OVEREXPRESSION OF A TRUNCATED GROWTH-HORMONE RECEPTOR IN THE SEX-LINKED DWARF CHICKEN - EVIDENCE FOR A SPLICE MUTATION

Sex-linked dwarfism in chickens is a term of GH resistance that resembles the Laron syndrome in humans. The dwarfism found in chickens is due to a mutant gene (dw) carried on the sex chromosome. The homozygous dwarf (dwdw) chicken is characterized by reductions in stature and plasma insulin-like growth factor-I (IGF-I) levels. Despite the absence of hepatic GH-binding activity, Southern blot analysis shows that there is no gross structural change in the gene for the GH receptor (GHR) in this strain of dwdw chicken. GH-dependent IGF-I production can be restored in cultured dwdw hepatocytes after transfection and transient expression of a chicken GHR (cGHR) cDNA, indicating that other factors that participate in GH-mediated IGF-I synthesis are intact. Northern blot analysis of liver, muscle, fat, and pituitary RNA from normal (DwDw) chickens shows a major transcript of 4.3 kilobases (kb) and three minor transcripts (0.8, 1.7, and 3.2 kb), which correspond to the cGHR. In contrast, the 0.8-kb transcript is the major cGHR transcript expressed in these tissues from dwdw chickens. Northern blot analysis with domain-specific probes shows that the 0.8-kb transcript in DwDw and dwdw liver contains only a small portion of the extracellular domain of the cGHR. A cDNA clone encoding this transcript has been isolated from a liver library prepared from a normal chicken. The 0.8-kb transcript corresponds to the first 325 nucleotides of the coding region of the cGHR, followed by a poly(A) tail. The presence of the poly(A) tail in the 0.8-kb transcript of both DwDw and dwdw birds was confirmed by an RNase H protection assay. Sequence analysis of the intron immediately downstream of this cleavage-polyade-nylation site shows a T-->C mutation of the GT in the 5'-splice donor site. These results suggest that the GHR deficiency of the sex-linked dwarf chicken is due to inappropriate splicing and destabilization of the full-length transcript, which would account for the absence of a functional GHR in these dwarfs.