The little (lit) mutation cosegregates with the growth hormone releasing factor receptor on mouse chromosome 6.

The little (lit) autosomal recessive mutation in the mouse causes dwarfism due to isolated growth hormone deficiency. The in vitro physiology of pituitary growth hormone release in lit/lit animals suggests that an abnormality in the growth hormone releasing factor (GRF) receptor (Ghrfr) is a very likely candidate for the lit mutation. We mapped Ghrfr to the region around lit on Chromosome (Chr) 6 in 100 chromosomes of an FVB x Czech II interspecific backcross. Molecular markers were Neuropeptide Y (Npy), homeobox (Hoxa2), immunoglobulin kappa chain (Igk), wingless-related MMTV integration site (Wnt-2), cystic fibrosis (Cftr), carboxypeptidase A (Cpa), and Ghrfr. Map order and distances were as follows: Cen-II-Wnt-2-(0 cM)-Cftr-(6.0 cM)-Cpa-(8.0 cM)-Npy-(1.0 cM)-Hoxa2-(3.0 cM)-Ghrfr-(2.0 cM)-Igk. We mapped lit (by phenotype) relative to Hoxa2 and Igk on 72 F2 chromosomes of offspring of a B6CZ lit/ + x B6FVB lit/ + intercross and 18 chromosomes of offspring of a B6FVB lit/ + intercross. Map order and distances were as follows: Hoxa2-(2.1 cM)-lit/Ghrfr-(3.7 cM)-Igk. No recombinations between lit and Ghrfr were detected. Thus, Ghrfr maps to mouse Chr 6 and may be allelic with lit. Amplification of a short segment at the 3' end of the Ghrfr mRNA by reverse transcription coupled to the polymerase chain reaction showed a greatly diminished level of GRF receptor mRNA in the pituitaries of lit/lit mice as compared with lit/ + controls. Low level of message could reflect a primary molecular defect or be due to secondary hypoplasia of somatotropes.(ABSTRACT TRUNCATED AT 250 WORDS)