A Novel Case of Concomitant PHEX and ALPL Mutation In a Family With Rickets

Currently, no published cases report concomitant X-linked hypophosphatemia (XLH) and adult hypophosphatasia (HPP). Both diseases share clinical phenotypes that are almost indistinguishable. The correct diagnosis may be missed without a standardized laboratory and genetic testing approach. Pathogenic variants in the phosphate regulating endopeptidases homolog X-linked gene (PHEX) and the tissue-nonspecific alkaline phosphatase gene (ALPL) are genes that cause XLH and HPP, respectively. We describe a concomitant yet undescribed genetic pathogenic variant in a family. A 61-year-old woman was referred by orthopedic surgery for the presence of bilateral leg bowing and short stature during the assessment of knee surgery. The patient had a biochemical workup relevant for low serum phosphorus and 1,25-dihydroxy vitamin D and normal alkaline phosphatase (ALP). Genetic analysis revealed pathogenic variants in PHEX and ALPL. Her 42-year-old daughter shared identical symptoms and genetic variants with her mother. Both patients started conventional treatment for XLH with phosphorus and vitamin D, and the daughter later switched to burosumab-twza. Adult XLH and HPP may have similarities in clinical presentation but differ in some essential laboratory findings. Normal ALP levels helped direct our diagnosis toward XLH. However, the diagnosis was challenging due to the presence of concurrent variants in the genes involved. These variants illustrate the significant heterogeneity of the clinical expression.