Mutations within the
growth hormone receptor (GHR) gene that lead to an inactivated or truncated GHR
protein cause abnormal growth and small adult size in a variety of species (
Laron dwarfism). We studied a line of miniature Bos indicus cattle that have phenotypic (small mature size) and endocrine (increased blood
growth hormone and decreased blood
insulin-like growth factor-I concentrations) similarities to Laron dwarfs. Liver
mRNA from miniature and control cattle was used to amplify a
cDNA within the coding region of the GHR. The miniature cattle had GHR
mRNA size (determined by Northern blot) and
cDNA sequence that were similar to control cattle and, therefore, were unlike most Laron dwarf genotypes in which the GHR gene is mutated. Amounts of
mRNA from liver as well as muscle (superficial neck and longissimus) were analyzed by
ribonuclease protection assay for
IGF-I, total GHR, GHR 1A (inducible, liver-specific GHR
mRNA), and GHR 1B (constitutive GHR
mRNA). Four control and five miniature bulls were tested. As expected, liver
IGF-I mRNA was decreased in the miniature cattle (approximately 12% of control; P < 0.01). The amount of the total GHR as well as GHR 1A
mRNA were also decreased in liver (17% and 19% of control, respectively; P < 0.01). Other GHR
mRNA, including GHR 1B
mRNA, were similar for miniature and control cattle. In muscle, there was a tendency (P < 0.10) for decreased
IGF-I mRNA and increased GHR
mRNA in miniature compared with control cattle. In summary, a novel phenotype for
Laron dwarfism in Bos indicus cattle was associated with underexpression of GHR 1A
mRNA, but not other GHR
mRNA variants in liver. In addition to decreased GHR 1A
mRNA, the miniature cattle had decreased liver
IGF-I mRNA. Full expression of GHR 1A in liver, therefore, may be required for full liver
IGF-I expression and normal growth.