* Yen remains on defensive, giving boost to Nikkei. * MSCI Asia ex-Japan little changed. * Most Asian markets closed for Christmas holiday. Uncertainty over whether U.S. lawmakers will strike a deal by an end-of-year deadline to avert a severe fiscal retrenchment undermined the yen and bolstered Japanese shares, but trading volume was low with many participants on Christmas holiday. House of Representatives Speaker John Boehner failed to gain support for a tax plan at the end of last week, raising fears that the United States may face the "fiscal cliff" of some $600 billion in automatic spending cuts and tax ... (full story)