DYSFUNCTIONAL GROWTH-HORMONE RECEPTOR IN A STRAIN OF SEX-LINKED DWARFCHICKEN - EVIDENCE FOR A MUTATION IN THE INTRACELLULAR DOMAIN

The sex-linked dwarf (dwdw) chicken represents a valuable animal model for studying GH insensitivity and the consequence of mutations in the GH receptor (GHR) gene. We have recently reported undetectable hepati c GH-binding activity and an aberrantly sized transcript in a strain o f dwdw chickens obtained from Arbor Acre Farms, Inc. (Glastonbury, CT, USA). Southern blot analysis of the chicken GHR (cGHR) gene revealed a restriction-fragment length polymorphism in HindIII and EcoRI digest s of genomic DNA in this strain of dwdw chicken. In order to localize the molecular mutation, we analysed the gene structure and determined the complete sequence of the 3' untranslated region (3' UTR) of the no rmal cGHR. With the use of this information, we located a large deleti on in the 3' end of the cGHR gene of the Connecticut (CT) strain of dw dw chicken. This deletion (1773 bp) contained 27 highly conserved amin o acids of the 3' end of the coding region, the in-frame stop codon, a less frequently used poly(A) signal that is normally found 445 bp dow nstream of the stop codon, and a large portion of the 3' UTR. Because of this deletion, 27 novel amino acids were substituted and the open r eading frame was extended for an additional 26 amino acids before reac hing the transcriptional termination site. The predicted amino acid se quence of the novel carboxyl-terminus of the dwdw cGHR is largely hydr ophobic with a polylysine tail, whereas the carboxyl-terminus of the w ild-type (DwDw) cGHR is composed of hydrophilic amino acids. Western b lot analysis using antisera directed against the extracellular domain of the cGHR confirmed that the mutant transcript was translated. These observations suggested that the mutation in the intracellular domain results in expression of a dysfunctional cGHR which could account for the phenotype of the CT dwdw chicken.