STRUCTURAL AND NUMERICAL CHROMOSOMAL-ABERRATIONS IN A METABOLICALLY COMPETENT HUMAN LYMPHOBLAST CELL-LINE (MCL-5)

MCL-5 cells are Epstein Barr virus-transformed human lymphoblasts whic h have been genetically engineered for use in mutagenicity testing. We have examined the modal chromosome number, karyotype and spontaneous micronucleus (MN) and sister chromatid exchange (SCE) frequencies of t he cell line. Replicate experiments were conducted on two different sh ipments purchased from Gentest Corp. Although the modal chromosome num ber was 48 (range 40-54, n = 400 metaphases) for both cell shipments, the second stock showed greater variation in chromosome number than th e first. a total of 60 G-banded metaphase cells was analyzed and seven karyotypes were prepared. Consistent structural abnormalities (transl ocations, deletions and isochromosomes) were found involving the X chr omosome and seven autosomes (1-3, 5, 6, 9 and 11), The karyotype typic al of this cell line was: 48,der(X)t(X;?)(p22.3;?) Y,t(1;2)(q23;p23),d el(3)(q12q12),+(3q),t(5;6) (q31;p23), +i(9p),der(11)t(11;13)(q23;q12). The mean MN frequency was 41.8 MN/1000 binucleate cells (n = 5000), W hen compared with our historical controls for primary lymphocyte cultu res this number (41.8) is significantly (8.4-fold) higher, The mean SC E frequency was 7.3 per metaphase (n = 100). We observed a hyperdiploi d chromosome number of 48 in the majority of metaphase spreads, indica ting a significant deviation from the normal diploid number characteri stic of the parent cells (RPMI 1788) established in 1969. The variatio n ire chromosome number distribution observed between shipments sugges ts the potential for further changes, The elevated MN frequency sugges ts that evaluating mutagenicity using this cytogenetic end-point may r equire excessive dosing to produce a significant response over backgro und. We conclude that careful interpretation of cytogenetic end-points is necessary when using MCL-5 cells in the light of the possibility o f clonal evolution presented here.