Mutational analyses of members of the fibroblast growth factor family and Hoxa-9 during mouse development

The complex pattern of events that govern embryogenesis and subsequent development is regulated by an intricate interplay of polypeptide growth factors and transcription factors. The work presented in this dissertation has focused on the role of one family of polypeptide growth factors-the fibroblast growth factors (FGFs)--and one family of transcription factors--the Hox homeoproteins. Using gene targeting and transgene insertion we have shown that members of both families are important during development of the mouse. Previously it was shown that FGF-3 is involved in the formation of the tail and inner ear. We have found that FGF-4 is required during postimplantation for the survival of the embryo and that FGF-5 is a negative regulator of hair growth. While Fgf-3, Fgf-4 and Fgf-5 are expressed in numerous tissue types throughout embryogenesis, mice homozygous for a targeted disruption in these genes show defects in only a subset of these tissues. This, in addition to the observation that these genes are often expressed in overlapping or adjacent domains, suggests that they have overlapping or redundant functions which might only be revealed in double mutants. To address this possibility, we used targeted mutagenesis to generate mice with mutations in two or more of these genes. The analysis of mice with disruptions of both alleles of Fgf-3 and one allele of Fgf-4 revealed that FGF-3 and FGF-4 function synergistically during the development of the embryonic vasculature system and the external ear. The analysis of mice with disruptions in both Fgf-3 and Fgf-5 showed that FGF-3 and FGF-5 also cooperate in the formation and/or maintenance of the embryonic vasculature. To investigate the function(s) of one member of the Hox gene family, we functionally inactivated Hoxa-9 in the mouse using gene targeting. In addition, we overexpressed Hoxa-9 via transgene insertion. We show that the loss-of-function mutation results in anterior homeotic transformations whereas the gain-of-function mutation results in posterior homeotic transformations. Thus, a critical dosage of Hoxa-9 is important for correct specification of regional identity along the anterior-posterior body axis.