Dec 17 (Reuters) - The U.S. Securities and Exchange Commission has delayed its decision on a spot bitcoin exchange-traded fund (ETF) proposal from Grayscale Bitcoin Trust, the world"s largest digital currency manager. In a notice dated Wednesday, the markets regulator said it now expects to review Grayscale"s proposal by Feb. 6, later than the original December deadline. In October, Grayscale, which has over $45 billion of cryptocurrency assets under management, had said it plans to convert its Grayscale Bitcoin Trust (GBTC.PK) into a spot bitcoin ETF. The first U.S. bitcoin futures-based exchange-traded fund also began trading that month, taking bitcoin"s price to a six-month high. However, the cryptocurrency has since eased and it is currently trading below $50,000.