The molecular defect in hypotransferrinemic mice

Hypotransferrinemic (Trf(hpx/hpx)) mice have a severe deficiency in serum t ransferrin (Trf) as the result of a spontaneous mutation linked to the muri ne Trf locus. They are born alive, but before weaning, die from severe anem ia if they are not treated with exogenous Trf or red blood cell transfusion s. We have determined the molecular basis of the hpx mutation. It results f rom a single point mutation, which alters an invariable nucleotide in the s plice donor site after exon 76 of the Trf gene. No normal Trf messenger RNA (mRNA) is made from the hpx allele, A small amount of mRNA results from th e usage of cryptic splice sites within exon 16, The predominant cryptic spl ice site produces a Trf mRNA carrying a 27-base pair (bp), in-frame deletio n, Less than 1% of normal levels of a Trf-like protein is found in the seru m of Trf(hpx/hpx) mice, most likely resulting from translation of the inter nally deleted mRNA, Despite their severe Trf deficiency, however, Trf(hpx/h px) mice initially treated with transferrin injections can survive after we aning without any further treatment. They have massive tissue iron overload develop in all nonhematopoietic tissues, while they continue to have sever e iron deficiency anemia. Their liver iron burden is 100-fold greater than that of wild-type mice and 15- to 20-fold more then that of mice lacking th e hemochromatosis gene, Hfe. Trf(hpx/hpx) mice thus provide an additional m odel with a defined molecular defect for the study of genetic iron disorder s. (C) 2000 by The American Society of Hematology.