21-P040 Tbx4 does not rescue limb-type specification in Pitx1−/− mouse

Vertebrates have two pairs of morphologically distinct appendages, the forelimbs and the hindlimbs. Although the mechanisms necessary for limb outgrowth and polarity are becoming fairly well-understood little is known about how limb-type identity is regulated. The homeobox containing transcription factor Pitx1 expressed in hindlimb bud mesenchyme and has been shown to be involved in limb-type determination. A complication to the interpretation of the Pitx1 phenotype has arisen following the demonstration that Pitx1 activity is required for normal expression of Tbx4. Since Tbx4 is required for normal limb outgrowth some of the abnormalities observed in the Pitx1 hindlimb could be explained by down regulation of Tbx4. We are using transgenic lines expressing Tbx4, Tbx5 or Pitx1 in combination with Pitx1 mouse to study the activity of Pitx1 independently of outgrowth defects. Both Tbx4 and Tbx5 rescued hindlimbs are closer to a wild-type size but none of the hindlimb characteristics are compensated by the expression of these T-box genes. Our data show that Pitx1 function is necessary for hindlimb patterning and is consistent with Tbx4 and Tbx5 being insufficient to determine limb-type but contributing to limb initiation and outgrowth.