When X Does Not Mark the Spot: Autosomal Dominant and Recessive Forms of Renal Hypophosphatemic Rickets and Osteomalacia

PURPOSE OF REVIEW: Conditions resulting in elevated fibroblast growth factor 23 (FGF23) cause hypophosphatemic rickets and osteomalacia. The most common of these is X-linked hypophosphatemia. In this review we will broadly discuss the other less common and clinically distinct forms of renal hypophosphatemia, with a focus on the autosomal dominant and autosomal recessive types. RECENT FINDINGS: Variants in multiple genes cause dominant (FGF23, SGK3, FGFR1), recessive (DMP1, ENPP1, FAM20C, INPPL1) or even somatic (NRAS, HRAS, GNAS, gene fusions) conditions of FGF23 excess, with important phenotypic differences. For example, in autosomal dominant hypophosphatemic rickets due to FGF23 variants, iron deficiency drives the phenotype, while ENPP1 variants cause phenotypes ranging from severe neonatal vascular calcifications to rickets or osteoporosis. Other gene abnormalities cause FGF23-independent hypophosphatemia, often involving kidney disease. Recognizing the different mechanisms and phenotypes of hypophosphatemic conditions is critical to prognosis, management and to developing more effective therapies.