Multifocal heterotopic ossification in a man with germline variants of LIM Mineralization Protein‐1 (LMP‐1)

Abstract

AbstractA 54‐year‐old man with a history of unimelic, post‐traumatic multifocal heterotopic ossification (HO) and normal genetic analysis of ACVR1 and GNAS had variants of unknown significance (VUS) in PDLIM‐7 (PDZ and LIM Domain Protein 7), the gene encoding LMP‐1 (LIM Mineralization Protein‐1), an intracellular protein involved in the bone morphogenetic protein (BMP) pathway signaling and ossification. In order to determine if the LMP‐1 variants were plausibly responsible for the phenotype observed, a series of in vitro experiments were conducted. C2C12 cells were co‐transfected with a BMP‐responsive reporter as well as the LMP‐1 wildtype (wt) construct or the LMP‐1T161I or the LMP‐1D181G constructs (herein designated as LMP‐161 or LMP‐181) corresponding to the coding variants detected in the patient. A significantly increased BMP‐reporter activity was observed in LMP‐161 or LMP‐181 transfected cells compared to the wt cells. The LMP‐181 variant exhibited BMP‐reporter activity with a four‐fold increase over the LMP‐1 wt protein. Similarly, mouse pre‐osteoblastic MC3T3 cells transfected with the patient's LMP‐1 variants expressed higher levels of osteoblast markers both at mRNA and protein levels and preferentially mineralized when stimulated with recombinant BMP‐2 compared to control cells. Presently, there are no pathogenic variants of LMP‐1 known to induce HO in humans. Our findings suggest that the germline variants in LMP‐1 detected in our patient are plausibly related to his multifocal HO (LMP1‐related multifocal HO). Further observations will be required to firmly establish this gene–disease relationship.