The human beta-globin gene and a functional viral thymidine kinase gene in developing mice

Two foreign cloned genes--one encoding a tissue-specific protein and one encoding a constitutive enzyme--were introduced into mouse eggs by microinjection into a pronucleus shortly after fertilization. They were the adult human genomic beta-globin gene and the thymidine kinase (TK; ATP:thymidine 5'-phosphotransferase, EC 2.7.1.21) gene of herpes simplex virus (HSV), ligated in the pBR322 plasmid. Thirty-three developing mice were autopsied in late fetal life; all appeared normal. Blot hybridization tests revealed that the DNA of as many as five (15%) of the fetuses (from separate litters), and of their corresponding placentas, contained copies of the human beta-globin gene and of the HSV tk gene that had been retained and replicated without significant loss or rearrangement. The estimated total numbers of copies per cell were 3-50 for the donor globin gene and 3-20 for the donor tk. In some of the fetuses, these totals included some copies of molecular weight higher than that of the intact sequence; the additional segments may have arisen through changes such as deletions or duplications. The foreign genes in the five positive fetuses appear to be present in high molecular weight DNA. Assays capable of distinguishing between foreign and native TK indicated that at least one of the fetuses with the HSV tk gene had some TK enzyme of the HSV type and, therefore, at least one gene copy that was being accurately transcribed and translated to produce a functional protein, despite the absence of selective pressure. Thus, pure recombinant genes introduced into mice at the onset of their development can remain intact and be stably incorporated and even expressed. These experiments provide a practical basis for novel investigations of the developmental control of normal gene expression in vivo of the causes and possible cures of genetic diseases.