Nickel is a known carcinogen. When in the diet in toxic amounts it contributes to oxidative stress, just as mercury and cadmium do, by reducing glutathione thereby interfering with cell membrane integrity and increasing lipid peroxidation (1). The oxidative damage, like from free iron or copper, can cause DNA damage (2).
1. Valko M, Morris H, Cronin MT. Metals, toxicity and oxidative stress. Curr Med Chem 2005;12:1161-208.
2. Tkeshelashvili LK, Reid TM, McBride TJ, Loeb LA. Nickel induces a signature mutation for oxygen free radical damage. Cancer Research; 53, 4172-4174, September 15, 1993.