Mouse ENU mutagenesis and molecular genetics approach to identify novel regulating bone homeostasis

[Truncated] Bone is a rigid organ, yet highly susceptible to metabolic changes throughout the adult life. A variety of bone cells; predominantly osteoclasts, osteoblasts, osteocytes and chondrocytes, regulate bone homeostasis and turnover rate through a series of multicellular signaling pathways. Bone homeostasis is continuously maintained by the bone remodeling process which is tightly regulated by two key activities: bone removal by osteoclasts and bone matrix formation by osteoblasts. Imbalances in either bone resorption or bone formation can lead to clinical diseases like osteoporosis, osteopetrosis and Paget`s disease of bone. Therefore, identification of novel genes and molecular pathways that regulate bone homeostasis may help us to develop new therapeutic strategies against bone diseases.

Osteoporosis is the most common pathological bone disorder observed in humans. It contributes to the risk of bone fragility and fractures associated with long-term disability and increased mortality rates. To gain insights into the molecular genetics and mechanisms of bone loss, a phenotype-driven N-ethyl-N-nitrosourea (ENU) mutant mouse screening approach, and molecular techniques, were employed to identify key molecules that regulate bone homeostasis. In the present study, we have identified two mutant mouse lines that exhibit reduced bone phenotypes.

The first mutant mouse strain (Morc3^+/-) has a heterozygous mutation at the splice donor site of intron 12 in the Microrchidia 3 (Morc3) gene. Morc3 is an epigenetic regulator of transcription and DNA damage response with previously unknown function in bone homeostasis. MicroCT analysis of 3 and 6 month old Morc3^+/- mice femurs revealed significant thinning of the cortical bone, with significantly reduced cortical area and cortical bone mineral density (BMD) (WT = 1.76 g/cm3 vs. Mutant = 1.71 g/cm3, p<0.05). However, no differences were observed in the trabecular bone parameters in 3 and 6 month old Morc3^+/- mice when compared to their WT littermates. Bone histomorphometry analysis of trabecular bone revealed similar results as determined by the microCT analysis with a significant reduction in Morc3^+/- osteoclast parameters in vivo.