In Portal 2, how did CEO of Aperture Science, Cave Johnson, presumably die?

In Portal 2, it is presumed that Cave Johnson, the CEO of Aperture Science, died from Moon Rock Poisoning. This is revealed through various audio recordings found throughout the game, where Cave Johnson talks about his experiments with moon rocks and their harmful effects on humans. He mentions how the scientists who worked with the moon rocks started developing strange symptoms and illnesses, which ultimately led to his own demise. Therefore, Moon Rock Poisoning is believed to be the cause of Cave Johnson's death in Portal 2.