THE AMES DWARF GENE, DF, IS REQUIRED EARLY IN PITUITARY ONTOGENY FOR THE EXTINCTION OF RPX TRANSCRIPTION AND INITIATION OF LINEAGE-SPECIFICCELL-PROLIFERATION

Two nonallelic dwarfing mutations in mice define genes important for p ituitary development and function. Mice homozygous for either the Ames (df) or Snell (Pit1(dw)) dwarf mutations exhibit severe proportional dwarfism, hypothyroidism, and infertility due to the cytodifferentiati on failure of three anterior pituitary cell types: thyrotropes, somato tropes, and lactotropes. Analysis of double heterozygotes and double m utants has provided evidence that the df and dw genes act sequentially in the same genetic pathway. Double heterozygotes had no reduction in growth rate or final adult size. Double homozygotes had essentially t he same phenotype as the single mutants and were recovered at the pred icted frequency, indicating that there are no previously unrecognized, redundant functions of the two genes. Several lines of evidence demon strate that df acts earlier in the differentiation pathway than Pit1. The df mutants fail to extinguish expression of the homeobox gene Rpx on embryonic day 13.5 (e13.5), and the size of their nascent pituitary glands is reduced by e14.5. In contrast, Pit1(dw) mutants down-regula te Rpx appropriately and exhibit normal cell proliferation up to e14.5 . The failure to extinguish Rpx and the concomitant hypocellutarity of df pituitaries suggest the importance of Rpx repression in lineage-sp ecific cell proliferation before the appearance of lineage-specific ma rkers. Later, Pit-1 and hypothalamic neuropeptides act sequentially to regulate marker gene transcription and cell proliferation. These resu lts establish the time of df action in a cascade of genes that regulat e pituitary ontogeny.