Pathogenesis of LRP6 High Bone Mass

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Project Summary LRP5 encodes low-density lipoprotein receptor-related protein 5 (LRP5). When LRP5 with a Frizzled receptor join on the surface of an osteoblast (OB) and bind a member of the Wnt family of ligands, canonical Wnt/β-catenin signaling occurs and increases bone formation. Eleven heterozygous gain-of-function missense mutations within LRP5 are known to prevent the LRP5 inhibitory ligands sclerostin and dickkopf1 from attaching to LRP5's first β-propeller, and thereby explain the rare autosomal dominant (AD) skeletal disorder “high bone mass” (HBM). LRP6 is a cognate co-receptor of LRP5 and similarly controls Wnt signaling in osteoblasts, yet the consequences of increased LRP6-mediated signaling remained unknown, until now. We investigated two multi-generational American families manifesting the clinical and routine laboratory features of LRP5 HBM but without an LRP5 defect and instead carrying a heterozygous LRP6 missense mutation (c.602C>T, p.A201V or c.553A>C, p.N185H). In both families, the LRP6 mutation co-segregated with striking generalized osteosclerosis and hyperostosis. Clinical features shared by the seven LRP6 HBM family members and ten LRP5 HBM patients included a broad jaw, torus palatinus, teeth encased in bone and, reportedly, resistance to fracturing and inability to float in water. However, there were significant clinical differences between our LRP5 and LRP6 families. DXA mean BMD Z-scores in LRP6 HBM versus LRP5 HBM were somewhat higher at the lumbar spine, and increased with age only in the LRP6 HBM families. Absence of adult maxillary lateral incisors was reported by some members in both LRP6 HBM families, but was not noted in LRP5 HBM. Hence, we have discovered mutations of LRP6 that cause a dento-osseous disorder similar to LRP5 HBM, but with some differences. We want to elucidate the mechanism of action for these LRP5 and LRP6 HBM mutations in both OBs and osteoclasts (OCs), since recent data shows that WNT signaling is important in inhibiting OC development and function. We propose to make induced pluripotent stem cells (iPSCs) from LRP5 and LRP6 HBM patient-derived cells (either kidney cells from urine or blood leukocytes). We will then compare and contrast OB development and function using LRP5 and LRP6 patient-derived iPSCs, along with control iPSCs. We will also assess Wnt signaling in OBs, for both mutant cell types to look for subtle differences in the mechanism of disease. We will also assess the impact of LRP5 and LRP6 mutations on OC development and function using patient-derived iPSCs, and initiate Wnt signaling studies in both mutant cell types. Our findings will lead to a better understanding of WNT signaling in bone development and turnover, and help guide development of better treatments for human diseases of both high and low (i.e. osteoporosis) bone mass.