Inactivation of the MouseHPRTLocus by a 203-bp Retroposon Insertion and a 55-kb Gene-Targeted Deletion: Establishment of New HPRT-Deficient Mouse Embryonic Stem Cell Lines

To obtain useful hypoxanthine phosphoribosyltransferase (HPRT)-deficient mouse ES cell lines, two different methods were employed: (i) selection of spontaneous 6-TG-resistant mutants and (ii) gene targeting of theHPRTlocus. The first approach resulted in the establishment of E14.1TG3B1, a spontaneous HPRT-deficient cell line with an insertional mutation of 203 bp in the third exon of theHPRTgene. The insert is highly homologous to the B2 mouse repetitive element and has all the expected retroposon characteristics, thus providing an example of gene inactivation by retroposon insertion. This clone exhibited stable 6-TG resistance and high germ-line transmission frequency. Thus E14.1TG3B1 is a useful ES cell line for modifying the mouse genome using theHPRTgene as a selection marker and for transmission at a high frequency into the mouse germ line. The second approach resulted in a 55-kb deletion of the mouseHPRTlocus, demonstrating the feasibility of replacement-targeting vectors to generate large genomic DNA deletions.