SPP1 and obesity due to melanocortin 4 receptor deficiency: Due to the multifunctional properties of OPN and its ubiquitous expression in the body, its pivotal role in coexistent disorders like obesity, renal dysfunctions and cardiovascular diseases (CVDs) is currently debated [15,16], in which simultaneous etiologic factors can promote OPN signaling deregulation and aggravate the cross-disorders, like cardiac or renal–fat diseases [15,16,17].