Mutations in sFRP1 or sFRP4 are not a common cause of craniotubular hyperostosis

Sclerosing bone dysplasias are a heterogeneous group of rare diseases marked by increased BMD caused by either increased bone formation or by decreased bone resorption. In this study we have focused on craniotubular hyperostoses mainly affecting the long bones and the skull. Currently, there are three causative genes identified namely LRP5, SOST and LRP4. All three genes are involved in the canonical Wnt signalling pathway. These findings support the role of this pathway in regulating bone formation. The secreted Frizzled related proteins (sFRPs) can modulate the Wnt signalling pathway by binding to Wnt ligands or Frizzled receptors. Studies using mice showed that two members of this family, sFRP1 and sFRP4, have an important effect on bone formation. Sfrp1-/- mice have increased BMD values especially after peak BMD was reached. On the contrary, sfrp4 overexpression mice exhibit reduced BMD. Therefore, we selected sFRP1 and sFRP4, two members of the secreted Frizzled related protein (sFRP) family, as candidate genes for mutation analysis in patients with craniotubular hyperostosis. Using Sanger sequencing we screened the exons and intron/exon boundaries of sFRP1 and sFRP4 in 53 patients. In all patients mutations in LRP5, SOST and LRP4 were excluded. We identified two unknown heterozygous variants both in sFRP1. The first variant in sFRP1 is an intronic variant which, according to prediction programs, does not affect the splicing of the gene. The second variant (p.Trp131Arg/-) was identified in a young boy whose healthy mother does not carry the variant. In conclusion, our studies indicate that mutations neither in sFRP1 nor in sFRP4 are a common cause of craniotubular hyperostoses. As a consequence, further research will be necessary to identify the disease causing gene(s) in this group of patients.