The SQSTM1 gene provides instructions for making a protein called p62. This protein plays an important role in bone remodeling, a normal process in which old bone is broken down and new bone is created to replace it. The p62 protein helps regulate this process through its role in a chemical signaling pathway that promotes the formation of osteoclasts. Osteoclasts are specialized cells that break down bone tissue during bone remodeling.
Studies suggest that p62 may have other functions in addition to its role in bone remodeling. It may be involved in recycling worn-out cell parts and unneeded proteins (autophagy), the self-destruction of cells (apoptosis), and the body's immune responses and inflammatory reactions.
More than 20 mutations in the SQSTM1 gene have been found to cause Paget disease of bone. Many SQSTM1 gene mutations change single protein building blocks (amino acids) in the p62 protein. The most common mutation replaces the amino acid proline with the amino acid leucine at protein position 392 (written as Pro392Leu or P392L).
Through a mechanism that is not well understood, SQSTM1 gene mutations appear to overactivate the chemical signaling pathway that promotes osteoclast formation. The increased signaling stimulates the production of too many osteoclasts and triggers these cells to break down bone abnormally. In people with Paget disease of bone, affected bone is broken down and replaced much faster than usual. When the new bone tissue grows, it is weaker and less organized than normal bone. These problems with bone remodeling cause certain bones to become unusually large, misshapen, and easily broken (fractured). It is unclear why the disease affects some bones but not others.
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