A treatment for systemic DNA mutation diseases accompanied with development of somatic mosaicism and elevation of blood extracellular DNA and, more particularly, to a treatment of diabetes mellitus and atherosclerosis. The inventive method consist from introducing a treatment agent into a circulating blood system of a patient diagnosed with systemic DNA mutation diseases when said treatment agent destroys extracellular DNA in said blood of said patient and wherein said treatment agent used to destroy said extracellular DNA is a DNASE enzyme: said agent might be administered in doses and regimens which sufficient to decrease number average molecular weight of circulating extracellular blood DNA in the blood of said patient; such decrease of number average molecular weight might be measured by gel electrophoresis of extracellular blood DNA fraction from the blood of said patient. A DNASE enzyme may be further applied in a dose and regime that provide a DNA hydrolytic activity measured in blood plasma exceeding 1.5 Kunitz units per 1 ml of blood plasma for more than 12 hours within a period of 24 hours.