Variegated gene expression in mice

Abstract

KENNETH DOBIE *~j, MAJID MEHTAH~, MARGARET McCLENAGHAN +~ AND RICHARD LATHE* kdobie@aim.salk.edu mehtali@tmnsgene.fr maggie.mcdenaghan@bbsrc.ac.uk dathe@ed.ac.uk *CENTRE FOR GENOME RESE~.RCH, UNIVERSITY OF EDINBURGH, WE.~ MAINS ROAD, EDINBURGH, UK EH9 3JQ. tRoSLIN INSTITUTE (EDINBL'RGH), ROSLIN, MIOLOTHIAN, UK EH25 9PS. CITI~'~SGENE SA, BUE DE MOhSHEt~L 67000 STRASBOERG, FRA.~CE. ~SALK lxs'n'~'rE, MBVL, 10010 N. TORP,~ PINES ROAD, LA JOLIA, CA 92027, USA. Tmnsgenic animals, particularly mice, are most-commonly generated by the microinjection of DNA into fertilized eggs. End-to-end ligation of the con- struct and/or homologous recombi- nation between circularized molecules is thought to generate a tmnsgene concatemer (often 5-50 copies) that integrates at a single site in the genome. While transgene expression can be stable and heritable, the tandem-repeat nature of the insertion appears to contribute to a phenom- enon akin to variegation, as described in insects and plants. This can gener- ate, by stochastic silencing of the trans- gene, a variable and mosaic pattem of transgene expression, Variegation can complicate the interpretation of some experiments using transgenic animals generated by microinjection. Below, we briefly discuss the evi- dence for variegation in mice, mecha- nisms of silencing and routes by which it can be avoided, and the possibility that some endogenous genes might be subject to variegation. Several different mechanisms can lead to variations in the level of (trans)gene expression within a single line. Variation can be due to strain- specific modifier genes, or to inte- gration or translocation into Y chro- mosome heterochromatinor to the X chromosome 1. Loss of function has also been observed when cells ex- pressing the construct have a selective disadvantage, so favouring loss of transgene DNA. For the most part, Wansgene loss appears uncommon although inverted repeats within a transgene array can favour deletion or rearrangement 2. Mosaic expression patterns have also been observed with tyrosinase fusion transgenes, although mosaic or striated expres- sion might be an imrinsic property of this locus3. As noted above, growing evidence now suggests that some transgenes can be subject to a phenomenon akin to the variegation found in insects and plants. Position effect variegation in