Evidently, there are no symptoms with osteopenia as it only relates to bone density. It can lead to breakage as it is related to osteoporsis which we all know leads to fracture if not successfully treated.
I don't know how much help that is to you. Mgus can cause problems though, if the blood viscosity increases where it causes mucus membranes to bleed, etc. I have pain, but I also have Lupus, fibromyalgia and a hemangioma, which all cause pain. So I couldn't say if my mgus causes pain or not. I have small bone lesions on my legs, but the doctor doesn't seem concerned as long as I don't have leg pain more than I do. He goes a lot by the blood light chain numbers.
Look up osteopenia on the web. It will give you several sites to read for more information.