Retardation of cochlear maturation and impaired hair cell function caused by deletion of all known thyroid hormone receptors

The deafness caused by early onset hypothyroidism indicates that thyroid hormone is essential for the development of hearing. We investigated the underlying roles of the TRalpha1 and TRbeta thyroid hormone receptors in the auditory system using receptor-deficient mice. TRalpha1 and TRbeta, which act as hormone-activated transcription factors, are encoded by the Thra and Thrb genes, respectively, and both are expressed in the developing cochlea. TRbeta is required for hearing because TRbeta-deficient (Thrb(tm1/tm1)) mice have a defective auditory-evoked brainstem response and retarded expression of a potassium current (I(K,f)) in the cochlear inner hair cells. Here, we show that although TRalpha1 is individually dispensable, TRalpha1 and TRbeta synergistically control an extended array of functions in postnatal cochlear development. Compared with Thrb(tm1/tm1) mice, the deletion of all TRs in Thra(tm1/tm1)Thrb(tm1/tm1) mice produces exacerbated and novel phenotypes, including delayed differentiation of the sensory epithelium, malformation of the tectorial membrane, impairment of electromechanical transduction in outer hair cells, and a low endocochlear potential. The induction of I(K,f) in inner hair cells was not markedly more retarded than in Thrb(tm1/tm1) mice, suggesting that this feature of hair cell maturation is primarily TRbeta-dependent. These results indicate that distinct pathways mediated by TRbeta alone or by TRbeta and TRalpha1 together facilitate control over an extended range of functions during the maturation of the cochlea.

The two thyroid hormone receptor genes have opposite effects on estrogen-stimulated sex behaviors

The two genes coding for thyroid hormone receptors (TR) alpha 1 and beta have opposite effects on female sex behaviors. Deletion of TRalpha 1 reduced them, whereas deletion of TRbeta actually increased them. These results could not be attributed to altered levels of hormones in the blood, general alterations in estrogen responsiveness or altered general activity. Instead, they indicate a previously unknown molecular mechanism upon which the two TR genes exert opposite influences.

Chicken epidermal growth factor (EGF) receptor: cDNA cloning, expression in mouse cells, and differential binding of EGF and transforming growth factor alpha

The primary structure of the chicken epidermal growth factor (EGF) receptor was deduced from the sequence of a cDNA clone containing the complete coding sequence and shown to be highly homologous to the human EGF receptor. NIH-3T3 cells devoid of endogenous EGF receptor were transfected with the appropriate cDNA constructs and shown to express either chicken or human EGF receptors. Like the human EGF receptor, the chicken EGF receptor is a glycoprotein with an apparent molecular weight of 170,000. Murine EGF bound to the chicken receptor with approximately 100-fold lower affinity than to the human receptor molecule. Surprisingly, human transforming growth factor alpha (TGF-alpha) bound equally well or even better to the chicken EGF receptor than to the human EGF receptor. Moreover, TGF-alpha stimulated DNA synthesis 100-fold better than did EGF in NIH 3T3 cells that expressed the chicken EGF receptor. The differential binding and potency of mammalian EGF and TGF-alpha by the avian EGF receptor contrasts with the similar affinities of the mammalian receptor for the two growth factors.

Common site of mutation in the erbB gene of avian erythroblastosis virus mutants that are temperature sensitive for transformation

The genome of the avian erythroblastosis virus temperature sensitive mutant ts34 was cloned from a cell line that was shown to contain a single integrated copy of the virus. The mutation was localized to the v-erbB gene by making chimeric viruses between the mutant genome and that of wildtype. Sequencing of the mutant v-erbB gene revealed a single amino acid change of a histidine to an aspartate residue at a position equivalent to amino-acid 826 of the human epidermal growth factor receptor. Interestingly this is the identical mutation to that recently reported for another temperature sensitive mutant ts167, indicating that this may be a hot spot for mutations in the v-erbB gene that give rise to ts transformation mutants. In addition, the different biological phenotypes of ts34 and ts167 are suggested to be due to an additional mutation in v-erbA in ts167.

Localization of the oncogene c-erbA2 to human chromosome 3

The human c-erbA1 gene has been previously mapped to chromosome 17. We have now mapped c-erbA2 to the short arm of chromosome 3, using a human genomic probe in Southern analysis of DNA from a panel of human/mouse somatic cell hybrids. In situ hybridization using the same probe on metaphase chromosomes has enabled fine chromosome mapping of c-erbA2 to the chromosome region 3p21-pter.

Nucleotide sequence of HBI, a novel recombinant MC29 derivative with altered pathogenic properties

HBI is a recombinant avian retrovirus with novel pathogenic properties that was derived from the myc-containing virus MC29. In contrast to MC29, which causes endotheliomas in chickens, HBI induces lymphoid tumors. The results of molecular cloning and nucleotide sequencing of HBI reported here show that the virus contains sequences derived from both c-myc and ring-neck pheasant virus, in addition to MC29. The 3' half of the myc gene was largely replaced by c-myc sequences, and most of the long terminal repeat and gag regions were replaced by ring-neck pheasant virus sequences. The long terminal repeat contained a triplicate sequence which was homologous to the core enhancer sequence of the simian virus 40 72-base-pair repeat. The significance of these changes in relation to the unusual biological properties of the virus are discussed.

Chromosomal localisation of the human homologues to the oncogenes erbA and B

Avian erythroblastosis virus (AEV) induces acute erythroleukemia and sarcomas in vivo and it transforms erythroblasts and fibroblasts in vitro. The virus has two host cell-derived genes, v-erbA and v-erbB. The latter encodes the oncogenic capacity of the virus, whereas v-erbA enhances the erythroblast transforming effects of v-erbB while being unable to induce neoplasms independently. Recently, human cellular homologues of these viral erb genes have been isolated. The chromosomal locations of two of these genes have been determined using EcoRI-digested DNA prepared from human-mouse somatic cell hybrids. The human c-erbA1 gene has been assigned to chromosome 17 and is located between 17p11 and 17q21. The human c-erbB sequence has been assigned to chromosome 7 and is located between 7pter and 7q22. Thus, in the human genome these genes are on two separate chromosomes. No evidence for the involvement of the human c-erb genes in neoplasia has been found.

Isolation and characterization of c-myc, a cellular homolog of the oncogene (v-myc) of avian myelocytomatosis virus strain 29

The chicken genome contains nucleotide sequences homologous to transforming genes (oncogenes) of a number of avian retroviruses. We have isolated chicken DNA (c-myc) that is homologous to the oncogene (v-myc) of the avian myelocytomatosis virus MC29 and have compared the structures of the cellular and viral genes. Results from restriction endonuclease mapping of c-myc and from analysis of heteroduplexes between the DNAs of the cellular and viral genes show that c-myc is homologous to 1,500 nucleotides in v-myc DNA. This homologous region is interrupted in c-myc by an intron-like sequence of 1,100 nucleotides which is absent from v-myc. Nuclear RNA from normal chicken cells contains at least five species of transcripts from c-myc ranging from 2.5 to 6.5 kilobases in length. By contrast, cytoplasm contains only the 2.5-kilobase c-myc RNA. These features of the c-myc gene and its nuclear transcripts are characteristic of normal cellular genes and suggest that the myc gene is of cellular rather than viral origin. The exons in c-myc may define two functional domains in the gene and may therefore facilitate the dissection of the different oncogenic potentials of the MC29 virus.