Expression of human and suppression of mouse nucleolus organizer activity in mouse-human somatic cell hybrids

Most mouse-human somatic cell hybrids show preferential loss of human chromosomes, absence of human 28S ribosomal RNA, and suppression of human nucleolus organizer activity, as visualized by the Ag-AS silver histochemical stain. In contrast, the mouse-human hybrids studied here show preferential loss of mouse chromosomes. The hybrids were made by fusion of HT-1080-6TG human fibrosarcoma cells with BALB/c mouse peritoneal macrophages or strain 129 mouse teratocarcinoma cells. The Ag-AS staining method shows nucleolus organizer activity of chromosomes 13, 14, 15, 21 (rarely), and 22 in the human parent and chromosomes 12, 15, 16 (rarely), and 18 in the BALB/c mouse parent. In the hybrid cells the human nucleolus organizer regions are active, as shown by Ag-AS staining and involvement in "satellite association." The mouse nucleolus organizer regions are not stained by the Ag-AS method even though mouse chromosomes 12, 15, and 18 are present in the BALB/c hybrids and at least one copy of each mouse chromosome is present in the teratocarcinoma-derived hybrids. Thus, in these mouse-human hybrids, unlike those that lose human chromosomes, only human nucleolus organizer activity is expressed, and mouse nucleolus organizer activity is suppressed.