Benezra 97-23522 The main objective of the research being proposed is to determine the function of the Id gene family during mammalian development using a genetic approach. The Id genes (Id 14) have been demonstrated both biochemically and in tissue culture models to be negative regulators of the basic helix-loop-helix (bHLH) family of transcription factors controlling cell fate decisions in such diverse processes as hematopoiesis, adipogenesis, myogenesis, osteogenesis and others. By analyzing the phenotypes of mice that harbor deletions in two of the Id genes (Idl and Id3) which are known to be expressed in similar patterns during development, the role of these proteins in controlling differentiation in vivo will be determined. In addition, it will be determined if disruption of the Id genes can compensate for the loss of the E protein family of bHLH proteins known to be targets for Id mediated repression biochemically. In this way, a direct genetic link between the Id and E protein families can be established genetically. Preliminary experiments indicate that indeed Idl and Id3 are required for post natal viability since double homozygous null mice developed in our laboratory usually die within one day of birth. These mice display evidence of proliferative and/or differentiation disorders in multiple tissue types and also a dramatic sex ratio distortion. These observations will be expanded by detailed molecular analyses both on tissue samples and fibroblasts derived from the mutant mice in order to define more clearly the role of the Id gene family during mammalian development. In addition, it has been observed that disruption of the Id genes can compensate for some of the defects observed in the E protein knockout mice. In particular, whereas homozygous disruption of the E proteins leads to postnatal lethality in a majority of mice, loss of both an E protein and Idl restores the postnatal viability of these mice. This observation will be extended to determine the cause of the lethali ty in the E protein knockout mice as well as the mechanism of rescue by the loss of Idl. These experiments will thus increase our understanding of the mechanism of action of the Id gene family and define more precisely the nature of the Id/ E protein interaction in viva.
