Alterations in Osteopontin Modify Muscle Size in Females in Both Humans and Mice.

PURPOSE: An osteopontin (OPN; SPP1) gene promoter polymorphism modifies disease severity in Duchenne muscular dystrophy, and we hypothesized that it might also modify muscle phenotypes in healthy volunteers. METHODS: Gene association studies were carried out for OPN (rs28357094) in the FAMuSS cohort (n=752; age 23.7±5.7 yrs). Phenotypes studied included muscle size (MRI), strength, and response to supervised resistance training. We also studied 147 young adults that had carried out a bout of eccentric elbow exercise (age 24.0 ± 5.2yrs). Phenotypes analyzed included strength, soreness, and serum muscle enzymes. RESULTS: In the FAMuSS cohort, the G allele was associated with 17% increase in baseline upper arm muscle volume only in women (F=26.32; p=5.32 × 10), explaining 5% of population variance. In the eccentric damage cohort, weak associations of the G allele were seen in women with both baseline myoglobin, and elevated CK. Sexually dimorphic effects of OPN on muscle were also seen in OPN null mice. Five of seven muscle groups examined showed smaller size in OPN null female mice, whereas two were smaller in males. Query of OPN gene transcription after experimental muscle damage in mice showed rapid induction within 12 hrs (100-fold increase from baseline), followed by sustained high level expression through 16 days of regeneration before falling to back to baseline. CONCLUSIONS: OPN is a sexually dimorphic modifier of muscle size in normal humans and mice, and responds to muscle damage. The OPN gene is known to be estrogen responsive, and this may explain the female-specific genotype effects in adult volunteers.