KREMEN1 Variants Associated with Ectodermal Dysplasia Impair Complex Formation of KREMEN1 with DKK1 and LRP6 and Attenuate WNT3A Response

Ectodermal dysplasia is a genetically and clinically heterogeneous condition that is caused by developmental defects of hair, teeth, nails, and certain glands. Biallelic KREMEN1 variants cause severe tooth agenesis and mild ectodermal features. KREMEN1 encodes a transmembrane receptor that binds DKK1 and LRP5/6 that are implicated in the WNT/β-catenin pathway. We report on 3 patients from 2 families with oligodontia and sparse scalp hair and eyebrows with the, to our knowledge, previously unreported homozygous KREMEN1 variants c.497G>A; p.(Gly166Asp) and c.136C>T; p.(Gln46∗). To gain insight into the functional consequences of KREMEN1 pathogenic variants, we ectopically expressed C-terminally Flag-tagged KREMEN1 wild-type and the protein variants Cys111Ser, Gly166Asp, Phe209Ser, and Phe258_Pro259del in human embryonic kidney 293T cells. KREMEN1 wild type was extensively N- and O-glycosylated, whereas the 4 protein variants showed a significant reduction in glycosylation, in particular of O-glycans. Ternary complex formation of all 4 KREMEN1 protein variants with DKK1 and LRP6 was reduced compared with that of wild type, whereas formation of the KREMEN1-LRP6 complex was not affected by the nonsynonymous variants. Primary fibroblasts from the 3 patients with KREMEN1 pathogenic variants showed a higher WNT pathway activity under starved culture condition that was followed by attenuated signaling response to WNT3A, suggesting a more general WNT pathway deregulation.