13-gene hypophosphatemia panel including XLH (PHEX gene) was performed and came back negative prompting a diagnostic search for a PMT causing TIO. A ⁶⁸Ga-DOTATATE PET/CT scan revealed the presence of a 9th right rib lesion, for which she underwent rib resection. The patient's laboratory values (notably serum phosphorus, calcium, and vitamin D) normalized, with FGF23 decreasing immediately after surgery, and symptoms resolving over the next three months. Chromogenic in situ hybridization (CISH) and RNA-sequencing of the tumor were positive for FGF23 (CISH) and the transcriptional marker FN1-FGFR1, a novel fusion gene between fibronectin (FN1) and Fibroblast Growth Factor Receptor 1 (FGFR1), previously determined to be present in the majority of TIO-associated tumors. This case demonstrates the notion that rare and diagnostically challenging disorders like TIO can be undiagnosed and/or misdiagnosed for many years, even by experienced clinicians and routine lab testing. It also underscores the power of novel tools available to clinicians such as gene panels, CISH, and RNA sequencing, and their ability to characterize TIO and its related tumors in the context of several phenotypically similar diseases.

Modulating phosphate consumption, a novel therapeutic approach for the control of cancer cell proliferation and tumorigenesis

Phosphorus, often in the form of inorganic phosphate (Pi), is critical to cellular function on many levels; it is required as an integral component of kinase signaling, in the formation and function of DNA and lipids, and energy metabolism in the form of ATP. Accordingly, crucial aspects of cell mitosis - such as DNA synthesis and ATP energy generation - elevate the cellular requirement for Pi, with rapidly dividing cells consuming increased levels. Mechanisms to sense, respond, acquire, accumulate, and potentially seek Pi have evolved to support highly proliferative cellular states such as injury and malignant transformation. As such, manipulating Pi availability to target rapidly dividing cells presents a novel strategy to reduce or prevent unrestrained cell growth. Currently, limited knowledge exists regarding how modulating Pi consumption by pre-cancerous cells might influence the initiation of aberrant growth during malignant transformation, and if reducing the bioavailability or suppressing Pi consumption by malignant cells could alter tumorigenesis. The concept of targeting Pi-regulated pathways and/or consumption by pre-cancerous or tumor cells represents a novel approach to cancer prevention and control, although current data remains insufficient as to rigorously assess the therapeutic value and physiological relevance of this strategy. With this review, we present a critical evaluation of the paradox of how an element critical to essential cellular functions can, when available in excess, influence and promote a cancer phenotype. Further, we conjecture how Pi manipulation could be utilized as a therapeutic intervention, either systemically or at the cell level, to ultimately suppress or treat cancer initiation and/or progression.