# Excess fibroblast growth factor 23 in alcoholic osteomalacia is derived from the bone

## TL;DR

This study shows that excess FGF23 in alcoholic osteomalacia comes from the bone, not other organs, using a new sensitive imaging method.

## Contribution

The study introduces a novel next-generation immunohistochemistry method to trace FGF23 origin in alcohol-induced osteomalacia.

## Key findings

- Bone-derived FGF23 is responsible for alcohol-induced hypophosphatemic osteomalacia.
- Next-generation IHC with PIDs enables precise quantification of FGF23 in bone tissue.
- High PID counts in bone samples indicate orthotopic FGF23 overproduction, while low counts suggest ectopic sources.

## Abstract

Excess fibroblast growth factor 23 (FGF23), a mature osteocyte-derived phosphaturic hormone, causes chronic hypophosphatemic osteomalacia in adults. This rare condition was recently reported in 2 alcoholic patients, with marked improvement upon cessation of alcohol consumption, suggesting a link between alcohol and FGF23-related hypophosphatemia within the highly limited cases. This study aimed to investigate whether the source of excess FGF23 in alcohol-induced FGF23-related hypophosphatemic osteomalacia is the bone or the other organs. To achieve this goal, an immunohistochemical approach for the bone obtained from a patient was employed. Initial attempts at quantifying FGF23 in the bone using conventional immunohistochemistry (IHC) faced issues in quantifiability and sensitivity for low FGF23 expression levels. Therefore, next-generation IHC with phosphor-integrated dots (PIDs) was applied, which enabled the quantification of FGF23 expression in the bone across a broad range. Preliminary analyses using IHC with PIDs on normal bone samples (n = 12) provided a reference level (154.5 PID particles per cell). IHC with PIDs quantified suppressed physiological FGF23 expression in the bone samples from 3 patients with tumor-induced osteomalacia, where FGF23 is oversecreted from a tumor (13.6 PID particles per cell). Subsequently, bone samples obtained from a 70-yr-old male with alcohol-induced FGF23-related hypophosphatemic osteomalacia were analyzed, showing a higher number of PID particles per cell (199.4 PID particles per cell) than the reference level. This study suggests that orthotopic, bone-derived FGF23 is implicated in alcohol-induced FGF23-related hypophosphatemic osteomalacia. Furthermore, the study also demonstrated that highly sensitive IHC with PIDs could aid in the differential diagnosis of FGF23-related hypophosphatemia of unknown origin. Specifically, a bone sample with a low number of PID particles per cell indicates an excess ectopic secretion of FGF23; a bone sample with a normal to high number of PID particles per cell indicates an excess orthotopic secretion of FGF23.