Therefore, we hypothesize that the decrease in BMD caused by brain atrophy (i.e., small volume ratio of BrainSegVol-to-eTIV) may be related to the decrease in the expression of genes such as COL1A1 and COL1A2. In addition, several potential biological mechanisms may underlie the causal association between brain structure and BMD. The gene discussed is COL1A2; the disease is Brain atrophy.