Differential Stability of a Human Mini-Chromosome in Mouse Cell Lines

. A 4 Mb human mini-chromosome, ΔΔ2, was transferred from Chinese hamster ovary (CHO) cells into a mouse L cell line. The mini-chromosome could be transferred intact into the L cells, with 112/119 clones maintaining a mini-chromosome of the same size as the original. Ten clones were grown for 30 days in continuous culture. The mini-chromosomes were maintained stably with or without selection at a copy number of 1–2 per cell and none experienced any size alterations, as determined by pulsed-field gel electrophoresis. Thus ΔΔ2 is structurally and mitotically stable in L cells. This contrasts with results in embryonic stem cells, in which ΔΔ2 is highly unstable. These findings indicate that established somatic cell lines, such as L cells and CHO cells, have less stringent controls over centromeric function than do normal embryonic cells.