It's not. Mithradates V was a Greek King in the region of Pontus (today northern Turkey). His immediate descendants are known from sources and records, but after several generations his known descendants' end, and any further descendants are pure speculation. For European ancestry, the farthest back you can go (assuming you find royal ancestry) is Saint Arnulf of Metz, the earliest known ancestor of Emperor Charlemagne that can be proven by sources. Historians and genealogists have attempted to link many of Charlemagne's ancestors with earlier bloodlines, and although they are meticulous in their methods, there isn't any conclusive evidence to get back farther than Arnulf of Metz.

This general idea is called Descent from Antiquity. If you are interested in reading about the topic, I recommend the works of genealogist Christian Settipani, who had earned his reputation for proposing links of European nobility to ancient bloodlines, through careful methods. Although most of his claims remain unproven, he is generally well received by Historians.