Abstract
Long Interspersed Element–1 (LINE-1, L1, L1TD1) are 6000 bp long retroviral elements and constitutes 21% of the human genome. Less than 100 L1 elements have retained the ability of autonomous retrotransposition. LINE-1 is an off-target of nucleoside reverse transcriptase inhibitors (NRTI) used in antiretroviral therapies whose feared major com-plication is marked reduction in bone mineral density (BMD) leading to osteoporosis. The role of LINE-1 in bone development and in disease, e.g. primary postmenopausal osteopo-rosis is unexplored. Bone marrow-derived mesenchymal stem cells (MSC) from healthy donors were differentiated ex vivo to mature osteoblasts and adipocytes. Gene expression, mineralization, lipid accumulation, and LINE-1 dynamics were assessed using NRTI-medi-ated inhibition of LINE-1 DNA expansion. LINE-1 DNA copy numbers were quantitated in 30 trans-iliac biopsies from postmenopausal women (16 osteoporotic; 14 age and BMI balanced healthy controls) using qPCR with centromeric satellite alpha (SATA) DNA as endogenous control. The results were correlated to clinical parameters of bone metabolism. Inhibition of LINE-1 genomic expansion in developing osteoblasts using NRTI Lamivudine 3TC, reduced matrix mineralization (-60%) along with transcript markers of differentiated bone cells: Osteopontin (SPP1) (-31%), Osterix (SP7) (-50%) and Bone Sialoprotein (IBSP) (-60%). LINE-1 DNA copy number in bone of osteoporotic women was reduced with 70% and 30% in Line-1 open reading frame (ORF)1 and ORF2 encoding regions, respectively, compared with healthy controls (p<0.0005). LINE-1 DNA copy numbers measured from ORF1and ORF2 were positively correlated with head BMD (p=0.006 and p=0.004), spine L1-L4 (p=0.0006 and p=0.001) and total hip (p=0.0005 and p=0.003). In contrast, MSC differentiating to adipocytes were not affected by alterations in L1 dynamics. In conclusion, low LINE-1 copy number distinguishes the genome of postmenopausal osteoporotic bone, correlates positively to BMD and reduces the osteogenic potential of MSC differentiating to osteoblasts ex vivo without affecting fat cell development. The inhibition of LINE-1 trans-position plays a hitherto unrecognized mechanistic role in osteoporosis development and in NRTI-induced bone loss in patients on antiretroviral therapy.