Pyle disease - Functional validation of disease-causing missense variants in SFRP4/sFRP4

Pyle disease is a rare autosomal recessive bone dysplasia characterized by a modeling defect of the tubular bones resulting in metaphyseal widening and cortical thinning, and bone fragility with fractures. Bi-allelic truncating variants in SFRP4 have been described as the primary molecular cause for Pyle disease. One report described a family with Pyle disease with compound heterozygous missense variants in SFRP4, yet no functional studies were performed. SFRP4 encodes the secreted Frizzled related protein 4 (sFRP4), which acts as a negative regulator of WNT signaling through the binding with WNT ligands or Frizzled receptors. We describe an adult woman with clinical and radiographic features of Pyle disease including wide metaphyses (Erlenmeyer-flask deformity), cortical thinning and multiple fractures. Genetic testing led to the identification of a novel homozygous missense variant (c.314G>C, p.(Arg105Pro)) in SFRP4, which is localized in the WNT-binding domain of sFRP4, similar to the previously reported missense variants (p.(Ala54Asp); p.(Cys125Ser)). Luciferase reporter experiments demonstrated that the p.(Arg105Pro) variant impairs the inhibitory potential of sFRP4 on WNT/β-catenin signaling. Similar impairment was also observed for the previously reported p.(Ala54Asp) and p.(Cys125Ser) variants. We therefore confirm that missense variants in SFRP4 may have a similar detrimental effect on sFRP4 function as truncating variants. This expands the mutational spectrum of Pyle disease and may improve the diagnosis of future families with this rare disorder. It also highlights the critical role of the WNT-binding domain of sFRP4 to act as a WNT signaling inhibitor.