LEP and obesity due to melanocortin 4 receptor deficiency: These findings are also in line with previous studies showing that leptin-deficient (ob/ob) and leptin receptor–deficient (db/db) transgenic mice, which naturally become obese on control chow diets, do not develop knee OA, further suggesting that body weight by itself may not be a risk factor for joint degeneration in obesity under a control diet [18].