The Nikkei stock index fell in early trading Tuesday as investors locked in gains after two days of advances, with some exporters weighed down by the yen’s appreciation against the U.S. dollar overnight. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 112.08 points, or 0.29 percent, from Monday to 38,810.95. The broader Topix index was up 0.07 point at 2,798.14. On the top-tier Prime Market, decliners were led by mining, rubber product and transportation equipment issues. At 9 a.m., the dollar fetched 156.08-11 yen compared with 155.98-156.08 yen in New York and 157…