Wnt signaling is involved in almost every aspect of embryonic development and controls homeostatic self-renewal in tissue regeneration and repair. It is an essential element during development of ectodermally-derived organs, including hair, teeth, skin, nails, and sweat glands (Nusse & Clevers, 2017). Genetic variants in the WNT signaling pathway, including WNT10A, WNT10B, KREMEN1, AXIN2, ANTXR1, DKK1, APC, and FZD6, lead to impaired WNT signaling and have been implicated in developmental disorders of ectodermal structures. These include autosomal dominant or autosomal recessive ectodermal dysplasias and isolated tooth agenesis (Doolan et al., 2021).

The WNT/β-catenin signaling pathway is initiated when a WNT protein binds to a FZD receptor and a coreceptor, LDL receptor-related protein 5 or 6 (LRP5/6) (Chu et al., 2013, MacDonald and He, 2012, Tamai et al., 2000), forming a WNT-FZD-LRP5/6 trimeric complex. WNT/β-catenin signaling is finely tuned as the result of interactions of WNTs and LRP6 with other proteins and phospholipids and through phosphorylation of LRP6 (Jeong and Jho, 2021). Defective WNT-FZD-LRP5/6 complexes, caused by genetic variants in LRP5/6 and WNT proteins, have been shown to impair WNT signaling and result in developmental disorders (Huang et al., 2021, MacDonald and He, 2012, Nusse and Clevers, 2017, Yu et al., 2021, Zhou et al., 2021). The WNT-FZD-LRP5/6 complex recruits Disshevelled (DVL) and AXIN through the intracellular domains of FZD and LRP5/6, leading to inhibition of phosphorylation and subsequent stabilization of the transcriptional activator β-catenin (MacDonald & He, 2012). In the presence of WNT proteins, the β-catenin destruction complex is recruited to the plasma membrane and becomes inactivated, leading to accumulation of β-catenin in the cytoplasm and nucleus, resulting in the activation of transcription of TCF/LEF and WNT responsive genes (Jeong and Jho, 2021, MacDonald and He, 2012).

LRP6 is a single-pass transmembrane receptor protein with multiple domains. The mouse Lrp6 and human LRP6 proteins share 98% identity, indicating that the protein is highly conserved during evolution. Lack of Arrow, a homolog to human LRP5 and LRP6 in Drosophila, has been shown to have an identical phenotype to that of Wingless (Wnt homolog) mutants (Wehrli et al., 2000). In addition, mice lacking Lrp6 have been shown to have phenotypes similar to those of Wnt mutants (Pinson et al., 2000). Overall, these studies in animals have demonstrated that LRP6 is a crucial element of WNT signaling.

The first 19 amino acids at the N-terminus of LRP6 are a signal peptide for export of the protein across the plasma membrane (Brown et al., 1998). The mature protein is generated by cleavage and removal of this signal peptide. LRP6 has an extracellular domain comprising four tandem (Tyr-Trp-Thr-Asp)-type β-propeller/epidermal growth factor (EGF) repeats followed by three LDLR type A repeats. A single LDLR β-propeller-EGF (PE) unit contains a six-bladed propeller interacting with the consecutive EGF repeat (Cheng et al., 2011, MacDonald and He, 2012). Interactions between the β-propeller-EGF structures are crucial for LRP6 biogenesis, maturation and transport to the plasma membrane. The functional structure of the entire LRP6 extracellular domain is a compact horseshoe platform configuration (Chen et al., 2011). The intracellular region contains five PPPS/TP (Pro-Pro-Pro-Ser/Thr-Pro) motifs (Tamai et al., 2004).

Mutations in LRP6 have been reported to be associated with a wide variety of defects, including coronary artery disease (Mani et al., 2007), neural tube defects (Lei et al., 2015), cleft lip/palate (Basha et al., 2018), osteoporosis (Williams & Insogna, 2009), and high bone mass (Brance et al., 2020). Recently tooth agenesis has been reported to be associated with mutations in LRP6 (Chu et al., 2021, Huang et al., 2021, Massink et al., 2015, Ockeloen et al., 2016, Wang et al., 2021, Whyte et al., 2019, Yu et al., 2021, Yu et al., 2019, Zhou et al., 2021).

Torus palatinus, torus mandibularis, and buccal exostoses have been reported to be associated with mutations in LRP5 and LRP6 (Rickels et al., 2005, Whyte et al., 2019). Such oral exostoses are benign intraoral osseous outgrowths of mature bone. They are not present in children but their occurrence is associated with increasing age. Torus palatinus is located in the midline of the hard palate, while torus mandibularis, the most common oral exostosis, is located on the lingual surface of the mandible. Buccal exostoses are bony outgrowths on the buccal aspects of the maxilla or mandible. Histologically oral exostoses consist of compact bone (Auškalnis et al., 2015, Jainkittivong and Langlais, 2000). A study in twins showed no difference of prevalence between men and women and very good concordance between monozygotic and moderate concordance between dizygotic twins. Inheritance appeared to be autosomal dominant (Auškalnis et al., 2015).

Here we report on five different LRP6 mutations in 14 patients from eight families with oral exostoses and dental anomalies. Each patient carried a heterozygous missense or nonsense mutation in LRP6. Two missense mutations have been reported in patients with coronary artery disease. Three of the five mutations are considered novel because they have not been previously reported to be pathogenic. Besides tooth agenesis and oral exostoses, our study shows, for the first time that mutations in LRP6 are also associated with mesiodens (a supernumerary tooth in the midline of the premaxilla), fusion of teeth, odontomas, microdontia, maldevelopment of roots, including long roots, molars with unseparated roots, and taurodontism (an elongation of the pulp chamber with the furcation area being displaced toward the apex of the root).