TOKYO, Jan 20 (Reuters) - Japan's Nikkei share average slipped to a one-week low on Monday as investors avoided taking risks as the weak-yen trend took a pause, while Nintendo Co Ltd plunged after the game maker forecast an annual loss. The Nikkei ended 0.6 percent lower at 15,641.68 after falling as low as 15,574.23, its lowest since Jan. 14. It has fallen for three straight days. The broader Topix fell 0.3 percent to 1,293.86 with only 2.2 billion shares changing hands, the lowest since Dec. 17, as trading was subdued with markets closed in the United States for a public holiday. Nintendo slid as much as 18.5 percent to 11,935 yen - its lowest since Nov. 8 - before ending down 6.2 percent. It was the most traded stock by turnover after the "Super Mario" creator said it now expects a full-year loss instead of a profit and slashed its Wii U sales forecast by almost 70 percent. The JPX-Nikkei Index 400, a recently introduced gauge comprised of firms with high return on equity and strong corporate governance, shed 0.3 percent to 11,676.73.