LRP6 and osteoarthritis: For example, a loss-of-function mutation in Lrp5 causes OPPG syndrome, a disorder involving low bone mass[12], whereas Lrp6 deficiency (Lrp6-/-) in mice is an embryonic lethal disorder[14], and a heterozygous loss-of-function mutation in Lrp6 (Lrp6+/-) is associated with decreased β-catenin signaling within articular cartilage and increased degenerative joint disease after ligament and meniscus injury[31].