U.S. stock index futures pointed to a lower open on Wall Street on Tuesday, ahead of a flurry of macro data that could shed light on the extent of the economic downturn. * At 0855 GMT S&P 500 March futures SPc1 were down 0.02 percent, Dow Jones futures DJc2 were down 0.2 percent and Nasdaq 100 futures NDc2 were down 0.4 percent. * Investors were bracing for quarterly final GDP figures, as well as U.S. monthly existing home sales. Any surprises on the data front could have a stronger-than-usual impact because of the holiday-thinned trading volumes. * The final figure for third quarter GDP should confirm that the economy contracted at an annual rate of 0.5 percent in the period. Forecasts among the 64 economists polled by Reuters ranged from minus 0.5 percent to minus 0.8 percent. * Energy shares won't get much relief on the commodity front, with oil prices extending their sharp fall to drop further below $40 a barrel on Tuesday, weakened anew by growing signs of deteriorating world oil demand. * Linux software provider Red Hat Inc (RHT.N) reported a quarterly profit on Monday that topped Wall Street's forecast and also issued a strong outlook, sending shares up more than 6 percent in extended trading. * On Monday, the Dow Jones industrial average .DJI fell 59.42 points, or 0.69 percent, to 8,519.69. The Standard & Poor's 500 Index .SPX was down 16.25 points, or 1.83 percent, at 871.63. The Nasdaq Composite Index .IXIC gave up 31.97 points, or 2.04 percent, at 1,532.35.

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