The loss of Profilin 1 causes early-onset Paget's disease of bone.

Paget's disease of bone (PDB) is a late-onset disorder frequently caused by mutations in theSQSTM1gene, leading to hyperactive osteoclasts and resulting in bone pain, deformities, and fractures. However, some more severe forms of PDB-negative forSQSTM1mutations-have been described, in which the disease degenerates into bone cancers and shows a poor prognosis. Osteosarcoma is the most frequent and aggressive tumor arising in PDB (OS/PDB), with a 5-year survival rate almost nil, but the underlying molecular mechanism is unknown. Here, we investigated an extended pedigree with 11 individuals affected by early onset and polyostotic PDB, mainly interesting the appendicular skeleton. Interestingly, three members also developed secondary osteosarcoma. We performed exome sequencing and identified a 4-bp deletion in thePFN1gene, resulting in the degradation of the mutant protein. Copy number screening on 218 PDB individuals of our biobank disclosed that four of them (2%) carry a germline heterozygous deletion ofPFN1. The identification of these subjects, who exhibit a particularly severe form of disease, emphasizes the diagnostic value of this genetic screening to identify PDB individuals predisposed to develop osteosarcoma. In fact, we detected allelic imbalance atPFN1locus also in 8 of 14 (57%) sporadic OS/PDB, further proving its causative role. in vitro experiments also confirmedPFN1involvement in this form of PDB. Indeed, CRISPR-Cas9-mediatedPfn1knockout in pre-osteoclasts resulted into enhanced osteoclast differentiation and resorption, with the formation of large osteoclasts never described before in PDB. In addition,Pfn1lacking pre-osteoblasts lost their differentiation capability and failed to efficiently mineralize bone. Moreover, they acquired features of malignant transformation, including loss of focal adhesions and increased invasion ability. In conclusion, these findings disclosePFN1haploinsufficiency as the pathological mechanism in OS/PDB. (c) 2020 American Society for Bone and Mineral Research.