The nucleotide and predicted amino acid sequences of two variant cDNAs [rat brain thyroid hormone receptor (rTRa) vI and vI], isolated from a rat brain cDNA library by using the Pst I fragment of v-erbA, showed virtual identity with the rat brain thyroid hormone receptor (rTRa)

Generalized thyroid hormone resistance (GTHR) is a disorder of thyroid hormone action that we have previously shown to be tightly linked to one of the two thyroid hormone receptor genes, c-erbA,8, in a single kindred, A. We now show that in two other kindreds, B and D, with differing phenotypes, there is also linkage between c-erbAft and GTHR. The combined maximum logarithm of the odds score for all three kindreds at a recombination fraction of 0 was 5.77. In vivo studies had shown a triiodothyronine (T3)-binding affinity abnormality in nuclear receptors of kindred A, and we therefore investigated the defect in c-erbA,8 in this kindred by sequencing a major portion of the T3-binding domain in the 3'-region of fibroblast c-erbAfl cDNA and leukocyte c-erbAf genomic DNA. A base substitution, cytosine to adenine, was found at cDNA position 1643 which altered the proline codon at position 448 to a histidine. By allelic-specific hybridization, this base substitution was found in only one allele of seven affected members, and not found in 10 unaffected members of kindred A, as expected for a dominant disease. Also, this altered base was not found in kindreds B or D, or in 92 random c-erbA,8 alleles. These results and the fact that the mutation is predicted to alter the secondary structure of the crucial T3-binding domain of the c-erbAft receptor suggest this mutation is an excellent candidate for the genetic cause of GTHR in kindred A. Different mutations in the c-erbAfl gene are likely responsible for the variant phenotypes of thyroid hormone resistance in kindreds B and D. (J. Clin. Invest. 1990.85:93-100.) linkage -candidate mutation -proline codon 448 * histidine -triiodothyronine-binding domain

Thyroid hormone resistance (THR) is primarily an autosomal dominant inherited disease characterized by resistance of pituitary and peripheral tissues to the action of thyroid hormone. We investigated whether the heterogeneous phenotypic features that occur not only among kindreds but also within the same kindred might be due to the expression of differing ratios of mutant and normal receptors in tissues. Using an allele-specific primer extension method, we determined the relative expression of normal and mutant mRNAs from the fibroblasts of affected and unaffected members of two kindreds with TRH: A-H and N-N. While two affected members of A-H, as expected, had nearly equal amounts of normal and mutant hTR(3 mRNA, two other members had mutant mRNA levels that accounted for at least 70% of the hTR# mRNA. Phenotypic variability within and between kindreds with generalized resistance to thyroid hormone GRTH may be due to this differential expression of the mutant and wild type mRNA. Furthermore, when several clinical parameters of THR were compared in several affected members from two kindreds with GRTH, we found that two cases in one kindred exhibited a high mutant-tonormal hTR68 ratio and had considerably more bone resistance during their development. In certain kindreds with THR, differing ratios of normal and mutant hTR receptors may be age and growth related and may account for the reported attenuation of phenotypic symptoms with age. (J. Clin. Invest. 1993.

The solubilization and delivery of lipids in plasma rely on both forms of apolipoprotein B (apo B): apo B-100 and apo B-48. Apo B-48 is the translational product of apo B-100 mRNA that undergoes peritranscriptional conversion of C-+U, replacing codon CAA (glntamine 2,153) with the infirame stop codon (UAA). We examined mRNA editing activity in the human and the rat by reverse transcriptionpolymerase chain reaction primer-extension analysis of intestine and liver total RNA. In rat intestine the percentage of apo B transcripts that undergo editing increases dramatically the day before birth (from -1% to 80%), whereas the rat liver acquires an adult level of editing activity during the third postnatal week (rising from -8% to 30%), when weaning is completed, bile acid composition matures, and plasma thyroid hormone levels peak. In contrast to the rat, the human intestine acquires adult levels of apo B mRNA editing relatively early in fetal development, rising from 10% at 10 weeks to -80% by the end of the second trimester. Our results establish that apo B mRNA editing is 1) developmentally regulated in a tissue-and species-specific manner; 2) fully developed prenatally in both human and rat intestine, suggesting a crucial role of apo B-48 in mammalian fetal adaptation to extrauterine life; and 3) acquired early in human fetal intestine, implying a potential role for apo B-48 in prenatal lipid metabolism. 1 In humans this large hydrophobic protein circulates in two forms: apo B-100 and apo B-48. In humans apo B-100 (Af r -512,000) is synthesized primarily by the liver, is the major lipoprotein in low density lipoproteins (LDLs), and functions as the ligand for the LDL receptor.