Christine M. Laine MD PhD, Maija Wessman PhD, Sanna Toiviainen-Salo MD PhD, Mari A. Kaunisto PhD, Mervi K. Mäyränpää MD PhD, Tero Laine MD PhD, Minna Pekkinen PhD, Heikki Kröger MD PhD, Ville-Valtteri Välimäki MD PhD, Matti J. Välimäki MD PhD, Anna-Elina Lehesjoki MD PhD, and Outi Mäkitie MD PhD
Genetic factors play an important role in the development of osteoporosis. Several monogenic forms of osteoporosis have been recognized; most recently an X-chromosomal form resulting from mutations in the gene encoding plastin 3 (PLS3). PLS3 is a protein involved in actin bundle formation in the cytoskeleton. We present a large family with early-onset osteoporosis and X-linked inheritance. Phenotyping was performed on 19 family members and whole-exome sequencing on seven family members; five with a diagnosis of early-onset osteoporosis and two with normal bone parameters. Osteoporosis had its onset in childhood and was characterized by recurrent peripheral fractures, low BMD, vertebral compression fractures, and significant height loss in adulthood. Males were in general more severely affected than females. Bone histomorphometry findings in four males and one female showed severe trabecular osteoporosis, low amount of osteoid and decreased mineral apposition rate indicating impaired bone formation; resorption parameters were increased in some. All affected subjects shared a single base substitution (c.73-24T > A) in intron 2 of PLS3 on Xq23. The mutation, confirmed by Sanger sequencing, segregated according to the skeletal phenotype. The mutation introduces a new acceptor splice site with a predicted splice score of 0.99 and thereby, as confirmed by cDNA sequencing, induces the insertion of 22 bases between exons 2 and 3, causing a frameshift and premature termination of mRNA translation (p.Asp25Alafs*17). The mutation affects the first N-terminal calcium-binding EF-hand domain and abolishes all calcium- and actin-binding domains of the protein. Our results confirm the role of PLS3 mutations in early-onset osteoporosis. The mechanism whereby PLS3 affects bone health is unclear, but it may be linked to osteocyte dendrite function and skeletal mechanosensing. Future studies are needed to elucidate the role of PLS3 in osteoporosis and to define optimal treatment.