Expression of the mouse GH-releasing hormone (GRH) gene is restricted to neurons within the hypothalamus and to placenta. In an attempt to generate immortalized mouse hypothalamic neurons expressing GRH, the proximal 872-nucleotide segment of the 5'-flanking region of the hypothalamic mouse GRH gene was cloned by polymerase chain reaction and ligated to a 2.7-kilobase DNA sequence encoding the simian virus-40 (SV40) T-antigen, so that regulation of SV40 T-antigen expression was dependent on sequences within the mGRH 5'-flanking region. This region contains both TATA and CAAT boxes. The mouse GRH/SV40 T-antigen fusion gene was injected into 1-cell mouse embryos, and SV40 T-antigen incorporation in the mouse genome was found in 11 of 77 live births (3 males and 8 females). Although no evidence of hypothalamic tumors was found, all mice that expressed the transgene also developed tumors originating in the adrenal medulla. Gene copy number varied from 1-20 and was inversely proportional to survival, which ranged from 7-16 weeks. Corticosterone levels were normal. The male transgenic mice were fertile, and their progeny expressed the transgene and developed similar tumors. Microscopic examination of the tumors revealed a primitive neuroectodermal neoplasm that exhibited hematogenous and lymph node metastases and contained 100 ng norepinephrine, 2.85 ng epinephrine, and 1.1 ng dopamine/mg tumor tissue. Primary culture of dispersed tumor cells released norepinephrine into the medium (180 pg/ml.24 h). Cell lines from 2 tumors were established and exhibited characteristics similar to those of mixed neuroblastoma or primitive neuroectodermal tumors. In conclusion, the proximal 872 nucleotides of the hypothalamic mouse GRH promoter contain elements directing tissue-specific expression limited to early adrenal neuroectodermal cells. Other GRH DNA sequences appear to be required for restricted expression of mouse GRH within the hypothalamus.
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