2 ACUTE MONOCYTIC LEUKEMIA (AML-M5A) CELL-LINES (MOLM-13 AND MOLM-14)WITH INTERCLONAL PHENOTYPIC HETEROGENEITY SHOWING MLL-AF9 FUSION RESULTING FROM AN OCCULT CHROMOSOME INSERTION, INS(11-9)(Q23-P22P23)

We describe two new human leukemia cell lines, MOLM-13 and MOLM-14, es tablished from the peripheral blood of a patient at relapse of acute m onocytic leukemia, FAB M5a, which had evolved from myelodysplastic syn drome (MDS). Both cell lines express monocyte-specific esterase (MSE) and MLL-AF9 fusion mRNA. Gene fusion is associated with a minute chrom osomal insertion, ins(11;9)(q23;p22p23). MOLM-13 and MOLM-14 are the f irst cell lines with, and represent the third reported case of, MLL ge ne rearrangement arising via chromosomal insertion. Both cell lines ca rry trisomy 8 which was also present during the MDS phase, as well as the most frequent trisomies associated with t(9;11), ie, +6, +13, +19 variously present in different subclones. Despite having these feature s in common, differences in antigen expression were noted between the two cell lines: that of MOLM-13 being CD34(+), CD13(-), CD14(-), CD15( +), CD33(+); whereas MOLM-14 was CD4(+), CD13(+), CD14(+), CD15(+), CD 33(+). Differentiation to macrophage-like morphology could be induced in both cell lines after stimulation with INF-gamma alone, or in combi nation with TNF-alpha, which treatment also induced or upregulated, ex pression of certain myelomonocyte-associated antigens, including CD13, CD14, CD15, CD64, CD65 and CD87. Together, these data confirm that bo th cell lines are likely to be novel in vitro models for studying mono cytic differentiation and leukemogenesis.